Imported Upstream version 1.0.0~beta

[rustc.git] / src / librustc_typeck / check / mod.rs

// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

/*

# check.rs

Within the check phase of type check, we check each item one at a time
(bodies of function expressions are checked as part of the containing
function).  Inference is used to supply types wherever they are
unknown.

By far the most complex case is checking the body of a function. This
can be broken down into several distinct phases:

- gather: creates type variables to represent the type of each local
  variable and pattern binding.

- main: the main pass does the lion's share of the work: it
  determines the types of all expressions, resolves
  methods, checks for most invalid conditions, and so forth.  In
  some cases, where a type is unknown, it may create a type or region
  variable and use that as the type of an expression.

  In the process of checking, various constraints will be placed on
  these type variables through the subtyping relationships requested
  through the `demand` module.  The `infer` module is in charge
  of resolving those constraints.

- regionck: after main is complete, the regionck pass goes over all
  types looking for regions and making sure that they did not escape
  into places they are not in scope.  This may also influence the
  final assignments of the various region variables if there is some
  flexibility.

- vtable: find and records the impls to use for each trait bound that
  appears on a type parameter.

- writeback: writes the final types within a function body, replacing
  type variables with their final inferred types.  These final types
  are written into the `tcx.node_types` table, which should *never* contain
  any reference to a type variable.

## Intermediate types

While type checking a function, the intermediate types for the
expressions, blocks, and so forth contained within the function are
stored in `fcx.node_types` and `fcx.item_substs`.  These types
may contain unresolved type variables.  After type checking is
complete, the functions in the writeback module are used to take the
types from this table, resolve them, and then write them into their
permanent home in the type context `ccx.tcx`.

This means that during inferencing you should use `fcx.write_ty()`
and `fcx.expr_ty()` / `fcx.node_ty()` to write/obtain the types of
nodes within the function.

The types of top-level items, which never contain unbound type
variables, are stored directly into the `tcx` tables.

n.b.: A type variable is not the same thing as a type parameter.  A
type variable is rather an "instance" of a type parameter: that is,
given a generic function `fn foo<T>(t: T)`: while checking the
function `foo`, the type `ty_param(0)` refers to the type `T`, which
is treated in abstract.  When `foo()` is called, however, `T` will be
substituted for a fresh type variable `N`.  This variable will
eventually be resolved to some concrete type (which might itself be
type parameter).

*/

pub use self::LvaluePreference::*;
pub use self::Expectation::*;
pub use self::compare_method::compare_impl_method;
use self::TupleArgumentsFlag::*;

use astconv::{self, ast_region_to_region, ast_ty_to_ty, AstConv, PathParamMode};
use check::_match::pat_ctxt;
use fmt_macros::{Parser, Piece, Position};
use middle::astconv_util::{check_path_args, NO_TPS, NO_REGIONS};
use middle::def;
use middle::infer;
use middle::mem_categorization as mc;
use middle::mem_categorization::McResult;
use middle::pat_util::{self, pat_id_map};
use middle::privacy::{AllPublic, LastMod};
use middle::region::{self, CodeExtent};
use middle::subst::{self, Subst, Substs, VecPerParamSpace, ParamSpace, TypeSpace};
use middle::traits;
use middle::ty::{FnSig, GenericPredicates, TypeScheme};
use middle::ty::{Disr, ParamTy, ParameterEnvironment};
use middle::ty::{self, HasProjectionTypes, RegionEscape, ToPolyTraitRef, Ty};
use middle::ty::liberate_late_bound_regions;
use middle::ty::{MethodCall, MethodCallee, MethodMap, ObjectCastMap};
use middle::ty_fold::{TypeFolder, TypeFoldable};
use rscope::RegionScope;
use session::Session;
use {CrateCtxt, lookup_full_def, require_same_types};
use TypeAndSubsts;
use lint;
use util::common::{block_query, ErrorReported, indenter, loop_query};
use util::ppaux::{self, Repr};
use util::nodemap::{DefIdMap, FnvHashMap, NodeMap};
use util::lev_distance::lev_distance;

use std::cell::{Cell, Ref, RefCell};
use std::mem::replace;
use std::rc::Rc;
use std::iter::repeat;
use std::slice;
use syntax::{self, abi, attr};
use syntax::attr::AttrMetaMethods;
use syntax::ast::{self, DefId, Visibility};
use syntax::ast_util::{self, local_def};
use syntax::codemap::{self, Span};
use syntax::feature_gate;
use syntax::owned_slice::OwnedSlice;
use syntax::parse::token;
use syntax::print::pprust;
use syntax::ptr::P;
use syntax::visit::{self, Visitor};

mod assoc;
pub mod dropck;
pub mod _match;
pub mod vtable;
pub mod writeback;
pub mod implicator;
pub mod regionck;
pub mod coercion;
pub mod demand;
pub mod method;
mod upvar;
pub mod wf;
mod closure;
mod callee;
mod compare_method;
mod op;

/// closures defined within the function.  For example:
///
///     fn foo() {
///         bar(move|| { ... })
///     }
///
/// Here, the function `foo()` and the closure passed to
/// `bar()` will each have their own `FnCtxt`, but they will
/// share the inherited fields.
pub struct Inherited<'a, 'tcx: 'a> {
    infcx: infer::InferCtxt<'a, 'tcx>,
    locals: RefCell<NodeMap<Ty<'tcx>>>,
    param_env: ty::ParameterEnvironment<'a, 'tcx>,

    // Temporary tables:
    node_types: RefCell<NodeMap<Ty<'tcx>>>,
    item_substs: RefCell<NodeMap<ty::ItemSubsts<'tcx>>>,
    adjustments: RefCell<NodeMap<ty::AutoAdjustment<'tcx>>>,
    method_map: MethodMap<'tcx>,
    upvar_capture_map: RefCell<ty::UpvarCaptureMap>,
    closure_tys: RefCell<DefIdMap<ty::ClosureTy<'tcx>>>,
    closure_kinds: RefCell<DefIdMap<ty::ClosureKind>>,
    object_cast_map: ObjectCastMap<'tcx>,

    // A mapping from each fn's id to its signature, with all bound
    // regions replaced with free ones. Unlike the other tables, this
    // one is never copied into the tcx: it is only used by regionck.
    fn_sig_map: RefCell<NodeMap<Vec<Ty<'tcx>>>>,

    // Tracks trait obligations incurred during this function body.
    fulfillment_cx: RefCell<traits::FulfillmentContext<'tcx>>,

    // When we process a call like `c()` where `c` is a closure type,
    // we may not have decided yet whether `c` is a `Fn`, `FnMut`, or
    // `FnOnce` closure. In that case, we defer full resolution of the
    // call until upvar inference can kick in and make the
    // decision. We keep these deferred resolutions grouped by the
    // def-id of the closure, so that once we decide, we can easily go
    // back and process them.
    deferred_call_resolutions: RefCell<DefIdMap<Vec<DeferredCallResolutionHandler<'tcx>>>>,

    deferred_cast_checks: RefCell<Vec<CastCheck<'tcx>>>,
}

trait DeferredCallResolution<'tcx> {
    fn resolve<'a>(&mut self, fcx: &FnCtxt<'a,'tcx>);
}

type DeferredCallResolutionHandler<'tcx> = Box<DeferredCallResolution<'tcx>+'tcx>;

/// Reifies a cast check to be checked once we have full type information for
/// a function context.
struct CastCheck<'tcx> {
    expr: ast::Expr,
    expr_ty: Ty<'tcx>,
    cast_ty: Ty<'tcx>,
    span: Span,
}

/// When type-checking an expression, we propagate downward
/// whatever type hint we are able in the form of an `Expectation`.
#[derive(Copy, Clone)]
pub enum Expectation<'tcx> {
    /// We know nothing about what type this expression should have.
    NoExpectation,

    /// This expression should have the type given (or some subtype)
    ExpectHasType(Ty<'tcx>),

    /// This expression will be cast to the `Ty`
    ExpectCastableToType(Ty<'tcx>),

    /// This rvalue expression will be wrapped in `&` or `Box` and coerced
    /// to `&Ty` or `Box<Ty>`, respectively. `Ty` is `[A]` or `Trait`.
    ExpectRvalueLikeUnsized(Ty<'tcx>),
}

impl<'tcx> Expectation<'tcx> {
    // Disregard "castable to" expectations because they
    // can lead us astray. Consider for example `if cond
    // {22} else {c} as u8` -- if we propagate the
    // "castable to u8" constraint to 22, it will pick the
    // type 22u8, which is overly constrained (c might not
    // be a u8). In effect, the problem is that the
    // "castable to" expectation is not the tightest thing
    // we can say, so we want to drop it in this case.
    // The tightest thing we can say is "must unify with
    // else branch". Note that in the case of a "has type"
    // constraint, this limitation does not hold.

    // If the expected type is just a type variable, then don't use
    // an expected type. Otherwise, we might write parts of the type
    // when checking the 'then' block which are incompatible with the
    // 'else' branch.
    fn adjust_for_branches<'a>(&self, fcx: &FnCtxt<'a, 'tcx>) -> Expectation<'tcx> {
        match *self {
            ExpectHasType(ety) => {
                let ety = fcx.infcx().shallow_resolve(ety);
                if !ty::type_is_ty_var(ety) {
                    ExpectHasType(ety)
                } else {
                    NoExpectation
                }
            }
            ExpectRvalueLikeUnsized(ety) => {
                ExpectRvalueLikeUnsized(ety)
            }
            _ => NoExpectation
        }
    }
}

#[derive(Copy, Clone)]
pub struct UnsafetyState {
    pub def: ast::NodeId,
    pub unsafety: ast::Unsafety,
    from_fn: bool
}

impl UnsafetyState {
    pub fn function(unsafety: ast::Unsafety, def: ast::NodeId) -> UnsafetyState {
        UnsafetyState { def: def, unsafety: unsafety, from_fn: true }
    }

    pub fn recurse(&mut self, blk: &ast::Block) -> UnsafetyState {
        match self.unsafety {
            // If this unsafe, then if the outer function was already marked as
            // unsafe we shouldn't attribute the unsafe'ness to the block. This
            // way the block can be warned about instead of ignoring this
            // extraneous block (functions are never warned about).
            ast::Unsafety::Unsafe if self.from_fn => *self,

            unsafety => {
                let (unsafety, def) = match blk.rules {
                    ast::UnsafeBlock(..) => (ast::Unsafety::Unsafe, blk.id),
                    ast::DefaultBlock => (unsafety, self.def),
                };
                UnsafetyState{ def: def,
                             unsafety: unsafety,
                             from_fn: false }
            }
        }
    }
}

#[derive(Clone)]
pub struct FnCtxt<'a, 'tcx: 'a> {
    body_id: ast::NodeId,

    // This flag is set to true if, during the writeback phase, we encounter
    // a type error in this function.
    writeback_errors: Cell<bool>,

    // Number of errors that had been reported when we started
    // checking this function. On exit, if we find that *more* errors
    // have been reported, we will skip regionck and other work that
    // expects the types within the function to be consistent.
    err_count_on_creation: usize,

    ret_ty: ty::FnOutput<'tcx>,

    ps: RefCell<UnsafetyState>,

    inh: &'a Inherited<'a, 'tcx>,

    ccx: &'a CrateCtxt<'a, 'tcx>,
}

impl<'a, 'tcx> mc::Typer<'tcx> for FnCtxt<'a, 'tcx> {
    fn node_ty(&self, id: ast::NodeId) -> McResult<Ty<'tcx>> {
        let ty = self.node_ty(id);
        self.resolve_type_vars_or_error(&ty)
    }
    fn expr_ty_adjusted(&self, expr: &ast::Expr) -> McResult<Ty<'tcx>> {
        let ty = self.adjust_expr_ty(expr, self.inh.adjustments.borrow().get(&expr.id));
        self.resolve_type_vars_or_error(&ty)
    }
    fn type_moves_by_default(&self, span: Span, ty: Ty<'tcx>) -> bool {
        let ty = self.infcx().resolve_type_vars_if_possible(&ty);
        !traits::type_known_to_meet_builtin_bound(self.infcx(), self, ty, ty::BoundCopy, span)
    }
    fn node_method_ty(&self, method_call: ty::MethodCall)
                      -> Option<Ty<'tcx>> {
        self.inh.method_map.borrow()
                           .get(&method_call)
                           .map(|method| method.ty)
                           .map(|ty| self.infcx().resolve_type_vars_if_possible(&ty))
    }
    fn node_method_origin(&self, method_call: ty::MethodCall)
                          -> Option<ty::MethodOrigin<'tcx>>
    {
        self.inh.method_map.borrow()
                           .get(&method_call)
                           .map(|method| method.origin.clone())
    }
    fn adjustments(&self) -> &RefCell<NodeMap<ty::AutoAdjustment<'tcx>>> {
        &self.inh.adjustments
    }
    fn is_method_call(&self, id: ast::NodeId) -> bool {
        self.inh.method_map.borrow().contains_key(&ty::MethodCall::expr(id))
    }
    fn temporary_scope(&self, rvalue_id: ast::NodeId) -> Option<CodeExtent> {
        self.param_env().temporary_scope(rvalue_id)
    }
    fn upvar_capture(&self, upvar_id: ty::UpvarId) -> Option<ty::UpvarCapture> {
        self.inh.upvar_capture_map.borrow().get(&upvar_id).cloned()
    }
}

impl<'a, 'tcx> ty::ClosureTyper<'tcx> for FnCtxt<'a, 'tcx> {
    fn param_env<'b>(&'b self) -> &'b ty::ParameterEnvironment<'b,'tcx> {
        &self.inh.param_env
    }

    fn closure_kind(&self,
                    def_id: ast::DefId)
                    -> Option<ty::ClosureKind>
    {
        self.inh.closure_kinds.borrow().get(&def_id).cloned()
    }

    fn closure_type(&self,
                    def_id: ast::DefId,
                    substs: &subst::Substs<'tcx>)
                    -> ty::ClosureTy<'tcx>
    {
        self.inh.closure_tys.borrow().get(&def_id).unwrap().subst(self.tcx(), substs)
    }

    fn closure_upvars(&self,
                      def_id: ast::DefId,
                      substs: &Substs<'tcx>)
                      -> Option<Vec<ty::ClosureUpvar<'tcx>>>
    {
        ty::closure_upvars(self, def_id, substs)
    }
}

impl<'a, 'tcx> Inherited<'a, 'tcx> {
    fn new(tcx: &'a ty::ctxt<'tcx>,
           param_env: ty::ParameterEnvironment<'a, 'tcx>)
           -> Inherited<'a, 'tcx> {
        Inherited {
            infcx: infer::new_infer_ctxt(tcx),
            locals: RefCell::new(NodeMap()),
            param_env: param_env,
            node_types: RefCell::new(NodeMap()),
            item_substs: RefCell::new(NodeMap()),
            adjustments: RefCell::new(NodeMap()),
            method_map: RefCell::new(FnvHashMap()),
            object_cast_map: RefCell::new(NodeMap()),
            upvar_capture_map: RefCell::new(FnvHashMap()),
            closure_tys: RefCell::new(DefIdMap()),
            closure_kinds: RefCell::new(DefIdMap()),
            fn_sig_map: RefCell::new(NodeMap()),
            fulfillment_cx: RefCell::new(traits::FulfillmentContext::new()),
            deferred_call_resolutions: RefCell::new(DefIdMap()),
            deferred_cast_checks: RefCell::new(Vec::new()),
        }
    }

    fn normalize_associated_types_in<T>(&self,
                                        typer: &ty::ClosureTyper<'tcx>,
                                        span: Span,
                                        body_id: ast::NodeId,
                                        value: &T)
                                        -> T
        where T : TypeFoldable<'tcx> + Clone + HasProjectionTypes + Repr<'tcx>
    {
        let mut fulfillment_cx = self.fulfillment_cx.borrow_mut();
        assoc::normalize_associated_types_in(&self.infcx,
                                             typer,
                                             &mut *fulfillment_cx, span,
                                             body_id,
                                             value)
    }

}

// Used by check_const and check_enum_variants
pub fn blank_fn_ctxt<'a, 'tcx>(ccx: &'a CrateCtxt<'a, 'tcx>,
                               inh: &'a Inherited<'a, 'tcx>,
                               rty: ty::FnOutput<'tcx>,
                               body_id: ast::NodeId)
                               -> FnCtxt<'a, 'tcx> {
    FnCtxt {
        body_id: body_id,
        writeback_errors: Cell::new(false),
        err_count_on_creation: ccx.tcx.sess.err_count(),
        ret_ty: rty,
        ps: RefCell::new(UnsafetyState::function(ast::Unsafety::Normal, 0)),
        inh: inh,
        ccx: ccx
    }
}

fn static_inherited_fields<'a, 'tcx>(ccx: &'a CrateCtxt<'a, 'tcx>)
                                    -> Inherited<'a, 'tcx> {
    // It's kind of a kludge to manufacture a fake function context
    // and statement context, but we might as well do write the code only once
    let param_env = ty::empty_parameter_environment(ccx.tcx);
    Inherited::new(ccx.tcx, param_env)
}

struct CheckItemTypesVisitor<'a, 'tcx: 'a> { ccx: &'a CrateCtxt<'a, 'tcx> }

impl<'a, 'tcx> Visitor<'tcx> for CheckItemTypesVisitor<'a, 'tcx> {
    fn visit_item(&mut self, i: &'tcx ast::Item) {
        check_item(self.ccx, i);
        visit::walk_item(self, i);
    }

    fn visit_ty(&mut self, t: &'tcx ast::Ty) {
        match t.node {
            ast::TyFixedLengthVec(_, ref expr) => {
                check_const_in_type(self.ccx, &**expr, self.ccx.tcx.types.usize);
            }
            _ => {}
        }

        visit::walk_ty(self, t);
    }
}

pub fn check_item_types(ccx: &CrateCtxt) {
    let krate = ccx.tcx.map.krate();
    let mut visit = wf::CheckTypeWellFormedVisitor::new(ccx);
    visit::walk_crate(&mut visit, krate);

    // If types are not well-formed, it leads to all manner of errors
    // downstream, so stop reporting errors at this point.
    ccx.tcx.sess.abort_if_errors();

    let mut visit = CheckItemTypesVisitor { ccx: ccx };
    visit::walk_crate(&mut visit, krate);

    ccx.tcx.sess.abort_if_errors();

    for drop_method_did in ccx.tcx.destructors.borrow().iter() {
        if drop_method_did.krate == ast::LOCAL_CRATE {
            let drop_impl_did = ccx.tcx.map.get_parent_did(drop_method_did.node);
            match dropck::check_drop_impl(ccx.tcx, drop_impl_did) {
                Ok(()) => {}
                Err(()) => {
                    assert!(ccx.tcx.sess.has_errors());
                }
            }
        }
    }

    ccx.tcx.sess.abort_if_errors();
}

fn check_bare_fn<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
                           decl: &'tcx ast::FnDecl,
                           body: &'tcx ast::Block,
                           fn_id: ast::NodeId,
                           fn_span: Span,
                           raw_fty: Ty<'tcx>,
                           param_env: ty::ParameterEnvironment<'a, 'tcx>)
{
    match raw_fty.sty {
        ty::ty_bare_fn(_, ref fn_ty) => {
            let inh = Inherited::new(ccx.tcx, param_env);

            // Compute the fty from point of view of inside fn.
            let fn_sig =
                fn_ty.sig.subst(ccx.tcx, &inh.param_env.free_substs);
            let fn_sig =
                liberate_late_bound_regions(ccx.tcx,
                                            region::DestructionScopeData::new(body.id),
                                            &fn_sig);
            let fn_sig =
                inh.normalize_associated_types_in(&inh.param_env, body.span, body.id, &fn_sig);

            let fcx = check_fn(ccx, fn_ty.unsafety, fn_id, &fn_sig,
                               decl, fn_id, body, &inh);

            vtable::select_all_fcx_obligations_and_apply_defaults(&fcx);
            upvar::closure_analyze_fn(&fcx, fn_id, decl, body);
            vtable::select_all_fcx_obligations_or_error(&fcx);
            fcx.check_casts();
            regionck::regionck_fn(&fcx, fn_id, fn_span, decl, body);
            writeback::resolve_type_vars_in_fn(&fcx, decl, body);
        }
        _ => ccx.tcx.sess.impossible_case(body.span,
                                 "check_bare_fn: function type expected")
    }
}

struct GatherLocalsVisitor<'a, 'tcx: 'a> {
    fcx: &'a FnCtxt<'a, 'tcx>
}

impl<'a, 'tcx> GatherLocalsVisitor<'a, 'tcx> {
    fn assign(&mut self, _span: Span, nid: ast::NodeId, ty_opt: Option<Ty<'tcx>>) -> Ty<'tcx> {
        match ty_opt {
            None => {
                // infer the variable's type
                let var_ty = self.fcx.infcx().next_ty_var();
                self.fcx.inh.locals.borrow_mut().insert(nid, var_ty);
                var_ty
            }
            Some(typ) => {
                // take type that the user specified
                self.fcx.inh.locals.borrow_mut().insert(nid, typ);
                typ
            }
        }
    }
}

impl<'a, 'tcx> Visitor<'tcx> for GatherLocalsVisitor<'a, 'tcx> {
    // Add explicitly-declared locals.
    fn visit_local(&mut self, local: &'tcx ast::Local) {
        let o_ty = match local.ty {
            Some(ref ty) => Some(self.fcx.to_ty(&**ty)),
            None => None
        };
        self.assign(local.span, local.id, o_ty);
        debug!("Local variable {} is assigned type {}",
               self.fcx.pat_to_string(&*local.pat),
               self.fcx.infcx().ty_to_string(
                   self.fcx.inh.locals.borrow().get(&local.id).unwrap().clone()));
        visit::walk_local(self, local);
    }

    // Add pattern bindings.
    fn visit_pat(&mut self, p: &'tcx ast::Pat) {
        if let ast::PatIdent(_, ref path1, _) = p.node {
            if pat_util::pat_is_binding(&self.fcx.ccx.tcx.def_map, p) {
                let var_ty = self.assign(p.span, p.id, None);

                self.fcx.require_type_is_sized(var_ty, p.span,
                                               traits::VariableType(p.id));

                debug!("Pattern binding {} is assigned to {} with type {}",
                       token::get_ident(path1.node),
                       self.fcx.infcx().ty_to_string(
                           self.fcx.inh.locals.borrow().get(&p.id).unwrap().clone()),
                       var_ty.repr(self.fcx.tcx()));
            }
        }
        visit::walk_pat(self, p);
    }

    fn visit_block(&mut self, b: &'tcx ast::Block) {
        // non-obvious: the `blk` variable maps to region lb, so
        // we have to keep this up-to-date.  This
        // is... unfortunate.  It'd be nice to not need this.
        visit::walk_block(self, b);
    }

    // Since an expr occurs as part of the type fixed size arrays we
    // need to record the type for that node
    fn visit_ty(&mut self, t: &'tcx ast::Ty) {
        match t.node {
            ast::TyFixedLengthVec(ref ty, ref count_expr) => {
                self.visit_ty(&**ty);
                check_expr_with_hint(self.fcx, &**count_expr, self.fcx.tcx().types.usize);
            }
            _ => visit::walk_ty(self, t)
        }
    }

    // Don't descend into fns and items
    fn visit_fn(&mut self, _: visit::FnKind<'tcx>, _: &'tcx ast::FnDecl,
                _: &'tcx ast::Block, _: Span, _: ast::NodeId) { }
    fn visit_item(&mut self, _: &ast::Item) { }

}

/// Helper used by check_bare_fn and check_expr_fn. Does the grungy work of checking a function
/// body and returns the function context used for that purpose, since in the case of a fn item
/// there is still a bit more to do.
///
/// * ...
/// * inherited: other fields inherited from the enclosing fn (if any)
fn check_fn<'a, 'tcx>(ccx: &'a CrateCtxt<'a, 'tcx>,
                      unsafety: ast::Unsafety,
                      unsafety_id: ast::NodeId,
                      fn_sig: &ty::FnSig<'tcx>,
                      decl: &'tcx ast::FnDecl,
                      fn_id: ast::NodeId,
                      body: &'tcx ast::Block,
                      inherited: &'a Inherited<'a, 'tcx>)
                      -> FnCtxt<'a, 'tcx>
{
    let tcx = ccx.tcx;
    let err_count_on_creation = tcx.sess.err_count();

    let arg_tys = &fn_sig.inputs;
    let ret_ty = fn_sig.output;

    debug!("check_fn(arg_tys={}, ret_ty={}, fn_id={})",
           arg_tys.repr(tcx),
           ret_ty.repr(tcx),
           fn_id);

    // Create the function context.  This is either derived from scratch or,
    // in the case of function expressions, based on the outer context.
    let fcx = FnCtxt {
        body_id: body.id,
        writeback_errors: Cell::new(false),
        err_count_on_creation: err_count_on_creation,
        ret_ty: ret_ty,
        ps: RefCell::new(UnsafetyState::function(unsafety, unsafety_id)),
        inh: inherited,
        ccx: ccx
    };

    // Remember return type so that regionck can access it later.
    let mut fn_sig_tys: Vec<Ty> =
        arg_tys.iter()
        .cloned()
        .collect();

    if let ty::FnConverging(ret_ty) = ret_ty {
        fcx.require_type_is_sized(ret_ty, decl.output.span(), traits::ReturnType);
        fn_sig_tys.push(ret_ty);
    }

    debug!("fn-sig-map: fn_id={} fn_sig_tys={}",
           fn_id,
           fn_sig_tys.repr(tcx));

    inherited.fn_sig_map.borrow_mut().insert(fn_id, fn_sig_tys);

    {
        let mut visit = GatherLocalsVisitor { fcx: &fcx, };

        // Add formal parameters.
        for (arg_ty, input) in arg_tys.iter().zip(decl.inputs.iter()) {
            // Create type variables for each argument.
            pat_util::pat_bindings(
                &tcx.def_map,
                &*input.pat,
                |_bm, pat_id, sp, _path| {
                    let var_ty = visit.assign(sp, pat_id, None);
                    fcx.require_type_is_sized(var_ty, sp,
                                              traits::VariableType(pat_id));
                });

            // Check the pattern.
            let pcx = pat_ctxt {
                fcx: &fcx,
                map: pat_id_map(&tcx.def_map, &*input.pat),
            };
            _match::check_pat(&pcx, &*input.pat, *arg_ty);
        }

        visit.visit_block(body);
    }

    check_block_with_expected(&fcx, body, match ret_ty {
        ty::FnConverging(result_type) => ExpectHasType(result_type),
        ty::FnDiverging => NoExpectation
    });

    for (input, arg) in decl.inputs.iter().zip(arg_tys.iter()) {
        fcx.write_ty(input.id, *arg);
    }

    fcx
}

pub fn check_struct(ccx: &CrateCtxt, id: ast::NodeId, span: Span) {
    let tcx = ccx.tcx;

    check_representable(tcx, span, id, "struct");
    check_instantiable(tcx, span, id);

    if ty::lookup_simd(tcx, local_def(id)) {
        check_simd(tcx, span, id);
    }
}

pub fn check_item<'a,'tcx>(ccx: &CrateCtxt<'a,'tcx>, it: &'tcx ast::Item) {
    debug!("check_item(it.id={}, it.ident={})",
           it.id,
           ty::item_path_str(ccx.tcx, local_def(it.id)));
    let _indenter = indenter();

    match it.node {
      ast::ItemStatic(_, _, ref e) |
      ast::ItemConst(_, ref e) => check_const(ccx, it.span, &**e, it.id),
      ast::ItemEnum(ref enum_definition, _) => {
        check_enum_variants(ccx,
                            it.span,
                            &enum_definition.variants,
                            it.id);
      }
      ast::ItemFn(ref decl, _, _, _, ref body) => {
        let fn_pty = ty::lookup_item_type(ccx.tcx, ast_util::local_def(it.id));
        let param_env = ParameterEnvironment::for_item(ccx.tcx, it.id);
        check_bare_fn(ccx, &**decl, &**body, it.id, it.span, fn_pty.ty, param_env);
      }
      ast::ItemImpl(_, _, _, _, _, ref impl_items) => {
        debug!("ItemImpl {} with id {}", token::get_ident(it.ident), it.id);

        let impl_pty = ty::lookup_item_type(ccx.tcx, ast_util::local_def(it.id));

          match ty::impl_trait_ref(ccx.tcx, local_def(it.id)) {
              Some(impl_trait_ref) => {
                check_impl_items_against_trait(ccx,
                                               it.span,
                                               &*impl_trait_ref,
                                               impl_items);
              }
              None => { }
          }

        for impl_item in impl_items {
            match impl_item.node {
                ast::MethodImplItem(ref sig, ref body) => {
                    check_method_body(ccx, &impl_pty.generics, sig, body,
                                      impl_item.id, impl_item.span);
                }
                ast::TypeImplItem(_) |
                ast::MacImplItem(_) => {
                    // Nothing to do here.
                }
            }
        }

      }
      ast::ItemTrait(_, ref generics, _, ref trait_items) => {
        check_trait_on_unimplemented(ccx, generics, it);
        let trait_def = ty::lookup_trait_def(ccx.tcx, local_def(it.id));
        for trait_item in trait_items {
            match trait_item.node {
                ast::MethodTraitItem(_, None) => {
                    // Nothing to do, since required methods don't have
                    // bodies to check.
                }
                ast::MethodTraitItem(ref sig, Some(ref body)) => {
                    check_method_body(ccx, &trait_def.generics, sig, body,
                                      trait_item.id, trait_item.span);
                }
                ast::TypeTraitItem(..) => {
                    // Nothing to do.
                }
            }
        }
      }
      ast::ItemStruct(..) => {
        check_struct(ccx, it.id, it.span);
      }
      ast::ItemTy(ref t, ref generics) => {
        let pty_ty = ty::node_id_to_type(ccx.tcx, it.id);
        check_bounds_are_used(ccx, t.span, &generics.ty_params, pty_ty);
      }
      ast::ItemForeignMod(ref m) => {
        if m.abi == abi::RustIntrinsic {
            for item in &m.items {
                check_intrinsic_type(ccx, &**item);
            }
        } else {
            for item in &m.items {
                let pty = ty::lookup_item_type(ccx.tcx, local_def(item.id));
                if !pty.generics.types.is_empty() {
                    span_err!(ccx.tcx.sess, item.span, E0044,
                        "foreign items may not have type parameters");
                }

                if let ast::ForeignItemFn(ref fn_decl, _) = item.node {
                    if fn_decl.variadic && m.abi != abi::C {
                        span_err!(ccx.tcx.sess, item.span, E0045,
                                  "variadic function must have C calling convention");
                    }
                }
            }
        }
      }
      _ => {/* nothing to do */ }
    }
}

fn check_trait_on_unimplemented<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
                               generics: &ast::Generics,
                               item: &ast::Item) {
    if let Some(ref attr) = item.attrs.iter().find(|a| {
        a.check_name("rustc_on_unimplemented")
    }) {
        if let Some(ref istring) = attr.value_str() {
            let parser = Parser::new(&istring);
            let types = &*generics.ty_params;
            for token in parser {
                match token {
                    Piece::String(_) => (), // Normal string, no need to check it
                    Piece::NextArgument(a) => match a.position {
                        // `{Self}` is allowed
                        Position::ArgumentNamed(s) if s == "Self" => (),
                        // So is `{A}` if A is a type parameter
                        Position::ArgumentNamed(s) => match types.iter().find(|t| {
                            t.ident.as_str() == s
                        }) {
                            Some(_) => (),
                            None => {
                                span_err!(ccx.tcx.sess, attr.span, E0230,
                                                 "there is no type parameter \
                                                          {} on trait {}",
                                                           s, item.ident.as_str());
                            }
                        },
                        // `{:1}` and `{}` are not to be used
                        Position::ArgumentIs(_) | Position::ArgumentNext => {
                            span_err!(ccx.tcx.sess, attr.span, E0231,
                                                  "only named substitution \
                                                   parameters are allowed");
                        }
                    }
                }
            }
        } else {
            span_err!(ccx.tcx.sess, attr.span, E0232,
                                  "this attribute must have a value, \
                                   eg `#[rustc_on_unimplemented = \"foo\"]`")
        }
    }
}

/// Type checks a method body.
///
/// # Parameters
///
/// * `item_generics`: generics defined on the impl/trait that contains
///   the method
/// * `self_bound`: bound for the `Self` type parameter, if any
/// * `method`: the method definition
fn check_method_body<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
                               item_generics: &ty::Generics<'tcx>,
                               sig: &'tcx ast::MethodSig,
                               body: &'tcx ast::Block,
                               id: ast::NodeId, span: Span) {
    debug!("check_method_body(item_generics={}, id={})",
            item_generics.repr(ccx.tcx), id);
    let param_env = ParameterEnvironment::for_item(ccx.tcx, id);

    let fty = ty::node_id_to_type(ccx.tcx, id);
    debug!("check_method_body: fty={}", fty.repr(ccx.tcx));

    check_bare_fn(ccx, &sig.decl, body, id, span, fty, param_env);
}

fn check_impl_items_against_trait<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
                                            impl_span: Span,
                                            impl_trait_ref: &ty::TraitRef<'tcx>,
                                            impl_items: &[P<ast::ImplItem>]) {
    // Locate trait methods
    let tcx = ccx.tcx;
    let trait_items = ty::trait_items(tcx, impl_trait_ref.def_id);

    // Check existing impl methods to see if they are both present in trait
    // and compatible with trait signature
    for impl_item in impl_items {
        match impl_item.node {
            ast::MethodImplItem(_, ref body) => {
                let impl_method_def_id = local_def(impl_item.id);
                let impl_item_ty = ty::impl_or_trait_item(ccx.tcx,
                                                          impl_method_def_id);

                // If this is an impl of a trait method, find the
                // corresponding method definition in the trait.
                let opt_trait_method_ty =
                    trait_items.iter()
                               .find(|ti| ti.name() == impl_item_ty.name());
                match opt_trait_method_ty {
                    Some(trait_method_ty) => {
                        match (trait_method_ty, &impl_item_ty) {
                            (&ty::MethodTraitItem(ref trait_method_ty),
                             &ty::MethodTraitItem(ref impl_method_ty)) => {
                                compare_impl_method(ccx.tcx,
                                                    &**impl_method_ty,
                                                    impl_item.span,
                                                    body.id,
                                                    &**trait_method_ty,
                                                    &*impl_trait_ref);
                            }
                            _ => {
                                // This is span_bug as it should have already been
                                // caught in resolve.
                                tcx.sess.span_bug(
                                    impl_item.span,
                                    &format!("item `{}` is of a different kind from its trait `{}`",
                                             token::get_name(impl_item_ty.name()),
                                             impl_trait_ref.repr(tcx)));
                            }
                        }
                    }
                    None => {
                        // This is span_bug as it should have already been
                        // caught in resolve.
                        tcx.sess.span_bug(
                            impl_item.span,
                            &format!("method `{}` is not a member of trait `{}`",
                                     token::get_name(impl_item_ty.name()),
                                     impl_trait_ref.repr(tcx)));
                    }
                }
            }
            ast::TypeImplItem(_) => {
                let typedef_def_id = local_def(impl_item.id);
                let typedef_ty = ty::impl_or_trait_item(ccx.tcx,
                                                        typedef_def_id);

                // If this is an impl of an associated type, find the
                // corresponding type definition in the trait.
                let opt_associated_type =
                    trait_items.iter()
                               .find(|ti| ti.name() == typedef_ty.name());
                match opt_associated_type {
                    Some(associated_type) => {
                        match (associated_type, &typedef_ty) {
                            (&ty::TypeTraitItem(_), &ty::TypeTraitItem(_)) => {}
                            _ => {
                                // This is `span_bug` as it should have
                                // already been caught in resolve.
                                tcx.sess.span_bug(
                                    impl_item.span,
                                    &format!("item `{}` is of a different kind from its trait `{}`",
                                             token::get_name(typedef_ty.name()),
                                             impl_trait_ref.repr(tcx)));
                            }
                        }
                    }
                    None => {
                        // This is `span_bug` as it should have already been
                        // caught in resolve.
                        tcx.sess.span_bug(
                            impl_item.span,
                            &format!(
                                "associated type `{}` is not a member of \
                                 trait `{}`",
                                token::get_name(typedef_ty.name()),
                                impl_trait_ref.repr(tcx)));
                    }
                }
            }
            ast::MacImplItem(_) => tcx.sess.span_bug(impl_item.span,
                                                     "unexpanded macro")
        }
    }

    // Check for missing items from trait
    let provided_methods = ty::provided_trait_methods(tcx, impl_trait_ref.def_id);
    let mut missing_methods = Vec::new();
    for trait_item in &*trait_items {
        match *trait_item {
            ty::MethodTraitItem(ref trait_method) => {
                let is_implemented =
                    impl_items.iter().any(|ii| {
                        match ii.node {
                            ast::MethodImplItem(..) => {
                                ii.ident.name == trait_method.name
                            }
                            ast::TypeImplItem(_) |
                            ast::MacImplItem(_) => false,
                        }
                    });
                let is_provided =
                    provided_methods.iter().any(|m| m.name == trait_method.name);
                if !is_implemented && !is_provided {
                    missing_methods.push(format!("`{}`", token::get_name(trait_method.name)));
                }
            }
            ty::TypeTraitItem(ref associated_type) => {
                let is_implemented = impl_items.iter().any(|ii| {
                    match ii.node {
                        ast::TypeImplItem(_) => {
                            ii.ident.name == associated_type.name
                        }
                        ast::MethodImplItem(..) |
                        ast::MacImplItem(_) => false,
                    }
                });
                if !is_implemented {
                    missing_methods.push(format!("`{}`", token::get_name(associated_type.name)));
                }
            }
        }
    }

    if !missing_methods.is_empty() {
        span_err!(tcx.sess, impl_span, E0046,
            "not all trait items implemented, missing: {}",
            missing_methods.connect(", "));
    }
}

fn report_cast_to_unsized_type<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                         span: Span,
                                         t_span: Span,
                                         e_span: Span,
                                         t_1: Ty<'tcx>,
                                         t_e: Ty<'tcx>,
                                         id: ast::NodeId) {
    let tstr = fcx.infcx().ty_to_string(t_1);
    fcx.type_error_message(span, |actual| {
        format!("cast to unsized type: `{}` as `{}`", actual, tstr)
    }, t_e, None);
    match t_e.sty {
        ty::ty_rptr(_, ty::mt { mutbl: mt, .. }) => {
            let mtstr = match mt {
                ast::MutMutable => "mut ",
                ast::MutImmutable => ""
            };
            if ty::type_is_trait(t_1) {
                span_help!(fcx.tcx().sess, t_span, "did you mean `&{}{}`?", mtstr, tstr);
            } else {
                span_help!(fcx.tcx().sess, span,
                           "consider using an implicit coercion to `&{}{}` instead",
                           mtstr, tstr);
            }
        }
        ty::ty_uniq(..) => {
            span_help!(fcx.tcx().sess, t_span, "did you mean `Box<{}>`?", tstr);
        }
        _ => {
            span_help!(fcx.tcx().sess, e_span,
                       "consider using a box or reference as appropriate");
        }
    }
    fcx.write_error(id);
}


fn check_cast<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>, cast: &CastCheck<'tcx>) {
    fn cast_through_integer_err<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                          span: Span,
                                          t_1: Ty<'tcx>,
                                          t_e: Ty<'tcx>) {
        fcx.type_error_message(span, |actual| {
            format!("illegal cast; cast through an \
                    integer first: `{}` as `{}`",
                    actual,
                    fcx.infcx().ty_to_string(t_1))
        }, t_e, None);
    }

    let span = cast.span;
    let e = &cast.expr;
    let t_e = structurally_resolved_type(fcx, span, cast.expr_ty);
    let t_1 = structurally_resolved_type(fcx, span, cast.cast_ty);

    // Check for trivial casts.
    if !ty::type_has_ty_infer(t_1) {
        if let Ok(()) = coercion::mk_assignty(fcx, e, t_e, t_1) {
            if ty::type_is_numeric(t_1) && ty::type_is_numeric(t_e) {
                fcx.tcx().sess.add_lint(lint::builtin::TRIVIAL_NUMERIC_CASTS,
                                        e.id,
                                        span,
                                        format!("trivial numeric cast: `{}` as `{}`. Cast can be \
                                                 replaced by coercion, this might require type \
                                                 ascription or a temporary variable",
                                                fcx.infcx().ty_to_string(t_e),
                                                fcx.infcx().ty_to_string(t_1)));
            } else {
                fcx.tcx().sess.add_lint(lint::builtin::TRIVIAL_CASTS,
                                        e.id,
                                        span,
                                        format!("trivial cast: `{}` as `{}`. Cast can be \
                                                 replaced by coercion, this might require type \
                                                 ascription or a temporary variable",
                                                fcx.infcx().ty_to_string(t_e),
                                                fcx.infcx().ty_to_string(t_1)));
            }
            return;
        }
    }

    let t_e_is_bare_fn_item = ty::type_is_bare_fn_item(t_e);
    let t_e_is_scalar = ty::type_is_scalar(t_e);
    let t_e_is_integral = ty::type_is_integral(t_e);
    let t_e_is_float = ty::type_is_floating_point(t_e);
    let t_e_is_c_enum = ty::type_is_c_like_enum(fcx.tcx(), t_e);

    let t_1_is_scalar = ty::type_is_scalar(t_1);
    let t_1_is_char = ty::type_is_char(t_1);
    let t_1_is_bare_fn = ty::type_is_bare_fn(t_1);
    let t_1_is_float = ty::type_is_floating_point(t_1);

    // casts to scalars other than `char` and `bare fn` are trivial
    let t_1_is_trivial = t_1_is_scalar && !t_1_is_char && !t_1_is_bare_fn;

    if t_e_is_bare_fn_item && t_1_is_bare_fn {
        demand::coerce(fcx, e.span, t_1, &e);
    } else if t_1_is_char {
        let t_e = fcx.infcx().shallow_resolve(t_e);
        if t_e.sty != ty::ty_uint(ast::TyU8) {
            fcx.type_error_message(span, |actual| {
                format!("only `u8` can be cast as `char`, not `{}`", actual)
            }, t_e, None);
        }
    } else if t_1.sty == ty::ty_bool {
        span_err!(fcx.tcx().sess, span, E0054,
                  "cannot cast as `bool`, compare with zero instead");
    } else if t_1_is_float && (t_e_is_scalar || t_e_is_c_enum) && !(
        t_e_is_integral || t_e_is_float || t_e.sty == ty::ty_bool) {
        // Casts to float must go through an integer or boolean
        cast_through_integer_err(fcx, span, t_1, t_e)
    } else if t_e_is_c_enum && t_1_is_trivial {
        if ty::type_is_unsafe_ptr(t_1) {
            // ... and likewise with C enum -> *T
            cast_through_integer_err(fcx, span, t_1, t_e)
        }
        // casts from C-like enums are allowed
    } else if ty::type_is_region_ptr(t_e) && ty::type_is_unsafe_ptr(t_1) {
        fn types_compatible<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>, sp: Span,
                                      t1: Ty<'tcx>, t2: Ty<'tcx>) -> bool {
            match t1.sty {
                ty::ty_vec(_, Some(_)) => {}
                _ => return false
            }
            if ty::type_needs_infer(t2) {
                // This prevents this special case from going off when casting
                // to a type that isn't fully specified; e.g. `as *_`. (Issue
                // #14893.)
                return false
            }

            let el = ty::sequence_element_type(fcx.tcx(), t1);
            infer::mk_eqty(fcx.infcx(),
                           false,
                           infer::Misc(sp),
                           el,
                           t2).is_ok()
        }

        // Due to the limitations of LLVM global constants,
        // region pointers end up pointing at copies of
        // vector elements instead of the original values.
        // To allow unsafe pointers to work correctly, we
        // need to special-case obtaining an unsafe pointer
        // from a region pointer to a vector.

        /* this cast is only allowed from &[T, ..n] to *T or
        &T to *T. */
        match (&t_e.sty, &t_1.sty) {
            (&ty::ty_rptr(_, ty::mt { ty: mt1, mutbl: ast::MutImmutable }),
             &ty::ty_ptr(ty::mt { ty: mt2, mutbl: ast::MutImmutable }))
            if types_compatible(fcx, e.span, mt1, mt2) => {
                /* this case is allowed */
            }
            _ => {
                demand::coerce(fcx, e.span, t_1, &e);
            }
        }
    } else if !(t_e_is_scalar && t_1_is_trivial) {
        /*
        If more type combinations should be supported than are
        supported here, then file an enhancement issue and
        record the issue number in this comment.
        */
        fcx.type_error_message(span, |actual| {
            format!("non-scalar cast: `{}` as `{}`",
                    actual,
                    fcx.infcx().ty_to_string(t_1))
        }, t_e, None);
    }
}

impl<'a, 'tcx> AstConv<'tcx> for FnCtxt<'a, 'tcx> {
    fn tcx(&self) -> &ty::ctxt<'tcx> { self.ccx.tcx }

    fn get_item_type_scheme(&self, _: Span, id: ast::DefId)
                            -> Result<ty::TypeScheme<'tcx>, ErrorReported>
    {
        Ok(ty::lookup_item_type(self.tcx(), id))
    }

    fn get_trait_def(&self, _: Span, id: ast::DefId)
                     -> Result<Rc<ty::TraitDef<'tcx>>, ErrorReported>
    {
        Ok(ty::lookup_trait_def(self.tcx(), id))
    }

    fn ensure_super_predicates(&self, _: Span, _: ast::DefId) -> Result<(), ErrorReported> {
        // all super predicates are ensured during collect pass
        Ok(())
    }

    fn get_free_substs(&self) -> Option<&Substs<'tcx>> {
        Some(&self.inh.param_env.free_substs)
    }

    fn get_type_parameter_bounds(&self,
                                 _: Span,
                                 node_id: ast::NodeId)
                                 -> Result<Vec<ty::PolyTraitRef<'tcx>>, ErrorReported>
    {
        let def = self.tcx().type_parameter_def(node_id);
        let r = self.inh.param_env.caller_bounds
                                  .iter()
                                  .filter_map(|predicate| {
                                      match *predicate {
                                          ty::Predicate::Trait(ref data) => {
                                              if data.0.self_ty().is_param(def.space, def.index) {
                                                  Some(data.to_poly_trait_ref())
                                              } else {
                                                  None
                                              }
                                          }
                                          _ => {
                                              None
                                          }
                                      }
                                  })
                                  .collect();
        Ok(r)
    }

    fn trait_defines_associated_type_named(&self,
                                           trait_def_id: ast::DefId,
                                           assoc_name: ast::Name)
                                           -> bool
    {
        let trait_def = ty::lookup_trait_def(self.ccx.tcx, trait_def_id);
        trait_def.associated_type_names.contains(&assoc_name)
    }

    fn ty_infer(&self, _span: Span) -> Ty<'tcx> {
        self.infcx().next_ty_var()
    }

    fn projected_ty_from_poly_trait_ref(&self,
                                        span: Span,
                                        poly_trait_ref: ty::PolyTraitRef<'tcx>,
                                        item_name: ast::Name)
                                        -> Ty<'tcx>
    {
        let (trait_ref, _) =
            self.infcx().replace_late_bound_regions_with_fresh_var(
                span,
                infer::LateBoundRegionConversionTime::AssocTypeProjection(item_name),
                &poly_trait_ref);

        self.normalize_associated_type(span, trait_ref, item_name)
    }

    fn projected_ty(&self,
                    span: Span,
                    trait_ref: Rc<ty::TraitRef<'tcx>>,
                    item_name: ast::Name)
                    -> Ty<'tcx>
    {
        self.normalize_associated_type(span, trait_ref, item_name)
    }
}

impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
    fn tcx(&self) -> &ty::ctxt<'tcx> { self.ccx.tcx }

    pub fn infcx(&self) -> &infer::InferCtxt<'a,'tcx> {
        &self.inh.infcx
    }

    pub fn param_env(&self) -> &ty::ParameterEnvironment<'a,'tcx> {
        &self.inh.param_env
    }

    pub fn sess(&self) -> &Session {
        &self.tcx().sess
    }

    pub fn err_count_since_creation(&self) -> usize {
        self.ccx.tcx.sess.err_count() - self.err_count_on_creation
    }

    /// Resolves type variables in `ty` if possible. Unlike the infcx
    /// version, this version will also select obligations if it seems
    /// useful, in an effort to get more type information.
    fn resolve_type_vars_if_possible(&self, mut ty: Ty<'tcx>) -> Ty<'tcx> {
        debug!("resolve_type_vars_if_possible(ty={})", ty.repr(self.tcx()));

        // No ty::infer()? Nothing needs doing.
        if !ty::type_has_ty_infer(ty) {
            debug!("resolve_type_vars_if_possible: ty={}", ty.repr(self.tcx()));
            return ty;
        }

        // If `ty` is a type variable, see whether we already know what it is.
        ty = self.infcx().resolve_type_vars_if_possible(&ty);
        if !ty::type_has_ty_infer(ty) {
            debug!("resolve_type_vars_if_possible: ty={}", ty.repr(self.tcx()));
            return ty;
        }

        // If not, try resolving any new fcx obligations that have cropped up.
        vtable::select_new_fcx_obligations(self);
        ty = self.infcx().resolve_type_vars_if_possible(&ty);
        if !ty::type_has_ty_infer(ty) {
            debug!("resolve_type_vars_if_possible: ty={}", ty.repr(self.tcx()));
            return ty;
        }

        // If not, try resolving *all* pending obligations as much as
        // possible. This can help substantially when there are
        // indirect dependencies that don't seem worth tracking
        // precisely.
        vtable::select_fcx_obligations_where_possible(self);
        ty = self.infcx().resolve_type_vars_if_possible(&ty);

        debug!("resolve_type_vars_if_possible: ty={}", ty.repr(self.tcx()));
        ty
    }

    /// Resolves all type variables in `t` and then, if any were left
    /// unresolved, substitutes an error type. This is used after the
    /// main checking when doing a second pass before writeback. The
    /// justification is that writeback will produce an error for
    /// these unconstrained type variables.
    fn resolve_type_vars_or_error(&self, t: &Ty<'tcx>) -> mc::McResult<Ty<'tcx>> {
        let t = self.infcx().resolve_type_vars_if_possible(t);
        if ty::type_has_ty_infer(t) || ty::type_is_error(t) { Err(()) } else { Ok(t) }
    }

    fn record_deferred_call_resolution(&self,
                                       closure_def_id: ast::DefId,
                                       r: DeferredCallResolutionHandler<'tcx>) {
        let mut deferred_call_resolutions = self.inh.deferred_call_resolutions.borrow_mut();
        deferred_call_resolutions.entry(closure_def_id).or_insert(vec![]).push(r);
    }

    fn remove_deferred_call_resolutions(&self,
                                        closure_def_id: ast::DefId)
                                        -> Vec<DeferredCallResolutionHandler<'tcx>>
    {
        let mut deferred_call_resolutions = self.inh.deferred_call_resolutions.borrow_mut();
        deferred_call_resolutions.remove(&closure_def_id).unwrap_or(Vec::new())
    }

    pub fn tag(&self) -> String {
        let self_ptr: *const FnCtxt = self;
        format!("{:?}", self_ptr)
    }

    pub fn local_ty(&self, span: Span, nid: ast::NodeId) -> Ty<'tcx> {
        match self.inh.locals.borrow().get(&nid) {
            Some(&t) => t,
            None => {
                self.tcx().sess.span_err(
                    span,
                    &format!("no type for local variable {}", nid));
                self.tcx().types.err
            }
        }
    }

    /// Apply "fallbacks" to some types
    /// ! gets replaced with (), unconstrained ints with i32, and unconstrained floats with f64.
    pub fn default_type_parameters(&self) {
        use middle::ty::UnconstrainedNumeric::{UnconstrainedInt, UnconstrainedFloat, Neither};
        for (_, &mut ref ty) in &mut *self.inh.node_types.borrow_mut() {
            let resolved = self.infcx().resolve_type_vars_if_possible(ty);
            if self.infcx().type_var_diverges(resolved) {
                demand::eqtype(self, codemap::DUMMY_SP, *ty, ty::mk_nil(self.tcx()));
            } else {
                match self.infcx().type_is_unconstrained_numeric(resolved) {
                    UnconstrainedInt => {
                        demand::eqtype(self, codemap::DUMMY_SP, *ty, self.tcx().types.i32)
                    },
                    UnconstrainedFloat => {
                        demand::eqtype(self, codemap::DUMMY_SP, *ty, self.tcx().types.f64)
                    }
                    Neither => { }
                }
            }
        }
    }

    #[inline]
    pub fn write_ty(&self, node_id: ast::NodeId, ty: Ty<'tcx>) {
        debug!("write_ty({}, {}) in fcx {}",
               node_id, ppaux::ty_to_string(self.tcx(), ty), self.tag());
        self.inh.node_types.borrow_mut().insert(node_id, ty);
    }

    pub fn write_substs(&self, node_id: ast::NodeId, substs: ty::ItemSubsts<'tcx>) {
        if !substs.substs.is_noop() {
            debug!("write_substs({}, {}) in fcx {}",
                   node_id,
                   substs.repr(self.tcx()),
                   self.tag());

            self.inh.item_substs.borrow_mut().insert(node_id, substs);
        }
    }

    pub fn write_autoderef_adjustment(&self,
                                      node_id: ast::NodeId,
                                      span: Span,
                                      derefs: usize) {
        if derefs == 0 { return; }
        self.write_adjustment(
            node_id,
            span,
            ty::AdjustDerefRef(ty::AutoDerefRef {
                autoderefs: derefs,
                autoref: None })
        );
    }

    pub fn write_adjustment(&self,
                            node_id: ast::NodeId,
                            span: Span,
                            adj: ty::AutoAdjustment<'tcx>) {
        debug!("write_adjustment(node_id={}, adj={})", node_id, adj.repr(self.tcx()));

        if adj.is_identity() {
            return;
        }

        // Careful: adjustments can imply trait obligations if we are
        // casting from a concrete type to an object type. I think
        // it'd probably be nicer to move the logic that creates the
        // obligation into the code that creates the adjustment, but
        // that's a bit awkward, so instead we go digging and pull the
        // obligation out here.
        self.register_adjustment_obligations(span, &adj);
        self.inh.adjustments.borrow_mut().insert(node_id, adj);
    }

    /// Basically whenever we are converting from a type scheme into
    /// the fn body space, we always want to normalize associated
    /// types as well. This function combines the two.
    fn instantiate_type_scheme<T>(&self,
                                  span: Span,
                                  substs: &Substs<'tcx>,
                                  value: &T)
                                  -> T
        where T : TypeFoldable<'tcx> + Clone + HasProjectionTypes + Repr<'tcx>
    {
        let value = value.subst(self.tcx(), substs);
        let result = self.normalize_associated_types_in(span, &value);
        debug!("instantiate_type_scheme(value={}, substs={}) = {}",
               value.repr(self.tcx()),
               substs.repr(self.tcx()),
               result.repr(self.tcx()));
        result
    }

    /// As `instantiate_type_scheme`, but for the bounds found in a
    /// generic type scheme.
    fn instantiate_bounds(&self,
                          span: Span,
                          substs: &Substs<'tcx>,
                          bounds: &ty::GenericPredicates<'tcx>)
                          -> ty::InstantiatedPredicates<'tcx>
    {
        ty::InstantiatedPredicates {
            predicates: self.instantiate_type_scheme(span, substs, &bounds.predicates)
        }
    }


    fn normalize_associated_types_in<T>(&self, span: Span, value: &T) -> T
        where T : TypeFoldable<'tcx> + Clone + HasProjectionTypes + Repr<'tcx>
    {
        self.inh.normalize_associated_types_in(self, span, self.body_id, value)
    }

    fn normalize_associated_type(&self,
                                 span: Span,
                                 trait_ref: Rc<ty::TraitRef<'tcx>>,
                                 item_name: ast::Name)
                                 -> Ty<'tcx>
    {
        let cause = traits::ObligationCause::new(span,
                                                 self.body_id,
                                                 traits::ObligationCauseCode::MiscObligation);
        self.inh.fulfillment_cx
            .borrow_mut()
            .normalize_projection_type(self.infcx(),
                                       self,
                                       ty::ProjectionTy {
                                           trait_ref: trait_ref,
                                           item_name: item_name,
                                       },
                                       cause)
    }

    fn register_adjustment_obligations(&self,
                                       span: Span,
                                       adj: &ty::AutoAdjustment<'tcx>) {
        match *adj {
            ty::AdjustReifyFnPointer(..) => { }
            ty::AdjustUnsafeFnPointer => { }
            ty::AdjustDerefRef(ref d_r) => {
                match d_r.autoref {
                    Some(ref a_r) => {
                        self.register_autoref_obligations(span, a_r);
                    }
                    None => {}
                }
            }
        }
    }

    fn register_autoref_obligations(&self,
                                    span: Span,
                                    autoref: &ty::AutoRef<'tcx>) {
        match *autoref {
            ty::AutoUnsize(ref unsize) => {
                self.register_unsize_obligations(span, unsize);
            }
            ty::AutoPtr(_, _, None) |
            ty::AutoUnsafe(_, None) => {
            }
            ty::AutoPtr(_, _, Some(ref a_r)) |
            ty::AutoUnsafe(_, Some(ref a_r)) => {
                self.register_autoref_obligations(span, &**a_r)
            }
            ty::AutoUnsizeUniq(ref unsize) => {
                self.register_unsize_obligations(span, unsize);
            }
        }
    }

    fn register_unsize_obligations(&self,
                                   span: Span,
                                   unsize: &ty::UnsizeKind<'tcx>) {
        debug!("register_unsize_obligations: unsize={:?}", unsize);

        match *unsize {
            ty::UnsizeLength(..) => {}
            ty::UnsizeStruct(ref u, _) => {
                self.register_unsize_obligations(span, &**u)
            }
            ty::UnsizeVtable(ref ty_trait, self_ty) => {
                vtable::check_object_safety(self.tcx(), ty_trait, span);

                // If the type is `Foo+'a`, ensures that the type
                // being cast to `Foo+'a` implements `Foo`:
                vtable::register_object_cast_obligations(self,
                                                         span,
                                                         ty_trait,
                                                         self_ty);

                // If the type is `Foo+'a`, ensures that the type
                // being cast to `Foo+'a` outlives `'a`:
                let cause = traits::ObligationCause { span: span,
                                                      body_id: self.body_id,
                                                      code: traits::ObjectCastObligation(self_ty) };
                self.register_region_obligation(self_ty, ty_trait.bounds.region_bound, cause);
            }
            ty::UnsizeUpcast(_) => { }
        }
    }

    /// Returns the type of `def_id` with all generics replaced by by fresh type/region variables.
    /// Also returns the substitution from the type parameters on `def_id` to the fresh variables.
    /// Registers any trait obligations specified on `def_id` at the same time.
    ///
    /// Note that function is only intended to be used with types (notably, not fns). This is
    /// because it doesn't do any instantiation of late-bound regions.
    pub fn instantiate_type(&self,
                            span: Span,
                            def_id: ast::DefId)
                            -> TypeAndSubsts<'tcx>
    {
        let type_scheme =
            ty::lookup_item_type(self.tcx(), def_id);
        let type_predicates =
            ty::lookup_predicates(self.tcx(), def_id);
        let substs =
            self.infcx().fresh_substs_for_generics(
                span,
                &type_scheme.generics);
        let bounds =
            self.instantiate_bounds(span, &substs, &type_predicates);
        self.add_obligations_for_parameters(
            traits::ObligationCause::new(
                span,
                self.body_id,
                traits::ItemObligation(def_id)),
            &bounds);
        let monotype =
            self.instantiate_type_scheme(span, &substs, &type_scheme.ty);

        TypeAndSubsts {
            ty: monotype,
            substs: substs
        }
    }

    /// Returns the type that this AST path refers to. If the path has no type
    /// parameters and the corresponding type has type parameters, fresh type
    /// and/or region variables are substituted.
    ///
    /// This is used when checking the constructor in struct literals.
    fn instantiate_struct_literal_ty(&self,
                                     did: ast::DefId,
                                     path: &ast::Path)
                                     -> TypeAndSubsts<'tcx>
    {
        let tcx = self.tcx();

        let ty::TypeScheme { generics, ty: decl_ty } =
            ty::lookup_item_type(tcx, did);

        let substs = astconv::ast_path_substs_for_ty(self, self,
                                                     path.span,
                                                     PathParamMode::Optional,
                                                     &generics,
                                                     path.segments.last().unwrap());

        let ty = self.instantiate_type_scheme(path.span, &substs, &decl_ty);

        TypeAndSubsts { substs: substs, ty: ty }
    }

    pub fn write_nil(&self, node_id: ast::NodeId) {
        self.write_ty(node_id, ty::mk_nil(self.tcx()));
    }
    pub fn write_error(&self, node_id: ast::NodeId) {
        self.write_ty(node_id, self.tcx().types.err);
    }

    pub fn require_type_meets(&self,
                              ty: Ty<'tcx>,
                              span: Span,
                              code: traits::ObligationCauseCode<'tcx>,
                              bound: ty::BuiltinBound)
    {
        self.register_builtin_bound(
            ty,
            bound,
            traits::ObligationCause::new(span, self.body_id, code));
    }

    pub fn require_type_is_sized(&self,
                                 ty: Ty<'tcx>,
                                 span: Span,
                                 code: traits::ObligationCauseCode<'tcx>)
    {
        self.require_type_meets(ty, span, code, ty::BoundSized);
    }

    pub fn require_expr_have_sized_type(&self,
                                        expr: &ast::Expr,
                                        code: traits::ObligationCauseCode<'tcx>)
    {
        self.require_type_is_sized(self.expr_ty(expr), expr.span, code);
    }

    pub fn type_is_known_to_be_sized(&self,
                                     ty: Ty<'tcx>,
                                     span: Span)
                                     -> bool
    {
        traits::type_known_to_meet_builtin_bound(self.infcx(),
                                                 self.param_env(),
                                                 ty,
                                                 ty::BoundSized,
                                                 span)
    }

    pub fn register_builtin_bound(&self,
                                  ty: Ty<'tcx>,
                                  builtin_bound: ty::BuiltinBound,
                                  cause: traits::ObligationCause<'tcx>)
    {
        self.inh.fulfillment_cx.borrow_mut()
            .register_builtin_bound(self.infcx(), ty, builtin_bound, cause);
    }

    pub fn register_predicate(&self,
                              obligation: traits::PredicateObligation<'tcx>)
    {
        debug!("register_predicate({})",
               obligation.repr(self.tcx()));
        self.inh.fulfillment_cx
            .borrow_mut()
            .register_predicate_obligation(self.infcx(), obligation);
    }

    pub fn to_ty(&self, ast_t: &ast::Ty) -> Ty<'tcx> {
        let t = ast_ty_to_ty(self, self, ast_t);

        let mut bounds_checker = wf::BoundsChecker::new(self,
                                                        ast_t.span,
                                                        self.body_id,
                                                        None);
        bounds_checker.check_ty(t);

        t
    }

    pub fn pat_to_string(&self, pat: &ast::Pat) -> String {
        pat.repr(self.tcx())
    }

    pub fn expr_ty(&self, ex: &ast::Expr) -> Ty<'tcx> {
        match self.inh.node_types.borrow().get(&ex.id) {
            Some(&t) => t,
            None => {
                self.tcx().sess.bug(&format!("no type for expr in fcx {}",
                                            self.tag()));
            }
        }
    }

    /// Apply `adjustment` to the type of `expr`
    pub fn adjust_expr_ty(&self,
                          expr: &ast::Expr,
                          adjustment: Option<&ty::AutoAdjustment<'tcx>>)
                          -> Ty<'tcx>
    {
        let raw_ty = self.expr_ty(expr);
        let raw_ty = self.infcx().shallow_resolve(raw_ty);
        let resolve_ty = |ty: Ty<'tcx>| self.infcx().resolve_type_vars_if_possible(&ty);
        ty::adjust_ty(self.tcx(),
                      expr.span,
                      expr.id,
                      raw_ty,
                      adjustment,
                      |method_call| self.inh.method_map.borrow()
                                                       .get(&method_call)
                                                       .map(|method| resolve_ty(method.ty)))
    }

    pub fn node_ty(&self, id: ast::NodeId) -> Ty<'tcx> {
        match self.inh.node_types.borrow().get(&id) {
            Some(&t) => t,
            None if self.err_count_since_creation() != 0 => self.tcx().types.err,
            None => {
                self.tcx().sess.bug(
                    &format!("no type for node {}: {} in fcx {}",
                            id, self.tcx().map.node_to_string(id),
                            self.tag()));
            }
        }
    }

    pub fn item_substs(&self) -> Ref<NodeMap<ty::ItemSubsts<'tcx>>> {
        self.inh.item_substs.borrow()
    }

    pub fn opt_node_ty_substs<F>(&self,
                                 id: ast::NodeId,
                                 f: F) where
        F: FnOnce(&ty::ItemSubsts<'tcx>),
    {
        match self.inh.item_substs.borrow().get(&id) {
            Some(s) => { f(s) }
            None => { }
        }
    }

    pub fn mk_subty(&self,
                    a_is_expected: bool,
                    origin: infer::TypeOrigin,
                    sub: Ty<'tcx>,
                    sup: Ty<'tcx>)
                    -> Result<(), ty::type_err<'tcx>> {
        infer::mk_subty(self.infcx(), a_is_expected, origin, sub, sup)
    }

    pub fn mk_eqty(&self,
                   a_is_expected: bool,
                   origin: infer::TypeOrigin,
                   sub: Ty<'tcx>,
                   sup: Ty<'tcx>)
                   -> Result<(), ty::type_err<'tcx>> {
        infer::mk_eqty(self.infcx(), a_is_expected, origin, sub, sup)
    }

    pub fn mk_subr(&self,
                   origin: infer::SubregionOrigin<'tcx>,
                   sub: ty::Region,
                   sup: ty::Region) {
        infer::mk_subr(self.infcx(), origin, sub, sup)
    }

    pub fn type_error_message<M>(&self,
                                 sp: Span,
                                 mk_msg: M,
                                 actual_ty: Ty<'tcx>,
                                 err: Option<&ty::type_err<'tcx>>) where
        M: FnOnce(String) -> String,
    {
        self.infcx().type_error_message(sp, mk_msg, actual_ty, err);
    }

    pub fn report_mismatched_types(&self,
                                   sp: Span,
                                   e: Ty<'tcx>,
                                   a: Ty<'tcx>,
                                   err: &ty::type_err<'tcx>) {
        self.infcx().report_mismatched_types(sp, e, a, err)
    }

    /// Registers an obligation for checking later, during regionck, that the type `ty` must
    /// outlive the region `r`.
    pub fn register_region_obligation(&self,
                                      ty: Ty<'tcx>,
                                      region: ty::Region,
                                      cause: traits::ObligationCause<'tcx>)
    {
        let mut fulfillment_cx = self.inh.fulfillment_cx.borrow_mut();
        fulfillment_cx.register_region_obligation(self.infcx(), ty, region, cause);
    }

    pub fn add_default_region_param_bounds(&self,
                                           substs: &Substs<'tcx>,
                                           expr: &ast::Expr)
    {
        for &ty in substs.types.iter() {
            let default_bound = ty::ReScope(CodeExtent::from_node_id(expr.id));
            let cause = traits::ObligationCause::new(expr.span, self.body_id,
                                                     traits::MiscObligation);
            self.register_region_obligation(ty, default_bound, cause);
        }
    }

    /// Given a fully substituted set of bounds (`generic_bounds`), and the values with which each
    /// type/region parameter was instantiated (`substs`), creates and registers suitable
    /// trait/region obligations.
    ///
    /// For example, if there is a function:
    ///
    /// ```
    /// fn foo<'a,T:'a>(...)
    /// ```
    ///
    /// and a reference:
    ///
    /// ```
    /// let f = foo;
    /// ```
    ///
    /// Then we will create a fresh region variable `'$0` and a fresh type variable `$1` for `'a`
    /// and `T`. This routine will add a region obligation `$1:'$0` and register it locally.
    pub fn add_obligations_for_parameters(&self,
                                          cause: traits::ObligationCause<'tcx>,
                                          predicates: &ty::InstantiatedPredicates<'tcx>)
    {
        assert!(!predicates.has_escaping_regions());

        debug!("add_obligations_for_parameters(predicates={})",
               predicates.repr(self.tcx()));

        let obligations = traits::predicates_for_generics(self.tcx(),
                                                          cause,
                                                          predicates);

        obligations.map_move(|o| self.register_predicate(o));
    }

    // Only for fields! Returns <none> for methods>
    // Indifferent to privacy flags
    pub fn lookup_field_ty(&self,
                           span: Span,
                           class_id: ast::DefId,
                           items: &[ty::field_ty],
                           fieldname: ast::Name,
                           substs: &subst::Substs<'tcx>)
                           -> Option<Ty<'tcx>>
    {
        let o_field = items.iter().find(|f| f.name == fieldname);
        o_field.map(|f| ty::lookup_field_type(self.tcx(), class_id, f.id, substs))
               .map(|t| self.normalize_associated_types_in(span, &t))
    }

    pub fn lookup_tup_field_ty(&self,
                               span: Span,
                               class_id: ast::DefId,
                               items: &[ty::field_ty],
                               idx: usize,
                               substs: &subst::Substs<'tcx>)
                               -> Option<Ty<'tcx>>
    {
        let o_field = if idx < items.len() { Some(&items[idx]) } else { None };
        o_field.map(|f| ty::lookup_field_type(self.tcx(), class_id, f.id, substs))
               .map(|t| self.normalize_associated_types_in(span, &t))
    }

    fn check_casts(&self) {
        let mut deferred_cast_checks = self.inh.deferred_cast_checks.borrow_mut();
        for check in deferred_cast_checks.iter() {
            check_cast(self, check);
        }

        deferred_cast_checks.clear();
    }
}

impl<'a, 'tcx> RegionScope for FnCtxt<'a, 'tcx> {
    fn object_lifetime_default(&self, span: Span) -> Option<ty::Region> {
        // RFC #599 specifies that object lifetime defaults take
        // precedence over other defaults. But within a fn body we
        // don't have a *default* region, rather we use inference to
        // find the *correct* region, which is strictly more general
        // (and anyway, within a fn body the right region may not even
        // be something the user can write explicitly, since it might
        // be some expression).
        Some(self.infcx().next_region_var(infer::MiscVariable(span)))
    }

    fn anon_regions(&self, span: Span, count: usize)
                    -> Result<Vec<ty::Region>, Option<Vec<(String, usize)>>> {
        Ok((0..count).map(|_| {
            self.infcx().next_region_var(infer::MiscVariable(span))
        }).collect())
    }
}

#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum LvaluePreference {
    PreferMutLvalue,
    NoPreference
}

/// Whether `autoderef` requires types to resolve.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum UnresolvedTypeAction {
    /// Produce an error and return `ty_err` whenever a type cannot
    /// be resolved (i.e. it is `ty_infer`).
    Error,
    /// Go on without emitting any errors, and return the unresolved
    /// type. Useful for probing, e.g. in coercions.
    Ignore
}

/// Executes an autoderef loop for the type `t`. At each step, invokes `should_stop` to decide
/// whether to terminate the loop. Returns the final type and number of derefs that it performed.
///
/// Note: this method does not modify the adjustments table. The caller is responsible for
/// inserting an AutoAdjustment record into the `fcx` using one of the suitable methods.
pub fn autoderef<'a, 'tcx, T, F>(fcx: &FnCtxt<'a, 'tcx>,
                                 sp: Span,
                                 base_ty: Ty<'tcx>,
                                 opt_expr: Option<&ast::Expr>,
                                 unresolved_type_action: UnresolvedTypeAction,
                                 mut lvalue_pref: LvaluePreference,
                                 mut should_stop: F)
                                 -> (Ty<'tcx>, usize, Option<T>)
    where F: FnMut(Ty<'tcx>, usize) -> Option<T>,
{
    debug!("autoderef(base_ty={}, opt_expr={}, lvalue_pref={:?})",
           base_ty.repr(fcx.tcx()),
           opt_expr.repr(fcx.tcx()),
           lvalue_pref);

    let mut t = base_ty;
    for autoderefs in 0..fcx.tcx().sess.recursion_limit.get() {
        let resolved_t = match unresolved_type_action {
            UnresolvedTypeAction::Error => {
                let resolved_t = structurally_resolved_type(fcx, sp, t);
                if ty::type_is_error(resolved_t) {
                    return (resolved_t, autoderefs, None);
                }
                resolved_t
            }
            UnresolvedTypeAction::Ignore => {
                // We can continue even when the type cannot be resolved
                // (i.e. it is an inference variable) because `ty::deref`
                // and `try_overloaded_deref` both simply return `None`
                // in such a case without producing spurious errors.
                fcx.resolve_type_vars_if_possible(t)
            }
        };

        match should_stop(resolved_t, autoderefs) {
            Some(x) => return (resolved_t, autoderefs, Some(x)),
            None => {}
        }

        // Otherwise, deref if type is derefable:
        let mt = match ty::deref(resolved_t, false) {
            Some(mt) => Some(mt),
            None => {
                let method_call = opt_expr.map(|expr| MethodCall::autoderef(expr.id, autoderefs));

                // Super subtle: it might seem as though we should
                // pass `opt_expr` to `try_overloaded_deref`, so that
                // the (implicit) autoref of using an overloaded deref
                // would get added to the adjustment table. However we
                // do not do that, because it's kind of a
                // "meta-adjustment" -- instead, we just leave it
                // unrecorded and know that there "will be" an
                // autoref. regionck and other bits of the code base,
                // when they encounter an overloaded autoderef, have
                // to do some reconstructive surgery. This is a pretty
                // complex mess that is begging for a proper MIR.
                try_overloaded_deref(fcx, sp, method_call, None, resolved_t, lvalue_pref)
            }
        };
        match mt {
            Some(mt) => {
                t = mt.ty;
                if mt.mutbl == ast::MutImmutable {
                    lvalue_pref = NoPreference;
                }
            }
            None => return (resolved_t, autoderefs, None)
        }
    }

    // We've reached the recursion limit, error gracefully.
    span_err!(fcx.tcx().sess, sp, E0055,
        "reached the recursion limit while auto-dereferencing {}",
        base_ty.repr(fcx.tcx()));
    (fcx.tcx().types.err, 0, None)
}

fn try_overloaded_deref<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                  span: Span,
                                  method_call: Option<MethodCall>,
                                  base_expr: Option<&ast::Expr>,
                                  base_ty: Ty<'tcx>,
                                  lvalue_pref: LvaluePreference)
                                  -> Option<ty::mt<'tcx>>
{
    // Try DerefMut first, if preferred.
    let method = match (lvalue_pref, fcx.tcx().lang_items.deref_mut_trait()) {
        (PreferMutLvalue, Some(trait_did)) => {
            method::lookup_in_trait(fcx, span, base_expr,
                                    token::intern("deref_mut"), trait_did,
                                    base_ty, None)
        }
        _ => None
    };

    // Otherwise, fall back to Deref.
    let method = match (method, fcx.tcx().lang_items.deref_trait()) {
        (None, Some(trait_did)) => {
            method::lookup_in_trait(fcx, span, base_expr,
                                    token::intern("deref"), trait_did,
                                    base_ty, None)
        }
        (method, _) => method
    };

    make_overloaded_lvalue_return_type(fcx, method_call, method)
}

/// For the overloaded lvalue expressions (`*x`, `x[3]`), the trait returns a type of `&T`, but the
/// actual type we assign to the *expression* is `T`. So this function just peels off the return
/// type by one layer to yield `T`. It also inserts the `method-callee` into the method map.
fn make_overloaded_lvalue_return_type<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                                method_call: Option<MethodCall>,
                                                method: Option<MethodCallee<'tcx>>)
                                                -> Option<ty::mt<'tcx>>
{
    match method {
        Some(method) => {
            // extract method method return type, which will be &T;
            // all LB regions should have been instantiated during method lookup
            let ret_ty = ty::ty_fn_ret(method.ty);
            let ret_ty = ty::no_late_bound_regions(fcx.tcx(), &ret_ty).unwrap().unwrap();

            if let Some(method_call) = method_call {
                fcx.inh.method_map.borrow_mut().insert(method_call, method);
            }

            // method returns &T, but the type as visible to user is T, so deref
            ty::deref(ret_ty, true)
        }
        None => None,
    }
}

fn autoderef_for_index<'a, 'tcx, T, F>(fcx: &FnCtxt<'a, 'tcx>,
                                       base_expr: &ast::Expr,
                                       base_ty: Ty<'tcx>,
                                       lvalue_pref: LvaluePreference,
                                       mut step: F)
                                       -> Option<T> where
    F: FnMut(Ty<'tcx>, ty::AutoDerefRef<'tcx>) -> Option<T>,
{
    // FIXME(#18741) -- this is almost but not quite the same as the
    // autoderef that normal method probing does. They could likely be
    // consolidated.

    let (ty, autoderefs, final_mt) = autoderef(fcx,
                                               base_expr.span,
                                               base_ty,
                                               Some(base_expr),
                                               UnresolvedTypeAction::Error,
                                               lvalue_pref,
                                               |adj_ty, idx| {
            let autoderefref = ty::AutoDerefRef { autoderefs: idx, autoref: None };
            step(adj_ty, autoderefref)
        });

    if final_mt.is_some() {
        return final_mt;
    }

    // After we have fully autoderef'd, if the resulting type is [T, ..n], then
    // do a final unsized coercion to yield [T].
    match ty.sty {
        ty::ty_vec(element_ty, Some(n)) => {
            let adjusted_ty = ty::mk_vec(fcx.tcx(), element_ty, None);
            let autoderefref = ty::AutoDerefRef {
                autoderefs: autoderefs,
                autoref: Some(ty::AutoUnsize(ty::UnsizeLength(n)))
            };
            step(adjusted_ty, autoderefref)
        }
        _ => {
            None
        }
    }
}

/// To type-check `base_expr[index_expr]`, we progressively autoderef (and otherwise adjust)
/// `base_expr`, looking for a type which either supports builtin indexing or overloaded indexing.
/// This loop implements one step in that search; the autoderef loop is implemented by
/// `autoderef_for_index`.
fn try_index_step<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                            method_call: MethodCall,
                            expr: &ast::Expr,
                            base_expr: &'tcx ast::Expr,
                            adjusted_ty: Ty<'tcx>,
                            adjustment: ty::AutoDerefRef<'tcx>,
                            lvalue_pref: LvaluePreference,
                            index_ty: Ty<'tcx>)
                            -> Option<(/*index type*/ Ty<'tcx>, /*element type*/ Ty<'tcx>)>
{
    let tcx = fcx.tcx();
    debug!("try_index_step(expr={}, base_expr.id={}, adjusted_ty={}, adjustment={:?}, index_ty={})",
           expr.repr(tcx),
           base_expr.repr(tcx),
           adjusted_ty.repr(tcx),
           adjustment,
           index_ty.repr(tcx));

    let input_ty = fcx.infcx().next_ty_var();

    // First, try built-in indexing.
    match (ty::index(adjusted_ty), &index_ty.sty) {
        (Some(ty), &ty::ty_uint(ast::TyUs)) | (Some(ty), &ty::ty_infer(ty::IntVar(_))) => {
            debug!("try_index_step: success, using built-in indexing");
            fcx.write_adjustment(base_expr.id, base_expr.span, ty::AdjustDerefRef(adjustment));
            return Some((tcx.types.usize, ty));
        }
        _ => {}
    }

    // Try `IndexMut` first, if preferred.
    let method = match (lvalue_pref, tcx.lang_items.index_mut_trait()) {
        (PreferMutLvalue, Some(trait_did)) => {
            method::lookup_in_trait_adjusted(fcx,
                                             expr.span,
                                             Some(&*base_expr),
                                             token::intern("index_mut"),
                                             trait_did,
                                             adjustment.clone(),
                                             adjusted_ty,
                                             Some(vec![input_ty]))
        }
        _ => None,
    };

    // Otherwise, fall back to `Index`.
    let method = match (method, tcx.lang_items.index_trait()) {
        (None, Some(trait_did)) => {
            method::lookup_in_trait_adjusted(fcx,
                                             expr.span,
                                             Some(&*base_expr),
                                             token::intern("index"),
                                             trait_did,
                                             adjustment.clone(),
                                             adjusted_ty,
                                             Some(vec![input_ty]))
        }
        (method, _) => method,
    };

    // If some lookup succeeds, write callee into table and extract index/element
    // type from the method signature.
    // If some lookup succeeded, install method in table
    method.and_then(|method| {
        debug!("try_index_step: success, using overloaded indexing");
        make_overloaded_lvalue_return_type(fcx, Some(method_call), Some(method)).
            map(|ret| (input_ty, ret.ty))
    })
}

fn check_method_argument_types<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                         sp: Span,
                                         method_fn_ty: Ty<'tcx>,
                                         callee_expr: &'tcx ast::Expr,
                                         args_no_rcvr: &'tcx [P<ast::Expr>],
                                         tuple_arguments: TupleArgumentsFlag,
                                         expected: Expectation<'tcx>)
                                         -> ty::FnOutput<'tcx> {
    if ty::type_is_error(method_fn_ty) {
        let err_inputs = err_args(fcx.tcx(), args_no_rcvr.len());

        let err_inputs = match tuple_arguments {
            DontTupleArguments => err_inputs,
            TupleArguments => vec![ty::mk_tup(fcx.tcx(), err_inputs)],
        };

        check_argument_types(fcx,
                             sp,
                             &err_inputs[..],
                             &[],
                             args_no_rcvr,
                             false,
                             tuple_arguments);
        ty::FnConverging(fcx.tcx().types.err)
    } else {
        match method_fn_ty.sty {
            ty::ty_bare_fn(_, ref fty) => {
                // HACK(eddyb) ignore self in the definition (see above).
                let expected_arg_tys = expected_types_for_fn_args(fcx,
                                                                  sp,
                                                                  expected,
                                                                  fty.sig.0.output,
                                                                  &fty.sig.0.inputs[1..]);
                check_argument_types(fcx,
                                     sp,
                                     &fty.sig.0.inputs[1..],
                                     &expected_arg_tys[..],
                                     args_no_rcvr,
                                     fty.sig.0.variadic,
                                     tuple_arguments);
                fty.sig.0.output
            }
            _ => {
                fcx.tcx().sess.span_bug(callee_expr.span,
                                        "method without bare fn type");
            }
        }
    }
}

/// Generic function that factors out common logic from function calls, method calls and overloaded
/// operators.
fn check_argument_types<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                  sp: Span,
                                  fn_inputs: &[Ty<'tcx>],
                                  expected_arg_tys: &[Ty<'tcx>],
                                  args: &'tcx [P<ast::Expr>],
                                  variadic: bool,
                                  tuple_arguments: TupleArgumentsFlag) {
    let tcx = fcx.ccx.tcx;

    // Grab the argument types, supplying fresh type variables
    // if the wrong number of arguments were supplied
    let supplied_arg_count = if tuple_arguments == DontTupleArguments {
        args.len()
    } else {
        1
    };

    let mut expected_arg_tys = expected_arg_tys;
    let expected_arg_count = fn_inputs.len();
    let formal_tys = if tuple_arguments == TupleArguments {
        let tuple_type = structurally_resolved_type(fcx, sp, fn_inputs[0]);
        match tuple_type.sty {
            ty::ty_tup(ref arg_types) => {
                if arg_types.len() != args.len() {
                    span_err!(tcx.sess, sp, E0057,
                        "this function takes {} parameter{} but {} parameter{} supplied",
                        arg_types.len(),
                        if arg_types.len() == 1 {""} else {"s"},
                        args.len(),
                        if args.len() == 1 {" was"} else {"s were"});
                    expected_arg_tys = &[];
                    err_args(fcx.tcx(), args.len())
                } else {
                    expected_arg_tys = match expected_arg_tys.get(0) {
                        Some(&ty) => match ty.sty {
                            ty::ty_tup(ref tys) => &**tys,
                            _ => &[]
                        },
                        None => &[]
                    };
                    (*arg_types).clone()
                }
            }
            _ => {
                span_err!(tcx.sess, sp, E0059,
                    "cannot use call notation; the first type parameter \
                     for the function trait is neither a tuple nor unit");
                expected_arg_tys = &[];
                err_args(fcx.tcx(), args.len())
            }
        }
    } else if expected_arg_count == supplied_arg_count {
        fn_inputs.to_vec()
    } else if variadic {
        if supplied_arg_count >= expected_arg_count {
            fn_inputs.to_vec()
        } else {
            span_err!(tcx.sess, sp, E0060,
                "this function takes at least {} parameter{} \
                 but {} parameter{} supplied",
                expected_arg_count,
                if expected_arg_count == 1 {""} else {"s"},
                supplied_arg_count,
                if supplied_arg_count == 1 {" was"} else {"s were"});
            expected_arg_tys = &[];
            err_args(fcx.tcx(), supplied_arg_count)
        }
    } else {
        span_err!(tcx.sess, sp, E0061,
            "this function takes {} parameter{} but {} parameter{} supplied",
            expected_arg_count,
            if expected_arg_count == 1 {""} else {"s"},
            supplied_arg_count,
            if supplied_arg_count == 1 {" was"} else {"s were"});
        expected_arg_tys = &[];
        err_args(fcx.tcx(), supplied_arg_count)
    };

    debug!("check_argument_types: formal_tys={:?}",
           formal_tys.iter().map(|t| fcx.infcx().ty_to_string(*t)).collect::<Vec<String>>());

    // Check the arguments.
    // We do this in a pretty awful way: first we typecheck any arguments
    // that are not anonymous functions, then we typecheck the anonymous
    // functions. This is so that we have more information about the types
    // of arguments when we typecheck the functions. This isn't really the
    // right way to do this.
    let xs = [false, true];
    for check_blocks in &xs {
        let check_blocks = *check_blocks;
        debug!("check_blocks={}", check_blocks);

        // More awful hacks: before we check argument types, try to do
        // an "opportunistic" vtable resolution of any trait bounds on
        // the call. This helps coercions.
        if check_blocks {
            vtable::select_new_fcx_obligations(fcx);
        }

        // For variadic functions, we don't have a declared type for all of
        // the arguments hence we only do our usual type checking with
        // the arguments who's types we do know.
        let t = if variadic {
            expected_arg_count
        } else if tuple_arguments == TupleArguments {
            args.len()
        } else {
            supplied_arg_count
        };
        for (i, arg) in args.iter().take(t).enumerate() {
            let is_block = match arg.node {
                ast::ExprClosure(..) => true,
                _ => false
            };

            if is_block == check_blocks {
                debug!("checking the argument");
                let formal_ty = formal_tys[i];

                // The special-cased logic below has three functions:
                // 1. Provide as good of an expected type as possible.
                let expected = expected_arg_tys.get(i).map(|&ty| {
                    Expectation::rvalue_hint(ty)
                });

                check_expr_with_unifier(fcx, &**arg,
                                        expected.unwrap_or(ExpectHasType(formal_ty)),
                                        NoPreference, || {
                    // 2. Coerce to the most detailed type that could be coerced
                    //    to, which is `expected_ty` if `rvalue_hint` returns an
                    //    `ExprHasType(expected_ty)`, or the `formal_ty` otherwise.
                    let coerce_ty = expected.and_then(|e| e.only_has_type(fcx));
                    demand::coerce(fcx, arg.span, coerce_ty.unwrap_or(formal_ty), &**arg);

                    // 3. Relate the expected type and the formal one,
                    //    if the expected type was used for the coercion.
                    coerce_ty.map(|ty| demand::suptype(fcx, arg.span, formal_ty, ty));
                });
            }
        }
    }

    // We also need to make sure we at least write the ty of the other
    // arguments which we skipped above.
    if variadic {
        for arg in args.iter().skip(expected_arg_count) {
            check_expr(fcx, &**arg);

            // There are a few types which get autopromoted when passed via varargs
            // in C but we just error out instead and require explicit casts.
            let arg_ty = structurally_resolved_type(fcx, arg.span,
                                                    fcx.expr_ty(&**arg));
            match arg_ty.sty {
                ty::ty_float(ast::TyF32) => {
                    fcx.type_error_message(arg.span,
                                           |t| {
                        format!("can't pass an {} to variadic \
                                 function, cast to c_double", t)
                    }, arg_ty, None);
                }
                ty::ty_int(ast::TyI8) | ty::ty_int(ast::TyI16) | ty::ty_bool => {
                    fcx.type_error_message(arg.span, |t| {
                        format!("can't pass {} to variadic \
                                 function, cast to c_int",
                                       t)
                    }, arg_ty, None);
                }
                ty::ty_uint(ast::TyU8) | ty::ty_uint(ast::TyU16) => {
                    fcx.type_error_message(arg.span, |t| {
                        format!("can't pass {} to variadic \
                                 function, cast to c_uint",
                                       t)
                    }, arg_ty, None);
                }
                _ => {}
            }
        }
    }
}

// FIXME(#17596) Ty<'tcx> is incorrectly invariant w.r.t 'tcx.
fn err_args<'tcx>(tcx: &ty::ctxt<'tcx>, len: usize) -> Vec<Ty<'tcx>> {
    (0..len).map(|_| tcx.types.err).collect()
}

fn write_call<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                        call_expr: &ast::Expr,
                        output: ty::FnOutput<'tcx>) {
    fcx.write_ty(call_expr.id, match output {
        ty::FnConverging(output_ty) => output_ty,
        ty::FnDiverging => fcx.infcx().next_diverging_ty_var()
    });
}

// AST fragment checking
fn check_lit<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                       lit: &ast::Lit,
                       expected: Expectation<'tcx>)
                       -> Ty<'tcx>
{
    let tcx = fcx.ccx.tcx;

    match lit.node {
        ast::LitStr(..) => ty::mk_str_slice(tcx, tcx.mk_region(ty::ReStatic), ast::MutImmutable),
        ast::LitBinary(ref v) => {
            ty::mk_rptr(tcx, tcx.mk_region(ty::ReStatic), ty::mt {
                ty: ty::mk_vec(tcx, tcx.types.u8, Some(v.len())),
                mutbl: ast::MutImmutable,
            })
        }
        ast::LitByte(_) => tcx.types.u8,
        ast::LitChar(_) => tcx.types.char,
        ast::LitInt(_, ast::SignedIntLit(t, _)) => ty::mk_mach_int(tcx, t),
        ast::LitInt(_, ast::UnsignedIntLit(t)) => ty::mk_mach_uint(tcx, t),
        ast::LitInt(_, ast::UnsuffixedIntLit(_)) => {
            let opt_ty = expected.to_option(fcx).and_then(|ty| {
                match ty.sty {
                    ty::ty_int(_) | ty::ty_uint(_) => Some(ty),
                    ty::ty_char => Some(tcx.types.u8),
                    ty::ty_ptr(..) => Some(tcx.types.usize),
                    ty::ty_bare_fn(..) => Some(tcx.types.usize),
                    _ => None
                }
            });
            opt_ty.unwrap_or_else(
                || ty::mk_int_var(tcx, fcx.infcx().next_int_var_id()))
        }
        ast::LitFloat(_, t) => ty::mk_mach_float(tcx, t),
        ast::LitFloatUnsuffixed(_) => {
            let opt_ty = expected.to_option(fcx).and_then(|ty| {
                match ty.sty {
                    ty::ty_float(_) => Some(ty),
                    _ => None
                }
            });
            opt_ty.unwrap_or_else(
                || ty::mk_float_var(tcx, fcx.infcx().next_float_var_id()))
        }
        ast::LitBool(_) => tcx.types.bool
    }
}

pub fn check_expr_has_type<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                     expr: &'tcx ast::Expr,
                                     expected: Ty<'tcx>) {
    check_expr_with_unifier(
        fcx, expr, ExpectHasType(expected), NoPreference,
        || demand::suptype(fcx, expr.span, expected, fcx.expr_ty(expr)));
}

fn check_expr_coercable_to_type<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                          expr: &'tcx ast::Expr,
                                          expected: Ty<'tcx>) {
    check_expr_with_unifier(
        fcx, expr, ExpectHasType(expected), NoPreference,
        || demand::coerce(fcx, expr.span, expected, expr));
}

fn check_expr_with_hint<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>, expr: &'tcx ast::Expr,
                                  expected: Ty<'tcx>) {
    check_expr_with_unifier(
        fcx, expr, ExpectHasType(expected), NoPreference,
        || ())
}

fn check_expr_with_expectation<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                         expr: &'tcx ast::Expr,
                                         expected: Expectation<'tcx>) {
    check_expr_with_unifier(
        fcx, expr, expected, NoPreference,
        || ())
}

fn check_expr_with_expectation_and_lvalue_pref<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                                         expr: &'tcx ast::Expr,
                                                         expected: Expectation<'tcx>,
                                                         lvalue_pref: LvaluePreference)
{
    check_expr_with_unifier(fcx, expr, expected, lvalue_pref, || ())
}

fn check_expr<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>, expr: &'tcx ast::Expr)  {
    check_expr_with_unifier(fcx, expr, NoExpectation, NoPreference, || ())
}

fn check_expr_with_lvalue_pref<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>, expr: &'tcx ast::Expr,
                                        lvalue_pref: LvaluePreference)  {
    check_expr_with_unifier(fcx, expr, NoExpectation, lvalue_pref, || ())
}

// determine the `self` type, using fresh variables for all variables
// declared on the impl declaration e.g., `impl<A,B> for ~[(A,B)]`
// would return ($0, $1) where $0 and $1 are freshly instantiated type
// variables.
pub fn impl_self_ty<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                              span: Span, // (potential) receiver for this impl
                              did: ast::DefId)
                              -> TypeAndSubsts<'tcx> {
    let tcx = fcx.tcx();

    let ity = ty::lookup_item_type(tcx, did);
    let (n_tps, rps, raw_ty) =
        (ity.generics.types.len(subst::TypeSpace),
         ity.generics.regions.get_slice(subst::TypeSpace),
         ity.ty);

    let rps = fcx.inh.infcx.region_vars_for_defs(span, rps);
    let tps = fcx.inh.infcx.next_ty_vars(n_tps);
    let substs = subst::Substs::new_type(tps, rps);
    let substd_ty = fcx.instantiate_type_scheme(span, &substs, &raw_ty);

    TypeAndSubsts { substs: substs, ty: substd_ty }
}

/// Controls whether the arguments are tupled. This is used for the call
/// operator.
///
/// Tupling means that all call-side arguments are packed into a tuple and
/// passed as a single parameter. For example, if tupling is enabled, this
/// function:
///
///     fn f(x: (isize, isize))
///
/// Can be called as:
///
///     f(1, 2);
///
/// Instead of:
///
///     f((1, 2));
#[derive(Clone, Eq, PartialEq)]
enum TupleArgumentsFlag {
    DontTupleArguments,
    TupleArguments,
}

/// Unifies the return type with the expected type early, for more coercions
/// and forward type information on the argument expressions.
fn expected_types_for_fn_args<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                        call_span: Span,
                                        expected_ret: Expectation<'tcx>,
                                        formal_ret: ty::FnOutput<'tcx>,
                                        formal_args: &[Ty<'tcx>])
                                        -> Vec<Ty<'tcx>> {
    let expected_args = expected_ret.only_has_type(fcx).and_then(|ret_ty| {
        if let ty::FnConverging(formal_ret_ty) = formal_ret {
            fcx.infcx().commit_regions_if_ok(|| {
                // Attempt to apply a subtyping relationship between the formal
                // return type (likely containing type variables if the function
                // is polymorphic) and the expected return type.
                // No argument expectations are produced if unification fails.
                let origin = infer::Misc(call_span);
                let ures = fcx.infcx().sub_types(false, origin, formal_ret_ty, ret_ty);
                // FIXME(#15760) can't use try! here, FromError doesn't default
                // to identity so the resulting type is not constrained.
                if let Err(e) = ures {
                    return Err(e);
                }

                // Record all the argument types, with the substitutions
                // produced from the above subtyping unification.
                Ok(formal_args.iter().map(|ty| {
                    fcx.infcx().resolve_type_vars_if_possible(ty)
                }).collect())
            }).ok()
        } else {
            None
        }
    }).unwrap_or(vec![]);
    debug!("expected_types_for_fn_args(formal={} -> {}, expected={} -> {})",
           formal_args.repr(fcx.tcx()), formal_ret.repr(fcx.tcx()),
           expected_args.repr(fcx.tcx()), expected_ret.repr(fcx.tcx()));
    expected_args
}

/// Invariant:
/// If an expression has any sub-expressions that result in a type error,
/// inspecting that expression's type with `ty::type_is_error` will return
/// true. Likewise, if an expression is known to diverge, inspecting its
/// type with `ty::type_is_bot` will return true (n.b.: since Rust is
/// strict, _|_ can appear in the type of an expression that does not,
/// itself, diverge: for example, fn() -> _|_.)
/// Note that inspecting a type's structure *directly* may expose the fact
/// that there are actually multiple representations for `ty_err`, so avoid
/// that when err needs to be handled differently.
fn check_expr_with_unifier<'a, 'tcx, F>(fcx: &FnCtxt<'a, 'tcx>,
                                        expr: &'tcx ast::Expr,
                                        expected: Expectation<'tcx>,
                                        lvalue_pref: LvaluePreference,
                                        unifier: F) where
    F: FnOnce(),
{
    debug!(">> typechecking: expr={} expected={}",
           expr.repr(fcx.tcx()), expected.repr(fcx.tcx()));

    // Checks a method call.
    fn check_method_call<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                   expr: &'tcx ast::Expr,
                                   method_name: ast::SpannedIdent,
                                   args: &'tcx [P<ast::Expr>],
                                   tps: &[P<ast::Ty>],
                                   expected: Expectation<'tcx>,
                                   lvalue_pref: LvaluePreference) {
        let rcvr = &*args[0];
        check_expr_with_lvalue_pref(fcx, &*rcvr, lvalue_pref);

        // no need to check for bot/err -- callee does that
        let expr_t = structurally_resolved_type(fcx,
                                                expr.span,
                                                fcx.expr_ty(&*rcvr));

        let tps = tps.iter().map(|ast_ty| fcx.to_ty(&**ast_ty)).collect::<Vec<_>>();
        let fn_ty = match method::lookup(fcx,
                                         method_name.span,
                                         method_name.node.name,
                                         expr_t,
                                         tps,
                                         expr,
                                         rcvr) {
            Ok(method) => {
                let method_ty = method.ty;
                let method_call = MethodCall::expr(expr.id);
                fcx.inh.method_map.borrow_mut().insert(method_call, method);
                method_ty
            }
            Err(error) => {
                method::report_error(fcx, method_name.span, expr_t,
                                     method_name.node.name, Some(rcvr), error);
                fcx.write_error(expr.id);
                fcx.tcx().types.err
            }
        };

        // Call the generic checker.
        let ret_ty = check_method_argument_types(fcx,
                                                 method_name.span,
                                                 fn_ty,
                                                 expr,
                                                 &args[1..],
                                                 DontTupleArguments,
                                                 expected);

        write_call(fcx, expr, ret_ty);
    }

    // A generic function for checking the then and else in an if
    // or if-else.
    fn check_then_else<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                 cond_expr: &'tcx ast::Expr,
                                 then_blk: &'tcx ast::Block,
                                 opt_else_expr: Option<&'tcx ast::Expr>,
                                 id: ast::NodeId,
                                 sp: Span,
                                 expected: Expectation<'tcx>) {
        check_expr_has_type(fcx, cond_expr, fcx.tcx().types.bool);

        let expected = expected.adjust_for_branches(fcx);
        check_block_with_expected(fcx, then_blk, expected);
        let then_ty = fcx.node_ty(then_blk.id);

        let branches_ty = match opt_else_expr {
            Some(ref else_expr) => {
                check_expr_with_expectation(fcx, &**else_expr, expected);
                let else_ty = fcx.expr_ty(&**else_expr);
                infer::common_supertype(fcx.infcx(),
                                        infer::IfExpression(sp),
                                        true,
                                        then_ty,
                                        else_ty)
            }
            None => {
                infer::common_supertype(fcx.infcx(),
                                        infer::IfExpressionWithNoElse(sp),
                                        false,
                                        then_ty,
                                        ty::mk_nil(fcx.tcx()))
            }
        };

        let cond_ty = fcx.expr_ty(cond_expr);
        let if_ty = if ty::type_is_error(cond_ty) {
            fcx.tcx().types.err
        } else {
            branches_ty
        };

        fcx.write_ty(id, if_ty);
    }

    // Check field access expressions
    fn check_field<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>,
                            expr: &'tcx ast::Expr,
                            lvalue_pref: LvaluePreference,
                            base: &'tcx ast::Expr,
                            field: &ast::SpannedIdent) {
        let tcx = fcx.ccx.tcx;
        check_expr_with_lvalue_pref(fcx, base, lvalue_pref);
        let expr_t = structurally_resolved_type(fcx, expr.span,
                                                fcx.expr_ty(base));
        // FIXME(eddyb) #12808 Integrate privacy into this auto-deref loop.
        let (_, autoderefs, field_ty) = autoderef(fcx,
                                                  expr.span,
                                                  expr_t,
                                                  Some(base),
                                                  UnresolvedTypeAction::Error,
                                                  lvalue_pref,
                                                  |base_t, _| {
                match base_t.sty {
                    ty::ty_struct(base_id, substs) => {
                        debug!("struct named {}", ppaux::ty_to_string(tcx, base_t));
                        let fields = ty::lookup_struct_fields(tcx, base_id);
                        fcx.lookup_field_ty(expr.span, base_id, &fields[..],
                                            field.node.name, &(*substs))
                    }
                    _ => None
                }
            });
        match field_ty {
            Some(field_ty) => {
                fcx.write_ty(expr.id, field_ty);
                fcx.write_autoderef_adjustment(base.id, base.span, autoderefs);
                return;
            }
            None => {}
        }

        if method::exists(fcx, field.span, field.node.name, expr_t, expr.id) {
            fcx.type_error_message(
                field.span,
                |actual| {
                    format!("attempted to take value of method `{}` on type \
                            `{}`", token::get_ident(field.node), actual)
                },
                expr_t, None);

            tcx.sess.fileline_help(field.span,
                               "maybe a `()` to call it is missing? \
                               If not, try an anonymous function");
        } else {
            fcx.type_error_message(
                expr.span,
                |actual| {
                    format!("attempted access of field `{}` on \
                            type `{}`, but no field with that \
                            name was found",
                            token::get_ident(field.node),
                            actual)
                },
                expr_t, None);
            if let Some(t) = ty::ty_to_def_id(expr_t) {
                suggest_field_names(t, field, tcx, vec![]);
            }
        }

        fcx.write_error(expr.id);
    }

    // displays hints about the closest matches in field names
    fn suggest_field_names<'tcx>(id : DefId,
                                 field : &ast::SpannedIdent,
                                 tcx : &ty::ctxt<'tcx>,
                                 skip : Vec<&str>) {
        let ident = token::get_ident(field.node);
        let name = &ident;
        // only find fits with at least one matching letter
        let mut best_dist = name.len();
        let fields = ty::lookup_struct_fields(tcx, id);
        let mut best = None;
        for elem in &fields {
            let n = elem.name.as_str();
            // ignore already set fields
            if skip.iter().any(|&x| x == n) {
                continue;
            }
            // ignore private fields from non-local crates
            if id.krate != ast::LOCAL_CRATE && elem.vis != Visibility::Public {
                continue;
            }
            let dist = lev_distance(n, name);
            if dist < best_dist {
                best = Some(n);
                best_dist = dist;
            }
        }
        if let Some(n) = best {
            tcx.sess.span_help(field.span,
                &format!("did you mean `{}`?", n));
        }
    }

    // Check tuple index expressions
    fn check_tup_field<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>,
                                expr: &'tcx ast::Expr,
                                lvalue_pref: LvaluePreference,
                                base: &'tcx ast::Expr,
                                idx: codemap::Spanned<usize>) {
        let tcx = fcx.ccx.tcx;
        check_expr_with_lvalue_pref(fcx, base, lvalue_pref);
        let expr_t = structurally_resolved_type(fcx, expr.span,
                                                fcx.expr_ty(base));
        let mut tuple_like = false;
        // FIXME(eddyb) #12808 Integrate privacy into this auto-deref loop.
        let (_, autoderefs, field_ty) = autoderef(fcx,
                                                  expr.span,
                                                  expr_t,
                                                  Some(base),
                                                  UnresolvedTypeAction::Error,
                                                  lvalue_pref,
                                                  |base_t, _| {
                match base_t.sty {
                    ty::ty_struct(base_id, substs) => {
                        tuple_like = ty::is_tuple_struct(tcx, base_id);
                        if tuple_like {
                            debug!("tuple struct named {}", ppaux::ty_to_string(tcx, base_t));
                            let fields = ty::lookup_struct_fields(tcx, base_id);
                            fcx.lookup_tup_field_ty(expr.span, base_id, &fields[..],
                                                    idx.node, &(*substs))
                        } else {
                            None
                        }
                    }
                    ty::ty_tup(ref v) => {
                        tuple_like = true;
                        if idx.node < v.len() { Some(v[idx.node]) } else { None }
                    }
                    _ => None
                }
            });
        match field_ty {
            Some(field_ty) => {
                fcx.write_ty(expr.id, field_ty);
                fcx.write_autoderef_adjustment(base.id, base.span, autoderefs);
                return;
            }
            None => {}
        }
        fcx.type_error_message(
            expr.span,
            |actual| {
                if tuple_like {
                    format!("attempted out-of-bounds tuple index `{}` on \
                                    type `{}`",
                                   idx.node,
                                   actual)
                } else {
                    format!("attempted tuple index `{}` on type `{}`, but the \
                                     type was not a tuple or tuple struct",
                                    idx.node,
                                    actual)
                }
            },
            expr_t, None);

        fcx.write_error(expr.id);
    }

    fn check_struct_or_variant_fields<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                                struct_ty: Ty<'tcx>,
                                                span: Span,
                                                class_id: ast::DefId,
                                                node_id: ast::NodeId,
                                                substitutions: &'tcx subst::Substs<'tcx>,
                                                field_types: &[ty::field_ty],
                                                ast_fields: &'tcx [ast::Field],
                                                check_completeness: bool,
                                                enum_id_opt: Option<ast::DefId>)  {
        let tcx = fcx.ccx.tcx;

        let mut class_field_map = FnvHashMap();
        let mut fields_found = 0;
        for field in field_types {
            class_field_map.insert(field.name, (field.id, false));
        }

        let mut error_happened = false;

        // Typecheck each field.
        for field in ast_fields {
            let mut expected_field_type = tcx.types.err;

            let pair = class_field_map.get(&field.ident.node.name).cloned();
            match pair {
                None => {
                    fcx.type_error_message(
                        field.ident.span,
                        |actual| match enum_id_opt {
                            Some(enum_id) => {
                                let variant_type = ty::enum_variant_with_id(tcx,
                                                                            enum_id,
                                                                            class_id);
                                format!("struct variant `{}::{}` has no field named `{}`",
                                        actual, variant_type.name.as_str(),
                                        token::get_ident(field.ident.node))
                            }
                            None => {
                                format!("structure `{}` has no field named `{}`",
                                        actual,
                                        token::get_ident(field.ident.node))
                            }
                        },
                        struct_ty,
                        None);
                    // prevent all specified fields from being suggested
                    let skip_fields = ast_fields.iter().map(|ref x| x.ident.node.name.as_str());
                    let actual_id = match enum_id_opt {
                        Some(_) => class_id,
                        None => ty::ty_to_def_id(struct_ty).unwrap()
                    };
                    suggest_field_names(actual_id, &field.ident, tcx, skip_fields.collect());
                    error_happened = true;
                }
                Some((_, true)) => {
                    span_err!(fcx.tcx().sess, field.ident.span, E0062,
                        "field `{}` specified more than once",
                        token::get_ident(field.ident.node));
                    error_happened = true;
                }
                Some((field_id, false)) => {
                    expected_field_type =
                        ty::lookup_field_type(
                            tcx, class_id, field_id, substitutions);
                    expected_field_type =
                        fcx.normalize_associated_types_in(
                            field.span, &expected_field_type);
                    class_field_map.insert(
                        field.ident.node.name, (field_id, true));
                    fields_found += 1;
                }
            }

            // Make sure to give a type to the field even if there's
            // an error, so we can continue typechecking
            check_expr_coercable_to_type(fcx, &*field.expr, expected_field_type);
        }

        if error_happened {
            fcx.write_error(node_id);
        }

        if check_completeness && !error_happened {
            // Make sure the programmer specified all the fields.
            assert!(fields_found <= field_types.len());
            if fields_found < field_types.len() {
                let mut missing_fields = Vec::new();
                for class_field in field_types {
                    let name = class_field.name;
                    let (_, seen) = *class_field_map.get(&name).unwrap();
                    if !seen {
                        missing_fields.push(
                            format!("`{}`", &token::get_name(name)))
                    }
                }

                span_err!(tcx.sess, span, E0063,
                    "missing field{}: {}",
                    if missing_fields.len() == 1 {""} else {"s"},
                    missing_fields.connect(", "));
             }
        }

        if !error_happened {
            fcx.write_ty(node_id, ty::mk_struct(fcx.ccx.tcx,
                                class_id, substitutions));
        }
    }

    fn check_struct_constructor<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>,
                                         id: ast::NodeId,
                                         span: codemap::Span,
                                         class_id: ast::DefId,
                                         fields: &'tcx [ast::Field],
                                         base_expr: Option<&'tcx ast::Expr>) {
        let tcx = fcx.ccx.tcx;

        // Generate the struct type.
        let TypeAndSubsts {
            ty: mut struct_type,
            substs: struct_substs
        } = fcx.instantiate_type(span, class_id);

        // Look up and check the fields.
        let class_fields = ty::lookup_struct_fields(tcx, class_id);
        check_struct_or_variant_fields(fcx,
                                       struct_type,
                                       span,
                                       class_id,
                                       id,
                                       fcx.ccx.tcx.mk_substs(struct_substs),
                                       &class_fields[..],
                                       fields,
                                       base_expr.is_none(),
                                       None);
        if ty::type_is_error(fcx.node_ty(id)) {
            struct_type = tcx.types.err;
        }

        // Check the base expression if necessary.
        match base_expr {
            None => {}
            Some(base_expr) => {
                check_expr_has_type(fcx, &*base_expr, struct_type);
            }
        }

        // Write in the resulting type.
        fcx.write_ty(id, struct_type);
    }

    fn check_struct_enum_variant<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>,
                                          id: ast::NodeId,
                                          span: codemap::Span,
                                          enum_id: ast::DefId,
                                          variant_id: ast::DefId,
                                          fields: &'tcx [ast::Field]) {
        let tcx = fcx.ccx.tcx;

        // Look up the number of type parameters and the raw type, and
        // determine whether the enum is region-parameterized.
        let TypeAndSubsts {
            ty: enum_type,
            substs: substitutions
        } = fcx.instantiate_type(span, enum_id);

        // Look up and check the enum variant fields.
        let variant_fields = ty::lookup_struct_fields(tcx, variant_id);
        check_struct_or_variant_fields(fcx,
                                       enum_type,
                                       span,
                                       variant_id,
                                       id,
                                       fcx.ccx.tcx.mk_substs(substitutions),
                                       &variant_fields[..],
                                       fields,
                                       true,
                                       Some(enum_id));
        fcx.write_ty(id, enum_type);
    }

    fn check_struct_fields_on_error<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>,
                                             id: ast::NodeId,
                                             fields: &'tcx [ast::Field],
                                             base_expr: &'tcx Option<P<ast::Expr>>) {
        // Make sure to still write the types
        // otherwise we might ICE
        fcx.write_error(id);
        for field in fields {
            check_expr(fcx, &*field.expr);
        }
        match *base_expr {
            Some(ref base) => check_expr(fcx, &**base),
            None => {}
        }
    }

    type ExprCheckerWithTy = fn(&FnCtxt, &ast::Expr, Ty);

    let tcx = fcx.ccx.tcx;
    let id = expr.id;
    match expr.node {
      ast::ExprBox(ref opt_place, ref subexpr) => {
          opt_place.as_ref().map(|place|check_expr(fcx, &**place));
          check_expr(fcx, &**subexpr);

          let mut checked = false;
          opt_place.as_ref().map(|place| match place.node {
              ast::ExprPath(None, ref path) => {
                  // FIXME(pcwalton): For now we hardcode the two permissible
                  // places: the exchange heap and the managed heap.
                  let definition = lookup_full_def(tcx, path.span, place.id);
                  let def_id = definition.def_id();
                  let referent_ty = fcx.expr_ty(&**subexpr);
                  if tcx.lang_items.exchange_heap() == Some(def_id) {
                      fcx.write_ty(id, ty::mk_uniq(tcx, referent_ty));
                      checked = true
                  }
              }
              _ => {}
          });

          if !checked {
              span_err!(tcx.sess, expr.span, E0066,
                  "only the managed heap and exchange heap are currently supported");
              fcx.write_ty(id, tcx.types.err);
          }
      }

      ast::ExprLit(ref lit) => {
        let typ = check_lit(fcx, &**lit, expected);
        fcx.write_ty(id, typ);
      }
      ast::ExprBinary(op, ref lhs, ref rhs) => {
        op::check_binop(fcx, expr, op, lhs, rhs);
      }
      ast::ExprAssignOp(op, ref lhs, ref rhs) => {
        op::check_binop_assign(fcx, expr, op, lhs, rhs);
      }
      ast::ExprUnary(unop, ref oprnd) => {
        let expected_inner = expected.to_option(fcx).map_or(NoExpectation, |ty| {
            match unop {
                ast::UnUniq => match ty.sty {
                    ty::ty_uniq(ty) => {
                        Expectation::rvalue_hint(ty)
                    }
                    _ => {
                        NoExpectation
                    }
                },
                ast::UnNot | ast::UnNeg => {
                    expected
                }
                ast::UnDeref => {
                    NoExpectation
                }
            }
        });
        let lvalue_pref = match unop {
            ast::UnDeref => lvalue_pref,
            _ => NoPreference
        };
        check_expr_with_expectation_and_lvalue_pref(
            fcx, &**oprnd, expected_inner, lvalue_pref);
        let mut oprnd_t = fcx.expr_ty(&**oprnd);

        if !ty::type_is_error(oprnd_t) {
            match unop {
                ast::UnUniq => {
                    oprnd_t = ty::mk_uniq(tcx, oprnd_t);
                }
                ast::UnDeref => {
                    oprnd_t = structurally_resolved_type(fcx, expr.span, oprnd_t);
                    oprnd_t = match ty::deref(oprnd_t, true) {
                        Some(mt) => mt.ty,
                        None => match try_overloaded_deref(fcx, expr.span,
                                                           Some(MethodCall::expr(expr.id)),
                                                           Some(&**oprnd), oprnd_t, lvalue_pref) {
                            Some(mt) => mt.ty,
                            None => {
                                let is_newtype = match oprnd_t.sty {
                                    ty::ty_struct(did, substs) => {
                                        let fields = ty::struct_fields(fcx.tcx(), did, substs);
                                        fields.len() == 1
                                        && fields[0].name ==
                                        token::special_idents::unnamed_field.name
                                    }
                                    _ => false
                                };
                                if is_newtype {
                                    // This is an obsolete struct deref
                                    span_err!(tcx.sess, expr.span, E0068,
                                        "single-field tuple-structs can \
                                         no longer be dereferenced");
                                } else {
                                    fcx.type_error_message(expr.span, |actual| {
                                        format!("type `{}` cannot be \
                                                dereferenced", actual)
                                    }, oprnd_t, None);
                                }
                                tcx.types.err
                            }
                        }
                    };
                }
                ast::UnNot => {
                    oprnd_t = structurally_resolved_type(fcx, oprnd.span,
                                                         oprnd_t);
                    if !(ty::type_is_integral(oprnd_t) ||
                         oprnd_t.sty == ty::ty_bool) {
                        oprnd_t = op::check_user_unop(fcx, "!", "not",
                                                      tcx.lang_items.not_trait(),
                                                      expr, &**oprnd, oprnd_t, unop);
                    }
                }
                ast::UnNeg => {
                    oprnd_t = structurally_resolved_type(fcx, oprnd.span,
                                                         oprnd_t);
                    if !(ty::type_is_integral(oprnd_t) ||
                         ty::type_is_fp(oprnd_t)) {
                        oprnd_t = op::check_user_unop(fcx, "-", "neg",
                                                      tcx.lang_items.neg_trait(),
                                                      expr, &**oprnd, oprnd_t, unop);
                    }
                    if let ty::ty_uint(_) = oprnd_t.sty {
                        if !tcx.sess.features.borrow().negate_unsigned {
                            feature_gate::emit_feature_err(
                                &tcx.sess.parse_sess.span_diagnostic,
                                "negate_unsigned",
                                expr.span,
                                "unary negation of unsigned integers may be removed in the future");
                        }
                    }
                }
            }
        }
        fcx.write_ty(id, oprnd_t);
      }
      ast::ExprAddrOf(mutbl, ref oprnd) => {
        let hint = expected.only_has_type(fcx).map_or(NoExpectation, |ty| {
            match ty.sty {
                ty::ty_rptr(_, ref mt) | ty::ty_ptr(ref mt) => {
                    if ty::expr_is_lval(fcx.tcx(), &**oprnd) {
                        // Lvalues may legitimately have unsized types.
                        // For example, dereferences of a fat pointer and
                        // the last field of a struct can be unsized.
                        ExpectHasType(mt.ty)
                    } else {
                        Expectation::rvalue_hint(mt.ty)
                    }
                }
                _ => NoExpectation
            }
        });
        let lvalue_pref = match mutbl {
            ast::MutMutable => PreferMutLvalue,
            ast::MutImmutable => NoPreference
        };
        check_expr_with_expectation_and_lvalue_pref(fcx,
                                                    &**oprnd,
                                                    hint,
                                                    lvalue_pref);

        let tm = ty::mt { ty: fcx.expr_ty(&**oprnd), mutbl: mutbl };
        let oprnd_t = if ty::type_is_error(tm.ty) {
            tcx.types.err
        } else {
            // Note: at this point, we cannot say what the best lifetime
            // is to use for resulting pointer.  We want to use the
            // shortest lifetime possible so as to avoid spurious borrowck
            // errors.  Moreover, the longest lifetime will depend on the
            // precise details of the value whose address is being taken
            // (and how long it is valid), which we don't know yet until type
            // inference is complete.
            //
            // Therefore, here we simply generate a region variable.  The
            // region inferencer will then select the ultimate value.
            // Finally, borrowck is charged with guaranteeing that the
            // value whose address was taken can actually be made to live
            // as long as it needs to live.
            let region = fcx.infcx().next_region_var(infer::AddrOfRegion(expr.span));
            ty::mk_rptr(tcx, tcx.mk_region(region), tm)
        };
        fcx.write_ty(id, oprnd_t);
      }
      ast::ExprPath(ref maybe_qself, ref path) => {
          let opt_self_ty = maybe_qself.as_ref().map(|qself| {
              fcx.to_ty(&qself.ty)
          });

          let path_res = if let Some(&d) = tcx.def_map.borrow().get(&id) {
              d
          } else if let Some(ast::QSelf { position: 0, .. }) = *maybe_qself {
                // Create some fake resolution that can't possibly be a type.
                def::PathResolution {
                    base_def: def::DefMod(local_def(ast::CRATE_NODE_ID)),
                    last_private: LastMod(AllPublic),
                    depth: path.segments.len()
                }
            } else {
              tcx.sess.span_bug(expr.span,
                                &format!("unbound path {}", expr.repr(tcx)))
          };

          let def = path_res.base_def;
          if path_res.depth == 0 {
              let (scheme, predicates) =
                  type_scheme_and_predicates_for_def(fcx, expr.span, def);
              instantiate_path(fcx, &path.segments,
                               scheme, &predicates,
                               opt_self_ty, def, expr.span, id);
          } else {
              let ty_segments = path.segments.init();
              let base_ty_end = path.segments.len() - path_res.depth;
              let ty = astconv::finish_resolving_def_to_ty(fcx,
                                                           fcx,
                                                           expr.span,
                                                           PathParamMode::Optional,
                                                           &def,
                                                           opt_self_ty,
                                                           &ty_segments[..base_ty_end],
                                                           &ty_segments[base_ty_end..]);
              let method_segment = path.segments.last().unwrap();
              let method_name = method_segment.identifier.name;
              match method::resolve_ufcs(fcx, expr.span, method_name, ty, id) {
                  Ok((def, lp)) => {
                      // Write back the new resolution.
                      tcx.def_map.borrow_mut().insert(id, def::PathResolution {
                          base_def: def,
                          last_private: path_res.last_private.or(lp),
                          depth: 0
                      });

                      let (scheme, predicates) =
                          type_scheme_and_predicates_for_def(fcx, expr.span, def);
                      instantiate_path(fcx, slice::ref_slice(method_segment),
                                       scheme, &predicates,
                                       Some(ty), def, expr.span, id);
                  }
                  Err(error) => {
                      method::report_error(fcx, expr.span, ty,
                                           method_name, None, error);
                      fcx.write_error(id);
                  }
              }
          }

          // We always require that the type provided as the value for
          // a type parameter outlives the moment of instantiation.
          constrain_path_type_parameters(fcx, expr);
      }
      ast::ExprInlineAsm(ref ia) => {
          for &(_, ref input) in &ia.inputs {
              check_expr(fcx, &**input);
          }
          for &(_, ref out, _) in &ia.outputs {
              check_expr(fcx, &**out);
          }
          fcx.write_nil(id);
      }
      ast::ExprMac(_) => tcx.sess.bug("unexpanded macro"),
      ast::ExprBreak(_) => { fcx.write_ty(id, fcx.infcx().next_diverging_ty_var()); }
      ast::ExprAgain(_) => { fcx.write_ty(id, fcx.infcx().next_diverging_ty_var()); }
      ast::ExprRet(ref expr_opt) => {
        match fcx.ret_ty {
            ty::FnConverging(result_type) => {
                match *expr_opt {
                    None =>
                        if let Err(_) = fcx.mk_eqty(false, infer::Misc(expr.span),
                                                    result_type, ty::mk_nil(fcx.tcx())) {
                            span_err!(tcx.sess, expr.span, E0069,
                                "`return;` in function returning non-nil");
                        },
                    Some(ref e) => {
                        check_expr_coercable_to_type(fcx, &**e, result_type);
                    }
                }
            }
            ty::FnDiverging => {
                if let Some(ref e) = *expr_opt {
                    check_expr(fcx, &**e);
                }
                span_err!(tcx.sess, expr.span, E0166,
                    "`return` in a function declared as diverging");
            }
        }
        fcx.write_ty(id, fcx.infcx().next_diverging_ty_var());
      }
      ast::ExprParen(ref a) => {
        check_expr_with_expectation_and_lvalue_pref(fcx,
                                                    &**a,
                                                    expected,
                                                    lvalue_pref);
        fcx.write_ty(id, fcx.expr_ty(&**a));
      }
      ast::ExprAssign(ref lhs, ref rhs) => {
        check_expr_with_lvalue_pref(fcx, &**lhs, PreferMutLvalue);

        let tcx = fcx.tcx();
        if !ty::expr_is_lval(tcx, &**lhs) {
            span_err!(tcx.sess, expr.span, E0070,
                "illegal left-hand side expression");
        }

        let lhs_ty = fcx.expr_ty(&**lhs);
        check_expr_coercable_to_type(fcx, &**rhs, lhs_ty);
        let rhs_ty = fcx.expr_ty(&**rhs);

        fcx.require_expr_have_sized_type(&**lhs, traits::AssignmentLhsSized);

        if ty::type_is_error(lhs_ty) || ty::type_is_error(rhs_ty) {
            fcx.write_error(id);
        } else {
            fcx.write_nil(id);
        }
      }
      ast::ExprIf(ref cond, ref then_blk, ref opt_else_expr) => {
        check_then_else(fcx, &**cond, &**then_blk, opt_else_expr.as_ref().map(|e| &**e),
                        id, expr.span, expected);
      }
      ast::ExprIfLet(..) => {
        tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
      }
      ast::ExprWhile(ref cond, ref body, _) => {
        check_expr_has_type(fcx, &**cond, tcx.types.bool);
        check_block_no_value(fcx, &**body);
        let cond_ty = fcx.expr_ty(&**cond);
        let body_ty = fcx.node_ty(body.id);
        if ty::type_is_error(cond_ty) || ty::type_is_error(body_ty) {
            fcx.write_error(id);
        }
        else {
            fcx.write_nil(id);
        }
      }
      ast::ExprWhileLet(..) => {
        tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
      }
      ast::ExprForLoop(..) => {
        tcx.sess.span_bug(expr.span, "non-desugared ExprForLoop");
      }
      ast::ExprLoop(ref body, _) => {
        check_block_no_value(fcx, &**body);
        if !may_break(tcx, expr.id, &**body) {
            fcx.write_ty(id, fcx.infcx().next_diverging_ty_var());
        } else {
            fcx.write_nil(id);
        }
      }
      ast::ExprMatch(ref discrim, ref arms, match_src) => {
        _match::check_match(fcx, expr, &**discrim, arms, expected, match_src);
      }
      ast::ExprClosure(capture, ref decl, ref body) => {
          closure::check_expr_closure(fcx, expr, capture, &**decl, &**body, expected);
      }
      ast::ExprBlock(ref b) => {
        check_block_with_expected(fcx, &**b, expected);
        fcx.write_ty(id, fcx.node_ty(b.id));
      }
      ast::ExprCall(ref callee, ref args) => {
          callee::check_call(fcx, expr, &**callee, &args[..], expected);
      }
      ast::ExprMethodCall(ident, ref tps, ref args) => {
        check_method_call(fcx, expr, ident, &args[..], &tps[..], expected, lvalue_pref);
        let arg_tys = args.iter().map(|a| fcx.expr_ty(&**a));
        let  args_err = arg_tys.fold(false,
             |rest_err, a| {
              rest_err || ty::type_is_error(a)});
        if args_err {
            fcx.write_error(id);
        }
      }
      ast::ExprCast(ref e, ref t) => {
        if let ast::TyFixedLengthVec(_, ref count_expr) = t.node {
            check_expr_with_hint(fcx, &**count_expr, tcx.types.usize);
        }

        // Find the type of `e`. Supply hints based on the type we are casting to,
        // if appropriate.
        let t_1 = fcx.to_ty(t);
        let t_1 = structurally_resolved_type(fcx, expr.span, t_1);
        check_expr_with_expectation(fcx, e, ExpectCastableToType(t_1));
        let t_e = fcx.expr_ty(e);

        // Eagerly check for some obvious errors.
        if ty::type_is_error(t_e) {
            fcx.write_error(id);
        } else if !fcx.type_is_known_to_be_sized(t_1, expr.span) {
            report_cast_to_unsized_type(fcx, expr.span, t.span, e.span, t_1, t_e, id);
        } else {
            // Write a type for the whole expression, assuming everything is going
            // to work out Ok.
            fcx.write_ty(id, t_1);

            // Defer other checks until we're done type checking.
            let mut deferred_cast_checks = fcx.inh.deferred_cast_checks.borrow_mut();
            deferred_cast_checks.push(CastCheck {
                expr: (**e).clone(),
                expr_ty: t_e,
                cast_ty: t_1,
                span: expr.span,
            });
        }
      }
      ast::ExprVec(ref args) => {
        let uty = expected.to_option(fcx).and_then(|uty| {
            match uty.sty {
                ty::ty_vec(ty, _) => Some(ty),
                _ => None
            }
        });

        let typ = match uty {
            Some(uty) => {
                for e in args {
                    check_expr_coercable_to_type(fcx, &**e, uty);
                }
                uty
            }
            None => {
                let t: Ty = fcx.infcx().next_ty_var();
                for e in args {
                    check_expr_has_type(fcx, &**e, t);
                }
                t
            }
        };
        let typ = ty::mk_vec(tcx, typ, Some(args.len()));
        fcx.write_ty(id, typ);
      }
      ast::ExprRepeat(ref element, ref count_expr) => {
        check_expr_has_type(fcx, &**count_expr, tcx.types.usize);
        let count = ty::eval_repeat_count(fcx.tcx(), &**count_expr);

        let uty = match expected {
            ExpectHasType(uty) => {
                match uty.sty {
                    ty::ty_vec(ty, _) => Some(ty),
                    _ => None
                }
            }
            _ => None
        };

        let (element_ty, t) = match uty {
            Some(uty) => {
                check_expr_coercable_to_type(fcx, &**element, uty);
                (uty, uty)
            }
            None => {
                let t: Ty = fcx.infcx().next_ty_var();
                check_expr_has_type(fcx, &**element, t);
                (fcx.expr_ty(&**element), t)
            }
        };

        if count > 1 {
            // For [foo, ..n] where n > 1, `foo` must have
            // Copy type:
            fcx.require_type_meets(
                t,
                expr.span,
                traits::RepeatVec,
                ty::BoundCopy);
        }

        if ty::type_is_error(element_ty) {
            fcx.write_error(id);
        } else {
            let t = ty::mk_vec(tcx, t, Some(count));
            fcx.write_ty(id, t);
        }
      }
      ast::ExprTup(ref elts) => {
        let flds = expected.only_has_type(fcx).and_then(|ty| {
            match ty.sty {
                ty::ty_tup(ref flds) => Some(&flds[..]),
                _ => None
            }
        });
        let mut err_field = false;

        let elt_ts = elts.iter().enumerate().map(|(i, e)| {
            let t = match flds {
                Some(ref fs) if i < fs.len() => {
                    let ety = fs[i];
                    check_expr_coercable_to_type(fcx, &**e, ety);
                    ety
                }
                _ => {
                    check_expr_with_expectation(fcx, &**e, NoExpectation);
                    fcx.expr_ty(&**e)
                }
            };
            err_field = err_field || ty::type_is_error(t);
            t
        }).collect();
        if err_field {
            fcx.write_error(id);
        } else {
            let typ = ty::mk_tup(tcx, elt_ts);
            fcx.write_ty(id, typ);
        }
      }
      ast::ExprStruct(ref path, ref fields, ref base_expr) => {
        // Resolve the path.
        let def = lookup_full_def(tcx, path.span, id);
        let struct_id = match def {
            def::DefVariant(enum_id, variant_id, true) => {
                check_struct_enum_variant(fcx, id, expr.span, enum_id,
                                          variant_id, &fields[..]);
                enum_id
            }
            def::DefTrait(def_id) => {
                span_err!(tcx.sess, path.span, E0159,
                    "use of trait `{}` as a struct constructor",
                    pprust::path_to_string(path));
                check_struct_fields_on_error(fcx,
                                             id,
                                             &fields[..],
                                             base_expr);
                def_id
            },
            def => {
                // Verify that this was actually a struct.
                let typ = ty::lookup_item_type(fcx.ccx.tcx, def.def_id());
                match typ.ty.sty {
                    ty::ty_struct(struct_did, _) => {
                        check_struct_constructor(fcx,
                                                 id,
                                                 expr.span,
                                                 struct_did,
                                                 &fields[..],
                                                 base_expr.as_ref().map(|e| &**e));
                    }
                    _ => {
                        span_err!(tcx.sess, path.span, E0071,
                            "`{}` does not name a structure",
                            pprust::path_to_string(path));
                        check_struct_fields_on_error(fcx,
                                                     id,
                                                     &fields[..],
                                                     base_expr);
                    }
                }

                def.def_id()
            }
        };

        // Turn the path into a type and verify that that type unifies with
        // the resulting structure type. This is needed to handle type
        // parameters correctly.
        let actual_structure_type = fcx.expr_ty(&*expr);
        if !ty::type_is_error(actual_structure_type) {
            let type_and_substs = fcx.instantiate_struct_literal_ty(struct_id, path);
            match fcx.mk_subty(false,
                               infer::Misc(path.span),
                               actual_structure_type,
                               type_and_substs.ty) {
                Ok(()) => {}
                Err(type_error) => {
                    let type_error_description =
                        ty::type_err_to_str(tcx, &type_error);
                    span_err!(fcx.tcx().sess, path.span, E0235,
                                 "structure constructor specifies a \
                                         structure of type `{}`, but this \
                                         structure has type `{}`: {}",
                                         fcx.infcx()
                                            .ty_to_string(type_and_substs.ty),
                                         fcx.infcx()
                                            .ty_to_string(
                                                actual_structure_type),
                                         type_error_description);
                    ty::note_and_explain_type_err(tcx, &type_error);
                }
            }
        }

        fcx.require_expr_have_sized_type(expr, traits::StructInitializerSized);
      }
      ast::ExprField(ref base, ref field) => {
        check_field(fcx, expr, lvalue_pref, &**base, field);
      }
      ast::ExprTupField(ref base, idx) => {
        check_tup_field(fcx, expr, lvalue_pref, &**base, idx);
      }
      ast::ExprIndex(ref base, ref idx) => {
          check_expr_with_lvalue_pref(fcx, &**base, lvalue_pref);
          let base_t = fcx.expr_ty(&**base);
          if ty::type_is_error(base_t) {
              fcx.write_ty(id, base_t);
          } else {
              check_expr(fcx, &**idx);
              let idx_t = fcx.expr_ty(&**idx);
              if ty::type_is_error(idx_t) {
                  fcx.write_ty(id, idx_t);
              } else {
                  let base_t = structurally_resolved_type(fcx, expr.span, base_t);

                  let result =
                      autoderef_for_index(fcx, &**base, base_t, lvalue_pref, |adj_ty, adj| {
                          try_index_step(fcx,
                                         MethodCall::expr(expr.id),
                                         expr,
                                         &**base,
                                         adj_ty,
                                         adj,
                                         lvalue_pref,
                                         idx_t)
                      });

                  match result {
                      Some((index_ty, element_ty)) => {
                          let idx_expr_ty = fcx.expr_ty(idx);
                          demand::eqtype(fcx, expr.span, index_ty, idx_expr_ty);
                          fcx.write_ty(id, element_ty);
                      }
                      _ => {
                          check_expr_has_type(fcx, &**idx, fcx.tcx().types.err);
                          fcx.type_error_message(
                              expr.span,
                              |actual| {
                                  format!("cannot index a value of type `{}`",
                                          actual)
                              },
                              base_t,
                              None);
                          fcx.write_ty(id, fcx.tcx().types.err);
                      }
                  }
              }
          }
       }
       ast::ExprRange(ref start, ref end) => {
          let t_start = start.as_ref().map(|e| {
            check_expr(fcx, &**e);
            fcx.expr_ty(&**e)
          });
          let t_end = end.as_ref().map(|e| {
            check_expr(fcx, &**e);
            fcx.expr_ty(&**e)
          });

          let idx_type = match (t_start, t_end) {
              (Some(ty), None) | (None, Some(ty)) => {
                  Some(ty)
              }
              (Some(t_start), Some(t_end)) if (ty::type_is_error(t_start) ||
                                               ty::type_is_error(t_end)) => {
                  Some(fcx.tcx().types.err)
              }
              (Some(t_start), Some(t_end)) => {
                  Some(infer::common_supertype(fcx.infcx(),
                                               infer::RangeExpression(expr.span),
                                               true,
                                               t_start,
                                               t_end))
              }
              _ => None
          };

          // Note that we don't check the type of start/end satisfy any
          // bounds because right now the range structs do not have any. If we add
          // some bounds, then we'll need to check `t_start` against them here.

          let range_type = match idx_type {
            Some(idx_type) if ty::type_is_error(idx_type) => {
                fcx.tcx().types.err
            }
            Some(idx_type) => {
                // Find the did from the appropriate lang item.
                let did = match (start, end) {
                    (&Some(_), &Some(_)) => tcx.lang_items.range_struct(),
                    (&Some(_), &None) => tcx.lang_items.range_from_struct(),
                    (&None, &Some(_)) => tcx.lang_items.range_to_struct(),
                    (&None, &None) => {
                        tcx.sess.span_bug(expr.span, "full range should be dealt with above")
                    }
                };

                if let Some(did) = did {
                    let predicates = ty::lookup_predicates(tcx, did);
                    let substs = Substs::new_type(vec![idx_type], vec![]);
                    let bounds = fcx.instantiate_bounds(expr.span, &substs, &predicates);
                    fcx.add_obligations_for_parameters(
                        traits::ObligationCause::new(expr.span,
                                                     fcx.body_id,
                                                     traits::ItemObligation(did)),
                        &bounds);

                    ty::mk_struct(tcx, did, tcx.mk_substs(substs))
                } else {
                    span_err!(tcx.sess, expr.span, E0236, "no lang item for range syntax");
                    fcx.tcx().types.err
                }
            }
            None => {
                // Neither start nor end => RangeFull
                if let Some(did) = tcx.lang_items.range_full_struct() {
                    let substs = Substs::new_type(vec![], vec![]);
                    ty::mk_struct(tcx, did, tcx.mk_substs(substs))
                } else {
                    span_err!(tcx.sess, expr.span, E0237, "no lang item for range syntax");
                    fcx.tcx().types.err
                }
            }
          };

          fcx.write_ty(id, range_type);
       }

    }

    debug!("type of expr({}) {} is...", expr.id,
           syntax::print::pprust::expr_to_string(expr));
    debug!("... {}, expected is {}",
           ppaux::ty_to_string(tcx, fcx.expr_ty(expr)),
           expected.repr(tcx));

    unifier();
}

fn constrain_path_type_parameters(fcx: &FnCtxt,
                                  expr: &ast::Expr)
{
    fcx.opt_node_ty_substs(expr.id, |item_substs| {
        fcx.add_default_region_param_bounds(&item_substs.substs, expr);
    });
}

impl<'tcx> Expectation<'tcx> {
    /// Provide an expectation for an rvalue expression given an *optional*
    /// hint, which is not required for type safety (the resulting type might
    /// be checked higher up, as is the case with `&expr` and `box expr`), but
    /// is useful in determining the concrete type.
    ///
    /// The primary use case is where the expected type is a fat pointer,
    /// like `&[isize]`. For example, consider the following statement:
    ///
    ///    let x: &[isize] = &[1, 2, 3];
    ///
    /// In this case, the expected type for the `&[1, 2, 3]` expression is
    /// `&[isize]`. If however we were to say that `[1, 2, 3]` has the
    /// expectation `ExpectHasType([isize])`, that would be too strong --
    /// `[1, 2, 3]` does not have the type `[isize]` but rather `[isize; 3]`.
    /// It is only the `&[1, 2, 3]` expression as a whole that can be coerced
    /// to the type `&[isize]`. Therefore, we propagate this more limited hint,
    /// which still is useful, because it informs integer literals and the like.
    /// See the test case `test/run-pass/coerce-expect-unsized.rs` and #20169
    /// for examples of where this comes up,.
    fn rvalue_hint(ty: Ty<'tcx>) -> Expectation<'tcx> {
        match ty.sty {
            ty::ty_vec(_, None) | ty::ty_trait(..) => {
                ExpectRvalueLikeUnsized(ty)
            }
            _ => ExpectHasType(ty)
        }
    }

    // Resolves `expected` by a single level if it is a variable. If
    // there is no expected type or resolution is not possible (e.g.,
    // no constraints yet present), just returns `None`.
    fn resolve<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Expectation<'tcx> {
        match self {
            NoExpectation => {
                NoExpectation
            }
            ExpectCastableToType(t) => {
                ExpectCastableToType(
                    fcx.infcx().resolve_type_vars_if_possible(&t))
            }
            ExpectHasType(t) => {
                ExpectHasType(
                    fcx.infcx().resolve_type_vars_if_possible(&t))
            }
            ExpectRvalueLikeUnsized(t) => {
                ExpectRvalueLikeUnsized(
                    fcx.infcx().resolve_type_vars_if_possible(&t))
            }
        }
    }

    fn to_option<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Option<Ty<'tcx>> {
        match self.resolve(fcx) {
            NoExpectation => None,
            ExpectCastableToType(ty) |
            ExpectHasType(ty) |
            ExpectRvalueLikeUnsized(ty) => Some(ty),
        }
    }

    fn only_has_type<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Option<Ty<'tcx>> {
        match self.resolve(fcx) {
            ExpectHasType(ty) => Some(ty),
            _ => None
        }
    }
}

impl<'tcx> Repr<'tcx> for Expectation<'tcx> {
    fn repr(&self, tcx: &ty::ctxt<'tcx>) -> String {
        match *self {
            NoExpectation => format!("NoExpectation"),
            ExpectHasType(t) => format!("ExpectHasType({})",
                                        t.repr(tcx)),
            ExpectCastableToType(t) => format!("ExpectCastableToType({})",
                                               t.repr(tcx)),
            ExpectRvalueLikeUnsized(t) => format!("ExpectRvalueLikeUnsized({})",
                                                  t.repr(tcx)),
        }
    }
}

pub fn check_decl_initializer<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>,
                                       local: &'tcx ast::Local,
                                       init: &'tcx ast::Expr)
{
    let ref_bindings = fcx.tcx().pat_contains_ref_binding(&local.pat);

    let local_ty = fcx.local_ty(init.span, local.id);
    if !ref_bindings {
        check_expr_coercable_to_type(fcx, init, local_ty)
    } else {
        // Somewhat subtle: if we have a `ref` binding in the pattern,
        // we want to avoid introducing coercions for the RHS. This is
        // both because it helps preserve sanity and, in the case of
        // ref mut, for soundness (issue #23116). In particular, in
        // the latter case, we need to be clear that the type of the
        // referent for the reference that results is *equal to* the
        // type of the lvalue it is referencing, and not some
        // supertype thereof.
        check_expr(fcx, init);
        let init_ty = fcx.expr_ty(init);
        demand::eqtype(fcx, init.span, init_ty, local_ty);
    };
}

pub fn check_decl_local<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>, local: &'tcx ast::Local)  {
    let tcx = fcx.ccx.tcx;

    let t = fcx.local_ty(local.span, local.id);
    fcx.write_ty(local.id, t);

    if let Some(ref init) = local.init {
        check_decl_initializer(fcx, local, &**init);
        let init_ty = fcx.expr_ty(&**init);
        if ty::type_is_error(init_ty) {
            fcx.write_ty(local.id, init_ty);
        }
    }

    let pcx = pat_ctxt {
        fcx: fcx,
        map: pat_id_map(&tcx.def_map, &*local.pat),
    };
    _match::check_pat(&pcx, &*local.pat, t);
    let pat_ty = fcx.node_ty(local.pat.id);
    if ty::type_is_error(pat_ty) {
        fcx.write_ty(local.id, pat_ty);
    }
}

pub fn check_stmt<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>, stmt: &'tcx ast::Stmt)  {
    let node_id;
    let mut saw_bot = false;
    let mut saw_err = false;
    match stmt.node {
      ast::StmtDecl(ref decl, id) => {
        node_id = id;
        match decl.node {
          ast::DeclLocal(ref l) => {
              check_decl_local(fcx, &**l);
              let l_t = fcx.node_ty(l.id);
              saw_bot = saw_bot || fcx.infcx().type_var_diverges(l_t);
              saw_err = saw_err || ty::type_is_error(l_t);
          }
          ast::DeclItem(_) => {/* ignore for now */ }
        }
      }
      ast::StmtExpr(ref expr, id) => {
        node_id = id;
        // Check with expected type of ()
        check_expr_has_type(fcx, &**expr, ty::mk_nil(fcx.tcx()));
        let expr_ty = fcx.expr_ty(&**expr);
        saw_bot = saw_bot || fcx.infcx().type_var_diverges(expr_ty);
        saw_err = saw_err || ty::type_is_error(expr_ty);
      }
      ast::StmtSemi(ref expr, id) => {
        node_id = id;
        check_expr(fcx, &**expr);
        let expr_ty = fcx.expr_ty(&**expr);
        saw_bot |= fcx.infcx().type_var_diverges(expr_ty);
        saw_err |= ty::type_is_error(expr_ty);
      }
      ast::StmtMac(..) => fcx.ccx.tcx.sess.bug("unexpanded macro")
    }
    if saw_bot {
        fcx.write_ty(node_id, fcx.infcx().next_diverging_ty_var());
    }
    else if saw_err {
        fcx.write_error(node_id);
    }
    else {
        fcx.write_nil(node_id)
    }
}

pub fn check_block_no_value<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>, blk: &'tcx ast::Block)  {
    check_block_with_expected(fcx, blk, ExpectHasType(ty::mk_nil(fcx.tcx())));
    let blkty = fcx.node_ty(blk.id);
    if ty::type_is_error(blkty) {
        fcx.write_error(blk.id);
    } else {
        let nilty = ty::mk_nil(fcx.tcx());
        demand::suptype(fcx, blk.span, nilty, blkty);
    }
}

fn check_block_with_expected<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                       blk: &'tcx ast::Block,
                                       expected: Expectation<'tcx>) {
    let prev = {
        let mut fcx_ps = fcx.ps.borrow_mut();
        let unsafety_state = fcx_ps.recurse(blk);
        replace(&mut *fcx_ps, unsafety_state)
    };

    let mut warned = false;
    let mut any_diverges = false;
    let mut any_err = false;
    for s in &blk.stmts {
        check_stmt(fcx, &**s);
        let s_id = ast_util::stmt_id(&**s);
        let s_ty = fcx.node_ty(s_id);
        if any_diverges && !warned && match s.node {
            ast::StmtDecl(ref decl, _) => {
                match decl.node {
                    ast::DeclLocal(_) => true,
                    _ => false,
                }
            }
            ast::StmtExpr(_, _) | ast::StmtSemi(_, _) => true,
            _ => false
        } {
            fcx.ccx
                .tcx
                .sess
                .add_lint(lint::builtin::UNREACHABLE_CODE,
                          s_id,
                          s.span,
                          "unreachable statement".to_string());
            warned = true;
        }
        any_diverges = any_diverges || fcx.infcx().type_var_diverges(s_ty);
        any_err = any_err || ty::type_is_error(s_ty);
    }
    match blk.expr {
        None => if any_err {
            fcx.write_error(blk.id);
        } else if any_diverges {
            fcx.write_ty(blk.id, fcx.infcx().next_diverging_ty_var());
        } else {
            fcx.write_nil(blk.id);
        },
        Some(ref e) => {
            if any_diverges && !warned {
                fcx.ccx
                    .tcx
                    .sess
                    .add_lint(lint::builtin::UNREACHABLE_CODE,
                              e.id,
                              e.span,
                              "unreachable expression".to_string());
            }
            let ety = match expected {
                ExpectHasType(ety) => {
                    check_expr_coercable_to_type(fcx, &**e, ety);
                    ety
                }
                _ => {
                    check_expr_with_expectation(fcx, &**e, expected);
                    fcx.expr_ty(&**e)
                }
            };

            if any_err {
                fcx.write_error(blk.id);
            } else if any_diverges {
                fcx.write_ty(blk.id, fcx.infcx().next_diverging_ty_var());
            } else {
                fcx.write_ty(blk.id, ety);
            }
        }
    };

    *fcx.ps.borrow_mut() = prev;
}

/// Checks a constant appearing in a type. At the moment this is just the
/// length expression in a fixed-length vector, but someday it might be
/// extended to type-level numeric literals.
fn check_const_in_type<'a,'tcx>(ccx: &'a CrateCtxt<'a,'tcx>,
                                expr: &'tcx ast::Expr,
                                expected_type: Ty<'tcx>) {
    let inh = static_inherited_fields(ccx);
    let fcx = blank_fn_ctxt(ccx, &inh, ty::FnConverging(expected_type), expr.id);
    check_const_with_ty(&fcx, expr.span, expr, expected_type);
}

fn check_const<'a,'tcx>(ccx: &CrateCtxt<'a,'tcx>,
                        sp: Span,
                        e: &'tcx ast::Expr,
                        id: ast::NodeId) {
    let inh = static_inherited_fields(ccx);
    let rty = ty::node_id_to_type(ccx.tcx, id);
    let fcx = blank_fn_ctxt(ccx, &inh, ty::FnConverging(rty), e.id);
    let declty = fcx.ccx.tcx.tcache.borrow().get(&local_def(id)).unwrap().ty;
    check_const_with_ty(&fcx, sp, e, declty);
}

fn check_const_with_ty<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                 _: Span,
                                 e: &'tcx ast::Expr,
                                 declty: Ty<'tcx>) {
    // Gather locals in statics (because of block expressions).
    // This is technically unnecessary because locals in static items are forbidden,
    // but prevents type checking from blowing up before const checking can properly
    // emit a error.
    GatherLocalsVisitor { fcx: fcx }.visit_expr(e);

    check_expr_with_hint(fcx, e, declty);
    demand::coerce(fcx, e.span, declty, e);
    vtable::select_all_fcx_obligations_or_error(fcx);
    fcx.check_casts();
    regionck::regionck_expr(fcx, e);
    writeback::resolve_type_vars_in_expr(fcx, e);
}

/// Checks whether a type can be represented in memory. In particular, it
/// identifies types that contain themselves without indirection through a
/// pointer, which would mean their size is unbounded. This is different from
/// the question of whether a type can be instantiated. See the definition of
/// `check_instantiable`.
pub fn check_representable(tcx: &ty::ctxt,
                           sp: Span,
                           item_id: ast::NodeId,
                           designation: &str) -> bool {
    let rty = ty::node_id_to_type(tcx, item_id);

    // Check that it is possible to represent this type. This call identifies
    // (1) types that contain themselves and (2) types that contain a different
    // recursive type. It is only necessary to throw an error on those that
    // contain themselves. For case 2, there must be an inner type that will be
    // caught by case 1.
    match ty::is_type_representable(tcx, sp, rty) {
      ty::SelfRecursive => {
        span_err!(tcx.sess, sp, E0072,
            "illegal recursive {} type; \
             wrap the inner value in a box to make it representable",
            designation);
        return false
      }
      ty::Representable | ty::ContainsRecursive => (),
    }
    return true
}

/// Checks whether a type can be created without an instance of itself.
/// This is similar but different from the question of whether a type
/// can be represented.  For example, the following type:
///
///     enum foo { None, Some(foo) }
///
/// is instantiable but is not representable.  Similarly, the type
///
///     enum foo { Some(@foo) }
///
/// is representable, but not instantiable.
pub fn check_instantiable(tcx: &ty::ctxt,
                          sp: Span,
                          item_id: ast::NodeId)
                          -> bool {
    let item_ty = ty::node_id_to_type(tcx, item_id);
    if !ty::is_instantiable(tcx, item_ty) {
        span_err!(tcx.sess, sp, E0073,
            "this type cannot be instantiated without an \
             instance of itself");
        fileline_help!(tcx.sess, sp, "consider using `Option<{}>`",
            ppaux::ty_to_string(tcx, item_ty));
        false
    } else {
        true
    }
}

pub fn check_simd(tcx: &ty::ctxt, sp: Span, id: ast::NodeId) {
    let t = ty::node_id_to_type(tcx, id);
    if ty::type_needs_subst(t) {
        span_err!(tcx.sess, sp, E0074, "SIMD vector cannot be generic");
        return;
    }
    match t.sty {
        ty::ty_struct(did, substs) => {
            let fields = ty::lookup_struct_fields(tcx, did);
            if fields.is_empty() {
                span_err!(tcx.sess, sp, E0075, "SIMD vector cannot be empty");
                return;
            }
            let e = ty::lookup_field_type(tcx, did, fields[0].id, substs);
            if !fields.iter().all(
                         |f| ty::lookup_field_type(tcx, did, f.id, substs) == e) {
                span_err!(tcx.sess, sp, E0076, "SIMD vector should be homogeneous");
                return;
            }
            if !ty::type_is_machine(e) {
                span_err!(tcx.sess, sp, E0077,
                    "SIMD vector element type should be machine type");
                return;
            }
        }
        _ => ()
    }
}

pub fn check_enum_variants<'a,'tcx>(ccx: &CrateCtxt<'a,'tcx>,
                                    sp: Span,
                                    vs: &'tcx [P<ast::Variant>],
                                    id: ast::NodeId) {

    fn disr_in_range(ccx: &CrateCtxt,
                     ty: attr::IntType,
                     disr: ty::Disr) -> bool {
        fn uint_in_range(ccx: &CrateCtxt, ty: ast::UintTy, disr: ty::Disr) -> bool {
            match ty {
                ast::TyU8 => disr as u8 as Disr == disr,
                ast::TyU16 => disr as u16 as Disr == disr,
                ast::TyU32 => disr as u32 as Disr == disr,
                ast::TyU64 => disr as u64 as Disr == disr,
                ast::TyUs => uint_in_range(ccx, ccx.tcx.sess.target.uint_type, disr)
            }
        }
        fn int_in_range(ccx: &CrateCtxt, ty: ast::IntTy, disr: ty::Disr) -> bool {
            match ty {
                ast::TyI8 => disr as i8 as Disr == disr,
                ast::TyI16 => disr as i16 as Disr == disr,
                ast::TyI32 => disr as i32 as Disr == disr,
                ast::TyI64 => disr as i64 as Disr == disr,
                ast::TyIs => int_in_range(ccx, ccx.tcx.sess.target.int_type, disr)
            }
        }
        match ty {
            attr::UnsignedInt(ty) => uint_in_range(ccx, ty, disr),
            attr::SignedInt(ty) => int_in_range(ccx, ty, disr)
        }
    }

    fn do_check<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
                          vs: &'tcx [P<ast::Variant>],
                          id: ast::NodeId,
                          hint: attr::ReprAttr) {
        #![allow(trivial_numeric_casts)]

        let rty = ty::node_id_to_type(ccx.tcx, id);
        let mut disr_vals: Vec<ty::Disr> = Vec::new();

        let inh = static_inherited_fields(ccx);
        let fcx = blank_fn_ctxt(ccx, &inh, ty::FnConverging(rty), id);

        let (_, repr_type_ty) = ty::enum_repr_type(ccx.tcx, Some(&hint));
        for v in vs {
            if let Some(ref e) = v.node.disr_expr {
                check_const_with_ty(&fcx, e.span, e, repr_type_ty);
            }
        }

        let def_id = local_def(id);

        // ty::enum_variants guards against discriminant overflows, so
        // we need not check for that.
        let variants = ty::enum_variants(ccx.tcx, def_id);

        for (v, variant) in vs.iter().zip(variants.iter()) {
            let current_disr_val = variant.disr_val;

            // Check for duplicate discriminant values
            match disr_vals.iter().position(|&x| x == current_disr_val) {
                Some(i) => {
                    span_err!(ccx.tcx.sess, v.span, E0081,
                        "discriminant value `{}` already exists", disr_vals[i]);
                    span_note!(ccx.tcx.sess, ccx.tcx.map.span(variants[i].id.node),
                        "conflicting discriminant here")
                }
                None => {}
            }
            // Check for unrepresentable discriminant values
            match hint {
                attr::ReprAny | attr::ReprExtern => (),
                attr::ReprInt(sp, ity) => {
                    if !disr_in_range(ccx, ity, current_disr_val) {
                        span_err!(ccx.tcx.sess, v.span, E0082,
                            "discriminant value outside specified type");
                        span_note!(ccx.tcx.sess, sp,
                            "discriminant type specified here");
                    }
                }
                attr::ReprPacked => {
                    ccx.tcx.sess.bug("range_to_inttype: found ReprPacked on an enum");
                }
            }
            disr_vals.push(current_disr_val);
        }
    }

    let hint = *ty::lookup_repr_hints(ccx.tcx, ast::DefId { krate: ast::LOCAL_CRATE, node: id })
        .get(0).unwrap_or(&attr::ReprAny);

    if hint != attr::ReprAny && vs.len() <= 1 {
        if vs.len() == 1 {
            span_err!(ccx.tcx.sess, sp, E0083,
                "unsupported representation for univariant enum");
        } else {
            span_err!(ccx.tcx.sess, sp, E0084,
                "unsupported representation for zero-variant enum");
        };
    }

    do_check(ccx, vs, id, hint);

    check_representable(ccx.tcx, sp, id, "enum");

    // Check that it is possible to instantiate this enum:
    //
    // This *sounds* like the same that as representable, but it's
    // not.  See def'n of `check_instantiable()` for details.
    check_instantiable(ccx.tcx, sp, id);
}

// Returns the type parameter count and the type for the given definition.
fn type_scheme_and_predicates_for_def<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                                sp: Span,
                                                defn: def::Def)
                                                -> (TypeScheme<'tcx>, GenericPredicates<'tcx>) {
    match defn {
        def::DefLocal(nid) | def::DefUpvar(nid, _) => {
            let typ = fcx.local_ty(sp, nid);
            (ty::TypeScheme { generics: ty::Generics::empty(), ty: typ },
             ty::GenericPredicates::empty())
        }
        def::DefFn(id, _) | def::DefMethod(id, _) |
        def::DefStatic(id, _) | def::DefVariant(_, id, _) |
        def::DefStruct(id) | def::DefConst(id) => {
            (ty::lookup_item_type(fcx.tcx(), id), ty::lookup_predicates(fcx.tcx(), id))
        }
        def::DefTrait(_) |
        def::DefTy(..) |
        def::DefAssociatedTy(..) |
        def::DefPrimTy(_) |
        def::DefTyParam(..) |
        def::DefMod(..) |
        def::DefForeignMod(..) |
        def::DefUse(..) |
        def::DefRegion(..) |
        def::DefLabel(..) |
        def::DefSelfTy(..) => {
            fcx.ccx.tcx.sess.span_bug(sp, &format!("expected value, found {:?}", defn));
        }
    }
}

// Instantiates the given path, which must refer to an item with the given
// number of type parameters and type.
pub fn instantiate_path<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                  segments: &[ast::PathSegment],
                                  type_scheme: TypeScheme<'tcx>,
                                  type_predicates: &ty::GenericPredicates<'tcx>,
                                  opt_self_ty: Option<Ty<'tcx>>,
                                  def: def::Def,
                                  span: Span,
                                  node_id: ast::NodeId) {
    debug!("instantiate_path(path={:?}, def={}, node_id={}, type_scheme={})",
           segments,
           def.repr(fcx.tcx()),
           node_id,
           type_scheme.repr(fcx.tcx()));

    // We need to extract the type parameters supplied by the user in
    // the path `path`. Due to the current setup, this is a bit of a
    // tricky-process; the problem is that resolve only tells us the
    // end-point of the path resolution, and not the intermediate steps.
    // Luckily, we can (at least for now) deduce the intermediate steps
    // just from the end-point.
    //
    // There are basically three cases to consider:
    //
    // 1. Reference to a *type*, such as a struct or enum:
    //
    //        mod a { struct Foo<T> { ... } }
    //
    //    Because we don't allow types to be declared within one
    //    another, a path that leads to a type will always look like
    //    `a::b::Foo<T>` where `a` and `b` are modules. This implies
    //    that only the final segment can have type parameters, and
    //    they are located in the TypeSpace.
    //
    //    *Note:* Generally speaking, references to types don't
    //    actually pass through this function, but rather the
    //    `ast_ty_to_ty` function in `astconv`. However, in the case
    //    of struct patterns (and maybe literals) we do invoke
    //    `instantiate_path` to get the general type of an instance of
    //    a struct. (In these cases, there are actually no type
    //    parameters permitted at present, but perhaps we will allow
    //    them in the future.)
    //
    // 1b. Reference to a enum variant or tuple-like struct:
    //
    //        struct foo<T>(...)
    //        enum E<T> { foo(...) }
    //
    //    In these cases, the parameters are declared in the type
    //    space.
    //
    // 2. Reference to a *fn item*:
    //
    //        fn foo<T>() { }
    //
    //    In this case, the path will again always have the form
    //    `a::b::foo::<T>` where only the final segment should have
    //    type parameters. However, in this case, those parameters are
    //    declared on a value, and hence are in the `FnSpace`.
    //
    // 3. Reference to a *method*:
    //
    //        impl<A> SomeStruct<A> {
    //            fn foo<B>(...)
    //        }
    //
    //    Here we can have a path like
    //    `a::b::SomeStruct::<A>::foo::<B>`, in which case parameters
    //    may appear in two places. The penultimate segment,
    //    `SomeStruct::<A>`, contains parameters in TypeSpace, and the
    //    final segment, `foo::<B>` contains parameters in fn space.
    //
    // The first step then is to categorize the segments appropriately.

    assert!(segments.len() >= 1);

    let mut ufcs_method = None;
    let mut segment_spaces: Vec<_>;
    match def {
        // Case 1 and 1b. Reference to a *type* or *enum variant*.
        def::DefSelfTy(..) |
        def::DefStruct(..) |
        def::DefVariant(..) |
        def::DefTy(..) |
        def::DefAssociatedTy(..) |
        def::DefTrait(..) |
        def::DefPrimTy(..) |
        def::DefTyParam(..) => {
            // Everything but the final segment should have no
            // parameters at all.
            segment_spaces = repeat(None).take(segments.len() - 1).collect();
            segment_spaces.push(Some(subst::TypeSpace));
        }

        // Case 2. Reference to a top-level value.
        def::DefFn(..) |
        def::DefConst(..) |
        def::DefStatic(..) => {
            segment_spaces = repeat(None).take(segments.len() - 1).collect();
            segment_spaces.push(Some(subst::FnSpace));
        }

        // Case 3. Reference to a method.
        def::DefMethod(_, provenance) => {
            match provenance {
                def::FromTrait(trait_did) => {
                    callee::check_legal_trait_for_method_call(fcx.ccx, span, trait_did)
                }
                def::FromImpl(_) => {}
            }

            if segments.len() >= 2 {
                segment_spaces = repeat(None).take(segments.len() - 2).collect();
                segment_spaces.push(Some(subst::TypeSpace));
                segment_spaces.push(Some(subst::FnSpace));
            } else {
                // `<T>::method` will end up here, and so can `T::method`.
                let self_ty = opt_self_ty.expect("UFCS sugared method missing Self");
                segment_spaces = vec![Some(subst::FnSpace)];
                ufcs_method = Some((provenance, self_ty));
            }
        }

        // Other cases. Various nonsense that really shouldn't show up
        // here. If they do, an error will have been reported
        // elsewhere. (I hope)
        def::DefMod(..) |
        def::DefForeignMod(..) |
        def::DefLocal(..) |
        def::DefUse(..) |
        def::DefRegion(..) |
        def::DefLabel(..) |
        def::DefUpvar(..) => {
            segment_spaces = repeat(None).take(segments.len()).collect();
        }
    }
    assert_eq!(segment_spaces.len(), segments.len());

    // In `<T as Trait<A, B>>::method`, `A` and `B` are mandatory, but
    // `opt_self_ty` can also be Some for `Foo::method`, where Foo's
    // type parameters are not mandatory.
    let require_type_space = opt_self_ty.is_some() && ufcs_method.is_none();

    debug!("segment_spaces={:?}", segment_spaces);

    // Next, examine the definition, and determine how many type
    // parameters we expect from each space.
    let type_defs = &type_scheme.generics.types;
    let region_defs = &type_scheme.generics.regions;

    // Now that we have categorized what space the parameters for each
    // segment belong to, let's sort out the parameters that the user
    // provided (if any) into their appropriate spaces. We'll also report
    // errors if type parameters are provided in an inappropriate place.
    let mut substs = Substs::empty();
    for (opt_space, segment) in segment_spaces.iter().zip(segments.iter()) {
        match *opt_space {
            None => {
                check_path_args(fcx.tcx(), slice::ref_slice(segment),
                                NO_TPS | NO_REGIONS);
            }

            Some(space) => {
                push_explicit_parameters_from_segment_to_substs(fcx,
                                                                space,
                                                                span,
                                                                type_defs,
                                                                region_defs,
                                                                segment,
                                                                &mut substs);
            }
        }
    }
    if let Some(self_ty) = opt_self_ty {
        if type_defs.len(subst::SelfSpace) == 1 {
            substs.types.push(subst::SelfSpace, self_ty);
        }
    }

    // Now we have to compare the types that the user *actually*
    // provided against the types that were *expected*. If the user
    // did not provide any types, then we want to substitute inference
    // variables. If the user provided some types, we may still need
    // to add defaults. If the user provided *too many* types, that's
    // a problem.
    for &space in &ParamSpace::all() {
        adjust_type_parameters(fcx, span, space, type_defs,
                               require_type_space, &mut substs);
        assert_eq!(substs.types.len(space), type_defs.len(space));

        adjust_region_parameters(fcx, span, space, region_defs, &mut substs);
        assert_eq!(substs.regions().len(space), region_defs.len(space));
    }

    // The things we are substituting into the type should not contain
    // escaping late-bound regions, and nor should the base type scheme.
    assert!(!substs.has_regions_escaping_depth(0));
    assert!(!type_scheme.has_escaping_regions());

    // Add all the obligations that are required, substituting and
    // normalized appropriately.
    let bounds = fcx.instantiate_bounds(span, &substs, &type_predicates);
    fcx.add_obligations_for_parameters(
        traits::ObligationCause::new(span, fcx.body_id, traits::ItemObligation(def.def_id())),
        &bounds);

    // Substitute the values for the type parameters into the type of
    // the referenced item.
    let ty_substituted = fcx.instantiate_type_scheme(span, &substs, &type_scheme.ty);


    if let Some((def::FromImpl(impl_def_id), self_ty)) = ufcs_method {
        // In the case of `Foo<T>::method` and `<Foo<T>>::method`, if `method`
        // is inherent, there is no `Self` parameter, instead, the impl needs
        // type parameters, which we can infer by unifying the provided `Self`
        // with the substituted impl type.
        let impl_scheme = ty::lookup_item_type(fcx.tcx(), impl_def_id);
        assert_eq!(substs.types.len(subst::TypeSpace),
                   impl_scheme.generics.types.len(subst::TypeSpace));
        assert_eq!(substs.regions().len(subst::TypeSpace),
                   impl_scheme.generics.regions.len(subst::TypeSpace));

        let impl_ty = fcx.instantiate_type_scheme(span, &substs, &impl_scheme.ty);
        if fcx.mk_subty(false, infer::Misc(span), self_ty, impl_ty).is_err() {
            fcx.tcx().sess.span_bug(span,
            &format!(
                "instantiate_path: (UFCS) {} was a subtype of {} but now is not?",
                self_ty.repr(fcx.tcx()),
                impl_ty.repr(fcx.tcx())));
        }
    }

    fcx.write_ty(node_id, ty_substituted);
    fcx.write_substs(node_id, ty::ItemSubsts { substs: substs });
    return;

    /// Finds the parameters that the user provided and adds them to `substs`. If too many
    /// parameters are provided, then reports an error and clears the output vector.
    ///
    /// We clear the output vector because that will cause the `adjust_XXX_parameters()` later to
    /// use inference variables. This seems less likely to lead to derived errors.
    ///
    /// Note that we *do not* check for *too few* parameters here. Due to the presence of defaults
    /// etc that is more complicated. I wanted however to do the reporting of *too many* parameters
    /// here because we can easily use the precise span of the N+1'th parameter.
    fn push_explicit_parameters_from_segment_to_substs<'a, 'tcx>(
        fcx: &FnCtxt<'a, 'tcx>,
        space: subst::ParamSpace,
        span: Span,
        type_defs: &VecPerParamSpace<ty::TypeParameterDef<'tcx>>,
        region_defs: &VecPerParamSpace<ty::RegionParameterDef>,
        segment: &ast::PathSegment,
        substs: &mut Substs<'tcx>)
    {
        match segment.parameters {
            ast::AngleBracketedParameters(ref data) => {
                push_explicit_angle_bracketed_parameters_from_segment_to_substs(
                    fcx, space, type_defs, region_defs, data, substs);
            }

            ast::ParenthesizedParameters(ref data) => {
                span_err!(fcx.tcx().sess, span, E0238,
                    "parenthesized parameters may only be used with a trait");
                push_explicit_parenthesized_parameters_from_segment_to_substs(
                    fcx, space, span, type_defs, data, substs);
            }
        }
    }

    fn push_explicit_angle_bracketed_parameters_from_segment_to_substs<'a, 'tcx>(
        fcx: &FnCtxt<'a, 'tcx>,
        space: subst::ParamSpace,
        type_defs: &VecPerParamSpace<ty::TypeParameterDef<'tcx>>,
        region_defs: &VecPerParamSpace<ty::RegionParameterDef>,
        data: &ast::AngleBracketedParameterData,
        substs: &mut Substs<'tcx>)
    {
        {
            let type_count = type_defs.len(space);
            assert_eq!(substs.types.len(space), 0);
            for (i, typ) in data.types.iter().enumerate() {
                let t = fcx.to_ty(&**typ);
                if i < type_count {
                    substs.types.push(space, t);
                } else if i == type_count {
                    span_err!(fcx.tcx().sess, typ.span, E0087,
                        "too many type parameters provided: \
                         expected at most {} parameter(s), \
                         found {} parameter(s)",
                         type_count, data.types.len());
                    substs.types.truncate(space, 0);
                    break;
                }
            }
        }

        if data.bindings.len() > 0 {
            span_err!(fcx.tcx().sess, data.bindings[0].span, E0182,
                      "unexpected binding of associated item in expression path \
                       (only allowed in type paths)");
        }

        {
            let region_count = region_defs.len(space);
            assert_eq!(substs.regions().len(space), 0);
            for (i, lifetime) in data.lifetimes.iter().enumerate() {
                let r = ast_region_to_region(fcx.tcx(), lifetime);
                if i < region_count {
                    substs.mut_regions().push(space, r);
                } else if i == region_count {
                    span_err!(fcx.tcx().sess, lifetime.span, E0088,
                        "too many lifetime parameters provided: \
                         expected {} parameter(s), found {} parameter(s)",
                        region_count,
                        data.lifetimes.len());
                    substs.mut_regions().truncate(space, 0);
                    break;
                }
            }
        }
    }

    /// As with
    /// `push_explicit_angle_bracketed_parameters_from_segment_to_substs`,
    /// but intended for `Foo(A,B) -> C` form. This expands to
    /// roughly the same thing as `Foo<(A,B),C>`. One important
    /// difference has to do with the treatment of anonymous
    /// regions, which are translated into bound regions (NYI).
    fn push_explicit_parenthesized_parameters_from_segment_to_substs<'a, 'tcx>(
        fcx: &FnCtxt<'a, 'tcx>,
        space: subst::ParamSpace,
        span: Span,
        type_defs: &VecPerParamSpace<ty::TypeParameterDef<'tcx>>,
        data: &ast::ParenthesizedParameterData,
        substs: &mut Substs<'tcx>)
    {
        let type_count = type_defs.len(space);
        if type_count < 2 {
            span_err!(fcx.tcx().sess, span, E0167,
                      "parenthesized form always supplies 2 type parameters, \
                      but only {} parameter(s) were expected",
                      type_count);
        }

        let input_tys: Vec<Ty> =
            data.inputs.iter().map(|ty| fcx.to_ty(&**ty)).collect();

        let tuple_ty =
            ty::mk_tup(fcx.tcx(), input_tys);

        if type_count >= 1 {
            substs.types.push(space, tuple_ty);
        }

        let output_ty: Option<Ty> =
            data.output.as_ref().map(|ty| fcx.to_ty(&**ty));

        let output_ty =
            output_ty.unwrap_or(ty::mk_nil(fcx.tcx()));

        if type_count >= 2 {
            substs.types.push(space, output_ty);
        }
    }

    fn adjust_type_parameters<'a, 'tcx>(
        fcx: &FnCtxt<'a, 'tcx>,
        span: Span,
        space: ParamSpace,
        defs: &VecPerParamSpace<ty::TypeParameterDef<'tcx>>,
        require_type_space: bool,
        substs: &mut Substs<'tcx>)
    {
        let provided_len = substs.types.len(space);
        let desired = defs.get_slice(space);
        let required_len = desired.iter()
                              .take_while(|d| d.default.is_none())
                              .count();

        debug!("adjust_type_parameters(space={:?}, \
               provided_len={}, \
               desired_len={}, \
               required_len={})",
               space,
               provided_len,
               desired.len(),
               required_len);

        // Enforced by `push_explicit_parameters_from_segment_to_substs()`.
        assert!(provided_len <= desired.len());

        // Nothing specified at all: supply inference variables for
        // everything.
        if provided_len == 0 && !(require_type_space && space == subst::TypeSpace) {
            substs.types.replace(space, fcx.infcx().next_ty_vars(desired.len()));
            return;
        }

        // Too few parameters specified: report an error and use Err
        // for everything.
        if provided_len < required_len {
            let qualifier =
                if desired.len() != required_len { "at least " } else { "" };
            span_err!(fcx.tcx().sess, span, E0089,
                "too few type parameters provided: expected {}{} parameter(s) \
                , found {} parameter(s)",
                qualifier, required_len, provided_len);
            substs.types.replace(space, repeat(fcx.tcx().types.err).take(desired.len()).collect());
            return;
        }

        // Otherwise, add in any optional parameters that the user
        // omitted. The case of *too many* parameters is handled
        // already by
        // push_explicit_parameters_from_segment_to_substs(). Note
        // that the *default* type are expressed in terms of all prior
        // parameters, so we have to substitute as we go with the
        // partial substitution that we have built up.
        for i in provided_len..desired.len() {
            let default = desired[i].default.unwrap();
            let default = default.subst_spanned(fcx.tcx(), substs, Some(span));
            substs.types.push(space, default);
        }
        assert_eq!(substs.types.len(space), desired.len());

        debug!("Final substs: {}", substs.repr(fcx.tcx()));
    }

    fn adjust_region_parameters(
        fcx: &FnCtxt,
        span: Span,
        space: ParamSpace,
        defs: &VecPerParamSpace<ty::RegionParameterDef>,
        substs: &mut Substs)
    {
        let provided_len = substs.mut_regions().len(space);
        let desired = defs.get_slice(space);

        // Enforced by `push_explicit_parameters_from_segment_to_substs()`.
        assert!(provided_len <= desired.len());

        // If nothing was provided, just use inference variables.
        if provided_len == 0 {
            substs.mut_regions().replace(
                space,
                fcx.infcx().region_vars_for_defs(span, desired));
            return;
        }

        // If just the right number were provided, everybody is happy.
        if provided_len == desired.len() {
            return;
        }

        // Otherwise, too few were provided. Report an error and then
        // use inference variables.
        span_err!(fcx.tcx().sess, span, E0090,
            "too few lifetime parameters provided: expected {} parameter(s), \
             found {} parameter(s)",
            desired.len(), provided_len);

        substs.mut_regions().replace(
            space,
            fcx.infcx().region_vars_for_defs(span, desired));
    }
}

fn structurally_resolve_type_or_else<'a, 'tcx, F>(fcx: &FnCtxt<'a, 'tcx>,
                                                  sp: Span,
                                                  ty: Ty<'tcx>,
                                                  f: F) -> Ty<'tcx>
    where F: Fn() -> Ty<'tcx>
{
    let mut ty = fcx.resolve_type_vars_if_possible(ty);

    if ty::type_is_ty_var(ty) {
        let alternative = f();

        // If not, error.
        if ty::type_is_ty_var(alternative) || ty::type_is_error(alternative) {
            fcx.type_error_message(sp, |_actual| {
                "the type of this value must be known in this context".to_string()
            }, ty, None);
            demand::suptype(fcx, sp, fcx.tcx().types.err, ty);
            ty = fcx.tcx().types.err;
        } else {
            demand::suptype(fcx, sp, alternative, ty);
            ty = alternative;
        }
    }

    ty
}

// Resolves `typ` by a single level if `typ` is a type variable.  If no
// resolution is possible, then an error is reported.
pub fn structurally_resolved_type<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                            sp: Span,
                                            ty: Ty<'tcx>)
                                            -> Ty<'tcx>
{
    structurally_resolve_type_or_else(fcx, sp, ty, || {
        fcx.tcx().types.err
    })
}

// Returns true if b contains a break that can exit from b
pub fn may_break(cx: &ty::ctxt, id: ast::NodeId, b: &ast::Block) -> bool {
    // First: is there an unlabeled break immediately
    // inside the loop?
    (loop_query(&*b, |e| {
        match *e {
            ast::ExprBreak(None) => true,
            _ => false
        }
    })) ||
    // Second: is there a labeled break with label
    // <id> nested anywhere inside the loop?
    (block_query(b, |e| {
        if let ast::ExprBreak(Some(_)) = e.node {
            lookup_full_def(cx, e.span, e.id) == def::DefLabel(id)
        } else {
            false
        }
    }))
}

pub fn check_bounds_are_used<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
                                       span: Span,
                                       tps: &OwnedSlice<ast::TyParam>,
                                       ty: Ty<'tcx>) {
    debug!("check_bounds_are_used(n_tps={}, ty={})",
           tps.len(), ppaux::ty_to_string(ccx.tcx, ty));

    // make a vector of booleans initially false, set to true when used
    if tps.len() == 0 { return; }
    let mut tps_used: Vec<_> = repeat(false).take(tps.len()).collect();

    ty::walk_ty(ty, |t| {
            match t.sty {
                ty::ty_param(ParamTy {idx, ..}) => {
                    debug!("Found use of ty param num {}", idx);
                    tps_used[idx as usize] = true;
                }
                _ => ()
            }
        });

    for (i, b) in tps_used.iter().enumerate() {
        if !*b {
            span_err!(ccx.tcx.sess, span, E0091,
                "type parameter `{}` is unused",
                token::get_ident(tps[i].ident));
        }
    }
}

pub fn check_intrinsic_type(ccx: &CrateCtxt, it: &ast::ForeignItem) {
    fn param<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>, n: u32) -> Ty<'tcx> {
        let name = token::intern(&format!("P{}", n));
        ty::mk_param(ccx.tcx, subst::FnSpace, n, name)
    }

    let tcx = ccx.tcx;
    let name = token::get_ident(it.ident);
    let (n_tps, inputs, output) = if name.starts_with("atomic_") {
        let split : Vec<&str> = name.split('_').collect();
        assert!(split.len() >= 2, "Atomic intrinsic not correct format");

        //We only care about the operation here
        let (n_tps, inputs, output) = match split[1] {
            "cxchg" => (1, vec!(ty::mk_mut_ptr(tcx, param(ccx, 0)),
                                param(ccx, 0),
                                param(ccx, 0)),
                        param(ccx, 0)),
            "load" => (1, vec!(ty::mk_imm_ptr(tcx, param(ccx, 0))),
                       param(ccx, 0)),
            "store" => (1, vec!(ty::mk_mut_ptr(tcx, param(ccx, 0)), param(ccx, 0)),
                        ty::mk_nil(tcx)),

            "xchg" | "xadd" | "xsub" | "and"  | "nand" | "or" | "xor" | "max" |
            "min"  | "umax" | "umin" => {
                (1, vec!(ty::mk_mut_ptr(tcx, param(ccx, 0)), param(ccx, 0)),
                 param(ccx, 0))
            }
            "fence" => {
                (0, Vec::new(), ty::mk_nil(tcx))
            }
            op => {
                span_err!(tcx.sess, it.span, E0092,
                    "unrecognized atomic operation function: `{}`", op);
                return;
            }
        };
        (n_tps, inputs, ty::FnConverging(output))
    } else if &name[..] == "abort" || &name[..] == "unreachable" {
        (0, Vec::new(), ty::FnDiverging)
    } else {
        let (n_tps, inputs, output) = match &name[..] {
            "breakpoint" => (0, Vec::new(), ty::mk_nil(tcx)),
            "size_of" |
            "pref_align_of" | "min_align_of" => (1, Vec::new(), ccx.tcx.types.usize),
            "init" | "init_dropped" => (1, Vec::new(), param(ccx, 0)),
            "uninit" => (1, Vec::new(), param(ccx, 0)),
            "forget" => (1, vec!( param(ccx, 0) ), ty::mk_nil(tcx)),
            "transmute" => (2, vec!( param(ccx, 0) ), param(ccx, 1)),
            "move_val_init" => {
                (1,
                 vec!(
                    ty::mk_mut_rptr(tcx,
                                    tcx.mk_region(ty::ReLateBound(ty::DebruijnIndex::new(1),
                                                                  ty::BrAnon(0))),
                                    param(ccx, 0)),
                    param(ccx, 0)
                  ),
               ty::mk_nil(tcx))
            }
            "needs_drop" => (1, Vec::new(), ccx.tcx.types.bool),
            "owns_managed" => (1, Vec::new(), ccx.tcx.types.bool),

            "type_name" => (1, Vec::new(), ty::mk_str_slice(tcx, tcx.mk_region(ty::ReStatic),
                                                             ast::MutImmutable)),
            "type_id" => (1, Vec::new(), ccx.tcx.types.u64),
            "offset" => {
              (1,
               vec!(
                  ty::mk_ptr(tcx, ty::mt {
                      ty: param(ccx, 0),
                      mutbl: ast::MutImmutable
                  }),
                  ccx.tcx.types.isize
               ),
               ty::mk_ptr(tcx, ty::mt {
                   ty: param(ccx, 0),
                   mutbl: ast::MutImmutable
               }))
            }
            "copy" | "copy_nonoverlapping" => {
              (1,
               vec!(
                  ty::mk_ptr(tcx, ty::mt {
                      ty: param(ccx, 0),
                      mutbl: ast::MutImmutable
                  }),
                  ty::mk_ptr(tcx, ty::mt {
                      ty: param(ccx, 0),
                      mutbl: ast::MutMutable
                  }),
                  tcx.types.usize,
               ),
               ty::mk_nil(tcx))
            }
            "volatile_copy_memory" | "volatile_copy_nonoverlapping_memory" => {
              (1,
               vec!(
                  ty::mk_ptr(tcx, ty::mt {
                      ty: param(ccx, 0),
                      mutbl: ast::MutMutable
                  }),
                  ty::mk_ptr(tcx, ty::mt {
                      ty: param(ccx, 0),
                      mutbl: ast::MutImmutable
                  }),
                  tcx.types.usize,
               ),
               ty::mk_nil(tcx))
            }
            "write_bytes" | "volatile_set_memory" => {
              (1,
               vec!(
                  ty::mk_ptr(tcx, ty::mt {
                      ty: param(ccx, 0),
                      mutbl: ast::MutMutable
                  }),
                  tcx.types.u8,
                  tcx.types.usize,
               ),
               ty::mk_nil(tcx))
            }
            "sqrtf32" => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "sqrtf64" => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "powif32" => {
               (0,
                vec!( tcx.types.f32, tcx.types.i32 ),
                tcx.types.f32)
            }
            "powif64" => {
               (0,
                vec!( tcx.types.f64, tcx.types.i32 ),
                tcx.types.f64)
            }
            "sinf32" => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "sinf64" => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "cosf32" => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "cosf64" => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "powf32" => {
               (0,
                vec!( tcx.types.f32, tcx.types.f32 ),
                tcx.types.f32)
            }
            "powf64" => {
               (0,
                vec!( tcx.types.f64, tcx.types.f64 ),
                tcx.types.f64)
            }
            "expf32"   => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "expf64"   => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "exp2f32"  => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "exp2f64"  => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "logf32"   => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "logf64"   => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "log10f32" => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "log10f64" => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "log2f32"  => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "log2f64"  => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "fmaf32" => {
                (0,
                 vec!( tcx.types.f32, tcx.types.f32, tcx.types.f32 ),
                 tcx.types.f32)
            }
            "fmaf64" => {
                (0,
                 vec!( tcx.types.f64, tcx.types.f64, tcx.types.f64 ),
                 tcx.types.f64)
            }
            "fabsf32"      => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "fabsf64"      => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "copysignf32"  => (0, vec!( tcx.types.f32, tcx.types.f32 ), tcx.types.f32),
            "copysignf64"  => (0, vec!( tcx.types.f64, tcx.types.f64 ), tcx.types.f64),
            "floorf32"     => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "floorf64"     => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "ceilf32"      => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "ceilf64"      => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "truncf32"     => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "truncf64"     => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "rintf32"      => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "rintf64"      => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "nearbyintf32" => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "nearbyintf64" => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "roundf32"     => (0, vec!( tcx.types.f32 ), tcx.types.f32),
            "roundf64"     => (0, vec!( tcx.types.f64 ), tcx.types.f64),
            "ctpop8"       => (0, vec!( tcx.types.u8  ), tcx.types.u8),
            "ctpop16"      => (0, vec!( tcx.types.u16 ), tcx.types.u16),
            "ctpop32"      => (0, vec!( tcx.types.u32 ), tcx.types.u32),
            "ctpop64"      => (0, vec!( tcx.types.u64 ), tcx.types.u64),
            "ctlz8"        => (0, vec!( tcx.types.u8  ), tcx.types.u8),
            "ctlz16"       => (0, vec!( tcx.types.u16 ), tcx.types.u16),
            "ctlz32"       => (0, vec!( tcx.types.u32 ), tcx.types.u32),
            "ctlz64"       => (0, vec!( tcx.types.u64 ), tcx.types.u64),
            "cttz8"        => (0, vec!( tcx.types.u8  ), tcx.types.u8),
            "cttz16"       => (0, vec!( tcx.types.u16 ), tcx.types.u16),
            "cttz32"       => (0, vec!( tcx.types.u32 ), tcx.types.u32),
            "cttz64"       => (0, vec!( tcx.types.u64 ), tcx.types.u64),
            "bswap16"      => (0, vec!( tcx.types.u16 ), tcx.types.u16),
            "bswap32"      => (0, vec!( tcx.types.u32 ), tcx.types.u32),
            "bswap64"      => (0, vec!( tcx.types.u64 ), tcx.types.u64),

            "volatile_load" =>
                (1, vec!( ty::mk_imm_ptr(tcx, param(ccx, 0)) ), param(ccx, 0)),
            "volatile_store" =>
                (1, vec!( ty::mk_mut_ptr(tcx, param(ccx, 0)), param(ccx, 0) ), ty::mk_nil(tcx)),

            "i8_add_with_overflow" | "i8_sub_with_overflow" | "i8_mul_with_overflow" =>
                (0, vec!(tcx.types.i8, tcx.types.i8),
                ty::mk_tup(tcx, vec!(tcx.types.i8, tcx.types.bool))),

            "i16_add_with_overflow" | "i16_sub_with_overflow" | "i16_mul_with_overflow" =>
                (0, vec!(tcx.types.i16, tcx.types.i16),
                ty::mk_tup(tcx, vec!(tcx.types.i16, tcx.types.bool))),

            "i32_add_with_overflow" | "i32_sub_with_overflow" | "i32_mul_with_overflow" =>
                (0, vec!(tcx.types.i32, tcx.types.i32),
                ty::mk_tup(tcx, vec!(tcx.types.i32, tcx.types.bool))),

            "i64_add_with_overflow" | "i64_sub_with_overflow" | "i64_mul_with_overflow" =>
                (0, vec!(tcx.types.i64, tcx.types.i64),
                ty::mk_tup(tcx, vec!(tcx.types.i64, tcx.types.bool))),

            "u8_add_with_overflow" | "u8_sub_with_overflow" | "u8_mul_with_overflow" =>
                (0, vec!(tcx.types.u8, tcx.types.u8),
                ty::mk_tup(tcx, vec!(tcx.types.u8, tcx.types.bool))),

            "u16_add_with_overflow" | "u16_sub_with_overflow" | "u16_mul_with_overflow" =>
                (0, vec!(tcx.types.u16, tcx.types.u16),
                ty::mk_tup(tcx, vec!(tcx.types.u16, tcx.types.bool))),

            "u32_add_with_overflow" | "u32_sub_with_overflow" | "u32_mul_with_overflow"=>
                (0, vec!(tcx.types.u32, tcx.types.u32),
                ty::mk_tup(tcx, vec!(tcx.types.u32, tcx.types.bool))),

            "u64_add_with_overflow" | "u64_sub_with_overflow"  | "u64_mul_with_overflow" =>
                (0, vec!(tcx.types.u64, tcx.types.u64),
                ty::mk_tup(tcx, vec!(tcx.types.u64, tcx.types.bool))),

            "overflowing_add" | "overflowing_sub" | "overflowing_mul" =>
                (1, vec![param(ccx, 0), param(ccx, 0)], param(ccx, 0)),

            "return_address" => (0, vec![], ty::mk_imm_ptr(tcx, tcx.types.u8)),

            "assume" => (0, vec![tcx.types.bool], ty::mk_nil(tcx)),

            ref other => {
                span_err!(tcx.sess, it.span, E0093,
                    "unrecognized intrinsic function: `{}`", *other);
                return;
            }
        };
        (n_tps, inputs, ty::FnConverging(output))
    };
    let fty = ty::mk_bare_fn(tcx, None, tcx.mk_bare_fn(ty::BareFnTy {
        unsafety: ast::Unsafety::Unsafe,
        abi: abi::RustIntrinsic,
        sig: ty::Binder(FnSig {
            inputs: inputs,
            output: output,
            variadic: false,
        }),
    }));
    let i_ty = ty::lookup_item_type(ccx.tcx, local_def(it.id));
    let i_n_tps = i_ty.generics.types.len(subst::FnSpace);
    if i_n_tps != n_tps {
        span_err!(tcx.sess, it.span, E0094,
            "intrinsic has wrong number of type \
             parameters: found {}, expected {}",
             i_n_tps, n_tps);
    } else {
        require_same_types(tcx,
                           None,
                           false,
                           it.span,
                           i_ty.ty,
                           fty,
                           || {
                format!("intrinsic has wrong type: expected `{}`",
                        ppaux::ty_to_string(ccx.tcx, fty))
            });
    }
}