New upstream version 1.30.0+dfsg1

[rustc.git] / src / librustc_mir / borrow_check / nll / region_infer / error_reporting / region_name.rs

// Copyright 2017 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.

use borrow_check::nll::region_infer::RegionInferenceContext;
use borrow_check::nll::universal_regions::DefiningTy;
use borrow_check::nll::ToRegionVid;
use rustc::hir;
use rustc::hir::def_id::DefId;
use rustc::infer::InferCtxt;
use rustc::mir::Mir;
use rustc::ty::subst::{Substs, UnpackedKind};
use rustc::ty::{self, RegionKind, RegionVid, Ty, TyCtxt};
use rustc::util::ppaux::with_highlight_region;
use rustc_errors::DiagnosticBuilder;
use syntax::ast::{Name, DUMMY_NODE_ID};
use syntax::symbol::keywords;
use syntax_pos::symbol::InternedString;

impl<'tcx> RegionInferenceContext<'tcx> {
    /// Maps from an internal MIR region vid to something that we can
    /// report to the user. In some cases, the region vids will map
    /// directly to lifetimes that the user has a name for (e.g.,
    /// `'static`). But frequently they will not, in which case we
    /// have to find some way to identify the lifetime to the user. To
    /// that end, this function takes a "diagnostic" so that it can
    /// create auxiliary notes as needed.
    ///
    /// Example (function arguments):
    ///
    /// Suppose we are trying to give a name to the lifetime of the
    /// reference `x`:
    ///
    /// ```
    /// fn foo(x: &u32) { .. }
    /// ```
    ///
    /// This function would create a label like this:
    ///
    /// ```
    ///  | fn foo(x: &u32) { .. }
    ///           ------- fully elaborated type of `x` is `&'1 u32`
    /// ```
    ///
    /// and then return the name `'1` for us to use.
    crate fn give_region_a_name(
        &self,
        infcx: &InferCtxt<'_, '_, 'tcx>,
        mir: &Mir<'tcx>,
        mir_def_id: DefId,
        fr: RegionVid,
        counter: &mut usize,
        diag: &mut DiagnosticBuilder,
    ) -> InternedString {
        debug!("give_region_a_name(fr={:?}, counter={})", fr, counter);

        assert!(self.universal_regions.is_universal_region(fr));

        let value = self.give_name_from_error_region(infcx.tcx, mir_def_id, fr, counter, diag)
            .or_else(|| {
                self.give_name_if_anonymous_region_appears_in_arguments(
                    infcx, mir, mir_def_id, fr, counter, diag,
                )
            })
            .or_else(|| {
                self.give_name_if_anonymous_region_appears_in_upvars(
                    infcx.tcx, mir, fr, counter, diag,
                )
            })
            .or_else(|| {
                self.give_name_if_anonymous_region_appears_in_output(
                    infcx, mir, mir_def_id, fr, counter, diag,
                )
            })
            .unwrap_or_else(|| span_bug!(mir.span, "can't make a name for free region {:?}", fr));

        debug!("give_region_a_name: gave name {:?}", value);
        value
    }

    /// Check for the case where `fr` maps to something that the
    /// *user* has a name for. In that case, we'll be able to map
    /// `fr` to a `Region<'tcx>`, and that region will be one of
    /// named variants.
    fn give_name_from_error_region(
        &self,
        tcx: TyCtxt<'_, '_, 'tcx>,
        mir_def_id: DefId,
        fr: RegionVid,
        counter: &mut usize,
        diag: &mut DiagnosticBuilder<'_>,
    ) -> Option<InternedString> {
        let error_region = self.to_error_region(fr)?;

        debug!("give_region_a_name: error_region = {:?}", error_region);
        match error_region {
            ty::ReEarlyBound(ebr) => {
                if ebr.has_name() {
                    self.highlight_named_span(tcx, error_region, &ebr.name, diag);
                    Some(ebr.name)
                } else {
                    None
                }
            }

            ty::ReStatic => Some(keywords::StaticLifetime.name().as_interned_str()),

            ty::ReFree(free_region) => match free_region.bound_region {
                ty::BoundRegion::BrNamed(_, name) => {
                    self.highlight_named_span(tcx, error_region, &name, diag);
                    Some(name)
                }

                ty::BoundRegion::BrEnv => {
                    let mir_node_id = tcx.hir.as_local_node_id(mir_def_id).expect("non-local mir");
                    let def_ty = self.universal_regions.defining_ty;

                    if let DefiningTy::Closure(def_id, substs) = def_ty {
                        let args_span = if let hir::ExprKind::Closure(_, _, _, span, _) =
                            tcx.hir.expect_expr(mir_node_id).node
                        {
                            span
                        } else {
                            bug!("Closure is not defined by a closure expr");
                        };
                        let region_name = self.synthesize_region_name(counter);
                        diag.span_label(
                            args_span,
                            format!("lifetime `{}` represents this closure's body", region_name),
                        );

                        let closure_kind_ty = substs.closure_kind_ty(def_id, tcx);
                        let note = match closure_kind_ty.to_opt_closure_kind() {
                            Some(ty::ClosureKind::Fn) => {
                                "closure implements `Fn`, so references to captured variables \
                                 can't escape the closure"
                            }
                            Some(ty::ClosureKind::FnMut) => {
                                "closure implements `FnMut`, so references to captured variables \
                                 can't escape the closure"
                            }
                            Some(ty::ClosureKind::FnOnce) => {
                                bug!("BrEnv in a `FnOnce` closure");
                            }
                            None => bug!("Closure kind not inferred in borrow check"),
                        };

                        diag.note(note);

                        Some(region_name)
                    } else {
                        // Can't have BrEnv in functions, constants or generators.
                        bug!("BrEnv outside of closure.");
                    }
                }

                ty::BoundRegion::BrAnon(_) | ty::BoundRegion::BrFresh(_) => None,
            },

            ty::ReLateBound(..)
            | ty::ReScope(..)
            | ty::ReVar(..)
            | ty::ReSkolemized(..)
            | ty::ReEmpty
            | ty::ReErased
            | ty::ReClosureBound(..)
            | ty::ReCanonical(..) => None,
        }
    }

    /// Highlight a named span to provide context for error messages that
    /// mention that span, for example:
    ///
    /// ```
    ///  |
    ///  | fn two_regions<'a, 'b, T>(cell: Cell<&'a ()>, t: T)
    ///  |                --  -- lifetime `'b` defined here
    ///  |                |
    ///  |                lifetime `'a` defined here
    ///  |
    ///  |     with_signature(cell, t, |cell, t| require(cell, t));
    ///  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'b` must
    ///  |                                                         outlive `'a`
    /// ```
    fn highlight_named_span(
        &self,
        tcx: TyCtxt<'_, '_, 'tcx>,
        error_region: &RegionKind,
        name: &InternedString,
        diag: &mut DiagnosticBuilder<'_>,
    ) {
        let cm = tcx.sess.source_map();

        let scope = error_region.free_region_binding_scope(tcx);
        let node = tcx.hir.as_local_node_id(scope).unwrap_or(DUMMY_NODE_ID);

        let mut sp = cm.def_span(tcx.hir.span(node));
        if let Some(param) = tcx.hir
            .get_generics(scope)
            .and_then(|generics| generics.get_named(name))
        {
            sp = param.span;
        }

        diag.span_label(sp, format!("lifetime `{}` defined here", name));
    }

    /// Find an argument that contains `fr` and label it with a fully
    /// elaborated type, returning something like `'1`. Result looks
    /// like:
    ///
    /// ```
    ///  | fn foo(x: &u32) { .. }
    ///           ------- fully elaborated type of `x` is `&'1 u32`
    /// ```
    fn give_name_if_anonymous_region_appears_in_arguments(
        &self,
        infcx: &InferCtxt<'_, '_, 'tcx>,
        mir: &Mir<'tcx>,
        mir_def_id: DefId,
        fr: RegionVid,
        counter: &mut usize,
        diag: &mut DiagnosticBuilder<'_>,
    ) -> Option<InternedString> {
        let implicit_inputs = self.universal_regions.defining_ty.implicit_inputs();
        let argument_index = self.get_argument_index_for_region(infcx.tcx, fr)?;

        let arg_ty =
            self.universal_regions.unnormalized_input_tys[implicit_inputs + argument_index];
        if let Some(region_name) = self.give_name_if_we_can_match_hir_ty_from_argument(
            infcx,
            mir,
            mir_def_id,
            fr,
            arg_ty,
            argument_index,
            counter,
            diag,
        ) {
            return Some(region_name);
        }

        self.give_name_if_we_cannot_match_hir_ty(infcx, mir, fr, arg_ty, counter, diag)
    }

    fn give_name_if_we_can_match_hir_ty_from_argument(
        &self,
        infcx: &InferCtxt<'_, '_, 'tcx>,
        mir: &Mir<'tcx>,
        mir_def_id: DefId,
        needle_fr: RegionVid,
        argument_ty: Ty<'tcx>,
        argument_index: usize,
        counter: &mut usize,
        diag: &mut DiagnosticBuilder<'_>,
    ) -> Option<InternedString> {
        let mir_node_id = infcx.tcx.hir.as_local_node_id(mir_def_id)?;
        let fn_decl = infcx.tcx.hir.fn_decl(mir_node_id)?;
        let argument_hir_ty: &hir::Ty = &fn_decl.inputs[argument_index];
        match argument_hir_ty.node {
            // This indicates a variable with no type annotation, like
            // `|x|`... in that case, we can't highlight the type but
            // must highlight the variable.
            hir::TyKind::Infer => self.give_name_if_we_cannot_match_hir_ty(
                infcx,
                mir,
                needle_fr,
                argument_ty,
                counter,
                diag,
            ),

            _ => self.give_name_if_we_can_match_hir_ty(
                infcx.tcx,
                needle_fr,
                argument_ty,
                argument_hir_ty,
                counter,
                diag,
            ),
        }
    }

    /// Attempts to highlight the specific part of a type in an argument
    /// that has no type annotation.
    /// For example, we might produce an annotation like this:
    ///
    /// ```
    ///  |     foo(|a, b| b)
    ///  |          -  -
    ///  |          |  |
    ///  |          |  has type `&'1 u32`
    ///  |          has type `&'2 u32`
    /// ```
    fn give_name_if_we_cannot_match_hir_ty(
        &self,
        infcx: &InferCtxt<'_, '_, 'tcx>,
        mir: &Mir<'tcx>,
        needle_fr: RegionVid,
        argument_ty: Ty<'tcx>,
        counter: &mut usize,
        diag: &mut DiagnosticBuilder<'_>,
    ) -> Option<InternedString> {
        let type_name = with_highlight_region(needle_fr, *counter, || {
            infcx.extract_type_name(&argument_ty)
        });

        debug!(
            "give_name_if_we_cannot_match_hir_ty: type_name={:?} needle_fr={:?}",
            type_name, needle_fr
        );
        let assigned_region_name = if type_name.find(&format!("'{}", counter)).is_some() {
            // Only add a label if we can confirm that a region was labelled.
            let argument_index = self.get_argument_index_for_region(infcx.tcx, needle_fr)?;
            let (_, span) = self.get_argument_name_and_span_for_region(mir, argument_index);
            diag.span_label(span, format!("has type `{}`", type_name));

            // This counter value will already have been used, so this function will increment it
            // so the next value will be used next and return the region name that would have been
            // used.
            Some(self.synthesize_region_name(counter))
        } else {
            None
        };

        assigned_region_name
    }

    /// Attempts to highlight the specific part of a type annotation
    /// that contains the anonymous reference we want to give a name
    /// to. For example, we might produce an annotation like this:
    ///
    /// ```
    ///  | fn a<T>(items: &[T]) -> Box<dyn Iterator<Item=&T>> {
    ///  |                - let's call the lifetime of this reference `'1`
    /// ```
    ///
    /// the way this works is that we match up `argument_ty`, which is
    /// a `Ty<'tcx>` (the internal form of the type) with
    /// `argument_hir_ty`, a `hir::Ty` (the syntax of the type
    /// annotation). We are descending through the types stepwise,
    /// looking in to find the region `needle_fr` in the internal
    /// type.  Once we find that, we can use the span of the `hir::Ty`
    /// to add the highlight.
    ///
    /// This is a somewhat imperfect process, so long the way we also
    /// keep track of the **closest** type we've found. If we fail to
    /// find the exact `&` or `'_` to highlight, then we may fall back
    /// to highlighting that closest type instead.
    fn give_name_if_we_can_match_hir_ty(
        &self,
        tcx: TyCtxt<'_, '_, 'tcx>,
        needle_fr: RegionVid,
        argument_ty: Ty<'tcx>,
        argument_hir_ty: &hir::Ty,
        counter: &mut usize,
        diag: &mut DiagnosticBuilder<'_>,
    ) -> Option<InternedString> {
        let search_stack: &mut Vec<(Ty<'tcx>, &hir::Ty)> = &mut Vec::new();

        search_stack.push((argument_ty, argument_hir_ty));

        while let Some((ty, hir_ty)) = search_stack.pop() {
            match (&ty.sty, &hir_ty.node) {
                // Check if the `argument_ty` is `&'X ..` where `'X`
                // is the region we are looking for -- if so, and we have a `&T`
                // on the RHS, then we want to highlight the `&` like so:
                //
                //     &
                //     - let's call the lifetime of this reference `'1`
                (
                    ty::Ref(region, referent_ty, _),
                    hir::TyKind::Rptr(_lifetime, referent_hir_ty),
                ) => {
                    if region.to_region_vid() == needle_fr {
                        let region_name = self.synthesize_region_name(counter);

                        // Just grab the first character, the `&`.
                        let source_map = tcx.sess.source_map();
                        let ampersand_span = source_map.start_point(hir_ty.span);

                        diag.span_label(
                            ampersand_span,
                            format!(
                                "let's call the lifetime of this reference `{}`",
                                region_name
                            ),
                        );

                        return Some(region_name);
                    }

                    // Otherwise, let's descend into the referent types.
                    search_stack.push((referent_ty, &referent_hir_ty.ty));
                }

                // Match up something like `Foo<'1>`
                (
                    ty::Adt(_adt_def, substs),
                    hir::TyKind::Path(hir::QPath::Resolved(None, path)),
                ) => {
                    if let Some(last_segment) = path.segments.last() {
                        if let Some(name) = self.match_adt_and_segment(
                            substs,
                            needle_fr,
                            last_segment,
                            counter,
                            diag,
                            search_stack,
                        ) {
                            return Some(name);
                        }
                    }
                }

                // The following cases don't have lifetimes, so we
                // just worry about trying to match up the rustc type
                // with the HIR types:
                (ty::Tuple(elem_tys), hir::TyKind::Tup(elem_hir_tys)) => {
                    search_stack.extend(elem_tys.iter().cloned().zip(elem_hir_tys));
                }

                (ty::Slice(elem_ty), hir::TyKind::Slice(elem_hir_ty))
                | (ty::Array(elem_ty, _), hir::TyKind::Array(elem_hir_ty, _)) => {
                    search_stack.push((elem_ty, elem_hir_ty));
                }

                (ty::RawPtr(mut_ty), hir::TyKind::Ptr(mut_hir_ty)) => {
                    search_stack.push((mut_ty.ty, &mut_hir_ty.ty));
                }

                _ => {
                    // FIXME there are other cases that we could trace
                }
            }
        }

        return None;
    }

    /// We've found an enum/struct/union type with the substitutions
    /// `substs` and -- in the HIR -- a path type with the final
    /// segment `last_segment`. Try to find a `'_` to highlight in
    /// the generic args (or, if not, to produce new zipped pairs of
    /// types+hir to search through).
    fn match_adt_and_segment<'hir>(
        &self,
        substs: &'tcx Substs<'tcx>,
        needle_fr: RegionVid,
        last_segment: &'hir hir::PathSegment,
        counter: &mut usize,
        diag: &mut DiagnosticBuilder<'_>,
        search_stack: &mut Vec<(Ty<'tcx>, &'hir hir::Ty)>,
    ) -> Option<InternedString> {
        // Did the user give explicit arguments? (e.g., `Foo<..>`)
        let args = last_segment.args.as_ref()?;
        let lifetime = self.try_match_adt_and_generic_args(substs, needle_fr, args, search_stack)?;
        match lifetime.name {
            hir::LifetimeName::Param(_)
            | hir::LifetimeName::Static
            | hir::LifetimeName::Underscore => {
                let region_name = self.synthesize_region_name(counter);
                let ampersand_span = lifetime.span;
                diag.span_label(ampersand_span, format!("let's call this `{}`", region_name));
                return Some(region_name);
            }

            hir::LifetimeName::Implicit => {
                // In this case, the user left off the lifetime; so
                // they wrote something like:
                //
                // ```
                // x: Foo<T>
                // ```
                //
                // where the fully elaborated form is `Foo<'_, '1,
                // T>`. We don't consider this a match; instead we let
                // the "fully elaborated" type fallback above handle
                // it.
                return None;
            }
        }
    }

    /// We've found an enum/struct/union type with the substitutions
    /// `substs` and -- in the HIR -- a path with the generic
    /// arguments `args`. If `needle_fr` appears in the args, return
    /// the `hir::Lifetime` that corresponds to it. If not, push onto
    /// `search_stack` the types+hir to search through.
    fn try_match_adt_and_generic_args<'hir>(
        &self,
        substs: &'tcx Substs<'tcx>,
        needle_fr: RegionVid,
        args: &'hir hir::GenericArgs,
        search_stack: &mut Vec<(Ty<'tcx>, &'hir hir::Ty)>,
    ) -> Option<&'hir hir::Lifetime> {
        for (kind, hir_arg) in substs.iter().zip(&args.args) {
            match (kind.unpack(), hir_arg) {
                (UnpackedKind::Lifetime(r), hir::GenericArg::Lifetime(lt)) => {
                    if r.to_region_vid() == needle_fr {
                        return Some(lt);
                    }
                }

                (UnpackedKind::Type(ty), hir::GenericArg::Type(hir_ty)) => {
                    search_stack.push((ty, hir_ty));
                }

                (UnpackedKind::Lifetime(_), _) | (UnpackedKind::Type(_), _) => {
                    // I *think* that HIR lowering should ensure this
                    // doesn't happen, even in erroneous
                    // programs. Else we should use delay-span-bug.
                    span_bug!(
                        hir_arg.span(),
                        "unmatched subst and hir arg: found {:?} vs {:?}",
                        kind,
                        hir_arg,
                    );
                }
            }
        }

        None
    }

    /// Find a closure upvar that contains `fr` and label it with a
    /// fully elaborated type, returning something like `'1`. Result
    /// looks like:
    ///
    /// ```
    ///  | let x = Some(&22);
    ///        - fully elaborated type of `x` is `Option<&'1 u32>`
    /// ```
    fn give_name_if_anonymous_region_appears_in_upvars(
        &self,
        tcx: TyCtxt<'_, '_, 'tcx>,
        mir: &Mir<'tcx>,
        fr: RegionVid,
        counter: &mut usize,
        diag: &mut DiagnosticBuilder<'_>,
    ) -> Option<InternedString> {
        let upvar_index = self.get_upvar_index_for_region(tcx, fr)?;
        let (upvar_name, upvar_span) =
            self.get_upvar_name_and_span_for_region(tcx, mir, upvar_index);
        let region_name = self.synthesize_region_name(counter);

        diag.span_label(
            upvar_span,
            format!(
                "lifetime `{}` appears in the type of `{}`",
                region_name, upvar_name
            ),
        );

        Some(region_name)
    }

    /// Check for arguments appearing in the (closure) return type. It
    /// must be a closure since, in a free fn, such an argument would
    /// have to either also appear in an argument (if using elision)
    /// or be early bound (named, not in argument).
    fn give_name_if_anonymous_region_appears_in_output(
        &self,
        infcx: &InferCtxt<'_, '_, 'tcx>,
        mir: &Mir<'tcx>,
        mir_def_id: DefId,
        fr: RegionVid,
        counter: &mut usize,
        diag: &mut DiagnosticBuilder<'_>,
    ) -> Option<InternedString> {
        let tcx = infcx.tcx;

        let return_ty = self.universal_regions.unnormalized_output_ty;
        debug!(
            "give_name_if_anonymous_region_appears_in_output: return_ty = {:?}",
            return_ty
        );
        if !infcx
            .tcx
            .any_free_region_meets(&return_ty, |r| r.to_region_vid() == fr)
        {
            return None;
        }

        let type_name = with_highlight_region(fr, *counter, || infcx.extract_type_name(&return_ty));

        let mir_node_id = tcx.hir.as_local_node_id(mir_def_id).expect("non-local mir");

        let (return_span, mir_description) =
            if let hir::ExprKind::Closure(_, _, _, span, gen_move) =
                tcx.hir.expect_expr(mir_node_id).node
            {
                (
                    tcx.sess.source_map().end_point(span),
                    if gen_move.is_some() {
                        " of generator"
                    } else {
                        " of closure"
                    },
                )
            } else {
                // unreachable?
                (mir.span, "")
            };

        diag.span_label(
            return_span,
            format!("return type{} is {}", mir_description, type_name),
        );

        // This counter value will already have been used, so this function will increment it
        // so the next value will be used next and return the region name that would have been
        // used.
        Some(self.synthesize_region_name(counter))
    }

    /// Create a synthetic region named `'1`, incrementing the
    /// counter.
    fn synthesize_region_name(&self, counter: &mut usize) -> InternedString {
        let c = *counter;
        *counter += 1;

        Name::intern(&format!("'{:?}", c)).as_interned_str()
    }
}