src/librustc/middle/check_const.rs

   1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 // Verifies that the types and values of const and static items
  12 // are safe. The rules enforced by this module are:
  13 //
  14 // - For each *mutable* static item, it checks that its **type**:
  15 //     - doesn't have a destructor
  16 //     - doesn't own a box
  17 //
  18 // - For each *immutable* static item, it checks that its **value**:
  19 //       - doesn't own a box
  20 //       - doesn't contain a struct literal or a call to an enum variant / struct constructor where
  21 //           - the type of the struct/enum has a dtor
  22 //
  23 // Rules Enforced Elsewhere:
  24 // - It's not possible to take the address of a static item with unsafe interior. This is enforced
  25 // by borrowck::gather_loans
  26
  27 use middle::cast::{CastKind};
  28 use middle::const_eval;
  29 use middle::const_eval::EvalHint::ExprTypeChecked;
  30 use middle::def;
  31 use middle::expr_use_visitor as euv;
  32 use middle::infer;
  33 use middle::mem_categorization as mc;
  34 use middle::traits;
  35 use middle::ty::{self, Ty};
  36 use util::nodemap::NodeMap;
  37
  38 use syntax::ast;
  39 use syntax::codemap::Span;
  40 use syntax::visit::{self, Visitor};
  41
  42 use std::collections::hash_map::Entry;
  43 use std::cmp::Ordering;
  44
  45 // Const qualification, from partial to completely promotable.
  46 bitflags! {
  47     #[derive(RustcEncodable, RustcDecodable)]
  48     flags ConstQualif: u8 {
  49         // Inner mutability (can not be placed behind a reference) or behind
  50         // &mut in a non-global expression. Can be copied from static memory.
  51         const MUTABLE_MEM        = 1 << 0,
  52         // Constant value with a type that implements Drop. Can be copied
  53         // from static memory, similar to MUTABLE_MEM.
  54         const NEEDS_DROP         = 1 << 1,
  55         // Even if the value can be placed in static memory, copying it from
  56         // there is more expensive than in-place instantiation, and/or it may
  57         // be too large. This applies to [T; N] and everything containing it.
  58         // N.B.: references need to clear this flag to not end up on the stack.
  59         const PREFER_IN_PLACE    = 1 << 2,
  60         // May use more than 0 bytes of memory, doesn't impact the constness
  61         // directly, but is not allowed to be borrowed mutably in a constant.
  62         const NON_ZERO_SIZED     = 1 << 3,
  63         // Actually borrowed, has to always be in static memory. Does not
  64         // propagate, and requires the expression to behave like a 'static
  65         // lvalue. The set of expressions with this flag is the minimum
  66         // that have to be promoted.
  67         const HAS_STATIC_BORROWS = 1 << 4,
  68         // Invalid const for miscellaneous reasons (e.g. not implemented).
  69         const NOT_CONST          = 1 << 5,
  70
  71         // Borrowing the expression won't produce &'static T if any of these
  72         // bits are set, though the value could be copied from static memory
  73         // if `NOT_CONST` isn't set.
  74         const NON_STATIC_BORROWS = ConstQualif::MUTABLE_MEM.bits |
  75                                    ConstQualif::NEEDS_DROP.bits |
  76                                    ConstQualif::NOT_CONST.bits
  77     }
  78 }
  79
  80 #[derive(Copy, Clone, Eq, PartialEq)]
  81 enum Mode {
  82     Const,
  83     ConstFn,
  84     Static,
  85     StaticMut,
  86
  87     // An expression that occurs outside of any constant context
  88     // (i.e. `const`, `static`, array lengths, etc.). The value
  89     // can be variable at runtime, but will be promotable to
  90     // static memory if we can prove it is actually constant.
  91     Var,
  92 }
  93
  94 struct CheckCrateVisitor<'a, 'tcx: 'a> {
  95     tcx: &'a ty::ctxt<'tcx>,
  96     mode: Mode,
  97     qualif: ConstQualif,
  98     rvalue_borrows: NodeMap<ast::Mutability>
  99 }
 100
 101 impl<'a, 'tcx> CheckCrateVisitor<'a, 'tcx> {
 102     fn with_mode<F, R>(&mut self, mode: Mode, f: F) -> R where
 103         F: FnOnce(&mut CheckCrateVisitor<'a, 'tcx>) -> R,
 104     {
 105         let (old_mode, old_qualif) = (self.mode, self.qualif);
 106         self.mode = mode;
 107         self.qualif = ConstQualif::empty();
 108         let r = f(self);
 109         self.mode = old_mode;
 110         self.qualif = old_qualif;
 111         r
 112     }
 113
 114     fn with_euv<'b, F, R>(&'b mut self, item_id: Option<ast::NodeId>, f: F) -> R where
 115         F: for<'t> FnOnce(&mut euv::ExprUseVisitor<'b, 't, 'b, 'tcx>) -> R,
 116     {
 117         let param_env = match item_id {
 118             Some(item_id) => ty::ParameterEnvironment::for_item(self.tcx, item_id),
 119             None => self.tcx.empty_parameter_environment()
 120         };
 121
 122         let infcx = infer::new_infer_ctxt(self.tcx, &self.tcx.tables, Some(param_env), false);
 123
 124         f(&mut euv::ExprUseVisitor::new(self, &infcx))
 125     }
 126
 127     fn global_expr(&mut self, mode: Mode, expr: &ast::Expr) -> ConstQualif {
 128         assert!(mode != Mode::Var);
 129         match self.tcx.const_qualif_map.borrow_mut().entry(expr.id) {
 130             Entry::Occupied(entry) => return *entry.get(),
 131             Entry::Vacant(entry) => {
 132                 // Prevent infinite recursion on re-entry.
 133                 entry.insert(ConstQualif::empty());
 134             }
 135         }
 136         self.with_mode(mode, |this| {
 137             this.with_euv(None, |euv| euv.consume_expr(expr));
 138             this.visit_expr(expr);
 139             this.qualif
 140         })
 141     }
 142
 143     fn fn_like(&mut self,
 144                fk: visit::FnKind,
 145                fd: &ast::FnDecl,
 146                b: &ast::Block,
 147                s: Span,
 148                fn_id: ast::NodeId)
 149                -> ConstQualif {
 150         match self.tcx.const_qualif_map.borrow_mut().entry(fn_id) {
 151             Entry::Occupied(entry) => return *entry.get(),
 152             Entry::Vacant(entry) => {
 153                 // Prevent infinite recursion on re-entry.
 154                 entry.insert(ConstQualif::empty());
 155             }
 156         }
 157
 158         let mode = match fk {
 159             visit::FkItemFn(_, _, _, ast::Constness::Const, _, _) => {
 160                 Mode::ConstFn
 161             }
 162             visit::FkMethod(_, m, _) => {
 163                 if m.constness == ast::Constness::Const {
 164                     Mode::ConstFn
 165                 } else {
 166                     Mode::Var
 167                 }
 168             }
 169             _ => Mode::Var
 170         };
 171
 172         // Ensure the arguments are simple, not mutable/by-ref or patterns.
 173         if mode == Mode::ConstFn {
 174             for arg in &fd.inputs {
 175                 match arg.pat.node {
 176                     ast::PatIdent(ast::BindByValue(ast::MutImmutable), _, None) => {}
 177                     _ => {
 178                         span_err!(self.tcx.sess, arg.pat.span, E0022,
 179                                   "arguments of constant functions can only \
 180                                    be immutable by-value bindings");
 181                     }
 182                 }
 183             }
 184         }
 185
 186         let qualif = self.with_mode(mode, |this| {
 187             this.with_euv(Some(fn_id), |euv| euv.walk_fn(fd, b));
 188             visit::walk_fn(this, fk, fd, b, s);
 189             this.qualif
 190         });
 191
 192         // Keep only bits that aren't affected by function body (NON_ZERO_SIZED),
 193         // and bits that don't change semantics, just optimizations (PREFER_IN_PLACE).
 194         let qualif = qualif & (ConstQualif::NON_ZERO_SIZED | ConstQualif::PREFER_IN_PLACE);
 195
 196         self.tcx.const_qualif_map.borrow_mut().insert(fn_id, qualif);
 197         qualif
 198     }
 199
 200     fn add_qualif(&mut self, qualif: ConstQualif) {
 201         self.qualif = self.qualif | qualif;
 202     }
 203
 204     /// Returns true if the call is to a const fn or method.
 205     fn handle_const_fn_call(&mut self,
 206                             expr: &ast::Expr,
 207                             def_id: ast::DefId,
 208                             ret_ty: Ty<'tcx>)
 209                             -> bool {
 210         if let Some(fn_like) = const_eval::lookup_const_fn_by_id(self.tcx, def_id) {
 211             if
 212                 // we are in a static/const initializer
 213                 self.mode != Mode::Var &&
 214
 215                 // feature-gate is not enabled
 216                 !self.tcx.sess.features.borrow().const_fn &&
 217
 218                 // this doesn't come from a macro that has #[allow_internal_unstable]
 219                 !self.tcx.sess.codemap().span_allows_unstable(expr.span)
 220             {
 221                 self.tcx.sess.span_err(
 222                     expr.span,
 223                     &format!("const fns are an unstable feature"));
 224                 fileline_help!(
 225                     self.tcx.sess,
 226                     expr.span,
 227                     "in Nightly builds, add `#![feature(const_fn)]` to the crate \
 228                      attributes to enable");
 229             }
 230
 231             let qualif = self.fn_like(fn_like.kind(),
 232                                       fn_like.decl(),
 233                                       fn_like.body(),
 234                                       fn_like.span(),
 235                                       fn_like.id());
 236             self.add_qualif(qualif);
 237
 238             if ret_ty.type_contents(self.tcx).interior_unsafe() {
 239                 self.add_qualif(ConstQualif::MUTABLE_MEM);
 240             }
 241
 242             true
 243         } else {
 244             false
 245         }
 246     }
 247
 248     fn record_borrow(&mut self, id: ast::NodeId, mutbl: ast::Mutability) {
 249         match self.rvalue_borrows.entry(id) {
 250             Entry::Occupied(mut entry) => {
 251                 // Merge the two borrows, taking the most demanding
 252                 // one, mutability-wise.
 253                 if mutbl == ast::MutMutable {
 254                     entry.insert(mutbl);
 255                 }
 256             }
 257             Entry::Vacant(entry) => {
 258                 entry.insert(mutbl);
 259             }
 260         }
 261     }
 262
 263     fn msg(&self) -> &'static str {
 264         match self.mode {
 265             Mode::Const => "constant",
 266             Mode::ConstFn => "constant function",
 267             Mode::StaticMut | Mode::Static => "static",
 268             Mode::Var => unreachable!(),
 269         }
 270     }
 271
 272     fn check_static_mut_type(&self, e: &ast::Expr) {
 273         let node_ty = self.tcx.node_id_to_type(e.id);
 274         let tcontents = node_ty.type_contents(self.tcx);
 275
 276         let suffix = if tcontents.has_dtor() {
 277             "destructors"
 278         } else if tcontents.owns_owned() {
 279             "boxes"
 280         } else {
 281             return
 282         };
 283
 284         span_err!(self.tcx.sess, e.span, E0397,
 285                  "mutable statics are not allowed to have {}", suffix);
 286     }
 287
 288     fn check_static_type(&self, e: &ast::Expr) {
 289         let ty = self.tcx.node_id_to_type(e.id);
 290         let infcx = infer::new_infer_ctxt(self.tcx, &self.tcx.tables, None, false);
 291         let cause = traits::ObligationCause::new(e.span, e.id, traits::SharedStatic);
 292         let mut fulfill_cx = infcx.fulfillment_cx.borrow_mut();
 293         fulfill_cx.register_builtin_bound(&infcx, ty, ty::BoundSync, cause);
 294         match fulfill_cx.select_all_or_error(&infcx) {
 295             Ok(()) => { },
 296             Err(ref errors) => {
 297                 traits::report_fulfillment_errors(&infcx, errors);
 298             }
 299         }
 300     }
 301 }
 302
 303 impl<'a, 'tcx, 'v> Visitor<'v> for CheckCrateVisitor<'a, 'tcx> {
 304     fn visit_item(&mut self, i: &ast::Item) {
 305         debug!("visit_item(item={})", self.tcx.map.node_to_string(i.id));
 306         match i.node {
 307             ast::ItemStatic(_, ast::MutImmutable, ref expr) => {
 308                 self.check_static_type(&**expr);
 309                 self.global_expr(Mode::Static, &**expr);
 310             }
 311             ast::ItemStatic(_, ast::MutMutable, ref expr) => {
 312                 self.check_static_mut_type(&**expr);
 313                 self.global_expr(Mode::StaticMut, &**expr);
 314             }
 315             ast::ItemConst(_, ref expr) => {
 316                 self.global_expr(Mode::Const, &**expr);
 317             }
 318             ast::ItemEnum(ref enum_definition, _) => {
 319                 for var in &enum_definition.variants {
 320                     if let Some(ref ex) = var.node.disr_expr {
 321                         self.global_expr(Mode::Const, &**ex);
 322                     }
 323                 }
 324             }
 325             _ => {
 326                 self.with_mode(Mode::Var, |v| visit::walk_item(v, i));
 327             }
 328         }
 329     }
 330
 331     fn visit_trait_item(&mut self, t: &'v ast::TraitItem) {
 332         match t.node {
 333             ast::ConstTraitItem(_, ref default) => {
 334                 if let Some(ref expr) = *default {
 335                     self.global_expr(Mode::Const, &*expr);
 336                 } else {
 337                     visit::walk_trait_item(self, t);
 338                 }
 339             }
 340             _ => self.with_mode(Mode::Var, |v| visit::walk_trait_item(v, t)),
 341         }
 342     }
 343
 344     fn visit_impl_item(&mut self, i: &'v ast::ImplItem) {
 345         match i.node {
 346             ast::ConstImplItem(_, ref expr) => {
 347                 self.global_expr(Mode::Const, &*expr);
 348             }
 349             _ => self.with_mode(Mode::Var, |v| visit::walk_impl_item(v, i)),
 350         }
 351     }
 352
 353     fn visit_fn(&mut self,
 354                 fk: visit::FnKind<'v>,
 355                 fd: &'v ast::FnDecl,
 356                 b: &'v ast::Block,
 357                 s: Span,
 358                 fn_id: ast::NodeId) {
 359         self.fn_like(fk, fd, b, s, fn_id);
 360     }
 361
 362     fn visit_pat(&mut self, p: &ast::Pat) {
 363         match p.node {
 364             ast::PatLit(ref lit) => {
 365                 self.global_expr(Mode::Const, &**lit);
 366             }
 367             ast::PatRange(ref start, ref end) => {
 368                 self.global_expr(Mode::Const, &**start);
 369                 self.global_expr(Mode::Const, &**end);
 370
 371                 match const_eval::compare_lit_exprs(self.tcx, start, end) {
 372                     Some(Ordering::Less) |
 373                     Some(Ordering::Equal) => {}
 374                     Some(Ordering::Greater) => {
 375                         span_err!(self.tcx.sess, start.span, E0030,
 376                             "lower range bound must be less than or equal to upper");
 377                     }
 378                     None => {
 379                         self.tcx.sess.span_bug(
 380                             start.span, "literals of different types in range pat");
 381                     }
 382                 }
 383             }
 384             _ => visit::walk_pat(self, p)
 385         }
 386     }
 387
 388     fn visit_block(&mut self, block: &ast::Block) {
 389         // Check all statements in the block
 390         for stmt in &block.stmts {
 391             let span = match stmt.node {
 392                 ast::StmtDecl(ref decl, _) => {
 393                     match decl.node {
 394                         ast::DeclLocal(_) => decl.span,
 395
 396                         // Item statements are allowed
 397                         ast::DeclItem(_) => continue
 398                     }
 399                 }
 400                 ast::StmtExpr(ref expr, _) => expr.span,
 401                 ast::StmtSemi(ref semi, _) => semi.span,
 402                 ast::StmtMac(..) => {
 403                     self.tcx.sess.span_bug(stmt.span, "unexpanded statement \
 404                                                        macro in const?!")
 405                 }
 406             };
 407             self.add_qualif(ConstQualif::NOT_CONST);
 408             if self.mode != Mode::Var {
 409                 span_err!(self.tcx.sess, span, E0016,
 410                           "blocks in {}s are limited to items and \
 411                            tail expressions", self.msg());
 412             }
 413         }
 414         visit::walk_block(self, block);
 415     }
 416
 417     fn visit_expr(&mut self, ex: &ast::Expr) {
 418         let mut outer = self.qualif;
 419         self.qualif = ConstQualif::empty();
 420
 421         let node_ty = self.tcx.node_id_to_type(ex.id);
 422         check_expr(self, ex, node_ty);
 423         check_adjustments(self, ex);
 424
 425         // Special-case some expressions to avoid certain flags bubbling up.
 426         match ex.node {
 427             ast::ExprCall(ref callee, ref args) => {
 428                 for arg in args {
 429                     self.visit_expr(&**arg)
 430                 }
 431
 432                 let inner = self.qualif;
 433                 self.visit_expr(&**callee);
 434                 // The callee's size doesn't count in the call.
 435                 let added = self.qualif - inner;
 436                 self.qualif = inner | (added - ConstQualif::NON_ZERO_SIZED);
 437             }
 438             ast::ExprRepeat(ref element, _) => {
 439                 self.visit_expr(&**element);
 440                 // The count is checked elsewhere (typeck).
 441                 let count = match node_ty.sty {
 442                     ty::TyArray(_, n) => n,
 443                     _ => unreachable!()
 444                 };
 445                 // [element; 0] is always zero-sized.
 446                 if count == 0 {
 447                     self.qualif.remove(ConstQualif::NON_ZERO_SIZED | ConstQualif::PREFER_IN_PLACE);
 448                 }
 449             }
 450             ast::ExprMatch(ref discr, ref arms, _) => {
 451                 // Compute the most demanding borrow from all the arms'
 452                 // patterns and set that on the discriminator.
 453                 let mut borrow = None;
 454                 for pat in arms.iter().flat_map(|arm| &arm.pats) {
 455                     let pat_borrow = self.rvalue_borrows.remove(&pat.id);
 456                     match (borrow, pat_borrow) {
 457                         (None, _) | (_, Some(ast::MutMutable)) => {
 458                             borrow = pat_borrow;
 459                         }
 460                         _ => {}
 461                     }
 462                 }
 463                 if let Some(mutbl) = borrow {
 464                     self.record_borrow(discr.id, mutbl);
 465                 }
 466                 visit::walk_expr(self, ex);
 467             }
 468             // Division by zero and overflow checking.
 469             ast::ExprBinary(op, _, _) => {
 470                 visit::walk_expr(self, ex);
 471                 let div_or_rem = op.node == ast::BiDiv || op.node == ast::BiRem;
 472                 match node_ty.sty {
 473                     ty::TyUint(_) | ty::TyInt(_) if div_or_rem => {
 474                         if !self.qualif.intersects(ConstQualif::NOT_CONST) {
 475                             match const_eval::eval_const_expr_partial(
 476                                     self.tcx, ex, ExprTypeChecked) {
 477                                 Ok(_) => {}
 478                                 Err(msg) => {
 479                                     span_err!(self.tcx.sess, msg.span, E0020,
 480                                               "{} in a constant expression",
 481                                               msg.description())
 482                                 }
 483                             }
 484                         }
 485                     }
 486                     _ => {}
 487                 }
 488             }
 489             _ => visit::walk_expr(self, ex)
 490         }
 491
 492         // Handle borrows on (or inside the autorefs of) this expression.
 493         match self.rvalue_borrows.remove(&ex.id) {
 494             Some(ast::MutImmutable) => {
 495                 // Constants cannot be borrowed if they contain interior mutability as
 496                 // it means that our "silent insertion of statics" could change
 497                 // initializer values (very bad).
 498                 // If the type doesn't have interior mutability, then `ConstQualif::MUTABLE_MEM` has
 499                 // propagated from another error, so erroring again would be just noise.
 500                 let tc = node_ty.type_contents(self.tcx);
 501                 if self.qualif.intersects(ConstQualif::MUTABLE_MEM) && tc.interior_unsafe() {
 502                     outer = outer | ConstQualif::NOT_CONST;
 503                     if self.mode != Mode::Var {
 504                         self.tcx.sess.span_err(ex.span,
 505                             "cannot borrow a constant which contains \
 506                              interior mutability, create a static instead");
 507                     }
 508                 }
 509                 // If the reference has to be 'static, avoid in-place initialization
 510                 // as that will end up pointing to the stack instead.
 511                 if !self.qualif.intersects(ConstQualif::NON_STATIC_BORROWS) {
 512                     self.qualif = self.qualif - ConstQualif::PREFER_IN_PLACE;
 513                     self.add_qualif(ConstQualif::HAS_STATIC_BORROWS);
 514                 }
 515             }
 516             Some(ast::MutMutable) => {
 517                 // `&mut expr` means expr could be mutated, unless it's zero-sized.
 518                 if self.qualif.intersects(ConstQualif::NON_ZERO_SIZED) {
 519                     if self.mode == Mode::Var {
 520                         outer = outer | ConstQualif::NOT_CONST;
 521                         self.add_qualif(ConstQualif::MUTABLE_MEM);
 522                     } else {
 523                         span_err!(self.tcx.sess, ex.span, E0017,
 524                             "references in {}s may only refer \
 525                              to immutable values", self.msg())
 526                     }
 527                 }
 528                 if !self.qualif.intersects(ConstQualif::NON_STATIC_BORROWS) {
 529                     self.add_qualif(ConstQualif::HAS_STATIC_BORROWS);
 530                 }
 531             }
 532             None => {}
 533         }
 534         self.tcx.const_qualif_map.borrow_mut().insert(ex.id, self.qualif);
 535         // Don't propagate certain flags.
 536         self.qualif = outer | (self.qualif - ConstQualif::HAS_STATIC_BORROWS);
 537     }
 538 }
 539
 540 /// This function is used to enforce the constraints on
 541 /// const/static items. It walks through the *value*
 542 /// of the item walking down the expression and evaluating
 543 /// every nested expression. If the expression is not part
 544 /// of a const/static item, it is qualified for promotion
 545 /// instead of producing errors.
 546 fn check_expr<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>,
 547                         e: &ast::Expr, node_ty: Ty<'tcx>) {
 548     match node_ty.sty {
 549         ty::TyStruct(did, _) |
 550         ty::TyEnum(did, _) if v.tcx.has_dtor(did) => {
 551             v.add_qualif(ConstQualif::NEEDS_DROP);
 552             if v.mode != Mode::Var {
 553                 v.tcx.sess.span_err(e.span,
 554                                     &format!("{}s are not allowed to have destructors",
 555                                              v.msg()));
 556             }
 557         }
 558         _ => {}
 559     }
 560
 561     let method_call = ty::MethodCall::expr(e.id);
 562     match e.node {
 563         ast::ExprUnary(..) |
 564         ast::ExprBinary(..) |
 565         ast::ExprIndex(..) if v.tcx.tables.borrow().method_map.contains_key(&method_call) => {
 566             v.add_qualif(ConstQualif::NOT_CONST);
 567             if v.mode != Mode::Var {
 568                 span_err!(v.tcx.sess, e.span, E0011,
 569                             "user-defined operators are not allowed in {}s", v.msg());
 570             }
 571         }
 572         ast::ExprBox(..) |
 573         ast::ExprUnary(ast::UnUniq, _) => {
 574             v.add_qualif(ConstQualif::NOT_CONST);
 575             if v.mode != Mode::Var {
 576                 span_err!(v.tcx.sess, e.span, E0010,
 577                           "allocations are not allowed in {}s", v.msg());
 578             }
 579         }
 580         ast::ExprUnary(op, ref inner) => {
 581             match v.tcx.node_id_to_type(inner.id).sty {
 582                 ty::TyRawPtr(_) => {
 583                     assert!(op == ast::UnDeref);
 584
 585                     v.add_qualif(ConstQualif::NOT_CONST);
 586                     if v.mode != Mode::Var {
 587                         span_err!(v.tcx.sess, e.span, E0396,
 588                                   "raw pointers cannot be dereferenced in {}s", v.msg());
 589                     }
 590                 }
 591                 _ => {}
 592             }
 593         }
 594         ast::ExprBinary(op, ref lhs, _) => {
 595             match v.tcx.node_id_to_type(lhs.id).sty {
 596                 ty::TyRawPtr(_) => {
 597                     assert!(op.node == ast::BiEq || op.node == ast::BiNe ||
 598                             op.node == ast::BiLe || op.node == ast::BiLt ||
 599                             op.node == ast::BiGe || op.node == ast::BiGt);
 600
 601                     v.add_qualif(ConstQualif::NOT_CONST);
 602                     if v.mode != Mode::Var {
 603                         span_err!(v.tcx.sess, e.span, E0395,
 604                                   "raw pointers cannot be compared in {}s", v.msg());
 605                     }
 606                 }
 607                 _ => {}
 608             }
 609         }
 610         ast::ExprCast(ref from, _) => {
 611             debug!("Checking const cast(id={})", from.id);
 612             match v.tcx.cast_kinds.borrow().get(&from.id) {
 613                 None => v.tcx.sess.span_bug(e.span, "no kind for cast"),
 614                 Some(&CastKind::PtrAddrCast) | Some(&CastKind::FnPtrAddrCast) => {
 615                     v.add_qualif(ConstQualif::NOT_CONST);
 616                     if v.mode != Mode::Var {
 617                         span_err!(v.tcx.sess, e.span, E0018,
 618                                   "raw pointers cannot be cast to integers in {}s", v.msg());
 619                     }
 620                 }
 621                 _ => {}
 622             }
 623         }
 624         ast::ExprPath(..) => {
 625             let def = v.tcx.def_map.borrow().get(&e.id).map(|d| d.full_def());
 626             match def {
 627                 Some(def::DefVariant(_, _, _)) => {
 628                     // Count the discriminator or function pointer.
 629                     v.add_qualif(ConstQualif::NON_ZERO_SIZED);
 630                 }
 631                 Some(def::DefStruct(_)) => {
 632                     if let ty::TyBareFn(..) = node_ty.sty {
 633                         // Count the function pointer.
 634                         v.add_qualif(ConstQualif::NON_ZERO_SIZED);
 635                     }
 636                 }
 637                 Some(def::DefFn(..)) | Some(def::DefMethod(..)) => {
 638                     // Count the function pointer.
 639                     v.add_qualif(ConstQualif::NON_ZERO_SIZED);
 640                 }
 641                 Some(def::DefStatic(..)) => {
 642                     match v.mode {
 643                         Mode::Static | Mode::StaticMut => {}
 644                         Mode::Const | Mode::ConstFn => {
 645                             span_err!(v.tcx.sess, e.span, E0013,
 646                                 "{}s cannot refer to other statics, insert \
 647                                  an intermediate constant instead", v.msg());
 648                         }
 649                         Mode::Var => v.add_qualif(ConstQualif::NOT_CONST)
 650                     }
 651                 }
 652                 Some(def::DefConst(did)) |
 653                 Some(def::DefAssociatedConst(did)) => {
 654                     if let Some(expr) = const_eval::lookup_const_by_id(v.tcx, did,
 655                                                                        Some(e.id)) {
 656                         let inner = v.global_expr(Mode::Const, expr);
 657                         v.add_qualif(inner);
 658                     } else {
 659                         v.tcx.sess.span_bug(e.span,
 660                                             "DefConst or DefAssociatedConst \
 661                                              doesn't point to a constant");
 662                     }
 663                 }
 664                 Some(def::DefLocal(_)) if v.mode == Mode::ConstFn => {
 665                     // Sadly, we can't determine whether the types are zero-sized.
 666                     v.add_qualif(ConstQualif::NOT_CONST | ConstQualif::NON_ZERO_SIZED);
 667                 }
 668                 def => {
 669                     v.add_qualif(ConstQualif::NOT_CONST);
 670                     if v.mode != Mode::Var {
 671                         debug!("(checking const) found bad def: {:?}", def);
 672                         span_err!(v.tcx.sess, e.span, E0014,
 673                                   "paths in {}s may only refer to constants \
 674                                    or functions", v.msg());
 675                     }
 676                 }
 677             }
 678         }
 679         ast::ExprCall(ref callee, _) => {
 680             let mut callee = &**callee;
 681             loop {
 682                 callee = match callee.node {
 683                     ast::ExprParen(ref inner) => &**inner,
 684                     ast::ExprBlock(ref block) => match block.expr {
 685                         Some(ref tail) => &**tail,
 686                         None => break
 687                     },
 688                     _ => break
 689                 };
 690             }
 691             let def = v.tcx.def_map.borrow().get(&callee.id).map(|d| d.full_def());
 692             let is_const = match def {
 693                 Some(def::DefStruct(..)) => true,
 694                 Some(def::DefVariant(..)) => {
 695                     // Count the discriminator.
 696                     v.add_qualif(ConstQualif::NON_ZERO_SIZED);
 697                     true
 698                 }
 699                 Some(def::DefFn(did, _)) => {
 700                     v.handle_const_fn_call(e, did, node_ty)
 701                 }
 702                 Some(def::DefMethod(did)) => {
 703                     match v.tcx.impl_or_trait_item(did).container() {
 704                         ty::ImplContainer(_) => {
 705                             v.handle_const_fn_call(e, did, node_ty)
 706                         }
 707                         ty::TraitContainer(_) => false
 708                     }
 709                 }
 710                 _ => false
 711             };
 712             if !is_const {
 713                 v.add_qualif(ConstQualif::NOT_CONST);
 714                 if v.mode != Mode::Var {
 715                     span_err!(v.tcx.sess, e.span, E0015,
 716                               "function calls in {}s are limited to \
 717                                constant functions, \
 718                                struct and enum constructors", v.msg());
 719                 }
 720             }
 721         }
 722         ast::ExprMethodCall(..) => {
 723             let method = v.tcx.tables.borrow().method_map[&method_call];
 724             let is_const = match v.tcx.impl_or_trait_item(method.def_id).container() {
 725                 ty::ImplContainer(_) => v.handle_const_fn_call(e, method.def_id, node_ty),
 726                 ty::TraitContainer(_) => false
 727             };
 728             if !is_const {
 729                 v.add_qualif(ConstQualif::NOT_CONST);
 730                 if v.mode != Mode::Var {
 731                     span_err!(v.tcx.sess, e.span, E0378,
 732                               "method calls in {}s are limited to \
 733                                constant inherent methods", v.msg());
 734                 }
 735             }
 736         }
 737         ast::ExprStruct(..) => {
 738             let did = v.tcx.def_map.borrow().get(&e.id).map(|def| def.def_id());
 739             if did == v.tcx.lang_items.unsafe_cell_type() {
 740                 v.add_qualif(ConstQualif::MUTABLE_MEM);
 741             }
 742         }
 743
 744         ast::ExprLit(_) |
 745         ast::ExprAddrOf(..) => {
 746             v.add_qualif(ConstQualif::NON_ZERO_SIZED);
 747         }
 748
 749         ast::ExprRepeat(..) => {
 750             v.add_qualif(ConstQualif::PREFER_IN_PLACE);
 751         }
 752
 753         ast::ExprClosure(..) => {
 754             // Paths in constant contexts cannot refer to local variables,
 755             // as there are none, and thus closures can't have upvars there.
 756             if v.tcx.with_freevars(e.id, |fv| !fv.is_empty()) {
 757                 assert!(v.mode == Mode::Var,
 758                         "global closures can't capture anything");
 759                 v.add_qualif(ConstQualif::NOT_CONST);
 760             }
 761         }
 762
 763         ast::ExprBlock(_) |
 764         ast::ExprIndex(..) |
 765         ast::ExprField(..) |
 766         ast::ExprTupField(..) |
 767         ast::ExprVec(_) |
 768         ast::ExprParen(..) |
 769         ast::ExprTup(..) => {}
 770
 771         // Conditional control flow (possible to implement).
 772         ast::ExprMatch(..) |
 773         ast::ExprIf(..) |
 774         ast::ExprIfLet(..) |
 775
 776         // Loops (not very meaningful in constants).
 777         ast::ExprWhile(..) |
 778         ast::ExprWhileLet(..) |
 779         ast::ExprForLoop(..) |
 780         ast::ExprLoop(..) |
 781
 782         // More control flow (also not very meaningful).
 783         ast::ExprBreak(_) |
 784         ast::ExprAgain(_) |
 785         ast::ExprRet(_) |
 786
 787         // Miscellaneous expressions that could be implemented.
 788         ast::ExprRange(..) |
 789
 790         // Expressions with side-effects.
 791         ast::ExprAssign(..) |
 792         ast::ExprAssignOp(..) |
 793         ast::ExprInlineAsm(_) |
 794         ast::ExprMac(_) => {
 795             v.add_qualif(ConstQualif::NOT_CONST);
 796             if v.mode != Mode::Var {
 797                 span_err!(v.tcx.sess, e.span, E0019,
 798                           "{} contains unimplemented expression type", v.msg());
 799             }
 800         }
 801     }
 802 }
 803
 804 /// Check the adjustments of an expression
 805 fn check_adjustments<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>, e: &ast::Expr) {
 806     match v.tcx.tables.borrow().adjustments.get(&e.id) {
 807         None | Some(&ty::AdjustReifyFnPointer) | Some(&ty::AdjustUnsafeFnPointer) => {}
 808         Some(&ty::AdjustDerefRef(ty::AutoDerefRef { autoderefs, .. })) => {
 809             if (0..autoderefs as u32).any(|autoderef| {
 810                     v.tcx.is_overloaded_autoderef(e.id, autoderef)
 811             }) {
 812                 v.add_qualif(ConstQualif::NOT_CONST);
 813                 if v.mode != Mode::Var {
 814                     span_err!(v.tcx.sess, e.span, E0400,
 815                               "user-defined dereference operators are not allowed in {}s",
 816                               v.msg());
 817                 }
 818             }
 819         }
 820     }
 821 }
 822
 823 pub fn check_crate(tcx: &ty::ctxt) {
 824     visit::walk_crate(&mut CheckCrateVisitor {
 825         tcx: tcx,
 826         mode: Mode::Var,
 827         qualif: ConstQualif::NOT_CONST,
 828         rvalue_borrows: NodeMap()
 829     }, tcx.map.krate());
 830
 831     tcx.sess.abort_if_errors();
 832 }
 833
 834 impl<'a, 'tcx> euv::Delegate<'tcx> for CheckCrateVisitor<'a, 'tcx> {
 835     fn consume(&mut self,
 836                _consume_id: ast::NodeId,
 837                consume_span: Span,
 838                cmt: mc::cmt,
 839                _mode: euv::ConsumeMode) {
 840         let mut cur = &cmt;
 841         loop {
 842             match cur.cat {
 843                 mc::cat_static_item => {
 844                     if self.mode != Mode::Var {
 845                         // statics cannot be consumed by value at any time, that would imply
 846                         // that they're an initializer (what a const is for) or kept in sync
 847                         // over time (not feasible), so deny it outright.
 848                         span_err!(self.tcx.sess, consume_span, E0394,
 849                                   "cannot refer to other statics by value, use the \
 850                                    address-of operator or a constant instead");
 851                     }
 852                     break;
 853                 }
 854                 mc::cat_deref(ref cmt, _, _) |
 855                 mc::cat_downcast(ref cmt, _) |
 856                 mc::cat_interior(ref cmt, _) => cur = cmt,
 857
 858                 mc::cat_rvalue(..) |
 859                 mc::cat_upvar(..) |
 860                 mc::cat_local(..) => break
 861             }
 862         }
 863     }
 864     fn borrow(&mut self,
 865               borrow_id: ast::NodeId,
 866               borrow_span: Span,
 867               cmt: mc::cmt<'tcx>,
 868               _loan_region: ty::Region,
 869               bk: ty::BorrowKind,
 870               loan_cause: euv::LoanCause)
 871     {
 872         // Kind of hacky, but we allow Unsafe coercions in constants.
 873         // These occur when we convert a &T or *T to a *U, as well as
 874         // when making a thin pointer (e.g., `*T`) into a fat pointer
 875         // (e.g., `*Trait`).
 876         match loan_cause {
 877             euv::LoanCause::AutoUnsafe => {
 878                 return;
 879             }
 880             _ => { }
 881         }
 882
 883         let mut cur = &cmt;
 884         let mut is_interior = false;
 885         loop {
 886             match cur.cat {
 887                 mc::cat_rvalue(..) => {
 888                     if loan_cause == euv::MatchDiscriminant {
 889                         // Ignore the dummy immutable borrow created by EUV.
 890                         break;
 891                     }
 892                     let mutbl = bk.to_mutbl_lossy();
 893                     if mutbl == ast::MutMutable && self.mode == Mode::StaticMut {
 894                         // Mutable slices are the only `&mut` allowed in
 895                         // globals, but only in `static mut`, nowhere else.
 896                         // FIXME: This exception is really weird... there isn't
 897                         // any fundamental reason to restrict this based on
 898                         // type of the expression.  `&mut [1]` has exactly the
 899                         // same representation as &mut 1.
 900                         match cmt.ty.sty {
 901                             ty::TyArray(_, _) | ty::TySlice(_) => break,
 902                             _ => {}
 903                         }
 904                     }
 905                     self.record_borrow(borrow_id, mutbl);
 906                     break;
 907                 }
 908                 mc::cat_static_item => {
 909                     if is_interior && self.mode != Mode::Var {
 910                         // Borrowed statics can specifically *only* have their address taken,
 911                         // not any number of other borrows such as borrowing fields, reading
 912                         // elements of an array, etc.
 913                         self.tcx.sess.span_err(borrow_span,
 914                             "cannot refer to the interior of another \
 915                              static, use a constant instead");
 916                     }
 917                     break;
 918                 }
 919                 mc::cat_deref(ref cmt, _, _) |
 920                 mc::cat_downcast(ref cmt, _) |
 921                 mc::cat_interior(ref cmt, _) => {
 922                     is_interior = true;
 923                     cur = cmt;
 924                 }
 925
 926                 mc::cat_upvar(..) |
 927                 mc::cat_local(..) => break
 928             }
 929         }
 930     }
 931
 932     fn decl_without_init(&mut self,
 933                          _id: ast::NodeId,
 934                          _span: Span) {}
 935     fn mutate(&mut self,
 936               _assignment_id: ast::NodeId,
 937               _assignment_span: Span,
 938               _assignee_cmt: mc::cmt,
 939               _mode: euv::MutateMode) {}
 940
 941     fn matched_pat(&mut self,
 942                    _: &ast::Pat,
 943                    _: mc::cmt,
 944                    _: euv::MatchMode) {}
 945
 946     fn consume_pat(&mut self,
 947                    _consume_pat: &ast::Pat,
 948                    _cmt: mc::cmt,
 949                    _mode: euv::ConsumeMode) {}
 950 }