src/librustc_passes/rvalue_promotion.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 rustc::ty::cast::CastKind;
  28 use rustc::hir::def::{Def, CtorKind};
  29 use rustc::hir::def_id::DefId;
  30 use rustc::hir::map::blocks::FnLikeNode;
  31 use rustc::middle::expr_use_visitor as euv;
  32 use rustc::middle::mem_categorization as mc;
  33 use rustc::middle::mem_categorization::Categorization;
  34 use rustc::ty::{self, Ty, TyCtxt};
  35 use rustc::ty::query::Providers;
  36 use rustc::ty::subst::Substs;
  37 use rustc::util::nodemap::{ItemLocalSet, NodeSet};
  38 use rustc::hir;
  39 use rustc_data_structures::sync::Lrc;
  40 use syntax::ast;
  41 use syntax::attr;
  42 use syntax_pos::{Span, DUMMY_SP};
  43 use self::Promotability::*;
  44 use std::ops::{BitAnd, BitAndAssign, BitOr};
  45
  46 pub fn provide(providers: &mut Providers) {
  47     *providers = Providers {
  48         rvalue_promotable_map,
  49         const_is_rvalue_promotable_to_static,
  50         ..*providers
  51     };
  52 }
  53
  54 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
  55     for &body_id in &tcx.hir.krate().body_ids {
  56         let def_id = tcx.hir.body_owner_def_id(body_id);
  57         tcx.const_is_rvalue_promotable_to_static(def_id);
  58     }
  59     tcx.sess.abort_if_errors();
  60 }
  61
  62 fn const_is_rvalue_promotable_to_static<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  63                                                   def_id: DefId)
  64                                                   -> bool
  65 {
  66     assert!(def_id.is_local());
  67
  68     let node_id = tcx.hir.as_local_node_id(def_id)
  69         .expect("rvalue_promotable_map invoked with non-local def-id");
  70     let body_id = tcx.hir.body_owned_by(node_id);
  71     let body_hir_id = tcx.hir.node_to_hir_id(body_id.node_id);
  72     tcx.rvalue_promotable_map(def_id).contains(&body_hir_id.local_id)
  73 }
  74
  75 fn rvalue_promotable_map<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  76                                    def_id: DefId)
  77                                    -> Lrc<ItemLocalSet>
  78 {
  79     let outer_def_id = tcx.closure_base_def_id(def_id);
  80     if outer_def_id != def_id {
  81         return tcx.rvalue_promotable_map(outer_def_id);
  82     }
  83
  84     let mut visitor = CheckCrateVisitor {
  85         tcx,
  86         tables: &ty::TypeckTables::empty(None),
  87         in_fn: false,
  88         in_static: false,
  89         mut_rvalue_borrows: NodeSet(),
  90         param_env: ty::ParamEnv::empty(),
  91         identity_substs: Substs::empty(),
  92         result: ItemLocalSet(),
  93     };
  94
  95     // `def_id` should be a `Body` owner
  96     let node_id = tcx.hir.as_local_node_id(def_id)
  97         .expect("rvalue_promotable_map invoked with non-local def-id");
  98     let body_id = tcx.hir.body_owned_by(node_id);
  99     let _ = visitor.check_nested_body(body_id);
 100
 101     Lrc::new(visitor.result)
 102 }
 103
 104 struct CheckCrateVisitor<'a, 'tcx: 'a> {
 105     tcx: TyCtxt<'a, 'tcx, 'tcx>,
 106     in_fn: bool,
 107     in_static: bool,
 108     mut_rvalue_borrows: NodeSet,
 109     param_env: ty::ParamEnv<'tcx>,
 110     identity_substs: &'tcx Substs<'tcx>,
 111     tables: &'a ty::TypeckTables<'tcx>,
 112     result: ItemLocalSet,
 113 }
 114
 115 #[must_use]
 116 #[derive(Debug, Clone, Copy, PartialEq)]
 117 enum Promotability {
 118     Promotable,
 119     NotPromotable
 120 }
 121
 122 impl BitAnd for Promotability {
 123     type Output = Self;
 124
 125     fn bitand(self, rhs: Self) -> Self {
 126         match (self, rhs) {
 127             (Promotable, Promotable) => Promotable,
 128             _ => NotPromotable,
 129         }
 130     }
 131 }
 132
 133 impl BitAndAssign for Promotability {
 134     fn bitand_assign(&mut self, rhs: Self) {
 135         *self = *self & rhs
 136     }
 137 }
 138
 139 impl BitOr for Promotability {
 140     type Output = Self;
 141
 142     fn bitor(self, rhs: Self) -> Self {
 143         match (self, rhs) {
 144             (NotPromotable, NotPromotable) => NotPromotable,
 145             _ => Promotable,
 146         }
 147     }
 148 }
 149
 150 impl<'a, 'gcx> CheckCrateVisitor<'a, 'gcx> {
 151     // Returns true iff all the values of the type are promotable.
 152     fn type_promotability(&mut self, ty: Ty<'gcx>) -> Promotability {
 153         debug!("type_promotability({})", ty);
 154
 155         if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) &&
 156             !ty.needs_drop(self.tcx, self.param_env) {
 157             Promotable
 158         } else {
 159             NotPromotable
 160         }
 161     }
 162
 163     fn handle_const_fn_call(&mut self, def_id: DefId,
 164                             ret_ty: Ty<'gcx>, span: Span) -> Promotability {
 165         if self.type_promotability(ret_ty) == NotPromotable {
 166             return NotPromotable;
 167         }
 168
 169         let node_check = if let Some(fn_id) = self.tcx.hir.as_local_node_id(def_id) {
 170             FnLikeNode::from_node(self.tcx.hir.get(fn_id)).map_or(false, |fn_like| {
 171                 fn_like.constness() == hir::Constness::Const
 172             })
 173         } else {
 174             self.tcx.is_const_fn(def_id)
 175         };
 176
 177         if !node_check {
 178             return NotPromotable
 179         }
 180
 181         if let Some(&attr::Stability {
 182             const_stability: Some(ref feature_name),
 183             .. }) = self.tcx.lookup_stability(def_id) {
 184             let stable_check =
 185                 // feature-gate is enabled,
 186                 self.tcx.features()
 187                     .declared_lib_features
 188                     .iter()
 189                     .any(|&(ref sym, _)| sym == feature_name) ||
 190
 191                     // this comes from a macro that has #[allow_internal_unstable]
 192                     span.allows_unstable();
 193             if !stable_check {
 194                 return NotPromotable
 195             }
 196         };
 197         Promotable
 198     }
 199
 200     /// While the `ExprUseVisitor` walks, we will identify which
 201     /// expressions are borrowed, and insert their ids into this
 202     /// table. Actually, we insert the "borrow-id", which is normally
 203     /// the id of the expession being borrowed: but in the case of
 204     /// `ref mut` borrows, the `id` of the pattern is
 205     /// inserted. Therefore later we remove that entry from the table
 206     /// and transfer it over to the value being matched. This will
 207     /// then prevent said value from being promoted.
 208     fn remove_mut_rvalue_borrow(&mut self, pat: &hir::Pat) -> bool {
 209         let mut any_removed = false;
 210         pat.walk(|p| {
 211             any_removed |= self.mut_rvalue_borrows.remove(&p.id);
 212             true
 213         });
 214         any_removed
 215     }
 216 }
 217
 218 impl<'a, 'tcx> CheckCrateVisitor<'a, 'tcx> {
 219     fn check_nested_body(&mut self, body_id: hir::BodyId) -> Promotability {
 220         let item_id = self.tcx.hir.body_owner(body_id);
 221         let item_def_id = self.tcx.hir.local_def_id(item_id);
 222
 223         let outer_in_fn = self.in_fn;
 224         let outer_tables = self.tables;
 225         let outer_param_env = self.param_env;
 226         let outer_identity_substs = self.identity_substs;
 227
 228         self.in_fn = false;
 229         self.in_static = false;
 230
 231         match self.tcx.hir.body_owner_kind(item_id) {
 232             hir::BodyOwnerKind::Fn => self.in_fn = true,
 233             hir::BodyOwnerKind::Static(_) => self.in_static = true,
 234             _ => {}
 235         };
 236
 237
 238         self.tables = self.tcx.typeck_tables_of(item_def_id);
 239         self.param_env = self.tcx.param_env(item_def_id);
 240         self.identity_substs = Substs::identity_for_item(self.tcx, item_def_id);
 241
 242         let body = self.tcx.hir.body(body_id);
 243
 244         let tcx = self.tcx;
 245         let param_env = self.param_env;
 246         let region_scope_tree = self.tcx.region_scope_tree(item_def_id);
 247         let tables = self.tables;
 248         euv::ExprUseVisitor::new(self, tcx, param_env, &region_scope_tree, tables, None)
 249             .consume_body(body);
 250
 251         let body_promotable = self.check_expr(&body.value);
 252         self.in_fn = outer_in_fn;
 253         self.tables = outer_tables;
 254         self.param_env = outer_param_env;
 255         self.identity_substs = outer_identity_substs;
 256         body_promotable
 257     }
 258
 259     fn check_stmt(&mut self, stmt: &'tcx hir::Stmt) -> Promotability {
 260         match stmt.node {
 261             hir::StmtKind::Decl(ref decl, _node_id) => {
 262                 match &decl.node {
 263                     hir::DeclKind::Local(local) => {
 264                         if self.remove_mut_rvalue_borrow(&local.pat) {
 265                             if let Some(init) = &local.init {
 266                                 self.mut_rvalue_borrows.insert(init.id);
 267                             }
 268                         }
 269
 270                         if let Some(ref expr) = local.init {
 271                             let _ = self.check_expr(&expr);
 272                         }
 273                         NotPromotable
 274                     }
 275                     // Item statements are allowed
 276                     hir::DeclKind::Item(_) => Promotable
 277                 }
 278             }
 279             hir::StmtKind::Expr(ref box_expr, _node_id) |
 280             hir::StmtKind::Semi(ref box_expr, _node_id) => {
 281                 let _ = self.check_expr(box_expr);
 282                 NotPromotable
 283             }
 284         }
 285     }
 286
 287     fn check_expr(&mut self, ex: &'tcx hir::Expr) -> Promotability {
 288         let node_ty = self.tables.node_id_to_type(ex.hir_id);
 289         let mut outer = check_expr_kind(self, ex, node_ty);
 290         outer &= check_adjustments(self, ex);
 291
 292         // Handle borrows on (or inside the autorefs of) this expression.
 293         if self.mut_rvalue_borrows.remove(&ex.id) {
 294             outer = NotPromotable
 295         }
 296
 297         if outer == Promotable {
 298             self.result.insert(ex.hir_id.local_id);
 299         }
 300         outer
 301     }
 302
 303     fn check_block(&mut self, block: &'tcx hir::Block) -> Promotability {
 304         let mut iter_result = Promotable;
 305         for index in block.stmts.iter() {
 306             iter_result &= self.check_stmt(index);
 307         }
 308         match block.expr {
 309             Some(ref box_expr) => iter_result & self.check_expr(&*box_expr),
 310             None => iter_result,
 311         }
 312     }
 313 }
 314
 315 /// This function is used to enforce the constraints on
 316 /// const/static items. It walks through the *value*
 317 /// of the item walking down the expression and evaluating
 318 /// every nested expression. If the expression is not part
 319 /// of a const/static item, it is qualified for promotion
 320 /// instead of producing errors.
 321 fn check_expr_kind<'a, 'tcx>(
 322     v: &mut CheckCrateVisitor<'a, 'tcx>,
 323     e: &'tcx hir::Expr, node_ty: Ty<'tcx>) -> Promotability {
 324
 325     let ty_result = match node_ty.sty {
 326         ty::Adt(def, _) if def.has_dtor(v.tcx) => {
 327             NotPromotable
 328         }
 329         _ => Promotable
 330     };
 331
 332     let node_result = match e.node {
 333         hir::ExprKind::Box(ref expr) => {
 334             let _ = v.check_expr(&expr);
 335             NotPromotable
 336         }
 337         hir::ExprKind::Unary(op, ref expr) => {
 338             let expr_promotability = v.check_expr(expr);
 339             if v.tables.is_method_call(e) || op == hir::UnDeref {
 340                 return NotPromotable;
 341             }
 342             expr_promotability
 343         }
 344         hir::ExprKind::Binary(op, ref lhs, ref rhs) => {
 345             let lefty = v.check_expr(lhs);
 346             let righty = v.check_expr(rhs);
 347             if v.tables.is_method_call(e) {
 348                 return NotPromotable;
 349             }
 350             match v.tables.node_id_to_type(lhs.hir_id).sty {
 351                 ty::RawPtr(_) => {
 352                     assert!(op.node == hir::BinOpKind::Eq || op.node == hir::BinOpKind::Ne ||
 353                             op.node == hir::BinOpKind::Le || op.node == hir::BinOpKind::Lt ||
 354                             op.node == hir::BinOpKind::Ge || op.node == hir::BinOpKind::Gt);
 355
 356                     NotPromotable
 357                 }
 358                 _ => lefty & righty
 359             }
 360         }
 361         hir::ExprKind::Cast(ref from, _) => {
 362             let expr_promotability = v.check_expr(from);
 363             debug!("Checking const cast(id={})", from.id);
 364             match v.tables.cast_kinds().get(from.hir_id) {
 365                 None => {
 366                     v.tcx.sess.delay_span_bug(e.span, "no kind for cast");
 367                     NotPromotable
 368                 },
 369                 Some(&CastKind::PtrAddrCast) | Some(&CastKind::FnPtrAddrCast) => {
 370                     NotPromotable
 371                 }
 372                 _ => expr_promotability
 373             }
 374         }
 375         hir::ExprKind::Path(ref qpath) => {
 376             let def = v.tables.qpath_def(qpath, e.hir_id);
 377             match def {
 378                 Def::VariantCtor(..) | Def::StructCtor(..) |
 379                 Def::Fn(..) | Def::Method(..) => Promotable,
 380
 381                 // References to a static that are themselves within a static
 382                 // are inherently promotable with the exception
 383                 //  of "#[thread_local]" statics, which may not
 384                 // outlive the current function
 385                 Def::Static(did, _) => {
 386
 387                     if v.in_static {
 388                         for attr in &v.tcx.get_attrs(did)[..] {
 389                             if attr.check_name("thread_local") {
 390                                 debug!("Reference to Static(id={:?}) is unpromotable \
 391                                        due to a #[thread_local] attribute", did);
 392                                 return NotPromotable;
 393                             }
 394                         }
 395                         Promotable
 396                     } else {
 397                         debug!("Reference to Static(id={:?}) is unpromotable as it is not \
 398                                referenced from a static", did);
 399                         NotPromotable
 400                     }
 401                 }
 402
 403                 Def::Const(did) |
 404                 Def::AssociatedConst(did) => {
 405                     let promotable = if v.tcx.trait_of_item(did).is_some() {
 406                         // Don't peek inside trait associated constants.
 407                         NotPromotable
 408                     } else if v.tcx.at(e.span).const_is_rvalue_promotable_to_static(did) {
 409                         Promotable
 410                     } else {
 411                         NotPromotable
 412                     };
 413                     // Just in case the type is more specific than the definition,
 414                     // e.g. impl associated const with type parameters, check it.
 415                     // Also, trait associated consts are relaxed by this.
 416                     promotable | v.type_promotability(node_ty)
 417                 }
 418                 _ => NotPromotable
 419             }
 420         }
 421         hir::ExprKind::Call(ref callee, ref hirvec) => {
 422             let mut call_result = v.check_expr(callee);
 423             for index in hirvec.iter() {
 424                 call_result &= v.check_expr(index);
 425             }
 426             let mut callee = &**callee;
 427             loop {
 428                 callee = match callee.node {
 429                     hir::ExprKind::Block(ref block, _) => match block.expr {
 430                         Some(ref tail) => &tail,
 431                         None => break
 432                     },
 433                     _ => break
 434                 };
 435             }
 436             // The callee is an arbitrary expression, it doesn't necessarily have a definition.
 437             let def = if let hir::ExprKind::Path(ref qpath) = callee.node {
 438                 v.tables.qpath_def(qpath, callee.hir_id)
 439             } else {
 440                 Def::Err
 441             };
 442             let def_result = match def {
 443                 Def::StructCtor(_, CtorKind::Fn) |
 444                 Def::VariantCtor(_, CtorKind::Fn) => Promotable,
 445                 Def::Fn(did) => {
 446                     v.handle_const_fn_call(did, node_ty, e.span)
 447                 }
 448                 Def::Method(did) => {
 449                     match v.tcx.associated_item(did).container {
 450                         ty::ImplContainer(_) => {
 451                             v.handle_const_fn_call(did, node_ty, e.span)
 452                         }
 453                         ty::TraitContainer(_) => NotPromotable,
 454                     }
 455                 }
 456                 _ => NotPromotable,
 457             };
 458             def_result & call_result
 459         }
 460         hir::ExprKind::MethodCall(ref _pathsegment, ref _span, ref hirvec) => {
 461             let mut method_call_result = Promotable;
 462             for index in hirvec.iter() {
 463                 method_call_result &= v.check_expr(index);
 464             }
 465             if let Some(def) = v.tables.type_dependent_defs().get(e.hir_id) {
 466                 let def_id = def.def_id();
 467                 match v.tcx.associated_item(def_id).container {
 468                     ty::ImplContainer(_) => {
 469                         method_call_result = method_call_result
 470                             & v.handle_const_fn_call(def_id, node_ty, e.span);
 471                     }
 472                     ty::TraitContainer(_) => return NotPromotable,
 473                 };
 474             } else {
 475                 v.tcx.sess.delay_span_bug(e.span, "no type-dependent def for method call");
 476             }
 477             method_call_result
 478         }
 479         hir::ExprKind::Struct(ref _qpath, ref hirvec, ref option_expr) => {
 480             let mut struct_result = Promotable;
 481             for index in hirvec.iter() {
 482                 struct_result &= v.check_expr(&index.expr);
 483             }
 484             if let Some(ref expr) = *option_expr {
 485                 struct_result &= v.check_expr(&expr);
 486             }
 487             if let ty::Adt(adt, ..) = v.tables.expr_ty(e).sty {
 488                 // unsafe_cell_type doesn't necessarily exist with no_core
 489                 if Some(adt.did) == v.tcx.lang_items().unsafe_cell_type() {
 490                     return NotPromotable;
 491                 }
 492             }
 493             struct_result
 494         }
 495
 496         hir::ExprKind::Lit(_) => Promotable,
 497
 498         hir::ExprKind::AddrOf(_, ref expr) |
 499         hir::ExprKind::Repeat(ref expr, _) => {
 500             v.check_expr(&expr)
 501         }
 502
 503         hir::ExprKind::Closure(_capture_clause, ref _box_fn_decl,
 504                                body_id, _span, _option_generator_movability) => {
 505             let nested_body_promotable = v.check_nested_body(body_id);
 506             // Paths in constant contexts cannot refer to local variables,
 507             // as there are none, and thus closures can't have upvars there.
 508             if v.tcx.with_freevars(e.id, |fv| !fv.is_empty()) {
 509                 NotPromotable
 510             } else {
 511                 nested_body_promotable
 512             }
 513         }
 514
 515         hir::ExprKind::Field(ref expr, _ident) => {
 516             let expr_promotability = v.check_expr(&expr);
 517             if let Some(def) = v.tables.expr_ty(expr).ty_adt_def() {
 518                 if def.is_union() {
 519                     return NotPromotable;
 520                 }
 521             }
 522             expr_promotability
 523         }
 524
 525         hir::ExprKind::Block(ref box_block, ref _option_label) => {
 526             v.check_block(box_block)
 527         }
 528
 529         hir::ExprKind::Index(ref lhs, ref rhs) => {
 530             let lefty = v.check_expr(lhs);
 531             let righty = v.check_expr(rhs);
 532             if v.tables.is_method_call(e) {
 533                 return NotPromotable;
 534             }
 535             lefty & righty
 536         }
 537
 538         hir::ExprKind::Array(ref hirvec) => {
 539             let mut array_result = Promotable;
 540             for index in hirvec.iter() {
 541                 array_result &= v.check_expr(index);
 542             }
 543             array_result
 544         }
 545
 546         hir::ExprKind::Type(ref expr, ref _ty) => {
 547             v.check_expr(&expr)
 548         }
 549
 550         hir::ExprKind::Tup(ref hirvec) => {
 551             let mut tup_result = Promotable;
 552             for index in hirvec.iter() {
 553                 tup_result &= v.check_expr(index);
 554             }
 555             tup_result
 556         }
 557
 558
 559         // Conditional control flow (possible to implement).
 560         hir::ExprKind::Match(ref expr, ref hirvec_arm, ref _match_source) => {
 561             // Compute the most demanding borrow from all the arms'
 562             // patterns and set that on the discriminator.
 563             let mut mut_borrow = false;
 564             for pat in hirvec_arm.iter().flat_map(|arm| &arm.pats) {
 565                 mut_borrow = v.remove_mut_rvalue_borrow(pat);
 566             }
 567             if mut_borrow {
 568                 v.mut_rvalue_borrows.insert(expr.id);
 569             }
 570
 571             let _ = v.check_expr(expr);
 572             for index in hirvec_arm.iter() {
 573                 let _ = v.check_expr(&*index.body);
 574                 if let Some(hir::Guard::If(ref expr)) = index.guard {
 575                     let _ = v.check_expr(&expr);
 576                 }
 577             }
 578             NotPromotable
 579         }
 580
 581         hir::ExprKind::If(ref lhs, ref rhs, ref option_expr) => {
 582             let _ = v.check_expr(lhs);
 583             let _ = v.check_expr(rhs);
 584             if let Some(ref expr) = option_expr {
 585                 let _ = v.check_expr(&expr);
 586             }
 587             NotPromotable
 588         }
 589
 590         // Loops (not very meaningful in constants).
 591         hir::ExprKind::While(ref expr, ref box_block, ref _option_label) => {
 592             let _ = v.check_expr(expr);
 593             let _ = v.check_block(box_block);
 594             NotPromotable
 595         }
 596
 597         hir::ExprKind::Loop(ref box_block, ref _option_label, ref _loop_source) => {
 598             let _ = v.check_block(box_block);
 599             NotPromotable
 600         }
 601
 602         // More control flow (also not very meaningful).
 603         hir::ExprKind::Break(_, ref option_expr) | hir::ExprKind::Ret(ref option_expr) => {
 604             if let Some(ref expr) = *option_expr {
 605                  let _ = v.check_expr(&expr);
 606             }
 607             NotPromotable
 608         }
 609
 610         hir::ExprKind::Continue(_) => {
 611             NotPromotable
 612         }
 613
 614         // Generator expressions
 615         hir::ExprKind::Yield(ref expr) => {
 616             let _ = v.check_expr(&expr);
 617             NotPromotable
 618         }
 619
 620         // Expressions with side-effects.
 621         hir::ExprKind::AssignOp(_, ref lhs, ref rhs) | hir::ExprKind::Assign(ref lhs, ref rhs) => {
 622             let _ = v.check_expr(lhs);
 623             let _ = v.check_expr(rhs);
 624             NotPromotable
 625         }
 626
 627         hir::ExprKind::InlineAsm(ref _inline_asm, ref hirvec_lhs, ref hirvec_rhs) => {
 628             for index in hirvec_lhs.iter().chain(hirvec_rhs.iter()) {
 629                 let _ = v.check_expr(index);
 630             }
 631             NotPromotable
 632         }
 633     };
 634     ty_result & node_result
 635 }
 636
 637 /// Check the adjustments of an expression
 638 fn check_adjustments<'a, 'tcx>(
 639     v: &mut CheckCrateVisitor<'a, 'tcx>,
 640     e: &hir::Expr) -> Promotability {
 641     use rustc::ty::adjustment::*;
 642
 643     let mut adjustments = v.tables.expr_adjustments(e).iter().peekable();
 644     while let Some(adjustment) = adjustments.next() {
 645         match adjustment.kind {
 646             Adjust::NeverToAny |
 647             Adjust::ReifyFnPointer |
 648             Adjust::UnsafeFnPointer |
 649             Adjust::ClosureFnPointer |
 650             Adjust::MutToConstPointer |
 651             Adjust::Borrow(_) |
 652             Adjust::Unsize => {}
 653
 654             Adjust::Deref(_) => {
 655                 if let Some(next_adjustment) = adjustments.peek() {
 656                     if let Adjust::Borrow(_) = next_adjustment.kind {
 657                         continue;
 658                     }
 659                 }
 660                 return NotPromotable;
 661             }
 662         }
 663     }
 664     Promotable
 665 }
 666
 667 impl<'a, 'gcx, 'tcx> euv::Delegate<'tcx> for CheckCrateVisitor<'a, 'gcx> {
 668     fn consume(&mut self,
 669                _consume_id: ast::NodeId,
 670                _consume_span: Span,
 671                _cmt: &mc::cmt_,
 672                _mode: euv::ConsumeMode) {}
 673
 674     fn borrow(&mut self,
 675               borrow_id: ast::NodeId,
 676               _borrow_span: Span,
 677               cmt: &mc::cmt_<'tcx>,
 678               _loan_region: ty::Region<'tcx>,
 679               bk: ty::BorrowKind,
 680               loan_cause: euv::LoanCause) {
 681         debug!(
 682             "borrow(borrow_id={:?}, cmt={:?}, bk={:?}, loan_cause={:?})",
 683             borrow_id,
 684             cmt,
 685             bk,
 686             loan_cause,
 687         );
 688
 689         // Kind of hacky, but we allow Unsafe coercions in constants.
 690         // These occur when we convert a &T or *T to a *U, as well as
 691         // when making a thin pointer (e.g., `*T`) into a fat pointer
 692         // (e.g., `*Trait`).
 693         if let euv::LoanCause::AutoUnsafe = loan_cause {
 694             return;
 695         }
 696
 697         let mut cur = cmt;
 698         loop {
 699             match cur.cat {
 700                 Categorization::Rvalue(..) => {
 701                     if loan_cause == euv::MatchDiscriminant {
 702                         // Ignore the dummy immutable borrow created by EUV.
 703                         break;
 704                     }
 705                     if bk.to_mutbl_lossy() == hir::MutMutable {
 706                         self.mut_rvalue_borrows.insert(borrow_id);
 707                     }
 708                     break;
 709                 }
 710                 Categorization::StaticItem => {
 711                     break;
 712                 }
 713                 Categorization::Deref(ref cmt, _) |
 714                 Categorization::Downcast(ref cmt, _) |
 715                 Categorization::Interior(ref cmt, _) => {
 716                     cur = cmt;
 717                 }
 718
 719                 Categorization::Upvar(..) |
 720                 Categorization::Local(..) => break,
 721             }
 722         }
 723     }
 724
 725     fn decl_without_init(&mut self, _id: ast::NodeId, _span: Span) {}
 726     fn mutate(&mut self,
 727               _assignment_id: ast::NodeId,
 728               _assignment_span: Span,
 729               _assignee_cmt: &mc::cmt_,
 730               _mode: euv::MutateMode) {
 731     }
 732
 733     fn matched_pat(&mut self, _: &hir::Pat, _: &mc::cmt_, _: euv::MatchMode) {}
 734
 735     fn consume_pat(&mut self, _consume_pat: &hir::Pat, _cmt: &mc::cmt_, _mode: euv::ConsumeMode) {}
 736 }