src/librustc_passes/rvalue_promotion.rs

   1 // Verifies that the types and values of const and static items
   2 // are safe. The rules enforced by this module are:
   3 //
   4 // - For each *mutable* static item, it checks that its **type**:
   5 //     - doesn't have a destructor
   6 //     - doesn't own a box
   7 //
   8 // - For each *immutable* static item, it checks that its **value**:
   9 //       - doesn't own a box
  10 //       - doesn't contain a struct literal or a call to an enum variant / struct constructor where
  11 //           - the type of the struct/enum has a dtor
  12 //
  13 // Rules Enforced Elsewhere:
  14 // - It's not possible to take the address of a static item with unsafe interior. This is enforced
  15 // by borrowck::gather_loans
  16
  17 use rustc::ty::cast::CastKind;
  18 use rustc::hir::def::{Def, CtorKind};
  19 use rustc::hir::def_id::DefId;
  20 use rustc::middle::expr_use_visitor as euv;
  21 use rustc::middle::mem_categorization as mc;
  22 use rustc::middle::mem_categorization::Categorization;
  23 use rustc::ty::{self, Ty, TyCtxt};
  24 use rustc::ty::query::Providers;
  25 use rustc::ty::subst::Substs;
  26 use rustc::util::nodemap::{ItemLocalSet, NodeSet};
  27 use rustc::hir;
  28 use rustc_data_structures::sync::Lrc;
  29 use syntax::ast;
  30 use syntax_pos::{Span, DUMMY_SP};
  31 use self::Promotability::*;
  32 use std::ops::{BitAnd, BitAndAssign, BitOr};
  33
  34 pub fn provide(providers: &mut Providers) {
  35     *providers = Providers {
  36         rvalue_promotable_map,
  37         const_is_rvalue_promotable_to_static,
  38         ..*providers
  39     };
  40 }
  41
  42 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
  43     for &body_id in &tcx.hir().krate().body_ids {
  44         let def_id = tcx.hir().body_owner_def_id(body_id);
  45         tcx.const_is_rvalue_promotable_to_static(def_id);
  46     }
  47     tcx.sess.abort_if_errors();
  48 }
  49
  50 fn const_is_rvalue_promotable_to_static<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  51                                                   def_id: DefId)
  52                                                   -> bool
  53 {
  54     assert!(def_id.is_local());
  55
  56     let node_id = tcx.hir().as_local_node_id(def_id)
  57         .expect("rvalue_promotable_map invoked with non-local def-id");
  58     let body_id = tcx.hir().body_owned_by(node_id);
  59     let body_hir_id = tcx.hir().node_to_hir_id(body_id.node_id);
  60     tcx.rvalue_promotable_map(def_id).contains(&body_hir_id.local_id)
  61 }
  62
  63 fn rvalue_promotable_map<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  64                                    def_id: DefId)
  65                                    -> Lrc<ItemLocalSet>
  66 {
  67     let outer_def_id = tcx.closure_base_def_id(def_id);
  68     if outer_def_id != def_id {
  69         return tcx.rvalue_promotable_map(outer_def_id);
  70     }
  71
  72     let mut visitor = CheckCrateVisitor {
  73         tcx,
  74         tables: &ty::TypeckTables::empty(None),
  75         in_fn: false,
  76         in_static: false,
  77         mut_rvalue_borrows: Default::default(),
  78         param_env: ty::ParamEnv::empty(),
  79         identity_substs: Substs::empty(),
  80         result: ItemLocalSet::default(),
  81     };
  82
  83     // `def_id` should be a `Body` owner
  84     let node_id = tcx.hir().as_local_node_id(def_id)
  85         .expect("rvalue_promotable_map invoked with non-local def-id");
  86     let body_id = tcx.hir().body_owned_by(node_id);
  87     let _ = visitor.check_nested_body(body_id);
  88
  89     Lrc::new(visitor.result)
  90 }
  91
  92 struct CheckCrateVisitor<'a, 'tcx: 'a> {
  93     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  94     in_fn: bool,
  95     in_static: bool,
  96     mut_rvalue_borrows: NodeSet,
  97     param_env: ty::ParamEnv<'tcx>,
  98     identity_substs: &'tcx Substs<'tcx>,
  99     tables: &'a ty::TypeckTables<'tcx>,
 100     result: ItemLocalSet,
 101 }
 102
 103 #[must_use]
 104 #[derive(Debug, Clone, Copy, PartialEq)]
 105 enum Promotability {
 106     Promotable,
 107     NotPromotable
 108 }
 109
 110 impl BitAnd for Promotability {
 111     type Output = Self;
 112
 113     fn bitand(self, rhs: Self) -> Self {
 114         match (self, rhs) {
 115             (Promotable, Promotable) => Promotable,
 116             _ => NotPromotable,
 117         }
 118     }
 119 }
 120
 121 impl BitAndAssign for Promotability {
 122     fn bitand_assign(&mut self, rhs: Self) {
 123         *self = *self & rhs
 124     }
 125 }
 126
 127 impl BitOr for Promotability {
 128     type Output = Self;
 129
 130     fn bitor(self, rhs: Self) -> Self {
 131         match (self, rhs) {
 132             (NotPromotable, NotPromotable) => NotPromotable,
 133             _ => Promotable,
 134         }
 135     }
 136 }
 137
 138 impl<'a, 'gcx> CheckCrateVisitor<'a, 'gcx> {
 139     // Returns true iff all the values of the type are promotable.
 140     fn type_promotability(&mut self, ty: Ty<'gcx>) -> Promotability {
 141         debug!("type_promotability({})", ty);
 142
 143         if ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) &&
 144             !ty.needs_drop(self.tcx, self.param_env) {
 145             Promotable
 146         } else {
 147             NotPromotable
 148         }
 149     }
 150
 151     fn handle_const_fn_call(
 152         &mut self,
 153         def_id: DefId,
 154     ) -> Promotability {
 155         if self.tcx.is_promotable_const_fn(def_id) {
 156             Promotable
 157         } else {
 158             NotPromotable
 159         }
 160     }
 161
 162     /// While the `ExprUseVisitor` walks, we will identify which
 163     /// expressions are borrowed, and insert their ids into this
 164     /// table. Actually, we insert the "borrow-id", which is normally
 165     /// the id of the expression being borrowed: but in the case of
 166     /// `ref mut` borrows, the `id` of the pattern is
 167     /// inserted. Therefore later we remove that entry from the table
 168     /// and transfer it over to the value being matched. This will
 169     /// then prevent said value from being promoted.
 170     fn remove_mut_rvalue_borrow(&mut self, pat: &hir::Pat) -> bool {
 171         let mut any_removed = false;
 172         pat.walk(|p| {
 173             any_removed |= self.mut_rvalue_borrows.remove(&p.id);
 174             true
 175         });
 176         any_removed
 177     }
 178 }
 179
 180 impl<'a, 'tcx> CheckCrateVisitor<'a, 'tcx> {
 181     fn check_nested_body(&mut self, body_id: hir::BodyId) -> Promotability {
 182         let item_id = self.tcx.hir().body_owner(body_id);
 183         let item_def_id = self.tcx.hir().local_def_id(item_id);
 184
 185         let outer_in_fn = self.in_fn;
 186         let outer_tables = self.tables;
 187         let outer_param_env = self.param_env;
 188         let outer_identity_substs = self.identity_substs;
 189
 190         self.in_fn = false;
 191         self.in_static = false;
 192
 193         match self.tcx.hir().body_owner_kind(item_id) {
 194             hir::BodyOwnerKind::Fn => self.in_fn = true,
 195             hir::BodyOwnerKind::Static(_) => self.in_static = true,
 196             _ => {}
 197         };
 198
 199
 200         self.tables = self.tcx.typeck_tables_of(item_def_id);
 201         self.param_env = self.tcx.param_env(item_def_id);
 202         self.identity_substs = Substs::identity_for_item(self.tcx, item_def_id);
 203
 204         let body = self.tcx.hir().body(body_id);
 205
 206         let tcx = self.tcx;
 207         let param_env = self.param_env;
 208         let region_scope_tree = self.tcx.region_scope_tree(item_def_id);
 209         let tables = self.tables;
 210         euv::ExprUseVisitor::new(self, tcx, param_env, &region_scope_tree, tables, None)
 211             .consume_body(body);
 212
 213         let body_promotable = self.check_expr(&body.value);
 214         self.in_fn = outer_in_fn;
 215         self.tables = outer_tables;
 216         self.param_env = outer_param_env;
 217         self.identity_substs = outer_identity_substs;
 218         body_promotable
 219     }
 220
 221     fn check_stmt(&mut self, stmt: &'tcx hir::Stmt) -> Promotability {
 222         match stmt.node {
 223             hir::StmtKind::Decl(ref decl, _node_id) => {
 224                 match &decl.node {
 225                     hir::DeclKind::Local(local) => {
 226                         if self.remove_mut_rvalue_borrow(&local.pat) {
 227                             if let Some(init) = &local.init {
 228                                 self.mut_rvalue_borrows.insert(init.id);
 229                             }
 230                         }
 231
 232                         if let Some(ref expr) = local.init {
 233                             let _ = self.check_expr(&expr);
 234                         }
 235                         NotPromotable
 236                     }
 237                     // Item statements are allowed
 238                     hir::DeclKind::Item(_) => Promotable
 239                 }
 240             }
 241             hir::StmtKind::Expr(ref box_expr, _node_id) |
 242             hir::StmtKind::Semi(ref box_expr, _node_id) => {
 243                 let _ = self.check_expr(box_expr);
 244                 NotPromotable
 245             }
 246         }
 247     }
 248
 249     fn check_expr(&mut self, ex: &'tcx hir::Expr) -> Promotability {
 250         let node_ty = self.tables.node_id_to_type(ex.hir_id);
 251         let mut outer = check_expr_kind(self, ex, node_ty);
 252         outer &= check_adjustments(self, ex);
 253
 254         // Handle borrows on (or inside the autorefs of) this expression.
 255         if self.mut_rvalue_borrows.remove(&ex.id) {
 256             outer = NotPromotable
 257         }
 258
 259         if outer == Promotable {
 260             self.result.insert(ex.hir_id.local_id);
 261         }
 262         outer
 263     }
 264
 265     fn check_block(&mut self, block: &'tcx hir::Block) -> Promotability {
 266         let mut iter_result = Promotable;
 267         for index in block.stmts.iter() {
 268             iter_result &= self.check_stmt(index);
 269         }
 270         match block.expr {
 271             Some(ref box_expr) => iter_result & self.check_expr(&*box_expr),
 272             None => iter_result,
 273         }
 274     }
 275 }
 276
 277 /// This function is used to enforce the constraints on
 278 /// const/static items. It walks through the *value*
 279 /// of the item walking down the expression and evaluating
 280 /// every nested expression. If the expression is not part
 281 /// of a const/static item, it is qualified for promotion
 282 /// instead of producing errors.
 283 fn check_expr_kind<'a, 'tcx>(
 284     v: &mut CheckCrateVisitor<'a, 'tcx>,
 285     e: &'tcx hir::Expr, node_ty: Ty<'tcx>) -> Promotability {
 286
 287     let ty_result = match node_ty.sty {
 288         ty::Adt(def, _) if def.has_dtor(v.tcx) => {
 289             NotPromotable
 290         }
 291         _ => Promotable
 292     };
 293
 294     let node_result = match e.node {
 295         hir::ExprKind::Box(ref expr) => {
 296             let _ = v.check_expr(&expr);
 297             NotPromotable
 298         }
 299         hir::ExprKind::Unary(op, ref expr) => {
 300             let expr_promotability = v.check_expr(expr);
 301             if v.tables.is_method_call(e) || op == hir::UnDeref {
 302                 return NotPromotable;
 303             }
 304             expr_promotability
 305         }
 306         hir::ExprKind::Binary(op, ref lhs, ref rhs) => {
 307             let lefty = v.check_expr(lhs);
 308             let righty = v.check_expr(rhs);
 309             if v.tables.is_method_call(e) {
 310                 return NotPromotable;
 311             }
 312             match v.tables.node_id_to_type(lhs.hir_id).sty {
 313                 ty::RawPtr(_) | ty::FnPtr(..) => {
 314                     assert!(op.node == hir::BinOpKind::Eq || op.node == hir::BinOpKind::Ne ||
 315                             op.node == hir::BinOpKind::Le || op.node == hir::BinOpKind::Lt ||
 316                             op.node == hir::BinOpKind::Ge || op.node == hir::BinOpKind::Gt);
 317
 318                     NotPromotable
 319                 }
 320                 _ => lefty & righty
 321             }
 322         }
 323         hir::ExprKind::Cast(ref from, _) => {
 324             let expr_promotability = v.check_expr(from);
 325             debug!("Checking const cast(id={})", from.id);
 326             match v.tables.cast_kinds().get(from.hir_id) {
 327                 None => {
 328                     v.tcx.sess.delay_span_bug(e.span, "no kind for cast");
 329                     NotPromotable
 330                 },
 331                 Some(&CastKind::PtrAddrCast) | Some(&CastKind::FnPtrAddrCast) => {
 332                     NotPromotable
 333                 }
 334                 _ => expr_promotability
 335             }
 336         }
 337         hir::ExprKind::Path(ref qpath) => {
 338             let def = v.tables.qpath_def(qpath, e.hir_id);
 339             match def {
 340                 Def::VariantCtor(..) | Def::StructCtor(..) |
 341                 Def::Fn(..) | Def::Method(..) | Def::SelfCtor(..) => Promotable,
 342
 343                 // References to a static that are themselves within a static
 344                 // are inherently promotable with the exception
 345                 //  of "#[thread_local]" statics, which may not
 346                 // outlive the current function
 347                 Def::Static(did, _) => {
 348
 349                     if v.in_static {
 350                         for attr in &v.tcx.get_attrs(did)[..] {
 351                             if attr.check_name("thread_local") {
 352                                 debug!("Reference to Static(id={:?}) is unpromotable \
 353                                        due to a #[thread_local] attribute", did);
 354                                 return NotPromotable;
 355                             }
 356                         }
 357                         Promotable
 358                     } else {
 359                         debug!("Reference to Static(id={:?}) is unpromotable as it is not \
 360                                referenced from a static", did);
 361                         NotPromotable
 362                     }
 363                 }
 364
 365                 Def::Const(did) |
 366                 Def::AssociatedConst(did) => {
 367                     let promotable = if v.tcx.trait_of_item(did).is_some() {
 368                         // Don't peek inside trait associated constants.
 369                         NotPromotable
 370                     } else if v.tcx.at(e.span).const_is_rvalue_promotable_to_static(did) {
 371                         Promotable
 372                     } else {
 373                         NotPromotable
 374                     };
 375                     // Just in case the type is more specific than the definition,
 376                     // e.g., impl associated const with type parameters, check it.
 377                     // Also, trait associated consts are relaxed by this.
 378                     promotable | v.type_promotability(node_ty)
 379                 }
 380                 _ => NotPromotable
 381             }
 382         }
 383         hir::ExprKind::Call(ref callee, ref hirvec) => {
 384             let mut call_result = v.check_expr(callee);
 385             for index in hirvec.iter() {
 386                 call_result &= v.check_expr(index);
 387             }
 388             let mut callee = &**callee;
 389             loop {
 390                 callee = match callee.node {
 391                     hir::ExprKind::Block(ref block, _) => match block.expr {
 392                         Some(ref tail) => &tail,
 393                         None => break
 394                     },
 395                     _ => break
 396                 };
 397             }
 398             // The callee is an arbitrary expression, it doesn't necessarily have a definition.
 399             let def = if let hir::ExprKind::Path(ref qpath) = callee.node {
 400                 v.tables.qpath_def(qpath, callee.hir_id)
 401             } else {
 402                 Def::Err
 403             };
 404             let def_result = match def {
 405                 Def::StructCtor(_, CtorKind::Fn) |
 406                 Def::VariantCtor(_, CtorKind::Fn) |
 407                 Def::SelfCtor(..) => Promotable,
 408                 Def::Fn(did) => v.handle_const_fn_call(did),
 409                 Def::Method(did) => {
 410                     match v.tcx.associated_item(did).container {
 411                         ty::ImplContainer(_) => v.handle_const_fn_call(did),
 412                         ty::TraitContainer(_) => NotPromotable,
 413                     }
 414                 }
 415                 _ => NotPromotable,
 416             };
 417             def_result & call_result
 418         }
 419         hir::ExprKind::MethodCall(ref _pathsegment, ref _span, ref hirvec) => {
 420             let mut method_call_result = Promotable;
 421             for index in hirvec.iter() {
 422                 method_call_result &= v.check_expr(index);
 423             }
 424             if let Some(def) = v.tables.type_dependent_defs().get(e.hir_id) {
 425                 let def_id = def.def_id();
 426                 match v.tcx.associated_item(def_id).container {
 427                     ty::ImplContainer(_) => method_call_result & v.handle_const_fn_call(def_id),
 428                     ty::TraitContainer(_) => NotPromotable,
 429                 }
 430             } else {
 431                 v.tcx.sess.delay_span_bug(e.span, "no type-dependent def for method call");
 432                 NotPromotable
 433             }
 434         }
 435         hir::ExprKind::Struct(ref _qpath, ref hirvec, ref option_expr) => {
 436             let mut struct_result = Promotable;
 437             for index in hirvec.iter() {
 438                 struct_result &= v.check_expr(&index.expr);
 439             }
 440             if let Some(ref expr) = *option_expr {
 441                 struct_result &= v.check_expr(&expr);
 442             }
 443             if let ty::Adt(adt, ..) = v.tables.expr_ty(e).sty {
 444                 // unsafe_cell_type doesn't necessarily exist with no_core
 445                 if Some(adt.did) == v.tcx.lang_items().unsafe_cell_type() {
 446                     return NotPromotable;
 447                 }
 448             }
 449             struct_result
 450         }
 451
 452         hir::ExprKind::Lit(_) => Promotable,
 453
 454         hir::ExprKind::AddrOf(_, ref expr) |
 455         hir::ExprKind::Repeat(ref expr, _) => {
 456             v.check_expr(&expr)
 457         }
 458
 459         hir::ExprKind::Closure(_capture_clause, ref _box_fn_decl,
 460                                body_id, _span, _option_generator_movability) => {
 461             let nested_body_promotable = v.check_nested_body(body_id);
 462             // Paths in constant contexts cannot refer to local variables,
 463             // as there are none, and thus closures can't have upvars there.
 464             if v.tcx.with_freevars(e.id, |fv| !fv.is_empty()) {
 465                 NotPromotable
 466             } else {
 467                 nested_body_promotable
 468             }
 469         }
 470
 471         hir::ExprKind::Field(ref expr, _ident) => {
 472             let expr_promotability = v.check_expr(&expr);
 473             if let Some(def) = v.tables.expr_ty(expr).ty_adt_def() {
 474                 if def.is_union() {
 475                     return NotPromotable;
 476                 }
 477             }
 478             expr_promotability
 479         }
 480
 481         hir::ExprKind::Block(ref box_block, ref _option_label) => {
 482             v.check_block(box_block)
 483         }
 484
 485         hir::ExprKind::Index(ref lhs, ref rhs) => {
 486             let lefty = v.check_expr(lhs);
 487             let righty = v.check_expr(rhs);
 488             if v.tables.is_method_call(e) {
 489                 return NotPromotable;
 490             }
 491             lefty & righty
 492         }
 493
 494         hir::ExprKind::Array(ref hirvec) => {
 495             let mut array_result = Promotable;
 496             for index in hirvec.iter() {
 497                 array_result &= v.check_expr(index);
 498             }
 499             array_result
 500         }
 501
 502         hir::ExprKind::Type(ref expr, ref _ty) => {
 503             v.check_expr(&expr)
 504         }
 505
 506         hir::ExprKind::Tup(ref hirvec) => {
 507             let mut tup_result = Promotable;
 508             for index in hirvec.iter() {
 509                 tup_result &= v.check_expr(index);
 510             }
 511             tup_result
 512         }
 513
 514
 515         // Conditional control flow (possible to implement).
 516         hir::ExprKind::Match(ref expr, ref hirvec_arm, ref _match_source) => {
 517             // Compute the most demanding borrow from all the arms'
 518             // patterns and set that on the discriminator.
 519             let mut mut_borrow = false;
 520             for pat in hirvec_arm.iter().flat_map(|arm| &arm.pats) {
 521                 mut_borrow = v.remove_mut_rvalue_borrow(pat);
 522             }
 523             if mut_borrow {
 524                 v.mut_rvalue_borrows.insert(expr.id);
 525             }
 526
 527             let _ = v.check_expr(expr);
 528             for index in hirvec_arm.iter() {
 529                 let _ = v.check_expr(&*index.body);
 530                 if let Some(hir::Guard::If(ref expr)) = index.guard {
 531                     let _ = v.check_expr(&expr);
 532                 }
 533             }
 534             NotPromotable
 535         }
 536
 537         hir::ExprKind::If(ref lhs, ref rhs, ref option_expr) => {
 538             let _ = v.check_expr(lhs);
 539             let _ = v.check_expr(rhs);
 540             if let Some(ref expr) = option_expr {
 541                 let _ = v.check_expr(&expr);
 542             }
 543             NotPromotable
 544         }
 545
 546         // Loops (not very meaningful in constants).
 547         hir::ExprKind::While(ref expr, ref box_block, ref _option_label) => {
 548             let _ = v.check_expr(expr);
 549             let _ = v.check_block(box_block);
 550             NotPromotable
 551         }
 552
 553         hir::ExprKind::Loop(ref box_block, ref _option_label, ref _loop_source) => {
 554             let _ = v.check_block(box_block);
 555             NotPromotable
 556         }
 557
 558         // More control flow (also not very meaningful).
 559         hir::ExprKind::Break(_, ref option_expr) | hir::ExprKind::Ret(ref option_expr) => {
 560             if let Some(ref expr) = *option_expr {
 561                  let _ = v.check_expr(&expr);
 562             }
 563             NotPromotable
 564         }
 565
 566         hir::ExprKind::Continue(_) => {
 567             NotPromotable
 568         }
 569
 570         // Generator expressions
 571         hir::ExprKind::Yield(ref expr) => {
 572             let _ = v.check_expr(&expr);
 573             NotPromotable
 574         }
 575
 576         // Expressions with side-effects.
 577         hir::ExprKind::AssignOp(_, ref lhs, ref rhs) | hir::ExprKind::Assign(ref lhs, ref rhs) => {
 578             let _ = v.check_expr(lhs);
 579             let _ = v.check_expr(rhs);
 580             NotPromotable
 581         }
 582
 583         hir::ExprKind::InlineAsm(ref _inline_asm, ref hirvec_lhs, ref hirvec_rhs) => {
 584             for index in hirvec_lhs.iter().chain(hirvec_rhs.iter()) {
 585                 let _ = v.check_expr(index);
 586             }
 587             NotPromotable
 588         }
 589     };
 590     ty_result & node_result
 591 }
 592
 593 /// Check the adjustments of an expression
 594 fn check_adjustments<'a, 'tcx>(
 595     v: &mut CheckCrateVisitor<'a, 'tcx>,
 596     e: &hir::Expr) -> Promotability {
 597     use rustc::ty::adjustment::*;
 598
 599     let mut adjustments = v.tables.expr_adjustments(e).iter().peekable();
 600     while let Some(adjustment) = adjustments.next() {
 601         match adjustment.kind {
 602             Adjust::NeverToAny |
 603             Adjust::ReifyFnPointer |
 604             Adjust::UnsafeFnPointer |
 605             Adjust::ClosureFnPointer |
 606             Adjust::MutToConstPointer |
 607             Adjust::Borrow(_) |
 608             Adjust::Unsize => {}
 609
 610             Adjust::Deref(_) => {
 611                 if let Some(next_adjustment) = adjustments.peek() {
 612                     if let Adjust::Borrow(_) = next_adjustment.kind {
 613                         continue;
 614                     }
 615                 }
 616                 return NotPromotable;
 617             }
 618         }
 619     }
 620     Promotable
 621 }
 622
 623 impl<'a, 'gcx, 'tcx> euv::Delegate<'tcx> for CheckCrateVisitor<'a, 'gcx> {
 624     fn consume(&mut self,
 625                _consume_id: ast::NodeId,
 626                _consume_span: Span,
 627                _cmt: &mc::cmt_,
 628                _mode: euv::ConsumeMode) {}
 629
 630     fn borrow(&mut self,
 631               borrow_id: ast::NodeId,
 632               _borrow_span: Span,
 633               cmt: &mc::cmt_<'tcx>,
 634               _loan_region: ty::Region<'tcx>,
 635               bk: ty::BorrowKind,
 636               loan_cause: euv::LoanCause) {
 637         debug!(
 638             "borrow(borrow_id={:?}, cmt={:?}, bk={:?}, loan_cause={:?})",
 639             borrow_id,
 640             cmt,
 641             bk,
 642             loan_cause,
 643         );
 644
 645         // Kind of hacky, but we allow Unsafe coercions in constants.
 646         // These occur when we convert a &T or *T to a *U, as well as
 647         // when making a thin pointer (e.g., `*T`) into a fat pointer
 648         // (e.g., `*Trait`).
 649         if let euv::LoanCause::AutoUnsafe = loan_cause {
 650             return;
 651         }
 652
 653         let mut cur = cmt;
 654         loop {
 655             match cur.cat {
 656                 Categorization::ThreadLocal(..) |
 657                 Categorization::Rvalue(..) => {
 658                     if loan_cause == euv::MatchDiscriminant {
 659                         // Ignore the dummy immutable borrow created by EUV.
 660                         break;
 661                     }
 662                     if bk.to_mutbl_lossy() == hir::MutMutable {
 663                         self.mut_rvalue_borrows.insert(borrow_id);
 664                     }
 665                     break;
 666                 }
 667                 Categorization::StaticItem => {
 668                     break;
 669                 }
 670                 Categorization::Deref(ref cmt, _) |
 671                 Categorization::Downcast(ref cmt, _) |
 672                 Categorization::Interior(ref cmt, _) => {
 673                     cur = cmt;
 674                 }
 675
 676                 Categorization::Upvar(..) |
 677                 Categorization::Local(..) => break,
 678             }
 679         }
 680     }
 681
 682     fn decl_without_init(&mut self, _id: ast::NodeId, _span: Span) {}
 683     fn mutate(&mut self,
 684               _assignment_id: ast::NodeId,
 685               _assignment_span: Span,
 686               _assignee_cmt: &mc::cmt_,
 687               _mode: euv::MutateMode) {
 688     }
 689
 690     fn matched_pat(&mut self, _: &hir::Pat, _: &mc::cmt_, _: euv::MatchMode) {}
 691
 692     fn consume_pat(&mut self, _consume_pat: &hir::Pat, _cmt: &mc::cmt_, _mode: euv::ConsumeMode) {}
 693 }