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