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