compiler/rustc_mir_build/src/build/matches/mod.rs

   1 //! Code related to match expressions. These are sufficiently complex to
   2 //! warrant their own module and submodules. :) This main module includes the
   3 //! high-level algorithm, the submodules contain the details.
   4 //!
   5 //! This also includes code for pattern bindings in `let` statements and
   6 //! function parameters.
   7
   8 use crate::build::expr::as_place::PlaceBuilder;
   9 use crate::build::scope::DropKind;
  10 use crate::build::ForGuard::{self, OutsideGuard, RefWithinGuard};
  11 use crate::build::{BlockAnd, BlockAndExtension, Builder};
  12 use crate::build::{GuardFrame, GuardFrameLocal, LocalsForNode};
  13 use rustc_data_structures::{
  14     fx::{FxHashSet, FxIndexMap},
  15     stack::ensure_sufficient_stack,
  16 };
  17 use rustc_hir::HirId;
  18 use rustc_index::bit_set::BitSet;
  19 use rustc_middle::middle::region;
  20 use rustc_middle::mir::*;
  21 use rustc_middle::thir::{self, *};
  22 use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty};
  23 use rustc_span::symbol::Symbol;
  24 use rustc_span::{BytePos, Pos, Span};
  25 use rustc_target::abi::VariantIdx;
  26 use smallvec::{smallvec, SmallVec};
  27
  28 // helper functions, broken out by category:
  29 mod simplify;
  30 mod test;
  31 mod util;
  32
  33 use std::borrow::Borrow;
  34 use std::convert::TryFrom;
  35 use std::mem;
  36
  37 impl<'a, 'tcx> Builder<'a, 'tcx> {
  38     pub(crate) fn then_else_break(
  39         &mut self,
  40         mut block: BasicBlock,
  41         expr: &Expr<'tcx>,
  42         temp_scope_override: Option<region::Scope>,
  43         break_scope: region::Scope,
  44         variable_scope_span: Span,
  45     ) -> BlockAnd<()> {
  46         let this = self;
  47         let expr_span = expr.span;
  48
  49         match expr.kind {
  50             ExprKind::LogicalOp { op: LogicalOp::And, lhs, rhs } => {
  51                 let lhs_then_block = unpack!(this.then_else_break(
  52                     block,
  53                     &this.thir[lhs],
  54                     temp_scope_override,
  55                     break_scope,
  56                     variable_scope_span,
  57                 ));
  58
  59                 let rhs_then_block = unpack!(this.then_else_break(
  60                     lhs_then_block,
  61                     &this.thir[rhs],
  62                     temp_scope_override,
  63                     break_scope,
  64                     variable_scope_span,
  65                 ));
  66
  67                 rhs_then_block.unit()
  68             }
  69             ExprKind::Scope { region_scope, lint_level, value } => {
  70                 let region_scope = (region_scope, this.source_info(expr_span));
  71                 this.in_scope(region_scope, lint_level, |this| {
  72                     this.then_else_break(
  73                         block,
  74                         &this.thir[value],
  75                         temp_scope_override,
  76                         break_scope,
  77                         variable_scope_span,
  78                     )
  79                 })
  80             }
  81             ExprKind::Let { expr, ref pat } => {
  82                 this.lower_let_expr(block, &this.thir[expr], pat, break_scope, variable_scope_span)
  83             }
  84             _ => {
  85                 let temp_scope = temp_scope_override.unwrap_or_else(|| this.local_scope());
  86                 let mutability = Mutability::Mut;
  87                 let place =
  88                     unpack!(block = this.as_temp(block, Some(temp_scope), expr, mutability));
  89                 let operand = Operand::Move(Place::from(place));
  90
  91                 let then_block = this.cfg.start_new_block();
  92                 let else_block = this.cfg.start_new_block();
  93                 let term = TerminatorKind::if_(this.tcx, operand, then_block, else_block);
  94
  95                 let source_info = this.source_info(expr_span);
  96                 this.cfg.terminate(block, source_info, term);
  97                 this.break_for_else(else_block, break_scope, source_info);
  98
  99                 then_block.unit()
 100             }
 101         }
 102     }
 103
 104     /// Generates MIR for a `match` expression.
 105     ///
 106     /// The MIR that we generate for a match looks like this.
 107     ///
 108     /// ```text
 109     /// [ 0. Pre-match ]
 110     ///        |
 111     /// [ 1. Evaluate Scrutinee (expression being matched on) ]
 112     /// [ (fake read of scrutinee) ]
 113     ///        |
 114     /// [ 2. Decision tree -- check discriminants ] <--------+
 115     ///        |                                             |
 116     ///        | (once a specific arm is chosen)             |
 117     ///        |                                             |
 118     /// [pre_binding_block]                           [otherwise_block]
 119     ///        |                                             |
 120     /// [ 3. Create "guard bindings" for arm ]               |
 121     /// [ (create fake borrows) ]                            |
 122     ///        |                                             |
 123     /// [ 4. Execute guard code ]                            |
 124     /// [ (read fake borrows) ] --(guard is false)-----------+
 125     ///        |
 126     ///        | (guard results in true)
 127     ///        |
 128     /// [ 5. Create real bindings and execute arm ]
 129     ///        |
 130     /// [ Exit match ]
 131     /// ```
 132     ///
 133     /// All of the different arms have been stacked on top of each other to
 134     /// simplify the diagram. For an arm with no guard the blocks marked 3 and
 135     /// 4 and the fake borrows are omitted.
 136     ///
 137     /// We generate MIR in the following steps:
 138     ///
 139     /// 1. Evaluate the scrutinee and add the fake read of it ([Builder::lower_scrutinee]).
 140     /// 2. Create the decision tree ([Builder::lower_match_tree]).
 141     /// 3. Determine the fake borrows that are needed from the places that were
 142     ///    matched against and create the required temporaries for them
 143     ///    ([Builder::calculate_fake_borrows]).
 144     /// 4. Create everything else: the guards and the arms ([Builder::lower_match_arms]).
 145     ///
 146     /// ## False edges
 147     ///
 148     /// We don't want to have the exact structure of the decision tree be
 149     /// visible through borrow checking. False edges ensure that the CFG as
 150     /// seen by borrow checking doesn't encode this. False edges are added:
 151     ///
 152     /// * From each pre-binding block to the next pre-binding block.
 153     /// * From each otherwise block to the next pre-binding block.
 154     crate fn match_expr(
 155         &mut self,
 156         destination: Place<'tcx>,
 157         span: Span,
 158         mut block: BasicBlock,
 159         scrutinee: &Expr<'tcx>,
 160         arms: &[ArmId],
 161     ) -> BlockAnd<()> {
 162         let scrutinee_span = scrutinee.span;
 163         let scrutinee_place =
 164             unpack!(block = self.lower_scrutinee(block, scrutinee, scrutinee_span,));
 165
 166         let mut arm_candidates = self.create_match_candidates(scrutinee_place.clone(), &arms);
 167
 168         let match_has_guard = arms.iter().copied().any(|arm| self.thir[arm].guard.is_some());
 169         let mut candidates =
 170             arm_candidates.iter_mut().map(|(_, candidate)| candidate).collect::<Vec<_>>();
 171
 172         let match_start_span = span.shrink_to_lo().to(scrutinee.span);
 173
 174         let fake_borrow_temps = self.lower_match_tree(
 175             block,
 176             scrutinee_span,
 177             match_start_span,
 178             match_has_guard,
 179             &mut candidates,
 180         );
 181
 182         self.lower_match_arms(
 183             destination,
 184             scrutinee_place,
 185             scrutinee_span,
 186             arm_candidates,
 187             self.source_info(span),
 188             fake_borrow_temps,
 189         )
 190     }
 191
 192     /// Evaluate the scrutinee and add the fake read of it.
 193     fn lower_scrutinee(
 194         &mut self,
 195         mut block: BasicBlock,
 196         scrutinee: &Expr<'tcx>,
 197         scrutinee_span: Span,
 198     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 199         let scrutinee_place_builder = unpack!(block = self.as_place_builder(block, scrutinee));
 200         // Matching on a `scrutinee_place` with an uninhabited type doesn't
 201         // generate any memory reads by itself, and so if the place "expression"
 202         // contains unsafe operations like raw pointer dereferences or union
 203         // field projections, we wouldn't know to require an `unsafe` block
 204         // around a `match` equivalent to `std::intrinsics::unreachable()`.
 205         // See issue #47412 for this hole being discovered in the wild.
 206         //
 207         // HACK(eddyb) Work around the above issue by adding a dummy inspection
 208         // of `scrutinee_place`, specifically by applying `ReadForMatch`.
 209         //
 210         // NOTE: ReadForMatch also checks that the scrutinee is initialized.
 211         // This is currently needed to not allow matching on an uninitialized,
 212         // uninhabited value. If we get never patterns, those will check that
 213         // the place is initialized, and so this read would only be used to
 214         // check safety.
 215         let cause_matched_place = FakeReadCause::ForMatchedPlace(None);
 216         let source_info = self.source_info(scrutinee_span);
 217
 218         if let Ok(scrutinee_builder) =
 219             scrutinee_place_builder.clone().try_upvars_resolved(self.tcx, self.typeck_results)
 220         {
 221             let scrutinee_place = scrutinee_builder.into_place(self.tcx, self.typeck_results);
 222             self.cfg.push_fake_read(block, source_info, cause_matched_place, scrutinee_place);
 223         }
 224
 225         block.and(scrutinee_place_builder)
 226     }
 227
 228     /// Create the initial `Candidate`s for a `match` expression.
 229     fn create_match_candidates<'pat>(
 230         &mut self,
 231         scrutinee: PlaceBuilder<'tcx>,
 232         arms: &'pat [ArmId],
 233     ) -> Vec<(&'pat Arm<'tcx>, Candidate<'pat, 'tcx>)>
 234     where
 235         'a: 'pat,
 236     {
 237         // Assemble a list of candidates: there is one candidate per pattern,
 238         // which means there may be more than one candidate *per arm*.
 239         arms.iter()
 240             .copied()
 241             .map(|arm| {
 242                 let arm = &self.thir[arm];
 243                 let arm_has_guard = arm.guard.is_some();
 244                 let arm_candidate = Candidate::new(scrutinee.clone(), &arm.pattern, arm_has_guard);
 245                 (arm, arm_candidate)
 246             })
 247             .collect()
 248     }
 249
 250     /// Create the decision tree for the match expression, starting from `block`.
 251     ///
 252     /// Modifies `candidates` to store the bindings and type ascriptions for
 253     /// that candidate.
 254     ///
 255     /// Returns the places that need fake borrows because we bind or test them.
 256     fn lower_match_tree<'pat>(
 257         &mut self,
 258         block: BasicBlock,
 259         scrutinee_span: Span,
 260         match_start_span: Span,
 261         match_has_guard: bool,
 262         candidates: &mut [&mut Candidate<'pat, 'tcx>],
 263     ) -> Vec<(Place<'tcx>, Local)> {
 264         // The set of places that we are creating fake borrows of. If there are
 265         // no match guards then we don't need any fake borrows, so don't track
 266         // them.
 267         let mut fake_borrows = match_has_guard.then(FxHashSet::default);
 268
 269         let mut otherwise = None;
 270
 271         // This will generate code to test scrutinee_place and
 272         // branch to the appropriate arm block
 273         self.match_candidates(
 274             match_start_span,
 275             scrutinee_span,
 276             block,
 277             &mut otherwise,
 278             candidates,
 279             &mut fake_borrows,
 280         );
 281
 282         if let Some(otherwise_block) = otherwise {
 283             // See the doc comment on `match_candidates` for why we may have an
 284             // otherwise block. Match checking will ensure this is actually
 285             // unreachable.
 286             let source_info = self.source_info(scrutinee_span);
 287             self.cfg.terminate(otherwise_block, source_info, TerminatorKind::Unreachable);
 288         }
 289
 290         // Link each leaf candidate to the `pre_binding_block` of the next one.
 291         let mut previous_candidate: Option<&mut Candidate<'_, '_>> = None;
 292
 293         for candidate in candidates {
 294             candidate.visit_leaves(|leaf_candidate| {
 295                 if let Some(ref mut prev) = previous_candidate {
 296                     prev.next_candidate_pre_binding_block = leaf_candidate.pre_binding_block;
 297                 }
 298                 previous_candidate = Some(leaf_candidate);
 299             });
 300         }
 301
 302         if let Some(ref borrows) = fake_borrows {
 303             self.calculate_fake_borrows(borrows, scrutinee_span)
 304         } else {
 305             Vec::new()
 306         }
 307     }
 308
 309     /// Lower the bindings, guards and arm bodies of a `match` expression.
 310     ///
 311     /// The decision tree should have already been created
 312     /// (by [Builder::lower_match_tree]).
 313     ///
 314     /// `outer_source_info` is the SourceInfo for the whole match.
 315     fn lower_match_arms(
 316         &mut self,
 317         destination: Place<'tcx>,
 318         scrutinee_place_builder: PlaceBuilder<'tcx>,
 319         scrutinee_span: Span,
 320         arm_candidates: Vec<(&'_ Arm<'tcx>, Candidate<'_, 'tcx>)>,
 321         outer_source_info: SourceInfo,
 322         fake_borrow_temps: Vec<(Place<'tcx>, Local)>,
 323     ) -> BlockAnd<()> {
 324         let arm_end_blocks: Vec<_> = arm_candidates
 325             .into_iter()
 326             .map(|(arm, candidate)| {
 327                 debug!("lowering arm {:?}\ncandidate = {:?}", arm, candidate);
 328
 329                 let arm_source_info = self.source_info(arm.span);
 330                 let arm_scope = (arm.scope, arm_source_info);
 331                 let match_scope = self.local_scope();
 332                 self.in_scope(arm_scope, arm.lint_level, |this| {
 333                     // `try_upvars_resolved` may fail if it is unable to resolve the given
 334                     // `PlaceBuilder` inside a closure. In this case, we don't want to include
 335                     // a scrutinee place. `scrutinee_place_builder` will fail to be resolved
 336                     // if the only match arm is a wildcard (`_`).
 337                     // Example:
 338                     // ```
 339                     // let foo = (0, 1);
 340                     // let c = || {
 341                     //    match foo { _ => () };
 342                     // };
 343                     // ```
 344                     let mut opt_scrutinee_place: Option<(Option<&Place<'tcx>>, Span)> = None;
 345                     let scrutinee_place: Place<'tcx>;
 346                     if let Ok(scrutinee_builder) = scrutinee_place_builder
 347                         .clone()
 348                         .try_upvars_resolved(this.tcx, this.typeck_results)
 349                     {
 350                         scrutinee_place =
 351                             scrutinee_builder.into_place(this.tcx, this.typeck_results);
 352                         opt_scrutinee_place = Some((Some(&scrutinee_place), scrutinee_span));
 353                     }
 354                     let scope = this.declare_bindings(
 355                         None,
 356                         arm.span,
 357                         &arm.pattern,
 358                         ArmHasGuard(arm.guard.is_some()),
 359                         opt_scrutinee_place,
 360                     );
 361
 362                     let arm_block = this.bind_pattern(
 363                         outer_source_info,
 364                         candidate,
 365                         arm.guard.as_ref(),
 366                         &fake_borrow_temps,
 367                         scrutinee_span,
 368                         Some(arm.span),
 369                         Some(arm.scope),
 370                         Some(match_scope),
 371                     );
 372
 373                     if let Some(source_scope) = scope {
 374                         this.source_scope = source_scope;
 375                     }
 376
 377                     this.expr_into_dest(destination, arm_block, &&this.thir[arm.body])
 378                 })
 379             })
 380             .collect();
 381
 382         // all the arm blocks will rejoin here
 383         let end_block = self.cfg.start_new_block();
 384
 385         let end_brace = self.source_info(
 386             outer_source_info.span.with_lo(outer_source_info.span.hi() - BytePos::from_usize(1)),
 387         );
 388         for arm_block in arm_end_blocks {
 389             let block = &self.cfg.basic_blocks[arm_block.0];
 390             let last_location = block.statements.last().map(|s| s.source_info);
 391
 392             self.cfg.goto(unpack!(arm_block), last_location.unwrap_or(end_brace), end_block);
 393         }
 394
 395         self.source_scope = outer_source_info.scope;
 396
 397         end_block.unit()
 398     }
 399
 400     /// Binds the variables and ascribes types for a given `match` arm or
 401     /// `let` binding.
 402     ///
 403     /// Also check if the guard matches, if it's provided.
 404     /// `arm_scope` should be `Some` if and only if this is called for a
 405     /// `match` arm.
 406     fn bind_pattern(
 407         &mut self,
 408         outer_source_info: SourceInfo,
 409         candidate: Candidate<'_, 'tcx>,
 410         guard: Option<&Guard<'tcx>>,
 411         fake_borrow_temps: &Vec<(Place<'tcx>, Local)>,
 412         scrutinee_span: Span,
 413         arm_span: Option<Span>,
 414         arm_scope: Option<region::Scope>,
 415         match_scope: Option<region::Scope>,
 416     ) -> BasicBlock {
 417         if candidate.subcandidates.is_empty() {
 418             // Avoid generating another `BasicBlock` when we only have one
 419             // candidate.
 420             self.bind_and_guard_matched_candidate(
 421                 candidate,
 422                 &[],
 423                 guard,
 424                 fake_borrow_temps,
 425                 scrutinee_span,
 426                 arm_span,
 427                 match_scope,
 428                 true,
 429             )
 430         } else {
 431             // It's helpful to avoid scheduling drops multiple times to save
 432             // drop elaboration from having to clean up the extra drops.
 433             //
 434             // If we are in a `let` then we only schedule drops for the first
 435             // candidate.
 436             //
 437             // If we're in a `match` arm then we could have a case like so:
 438             //
 439             // Ok(x) | Err(x) if return => { /* ... */ }
 440             //
 441             // In this case we don't want a drop of `x` scheduled when we
 442             // return: it isn't bound by move until right before enter the arm.
 443             // To handle this we instead unschedule it's drop after each time
 444             // we lower the guard.
 445             let target_block = self.cfg.start_new_block();
 446             let mut schedule_drops = true;
 447             // We keep a stack of all of the bindings and type asciptions
 448             // from the parent candidates that we visit, that also need to
 449             // be bound for each candidate.
 450             traverse_candidate(
 451                 candidate,
 452                 &mut Vec::new(),
 453                 &mut |leaf_candidate, parent_bindings| {
 454                     if let Some(arm_scope) = arm_scope {
 455                         self.clear_top_scope(arm_scope);
 456                     }
 457                     let binding_end = self.bind_and_guard_matched_candidate(
 458                         leaf_candidate,
 459                         parent_bindings,
 460                         guard,
 461                         &fake_borrow_temps,
 462                         scrutinee_span,
 463                         arm_span,
 464                         match_scope,
 465                         schedule_drops,
 466                     );
 467                     if arm_scope.is_none() {
 468                         schedule_drops = false;
 469                     }
 470                     self.cfg.goto(binding_end, outer_source_info, target_block);
 471                 },
 472                 |inner_candidate, parent_bindings| {
 473                     parent_bindings.push((inner_candidate.bindings, inner_candidate.ascriptions));
 474                     inner_candidate.subcandidates.into_iter()
 475                 },
 476                 |parent_bindings| {
 477                     parent_bindings.pop();
 478                 },
 479             );
 480
 481             target_block
 482         }
 483     }
 484
 485     pub(super) fn expr_into_pattern(
 486         &mut self,
 487         mut block: BasicBlock,
 488         irrefutable_pat: Pat<'tcx>,
 489         initializer: &Expr<'tcx>,
 490     ) -> BlockAnd<()> {
 491         match *irrefutable_pat.kind {
 492             // Optimize the case of `let x = ...` to write directly into `x`
 493             PatKind::Binding { mode: BindingMode::ByValue, var, subpattern: None, .. } => {
 494                 let place =
 495                     self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
 496                 unpack!(block = self.expr_into_dest(place, block, initializer));
 497
 498                 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
 499                 let source_info = self.source_info(irrefutable_pat.span);
 500                 self.cfg.push_fake_read(block, source_info, FakeReadCause::ForLet(None), place);
 501
 502                 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
 503                 block.unit()
 504             }
 505
 506             // Optimize the case of `let x: T = ...` to write directly
 507             // into `x` and then require that `T == typeof(x)`.
 508             //
 509             // Weirdly, this is needed to prevent the
 510             // `intrinsic-move-val.rs` test case from crashing. That
 511             // test works with uninitialized values in a rather
 512             // dubious way, so it may be that the test is kind of
 513             // broken.
 514             PatKind::AscribeUserType {
 515                 subpattern:
 516                     Pat {
 517                         kind:
 518                             box PatKind::Binding {
 519                                 mode: BindingMode::ByValue,
 520                                 var,
 521                                 subpattern: None,
 522                                 ..
 523                             },
 524                         ..
 525                     },
 526                 ascription:
 527                     thir::Ascription { user_ty: pat_ascription_ty, variance: _, user_ty_span },
 528             } => {
 529                 let place =
 530                     self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
 531                 unpack!(block = self.expr_into_dest(place, block, initializer));
 532
 533                 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
 534                 let pattern_source_info = self.source_info(irrefutable_pat.span);
 535                 let cause_let = FakeReadCause::ForLet(None);
 536                 self.cfg.push_fake_read(block, pattern_source_info, cause_let, place);
 537
 538                 let ty_source_info = self.source_info(user_ty_span);
 539                 let user_ty = pat_ascription_ty.user_ty(
 540                     &mut self.canonical_user_type_annotations,
 541                     place.ty(&self.local_decls, self.tcx).ty,
 542                     ty_source_info.span,
 543                 );
 544                 self.cfg.push(
 545                     block,
 546                     Statement {
 547                         source_info: ty_source_info,
 548                         kind: StatementKind::AscribeUserType(
 549                             Box::new((place, user_ty)),
 550                             // We always use invariant as the variance here. This is because the
 551                             // variance field from the ascription refers to the variance to use
 552                             // when applying the type to the value being matched, but this
 553                             // ascription applies rather to the type of the binding. e.g., in this
 554                             // example:
 555                             //
 556                             // ```
 557                             // let x: T = <expr>
 558                             // ```
 559                             //
 560                             // We are creating an ascription that defines the type of `x` to be
 561                             // exactly `T` (i.e., with invariance). The variance field, in
 562                             // contrast, is intended to be used to relate `T` to the type of
 563                             // `<expr>`.
 564                             ty::Variance::Invariant,
 565                         ),
 566                     },
 567                 );
 568
 569                 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
 570                 block.unit()
 571             }
 572
 573             _ => {
 574                 let place_builder = unpack!(block = self.as_place_builder(block, initializer));
 575                 self.place_into_pattern(block, irrefutable_pat, place_builder, true)
 576             }
 577         }
 578     }
 579
 580     crate fn place_into_pattern(
 581         &mut self,
 582         block: BasicBlock,
 583         irrefutable_pat: Pat<'tcx>,
 584         initializer: PlaceBuilder<'tcx>,
 585         set_match_place: bool,
 586     ) -> BlockAnd<()> {
 587         let mut candidate = Candidate::new(initializer.clone(), &irrefutable_pat, false);
 588         let fake_borrow_temps = self.lower_match_tree(
 589             block,
 590             irrefutable_pat.span,
 591             irrefutable_pat.span,
 592             false,
 593             &mut [&mut candidate],
 594         );
 595         // For matches and function arguments, the place that is being matched
 596         // can be set when creating the variables. But the place for
 597         // let PATTERN = ... might not even exist until we do the assignment.
 598         // so we set it here instead.
 599         if set_match_place {
 600             let mut candidate_ref = &candidate;
 601             while let Some(next) = {
 602                 for binding in &candidate_ref.bindings {
 603                     let local = self.var_local_id(binding.var_id, OutsideGuard);
 604
 605                     let Some(box LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
 606                         VarBindingForm { opt_match_place: Some((ref mut match_place, _)), .. },
 607                     )))) = self.local_decls[local].local_info else {
 608                         bug!("Let binding to non-user variable.")
 609                     };
 610                     // `try_upvars_resolved` may fail if it is unable to resolve the given
 611                     // `PlaceBuilder` inside a closure. In this case, we don't want to include
 612                     // a scrutinee place. `scrutinee_place_builder` will fail for destructured
 613                     // assignments. This is because a closure only captures the precise places
 614                     // that it will read and as a result a closure may not capture the entire
 615                     // tuple/struct and rather have individual places that will be read in the
 616                     // final MIR.
 617                     // Example:
 618                     // ```
 619                     // let foo = (0, 1);
 620                     // let c = || {
 621                     //    let (v1, v2) = foo;
 622                     // };
 623                     // ```
 624                     if let Ok(match_pair_resolved) =
 625                         initializer.clone().try_upvars_resolved(self.tcx, self.typeck_results)
 626                     {
 627                         let place = match_pair_resolved.into_place(self.tcx, self.typeck_results);
 628                         *match_place = Some(place);
 629                     }
 630                 }
 631                 // All of the subcandidates should bind the same locals, so we
 632                 // only visit the first one.
 633                 candidate_ref.subcandidates.get(0)
 634             } {
 635                 candidate_ref = next;
 636             }
 637         }
 638
 639         self.bind_pattern(
 640             self.source_info(irrefutable_pat.span),
 641             candidate,
 642             None,
 643             &fake_borrow_temps,
 644             irrefutable_pat.span,
 645             None,
 646             None,
 647             None,
 648         )
 649         .unit()
 650     }
 651
 652     /// Declares the bindings of the given patterns and returns the visibility
 653     /// scope for the bindings in these patterns, if such a scope had to be
 654     /// created. NOTE: Declaring the bindings should always be done in their
 655     /// drop scope.
 656     crate fn declare_bindings(
 657         &mut self,
 658         mut visibility_scope: Option<SourceScope>,
 659         scope_span: Span,
 660         pattern: &Pat<'tcx>,
 661         has_guard: ArmHasGuard,
 662         opt_match_place: Option<(Option<&Place<'tcx>>, Span)>,
 663     ) -> Option<SourceScope> {
 664         debug!("declare_bindings: pattern={:?}", pattern);
 665         self.visit_primary_bindings(
 666             &pattern,
 667             UserTypeProjections::none(),
 668             &mut |this, mutability, name, mode, var, span, ty, user_ty| {
 669                 if visibility_scope.is_none() {
 670                     visibility_scope =
 671                         Some(this.new_source_scope(scope_span, LintLevel::Inherited, None));
 672                 }
 673                 let source_info = SourceInfo { span, scope: this.source_scope };
 674                 let visibility_scope = visibility_scope.unwrap();
 675                 this.declare_binding(
 676                     source_info,
 677                     visibility_scope,
 678                     mutability,
 679                     name,
 680                     mode,
 681                     var,
 682                     ty,
 683                     user_ty,
 684                     has_guard,
 685                     opt_match_place.map(|(x, y)| (x.cloned(), y)),
 686                     pattern.span,
 687                 );
 688             },
 689         );
 690         visibility_scope
 691     }
 692
 693     crate fn storage_live_binding(
 694         &mut self,
 695         block: BasicBlock,
 696         var: HirId,
 697         span: Span,
 698         for_guard: ForGuard,
 699         schedule_drop: bool,
 700     ) -> Place<'tcx> {
 701         let local_id = self.var_local_id(var, for_guard);
 702         let source_info = self.source_info(span);
 703         self.cfg.push(block, Statement { source_info, kind: StatementKind::StorageLive(local_id) });
 704         let region_scope = self.region_scope_tree.var_scope(var.local_id);
 705         if schedule_drop {
 706             self.schedule_drop(span, region_scope, local_id, DropKind::Storage);
 707         }
 708         Place::from(local_id)
 709     }
 710
 711     crate fn schedule_drop_for_binding(&mut self, var: HirId, span: Span, for_guard: ForGuard) {
 712         let local_id = self.var_local_id(var, for_guard);
 713         let region_scope = self.region_scope_tree.var_scope(var.local_id);
 714         self.schedule_drop(span, region_scope, local_id, DropKind::Value);
 715     }
 716
 717     /// Visit all of the primary bindings in a patterns, that is, visit the
 718     /// leftmost occurrence of each variable bound in a pattern. A variable
 719     /// will occur more than once in an or-pattern.
 720     pub(super) fn visit_primary_bindings(
 721         &mut self,
 722         pattern: &Pat<'tcx>,
 723         pattern_user_ty: UserTypeProjections,
 724         f: &mut impl FnMut(
 725             &mut Self,
 726             Mutability,
 727             Symbol,
 728             BindingMode,
 729             HirId,
 730             Span,
 731             Ty<'tcx>,
 732             UserTypeProjections,
 733         ),
 734     ) {
 735         debug!(
 736             "visit_primary_bindings: pattern={:?} pattern_user_ty={:?}",
 737             pattern, pattern_user_ty
 738         );
 739         match *pattern.kind {
 740             PatKind::Binding {
 741                 mutability,
 742                 name,
 743                 mode,
 744                 var,
 745                 ty,
 746                 ref subpattern,
 747                 is_primary,
 748                 ..
 749             } => {
 750                 if is_primary {
 751                     f(self, mutability, name, mode, var, pattern.span, ty, pattern_user_ty.clone());
 752                 }
 753                 if let Some(subpattern) = subpattern.as_ref() {
 754                     self.visit_primary_bindings(subpattern, pattern_user_ty, f);
 755                 }
 756             }
 757
 758             PatKind::Array { ref prefix, ref slice, ref suffix }
 759             | PatKind::Slice { ref prefix, ref slice, ref suffix } => {
 760                 let from = u64::try_from(prefix.len()).unwrap();
 761                 let to = u64::try_from(suffix.len()).unwrap();
 762                 for subpattern in prefix {
 763                     self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
 764                 }
 765                 for subpattern in slice {
 766                     self.visit_primary_bindings(
 767                         subpattern,
 768                         pattern_user_ty.clone().subslice(from, to),
 769                         f,
 770                     );
 771                 }
 772                 for subpattern in suffix {
 773                     self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
 774                 }
 775             }
 776
 777             PatKind::Constant { .. } | PatKind::Range { .. } | PatKind::Wild => {}
 778
 779             PatKind::Deref { ref subpattern } => {
 780                 self.visit_primary_bindings(subpattern, pattern_user_ty.deref(), f);
 781             }
 782
 783             PatKind::AscribeUserType {
 784                 ref subpattern,
 785                 ascription: thir::Ascription { ref user_ty, user_ty_span, variance: _ },
 786             } => {
 787                 // This corresponds to something like
 788                 //
 789                 // ```
 790                 // let A::<'a>(_): A<'static> = ...;
 791                 // ```
 792                 //
 793                 // Note that the variance doesn't apply here, as we are tracking the effect
 794                 // of `user_ty` on any bindings contained with subpattern.
 795                 let annotation = CanonicalUserTypeAnnotation {
 796                     span: user_ty_span,
 797                     user_ty: user_ty.user_ty,
 798                     inferred_ty: subpattern.ty,
 799                 };
 800                 let projection = UserTypeProjection {
 801                     base: self.canonical_user_type_annotations.push(annotation),
 802                     projs: Vec::new(),
 803                 };
 804                 let subpattern_user_ty = pattern_user_ty.push_projection(&projection, user_ty_span);
 805                 self.visit_primary_bindings(subpattern, subpattern_user_ty, f)
 806             }
 807
 808             PatKind::Leaf { ref subpatterns } => {
 809                 for subpattern in subpatterns {
 810                     let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field);
 811                     debug!("visit_primary_bindings: subpattern_user_ty={:?}", subpattern_user_ty);
 812                     self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
 813                 }
 814             }
 815
 816             PatKind::Variant { adt_def, substs: _, variant_index, ref subpatterns } => {
 817                 for subpattern in subpatterns {
 818                     let subpattern_user_ty =
 819                         pattern_user_ty.clone().variant(adt_def, variant_index, subpattern.field);
 820                     self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
 821                 }
 822             }
 823             PatKind::Or { ref pats } => {
 824                 // In cases where we recover from errors the primary bindings
 825                 // may not all be in the leftmost subpattern. For example in
 826                 // `let (x | y) = ...`, the primary binding of `y` occurs in
 827                 // the right subpattern
 828                 for subpattern in pats {
 829                     self.visit_primary_bindings(subpattern, pattern_user_ty.clone(), f);
 830                 }
 831             }
 832         }
 833     }
 834 }
 835
 836 #[derive(Debug)]
 837 struct Candidate<'pat, 'tcx> {
 838     /// [`Span`] of the original pattern that gave rise to this candidate.
 839     span: Span,
 840
 841     /// Whether this `Candidate` has a guard.
 842     has_guard: bool,
 843
 844     /// All of these must be satisfied...
 845     match_pairs: SmallVec<[MatchPair<'pat, 'tcx>; 1]>,
 846
 847     /// ...these bindings established...
 848     bindings: Vec<Binding<'tcx>>,
 849
 850     /// ...and these types asserted...
 851     ascriptions: Vec<Ascription<'tcx>>,
 852
 853     /// ...and if this is non-empty, one of these subcandidates also has to match...
 854     subcandidates: Vec<Candidate<'pat, 'tcx>>,
 855
 856     /// ...and the guard must be evaluated; if it's `false` then branch to `otherwise_block`.
 857     otherwise_block: Option<BasicBlock>,
 858
 859     /// The block before the `bindings` have been established.
 860     pre_binding_block: Option<BasicBlock>,
 861     /// The pre-binding block of the next candidate.
 862     next_candidate_pre_binding_block: Option<BasicBlock>,
 863 }
 864
 865 impl<'tcx, 'pat> Candidate<'pat, 'tcx> {
 866     fn new(place: PlaceBuilder<'tcx>, pattern: &'pat Pat<'tcx>, has_guard: bool) -> Self {
 867         Candidate {
 868             span: pattern.span,
 869             has_guard,
 870             match_pairs: smallvec![MatchPair { place, pattern }],
 871             bindings: Vec::new(),
 872             ascriptions: Vec::new(),
 873             subcandidates: Vec::new(),
 874             otherwise_block: None,
 875             pre_binding_block: None,
 876             next_candidate_pre_binding_block: None,
 877         }
 878     }
 879
 880     /// Visit the leaf candidates (those with no subcandidates) contained in
 881     /// this candidate.
 882     fn visit_leaves<'a>(&'a mut self, mut visit_leaf: impl FnMut(&'a mut Self)) {
 883         traverse_candidate(
 884             self,
 885             &mut (),
 886             &mut move |c, _| visit_leaf(c),
 887             move |c, _| c.subcandidates.iter_mut(),
 888             |_| {},
 889         );
 890     }
 891 }
 892
 893 /// A depth-first traversal of the `Candidate` and all of its recursive
 894 /// subcandidates.
 895 fn traverse_candidate<'pat, 'tcx: 'pat, C, T, I>(
 896     candidate: C,
 897     context: &mut T,
 898     visit_leaf: &mut impl FnMut(C, &mut T),
 899     get_children: impl Copy + Fn(C, &mut T) -> I,
 900     complete_children: impl Copy + Fn(&mut T),
 901 ) where
 902     C: Borrow<Candidate<'pat, 'tcx>>,
 903     I: Iterator<Item = C>,
 904 {
 905     if candidate.borrow().subcandidates.is_empty() {
 906         visit_leaf(candidate, context)
 907     } else {
 908         for child in get_children(candidate, context) {
 909             traverse_candidate(child, context, visit_leaf, get_children, complete_children);
 910         }
 911         complete_children(context)
 912     }
 913 }
 914
 915 #[derive(Clone, Debug)]
 916 struct Binding<'tcx> {
 917     span: Span,
 918     source: Place<'tcx>,
 919     var_id: HirId,
 920     binding_mode: BindingMode,
 921 }
 922
 923 /// Indicates that the type of `source` must be a subtype of the
 924 /// user-given type `user_ty`; this is basically a no-op but can
 925 /// influence region inference.
 926 #[derive(Clone, Debug)]
 927 struct Ascription<'tcx> {
 928     span: Span,
 929     source: Place<'tcx>,
 930     user_ty: PatTyProj<'tcx>,
 931     variance: ty::Variance,
 932 }
 933
 934 #[derive(Clone, Debug)]
 935 crate struct MatchPair<'pat, 'tcx> {
 936     // this place...
 937     place: PlaceBuilder<'tcx>,
 938
 939     // ... must match this pattern.
 940     pattern: &'pat Pat<'tcx>,
 941 }
 942
 943 /// See [`Test`] for more.
 944 #[derive(Clone, Debug, PartialEq)]
 945 enum TestKind<'tcx> {
 946     /// Test what enum variant a value is.
 947     Switch {
 948         /// The enum type being tested.
 949         adt_def: &'tcx ty::AdtDef,
 950         /// The set of variants that we should create a branch for. We also
 951         /// create an additional "otherwise" case.
 952         variants: BitSet<VariantIdx>,
 953     },
 954
 955     /// Test what value an integer, `bool`, or `char` has.
 956     SwitchInt {
 957         /// The type of the value that we're testing.
 958         switch_ty: Ty<'tcx>,
 959         /// The (ordered) set of values that we test for.
 960         ///
 961         /// For integers and `char`s we create a branch to each of the values in
 962         /// `options`, as well as an "otherwise" branch for all other values, even
 963         /// in the (rare) case that `options` is exhaustive.
 964         ///
 965         /// For `bool` we always generate two edges, one for `true` and one for
 966         /// `false`.
 967         options: FxIndexMap<ty::Const<'tcx>, u128>,
 968     },
 969
 970     /// Test for equality with value, possibly after an unsizing coercion to
 971     /// `ty`,
 972     Eq {
 973         value: ty::Const<'tcx>,
 974         // Integer types are handled by `SwitchInt`, and constants with ADT
 975         // types are converted back into patterns, so this can only be `&str`,
 976         // `&[T]`, `f32` or `f64`.
 977         ty: Ty<'tcx>,
 978     },
 979
 980     /// Test whether the value falls within an inclusive or exclusive range
 981     Range(PatRange<'tcx>),
 982
 983     /// Test that the length of the slice is equal to `len`.
 984     Len { len: u64, op: BinOp },
 985 }
 986
 987 /// A test to perform to determine which [`Candidate`] matches a value.
 988 ///
 989 /// [`Test`] is just the test to perform; it does not include the value
 990 /// to be tested.
 991 #[derive(Debug)]
 992 crate struct Test<'tcx> {
 993     span: Span,
 994     kind: TestKind<'tcx>,
 995 }
 996
 997 /// `ArmHasGuard` is a wrapper around a boolean flag. It indicates whether
 998 /// a match arm has a guard expression attached to it.
 999 #[derive(Copy, Clone, Debug)]
1000 crate struct ArmHasGuard(crate bool);
1001
1002 ///////////////////////////////////////////////////////////////////////////
1003 // Main matching algorithm
1004
1005 impl<'a, 'tcx> Builder<'a, 'tcx> {
1006     /// The main match algorithm. It begins with a set of candidates
1007     /// `candidates` and has the job of generating code to determine
1008     /// which of these candidates, if any, is the correct one. The
1009     /// candidates are sorted such that the first item in the list
1010     /// has the highest priority. When a candidate is found to match
1011     /// the value, we will set and generate a branch to the appropriate
1012     /// pre-binding block.
1013     ///
1014     /// If we find that *NONE* of the candidates apply, we branch to the
1015     /// `otherwise_block`, setting it to `Some` if required. In principle, this
1016     /// means that the input list was not exhaustive, though at present we
1017     /// sometimes are not smart enough to recognize all exhaustive inputs.
1018     ///
1019     /// It might be surprising that the input can be non-exhaustive.
1020     /// Indeed, initially, it is not, because all matches are
1021     /// exhaustive in Rust. But during processing we sometimes divide
1022     /// up the list of candidates and recurse with a non-exhaustive
1023     /// list. This is important to keep the size of the generated code
1024     /// under control. See [`Builder::test_candidates`] for more details.
1025     ///
1026     /// If `fake_borrows` is `Some`, then places which need fake borrows
1027     /// will be added to it.
1028     ///
1029     /// For an example of a case where we set `otherwise_block`, even for an
1030     /// exhaustive match, consider:
1031     ///
1032     /// ```
1033     /// match x {
1034     ///     (true, true) => (),
1035     ///     (_, false) => (),
1036     ///     (false, true) => (),
1037     /// }
1038     /// ```
1039     ///
1040     /// For this match, we check if `x.0` matches `true` (for the first
1041     /// arm). If it doesn't match, we check `x.1`. If `x.1` is `true` we check
1042     /// if `x.0` matches `false` (for the third arm). In the (impossible at
1043     /// runtime) case when `x.0` is now `true`, we branch to
1044     /// `otherwise_block`.
1045     fn match_candidates<'pat>(
1046         &mut self,
1047         span: Span,
1048         scrutinee_span: Span,
1049         start_block: BasicBlock,
1050         otherwise_block: &mut Option<BasicBlock>,
1051         candidates: &mut [&mut Candidate<'pat, 'tcx>],
1052         fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>,
1053     ) {
1054         debug!(
1055             "matched_candidate(span={:?}, candidates={:?}, start_block={:?}, otherwise_block={:?})",
1056             span, candidates, start_block, otherwise_block,
1057         );
1058
1059         // Start by simplifying candidates. Once this process is complete, all
1060         // the match pairs which remain require some form of test, whether it
1061         // be a switch or pattern comparison.
1062         let mut split_or_candidate = false;
1063         for candidate in &mut *candidates {
1064             split_or_candidate |= self.simplify_candidate(candidate);
1065         }
1066
1067         ensure_sufficient_stack(|| {
1068             if split_or_candidate {
1069                 // At least one of the candidates has been split into subcandidates.
1070                 // We need to change the candidate list to include those.
1071                 let mut new_candidates = Vec::new();
1072
1073                 for candidate in candidates {
1074                     candidate.visit_leaves(|leaf_candidate| new_candidates.push(leaf_candidate));
1075                 }
1076                 self.match_simplified_candidates(
1077                     span,
1078                     scrutinee_span,
1079                     start_block,
1080                     otherwise_block,
1081                     &mut *new_candidates,
1082                     fake_borrows,
1083                 );
1084             } else {
1085                 self.match_simplified_candidates(
1086                     span,
1087                     scrutinee_span,
1088                     start_block,
1089                     otherwise_block,
1090                     candidates,
1091                     fake_borrows,
1092                 );
1093             }
1094         });
1095     }
1096
1097     fn match_simplified_candidates(
1098         &mut self,
1099         span: Span,
1100         scrutinee_span: Span,
1101         start_block: BasicBlock,
1102         otherwise_block: &mut Option<BasicBlock>,
1103         candidates: &mut [&mut Candidate<'_, 'tcx>],
1104         fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>,
1105     ) {
1106         // The candidates are sorted by priority. Check to see whether the
1107         // higher priority candidates (and hence at the front of the slice)
1108         // have satisfied all their match pairs.
1109         let fully_matched = candidates.iter().take_while(|c| c.match_pairs.is_empty()).count();
1110         debug!("match_candidates: {:?} candidates fully matched", fully_matched);
1111         let (matched_candidates, unmatched_candidates) = candidates.split_at_mut(fully_matched);
1112
1113         let block = if !matched_candidates.is_empty() {
1114             let otherwise_block =
1115                 self.select_matched_candidates(matched_candidates, start_block, fake_borrows);
1116
1117             if let Some(last_otherwise_block) = otherwise_block {
1118                 last_otherwise_block
1119             } else {
1120                 // Any remaining candidates are unreachable.
1121                 if unmatched_candidates.is_empty() {
1122                     return;
1123                 }
1124                 self.cfg.start_new_block()
1125             }
1126         } else {
1127             start_block
1128         };
1129
1130         // If there are no candidates that still need testing, we're
1131         // done. Since all matches are exhaustive, execution should
1132         // never reach this point.
1133         if unmatched_candidates.is_empty() {
1134             let source_info = self.source_info(span);
1135             if let Some(otherwise) = *otherwise_block {
1136                 self.cfg.goto(block, source_info, otherwise);
1137             } else {
1138                 *otherwise_block = Some(block);
1139             }
1140             return;
1141         }
1142
1143         // Test for the remaining candidates.
1144         self.test_candidates_with_or(
1145             span,
1146             scrutinee_span,
1147             unmatched_candidates,
1148             block,
1149             otherwise_block,
1150             fake_borrows,
1151         );
1152     }
1153
1154     /// Link up matched candidates.
1155     ///
1156     /// For example, if we have something like this:
1157     ///
1158     /// ```rust
1159     /// ...
1160     /// Some(x) if cond1 => ...
1161     /// Some(x) => ...
1162     /// Some(x) if cond2 => ...
1163     /// ...
1164     /// ```
1165     ///
1166     /// We generate real edges from:
1167     ///
1168     /// * `start_block` to the [pre-binding block] of the first pattern,
1169     /// * the [otherwise block] of the first pattern to the second pattern,
1170     /// * the [otherwise block] of the third pattern to a block with an
1171     ///   [`Unreachable` terminator](TerminatorKind::Unreachable).
1172     ///
1173     /// In addition, we add fake edges from the otherwise blocks to the
1174     /// pre-binding block of the next candidate in the original set of
1175     /// candidates.
1176     ///
1177     /// [pre-binding block]: Candidate::pre_binding_block
1178     /// [otherwise block]: Candidate::otherwise_block
1179     fn select_matched_candidates(
1180         &mut self,
1181         matched_candidates: &mut [&mut Candidate<'_, 'tcx>],
1182         start_block: BasicBlock,
1183         fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>,
1184     ) -> Option<BasicBlock> {
1185         debug_assert!(
1186             !matched_candidates.is_empty(),
1187             "select_matched_candidates called with no candidates",
1188         );
1189         debug_assert!(
1190             matched_candidates.iter().all(|c| c.subcandidates.is_empty()),
1191             "subcandidates should be empty in select_matched_candidates",
1192         );
1193
1194         // Insert a borrows of prefixes of places that are bound and are
1195         // behind a dereference projection.
1196         //
1197         // These borrows are taken to avoid situations like the following:
1198         //
1199         // match x[10] {
1200         //     _ if { x = &[0]; false } => (),
1201         //     y => (), // Out of bounds array access!
1202         // }
1203         //
1204         // match *x {
1205         //     // y is bound by reference in the guard and then by copy in the
1206         //     // arm, so y is 2 in the arm!
1207         //     y if { y == 1 && (x = &2) == () } => y,
1208         //     _ => 3,
1209         // }
1210         if let Some(fake_borrows) = fake_borrows {
1211             for Binding { source, .. } in
1212                 matched_candidates.iter().flat_map(|candidate| &candidate.bindings)
1213             {
1214                 if let Some(i) =
1215                     source.projection.iter().rposition(|elem| elem == ProjectionElem::Deref)
1216                 {
1217                     let proj_base = &source.projection[..i];
1218
1219                     fake_borrows.insert(Place {
1220                         local: source.local,
1221                         projection: self.tcx.intern_place_elems(proj_base),
1222                     });
1223                 }
1224             }
1225         }
1226
1227         let fully_matched_with_guard = matched_candidates
1228             .iter()
1229             .position(|c| !c.has_guard)
1230             .unwrap_or(matched_candidates.len() - 1);
1231
1232         let (reachable_candidates, unreachable_candidates) =
1233             matched_candidates.split_at_mut(fully_matched_with_guard + 1);
1234
1235         let mut next_prebinding = start_block;
1236
1237         for candidate in reachable_candidates.iter_mut() {
1238             assert!(candidate.otherwise_block.is_none());
1239             assert!(candidate.pre_binding_block.is_none());
1240             candidate.pre_binding_block = Some(next_prebinding);
1241             if candidate.has_guard {
1242                 // Create the otherwise block for this candidate, which is the
1243                 // pre-binding block for the next candidate.
1244                 next_prebinding = self.cfg.start_new_block();
1245                 candidate.otherwise_block = Some(next_prebinding);
1246             }
1247         }
1248
1249         debug!(
1250             "match_candidates: add pre_binding_blocks for unreachable {:?}",
1251             unreachable_candidates,
1252         );
1253         for candidate in unreachable_candidates {
1254             assert!(candidate.pre_binding_block.is_none());
1255             candidate.pre_binding_block = Some(self.cfg.start_new_block());
1256         }
1257
1258         reachable_candidates.last_mut().unwrap().otherwise_block
1259     }
1260
1261     /// Tests a candidate where there are only or-patterns left to test, or
1262     /// forwards to [Builder::test_candidates].
1263     ///
1264     /// Given a pattern `(P | Q, R | S)` we (in principle) generate a CFG like
1265     /// so:
1266     ///
1267     /// ```text
1268     /// [ start ]
1269     ///      |
1270     /// [ match P, Q ]
1271     ///      |
1272     ///      +----------------------------------------+------------------------------------+
1273     ///      |                                        |                                    |
1274     ///      V                                        V                                    V
1275     /// [ P matches ]                           [ Q matches ]                        [ otherwise ]
1276     ///      |                                        |                                    |
1277     ///      V                                        V                                    |
1278     /// [ match R, S ]                          [ match R, S ]                             |
1279     ///      |                                        |                                    |
1280     ///      +--------------+------------+            +--------------+------------+        |
1281     ///      |              |            |            |              |            |        |
1282     ///      V              V            V            V              V            V        |
1283     /// [ R matches ] [ S matches ] [otherwise ] [ R matches ] [ S matches ] [otherwise ]  |
1284     ///      |              |            |            |              |            |        |
1285     ///      +--------------+------------|------------+--------------+            |        |
1286     ///      |                           |                                        |        |
1287     ///      |                           +----------------------------------------+--------+
1288     ///      |                           |
1289     ///      V                           V
1290     /// [ Success ]                 [ Failure ]
1291     /// ```
1292     ///
1293     /// In practice there are some complications:
1294     ///
1295     /// * If there's a guard, then the otherwise branch of the first match on
1296     ///   `R | S` goes to a test for whether `Q` matches, and the control flow
1297     ///   doesn't merge into a single success block until after the guard is
1298     ///   tested.
1299     /// * If neither `P` or `Q` has any bindings or type ascriptions and there
1300     ///   isn't a match guard, then we create a smaller CFG like:
1301     ///
1302     /// ```text
1303     ///     ...
1304     ///      +---------------+------------+
1305     ///      |               |            |
1306     /// [ P matches ] [ Q matches ] [ otherwise ]
1307     ///      |               |            |
1308     ///      +---------------+            |
1309     ///      |                           ...
1310     /// [ match R, S ]
1311     ///      |
1312     ///     ...
1313     /// ```
1314     fn test_candidates_with_or(
1315         &mut self,
1316         span: Span,
1317         scrutinee_span: Span,
1318         candidates: &mut [&mut Candidate<'_, 'tcx>],
1319         block: BasicBlock,
1320         otherwise_block: &mut Option<BasicBlock>,
1321         fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>,
1322     ) {
1323         let (first_candidate, remaining_candidates) = candidates.split_first_mut().unwrap();
1324
1325         // All of the or-patterns have been sorted to the end, so if the first
1326         // pattern is an or-pattern we only have or-patterns.
1327         match *first_candidate.match_pairs[0].pattern.kind {
1328             PatKind::Or { .. } => (),
1329             _ => {
1330                 self.test_candidates(
1331                     span,
1332                     scrutinee_span,
1333                     candidates,
1334                     block,
1335                     otherwise_block,
1336                     fake_borrows,
1337                 );
1338                 return;
1339             }
1340         }
1341
1342         let match_pairs = mem::take(&mut first_candidate.match_pairs);
1343         first_candidate.pre_binding_block = Some(block);
1344
1345         let mut otherwise = None;
1346         for match_pair in match_pairs {
1347             let PatKind::Or { ref pats } = &*match_pair.pattern.kind else {
1348                 bug!("Or-patterns should have been sorted to the end");
1349             };
1350             let or_span = match_pair.pattern.span;
1351             let place = match_pair.place;
1352
1353             first_candidate.visit_leaves(|leaf_candidate| {
1354                 self.test_or_pattern(
1355                     leaf_candidate,
1356                     &mut otherwise,
1357                     pats,
1358                     or_span,
1359                     place.clone(),
1360                     fake_borrows,
1361                 );
1362             });
1363         }
1364
1365         let remainder_start = otherwise.unwrap_or_else(|| self.cfg.start_new_block());
1366
1367         self.match_candidates(
1368             span,
1369             scrutinee_span,
1370             remainder_start,
1371             otherwise_block,
1372             remaining_candidates,
1373             fake_borrows,
1374         )
1375     }
1376
1377     fn test_or_pattern<'pat>(
1378         &mut self,
1379         candidate: &mut Candidate<'pat, 'tcx>,
1380         otherwise: &mut Option<BasicBlock>,
1381         pats: &'pat [Pat<'tcx>],
1382         or_span: Span,
1383         place: PlaceBuilder<'tcx>,
1384         fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>,
1385     ) {
1386         debug!("test_or_pattern:\ncandidate={:#?}\npats={:#?}", candidate, pats);
1387         let mut or_candidates: Vec<_> = pats
1388             .iter()
1389             .map(|pat| Candidate::new(place.clone(), pat, candidate.has_guard))
1390             .collect();
1391         let mut or_candidate_refs: Vec<_> = or_candidates.iter_mut().collect();
1392         let otherwise = if candidate.otherwise_block.is_some() {
1393             &mut candidate.otherwise_block
1394         } else {
1395             otherwise
1396         };
1397         self.match_candidates(
1398             or_span,
1399             or_span,
1400             candidate.pre_binding_block.unwrap(),
1401             otherwise,
1402             &mut or_candidate_refs,
1403             fake_borrows,
1404         );
1405         candidate.subcandidates = or_candidates;
1406         self.merge_trivial_subcandidates(candidate, self.source_info(or_span));
1407     }
1408
1409     /// Try to merge all of the subcandidates of the given candidate into one.
1410     /// This avoids exponentially large CFGs in cases like `(1 | 2, 3 | 4, ...)`.
1411     fn merge_trivial_subcandidates(
1412         &mut self,
1413         candidate: &mut Candidate<'_, 'tcx>,
1414         source_info: SourceInfo,
1415     ) {
1416         if candidate.subcandidates.is_empty() || candidate.has_guard {
1417             // FIXME(or_patterns; matthewjasper) Don't give up if we have a guard.
1418             return;
1419         }
1420
1421         let mut can_merge = true;
1422
1423         // Not `Iterator::all` because we don't want to short-circuit.
1424         for subcandidate in &mut candidate.subcandidates {
1425             self.merge_trivial_subcandidates(subcandidate, source_info);
1426
1427             // FIXME(or_patterns; matthewjasper) Try to be more aggressive here.
1428             can_merge &= subcandidate.subcandidates.is_empty()
1429                 && subcandidate.bindings.is_empty()
1430                 && subcandidate.ascriptions.is_empty();
1431         }
1432
1433         if can_merge {
1434             let any_matches = self.cfg.start_new_block();
1435             for subcandidate in mem::take(&mut candidate.subcandidates) {
1436                 let or_block = subcandidate.pre_binding_block.unwrap();
1437                 self.cfg.goto(or_block, source_info, any_matches);
1438             }
1439             candidate.pre_binding_block = Some(any_matches);
1440         }
1441     }
1442
1443     /// This is the most subtle part of the matching algorithm. At
1444     /// this point, the input candidates have been fully simplified,
1445     /// and so we know that all remaining match-pairs require some
1446     /// sort of test. To decide what test to perform, we take the highest
1447     /// priority candidate (the first one in the list, as of January 2021)
1448     /// and extract the first match-pair from the list. From this we decide
1449     /// what kind of test is needed using [`Builder::test`], defined in the
1450     /// [`test` module](mod@test).
1451     ///
1452     /// *Note:* taking the first match pair is somewhat arbitrary, and
1453     /// we might do better here by choosing more carefully what to
1454     /// test.
1455     ///
1456     /// For example, consider the following possible match-pairs:
1457     ///
1458     /// 1. `x @ Some(P)` -- we will do a [`Switch`] to decide what variant `x` has
1459     /// 2. `x @ 22` -- we will do a [`SwitchInt`] to decide what value `x` has
1460     /// 3. `x @ 3..5` -- we will do a [`Range`] test to decide what range `x` falls in
1461     /// 4. etc.
1462     ///
1463     /// [`Switch`]: TestKind::Switch
1464     /// [`SwitchInt`]: TestKind::SwitchInt
1465     /// [`Range`]: TestKind::Range
1466     ///
1467     /// Once we know what sort of test we are going to perform, this
1468     /// test may also help us winnow down our candidates. So we walk over
1469     /// the candidates (from high to low priority) and check. This
1470     /// gives us, for each outcome of the test, a transformed list of
1471     /// candidates. For example, if we are testing `x.0`'s variant,
1472     /// and we have a candidate `(x.0 @ Some(v), x.1 @ 22)`,
1473     /// then we would have a resulting candidate of `((x.0 as Some).0 @ v, x.1 @ 22)`.
1474     /// Note that the first match-pair is now simpler (and, in fact, irrefutable).
1475     ///
1476     /// But there may also be candidates that the test just doesn't
1477     /// apply to. The classical example involves wildcards:
1478     ///
1479     /// ```
1480     /// # let (x, y, z) = (true, true, true);
1481     /// match (x, y, z) {
1482     ///     (true, _, true) => true,    // (0)
1483     ///     (_, true, _) => true,       // (1)
1484     ///     (false, false, _) => false, // (2)
1485     ///     (true, _, false) => false,  // (3)
1486     /// }
1487     /// ```
1488     ///
1489     /// In that case, after we test on `x`, there are 2 overlapping candidate
1490     /// sets:
1491     ///
1492     /// - If the outcome is that `x` is true, candidates 0, 1, and 3
1493     /// - If the outcome is that `x` is false, candidates 1 and 2
1494     ///
1495     /// Here, the traditional "decision tree" method would generate 2
1496     /// separate code-paths for the 2 separate cases.
1497     ///
1498     /// In some cases, this duplication can create an exponential amount of
1499     /// code. This is most easily seen by noticing that this method terminates
1500     /// with precisely the reachable arms being reachable - but that problem
1501     /// is trivially NP-complete:
1502     ///
1503     /// ```rust
1504     ///     match (var0, var1, var2, var3, ...) {
1505     ///         (true, _, _, false, true, ...) => false,
1506     ///         (_, true, true, false, _, ...) => false,
1507     ///         (false, _, false, false, _, ...) => false,
1508     ///         ...
1509     ///         _ => true
1510     ///     }
1511     /// ```
1512     ///
1513     /// Here the last arm is reachable only if there is an assignment to
1514     /// the variables that does not match any of the literals. Therefore,
1515     /// compilation would take an exponential amount of time in some cases.
1516     ///
1517     /// That kind of exponential worst-case might not occur in practice, but
1518     /// our simplistic treatment of constants and guards would make it occur
1519     /// in very common situations - for example [#29740]:
1520     ///
1521     /// ```rust
1522     /// match x {
1523     ///     "foo" if foo_guard => ...,
1524     ///     "bar" if bar_guard => ...,
1525     ///     "baz" if baz_guard => ...,
1526     ///     ...
1527     /// }
1528     /// ```
1529     ///
1530     /// [#29740]: https://github.com/rust-lang/rust/issues/29740
1531     ///
1532     /// Here we first test the match-pair `x @ "foo"`, which is an [`Eq` test].
1533     ///
1534     /// [`Eq` test]: TestKind::Eq
1535     ///
1536     /// It might seem that we would end up with 2 disjoint candidate
1537     /// sets, consisting of the first candidate or the other two, but our
1538     /// algorithm doesn't reason about `"foo"` being distinct from the other
1539     /// constants; it considers the latter arms to potentially match after
1540     /// both outcomes, which obviously leads to an exponential number
1541     /// of tests.
1542     ///
1543     /// To avoid these kinds of problems, our algorithm tries to ensure
1544     /// the amount of generated tests is linear. When we do a k-way test,
1545     /// we return an additional "unmatched" set alongside the obvious `k`
1546     /// sets. When we encounter a candidate that would be present in more
1547     /// than one of the sets, we put it and all candidates below it into the
1548     /// "unmatched" set. This ensures these `k+1` sets are disjoint.
1549     ///
1550     /// After we perform our test, we branch into the appropriate candidate
1551     /// set and recurse with `match_candidates`. These sub-matches are
1552     /// obviously non-exhaustive - as we discarded our otherwise set - so
1553     /// we set their continuation to do `match_candidates` on the
1554     /// "unmatched" set (which is again non-exhaustive).
1555     ///
1556     /// If you apply this to the above test, you basically wind up
1557     /// with an if-else-if chain, testing each candidate in turn,
1558     /// which is precisely what we want.
1559     ///
1560     /// In addition to avoiding exponential-time blowups, this algorithm
1561     /// also has the nice property that each guard and arm is only generated
1562     /// once.
1563     fn test_candidates<'pat, 'b, 'c>(
1564         &mut self,
1565         span: Span,
1566         scrutinee_span: Span,
1567         mut candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>],
1568         block: BasicBlock,
1569         otherwise_block: &mut Option<BasicBlock>,
1570         fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>,
1571     ) {
1572         // extract the match-pair from the highest priority candidate
1573         let match_pair = &candidates.first().unwrap().match_pairs[0];
1574         let mut test = self.test(match_pair);
1575         let match_place = match_pair.place.clone();
1576
1577         // most of the time, the test to perform is simply a function
1578         // of the main candidate; but for a test like SwitchInt, we
1579         // may want to add cases based on the candidates that are
1580         // available
1581         match test.kind {
1582             TestKind::SwitchInt { switch_ty, ref mut options } => {
1583                 for candidate in candidates.iter() {
1584                     if !self.add_cases_to_switch(&match_place, candidate, switch_ty, options) {
1585                         break;
1586                     }
1587                 }
1588             }
1589             TestKind::Switch { adt_def: _, ref mut variants } => {
1590                 for candidate in candidates.iter() {
1591                     if !self.add_variants_to_switch(&match_place, candidate, variants) {
1592                         break;
1593                     }
1594                 }
1595             }
1596             _ => {}
1597         }
1598
1599         // Insert a Shallow borrow of any places that is switched on.
1600         if let Some(fb) = fake_borrows {
1601             if let Ok(match_place_resolved) =
1602                 match_place.clone().try_upvars_resolved(self.tcx, self.typeck_results)
1603             {
1604                 let resolved_place = match_place_resolved.into_place(self.tcx, self.typeck_results);
1605                 fb.insert(resolved_place);
1606             }
1607         }
1608
1609         // perform the test, branching to one of N blocks. For each of
1610         // those N possible outcomes, create a (initially empty)
1611         // vector of candidates. Those are the candidates that still
1612         // apply if the test has that particular outcome.
1613         debug!("match_candidates: test={:?} match_pair={:?}", test, match_pair);
1614         let mut target_candidates: Vec<Vec<&mut Candidate<'pat, 'tcx>>> = vec![];
1615         target_candidates.resize_with(test.targets(), Default::default);
1616
1617         let total_candidate_count = candidates.len();
1618
1619         // Sort the candidates into the appropriate vector in
1620         // `target_candidates`. Note that at some point we may
1621         // encounter a candidate where the test is not relevant; at
1622         // that point, we stop sorting.
1623         while let Some(candidate) = candidates.first_mut() {
1624             let Some(idx) = self.sort_candidate(&match_place.clone(), &test, candidate) else {
1625                 break;
1626             };
1627             let (candidate, rest) = candidates.split_first_mut().unwrap();
1628             target_candidates[idx].push(candidate);
1629             candidates = rest;
1630         }
1631         // at least the first candidate ought to be tested
1632         assert!(total_candidate_count > candidates.len());
1633         debug!("tested_candidates: {}", total_candidate_count - candidates.len());
1634         debug!("untested_candidates: {}", candidates.len());
1635
1636         // HACK(matthewjasper) This is a closure so that we can let the test
1637         // create its blocks before the rest of the match. This currently
1638         // improves the speed of llvm when optimizing long string literal
1639         // matches
1640         let make_target_blocks = move |this: &mut Self| -> Vec<BasicBlock> {
1641             // The block that we should branch to if none of the
1642             // `target_candidates` match. This is either the block where we
1643             // start matching the untested candidates if there are any,
1644             // otherwise it's the `otherwise_block`.
1645             let remainder_start = &mut None;
1646             let remainder_start =
1647                 if candidates.is_empty() { &mut *otherwise_block } else { remainder_start };
1648
1649             // For each outcome of test, process the candidates that still
1650             // apply. Collect a list of blocks where control flow will
1651             // branch if one of the `target_candidate` sets is not
1652             // exhaustive.
1653             let target_blocks: Vec<_> = target_candidates
1654                 .into_iter()
1655                 .map(|mut candidates| {
1656                     if !candidates.is_empty() {
1657                         let candidate_start = this.cfg.start_new_block();
1658                         this.match_candidates(
1659                             span,
1660                             scrutinee_span,
1661                             candidate_start,
1662                             remainder_start,
1663                             &mut *candidates,
1664                             fake_borrows,
1665                         );
1666                         candidate_start
1667                     } else {
1668                         *remainder_start.get_or_insert_with(|| this.cfg.start_new_block())
1669                     }
1670                 })
1671                 .collect();
1672
1673             if !candidates.is_empty() {
1674                 let remainder_start = remainder_start.unwrap_or_else(|| this.cfg.start_new_block());
1675                 this.match_candidates(
1676                     span,
1677                     scrutinee_span,
1678                     remainder_start,
1679                     otherwise_block,
1680                     candidates,
1681                     fake_borrows,
1682                 );
1683             };
1684
1685             target_blocks
1686         };
1687
1688         self.perform_test(span, scrutinee_span, block, match_place, &test, make_target_blocks);
1689     }
1690
1691     /// Determine the fake borrows that are needed from a set of places that
1692     /// have to be stable across match guards.
1693     ///
1694     /// Returns a list of places that need a fake borrow and the temporary
1695     /// that's used to store the fake borrow.
1696     ///
1697     /// Match exhaustiveness checking is not able to handle the case where the
1698     /// place being matched on is mutated in the guards. We add "fake borrows"
1699     /// to the guards that prevent any mutation of the place being matched.
1700     /// There are a some subtleties:
1701     ///
1702     /// 1. Borrowing `*x` doesn't prevent assigning to `x`. If `x` is a shared
1703     ///    reference, the borrow isn't even tracked. As such we have to add fake
1704     ///    borrows of any prefixes of a place
1705     /// 2. We don't want `match x { _ => (), }` to conflict with mutable
1706     ///    borrows of `x`, so we only add fake borrows for places which are
1707     ///    bound or tested by the match.
1708     /// 3. We don't want the fake borrows to conflict with `ref mut` bindings,
1709     ///    so we use a special BorrowKind for them.
1710     /// 4. The fake borrows may be of places in inactive variants, so it would
1711     ///    be UB to generate code for them. They therefore have to be removed
1712     ///    by a MIR pass run after borrow checking.
1713     fn calculate_fake_borrows<'b>(
1714         &mut self,
1715         fake_borrows: &'b FxHashSet<Place<'tcx>>,
1716         temp_span: Span,
1717     ) -> Vec<(Place<'tcx>, Local)> {
1718         let tcx = self.tcx;
1719
1720         debug!("add_fake_borrows fake_borrows = {:?}", fake_borrows);
1721
1722         let mut all_fake_borrows = Vec::with_capacity(fake_borrows.len());
1723
1724         // Insert a Shallow borrow of the prefixes of any fake borrows.
1725         for place in fake_borrows {
1726             let mut cursor = place.projection.as_ref();
1727             while let [proj_base @ .., elem] = cursor {
1728                 cursor = proj_base;
1729
1730                 if let ProjectionElem::Deref = elem {
1731                     // Insert a shallow borrow after a deref. For other
1732                     // projections the borrow of prefix_cursor will
1733                     // conflict with any mutation of base.
1734                     all_fake_borrows.push(PlaceRef { local: place.local, projection: proj_base });
1735                 }
1736             }
1737
1738             all_fake_borrows.push(place.as_ref());
1739         }
1740
1741         // Deduplicate and ensure a deterministic order.
1742         all_fake_borrows.sort();
1743         all_fake_borrows.dedup();
1744
1745         debug!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows);
1746
1747         all_fake_borrows
1748             .into_iter()
1749             .map(|matched_place_ref| {
1750                 let matched_place = Place {
1751                     local: matched_place_ref.local,
1752                     projection: tcx.intern_place_elems(matched_place_ref.projection),
1753                 };
1754                 let fake_borrow_deref_ty = matched_place.ty(&self.local_decls, tcx).ty;
1755                 let fake_borrow_ty = tcx.mk_imm_ref(tcx.lifetimes.re_erased, fake_borrow_deref_ty);
1756                 let fake_borrow_temp =
1757                     self.local_decls.push(LocalDecl::new(fake_borrow_ty, temp_span));
1758
1759                 (matched_place, fake_borrow_temp)
1760             })
1761             .collect()
1762     }
1763 }
1764
1765 ///////////////////////////////////////////////////////////////////////////
1766 // Pat binding - used for `let` and function parameters as well.
1767
1768 impl<'a, 'tcx> Builder<'a, 'tcx> {
1769     crate fn lower_let_expr(
1770         &mut self,
1771         mut block: BasicBlock,
1772         expr: &Expr<'tcx>,
1773         pat: &Pat<'tcx>,
1774         else_target: region::Scope,
1775         span: Span,
1776     ) -> BlockAnd<()> {
1777         let expr_span = expr.span;
1778         let expr_place_builder = unpack!(block = self.lower_scrutinee(block, expr, expr_span));
1779         let wildcard = Pat::wildcard_from_ty(pat.ty);
1780         let mut guard_candidate = Candidate::new(expr_place_builder.clone(), &pat, false);
1781         let mut otherwise_candidate = Candidate::new(expr_place_builder.clone(), &wildcard, false);
1782         let fake_borrow_temps = self.lower_match_tree(
1783             block,
1784             pat.span,
1785             pat.span,
1786             false,
1787             &mut [&mut guard_candidate, &mut otherwise_candidate],
1788         );
1789         let mut opt_expr_place: Option<(Option<&Place<'tcx>>, Span)> = None;
1790         let expr_place: Place<'tcx>;
1791         if let Ok(expr_builder) =
1792             expr_place_builder.try_upvars_resolved(self.tcx, self.typeck_results)
1793         {
1794             expr_place = expr_builder.into_place(self.tcx, self.typeck_results);
1795             opt_expr_place = Some((Some(&expr_place), expr_span));
1796         }
1797         let otherwise_post_guard_block = otherwise_candidate.pre_binding_block.unwrap();
1798         self.break_for_else(otherwise_post_guard_block, else_target, self.source_info(expr_span));
1799
1800         self.declare_bindings(None, pat.span.to(span), pat, ArmHasGuard(false), opt_expr_place);
1801         let post_guard_block = self.bind_pattern(
1802             self.source_info(pat.span),
1803             guard_candidate,
1804             None,
1805             &fake_borrow_temps,
1806             expr.span,
1807             None,
1808             None,
1809             None,
1810         );
1811
1812         post_guard_block.unit()
1813     }
1814
1815     /// Initializes each of the bindings from the candidate by
1816     /// moving/copying/ref'ing the source as appropriate. Tests the guard, if
1817     /// any, and then branches to the arm. Returns the block for the case where
1818     /// the guard succeeds.
1819     ///
1820     /// Note: we do not check earlier that if there is a guard,
1821     /// there cannot be move bindings. We avoid a use-after-move by only
1822     /// moving the binding once the guard has evaluated to true (see below).
1823     fn bind_and_guard_matched_candidate<'pat>(
1824         &mut self,
1825         candidate: Candidate<'pat, 'tcx>,
1826         parent_bindings: &[(Vec<Binding<'tcx>>, Vec<Ascription<'tcx>>)],
1827         guard: Option<&Guard<'tcx>>,
1828         fake_borrows: &Vec<(Place<'tcx>, Local)>,
1829         scrutinee_span: Span,
1830         arm_span: Option<Span>,
1831         match_scope: Option<region::Scope>,
1832         schedule_drops: bool,
1833     ) -> BasicBlock {
1834         debug!("bind_and_guard_matched_candidate(candidate={:?})", candidate);
1835
1836         debug_assert!(candidate.match_pairs.is_empty());
1837
1838         let candidate_source_info = self.source_info(candidate.span);
1839
1840         let mut block = candidate.pre_binding_block.unwrap();
1841
1842         if candidate.next_candidate_pre_binding_block.is_some() {
1843             let fresh_block = self.cfg.start_new_block();
1844             self.false_edges(
1845                 block,
1846                 fresh_block,
1847                 candidate.next_candidate_pre_binding_block,
1848                 candidate_source_info,
1849             );
1850             block = fresh_block;
1851         }
1852
1853         self.ascribe_types(
1854             block,
1855             parent_bindings
1856                 .iter()
1857                 .flat_map(|(_, ascriptions)| ascriptions)
1858                 .chain(&candidate.ascriptions),
1859         );
1860
1861         // rust-lang/rust#27282: The `autoref` business deserves some
1862         // explanation here.
1863         //
1864         // The intent of the `autoref` flag is that when it is true,
1865         // then any pattern bindings of type T will map to a `&T`
1866         // within the context of the guard expression, but will
1867         // continue to map to a `T` in the context of the arm body. To
1868         // avoid surfacing this distinction in the user source code
1869         // (which would be a severe change to the language and require
1870         // far more revision to the compiler), when `autoref` is true,
1871         // then any occurrence of the identifier in the guard
1872         // expression will automatically get a deref op applied to it.
1873         //
1874         // So an input like:
1875         //
1876         // ```
1877         // let place = Foo::new();
1878         // match place { foo if inspect(foo)
1879         //     => feed(foo), ...  }
1880         // ```
1881         //
1882         // will be treated as if it were really something like:
1883         //
1884         // ```
1885         // let place = Foo::new();
1886         // match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) }
1887         //     => { let tmp2 = place; feed(tmp2) }, ... }
1888         //
1889         // And an input like:
1890         //
1891         // ```
1892         // let place = Foo::new();
1893         // match place { ref mut foo if inspect(foo)
1894         //     => feed(foo), ...  }
1895         // ```
1896         //
1897         // will be treated as if it were really something like:
1898         //
1899         // ```
1900         // let place = Foo::new();
1901         // match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) }
1902         //     => { let tmp2 = &mut place; feed(tmp2) }, ... }
1903         // ```
1904         //
1905         // In short, any pattern binding will always look like *some*
1906         // kind of `&T` within the guard at least in terms of how the
1907         // MIR-borrowck views it, and this will ensure that guard
1908         // expressions cannot mutate their the match inputs via such
1909         // bindings. (It also ensures that guard expressions can at
1910         // most *copy* values from such bindings; non-Copy things
1911         // cannot be moved via pattern bindings in guard expressions.)
1912         //
1913         // ----
1914         //
1915         // Implementation notes (under assumption `autoref` is true).
1916         //
1917         // To encode the distinction above, we must inject the
1918         // temporaries `tmp1` and `tmp2`.
1919         //
1920         // There are two cases of interest: binding by-value, and binding by-ref.
1921         //
1922         // 1. Binding by-value: Things are simple.
1923         //
1924         //    * Establishing `tmp1` creates a reference into the
1925         //      matched place. This code is emitted by
1926         //      bind_matched_candidate_for_guard.
1927         //
1928         //    * `tmp2` is only initialized "lazily", after we have
1929         //      checked the guard. Thus, the code that can trigger
1930         //      moves out of the candidate can only fire after the
1931         //      guard evaluated to true. This initialization code is
1932         //      emitted by bind_matched_candidate_for_arm.
1933         //
1934         // 2. Binding by-reference: Things are tricky.
1935         //
1936         //    * Here, the guard expression wants a `&&` or `&&mut`
1937         //      into the original input. This means we need to borrow
1938         //      the reference that we create for the arm.
1939         //    * So we eagerly create the reference for the arm and then take a
1940         //      reference to that.
1941         if let Some(guard) = guard {
1942             let tcx = self.tcx;
1943             let bindings = parent_bindings
1944                 .iter()
1945                 .flat_map(|(bindings, _)| bindings)
1946                 .chain(&candidate.bindings);
1947
1948             self.bind_matched_candidate_for_guard(block, schedule_drops, bindings.clone());
1949             let guard_frame = GuardFrame {
1950                 locals: bindings.map(|b| GuardFrameLocal::new(b.var_id, b.binding_mode)).collect(),
1951             };
1952             debug!("entering guard building context: {:?}", guard_frame);
1953             self.guard_context.push(guard_frame);
1954
1955             let re_erased = tcx.lifetimes.re_erased;
1956             let scrutinee_source_info = self.source_info(scrutinee_span);
1957             for &(place, temp) in fake_borrows {
1958                 let borrow = Rvalue::Ref(re_erased, BorrowKind::Shallow, place);
1959                 self.cfg.push_assign(block, scrutinee_source_info, Place::from(temp), borrow);
1960             }
1961
1962             let arm_span = arm_span.unwrap();
1963             let match_scope = match_scope.unwrap();
1964             let mut guard_span = rustc_span::DUMMY_SP;
1965
1966             let (post_guard_block, otherwise_post_guard_block) =
1967                 self.in_if_then_scope(match_scope, |this| match *guard {
1968                     Guard::If(e) => {
1969                         let e = &this.thir[e];
1970                         guard_span = e.span;
1971                         this.then_else_break(block, e, None, match_scope, arm_span)
1972                     }
1973                     Guard::IfLet(ref pat, scrutinee) => {
1974                         let s = &this.thir[scrutinee];
1975                         guard_span = s.span;
1976                         this.lower_let_expr(block, s, pat, match_scope, arm_span)
1977                     }
1978                 });
1979
1980             let source_info = self.source_info(guard_span);
1981             let guard_end = self.source_info(tcx.sess.source_map().end_point(guard_span));
1982             let guard_frame = self.guard_context.pop().unwrap();
1983             debug!("Exiting guard building context with locals: {:?}", guard_frame);
1984
1985             for &(_, temp) in fake_borrows {
1986                 let cause = FakeReadCause::ForMatchGuard;
1987                 self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(temp));
1988             }
1989
1990             let otherwise_block = candidate.otherwise_block.unwrap_or_else(|| {
1991                 let unreachable = self.cfg.start_new_block();
1992                 self.cfg.terminate(unreachable, source_info, TerminatorKind::Unreachable);
1993                 unreachable
1994             });
1995             self.false_edges(
1996                 otherwise_post_guard_block,
1997                 otherwise_block,
1998                 candidate.next_candidate_pre_binding_block,
1999                 source_info,
2000             );
2001
2002             // We want to ensure that the matched candidates are bound
2003             // after we have confirmed this candidate *and* any
2004             // associated guard; Binding them on `block` is too soon,
2005             // because that would be before we've checked the result
2006             // from the guard.
2007             //
2008             // But binding them on the arm is *too late*, because
2009             // then all of the candidates for a single arm would be
2010             // bound in the same place, that would cause a case like:
2011             //
2012             // ```rust
2013             // match (30, 2) {
2014             //     (mut x, 1) | (2, mut x) if { true } => { ... }
2015             //     ...                                 // ^^^^^^^ (this is `arm_block`)
2016             // }
2017             // ```
2018             //
2019             // would yield an `arm_block` something like:
2020             //
2021             // ```
2022             // StorageLive(_4);        // _4 is `x`
2023             // _4 = &mut (_1.0: i32);  // this is handling `(mut x, 1)` case
2024             // _4 = &mut (_1.1: i32);  // this is handling `(2, mut x)` case
2025             // ```
2026             //
2027             // and that is clearly not correct.
2028             let by_value_bindings = parent_bindings
2029                 .iter()
2030                 .flat_map(|(bindings, _)| bindings)
2031                 .chain(&candidate.bindings)
2032                 .filter(|binding| matches!(binding.binding_mode, BindingMode::ByValue));
2033             // Read all of the by reference bindings to ensure that the
2034             // place they refer to can't be modified by the guard.
2035             for binding in by_value_bindings.clone() {
2036                 let local_id = self.var_local_id(binding.var_id, RefWithinGuard);
2037                 let cause = FakeReadCause::ForGuardBinding;
2038                 self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(local_id));
2039             }
2040             assert!(schedule_drops, "patterns with guards must schedule drops");
2041             self.bind_matched_candidate_for_arm_body(post_guard_block, true, by_value_bindings);
2042
2043             post_guard_block
2044         } else {
2045             // (Here, it is not too early to bind the matched
2046             // candidate on `block`, because there is no guard result
2047             // that we have to inspect before we bind them.)
2048             self.bind_matched_candidate_for_arm_body(
2049                 block,
2050                 schedule_drops,
2051                 parent_bindings
2052                     .iter()
2053                     .flat_map(|(bindings, _)| bindings)
2054                     .chain(&candidate.bindings),
2055             );
2056             block
2057         }
2058     }
2059
2060     /// Append `AscribeUserType` statements onto the end of `block`
2061     /// for each ascription
2062     fn ascribe_types<'b>(
2063         &mut self,
2064         block: BasicBlock,
2065         ascriptions: impl IntoIterator<Item = &'b Ascription<'tcx>>,
2066     ) where
2067         'tcx: 'b,
2068     {
2069         for ascription in ascriptions {
2070             let source_info = self.source_info(ascription.span);
2071
2072             debug!(
2073                 "adding user ascription at span {:?} of place {:?} and {:?}",
2074                 source_info.span, ascription.source, ascription.user_ty,
2075             );
2076
2077             let user_ty = ascription.user_ty.user_ty(
2078                 &mut self.canonical_user_type_annotations,
2079                 ascription.source.ty(&self.local_decls, self.tcx).ty,
2080                 source_info.span,
2081             );
2082             self.cfg.push(
2083                 block,
2084                 Statement {
2085                     source_info,
2086                     kind: StatementKind::AscribeUserType(
2087                         Box::new((ascription.source, user_ty)),
2088                         ascription.variance,
2089                     ),
2090                 },
2091             );
2092         }
2093     }
2094
2095     fn bind_matched_candidate_for_guard<'b>(
2096         &mut self,
2097         block: BasicBlock,
2098         schedule_drops: bool,
2099         bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
2100     ) where
2101         'tcx: 'b,
2102     {
2103         debug!("bind_matched_candidate_for_guard(block={:?})", block);
2104
2105         // Assign each of the bindings. Since we are binding for a
2106         // guard expression, this will never trigger moves out of the
2107         // candidate.
2108         let re_erased = self.tcx.lifetimes.re_erased;
2109         for binding in bindings {
2110             debug!("bind_matched_candidate_for_guard(binding={:?})", binding);
2111             let source_info = self.source_info(binding.span);
2112
2113             // For each pattern ident P of type T, `ref_for_guard` is
2114             // a reference R: &T pointing to the location matched by
2115             // the pattern, and every occurrence of P within a guard
2116             // denotes *R.
2117             let ref_for_guard = self.storage_live_binding(
2118                 block,
2119                 binding.var_id,
2120                 binding.span,
2121                 RefWithinGuard,
2122                 schedule_drops,
2123             );
2124             match binding.binding_mode {
2125                 BindingMode::ByValue => {
2126                     let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, binding.source);
2127                     self.cfg.push_assign(block, source_info, ref_for_guard, rvalue);
2128                 }
2129                 BindingMode::ByRef(borrow_kind) => {
2130                     let value_for_arm = self.storage_live_binding(
2131                         block,
2132                         binding.var_id,
2133                         binding.span,
2134                         OutsideGuard,
2135                         schedule_drops,
2136                     );
2137
2138                     let rvalue = Rvalue::Ref(re_erased, borrow_kind, binding.source);
2139                     self.cfg.push_assign(block, source_info, value_for_arm, rvalue);
2140                     let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, value_for_arm);
2141                     self.cfg.push_assign(block, source_info, ref_for_guard, rvalue);
2142                 }
2143             }
2144         }
2145     }
2146
2147     fn bind_matched_candidate_for_arm_body<'b>(
2148         &mut self,
2149         block: BasicBlock,
2150         schedule_drops: bool,
2151         bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
2152     ) where
2153         'tcx: 'b,
2154     {
2155         debug!("bind_matched_candidate_for_arm_body(block={:?})", block);
2156
2157         let re_erased = self.tcx.lifetimes.re_erased;
2158         // Assign each of the bindings. This may trigger moves out of the candidate.
2159         for binding in bindings {
2160             let source_info = self.source_info(binding.span);
2161             let local = self.storage_live_binding(
2162                 block,
2163                 binding.var_id,
2164                 binding.span,
2165                 OutsideGuard,
2166                 schedule_drops,
2167             );
2168             if schedule_drops {
2169                 self.schedule_drop_for_binding(binding.var_id, binding.span, OutsideGuard);
2170             }
2171             let rvalue = match binding.binding_mode {
2172                 BindingMode::ByValue => Rvalue::Use(self.consume_by_copy_or_move(binding.source)),
2173                 BindingMode::ByRef(borrow_kind) => {
2174                     Rvalue::Ref(re_erased, borrow_kind, binding.source)
2175                 }
2176             };
2177             self.cfg.push_assign(block, source_info, local, rvalue);
2178         }
2179     }
2180
2181     /// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where the bound
2182     /// `var` has type `T` in the arm body) in a pattern maps to 2 locals. The
2183     /// first local is a binding for occurrences of `var` in the guard, which
2184     /// will have type `&T`. The second local is a binding for occurrences of
2185     /// `var` in the arm body, which will have type `T`.
2186     fn declare_binding(
2187         &mut self,
2188         source_info: SourceInfo,
2189         visibility_scope: SourceScope,
2190         mutability: Mutability,
2191         name: Symbol,
2192         mode: BindingMode,
2193         var_id: HirId,
2194         var_ty: Ty<'tcx>,
2195         user_ty: UserTypeProjections,
2196         has_guard: ArmHasGuard,
2197         opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
2198         pat_span: Span,
2199     ) {
2200         debug!(
2201             "declare_binding(var_id={:?}, name={:?}, mode={:?}, var_ty={:?}, \
2202              visibility_scope={:?}, source_info={:?})",
2203             var_id, name, mode, var_ty, visibility_scope, source_info
2204         );
2205
2206         let tcx = self.tcx;
2207         let debug_source_info = SourceInfo { span: source_info.span, scope: visibility_scope };
2208         let binding_mode = match mode {
2209             BindingMode::ByValue => ty::BindingMode::BindByValue(mutability),
2210             BindingMode::ByRef(_) => ty::BindingMode::BindByReference(mutability),
2211         };
2212         debug!("declare_binding: user_ty={:?}", user_ty);
2213         let local = LocalDecl::<'tcx> {
2214             mutability,
2215             ty: var_ty,
2216             user_ty: if user_ty.is_empty() { None } else { Some(Box::new(user_ty)) },
2217             source_info,
2218             internal: false,
2219             is_block_tail: None,
2220             local_info: Some(Box::new(LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
2221                 VarBindingForm {
2222                     binding_mode,
2223                     // hypothetically, `visit_primary_bindings` could try to unzip
2224                     // an outermost hir::Ty as we descend, matching up
2225                     // idents in pat; but complex w/ unclear UI payoff.
2226                     // Instead, just abandon providing diagnostic info.
2227                     opt_ty_info: None,
2228                     opt_match_place,
2229                     pat_span,
2230                 },
2231             ))))),
2232         };
2233         let for_arm_body = self.local_decls.push(local);
2234         self.var_debug_info.push(VarDebugInfo {
2235             name,
2236             source_info: debug_source_info,
2237             value: VarDebugInfoContents::Place(for_arm_body.into()),
2238         });
2239         let locals = if has_guard.0 {
2240             let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
2241                 // This variable isn't mutated but has a name, so has to be
2242                 // immutable to avoid the unused mut lint.
2243                 mutability: Mutability::Not,
2244                 ty: tcx.mk_imm_ref(tcx.lifetimes.re_erased, var_ty),
2245                 user_ty: None,
2246                 source_info,
2247                 internal: false,
2248                 is_block_tail: None,
2249                 local_info: Some(Box::new(LocalInfo::User(ClearCrossCrate::Set(
2250                     BindingForm::RefForGuard,
2251                 )))),
2252             });
2253             self.var_debug_info.push(VarDebugInfo {
2254                 name,
2255                 source_info: debug_source_info,
2256                 value: VarDebugInfoContents::Place(ref_for_guard.into()),
2257             });
2258             LocalsForNode::ForGuard { ref_for_guard, for_arm_body }
2259         } else {
2260             LocalsForNode::One(for_arm_body)
2261         };
2262         debug!("declare_binding: vars={:?}", locals);
2263         self.var_indices.insert(var_id, locals);
2264     }
2265 }