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, FxIndexSet},
  15     stack::ensure_sufficient_stack,
  16 };
  17 use rustc_index::bit_set::BitSet;
  18 use rustc_middle::middle::region;
  19 use rustc_middle::mir::*;
  20 use rustc_middle::thir::{self, *};
  21 use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty};
  22 use rustc_span::symbol::Symbol;
  23 use rustc_span::{BytePos, Pos, Span};
  24 use rustc_target::abi::VariantIdx;
  25 use smallvec::{smallvec, SmallVec};
  26
  27 // helper functions, broken out by category:
  28 mod simplify;
  29 mod test;
  30 mod util;
  31
  32 use std::borrow::Borrow;
  33 use std::convert::TryFrom;
  34 use std::mem;
  35
  36 impl<'a, 'tcx> Builder<'a, 'tcx> {
  37     pub(crate) fn then_else_break(
  38         &mut self,
  39         mut block: BasicBlock,
  40         expr: &Expr<'tcx>,
  41         temp_scope_override: Option<region::Scope>,
  42         break_scope: region::Scope,
  43         variable_source_info: SourceInfo,
  44     ) -> BlockAnd<()> {
  45         let this = self;
  46         let expr_span = expr.span;
  47
  48         match expr.kind {
  49             ExprKind::LogicalOp { op: LogicalOp::And, lhs, rhs } => {
  50                 let lhs_then_block = unpack!(this.then_else_break(
  51                     block,
  52                     &this.thir[lhs],
  53                     temp_scope_override,
  54                     break_scope,
  55                     variable_source_info,
  56                 ));
  57
  58                 let rhs_then_block = unpack!(this.then_else_break(
  59                     lhs_then_block,
  60                     &this.thir[rhs],
  61                     temp_scope_override,
  62                     break_scope,
  63                     variable_source_info,
  64                 ));
  65
  66                 rhs_then_block.unit()
  67             }
  68             ExprKind::Scope { region_scope, lint_level, value } => {
  69                 let region_scope = (region_scope, this.source_info(expr_span));
  70                 this.in_scope(region_scope, lint_level, |this| {
  71                     this.then_else_break(
  72                         block,
  73                         &this.thir[value],
  74                         temp_scope_override,
  75                         break_scope,
  76                         variable_source_info,
  77                     )
  78                 })
  79             }
  80             ExprKind::Let { expr, ref pat } => this.lower_let_expr(
  81                 block,
  82                 &this.thir[expr],
  83                 pat,
  84                 break_scope,
  85                 Some(variable_source_info.scope),
  86                 variable_source_info.span,
  87             ),
  88             _ => {
  89                 let temp_scope = temp_scope_override.unwrap_or_else(|| this.local_scope());
  90                 let mutability = Mutability::Mut;
  91                 let place =
  92                     unpack!(block = this.as_temp(block, Some(temp_scope), expr, mutability));
  93                 let operand = Operand::Move(Place::from(place));
  94
  95                 let then_block = this.cfg.start_new_block();
  96                 let else_block = this.cfg.start_new_block();
  97                 let term = TerminatorKind::if_(this.tcx, operand, then_block, else_block);
  98
  99                 let source_info = this.source_info(expr_span);
 100                 this.cfg.terminate(block, source_info, term);
 101                 this.break_for_else(else_block, break_scope, source_info);
 102
 103                 then_block.unit()
 104             }
 105         }
 106     }
 107
 108     /// Generates MIR for a `match` expression.
 109     ///
 110     /// The MIR that we generate for a match looks like this.
 111     ///
 112     /// ```text
 113     /// [ 0. Pre-match ]
 114     ///        |
 115     /// [ 1. Evaluate Scrutinee (expression being matched on) ]
 116     /// [ (fake read of scrutinee) ]
 117     ///        |
 118     /// [ 2. Decision tree -- check discriminants ] <--------+
 119     ///        |                                             |
 120     ///        | (once a specific arm is chosen)             |
 121     ///        |                                             |
 122     /// [pre_binding_block]                           [otherwise_block]
 123     ///        |                                             |
 124     /// [ 3. Create "guard bindings" for arm ]               |
 125     /// [ (create fake borrows) ]                            |
 126     ///        |                                             |
 127     /// [ 4. Execute guard code ]                            |
 128     /// [ (read fake borrows) ] --(guard is false)-----------+
 129     ///        |
 130     ///        | (guard results in true)
 131     ///        |
 132     /// [ 5. Create real bindings and execute arm ]
 133     ///        |
 134     /// [ Exit match ]
 135     /// ```
 136     ///
 137     /// All of the different arms have been stacked on top of each other to
 138     /// simplify the diagram. For an arm with no guard the blocks marked 3 and
 139     /// 4 and the fake borrows are omitted.
 140     ///
 141     /// We generate MIR in the following steps:
 142     ///
 143     /// 1. Evaluate the scrutinee and add the fake read of it ([Builder::lower_scrutinee]).
 144     /// 2. Create the decision tree ([Builder::lower_match_tree]).
 145     /// 3. Determine the fake borrows that are needed from the places that were
 146     ///    matched against and create the required temporaries for them
 147     ///    ([Builder::calculate_fake_borrows]).
 148     /// 4. Create everything else: the guards and the arms ([Builder::lower_match_arms]).
 149     ///
 150     /// ## False edges
 151     ///
 152     /// We don't want to have the exact structure of the decision tree be
 153     /// visible through borrow checking. False edges ensure that the CFG as
 154     /// seen by borrow checking doesn't encode this. False edges are added:
 155     ///
 156     /// * From each pre-binding block to the next pre-binding block.
 157     /// * From each otherwise block to the next pre-binding block.
 158     #[instrument(level = "debug", skip(self, arms))]
 159     pub(crate) fn match_expr(
 160         &mut self,
 161         destination: Place<'tcx>,
 162         span: Span,
 163         mut block: BasicBlock,
 164         scrutinee: &Expr<'tcx>,
 165         arms: &[ArmId],
 166     ) -> BlockAnd<()> {
 167         let scrutinee_span = scrutinee.span;
 168         let scrutinee_place =
 169             unpack!(block = self.lower_scrutinee(block, scrutinee, scrutinee_span,));
 170
 171         let mut arm_candidates = self.create_match_candidates(scrutinee_place.clone(), &arms);
 172
 173         let match_has_guard = arm_candidates.iter().any(|(_, candidate)| candidate.has_guard);
 174         let mut candidates =
 175             arm_candidates.iter_mut().map(|(_, candidate)| candidate).collect::<Vec<_>>();
 176
 177         let match_start_span = span.shrink_to_lo().to(scrutinee.span);
 178
 179         let fake_borrow_temps = self.lower_match_tree(
 180             block,
 181             scrutinee_span,
 182             match_start_span,
 183             match_has_guard,
 184             &mut candidates,
 185         );
 186
 187         self.lower_match_arms(
 188             destination,
 189             scrutinee_place,
 190             scrutinee_span,
 191             arm_candidates,
 192             self.source_info(span),
 193             fake_borrow_temps,
 194         )
 195     }
 196
 197     /// Evaluate the scrutinee and add the fake read of it.
 198     fn lower_scrutinee(
 199         &mut self,
 200         mut block: BasicBlock,
 201         scrutinee: &Expr<'tcx>,
 202         scrutinee_span: Span,
 203     ) -> BlockAnd<PlaceBuilder<'tcx>> {
 204         let scrutinee_place_builder = unpack!(block = self.as_place_builder(block, scrutinee));
 205         // Matching on a `scrutinee_place` with an uninhabited type doesn't
 206         // generate any memory reads by itself, and so if the place "expression"
 207         // contains unsafe operations like raw pointer dereferences or union
 208         // field projections, we wouldn't know to require an `unsafe` block
 209         // around a `match` equivalent to `std::intrinsics::unreachable()`.
 210         // See issue #47412 for this hole being discovered in the wild.
 211         //
 212         // HACK(eddyb) Work around the above issue by adding a dummy inspection
 213         // of `scrutinee_place`, specifically by applying `ReadForMatch`.
 214         //
 215         // NOTE: ReadForMatch also checks that the scrutinee is initialized.
 216         // This is currently needed to not allow matching on an uninitialized,
 217         // uninhabited value. If we get never patterns, those will check that
 218         // the place is initialized, and so this read would only be used to
 219         // check safety.
 220         let cause_matched_place = FakeReadCause::ForMatchedPlace(None);
 221         let source_info = self.source_info(scrutinee_span);
 222
 223         if let Ok(scrutinee_builder) =
 224             scrutinee_place_builder.clone().try_upvars_resolved(self.tcx, self.typeck_results)
 225         {
 226             let scrutinee_place = scrutinee_builder.into_place(self.tcx, self.typeck_results);
 227             self.cfg.push_fake_read(block, source_info, cause_matched_place, scrutinee_place);
 228         }
 229
 230         block.and(scrutinee_place_builder)
 231     }
 232
 233     /// Create the initial `Candidate`s for a `match` expression.
 234     fn create_match_candidates<'pat>(
 235         &mut self,
 236         scrutinee: PlaceBuilder<'tcx>,
 237         arms: &'pat [ArmId],
 238     ) -> Vec<(&'pat Arm<'tcx>, Candidate<'pat, 'tcx>)>
 239     where
 240         'a: 'pat,
 241     {
 242         // Assemble a list of candidates: there is one candidate per pattern,
 243         // which means there may be more than one candidate *per arm*.
 244         arms.iter()
 245             .copied()
 246             .map(|arm| {
 247                 let arm = &self.thir[arm];
 248                 let arm_has_guard = arm.guard.is_some();
 249                 let arm_candidate = Candidate::new(scrutinee.clone(), &arm.pattern, arm_has_guard);
 250                 (arm, arm_candidate)
 251             })
 252             .collect()
 253     }
 254
 255     /// Create the decision tree for the match expression, starting from `block`.
 256     ///
 257     /// Modifies `candidates` to store the bindings and type ascriptions for
 258     /// that candidate.
 259     ///
 260     /// Returns the places that need fake borrows because we bind or test them.
 261     fn lower_match_tree<'pat>(
 262         &mut self,
 263         block: BasicBlock,
 264         scrutinee_span: Span,
 265         match_start_span: Span,
 266         match_has_guard: bool,
 267         candidates: &mut [&mut Candidate<'pat, 'tcx>],
 268     ) -> Vec<(Place<'tcx>, Local)> {
 269         // The set of places that we are creating fake borrows of. If there are
 270         // no match guards then we don't need any fake borrows, so don't track
 271         // them.
 272         let mut fake_borrows = match_has_guard.then(FxIndexSet::default);
 273
 274         let mut otherwise = None;
 275
 276         // This will generate code to test scrutinee_place and
 277         // branch to the appropriate arm block
 278         self.match_candidates(
 279             match_start_span,
 280             scrutinee_span,
 281             block,
 282             &mut otherwise,
 283             candidates,
 284             &mut fake_borrows,
 285         );
 286
 287         if let Some(otherwise_block) = otherwise {
 288             // See the doc comment on `match_candidates` for why we may have an
 289             // otherwise block. Match checking will ensure this is actually
 290             // unreachable.
 291             let source_info = self.source_info(scrutinee_span);
 292             self.cfg.terminate(otherwise_block, source_info, TerminatorKind::Unreachable);
 293         }
 294
 295         // Link each leaf candidate to the `pre_binding_block` of the next one.
 296         let mut previous_candidate: Option<&mut Candidate<'_, '_>> = None;
 297
 298         for candidate in candidates {
 299             candidate.visit_leaves(|leaf_candidate| {
 300                 if let Some(ref mut prev) = previous_candidate {
 301                     prev.next_candidate_pre_binding_block = leaf_candidate.pre_binding_block;
 302                 }
 303                 previous_candidate = Some(leaf_candidate);
 304             });
 305         }
 306
 307         if let Some(ref borrows) = fake_borrows {
 308             self.calculate_fake_borrows(borrows, scrutinee_span)
 309         } else {
 310             Vec::new()
 311         }
 312     }
 313
 314     /// Lower the bindings, guards and arm bodies of a `match` expression.
 315     ///
 316     /// The decision tree should have already been created
 317     /// (by [Builder::lower_match_tree]).
 318     ///
 319     /// `outer_source_info` is the SourceInfo for the whole match.
 320     fn lower_match_arms(
 321         &mut self,
 322         destination: Place<'tcx>,
 323         scrutinee_place_builder: PlaceBuilder<'tcx>,
 324         scrutinee_span: Span,
 325         arm_candidates: Vec<(&'_ Arm<'tcx>, Candidate<'_, 'tcx>)>,
 326         outer_source_info: SourceInfo,
 327         fake_borrow_temps: Vec<(Place<'tcx>, Local)>,
 328     ) -> BlockAnd<()> {
 329         let arm_end_blocks: Vec<_> = arm_candidates
 330             .into_iter()
 331             .map(|(arm, candidate)| {
 332                 debug!("lowering arm {:?}\ncandidate = {:?}", arm, candidate);
 333
 334                 let arm_source_info = self.source_info(arm.span);
 335                 let arm_scope = (arm.scope, arm_source_info);
 336                 let match_scope = self.local_scope();
 337                 self.in_scope(arm_scope, arm.lint_level, |this| {
 338                     // `try_upvars_resolved` may fail if it is unable to resolve the given
 339                     // `PlaceBuilder` inside a closure. In this case, we don't want to include
 340                     // a scrutinee place. `scrutinee_place_builder` will fail to be resolved
 341                     // if the only match arm is a wildcard (`_`).
 342                     // Example:
 343                     // ```
 344                     // let foo = (0, 1);
 345                     // let c = || {
 346                     //    match foo { _ => () };
 347                     // };
 348                     // ```
 349                     let mut opt_scrutinee_place: Option<(Option<&Place<'tcx>>, Span)> = None;
 350                     let scrutinee_place: Place<'tcx>;
 351                     if let Ok(scrutinee_builder) = scrutinee_place_builder
 352                         .clone()
 353                         .try_upvars_resolved(this.tcx, this.typeck_results)
 354                     {
 355                         scrutinee_place =
 356                             scrutinee_builder.into_place(this.tcx, this.typeck_results);
 357                         opt_scrutinee_place = Some((Some(&scrutinee_place), scrutinee_span));
 358                     }
 359                     let scope = this.declare_bindings(
 360                         None,
 361                         arm.span,
 362                         &arm.pattern,
 363                         ArmHasGuard(arm.guard.is_some()),
 364                         opt_scrutinee_place,
 365                     );
 366
 367                     let arm_block = this.bind_pattern(
 368                         outer_source_info,
 369                         candidate,
 370                         arm.guard.as_ref(),
 371                         &fake_borrow_temps,
 372                         scrutinee_span,
 373                         Some(arm.span),
 374                         Some(arm.scope),
 375                         Some(match_scope),
 376                     );
 377
 378                     if let Some(source_scope) = scope {
 379                         this.source_scope = source_scope;
 380                     }
 381
 382                     this.expr_into_dest(destination, arm_block, &&this.thir[arm.body])
 383                 })
 384             })
 385             .collect();
 386
 387         // all the arm blocks will rejoin here
 388         let end_block = self.cfg.start_new_block();
 389
 390         let end_brace = self.source_info(
 391             outer_source_info.span.with_lo(outer_source_info.span.hi() - BytePos::from_usize(1)),
 392         );
 393         for arm_block in arm_end_blocks {
 394             let block = &self.cfg.basic_blocks[arm_block.0];
 395             let last_location = block.statements.last().map(|s| s.source_info);
 396
 397             self.cfg.goto(unpack!(arm_block), last_location.unwrap_or(end_brace), end_block);
 398         }
 399
 400         self.source_scope = outer_source_info.scope;
 401
 402         end_block.unit()
 403     }
 404
 405     /// Binds the variables and ascribes types for a given `match` arm or
 406     /// `let` binding.
 407     ///
 408     /// Also check if the guard matches, if it's provided.
 409     /// `arm_scope` should be `Some` if and only if this is called for a
 410     /// `match` arm.
 411     fn bind_pattern(
 412         &mut self,
 413         outer_source_info: SourceInfo,
 414         candidate: Candidate<'_, 'tcx>,
 415         guard: Option<&Guard<'tcx>>,
 416         fake_borrow_temps: &[(Place<'tcx>, Local)],
 417         scrutinee_span: Span,
 418         arm_span: Option<Span>,
 419         arm_scope: Option<region::Scope>,
 420         match_scope: Option<region::Scope>,
 421     ) -> BasicBlock {
 422         if candidate.subcandidates.is_empty() {
 423             // Avoid generating another `BasicBlock` when we only have one
 424             // candidate.
 425             self.bind_and_guard_matched_candidate(
 426                 candidate,
 427                 &[],
 428                 guard,
 429                 fake_borrow_temps,
 430                 scrutinee_span,
 431                 arm_span,
 432                 match_scope,
 433                 true,
 434             )
 435         } else {
 436             // It's helpful to avoid scheduling drops multiple times to save
 437             // drop elaboration from having to clean up the extra drops.
 438             //
 439             // If we are in a `let` then we only schedule drops for the first
 440             // candidate.
 441             //
 442             // If we're in a `match` arm then we could have a case like so:
 443             //
 444             // Ok(x) | Err(x) if return => { /* ... */ }
 445             //
 446             // In this case we don't want a drop of `x` scheduled when we
 447             // return: it isn't bound by move until right before enter the arm.
 448             // To handle this we instead unschedule it's drop after each time
 449             // we lower the guard.
 450             let target_block = self.cfg.start_new_block();
 451             let mut schedule_drops = true;
 452             // We keep a stack of all of the bindings and type ascriptions
 453             // from the parent candidates that we visit, that also need to
 454             // be bound for each candidate.
 455             traverse_candidate(
 456                 candidate,
 457                 &mut Vec::new(),
 458                 &mut |leaf_candidate, parent_bindings| {
 459                     if let Some(arm_scope) = arm_scope {
 460                         self.clear_top_scope(arm_scope);
 461                     }
 462                     let binding_end = self.bind_and_guard_matched_candidate(
 463                         leaf_candidate,
 464                         parent_bindings,
 465                         guard,
 466                         &fake_borrow_temps,
 467                         scrutinee_span,
 468                         arm_span,
 469                         match_scope,
 470                         schedule_drops,
 471                     );
 472                     if arm_scope.is_none() {
 473                         schedule_drops = false;
 474                     }
 475                     self.cfg.goto(binding_end, outer_source_info, target_block);
 476                 },
 477                 |inner_candidate, parent_bindings| {
 478                     parent_bindings.push((inner_candidate.bindings, inner_candidate.ascriptions));
 479                     inner_candidate.subcandidates.into_iter()
 480                 },
 481                 |parent_bindings| {
 482                     parent_bindings.pop();
 483                 },
 484             );
 485
 486             target_block
 487         }
 488     }
 489
 490     pub(super) fn expr_into_pattern(
 491         &mut self,
 492         mut block: BasicBlock,
 493         irrefutable_pat: Pat<'tcx>,
 494         initializer: &Expr<'tcx>,
 495     ) -> BlockAnd<()> {
 496         match *irrefutable_pat.kind {
 497             // Optimize the case of `let x = ...` to write directly into `x`
 498             PatKind::Binding { mode: BindingMode::ByValue, var, subpattern: None, .. } => {
 499                 let place =
 500                     self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
 501                 unpack!(block = self.expr_into_dest(place, block, initializer));
 502
 503                 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
 504                 let source_info = self.source_info(irrefutable_pat.span);
 505                 self.cfg.push_fake_read(block, source_info, FakeReadCause::ForLet(None), place);
 506
 507                 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
 508                 block.unit()
 509             }
 510
 511             // Optimize the case of `let x: T = ...` to write directly
 512             // into `x` and then require that `T == typeof(x)`.
 513             //
 514             // Weirdly, this is needed to prevent the
 515             // `intrinsic-move-val.rs` test case from crashing. That
 516             // test works with uninitialized values in a rather
 517             // dubious way, so it may be that the test is kind of
 518             // broken.
 519             PatKind::AscribeUserType {
 520                 subpattern:
 521                     Pat {
 522                         kind:
 523                             box PatKind::Binding {
 524                                 mode: BindingMode::ByValue,
 525                                 var,
 526                                 subpattern: None,
 527                                 ..
 528                             },
 529                         ..
 530                     },
 531                 ascription: thir::Ascription { annotation, variance: _ },
 532             } => {
 533                 let place =
 534                     self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
 535                 unpack!(block = self.expr_into_dest(place, block, initializer));
 536
 537                 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
 538                 let pattern_source_info = self.source_info(irrefutable_pat.span);
 539                 let cause_let = FakeReadCause::ForLet(None);
 540                 self.cfg.push_fake_read(block, pattern_source_info, cause_let, place);
 541
 542                 let ty_source_info = self.source_info(annotation.span);
 543
 544                 let base = self.canonical_user_type_annotations.push(annotation);
 545                 self.cfg.push(
 546                     block,
 547                     Statement {
 548                         source_info: ty_source_info,
 549                         kind: StatementKind::AscribeUserType(
 550                             Box::new((place, UserTypeProjection { base, projs: Vec::new() })),
 551                             // We always use invariant as the variance here. This is because the
 552                             // variance field from the ascription refers to the variance to use
 553                             // when applying the type to the value being matched, but this
 554                             // ascription applies rather to the type of the binding. e.g., in this
 555                             // example:
 556                             //
 557                             // ```
 558                             // let x: T = <expr>
 559                             // ```
 560                             //
 561                             // We are creating an ascription that defines the type of `x` to be
 562                             // exactly `T` (i.e., with invariance). The variance field, in
 563                             // contrast, is intended to be used to relate `T` to the type of
 564                             // `<expr>`.
 565                             ty::Variance::Invariant,
 566                         ),
 567                     },
 568                 );
 569
 570                 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
 571                 block.unit()
 572             }
 573
 574             _ => {
 575                 let place_builder = unpack!(block = self.as_place_builder(block, initializer));
 576                 self.place_into_pattern(block, irrefutable_pat, place_builder, true)
 577             }
 578         }
 579     }
 580
 581     pub(crate) fn place_into_pattern(
 582         &mut self,
 583         block: BasicBlock,
 584         irrefutable_pat: Pat<'tcx>,
 585         initializer: PlaceBuilder<'tcx>,
 586         set_match_place: bool,
 587     ) -> BlockAnd<()> {
 588         let mut candidate = Candidate::new(initializer.clone(), &irrefutable_pat, false);
 589         let fake_borrow_temps = self.lower_match_tree(
 590             block,
 591             irrefutable_pat.span,
 592             irrefutable_pat.span,
 593             false,
 594             &mut [&mut candidate],
 595         );
 596         // For matches and function arguments, the place that is being matched
 597         // can be set when creating the variables. But the place for
 598         // let PATTERN = ... might not even exist until we do the assignment.
 599         // so we set it here instead.
 600         if set_match_place {
 601             let mut candidate_ref = &candidate;
 602             while let Some(next) = {
 603                 for binding in &candidate_ref.bindings {
 604                     let local = self.var_local_id(binding.var_id, OutsideGuard);
 605
 606                     let Some(box LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
 607                         VarBindingForm { opt_match_place: Some((ref mut match_place, _)), .. },
 608                     )))) = self.local_decls[local].local_info else {
 609                         bug!("Let binding to non-user variable.")
 610                     };
 611                     // `try_upvars_resolved` may fail if it is unable to resolve the given
 612                     // `PlaceBuilder` inside a closure. In this case, we don't want to include
 613                     // a scrutinee place. `scrutinee_place_builder` will fail for destructured
 614                     // assignments. This is because a closure only captures the precise places
 615                     // that it will read and as a result a closure may not capture the entire
 616                     // tuple/struct and rather have individual places that will be read in the
 617                     // final MIR.
 618                     // Example:
 619                     // ```
 620                     // let foo = (0, 1);
 621                     // let c = || {
 622                     //    let (v1, v2) = foo;
 623                     // };
 624                     // ```
 625                     if let Ok(match_pair_resolved) =
 626                         initializer.clone().try_upvars_resolved(self.tcx, self.typeck_results)
 627                     {
 628                         let place = match_pair_resolved.into_place(self.tcx, self.typeck_results);
 629                         *match_place = Some(place);
 630                     }
 631                 }
 632                 // All of the subcandidates should bind the same locals, so we
 633                 // only visit the first one.
 634                 candidate_ref.subcandidates.get(0)
 635             } {
 636                 candidate_ref = next;
 637             }
 638         }
 639
 640         self.bind_pattern(
 641             self.source_info(irrefutable_pat.span),
 642             candidate,
 643             None,
 644             &fake_borrow_temps,
 645             irrefutable_pat.span,
 646             None,
 647             None,
 648             None,
 649         )
 650         .unit()
 651     }
 652
 653     /// Declares the bindings of the given patterns and returns the visibility
 654     /// scope for the bindings in these patterns, if such a scope had to be
 655     /// created. NOTE: Declaring the bindings should always be done in their
 656     /// drop scope.
 657     pub(crate) fn declare_bindings(
 658         &mut self,
 659         mut visibility_scope: Option<SourceScope>,
 660         scope_span: Span,
 661         pattern: &Pat<'tcx>,
 662         has_guard: ArmHasGuard,
 663         opt_match_place: Option<(Option<&Place<'tcx>>, Span)>,
 664     ) -> Option<SourceScope> {
 665         debug!("declare_bindings: pattern={:?}", pattern);
 666         self.visit_primary_bindings(
 667             &pattern,
 668             UserTypeProjections::none(),
 669             &mut |this, mutability, name, mode, var, span, ty, user_ty| {
 670                 if visibility_scope.is_none() {
 671                     visibility_scope =
 672                         Some(this.new_source_scope(scope_span, LintLevel::Inherited, None));
 673                 }
 674                 let source_info = SourceInfo { span, scope: this.source_scope };
 675                 let visibility_scope = visibility_scope.unwrap();
 676                 this.declare_binding(
 677                     source_info,
 678                     visibility_scope,
 679                     mutability,
 680                     name,
 681                     mode,
 682                     var,
 683                     ty,
 684                     user_ty,
 685                     has_guard,
 686                     opt_match_place.map(|(x, y)| (x.cloned(), y)),
 687                     pattern.span,
 688                 );
 689             },
 690         );
 691         visibility_scope
 692     }
 693
 694     pub(crate) fn storage_live_binding(
 695         &mut self,
 696         block: BasicBlock,
 697         var: LocalVarId,
 698         span: Span,
 699         for_guard: ForGuard,
 700         schedule_drop: bool,
 701     ) -> Place<'tcx> {
 702         let local_id = self.var_local_id(var, for_guard);
 703         let source_info = self.source_info(span);
 704         self.cfg.push(block, Statement { source_info, kind: StatementKind::StorageLive(local_id) });
 705         // Although there is almost always scope for given variable in corner cases
 706         // like #92893 we might get variable with no scope.
 707         if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) && schedule_drop{
 708             self.schedule_drop(span, region_scope, local_id, DropKind::Storage);
 709         }
 710         Place::from(local_id)
 711     }
 712
 713     pub(crate) fn schedule_drop_for_binding(
 714         &mut self,
 715         var: LocalVarId,
 716         span: Span,
 717         for_guard: ForGuard,
 718     ) {
 719         let local_id = self.var_local_id(var, for_guard);
 720         if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) {
 721             self.schedule_drop(span, region_scope, local_id, DropKind::Value);
 722         }
 723     }
 724
 725     /// Visit all of the primary bindings in a patterns, that is, visit the
 726     /// leftmost occurrence of each variable bound in a pattern. A variable
 727     /// will occur more than once in an or-pattern.
 728     pub(super) fn visit_primary_bindings(
 729         &mut self,
 730         pattern: &Pat<'tcx>,
 731         pattern_user_ty: UserTypeProjections,
 732         f: &mut impl FnMut(
 733             &mut Self,
 734             Mutability,
 735             Symbol,
 736             BindingMode,
 737             LocalVarId,
 738             Span,
 739             Ty<'tcx>,
 740             UserTypeProjections,
 741         ),
 742     ) {
 743         debug!(
 744             "visit_primary_bindings: pattern={:?} pattern_user_ty={:?}",
 745             pattern, pattern_user_ty
 746         );
 747         match *pattern.kind {
 748             PatKind::Binding {
 749                 mutability,
 750                 name,
 751                 mode,
 752                 var,
 753                 ty,
 754                 ref subpattern,
 755                 is_primary,
 756                 ..
 757             } => {
 758                 if is_primary {
 759                     f(self, mutability, name, mode, var, pattern.span, ty, pattern_user_ty.clone());
 760                 }
 761                 if let Some(subpattern) = subpattern.as_ref() {
 762                     self.visit_primary_bindings(subpattern, pattern_user_ty, f);
 763                 }
 764             }
 765
 766             PatKind::Array { ref prefix, ref slice, ref suffix }
 767             | PatKind::Slice { ref prefix, ref slice, ref suffix } => {
 768                 let from = u64::try_from(prefix.len()).unwrap();
 769                 let to = u64::try_from(suffix.len()).unwrap();
 770                 for subpattern in prefix {
 771                     self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
 772                 }
 773                 for subpattern in slice {
 774                     self.visit_primary_bindings(
 775                         subpattern,
 776                         pattern_user_ty.clone().subslice(from, to),
 777                         f,
 778                     );
 779                 }
 780                 for subpattern in suffix {
 781                     self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
 782                 }
 783             }
 784
 785             PatKind::Constant { .. } | PatKind::Range { .. } | PatKind::Wild => {}
 786
 787             PatKind::Deref { ref subpattern } => {
 788                 self.visit_primary_bindings(subpattern, pattern_user_ty.deref(), f);
 789             }
 790
 791             PatKind::AscribeUserType {
 792                 ref subpattern,
 793                 ascription: thir::Ascription { ref annotation, variance: _ },
 794             } => {
 795                 // This corresponds to something like
 796                 //
 797                 // ```
 798                 // let A::<'a>(_): A<'static> = ...;
 799                 // ```
 800                 //
 801                 // Note that the variance doesn't apply here, as we are tracking the effect
 802                 // of `user_ty` on any bindings contained with subpattern.
 803
 804                 let projection = UserTypeProjection {
 805                     base: self.canonical_user_type_annotations.push(annotation.clone()),
 806                     projs: Vec::new(),
 807                 };
 808                 let subpattern_user_ty =
 809                     pattern_user_ty.push_projection(&projection, annotation.span);
 810                 self.visit_primary_bindings(subpattern, subpattern_user_ty, f)
 811             }
 812
 813             PatKind::Leaf { ref subpatterns } => {
 814                 for subpattern in subpatterns {
 815                     let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field);
 816                     debug!("visit_primary_bindings: subpattern_user_ty={:?}", subpattern_user_ty);
 817                     self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
 818                 }
 819             }
 820
 821             PatKind::Variant { adt_def, substs: _, variant_index, ref subpatterns } => {
 822                 for subpattern in subpatterns {
 823                     let subpattern_user_ty =
 824                         pattern_user_ty.clone().variant(adt_def, variant_index, subpattern.field);
 825                     self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
 826                 }
 827             }
 828             PatKind::Or { ref pats } => {
 829                 // In cases where we recover from errors the primary bindings
 830                 // may not all be in the leftmost subpattern. For example in
 831                 // `let (x | y) = ...`, the primary binding of `y` occurs in
 832                 // the right subpattern
 833                 for subpattern in pats {
 834                     self.visit_primary_bindings(subpattern, pattern_user_ty.clone(), f);
 835                 }
 836             }
 837         }
 838     }
 839 }
 840
 841 #[derive(Debug)]
 842 struct Candidate<'pat, 'tcx> {
 843     /// [`Span`] of the original pattern that gave rise to this candidate.
 844     span: Span,
 845
 846     /// Whether this `Candidate` has a guard.
 847     has_guard: bool,
 848
 849     /// All of these must be satisfied...
 850     match_pairs: SmallVec<[MatchPair<'pat, 'tcx>; 1]>,
 851
 852     /// ...these bindings established...
 853     bindings: Vec<Binding<'tcx>>,
 854
 855     /// ...and these types asserted...
 856     ascriptions: Vec<Ascription<'tcx>>,
 857
 858     /// ...and if this is non-empty, one of these subcandidates also has to match...
 859     subcandidates: Vec<Candidate<'pat, 'tcx>>,
 860
 861     /// ...and the guard must be evaluated; if it's `false` then branch to `otherwise_block`.
 862     otherwise_block: Option<BasicBlock>,
 863
 864     /// The block before the `bindings` have been established.
 865     pre_binding_block: Option<BasicBlock>,
 866     /// The pre-binding block of the next candidate.
 867     next_candidate_pre_binding_block: Option<BasicBlock>,
 868 }
 869
 870 impl<'tcx, 'pat> Candidate<'pat, 'tcx> {
 871     fn new(place: PlaceBuilder<'tcx>, pattern: &'pat Pat<'tcx>, has_guard: bool) -> Self {
 872         Candidate {
 873             span: pattern.span,
 874             has_guard,
 875             match_pairs: smallvec![MatchPair { place, pattern }],
 876             bindings: Vec::new(),
 877             ascriptions: Vec::new(),
 878             subcandidates: Vec::new(),
 879             otherwise_block: None,
 880             pre_binding_block: None,
 881             next_candidate_pre_binding_block: None,
 882         }
 883     }
 884
 885     /// Visit the leaf candidates (those with no subcandidates) contained in
 886     /// this candidate.
 887     fn visit_leaves<'a>(&'a mut self, mut visit_leaf: impl FnMut(&'a mut Self)) {
 888         traverse_candidate(
 889             self,
 890             &mut (),
 891             &mut move |c, _| visit_leaf(c),
 892             move |c, _| c.subcandidates.iter_mut(),
 893             |_| {},
 894         );
 895     }
 896 }
 897
 898 /// A depth-first traversal of the `Candidate` and all of its recursive
 899 /// subcandidates.
 900 fn traverse_candidate<'pat, 'tcx: 'pat, C, T, I>(
 901     candidate: C,
 902     context: &mut T,
 903     visit_leaf: &mut impl FnMut(C, &mut T),
 904     get_children: impl Copy + Fn(C, &mut T) -> I,
 905     complete_children: impl Copy + Fn(&mut T),
 906 ) where
 907     C: Borrow<Candidate<'pat, 'tcx>>,
 908     I: Iterator<Item = C>,
 909 {
 910     if candidate.borrow().subcandidates.is_empty() {
 911         visit_leaf(candidate, context)
 912     } else {
 913         for child in get_children(candidate, context) {
 914             traverse_candidate(child, context, visit_leaf, get_children, complete_children);
 915         }
 916         complete_children(context)
 917     }
 918 }
 919
 920 #[derive(Clone, Debug)]
 921 struct Binding<'tcx> {
 922     span: Span,
 923     source: Place<'tcx>,
 924     var_id: LocalVarId,
 925     binding_mode: BindingMode,
 926 }
 927
 928 /// Indicates that the type of `source` must be a subtype of the
 929 /// user-given type `user_ty`; this is basically a no-op but can
 930 /// influence region inference.
 931 #[derive(Clone, Debug)]
 932 struct Ascription<'tcx> {
 933     source: Place<'tcx>,
 934     annotation: CanonicalUserTypeAnnotation<'tcx>,
 935     variance: ty::Variance,
 936 }
 937
 938 #[derive(Clone, Debug)]
 939 pub(crate) struct MatchPair<'pat, 'tcx> {
 940     // this place...
 941     place: PlaceBuilder<'tcx>,
 942
 943     // ... must match this pattern.
 944     pattern: &'pat Pat<'tcx>,
 945 }
 946
 947 /// See [`Test`] for more.
 948 #[derive(Clone, Debug, PartialEq)]
 949 enum TestKind<'tcx> {
 950     /// Test what enum variant a value is.
 951     Switch {
 952         /// The enum type being tested.
 953         adt_def: ty::AdtDef<'tcx>,
 954         /// The set of variants that we should create a branch for. We also
 955         /// create an additional "otherwise" case.
 956         variants: BitSet<VariantIdx>,
 957     },
 958
 959     /// Test what value an integer, `bool`, or `char` has.
 960     SwitchInt {
 961         /// The type of the value that we're testing.
 962         switch_ty: Ty<'tcx>,
 963         /// The (ordered) set of values that we test for.
 964         ///
 965         /// For integers and `char`s we create a branch to each of the values in
 966         /// `options`, as well as an "otherwise" branch for all other values, even
 967         /// in the (rare) case that `options` is exhaustive.
 968         ///
 969         /// For `bool` we always generate two edges, one for `true` and one for
 970         /// `false`.
 971         options: FxIndexMap<ConstantKind<'tcx>, u128>,
 972     },
 973
 974     /// Test for equality with value, possibly after an unsizing coercion to
 975     /// `ty`,
 976     Eq {
 977         value: ConstantKind<'tcx>,
 978         // Integer types are handled by `SwitchInt`, and constants with ADT
 979         // types are converted back into patterns, so this can only be `&str`,
 980         // `&[T]`, `f32` or `f64`.
 981         ty: Ty<'tcx>,
 982     },
 983
 984     /// Test whether the value falls within an inclusive or exclusive range
 985     Range(PatRange<'tcx>),
 986
 987     /// Test that the length of the slice is equal to `len`.
 988     Len { len: u64, op: BinOp },
 989 }
 990
 991 /// A test to perform to determine which [`Candidate`] matches a value.
 992 ///
 993 /// [`Test`] is just the test to perform; it does not include the value
 994 /// to be tested.
 995 #[derive(Debug)]
 996 pub(crate) struct Test<'tcx> {
 997     span: Span,
 998     kind: TestKind<'tcx>,
 999 }
1000
1001 /// `ArmHasGuard` is a wrapper around a boolean flag. It indicates whether
1002 /// a match arm has a guard expression attached to it.
1003 #[derive(Copy, Clone, Debug)]
1004 pub(crate) struct ArmHasGuard(pub(crate) bool);
1005
1006 ///////////////////////////////////////////////////////////////////////////
1007 // Main matching algorithm
1008
1009 impl<'a, 'tcx> Builder<'a, 'tcx> {
1010     /// The main match algorithm. It begins with a set of candidates
1011     /// `candidates` and has the job of generating code to determine
1012     /// which of these candidates, if any, is the correct one. The
1013     /// candidates are sorted such that the first item in the list
1014     /// has the highest priority. When a candidate is found to match
1015     /// the value, we will set and generate a branch to the appropriate
1016     /// pre-binding block.
1017     ///
1018     /// If we find that *NONE* of the candidates apply, we branch to the
1019     /// `otherwise_block`, setting it to `Some` if required. In principle, this
1020     /// means that the input list was not exhaustive, though at present we
1021     /// sometimes are not smart enough to recognize all exhaustive inputs.
1022     ///
1023     /// It might be surprising that the input can be non-exhaustive.
1024     /// Indeed, initially, it is not, because all matches are
1025     /// exhaustive in Rust. But during processing we sometimes divide
1026     /// up the list of candidates and recurse with a non-exhaustive
1027     /// list. This is important to keep the size of the generated code
1028     /// under control. See [`Builder::test_candidates`] for more details.
1029     ///
1030     /// If `fake_borrows` is `Some`, then places which need fake borrows
1031     /// will be added to it.
1032     ///
1033     /// For an example of a case where we set `otherwise_block`, even for an
1034     /// exhaustive match, consider:
1035     ///
1036     /// ```
1037     /// # fn foo(x: (bool, bool)) {
1038     /// match x {
1039     ///     (true, true) => (),
1040     ///     (_, false) => (),
1041     ///     (false, true) => (),
1042     /// }
1043     /// # }
1044     /// ```
1045     ///
1046     /// For this match, we check if `x.0` matches `true` (for the first
1047     /// arm). If it doesn't match, we check `x.1`. If `x.1` is `true` we check
1048     /// if `x.0` matches `false` (for the third arm). In the (impossible at
1049     /// runtime) case when `x.0` is now `true`, we branch to
1050     /// `otherwise_block`.
1051     fn match_candidates<'pat>(
1052         &mut self,
1053         span: Span,
1054         scrutinee_span: Span,
1055         start_block: BasicBlock,
1056         otherwise_block: &mut Option<BasicBlock>,
1057         candidates: &mut [&mut Candidate<'pat, 'tcx>],
1058         fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1059     ) {
1060         debug!(
1061             "matched_candidate(span={:?}, candidates={:?}, start_block={:?}, otherwise_block={:?})",
1062             span, candidates, start_block, otherwise_block,
1063         );
1064
1065         // Start by simplifying candidates. Once this process is complete, all
1066         // the match pairs which remain require some form of test, whether it
1067         // be a switch or pattern comparison.
1068         let mut split_or_candidate = false;
1069         for candidate in &mut *candidates {
1070             split_or_candidate |= self.simplify_candidate(candidate);
1071         }
1072
1073         ensure_sufficient_stack(|| {
1074             if split_or_candidate {
1075                 // At least one of the candidates has been split into subcandidates.
1076                 // We need to change the candidate list to include those.
1077                 let mut new_candidates = Vec::new();
1078
1079                 for candidate in candidates {
1080                     candidate.visit_leaves(|leaf_candidate| new_candidates.push(leaf_candidate));
1081                 }
1082                 self.match_simplified_candidates(
1083                     span,
1084                     scrutinee_span,
1085                     start_block,
1086                     otherwise_block,
1087                     &mut *new_candidates,
1088                     fake_borrows,
1089                 );
1090             } else {
1091                 self.match_simplified_candidates(
1092                     span,
1093                     scrutinee_span,
1094                     start_block,
1095                     otherwise_block,
1096                     candidates,
1097                     fake_borrows,
1098                 );
1099             }
1100         });
1101     }
1102
1103     fn match_simplified_candidates(
1104         &mut self,
1105         span: Span,
1106         scrutinee_span: Span,
1107         start_block: BasicBlock,
1108         otherwise_block: &mut Option<BasicBlock>,
1109         candidates: &mut [&mut Candidate<'_, 'tcx>],
1110         fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1111     ) {
1112         // The candidates are sorted by priority. Check to see whether the
1113         // higher priority candidates (and hence at the front of the slice)
1114         // have satisfied all their match pairs.
1115         let fully_matched = candidates.iter().take_while(|c| c.match_pairs.is_empty()).count();
1116         debug!("match_candidates: {:?} candidates fully matched", fully_matched);
1117         let (matched_candidates, unmatched_candidates) = candidates.split_at_mut(fully_matched);
1118
1119         let block = if !matched_candidates.is_empty() {
1120             let otherwise_block =
1121                 self.select_matched_candidates(matched_candidates, start_block, fake_borrows);
1122
1123             if let Some(last_otherwise_block) = otherwise_block {
1124                 last_otherwise_block
1125             } else {
1126                 // Any remaining candidates are unreachable.
1127                 if unmatched_candidates.is_empty() {
1128                     return;
1129                 }
1130                 self.cfg.start_new_block()
1131             }
1132         } else {
1133             start_block
1134         };
1135
1136         // If there are no candidates that still need testing, we're
1137         // done. Since all matches are exhaustive, execution should
1138         // never reach this point.
1139         if unmatched_candidates.is_empty() {
1140             let source_info = self.source_info(span);
1141             if let Some(otherwise) = *otherwise_block {
1142                 self.cfg.goto(block, source_info, otherwise);
1143             } else {
1144                 *otherwise_block = Some(block);
1145             }
1146             return;
1147         }
1148
1149         // Test for the remaining candidates.
1150         self.test_candidates_with_or(
1151             span,
1152             scrutinee_span,
1153             unmatched_candidates,
1154             block,
1155             otherwise_block,
1156             fake_borrows,
1157         );
1158     }
1159
1160     /// Link up matched candidates.
1161     ///
1162     /// For example, if we have something like this:
1163     ///
1164     /// ```ignore (illustrative)
1165     /// ...
1166     /// Some(x) if cond1 => ...
1167     /// Some(x) => ...
1168     /// Some(x) if cond2 => ...
1169     /// ...
1170     /// ```
1171     ///
1172     /// We generate real edges from:
1173     ///
1174     /// * `start_block` to the [pre-binding block] of the first pattern,
1175     /// * the [otherwise block] of the first pattern to the second pattern,
1176     /// * the [otherwise block] of the third pattern to a block with an
1177     ///   [`Unreachable` terminator](TerminatorKind::Unreachable).
1178     ///
1179     /// In addition, we add fake edges from the otherwise blocks to the
1180     /// pre-binding block of the next candidate in the original set of
1181     /// candidates.
1182     ///
1183     /// [pre-binding block]: Candidate::pre_binding_block
1184     /// [otherwise block]: Candidate::otherwise_block
1185     fn select_matched_candidates(
1186         &mut self,
1187         matched_candidates: &mut [&mut Candidate<'_, 'tcx>],
1188         start_block: BasicBlock,
1189         fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1190     ) -> Option<BasicBlock> {
1191         debug_assert!(
1192             !matched_candidates.is_empty(),
1193             "select_matched_candidates called with no candidates",
1194         );
1195         debug_assert!(
1196             matched_candidates.iter().all(|c| c.subcandidates.is_empty()),
1197             "subcandidates should be empty in select_matched_candidates",
1198         );
1199
1200         // Insert a borrows of prefixes of places that are bound and are
1201         // behind a dereference projection.
1202         //
1203         // These borrows are taken to avoid situations like the following:
1204         //
1205         // match x[10] {
1206         //     _ if { x = &[0]; false } => (),
1207         //     y => (), // Out of bounds array access!
1208         // }
1209         //
1210         // match *x {
1211         //     // y is bound by reference in the guard and then by copy in the
1212         //     // arm, so y is 2 in the arm!
1213         //     y if { y == 1 && (x = &2) == () } => y,
1214         //     _ => 3,
1215         // }
1216         if let Some(fake_borrows) = fake_borrows {
1217             for Binding { source, .. } in
1218                 matched_candidates.iter().flat_map(|candidate| &candidate.bindings)
1219             {
1220                 if let Some(i) =
1221                     source.projection.iter().rposition(|elem| elem == ProjectionElem::Deref)
1222                 {
1223                     let proj_base = &source.projection[..i];
1224
1225                     fake_borrows.insert(Place {
1226                         local: source.local,
1227                         projection: self.tcx.intern_place_elems(proj_base),
1228                     });
1229                 }
1230             }
1231         }
1232
1233         let fully_matched_with_guard = matched_candidates
1234             .iter()
1235             .position(|c| !c.has_guard)
1236             .unwrap_or(matched_candidates.len() - 1);
1237
1238         let (reachable_candidates, unreachable_candidates) =
1239             matched_candidates.split_at_mut(fully_matched_with_guard + 1);
1240
1241         let mut next_prebinding = start_block;
1242
1243         for candidate in reachable_candidates.iter_mut() {
1244             assert!(candidate.otherwise_block.is_none());
1245             assert!(candidate.pre_binding_block.is_none());
1246             candidate.pre_binding_block = Some(next_prebinding);
1247             if candidate.has_guard {
1248                 // Create the otherwise block for this candidate, which is the
1249                 // pre-binding block for the next candidate.
1250                 next_prebinding = self.cfg.start_new_block();
1251                 candidate.otherwise_block = Some(next_prebinding);
1252             }
1253         }
1254
1255         debug!(
1256             "match_candidates: add pre_binding_blocks for unreachable {:?}",
1257             unreachable_candidates,
1258         );
1259         for candidate in unreachable_candidates {
1260             assert!(candidate.pre_binding_block.is_none());
1261             candidate.pre_binding_block = Some(self.cfg.start_new_block());
1262         }
1263
1264         reachable_candidates.last_mut().unwrap().otherwise_block
1265     }
1266
1267     /// Tests a candidate where there are only or-patterns left to test, or
1268     /// forwards to [Builder::test_candidates].
1269     ///
1270     /// Given a pattern `(P | Q, R | S)` we (in principle) generate a CFG like
1271     /// so:
1272     ///
1273     /// ```text
1274     /// [ start ]
1275     ///      |
1276     /// [ match P, Q ]
1277     ///      |
1278     ///      +----------------------------------------+------------------------------------+
1279     ///      |                                        |                                    |
1280     ///      V                                        V                                    V
1281     /// [ P matches ]                           [ Q matches ]                        [ otherwise ]
1282     ///      |                                        |                                    |
1283     ///      V                                        V                                    |
1284     /// [ match R, S ]                          [ match R, S ]                             |
1285     ///      |                                        |                                    |
1286     ///      +--------------+------------+            +--------------+------------+        |
1287     ///      |              |            |            |              |            |        |
1288     ///      V              V            V            V              V            V        |
1289     /// [ R matches ] [ S matches ] [otherwise ] [ R matches ] [ S matches ] [otherwise ]  |
1290     ///      |              |            |            |              |            |        |
1291     ///      +--------------+------------|------------+--------------+            |        |
1292     ///      |                           |                                        |        |
1293     ///      |                           +----------------------------------------+--------+
1294     ///      |                           |
1295     ///      V                           V
1296     /// [ Success ]                 [ Failure ]
1297     /// ```
1298     ///
1299     /// In practice there are some complications:
1300     ///
1301     /// * If there's a guard, then the otherwise branch of the first match on
1302     ///   `R | S` goes to a test for whether `Q` matches, and the control flow
1303     ///   doesn't merge into a single success block until after the guard is
1304     ///   tested.
1305     /// * If neither `P` or `Q` has any bindings or type ascriptions and there
1306     ///   isn't a match guard, then we create a smaller CFG like:
1307     ///
1308     /// ```text
1309     ///     ...
1310     ///      +---------------+------------+
1311     ///      |               |            |
1312     /// [ P matches ] [ Q matches ] [ otherwise ]
1313     ///      |               |            |
1314     ///      +---------------+            |
1315     ///      |                           ...
1316     /// [ match R, S ]
1317     ///      |
1318     ///     ...
1319     /// ```
1320     fn test_candidates_with_or(
1321         &mut self,
1322         span: Span,
1323         scrutinee_span: Span,
1324         candidates: &mut [&mut Candidate<'_, 'tcx>],
1325         block: BasicBlock,
1326         otherwise_block: &mut Option<BasicBlock>,
1327         fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1328     ) {
1329         let (first_candidate, remaining_candidates) = candidates.split_first_mut().unwrap();
1330
1331         // All of the or-patterns have been sorted to the end, so if the first
1332         // pattern is an or-pattern we only have or-patterns.
1333         match *first_candidate.match_pairs[0].pattern.kind {
1334             PatKind::Or { .. } => (),
1335             _ => {
1336                 self.test_candidates(
1337                     span,
1338                     scrutinee_span,
1339                     candidates,
1340                     block,
1341                     otherwise_block,
1342                     fake_borrows,
1343                 );
1344                 return;
1345             }
1346         }
1347
1348         let match_pairs = mem::take(&mut first_candidate.match_pairs);
1349         first_candidate.pre_binding_block = Some(block);
1350
1351         let mut otherwise = None;
1352         for match_pair in match_pairs {
1353             let PatKind::Or { ref pats } = &*match_pair.pattern.kind else {
1354                 bug!("Or-patterns should have been sorted to the end");
1355             };
1356             let or_span = match_pair.pattern.span;
1357             let place = match_pair.place;
1358
1359             first_candidate.visit_leaves(|leaf_candidate| {
1360                 self.test_or_pattern(
1361                     leaf_candidate,
1362                     &mut otherwise,
1363                     pats,
1364                     or_span,
1365                     place.clone(),
1366                     fake_borrows,
1367                 );
1368             });
1369         }
1370
1371         let remainder_start = otherwise.unwrap_or_else(|| self.cfg.start_new_block());
1372
1373         self.match_candidates(
1374             span,
1375             scrutinee_span,
1376             remainder_start,
1377             otherwise_block,
1378             remaining_candidates,
1379             fake_borrows,
1380         )
1381     }
1382
1383     fn test_or_pattern<'pat>(
1384         &mut self,
1385         candidate: &mut Candidate<'pat, 'tcx>,
1386         otherwise: &mut Option<BasicBlock>,
1387         pats: &'pat [Pat<'tcx>],
1388         or_span: Span,
1389         place: PlaceBuilder<'tcx>,
1390         fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1391     ) {
1392         debug!("test_or_pattern:\ncandidate={:#?}\npats={:#?}", candidate, pats);
1393         let mut or_candidates: Vec<_> = pats
1394             .iter()
1395             .map(|pat| Candidate::new(place.clone(), pat, candidate.has_guard))
1396             .collect();
1397         let mut or_candidate_refs: Vec<_> = or_candidates.iter_mut().collect();
1398         let otherwise = if candidate.otherwise_block.is_some() {
1399             &mut candidate.otherwise_block
1400         } else {
1401             otherwise
1402         };
1403         self.match_candidates(
1404             or_span,
1405             or_span,
1406             candidate.pre_binding_block.unwrap(),
1407             otherwise,
1408             &mut or_candidate_refs,
1409             fake_borrows,
1410         );
1411         candidate.subcandidates = or_candidates;
1412         self.merge_trivial_subcandidates(candidate, self.source_info(or_span));
1413     }
1414
1415     /// Try to merge all of the subcandidates of the given candidate into one.
1416     /// This avoids exponentially large CFGs in cases like `(1 | 2, 3 | 4, ...)`.
1417     fn merge_trivial_subcandidates(
1418         &mut self,
1419         candidate: &mut Candidate<'_, 'tcx>,
1420         source_info: SourceInfo,
1421     ) {
1422         if candidate.subcandidates.is_empty() || candidate.has_guard {
1423             // FIXME(or_patterns; matthewjasper) Don't give up if we have a guard.
1424             return;
1425         }
1426
1427         let mut can_merge = true;
1428
1429         // Not `Iterator::all` because we don't want to short-circuit.
1430         for subcandidate in &mut candidate.subcandidates {
1431             self.merge_trivial_subcandidates(subcandidate, source_info);
1432
1433             // FIXME(or_patterns; matthewjasper) Try to be more aggressive here.
1434             can_merge &= subcandidate.subcandidates.is_empty()
1435                 && subcandidate.bindings.is_empty()
1436                 && subcandidate.ascriptions.is_empty();
1437         }
1438
1439         if can_merge {
1440             let any_matches = self.cfg.start_new_block();
1441             for subcandidate in mem::take(&mut candidate.subcandidates) {
1442                 let or_block = subcandidate.pre_binding_block.unwrap();
1443                 self.cfg.goto(or_block, source_info, any_matches);
1444             }
1445             candidate.pre_binding_block = Some(any_matches);
1446         }
1447     }
1448
1449     /// This is the most subtle part of the matching algorithm. At
1450     /// this point, the input candidates have been fully simplified,
1451     /// and so we know that all remaining match-pairs require some
1452     /// sort of test. To decide what test to perform, we take the highest
1453     /// priority candidate (the first one in the list, as of January 2021)
1454     /// and extract the first match-pair from the list. From this we decide
1455     /// what kind of test is needed using [`Builder::test`], defined in the
1456     /// [`test` module](mod@test).
1457     ///
1458     /// *Note:* taking the first match pair is somewhat arbitrary, and
1459     /// we might do better here by choosing more carefully what to
1460     /// test.
1461     ///
1462     /// For example, consider the following possible match-pairs:
1463     ///
1464     /// 1. `x @ Some(P)` -- we will do a [`Switch`] to decide what variant `x` has
1465     /// 2. `x @ 22` -- we will do a [`SwitchInt`] to decide what value `x` has
1466     /// 3. `x @ 3..5` -- we will do a [`Range`] test to decide what range `x` falls in
1467     /// 4. etc.
1468     ///
1469     /// [`Switch`]: TestKind::Switch
1470     /// [`SwitchInt`]: TestKind::SwitchInt
1471     /// [`Range`]: TestKind::Range
1472     ///
1473     /// Once we know what sort of test we are going to perform, this
1474     /// test may also help us winnow down our candidates. So we walk over
1475     /// the candidates (from high to low priority) and check. This
1476     /// gives us, for each outcome of the test, a transformed list of
1477     /// candidates. For example, if we are testing `x.0`'s variant,
1478     /// and we have a candidate `(x.0 @ Some(v), x.1 @ 22)`,
1479     /// then we would have a resulting candidate of `((x.0 as Some).0 @ v, x.1 @ 22)`.
1480     /// Note that the first match-pair is now simpler (and, in fact, irrefutable).
1481     ///
1482     /// But there may also be candidates that the test just doesn't
1483     /// apply to. The classical example involves wildcards:
1484     ///
1485     /// ```
1486     /// # let (x, y, z) = (true, true, true);
1487     /// match (x, y, z) {
1488     ///     (true , _    , true ) => true,  // (0)
1489     ///     (_    , true , _    ) => true,  // (1)
1490     ///     (false, false, _    ) => false, // (2)
1491     ///     (true , _    , false) => false, // (3)
1492     /// }
1493     /// # ;
1494     /// ```
1495     ///
1496     /// In that case, after we test on `x`, there are 2 overlapping candidate
1497     /// sets:
1498     ///
1499     /// - If the outcome is that `x` is true, candidates 0, 1, and 3
1500     /// - If the outcome is that `x` is false, candidates 1 and 2
1501     ///
1502     /// Here, the traditional "decision tree" method would generate 2
1503     /// separate code-paths for the 2 separate cases.
1504     ///
1505     /// In some cases, this duplication can create an exponential amount of
1506     /// code. This is most easily seen by noticing that this method terminates
1507     /// with precisely the reachable arms being reachable - but that problem
1508     /// is trivially NP-complete:
1509     ///
1510     /// ```ignore (illustrative)
1511     /// match (var0, var1, var2, var3, ...) {
1512     ///     (true , _   , _    , false, true, ...) => false,
1513     ///     (_    , true, true , false, _   , ...) => false,
1514     ///     (false, _   , false, false, _   , ...) => false,
1515     ///     ...
1516     ///     _ => true
1517     /// }
1518     /// ```
1519     ///
1520     /// Here the last arm is reachable only if there is an assignment to
1521     /// the variables that does not match any of the literals. Therefore,
1522     /// compilation would take an exponential amount of time in some cases.
1523     ///
1524     /// That kind of exponential worst-case might not occur in practice, but
1525     /// our simplistic treatment of constants and guards would make it occur
1526     /// in very common situations - for example [#29740]:
1527     ///
1528     /// ```ignore (illustrative)
1529     /// match x {
1530     ///     "foo" if foo_guard => ...,
1531     ///     "bar" if bar_guard => ...,
1532     ///     "baz" if baz_guard => ...,
1533     ///     ...
1534     /// }
1535     /// ```
1536     ///
1537     /// [#29740]: https://github.com/rust-lang/rust/issues/29740
1538     ///
1539     /// Here we first test the match-pair `x @ "foo"`, which is an [`Eq` test].
1540     ///
1541     /// [`Eq` test]: TestKind::Eq
1542     ///
1543     /// It might seem that we would end up with 2 disjoint candidate
1544     /// sets, consisting of the first candidate or the other two, but our
1545     /// algorithm doesn't reason about `"foo"` being distinct from the other
1546     /// constants; it considers the latter arms to potentially match after
1547     /// both outcomes, which obviously leads to an exponential number
1548     /// of tests.
1549     ///
1550     /// To avoid these kinds of problems, our algorithm tries to ensure
1551     /// the amount of generated tests is linear. When we do a k-way test,
1552     /// we return an additional "unmatched" set alongside the obvious `k`
1553     /// sets. When we encounter a candidate that would be present in more
1554     /// than one of the sets, we put it and all candidates below it into the
1555     /// "unmatched" set. This ensures these `k+1` sets are disjoint.
1556     ///
1557     /// After we perform our test, we branch into the appropriate candidate
1558     /// set and recurse with `match_candidates`. These sub-matches are
1559     /// obviously non-exhaustive - as we discarded our otherwise set - so
1560     /// we set their continuation to do `match_candidates` on the
1561     /// "unmatched" set (which is again non-exhaustive).
1562     ///
1563     /// If you apply this to the above test, you basically wind up
1564     /// with an if-else-if chain, testing each candidate in turn,
1565     /// which is precisely what we want.
1566     ///
1567     /// In addition to avoiding exponential-time blowups, this algorithm
1568     /// also has the nice property that each guard and arm is only generated
1569     /// once.
1570     fn test_candidates<'pat, 'b, 'c>(
1571         &mut self,
1572         span: Span,
1573         scrutinee_span: Span,
1574         mut candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>],
1575         block: BasicBlock,
1576         otherwise_block: &mut Option<BasicBlock>,
1577         fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1578     ) {
1579         // extract the match-pair from the highest priority candidate
1580         let match_pair = &candidates.first().unwrap().match_pairs[0];
1581         let mut test = self.test(match_pair);
1582         let match_place = match_pair.place.clone();
1583
1584         // most of the time, the test to perform is simply a function
1585         // of the main candidate; but for a test like SwitchInt, we
1586         // may want to add cases based on the candidates that are
1587         // available
1588         match test.kind {
1589             TestKind::SwitchInt { switch_ty, ref mut options } => {
1590                 for candidate in candidates.iter() {
1591                     if !self.add_cases_to_switch(&match_place, candidate, switch_ty, options) {
1592                         break;
1593                     }
1594                 }
1595             }
1596             TestKind::Switch { adt_def: _, ref mut variants } => {
1597                 for candidate in candidates.iter() {
1598                     if !self.add_variants_to_switch(&match_place, candidate, variants) {
1599                         break;
1600                     }
1601                 }
1602             }
1603             _ => {}
1604         }
1605
1606         // Insert a Shallow borrow of any places that is switched on.
1607         if let Some(fb) = fake_borrows && let Ok(match_place_resolved) =
1608             match_place.clone().try_upvars_resolved(self.tcx, self.typeck_results)
1609         {
1610             let resolved_place = match_place_resolved.into_place(self.tcx, self.typeck_results);
1611             fb.insert(resolved_place);
1612         }
1613
1614         // perform the test, branching to one of N blocks. For each of
1615         // those N possible outcomes, create a (initially empty)
1616         // vector of candidates. Those are the candidates that still
1617         // apply if the test has that particular outcome.
1618         debug!("test_candidates: test={:?} match_pair={:?}", test, match_pair);
1619         let mut target_candidates: Vec<Vec<&mut Candidate<'pat, 'tcx>>> = vec![];
1620         target_candidates.resize_with(test.targets(), Default::default);
1621
1622         let total_candidate_count = candidates.len();
1623
1624         // Sort the candidates into the appropriate vector in
1625         // `target_candidates`. Note that at some point we may
1626         // encounter a candidate where the test is not relevant; at
1627         // that point, we stop sorting.
1628         while let Some(candidate) = candidates.first_mut() {
1629             let Some(idx) = self.sort_candidate(&match_place.clone(), &test, candidate) else {
1630                 break;
1631             };
1632             let (candidate, rest) = candidates.split_first_mut().unwrap();
1633             target_candidates[idx].push(candidate);
1634             candidates = rest;
1635         }
1636         // at least the first candidate ought to be tested
1637         assert!(total_candidate_count > candidates.len());
1638         debug!("test_candidates: tested_candidates: {}", total_candidate_count - candidates.len());
1639         debug!("test_candidates: untested_candidates: {}", candidates.len());
1640
1641         // HACK(matthewjasper) This is a closure so that we can let the test
1642         // create its blocks before the rest of the match. This currently
1643         // improves the speed of llvm when optimizing long string literal
1644         // matches
1645         let make_target_blocks = move |this: &mut Self| -> Vec<BasicBlock> {
1646             // The block that we should branch to if none of the
1647             // `target_candidates` match. This is either the block where we
1648             // start matching the untested candidates if there are any,
1649             // otherwise it's the `otherwise_block`.
1650             let remainder_start = &mut None;
1651             let remainder_start =
1652                 if candidates.is_empty() { &mut *otherwise_block } else { remainder_start };
1653
1654             // For each outcome of test, process the candidates that still
1655             // apply. Collect a list of blocks where control flow will
1656             // branch if one of the `target_candidate` sets is not
1657             // exhaustive.
1658             let target_blocks: Vec<_> = target_candidates
1659                 .into_iter()
1660                 .map(|mut candidates| {
1661                     if !candidates.is_empty() {
1662                         let candidate_start = this.cfg.start_new_block();
1663                         this.match_candidates(
1664                             span,
1665                             scrutinee_span,
1666                             candidate_start,
1667                             remainder_start,
1668                             &mut *candidates,
1669                             fake_borrows,
1670                         );
1671                         candidate_start
1672                     } else {
1673                         *remainder_start.get_or_insert_with(|| this.cfg.start_new_block())
1674                     }
1675                 })
1676                 .collect();
1677
1678             if !candidates.is_empty() {
1679                 let remainder_start = remainder_start.unwrap_or_else(|| this.cfg.start_new_block());
1680                 this.match_candidates(
1681                     span,
1682                     scrutinee_span,
1683                     remainder_start,
1684                     otherwise_block,
1685                     candidates,
1686                     fake_borrows,
1687                 );
1688             };
1689
1690             target_blocks
1691         };
1692
1693         self.perform_test(span, scrutinee_span, block, match_place, &test, make_target_blocks);
1694     }
1695
1696     /// Determine the fake borrows that are needed from a set of places that
1697     /// have to be stable across match guards.
1698     ///
1699     /// Returns a list of places that need a fake borrow and the temporary
1700     /// that's used to store the fake borrow.
1701     ///
1702     /// Match exhaustiveness checking is not able to handle the case where the
1703     /// place being matched on is mutated in the guards. We add "fake borrows"
1704     /// to the guards that prevent any mutation of the place being matched.
1705     /// There are a some subtleties:
1706     ///
1707     /// 1. Borrowing `*x` doesn't prevent assigning to `x`. If `x` is a shared
1708     ///    reference, the borrow isn't even tracked. As such we have to add fake
1709     ///    borrows of any prefixes of a place
1710     /// 2. We don't want `match x { _ => (), }` to conflict with mutable
1711     ///    borrows of `x`, so we only add fake borrows for places which are
1712     ///    bound or tested by the match.
1713     /// 3. We don't want the fake borrows to conflict with `ref mut` bindings,
1714     ///    so we use a special BorrowKind for them.
1715     /// 4. The fake borrows may be of places in inactive variants, so it would
1716     ///    be UB to generate code for them. They therefore have to be removed
1717     ///    by a MIR pass run after borrow checking.
1718     fn calculate_fake_borrows<'b>(
1719         &mut self,
1720         fake_borrows: &'b FxIndexSet<Place<'tcx>>,
1721         temp_span: Span,
1722     ) -> Vec<(Place<'tcx>, Local)> {
1723         let tcx = self.tcx;
1724
1725         debug!("add_fake_borrows fake_borrows = {:?}", fake_borrows);
1726
1727         let mut all_fake_borrows = Vec::with_capacity(fake_borrows.len());
1728
1729         // Insert a Shallow borrow of the prefixes of any fake borrows.
1730         for place in fake_borrows {
1731             let mut cursor = place.projection.as_ref();
1732             while let [proj_base @ .., elem] = cursor {
1733                 cursor = proj_base;
1734
1735                 if let ProjectionElem::Deref = elem {
1736                     // Insert a shallow borrow after a deref. For other
1737                     // projections the borrow of prefix_cursor will
1738                     // conflict with any mutation of base.
1739                     all_fake_borrows.push(PlaceRef { local: place.local, projection: proj_base });
1740                 }
1741             }
1742
1743             all_fake_borrows.push(place.as_ref());
1744         }
1745
1746         // Deduplicate
1747         let mut dedup = FxHashSet::default();
1748         all_fake_borrows.retain(|b| dedup.insert(*b));
1749
1750         debug!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows);
1751
1752         all_fake_borrows
1753             .into_iter()
1754             .map(|matched_place_ref| {
1755                 let matched_place = Place {
1756                     local: matched_place_ref.local,
1757                     projection: tcx.intern_place_elems(matched_place_ref.projection),
1758                 };
1759                 let fake_borrow_deref_ty = matched_place.ty(&self.local_decls, tcx).ty;
1760                 let fake_borrow_ty = tcx.mk_imm_ref(tcx.lifetimes.re_erased, fake_borrow_deref_ty);
1761                 let fake_borrow_temp =
1762                     self.local_decls.push(LocalDecl::new(fake_borrow_ty, temp_span));
1763
1764                 (matched_place, fake_borrow_temp)
1765             })
1766             .collect()
1767     }
1768 }
1769
1770 ///////////////////////////////////////////////////////////////////////////
1771 // Pat binding - used for `let` and function parameters as well.
1772
1773 impl<'a, 'tcx> Builder<'a, 'tcx> {
1774     pub(crate) fn lower_let_expr(
1775         &mut self,
1776         mut block: BasicBlock,
1777         expr: &Expr<'tcx>,
1778         pat: &Pat<'tcx>,
1779         else_target: region::Scope,
1780         source_scope: Option<SourceScope>,
1781         span: Span,
1782     ) -> BlockAnd<()> {
1783         let expr_span = expr.span;
1784         let expr_place_builder = unpack!(block = self.lower_scrutinee(block, expr, expr_span));
1785         let wildcard = Pat::wildcard_from_ty(pat.ty);
1786         let mut guard_candidate = Candidate::new(expr_place_builder.clone(), &pat, false);
1787         let mut otherwise_candidate = Candidate::new(expr_place_builder.clone(), &wildcard, false);
1788         let fake_borrow_temps = self.lower_match_tree(
1789             block,
1790             pat.span,
1791             pat.span,
1792             false,
1793             &mut [&mut guard_candidate, &mut otherwise_candidate],
1794         );
1795         let mut opt_expr_place: Option<(Option<&Place<'tcx>>, Span)> = None;
1796         let expr_place: Place<'tcx>;
1797         if let Ok(expr_builder) =
1798             expr_place_builder.try_upvars_resolved(self.tcx, self.typeck_results)
1799         {
1800             expr_place = expr_builder.into_place(self.tcx, self.typeck_results);
1801             opt_expr_place = Some((Some(&expr_place), expr_span));
1802         }
1803         let otherwise_post_guard_block = otherwise_candidate.pre_binding_block.unwrap();
1804         self.break_for_else(otherwise_post_guard_block, else_target, self.source_info(expr_span));
1805
1806         self.declare_bindings(
1807             source_scope,
1808             pat.span.to(span),
1809             pat,
1810             ArmHasGuard(false),
1811             opt_expr_place,
1812         );
1813
1814         let post_guard_block = self.bind_pattern(
1815             self.source_info(pat.span),
1816             guard_candidate,
1817             None,
1818             &fake_borrow_temps,
1819             expr.span,
1820             None,
1821             None,
1822             None,
1823         );
1824
1825         post_guard_block.unit()
1826     }
1827
1828     /// Initializes each of the bindings from the candidate by
1829     /// moving/copying/ref'ing the source as appropriate. Tests the guard, if
1830     /// any, and then branches to the arm. Returns the block for the case where
1831     /// the guard succeeds.
1832     ///
1833     /// Note: we do not check earlier that if there is a guard,
1834     /// there cannot be move bindings. We avoid a use-after-move by only
1835     /// moving the binding once the guard has evaluated to true (see below).
1836     fn bind_and_guard_matched_candidate<'pat>(
1837         &mut self,
1838         candidate: Candidate<'pat, 'tcx>,
1839         parent_bindings: &[(Vec<Binding<'tcx>>, Vec<Ascription<'tcx>>)],
1840         guard: Option<&Guard<'tcx>>,
1841         fake_borrows: &[(Place<'tcx>, Local)],
1842         scrutinee_span: Span,
1843         arm_span: Option<Span>,
1844         match_scope: Option<region::Scope>,
1845         schedule_drops: bool,
1846     ) -> BasicBlock {
1847         debug!("bind_and_guard_matched_candidate(candidate={:?})", candidate);
1848
1849         debug_assert!(candidate.match_pairs.is_empty());
1850
1851         let candidate_source_info = self.source_info(candidate.span);
1852
1853         let mut block = candidate.pre_binding_block.unwrap();
1854
1855         if candidate.next_candidate_pre_binding_block.is_some() {
1856             let fresh_block = self.cfg.start_new_block();
1857             self.false_edges(
1858                 block,
1859                 fresh_block,
1860                 candidate.next_candidate_pre_binding_block,
1861                 candidate_source_info,
1862             );
1863             block = fresh_block;
1864         }
1865
1866         self.ascribe_types(
1867             block,
1868             parent_bindings
1869                 .iter()
1870                 .flat_map(|(_, ascriptions)| ascriptions)
1871                 .cloned()
1872                 .chain(candidate.ascriptions),
1873         );
1874
1875         // rust-lang/rust#27282: The `autoref` business deserves some
1876         // explanation here.
1877         //
1878         // The intent of the `autoref` flag is that when it is true,
1879         // then any pattern bindings of type T will map to a `&T`
1880         // within the context of the guard expression, but will
1881         // continue to map to a `T` in the context of the arm body. To
1882         // avoid surfacing this distinction in the user source code
1883         // (which would be a severe change to the language and require
1884         // far more revision to the compiler), when `autoref` is true,
1885         // then any occurrence of the identifier in the guard
1886         // expression will automatically get a deref op applied to it.
1887         //
1888         // So an input like:
1889         //
1890         // ```
1891         // let place = Foo::new();
1892         // match place { foo if inspect(foo)
1893         //     => feed(foo), ...  }
1894         // ```
1895         //
1896         // will be treated as if it were really something like:
1897         //
1898         // ```
1899         // let place = Foo::new();
1900         // match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) }
1901         //     => { let tmp2 = place; feed(tmp2) }, ... }
1902         //
1903         // And an input like:
1904         //
1905         // ```
1906         // let place = Foo::new();
1907         // match place { ref mut foo if inspect(foo)
1908         //     => feed(foo), ...  }
1909         // ```
1910         //
1911         // will be treated as if it were really something like:
1912         //
1913         // ```
1914         // let place = Foo::new();
1915         // match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) }
1916         //     => { let tmp2 = &mut place; feed(tmp2) }, ... }
1917         // ```
1918         //
1919         // In short, any pattern binding will always look like *some*
1920         // kind of `&T` within the guard at least in terms of how the
1921         // MIR-borrowck views it, and this will ensure that guard
1922         // expressions cannot mutate their the match inputs via such
1923         // bindings. (It also ensures that guard expressions can at
1924         // most *copy* values from such bindings; non-Copy things
1925         // cannot be moved via pattern bindings in guard expressions.)
1926         //
1927         // ----
1928         //
1929         // Implementation notes (under assumption `autoref` is true).
1930         //
1931         // To encode the distinction above, we must inject the
1932         // temporaries `tmp1` and `tmp2`.
1933         //
1934         // There are two cases of interest: binding by-value, and binding by-ref.
1935         //
1936         // 1. Binding by-value: Things are simple.
1937         //
1938         //    * Establishing `tmp1` creates a reference into the
1939         //      matched place. This code is emitted by
1940         //      bind_matched_candidate_for_guard.
1941         //
1942         //    * `tmp2` is only initialized "lazily", after we have
1943         //      checked the guard. Thus, the code that can trigger
1944         //      moves out of the candidate can only fire after the
1945         //      guard evaluated to true. This initialization code is
1946         //      emitted by bind_matched_candidate_for_arm.
1947         //
1948         // 2. Binding by-reference: Things are tricky.
1949         //
1950         //    * Here, the guard expression wants a `&&` or `&&mut`
1951         //      into the original input. This means we need to borrow
1952         //      the reference that we create for the arm.
1953         //    * So we eagerly create the reference for the arm and then take a
1954         //      reference to that.
1955         if let Some(guard) = guard {
1956             let tcx = self.tcx;
1957             let bindings = parent_bindings
1958                 .iter()
1959                 .flat_map(|(bindings, _)| bindings)
1960                 .chain(&candidate.bindings);
1961
1962             self.bind_matched_candidate_for_guard(block, schedule_drops, bindings.clone());
1963             let guard_frame = GuardFrame {
1964                 locals: bindings.map(|b| GuardFrameLocal::new(b.var_id, b.binding_mode)).collect(),
1965             };
1966             debug!("entering guard building context: {:?}", guard_frame);
1967             self.guard_context.push(guard_frame);
1968
1969             let re_erased = tcx.lifetimes.re_erased;
1970             let scrutinee_source_info = self.source_info(scrutinee_span);
1971             for &(place, temp) in fake_borrows {
1972                 let borrow = Rvalue::Ref(re_erased, BorrowKind::Shallow, place);
1973                 self.cfg.push_assign(block, scrutinee_source_info, Place::from(temp), borrow);
1974             }
1975
1976             let arm_span = arm_span.unwrap();
1977             let match_scope = match_scope.unwrap();
1978             let mut guard_span = rustc_span::DUMMY_SP;
1979
1980             let (post_guard_block, otherwise_post_guard_block) =
1981                 self.in_if_then_scope(match_scope, |this| match *guard {
1982                     Guard::If(e) => {
1983                         let e = &this.thir[e];
1984                         guard_span = e.span;
1985                         this.then_else_break(
1986                             block,
1987                             e,
1988                             None,
1989                             match_scope,
1990                             this.source_info(arm_span),
1991                         )
1992                     }
1993                     Guard::IfLet(ref pat, scrutinee) => {
1994                         let s = &this.thir[scrutinee];
1995                         guard_span = s.span;
1996                         this.lower_let_expr(block, s, pat, match_scope, None, arm_span)
1997                     }
1998                 });
1999
2000             let source_info = self.source_info(guard_span);
2001             let guard_end = self.source_info(tcx.sess.source_map().end_point(guard_span));
2002             let guard_frame = self.guard_context.pop().unwrap();
2003             debug!("Exiting guard building context with locals: {:?}", guard_frame);
2004
2005             for &(_, temp) in fake_borrows {
2006                 let cause = FakeReadCause::ForMatchGuard;
2007                 self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(temp));
2008             }
2009
2010             let otherwise_block = candidate.otherwise_block.unwrap_or_else(|| {
2011                 let unreachable = self.cfg.start_new_block();
2012                 self.cfg.terminate(unreachable, source_info, TerminatorKind::Unreachable);
2013                 unreachable
2014             });
2015             self.false_edges(
2016                 otherwise_post_guard_block,
2017                 otherwise_block,
2018                 candidate.next_candidate_pre_binding_block,
2019                 source_info,
2020             );
2021
2022             // We want to ensure that the matched candidates are bound
2023             // after we have confirmed this candidate *and* any
2024             // associated guard; Binding them on `block` is too soon,
2025             // because that would be before we've checked the result
2026             // from the guard.
2027             //
2028             // But binding them on the arm is *too late*, because
2029             // then all of the candidates for a single arm would be
2030             // bound in the same place, that would cause a case like:
2031             //
2032             // ```rust
2033             // match (30, 2) {
2034             //     (mut x, 1) | (2, mut x) if { true } => { ... }
2035             //     ...                                 // ^^^^^^^ (this is `arm_block`)
2036             // }
2037             // ```
2038             //
2039             // would yield an `arm_block` something like:
2040             //
2041             // ```
2042             // StorageLive(_4);        // _4 is `x`
2043             // _4 = &mut (_1.0: i32);  // this is handling `(mut x, 1)` case
2044             // _4 = &mut (_1.1: i32);  // this is handling `(2, mut x)` case
2045             // ```
2046             //
2047             // and that is clearly not correct.
2048             let by_value_bindings = parent_bindings
2049                 .iter()
2050                 .flat_map(|(bindings, _)| bindings)
2051                 .chain(&candidate.bindings)
2052                 .filter(|binding| matches!(binding.binding_mode, BindingMode::ByValue));
2053             // Read all of the by reference bindings to ensure that the
2054             // place they refer to can't be modified by the guard.
2055             for binding in by_value_bindings.clone() {
2056                 let local_id = self.var_local_id(binding.var_id, RefWithinGuard);
2057                 let cause = FakeReadCause::ForGuardBinding;
2058                 self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(local_id));
2059             }
2060             assert!(schedule_drops, "patterns with guards must schedule drops");
2061             self.bind_matched_candidate_for_arm_body(post_guard_block, true, by_value_bindings);
2062
2063             post_guard_block
2064         } else {
2065             // (Here, it is not too early to bind the matched
2066             // candidate on `block`, because there is no guard result
2067             // that we have to inspect before we bind them.)
2068             self.bind_matched_candidate_for_arm_body(
2069                 block,
2070                 schedule_drops,
2071                 parent_bindings
2072                     .iter()
2073                     .flat_map(|(bindings, _)| bindings)
2074                     .chain(&candidate.bindings),
2075             );
2076             block
2077         }
2078     }
2079
2080     /// Append `AscribeUserType` statements onto the end of `block`
2081     /// for each ascription
2082     fn ascribe_types(
2083         &mut self,
2084         block: BasicBlock,
2085         ascriptions: impl IntoIterator<Item = Ascription<'tcx>>,
2086     ) {
2087         for ascription in ascriptions {
2088             let source_info = self.source_info(ascription.annotation.span);
2089
2090             let base = self.canonical_user_type_annotations.push(ascription.annotation);
2091             self.cfg.push(
2092                 block,
2093                 Statement {
2094                     source_info,
2095                     kind: StatementKind::AscribeUserType(
2096                         Box::new((
2097                             ascription.source,
2098                             UserTypeProjection { base, projs: Vec::new() },
2099                         )),
2100                         ascription.variance,
2101                     ),
2102                 },
2103             );
2104         }
2105     }
2106
2107     fn bind_matched_candidate_for_guard<'b>(
2108         &mut self,
2109         block: BasicBlock,
2110         schedule_drops: bool,
2111         bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
2112     ) where
2113         'tcx: 'b,
2114     {
2115         debug!("bind_matched_candidate_for_guard(block={:?})", block);
2116
2117         // Assign each of the bindings. Since we are binding for a
2118         // guard expression, this will never trigger moves out of the
2119         // candidate.
2120         let re_erased = self.tcx.lifetimes.re_erased;
2121         for binding in bindings {
2122             debug!("bind_matched_candidate_for_guard(binding={:?})", binding);
2123             let source_info = self.source_info(binding.span);
2124
2125             // For each pattern ident P of type T, `ref_for_guard` is
2126             // a reference R: &T pointing to the location matched by
2127             // the pattern, and every occurrence of P within a guard
2128             // denotes *R.
2129             let ref_for_guard = self.storage_live_binding(
2130                 block,
2131                 binding.var_id,
2132                 binding.span,
2133                 RefWithinGuard,
2134                 schedule_drops,
2135             );
2136             match binding.binding_mode {
2137                 BindingMode::ByValue => {
2138                     let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, binding.source);
2139                     self.cfg.push_assign(block, source_info, ref_for_guard, rvalue);
2140                 }
2141                 BindingMode::ByRef(borrow_kind) => {
2142                     let value_for_arm = self.storage_live_binding(
2143                         block,
2144                         binding.var_id,
2145                         binding.span,
2146                         OutsideGuard,
2147                         schedule_drops,
2148                     );
2149
2150                     let rvalue = Rvalue::Ref(re_erased, borrow_kind, binding.source);
2151                     self.cfg.push_assign(block, source_info, value_for_arm, rvalue);
2152                     let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, value_for_arm);
2153                     self.cfg.push_assign(block, source_info, ref_for_guard, rvalue);
2154                 }
2155             }
2156         }
2157     }
2158
2159     fn bind_matched_candidate_for_arm_body<'b>(
2160         &mut self,
2161         block: BasicBlock,
2162         schedule_drops: bool,
2163         bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
2164     ) where
2165         'tcx: 'b,
2166     {
2167         debug!("bind_matched_candidate_for_arm_body(block={:?})", block);
2168
2169         let re_erased = self.tcx.lifetimes.re_erased;
2170         // Assign each of the bindings. This may trigger moves out of the candidate.
2171         for binding in bindings {
2172             let source_info = self.source_info(binding.span);
2173             let local = self.storage_live_binding(
2174                 block,
2175                 binding.var_id,
2176                 binding.span,
2177                 OutsideGuard,
2178                 schedule_drops,
2179             );
2180             if schedule_drops {
2181                 self.schedule_drop_for_binding(binding.var_id, binding.span, OutsideGuard);
2182             }
2183             let rvalue = match binding.binding_mode {
2184                 BindingMode::ByValue => Rvalue::Use(self.consume_by_copy_or_move(binding.source)),
2185                 BindingMode::ByRef(borrow_kind) => {
2186                     Rvalue::Ref(re_erased, borrow_kind, binding.source)
2187                 }
2188             };
2189             self.cfg.push_assign(block, source_info, local, rvalue);
2190         }
2191     }
2192
2193     /// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where the bound
2194     /// `var` has type `T` in the arm body) in a pattern maps to 2 locals. The
2195     /// first local is a binding for occurrences of `var` in the guard, which
2196     /// will have type `&T`. The second local is a binding for occurrences of
2197     /// `var` in the arm body, which will have type `T`.
2198     fn declare_binding(
2199         &mut self,
2200         source_info: SourceInfo,
2201         visibility_scope: SourceScope,
2202         mutability: Mutability,
2203         name: Symbol,
2204         mode: BindingMode,
2205         var_id: LocalVarId,
2206         var_ty: Ty<'tcx>,
2207         user_ty: UserTypeProjections,
2208         has_guard: ArmHasGuard,
2209         opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
2210         pat_span: Span,
2211     ) {
2212         debug!(
2213             "declare_binding(var_id={:?}, name={:?}, mode={:?}, var_ty={:?}, \
2214              visibility_scope={:?}, source_info={:?})",
2215             var_id, name, mode, var_ty, visibility_scope, source_info
2216         );
2217
2218         let tcx = self.tcx;
2219         let debug_source_info = SourceInfo { span: source_info.span, scope: visibility_scope };
2220         let binding_mode = match mode {
2221             BindingMode::ByValue => ty::BindingMode::BindByValue(mutability),
2222             BindingMode::ByRef(_) => ty::BindingMode::BindByReference(mutability),
2223         };
2224         debug!("declare_binding: user_ty={:?}", user_ty);
2225         let local = LocalDecl::<'tcx> {
2226             mutability,
2227             ty: var_ty,
2228             user_ty: if user_ty.is_empty() { None } else { Some(Box::new(user_ty)) },
2229             source_info,
2230             internal: false,
2231             is_block_tail: None,
2232             local_info: Some(Box::new(LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
2233                 VarBindingForm {
2234                     binding_mode,
2235                     // hypothetically, `visit_primary_bindings` could try to unzip
2236                     // an outermost hir::Ty as we descend, matching up
2237                     // idents in pat; but complex w/ unclear UI payoff.
2238                     // Instead, just abandon providing diagnostic info.
2239                     opt_ty_info: None,
2240                     opt_match_place,
2241                     pat_span,
2242                 },
2243             ))))),
2244         };
2245         let for_arm_body = self.local_decls.push(local);
2246         self.var_debug_info.push(VarDebugInfo {
2247             name,
2248             source_info: debug_source_info,
2249             value: VarDebugInfoContents::Place(for_arm_body.into()),
2250         });
2251         let locals = if has_guard.0 {
2252             let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
2253                 // This variable isn't mutated but has a name, so has to be
2254                 // immutable to avoid the unused mut lint.
2255                 mutability: Mutability::Not,
2256                 ty: tcx.mk_imm_ref(tcx.lifetimes.re_erased, var_ty),
2257                 user_ty: None,
2258                 source_info,
2259                 internal: false,
2260                 is_block_tail: None,
2261                 local_info: Some(Box::new(LocalInfo::User(ClearCrossCrate::Set(
2262                     BindingForm::RefForGuard,
2263                 )))),
2264             });
2265             self.var_debug_info.push(VarDebugInfo {
2266                 name,
2267                 source_info: debug_source_info,
2268                 value: VarDebugInfoContents::Place(ref_for_guard.into()),
2269             });
2270             LocalsForNode::ForGuard { ref_for_guard, for_arm_body }
2271         } else {
2272             LocalsForNode::One(for_arm_body)
2273         };
2274         debug!("declare_binding: vars={:?}", locals);
2275         self.var_indices.insert(var_id, locals);
2276     }
2277
2278     pub(crate) fn ast_let_else(
2279         &mut self,
2280         mut block: BasicBlock,
2281         init: &Expr<'tcx>,
2282         initializer_span: Span,
2283         else_block: BlockId,
2284         visibility_scope: Option<SourceScope>,
2285         remainder_scope: region::Scope,
2286         remainder_span: Span,
2287         pattern: &Pat<'tcx>,
2288     ) -> BlockAnd<()> {
2289         let else_block_span = self.thir[else_block].span;
2290         let (matching, failure) = self.in_if_then_scope(remainder_scope, |this| {
2291             let scrutinee = unpack!(block = this.lower_scrutinee(block, init, initializer_span));
2292             let pat = Pat { ty: init.ty, span: else_block_span, kind: Box::new(PatKind::Wild) };
2293             let mut wildcard = Candidate::new(scrutinee.clone(), &pat, false);
2294             this.declare_bindings(
2295                 visibility_scope,
2296                 remainder_span,
2297                 pattern,
2298                 ArmHasGuard(false),
2299                 Some((None, initializer_span)),
2300             );
2301             let mut candidate = Candidate::new(scrutinee.clone(), pattern, false);
2302             let fake_borrow_temps = this.lower_match_tree(
2303                 block,
2304                 initializer_span,
2305                 pattern.span,
2306                 false,
2307                 &mut [&mut candidate, &mut wildcard],
2308             );
2309             // This block is for the matching case
2310             let matching = this.bind_pattern(
2311                 this.source_info(pattern.span),
2312                 candidate,
2313                 None,
2314                 &fake_borrow_temps,
2315                 initializer_span,
2316                 None,
2317                 None,
2318                 None,
2319             );
2320             // This block is for the failure case
2321             let failure = this.bind_pattern(
2322                 this.source_info(else_block_span),
2323                 wildcard,
2324                 None,
2325                 &fake_borrow_temps,
2326                 initializer_span,
2327                 None,
2328                 None,
2329                 None,
2330             );
2331             this.break_for_else(failure, remainder_scope, this.source_info(initializer_span));
2332             matching.unit()
2333         });
2334
2335         // This place is not really used because this destination place
2336         // should never be used to take values at the end of the failure
2337         // block.
2338         let dummy_place = self.temp(self.tcx.types.never, else_block_span);
2339         let failure_block;
2340         unpack!(
2341             failure_block = self.ast_block(
2342                 dummy_place,
2343                 failure,
2344                 else_block,
2345                 self.source_info(else_block_span),
2346             )
2347         );
2348         self.cfg.terminate(
2349             failure_block,
2350             self.source_info(else_block_span),
2351             TerminatorKind::Unreachable,
2352         );
2353         matching.unit()
2354     }
2355 }