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.
5 //! This also includes code for pattern bindings in `let` statements and
6 //! function parameters.
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,
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};
27 // helper functions, broken out by category:
32 use std::borrow::Borrow;
33 use std::convert::TryFrom;
36 impl<'a, 'tcx> Builder<'a, 'tcx> {
37 pub(crate) fn then_else_break(
39 mut block: BasicBlock,
41 temp_scope_override: Option<region::Scope>,
42 break_scope: region::Scope,
43 variable_source_info: SourceInfo,
46 let expr_span = expr.span;
49 ExprKind::LogicalOp { op: LogicalOp::And, lhs, rhs } => {
50 let lhs_then_block = unpack!(this.then_else_break(
58 let rhs_then_block = unpack!(this.then_else_break(
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| {
80 ExprKind::Let { expr, ref pat } => this.lower_let_expr(
85 Some(variable_source_info.scope),
86 variable_source_info.span,
89 let temp_scope = temp_scope_override.unwrap_or_else(|| this.local_scope());
90 let mutability = Mutability::Mut;
92 unpack!(block = this.as_temp(block, Some(temp_scope), expr, mutability));
93 let operand = Operand::Move(Place::from(place));
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);
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);
108 /// Generates MIR for a `match` expression.
110 /// The MIR that we generate for a match looks like this.
115 /// [ 1. Evaluate Scrutinee (expression being matched on) ]
116 /// [ (fake read of scrutinee) ]
118 /// [ 2. Decision tree -- check discriminants ] <--------+
120 /// | (once a specific arm is chosen) |
122 /// [pre_binding_block] [otherwise_block]
124 /// [ 3. Create "guard bindings" for arm ] |
125 /// [ (create fake borrows) ] |
127 /// [ 4. Execute guard code ] |
128 /// [ (read fake borrows) ] --(guard is false)-----------+
130 /// | (guard results in true)
132 /// [ 5. Create real bindings and execute arm ]
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.
141 /// We generate MIR in the following steps:
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]).
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:
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(
161 destination: Place<'tcx>,
163 mut block: BasicBlock,
164 scrutinee: &Expr<'tcx>,
167 let scrutinee_span = scrutinee.span;
168 let scrutinee_place =
169 unpack!(block = self.lower_scrutinee(block, scrutinee, scrutinee_span,));
171 let mut arm_candidates = self.create_match_candidates(scrutinee_place.clone(), &arms);
173 let match_has_guard = arm_candidates.iter().any(|(_, candidate)| candidate.has_guard);
175 arm_candidates.iter_mut().map(|(_, candidate)| candidate).collect::<Vec<_>>();
177 let match_start_span = span.shrink_to_lo().to(scrutinee.span);
179 let fake_borrow_temps = self.lower_match_tree(
187 self.lower_match_arms(
192 self.source_info(span),
197 /// Evaluate the scrutinee and add the fake read of it.
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.
212 // HACK(eddyb) Work around the above issue by adding a dummy inspection
213 // of `scrutinee_place`, specifically by applying `ReadForMatch`.
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
220 let cause_matched_place = FakeReadCause::ForMatchedPlace(None);
221 let source_info = self.source_info(scrutinee_span);
223 if let Ok(scrutinee_builder) = scrutinee_place_builder.clone().try_upvars_resolved(self) {
224 let scrutinee_place = scrutinee_builder.into_place(self);
225 self.cfg.push_fake_read(block, source_info, cause_matched_place, scrutinee_place);
228 block.and(scrutinee_place_builder)
231 /// Create the initial `Candidate`s for a `match` expression.
232 fn create_match_candidates<'pat>(
234 scrutinee: PlaceBuilder<'tcx>,
236 ) -> Vec<(&'pat Arm<'tcx>, Candidate<'pat, 'tcx>)>
240 // Assemble a list of candidates: there is one candidate per pattern,
241 // which means there may be more than one candidate *per arm*.
245 let arm = &self.thir[arm];
246 let arm_has_guard = arm.guard.is_some();
248 Candidate::new(scrutinee.clone(), &arm.pattern, arm_has_guard, self);
254 /// Create the decision tree for the match expression, starting from `block`.
256 /// Modifies `candidates` to store the bindings and type ascriptions for
259 /// Returns the places that need fake borrows because we bind or test them.
260 fn lower_match_tree<'pat>(
263 scrutinee_span: Span,
264 match_start_span: Span,
265 match_has_guard: bool,
266 candidates: &mut [&mut Candidate<'pat, 'tcx>],
267 ) -> Vec<(Place<'tcx>, Local)> {
268 // The set of places that we are creating fake borrows of. If there are
269 // no match guards then we don't need any fake borrows, so don't track
271 let mut fake_borrows = match_has_guard.then(FxIndexSet::default);
273 let mut otherwise = None;
275 // This will generate code to test scrutinee_place and
276 // branch to the appropriate arm block
277 self.match_candidates(
286 if let Some(otherwise_block) = otherwise {
287 // See the doc comment on `match_candidates` for why we may have an
288 // otherwise block. Match checking will ensure this is actually
290 let source_info = self.source_info(scrutinee_span);
291 self.cfg.terminate(otherwise_block, source_info, TerminatorKind::Unreachable);
294 // Link each leaf candidate to the `pre_binding_block` of the next one.
295 let mut previous_candidate: Option<&mut Candidate<'_, '_>> = None;
297 for candidate in candidates {
298 candidate.visit_leaves(|leaf_candidate| {
299 if let Some(ref mut prev) = previous_candidate {
300 prev.next_candidate_pre_binding_block = leaf_candidate.pre_binding_block;
302 previous_candidate = Some(leaf_candidate);
306 if let Some(ref borrows) = fake_borrows {
307 self.calculate_fake_borrows(borrows, scrutinee_span)
313 /// Lower the bindings, guards and arm bodies of a `match` expression.
315 /// The decision tree should have already been created
316 /// (by [Builder::lower_match_tree]).
318 /// `outer_source_info` is the SourceInfo for the whole match.
321 destination: Place<'tcx>,
322 scrutinee_place_builder: PlaceBuilder<'tcx>,
323 scrutinee_span: Span,
324 arm_candidates: Vec<(&'_ Arm<'tcx>, Candidate<'_, 'tcx>)>,
325 outer_source_info: SourceInfo,
326 fake_borrow_temps: Vec<(Place<'tcx>, Local)>,
328 let arm_end_blocks: Vec<_> = arm_candidates
330 .map(|(arm, candidate)| {
331 debug!("lowering arm {:?}\ncandidate = {:?}", arm, candidate);
333 let arm_source_info = self.source_info(arm.span);
334 let arm_scope = (arm.scope, arm_source_info);
335 let match_scope = self.local_scope();
336 self.in_scope(arm_scope, arm.lint_level, |this| {
337 // `try_upvars_resolved` may fail if it is unable to resolve the given
338 // `PlaceBuilder` inside a closure. In this case, we don't want to include
339 // a scrutinee place. `scrutinee_place_builder` will fail to be resolved
340 // if the only match arm is a wildcard (`_`).
345 // match foo { _ => () };
348 let mut opt_scrutinee_place: Option<(Option<&Place<'tcx>>, Span)> = None;
349 let scrutinee_place: Place<'tcx>;
350 if let Ok(scrutinee_builder) =
351 scrutinee_place_builder.clone().try_upvars_resolved(this)
353 scrutinee_place = scrutinee_builder.into_place(this);
354 opt_scrutinee_place = Some((Some(&scrutinee_place), scrutinee_span));
356 let scope = this.declare_bindings(
360 ArmHasGuard(arm.guard.is_some()),
364 let arm_block = this.bind_pattern(
369 Some((arm, match_scope)),
373 if let Some(source_scope) = scope {
374 this.source_scope = source_scope;
377 this.expr_into_dest(destination, arm_block, &&this.thir[arm.body])
382 // all the arm blocks will rejoin here
383 let end_block = self.cfg.start_new_block();
385 let end_brace = self.source_info(
386 outer_source_info.span.with_lo(outer_source_info.span.hi() - BytePos::from_usize(1)),
388 for arm_block in arm_end_blocks {
389 let block = &self.cfg.basic_blocks[arm_block.0];
390 let last_location = block.statements.last().map(|s| s.source_info);
392 self.cfg.goto(unpack!(arm_block), last_location.unwrap_or(end_brace), end_block);
395 self.source_scope = outer_source_info.scope;
400 /// Binds the variables and ascribes types for a given `match` arm or
403 /// Also check if the guard matches, if it's provided.
404 /// `arm_scope` should be `Some` if and only if this is called for a
408 outer_source_info: SourceInfo,
409 candidate: Candidate<'_, 'tcx>,
410 fake_borrow_temps: &[(Place<'tcx>, Local)],
411 scrutinee_span: Span,
412 arm_match_scope: Option<(&Arm<'tcx>, region::Scope)>,
413 storages_alive: bool,
415 if candidate.subcandidates.is_empty() {
416 // Avoid generating another `BasicBlock` when we only have one
418 self.bind_and_guard_matched_candidate(
428 // It's helpful to avoid scheduling drops multiple times to save
429 // drop elaboration from having to clean up the extra drops.
431 // If we are in a `let` then we only schedule drops for the first
434 // If we're in a `match` arm then we could have a case like so:
436 // Ok(x) | Err(x) if return => { /* ... */ }
438 // In this case we don't want a drop of `x` scheduled when we
439 // return: it isn't bound by move until right before enter the arm.
440 // To handle this we instead unschedule it's drop after each time
441 // we lower the guard.
442 let target_block = self.cfg.start_new_block();
443 let mut schedule_drops = true;
444 let arm = arm_match_scope.unzip().0;
445 // We keep a stack of all of the bindings and type ascriptions
446 // from the parent candidates that we visit, that also need to
447 // be bound for each candidate.
451 &mut |leaf_candidate, parent_bindings| {
452 if let Some(arm) = arm {
453 self.clear_top_scope(arm.scope);
455 let binding_end = self.bind_and_guard_matched_candidate(
465 schedule_drops = false;
467 self.cfg.goto(binding_end, outer_source_info, target_block);
469 |inner_candidate, parent_bindings| {
470 parent_bindings.push((inner_candidate.bindings, inner_candidate.ascriptions));
471 inner_candidate.subcandidates.into_iter()
474 parent_bindings.pop();
482 pub(super) fn expr_into_pattern(
484 mut block: BasicBlock,
485 irrefutable_pat: &Pat<'tcx>,
486 initializer: &Expr<'tcx>,
488 match irrefutable_pat.kind {
489 // Optimize the case of `let x = ...` to write directly into `x`
490 PatKind::Binding { mode: BindingMode::ByValue, var, subpattern: None, .. } => {
492 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
493 unpack!(block = self.expr_into_dest(place, block, initializer));
495 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
496 let source_info = self.source_info(irrefutable_pat.span);
497 self.cfg.push_fake_read(block, source_info, FakeReadCause::ForLet(None), place);
499 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
503 // Optimize the case of `let x: T = ...` to write directly
504 // into `x` and then require that `T == typeof(x)`.
506 // Weirdly, this is needed to prevent the
507 // `intrinsic-move-val.rs` test case from crashing. That
508 // test works with uninitialized values in a rather
509 // dubious way, so it may be that the test is kind of
511 PatKind::AscribeUserType {
516 mode: BindingMode::ByValue, var, subpattern: None, ..
520 ascription: thir::Ascription { ref annotation, variance: _ },
523 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
524 unpack!(block = self.expr_into_dest(place, block, initializer));
526 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
527 let pattern_source_info = self.source_info(irrefutable_pat.span);
528 let cause_let = FakeReadCause::ForLet(None);
529 self.cfg.push_fake_read(block, pattern_source_info, cause_let, place);
531 let ty_source_info = self.source_info(annotation.span);
533 let base = self.canonical_user_type_annotations.push(annotation.clone());
537 source_info: ty_source_info,
538 kind: StatementKind::AscribeUserType(
539 Box::new((place, UserTypeProjection { base, projs: Vec::new() })),
540 // We always use invariant as the variance here. This is because the
541 // variance field from the ascription refers to the variance to use
542 // when applying the type to the value being matched, but this
543 // ascription applies rather to the type of the binding. e.g., in this
550 // We are creating an ascription that defines the type of `x` to be
551 // exactly `T` (i.e., with invariance). The variance field, in
552 // contrast, is intended to be used to relate `T` to the type of
554 ty::Variance::Invariant,
559 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
564 let place_builder = unpack!(block = self.as_place_builder(block, initializer));
565 self.place_into_pattern(block, &irrefutable_pat, place_builder, true)
570 pub(crate) fn place_into_pattern(
573 irrefutable_pat: &Pat<'tcx>,
574 initializer: PlaceBuilder<'tcx>,
575 set_match_place: bool,
577 let mut candidate = Candidate::new(initializer.clone(), &irrefutable_pat, false, self);
578 let fake_borrow_temps = self.lower_match_tree(
580 irrefutable_pat.span,
581 irrefutable_pat.span,
583 &mut [&mut candidate],
585 // For matches and function arguments, the place that is being matched
586 // can be set when creating the variables. But the place for
587 // let PATTERN = ... might not even exist until we do the assignment.
588 // so we set it here instead.
590 let mut candidate_ref = &candidate;
591 while let Some(next) = {
592 for binding in &candidate_ref.bindings {
593 let local = self.var_local_id(binding.var_id, OutsideGuard);
594 // `try_upvars_resolved` may fail if it is unable to resolve the given
595 // `PlaceBuilder` inside a closure. In this case, we don't want to include
596 // a scrutinee place. `scrutinee_place_builder` will fail for destructured
597 // assignments. This is because a closure only captures the precise places
598 // that it will read and as a result a closure may not capture the entire
599 // tuple/struct and rather have individual places that will be read in the
605 // let (v1, v2) = foo;
608 if let Ok(match_pair_resolved) = initializer.clone().try_upvars_resolved(self) {
609 let place = match_pair_resolved.into_place(self);
611 let Some(box LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
612 VarBindingForm { opt_match_place: Some((ref mut match_place, _)), .. },
613 )))) = self.local_decls[local].local_info else {
614 bug!("Let binding to non-user variable.")
616 *match_place = Some(place);
619 // All of the subcandidates should bind the same locals, so we
620 // only visit the first one.
621 candidate_ref.subcandidates.get(0)
623 candidate_ref = next;
628 self.source_info(irrefutable_pat.span),
631 irrefutable_pat.span,
638 /// Declares the bindings of the given patterns and returns the visibility
639 /// scope for the bindings in these patterns, if such a scope had to be
640 /// created. NOTE: Declaring the bindings should always be done in their
642 #[instrument(skip(self), level = "debug")]
643 pub(crate) fn declare_bindings(
645 mut visibility_scope: Option<SourceScope>,
648 has_guard: ArmHasGuard,
649 opt_match_place: Option<(Option<&Place<'tcx>>, Span)>,
650 ) -> Option<SourceScope> {
651 self.visit_primary_bindings(
653 UserTypeProjections::none(),
654 &mut |this, mutability, name, mode, var, span, ty, user_ty| {
655 if visibility_scope.is_none() {
657 Some(this.new_source_scope(scope_span, LintLevel::Inherited, None));
659 let source_info = SourceInfo { span, scope: this.source_scope };
660 let visibility_scope = visibility_scope.unwrap();
661 this.declare_binding(
671 opt_match_place.map(|(x, y)| (x.cloned(), y)),
679 pub(crate) fn storage_live_binding(
687 let local_id = self.var_local_id(var, for_guard);
688 let source_info = self.source_info(span);
689 self.cfg.push(block, Statement { source_info, kind: StatementKind::StorageLive(local_id) });
690 // Although there is almost always scope for given variable in corner cases
691 // like #92893 we might get variable with no scope.
692 if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) && schedule_drop {
693 self.schedule_drop(span, region_scope, local_id, DropKind::Storage);
695 Place::from(local_id)
698 pub(crate) fn schedule_drop_for_binding(
704 let local_id = self.var_local_id(var, for_guard);
705 if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) {
706 self.schedule_drop(span, region_scope, local_id, DropKind::Value);
710 /// Visit all of the primary bindings in a patterns, that is, visit the
711 /// leftmost occurrence of each variable bound in a pattern. A variable
712 /// will occur more than once in an or-pattern.
713 pub(super) fn visit_primary_bindings(
716 pattern_user_ty: UserTypeProjections,
729 "visit_primary_bindings: pattern={:?} pattern_user_ty={:?}",
730 pattern, pattern_user_ty
744 f(self, mutability, name, mode, var, pattern.span, ty, pattern_user_ty.clone());
746 if let Some(subpattern) = subpattern.as_ref() {
747 self.visit_primary_bindings(subpattern, pattern_user_ty, f);
751 PatKind::Array { ref prefix, ref slice, ref suffix }
752 | PatKind::Slice { ref prefix, ref slice, ref suffix } => {
753 let from = u64::try_from(prefix.len()).unwrap();
754 let to = u64::try_from(suffix.len()).unwrap();
755 for subpattern in prefix.iter() {
756 self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
758 for subpattern in slice {
759 self.visit_primary_bindings(
761 pattern_user_ty.clone().subslice(from, to),
765 for subpattern in suffix.iter() {
766 self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
770 PatKind::Constant { .. } | PatKind::Range { .. } | PatKind::Wild => {}
772 PatKind::Deref { ref subpattern } => {
773 self.visit_primary_bindings(subpattern, pattern_user_ty.deref(), f);
776 PatKind::AscribeUserType {
778 ascription: thir::Ascription { ref annotation, variance: _ },
780 // This corresponds to something like
783 // let A::<'a>(_): A<'static> = ...;
786 // Note that the variance doesn't apply here, as we are tracking the effect
787 // of `user_ty` on any bindings contained with subpattern.
789 let projection = UserTypeProjection {
790 base: self.canonical_user_type_annotations.push(annotation.clone()),
793 let subpattern_user_ty =
794 pattern_user_ty.push_projection(&projection, annotation.span);
795 self.visit_primary_bindings(subpattern, subpattern_user_ty, f)
798 PatKind::Leaf { ref subpatterns } => {
799 for subpattern in subpatterns {
800 let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field);
801 debug!("visit_primary_bindings: subpattern_user_ty={:?}", subpattern_user_ty);
802 self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
806 PatKind::Variant { adt_def, substs: _, variant_index, ref subpatterns } => {
807 for subpattern in subpatterns {
808 let subpattern_user_ty =
809 pattern_user_ty.clone().variant(adt_def, variant_index, subpattern.field);
810 self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
813 PatKind::Or { ref pats } => {
814 // In cases where we recover from errors the primary bindings
815 // may not all be in the leftmost subpattern. For example in
816 // `let (x | y) = ...`, the primary binding of `y` occurs in
817 // the right subpattern
818 for subpattern in pats.iter() {
819 self.visit_primary_bindings(subpattern, pattern_user_ty.clone(), f);
827 struct Candidate<'pat, 'tcx> {
828 /// [`Span`] of the original pattern that gave rise to this candidate.
831 /// Whether this `Candidate` has a guard.
834 /// All of these must be satisfied...
835 match_pairs: SmallVec<[MatchPair<'pat, 'tcx>; 1]>,
837 /// ...these bindings established...
838 bindings: Vec<Binding<'tcx>>,
840 /// ...and these types asserted...
841 ascriptions: Vec<Ascription<'tcx>>,
843 /// ...and if this is non-empty, one of these subcandidates also has to match...
844 subcandidates: Vec<Candidate<'pat, 'tcx>>,
846 /// ...and the guard must be evaluated; if it's `false` then branch to `otherwise_block`.
847 otherwise_block: Option<BasicBlock>,
849 /// The block before the `bindings` have been established.
850 pre_binding_block: Option<BasicBlock>,
851 /// The pre-binding block of the next candidate.
852 next_candidate_pre_binding_block: Option<BasicBlock>,
855 impl<'tcx, 'pat> Candidate<'pat, 'tcx> {
857 place: PlaceBuilder<'tcx>,
858 pattern: &'pat Pat<'tcx>,
860 cx: &Builder<'_, 'tcx>,
865 match_pairs: smallvec![MatchPair::new(place, pattern, cx)],
866 bindings: Vec::new(),
867 ascriptions: Vec::new(),
868 subcandidates: Vec::new(),
869 otherwise_block: None,
870 pre_binding_block: None,
871 next_candidate_pre_binding_block: None,
875 /// Visit the leaf candidates (those with no subcandidates) contained in
877 fn visit_leaves<'a>(&'a mut self, mut visit_leaf: impl FnMut(&'a mut Self)) {
881 &mut move |c, _| visit_leaf(c),
882 move |c, _| c.subcandidates.iter_mut(),
888 /// A depth-first traversal of the `Candidate` and all of its recursive
890 fn traverse_candidate<'pat, 'tcx: 'pat, C, T, I>(
893 visit_leaf: &mut impl FnMut(C, &mut T),
894 get_children: impl Copy + Fn(C, &mut T) -> I,
895 complete_children: impl Copy + Fn(&mut T),
897 C: Borrow<Candidate<'pat, 'tcx>>,
898 I: Iterator<Item = C>,
900 if candidate.borrow().subcandidates.is_empty() {
901 visit_leaf(candidate, context)
903 for child in get_children(candidate, context) {
904 traverse_candidate(child, context, visit_leaf, get_children, complete_children);
906 complete_children(context)
910 #[derive(Clone, Debug)]
911 struct Binding<'tcx> {
915 binding_mode: BindingMode,
918 /// Indicates that the type of `source` must be a subtype of the
919 /// user-given type `user_ty`; this is basically a no-op but can
920 /// influence region inference.
921 #[derive(Clone, Debug)]
922 struct Ascription<'tcx> {
924 annotation: CanonicalUserTypeAnnotation<'tcx>,
925 variance: ty::Variance,
928 #[derive(Clone, Debug)]
929 pub(crate) struct MatchPair<'pat, 'tcx> {
931 place: PlaceBuilder<'tcx>,
933 // ... must match this pattern.
934 pattern: &'pat Pat<'tcx>,
937 /// See [`Test`] for more.
938 #[derive(Clone, Debug, PartialEq)]
939 enum TestKind<'tcx> {
940 /// Test what enum variant a value is.
942 /// The enum type being tested.
943 adt_def: ty::AdtDef<'tcx>,
944 /// The set of variants that we should create a branch for. We also
945 /// create an additional "otherwise" case.
946 variants: BitSet<VariantIdx>,
949 /// Test what value an integer, `bool`, or `char` has.
951 /// The type of the value that we're testing.
953 /// The (ordered) set of values that we test for.
955 /// For integers and `char`s we create a branch to each of the values in
956 /// `options`, as well as an "otherwise" branch for all other values, even
957 /// in the (rare) case that `options` is exhaustive.
959 /// For `bool` we always generate two edges, one for `true` and one for
961 options: FxIndexMap<ConstantKind<'tcx>, u128>,
964 /// Test for equality with value, possibly after an unsizing coercion to
967 value: ConstantKind<'tcx>,
968 // Integer types are handled by `SwitchInt`, and constants with ADT
969 // types are converted back into patterns, so this can only be `&str`,
970 // `&[T]`, `f32` or `f64`.
974 /// Test whether the value falls within an inclusive or exclusive range
975 Range(Box<PatRange<'tcx>>),
977 /// Test that the length of the slice is equal to `len`.
978 Len { len: u64, op: BinOp },
981 /// A test to perform to determine which [`Candidate`] matches a value.
983 /// [`Test`] is just the test to perform; it does not include the value
986 pub(crate) struct Test<'tcx> {
988 kind: TestKind<'tcx>,
991 /// `ArmHasGuard` is a wrapper around a boolean flag. It indicates whether
992 /// a match arm has a guard expression attached to it.
993 #[derive(Copy, Clone, Debug)]
994 pub(crate) struct ArmHasGuard(pub(crate) bool);
996 ///////////////////////////////////////////////////////////////////////////
997 // Main matching algorithm
999 impl<'a, 'tcx> Builder<'a, 'tcx> {
1000 /// The main match algorithm. It begins with a set of candidates
1001 /// `candidates` and has the job of generating code to determine
1002 /// which of these candidates, if any, is the correct one. The
1003 /// candidates are sorted such that the first item in the list
1004 /// has the highest priority. When a candidate is found to match
1005 /// the value, we will set and generate a branch to the appropriate
1006 /// pre-binding block.
1008 /// If we find that *NONE* of the candidates apply, we branch to the
1009 /// `otherwise_block`, setting it to `Some` if required. In principle, this
1010 /// means that the input list was not exhaustive, though at present we
1011 /// sometimes are not smart enough to recognize all exhaustive inputs.
1013 /// It might be surprising that the input can be non-exhaustive.
1014 /// Indeed, initially, it is not, because all matches are
1015 /// exhaustive in Rust. But during processing we sometimes divide
1016 /// up the list of candidates and recurse with a non-exhaustive
1017 /// list. This is important to keep the size of the generated code
1018 /// under control. See [`Builder::test_candidates`] for more details.
1020 /// If `fake_borrows` is `Some`, then places which need fake borrows
1021 /// will be added to it.
1023 /// For an example of a case where we set `otherwise_block`, even for an
1024 /// exhaustive match, consider:
1027 /// # fn foo(x: (bool, bool)) {
1029 /// (true, true) => (),
1030 /// (_, false) => (),
1031 /// (false, true) => (),
1036 /// For this match, we check if `x.0` matches `true` (for the first
1037 /// arm). If it doesn't match, we check `x.1`. If `x.1` is `true` we check
1038 /// if `x.0` matches `false` (for the third arm). In the (impossible at
1039 /// runtime) case when `x.0` is now `true`, we branch to
1040 /// `otherwise_block`.
1041 #[instrument(skip(self, fake_borrows), level = "debug")]
1042 fn match_candidates<'pat>(
1045 scrutinee_span: Span,
1046 start_block: BasicBlock,
1047 otherwise_block: &mut Option<BasicBlock>,
1048 candidates: &mut [&mut Candidate<'pat, 'tcx>],
1049 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1051 // Start by simplifying candidates. Once this process is complete, all
1052 // the match pairs which remain require some form of test, whether it
1053 // be a switch or pattern comparison.
1054 let mut split_or_candidate = false;
1055 for candidate in &mut *candidates {
1056 split_or_candidate |= self.simplify_candidate(candidate);
1059 ensure_sufficient_stack(|| {
1060 if split_or_candidate {
1061 // At least one of the candidates has been split into subcandidates.
1062 // We need to change the candidate list to include those.
1063 let mut new_candidates = Vec::new();
1065 for candidate in candidates {
1066 candidate.visit_leaves(|leaf_candidate| new_candidates.push(leaf_candidate));
1068 self.match_simplified_candidates(
1073 &mut *new_candidates,
1077 self.match_simplified_candidates(
1089 fn match_simplified_candidates(
1092 scrutinee_span: Span,
1093 start_block: BasicBlock,
1094 otherwise_block: &mut Option<BasicBlock>,
1095 candidates: &mut [&mut Candidate<'_, 'tcx>],
1096 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1098 // The candidates are sorted by priority. Check to see whether the
1099 // higher priority candidates (and hence at the front of the slice)
1100 // have satisfied all their match pairs.
1101 let fully_matched = candidates.iter().take_while(|c| c.match_pairs.is_empty()).count();
1102 debug!("match_candidates: {:?} candidates fully matched", fully_matched);
1103 let (matched_candidates, unmatched_candidates) = candidates.split_at_mut(fully_matched);
1105 let block = if !matched_candidates.is_empty() {
1106 let otherwise_block =
1107 self.select_matched_candidates(matched_candidates, start_block, fake_borrows);
1109 if let Some(last_otherwise_block) = otherwise_block {
1110 last_otherwise_block
1112 // Any remaining candidates are unreachable.
1113 if unmatched_candidates.is_empty() {
1116 self.cfg.start_new_block()
1122 // If there are no candidates that still need testing, we're
1123 // done. Since all matches are exhaustive, execution should
1124 // never reach this point.
1125 if unmatched_candidates.is_empty() {
1126 let source_info = self.source_info(span);
1127 if let Some(otherwise) = *otherwise_block {
1128 self.cfg.goto(block, source_info, otherwise);
1130 *otherwise_block = Some(block);
1135 // Test for the remaining candidates.
1136 self.test_candidates_with_or(
1139 unmatched_candidates,
1146 /// Link up matched candidates.
1148 /// For example, if we have something like this:
1150 /// ```ignore (illustrative)
1152 /// Some(x) if cond1 => ...
1154 /// Some(x) if cond2 => ...
1158 /// We generate real edges from:
1160 /// * `start_block` to the [pre-binding block] of the first pattern,
1161 /// * the [otherwise block] of the first pattern to the second pattern,
1162 /// * the [otherwise block] of the third pattern to a block with an
1163 /// [`Unreachable` terminator](TerminatorKind::Unreachable).
1165 /// In addition, we add fake edges from the otherwise blocks to the
1166 /// pre-binding block of the next candidate in the original set of
1169 /// [pre-binding block]: Candidate::pre_binding_block
1170 /// [otherwise block]: Candidate::otherwise_block
1171 fn select_matched_candidates(
1173 matched_candidates: &mut [&mut Candidate<'_, 'tcx>],
1174 start_block: BasicBlock,
1175 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1176 ) -> Option<BasicBlock> {
1178 !matched_candidates.is_empty(),
1179 "select_matched_candidates called with no candidates",
1182 matched_candidates.iter().all(|c| c.subcandidates.is_empty()),
1183 "subcandidates should be empty in select_matched_candidates",
1186 // Insert a borrows of prefixes of places that are bound and are
1187 // behind a dereference projection.
1189 // These borrows are taken to avoid situations like the following:
1192 // _ if { x = &[0]; false } => (),
1193 // y => (), // Out of bounds array access!
1197 // // y is bound by reference in the guard and then by copy in the
1198 // // arm, so y is 2 in the arm!
1199 // y if { y == 1 && (x = &2) == () } => y,
1202 if let Some(fake_borrows) = fake_borrows {
1203 for Binding { source, .. } in
1204 matched_candidates.iter().flat_map(|candidate| &candidate.bindings)
1207 source.projection.iter().rposition(|elem| elem == ProjectionElem::Deref)
1209 let proj_base = &source.projection[..i];
1211 fake_borrows.insert(Place {
1212 local: source.local,
1213 projection: self.tcx.intern_place_elems(proj_base),
1219 let fully_matched_with_guard = matched_candidates
1221 .position(|c| !c.has_guard)
1222 .unwrap_or(matched_candidates.len() - 1);
1224 let (reachable_candidates, unreachable_candidates) =
1225 matched_candidates.split_at_mut(fully_matched_with_guard + 1);
1227 let mut next_prebinding = start_block;
1229 for candidate in reachable_candidates.iter_mut() {
1230 assert!(candidate.otherwise_block.is_none());
1231 assert!(candidate.pre_binding_block.is_none());
1232 candidate.pre_binding_block = Some(next_prebinding);
1233 if candidate.has_guard {
1234 // Create the otherwise block for this candidate, which is the
1235 // pre-binding block for the next candidate.
1236 next_prebinding = self.cfg.start_new_block();
1237 candidate.otherwise_block = Some(next_prebinding);
1242 "match_candidates: add pre_binding_blocks for unreachable {:?}",
1243 unreachable_candidates,
1245 for candidate in unreachable_candidates {
1246 assert!(candidate.pre_binding_block.is_none());
1247 candidate.pre_binding_block = Some(self.cfg.start_new_block());
1250 reachable_candidates.last_mut().unwrap().otherwise_block
1253 /// Tests a candidate where there are only or-patterns left to test, or
1254 /// forwards to [Builder::test_candidates].
1256 /// Given a pattern `(P | Q, R | S)` we (in principle) generate a CFG like
1264 /// +----------------------------------------+------------------------------------+
1267 /// [ P matches ] [ Q matches ] [ otherwise ]
1270 /// [ match R, S ] [ match R, S ] |
1272 /// +--------------+------------+ +--------------+------------+ |
1275 /// [ R matches ] [ S matches ] [otherwise ] [ R matches ] [ S matches ] [otherwise ] |
1277 /// +--------------+------------|------------+--------------+ | |
1279 /// | +----------------------------------------+--------+
1282 /// [ Success ] [ Failure ]
1285 /// In practice there are some complications:
1287 /// * If there's a guard, then the otherwise branch of the first match on
1288 /// `R | S` goes to a test for whether `Q` matches, and the control flow
1289 /// doesn't merge into a single success block until after the guard is
1291 /// * If neither `P` or `Q` has any bindings or type ascriptions and there
1292 /// isn't a match guard, then we create a smaller CFG like:
1296 /// +---------------+------------+
1298 /// [ P matches ] [ Q matches ] [ otherwise ]
1300 /// +---------------+ |
1306 fn test_candidates_with_or(
1309 scrutinee_span: Span,
1310 candidates: &mut [&mut Candidate<'_, 'tcx>],
1312 otherwise_block: &mut Option<BasicBlock>,
1313 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1315 let (first_candidate, remaining_candidates) = candidates.split_first_mut().unwrap();
1317 // All of the or-patterns have been sorted to the end, so if the first
1318 // pattern is an or-pattern we only have or-patterns.
1319 match first_candidate.match_pairs[0].pattern.kind {
1320 PatKind::Or { .. } => (),
1322 self.test_candidates(
1334 let match_pairs = mem::take(&mut first_candidate.match_pairs);
1335 first_candidate.pre_binding_block = Some(block);
1337 let mut otherwise = None;
1338 for match_pair in match_pairs {
1339 let PatKind::Or { ref pats } = &match_pair.pattern.kind else {
1340 bug!("Or-patterns should have been sorted to the end");
1342 let or_span = match_pair.pattern.span;
1343 let place = match_pair.place;
1345 first_candidate.visit_leaves(|leaf_candidate| {
1346 self.test_or_pattern(
1357 let remainder_start = otherwise.unwrap_or_else(|| self.cfg.start_new_block());
1359 self.match_candidates(
1364 remaining_candidates,
1370 skip(self, otherwise, or_span, place, fake_borrows, candidate, pats),
1373 fn test_or_pattern<'pat>(
1375 candidate: &mut Candidate<'pat, 'tcx>,
1376 otherwise: &mut Option<BasicBlock>,
1377 pats: &'pat [Box<Pat<'tcx>>],
1379 place: PlaceBuilder<'tcx>,
1380 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1382 debug!("candidate={:#?}\npats={:#?}", candidate, pats);
1383 let mut or_candidates: Vec<_> = pats
1385 .map(|pat| Candidate::new(place.clone(), pat, candidate.has_guard, self))
1387 let mut or_candidate_refs: Vec<_> = or_candidates.iter_mut().collect();
1388 let otherwise = if candidate.otherwise_block.is_some() {
1389 &mut candidate.otherwise_block
1393 self.match_candidates(
1396 candidate.pre_binding_block.unwrap(),
1398 &mut or_candidate_refs,
1401 candidate.subcandidates = or_candidates;
1402 self.merge_trivial_subcandidates(candidate, self.source_info(or_span));
1405 /// Try to merge all of the subcandidates of the given candidate into one.
1406 /// This avoids exponentially large CFGs in cases like `(1 | 2, 3 | 4, ...)`.
1407 fn merge_trivial_subcandidates(
1409 candidate: &mut Candidate<'_, 'tcx>,
1410 source_info: SourceInfo,
1412 if candidate.subcandidates.is_empty() || candidate.has_guard {
1413 // FIXME(or_patterns; matthewjasper) Don't give up if we have a guard.
1417 let mut can_merge = true;
1419 // Not `Iterator::all` because we don't want to short-circuit.
1420 for subcandidate in &mut candidate.subcandidates {
1421 self.merge_trivial_subcandidates(subcandidate, source_info);
1423 // FIXME(or_patterns; matthewjasper) Try to be more aggressive here.
1424 can_merge &= subcandidate.subcandidates.is_empty()
1425 && subcandidate.bindings.is_empty()
1426 && subcandidate.ascriptions.is_empty();
1430 let any_matches = self.cfg.start_new_block();
1431 for subcandidate in mem::take(&mut candidate.subcandidates) {
1432 let or_block = subcandidate.pre_binding_block.unwrap();
1433 self.cfg.goto(or_block, source_info, any_matches);
1435 candidate.pre_binding_block = Some(any_matches);
1439 /// This is the most subtle part of the matching algorithm. At
1440 /// this point, the input candidates have been fully simplified,
1441 /// and so we know that all remaining match-pairs require some
1442 /// sort of test. To decide what test to perform, we take the highest
1443 /// priority candidate (the first one in the list, as of January 2021)
1444 /// and extract the first match-pair from the list. From this we decide
1445 /// what kind of test is needed using [`Builder::test`], defined in the
1446 /// [`test` module](mod@test).
1448 /// *Note:* taking the first match pair is somewhat arbitrary, and
1449 /// we might do better here by choosing more carefully what to
1452 /// For example, consider the following possible match-pairs:
1454 /// 1. `x @ Some(P)` -- we will do a [`Switch`] to decide what variant `x` has
1455 /// 2. `x @ 22` -- we will do a [`SwitchInt`] to decide what value `x` has
1456 /// 3. `x @ 3..5` -- we will do a [`Range`] test to decide what range `x` falls in
1459 /// [`Switch`]: TestKind::Switch
1460 /// [`SwitchInt`]: TestKind::SwitchInt
1461 /// [`Range`]: TestKind::Range
1463 /// Once we know what sort of test we are going to perform, this
1464 /// test may also help us winnow down our candidates. So we walk over
1465 /// the candidates (from high to low priority) and check. This
1466 /// gives us, for each outcome of the test, a transformed list of
1467 /// candidates. For example, if we are testing `x.0`'s variant,
1468 /// and we have a candidate `(x.0 @ Some(v), x.1 @ 22)`,
1469 /// then we would have a resulting candidate of `((x.0 as Some).0 @ v, x.1 @ 22)`.
1470 /// Note that the first match-pair is now simpler (and, in fact, irrefutable).
1472 /// But there may also be candidates that the test just doesn't
1473 /// apply to. The classical example involves wildcards:
1476 /// # let (x, y, z) = (true, true, true);
1477 /// match (x, y, z) {
1478 /// (true , _ , true ) => true, // (0)
1479 /// (_ , true , _ ) => true, // (1)
1480 /// (false, false, _ ) => false, // (2)
1481 /// (true , _ , false) => false, // (3)
1486 /// In that case, after we test on `x`, there are 2 overlapping candidate
1489 /// - If the outcome is that `x` is true, candidates 0, 1, and 3
1490 /// - If the outcome is that `x` is false, candidates 1 and 2
1492 /// Here, the traditional "decision tree" method would generate 2
1493 /// separate code-paths for the 2 separate cases.
1495 /// In some cases, this duplication can create an exponential amount of
1496 /// code. This is most easily seen by noticing that this method terminates
1497 /// with precisely the reachable arms being reachable - but that problem
1498 /// is trivially NP-complete:
1500 /// ```ignore (illustrative)
1501 /// match (var0, var1, var2, var3, ...) {
1502 /// (true , _ , _ , false, true, ...) => false,
1503 /// (_ , true, true , false, _ , ...) => false,
1504 /// (false, _ , false, false, _ , ...) => false,
1510 /// Here the last arm is reachable only if there is an assignment to
1511 /// the variables that does not match any of the literals. Therefore,
1512 /// compilation would take an exponential amount of time in some cases.
1514 /// That kind of exponential worst-case might not occur in practice, but
1515 /// our simplistic treatment of constants and guards would make it occur
1516 /// in very common situations - for example [#29740]:
1518 /// ```ignore (illustrative)
1520 /// "foo" if foo_guard => ...,
1521 /// "bar" if bar_guard => ...,
1522 /// "baz" if baz_guard => ...,
1527 /// [#29740]: https://github.com/rust-lang/rust/issues/29740
1529 /// Here we first test the match-pair `x @ "foo"`, which is an [`Eq` test].
1531 /// [`Eq` test]: TestKind::Eq
1533 /// It might seem that we would end up with 2 disjoint candidate
1534 /// sets, consisting of the first candidate or the other two, but our
1535 /// algorithm doesn't reason about `"foo"` being distinct from the other
1536 /// constants; it considers the latter arms to potentially match after
1537 /// both outcomes, which obviously leads to an exponential number
1540 /// To avoid these kinds of problems, our algorithm tries to ensure
1541 /// the amount of generated tests is linear. When we do a k-way test,
1542 /// we return an additional "unmatched" set alongside the obvious `k`
1543 /// sets. When we encounter a candidate that would be present in more
1544 /// than one of the sets, we put it and all candidates below it into the
1545 /// "unmatched" set. This ensures these `k+1` sets are disjoint.
1547 /// After we perform our test, we branch into the appropriate candidate
1548 /// set and recurse with `match_candidates`. These sub-matches are
1549 /// obviously non-exhaustive - as we discarded our otherwise set - so
1550 /// we set their continuation to do `match_candidates` on the
1551 /// "unmatched" set (which is again non-exhaustive).
1553 /// If you apply this to the above test, you basically wind up
1554 /// with an if-else-if chain, testing each candidate in turn,
1555 /// which is precisely what we want.
1557 /// In addition to avoiding exponential-time blowups, this algorithm
1558 /// also has the nice property that each guard and arm is only generated
1560 fn test_candidates<'pat, 'b, 'c>(
1563 scrutinee_span: Span,
1564 mut candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>],
1566 otherwise_block: &mut Option<BasicBlock>,
1567 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1569 // extract the match-pair from the highest priority candidate
1570 let match_pair = &candidates.first().unwrap().match_pairs[0];
1571 let mut test = self.test(match_pair);
1572 let match_place = match_pair.place.clone();
1574 // most of the time, the test to perform is simply a function
1575 // of the main candidate; but for a test like SwitchInt, we
1576 // may want to add cases based on the candidates that are
1579 TestKind::SwitchInt { switch_ty, ref mut options } => {
1580 for candidate in candidates.iter() {
1581 if !self.add_cases_to_switch(&match_place, candidate, switch_ty, options) {
1586 TestKind::Switch { adt_def: _, ref mut variants } => {
1587 for candidate in candidates.iter() {
1588 if !self.add_variants_to_switch(&match_place, candidate, variants) {
1596 // Insert a Shallow borrow of any places that is switched on.
1597 if let Some(fb) = fake_borrows && let Ok(match_place_resolved) =
1598 match_place.clone().try_upvars_resolved(self)
1600 let resolved_place = match_place_resolved.into_place(self);
1601 fb.insert(resolved_place);
1604 // perform the test, branching to one of N blocks. For each of
1605 // those N possible outcomes, create a (initially empty)
1606 // vector of candidates. Those are the candidates that still
1607 // apply if the test has that particular outcome.
1608 debug!("test_candidates: test={:?} match_pair={:?}", test, match_pair);
1609 let mut target_candidates: Vec<Vec<&mut Candidate<'pat, 'tcx>>> = vec![];
1610 target_candidates.resize_with(test.targets(), Default::default);
1612 let total_candidate_count = candidates.len();
1614 // Sort the candidates into the appropriate vector in
1615 // `target_candidates`. Note that at some point we may
1616 // encounter a candidate where the test is not relevant; at
1617 // that point, we stop sorting.
1618 while let Some(candidate) = candidates.first_mut() {
1619 let Some(idx) = self.sort_candidate(&match_place.clone(), &test, candidate) else {
1622 let (candidate, rest) = candidates.split_first_mut().unwrap();
1623 target_candidates[idx].push(candidate);
1626 // at least the first candidate ought to be tested
1628 total_candidate_count > candidates.len(),
1630 total_candidate_count,
1633 debug!("tested_candidates: {}", total_candidate_count - candidates.len());
1634 debug!("untested_candidates: {}", candidates.len());
1636 // HACK(matthewjasper) This is a closure so that we can let the test
1637 // create its blocks before the rest of the match. This currently
1638 // improves the speed of llvm when optimizing long string literal
1640 let make_target_blocks = move |this: &mut Self| -> Vec<BasicBlock> {
1641 // The block that we should branch to if none of the
1642 // `target_candidates` match. This is either the block where we
1643 // start matching the untested candidates if there are any,
1644 // otherwise it's the `otherwise_block`.
1645 let remainder_start = &mut None;
1646 let remainder_start =
1647 if candidates.is_empty() { &mut *otherwise_block } else { remainder_start };
1649 // For each outcome of test, process the candidates that still
1650 // apply. Collect a list of blocks where control flow will
1651 // branch if one of the `target_candidate` sets is not
1653 let target_blocks: Vec<_> = target_candidates
1655 .map(|mut candidates| {
1656 if !candidates.is_empty() {
1657 let candidate_start = this.cfg.start_new_block();
1658 this.match_candidates(
1668 *remainder_start.get_or_insert_with(|| this.cfg.start_new_block())
1673 if !candidates.is_empty() {
1674 let remainder_start = remainder_start.unwrap_or_else(|| this.cfg.start_new_block());
1675 this.match_candidates(
1688 self.perform_test(span, scrutinee_span, block, match_place, &test, make_target_blocks);
1691 /// Determine the fake borrows that are needed from a set of places that
1692 /// have to be stable across match guards.
1694 /// Returns a list of places that need a fake borrow and the temporary
1695 /// that's used to store the fake borrow.
1697 /// Match exhaustiveness checking is not able to handle the case where the
1698 /// place being matched on is mutated in the guards. We add "fake borrows"
1699 /// to the guards that prevent any mutation of the place being matched.
1700 /// There are a some subtleties:
1702 /// 1. Borrowing `*x` doesn't prevent assigning to `x`. If `x` is a shared
1703 /// reference, the borrow isn't even tracked. As such we have to add fake
1704 /// borrows of any prefixes of a place
1705 /// 2. We don't want `match x { _ => (), }` to conflict with mutable
1706 /// borrows of `x`, so we only add fake borrows for places which are
1707 /// bound or tested by the match.
1708 /// 3. We don't want the fake borrows to conflict with `ref mut` bindings,
1709 /// so we use a special BorrowKind for them.
1710 /// 4. The fake borrows may be of places in inactive variants, so it would
1711 /// be UB to generate code for them. They therefore have to be removed
1712 /// by a MIR pass run after borrow checking.
1713 fn calculate_fake_borrows<'b>(
1715 fake_borrows: &'b FxIndexSet<Place<'tcx>>,
1717 ) -> Vec<(Place<'tcx>, Local)> {
1720 debug!("add_fake_borrows fake_borrows = {:?}", fake_borrows);
1722 let mut all_fake_borrows = Vec::with_capacity(fake_borrows.len());
1724 // Insert a Shallow borrow of the prefixes of any fake borrows.
1725 for place in fake_borrows {
1726 let mut cursor = place.projection.as_ref();
1727 while let [proj_base @ .., elem] = cursor {
1730 if let ProjectionElem::Deref = elem {
1731 // Insert a shallow borrow after a deref. For other
1732 // projections the borrow of prefix_cursor will
1733 // conflict with any mutation of base.
1734 all_fake_borrows.push(PlaceRef { local: place.local, projection: proj_base });
1738 all_fake_borrows.push(place.as_ref());
1742 let mut dedup = FxHashSet::default();
1743 all_fake_borrows.retain(|b| dedup.insert(*b));
1745 debug!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows);
1749 .map(|matched_place_ref| {
1750 let matched_place = Place {
1751 local: matched_place_ref.local,
1752 projection: tcx.intern_place_elems(matched_place_ref.projection),
1754 let fake_borrow_deref_ty = matched_place.ty(&self.local_decls, tcx).ty;
1755 let fake_borrow_ty = tcx.mk_imm_ref(tcx.lifetimes.re_erased, fake_borrow_deref_ty);
1756 let fake_borrow_temp =
1757 self.local_decls.push(LocalDecl::new(fake_borrow_ty, temp_span));
1759 (matched_place, fake_borrow_temp)
1765 ///////////////////////////////////////////////////////////////////////////
1766 // Pat binding - used for `let` and function parameters as well.
1768 impl<'a, 'tcx> Builder<'a, 'tcx> {
1769 pub(crate) fn lower_let_expr(
1771 mut block: BasicBlock,
1774 else_target: region::Scope,
1775 source_scope: Option<SourceScope>,
1778 let expr_span = expr.span;
1779 let expr_place_builder = unpack!(block = self.lower_scrutinee(block, expr, expr_span));
1780 let wildcard = Pat::wildcard_from_ty(pat.ty);
1781 let mut guard_candidate = Candidate::new(expr_place_builder.clone(), &pat, false, self);
1782 let mut otherwise_candidate =
1783 Candidate::new(expr_place_builder.clone(), &wildcard, false, self);
1784 let fake_borrow_temps = self.lower_match_tree(
1789 &mut [&mut guard_candidate, &mut otherwise_candidate],
1791 let mut opt_expr_place: Option<(Option<&Place<'tcx>>, Span)> = None;
1792 let expr_place: Place<'tcx>;
1793 if let Ok(expr_builder) = expr_place_builder.try_upvars_resolved(self) {
1794 expr_place = expr_builder.into_place(self);
1795 opt_expr_place = Some((Some(&expr_place), expr_span));
1797 let otherwise_post_guard_block = otherwise_candidate.pre_binding_block.unwrap();
1798 self.break_for_else(otherwise_post_guard_block, else_target, self.source_info(expr_span));
1800 self.declare_bindings(
1808 let post_guard_block = self.bind_pattern(
1809 self.source_info(pat.span),
1817 post_guard_block.unit()
1820 /// Initializes each of the bindings from the candidate by
1821 /// moving/copying/ref'ing the source as appropriate. Tests the guard, if
1822 /// any, and then branches to the arm. Returns the block for the case where
1823 /// the guard succeeds.
1825 /// Note: we do not check earlier that if there is a guard,
1826 /// there cannot be move bindings. We avoid a use-after-move by only
1827 /// moving the binding once the guard has evaluated to true (see below).
1828 fn bind_and_guard_matched_candidate<'pat>(
1830 candidate: Candidate<'pat, 'tcx>,
1831 parent_bindings: &[(Vec<Binding<'tcx>>, Vec<Ascription<'tcx>>)],
1832 fake_borrows: &[(Place<'tcx>, Local)],
1833 scrutinee_span: Span,
1834 arm_match_scope: Option<(&Arm<'tcx>, region::Scope)>,
1835 schedule_drops: bool,
1836 storages_alive: bool,
1838 debug!("bind_and_guard_matched_candidate(candidate={:?})", candidate);
1840 debug_assert!(candidate.match_pairs.is_empty());
1842 let candidate_source_info = self.source_info(candidate.span);
1844 let mut block = candidate.pre_binding_block.unwrap();
1846 if candidate.next_candidate_pre_binding_block.is_some() {
1847 let fresh_block = self.cfg.start_new_block();
1851 candidate.next_candidate_pre_binding_block,
1852 candidate_source_info,
1854 block = fresh_block;
1861 .flat_map(|(_, ascriptions)| ascriptions)
1863 .chain(candidate.ascriptions),
1866 // rust-lang/rust#27282: The `autoref` business deserves some
1867 // explanation here.
1869 // The intent of the `autoref` flag is that when it is true,
1870 // then any pattern bindings of type T will map to a `&T`
1871 // within the context of the guard expression, but will
1872 // continue to map to a `T` in the context of the arm body. To
1873 // avoid surfacing this distinction in the user source code
1874 // (which would be a severe change to the language and require
1875 // far more revision to the compiler), when `autoref` is true,
1876 // then any occurrence of the identifier in the guard
1877 // expression will automatically get a deref op applied to it.
1879 // So an input like:
1882 // let place = Foo::new();
1883 // match place { foo if inspect(foo)
1884 // => feed(foo), ... }
1887 // will be treated as if it were really something like:
1890 // let place = Foo::new();
1891 // match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) }
1892 // => { let tmp2 = place; feed(tmp2) }, ... }
1894 // And an input like:
1897 // let place = Foo::new();
1898 // match place { ref mut foo if inspect(foo)
1899 // => feed(foo), ... }
1902 // will be treated as if it were really something like:
1905 // let place = Foo::new();
1906 // match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) }
1907 // => { let tmp2 = &mut place; feed(tmp2) }, ... }
1910 // In short, any pattern binding will always look like *some*
1911 // kind of `&T` within the guard at least in terms of how the
1912 // MIR-borrowck views it, and this will ensure that guard
1913 // expressions cannot mutate their the match inputs via such
1914 // bindings. (It also ensures that guard expressions can at
1915 // most *copy* values from such bindings; non-Copy things
1916 // cannot be moved via pattern bindings in guard expressions.)
1920 // Implementation notes (under assumption `autoref` is true).
1922 // To encode the distinction above, we must inject the
1923 // temporaries `tmp1` and `tmp2`.
1925 // There are two cases of interest: binding by-value, and binding by-ref.
1927 // 1. Binding by-value: Things are simple.
1929 // * Establishing `tmp1` creates a reference into the
1930 // matched place. This code is emitted by
1931 // bind_matched_candidate_for_guard.
1933 // * `tmp2` is only initialized "lazily", after we have
1934 // checked the guard. Thus, the code that can trigger
1935 // moves out of the candidate can only fire after the
1936 // guard evaluated to true. This initialization code is
1937 // emitted by bind_matched_candidate_for_arm.
1939 // 2. Binding by-reference: Things are tricky.
1941 // * Here, the guard expression wants a `&&` or `&&mut`
1942 // into the original input. This means we need to borrow
1943 // the reference that we create for the arm.
1944 // * So we eagerly create the reference for the arm and then take a
1945 // reference to that.
1946 if let Some((arm, match_scope)) = arm_match_scope
1947 && let Some(guard) = &arm.guard
1950 let bindings = parent_bindings
1952 .flat_map(|(bindings, _)| bindings)
1953 .chain(&candidate.bindings);
1955 self.bind_matched_candidate_for_guard(block, schedule_drops, bindings.clone());
1956 let guard_frame = GuardFrame {
1957 locals: bindings.map(|b| GuardFrameLocal::new(b.var_id, b.binding_mode)).collect(),
1959 debug!("entering guard building context: {:?}", guard_frame);
1960 self.guard_context.push(guard_frame);
1962 let re_erased = tcx.lifetimes.re_erased;
1963 let scrutinee_source_info = self.source_info(scrutinee_span);
1964 for &(place, temp) in fake_borrows {
1965 let borrow = Rvalue::Ref(re_erased, BorrowKind::Shallow, place);
1966 self.cfg.push_assign(block, scrutinee_source_info, Place::from(temp), borrow);
1969 let mut guard_span = rustc_span::DUMMY_SP;
1971 let (post_guard_block, otherwise_post_guard_block) =
1972 self.in_if_then_scope(match_scope, guard_span, |this| match *guard {
1974 let e = &this.thir[e];
1975 guard_span = e.span;
1976 this.then_else_break(
1981 this.source_info(arm.span),
1984 Guard::IfLet(ref pat, scrutinee) => {
1985 let s = &this.thir[scrutinee];
1986 guard_span = s.span;
1987 this.lower_let_expr(block, s, pat, match_scope, None, arm.span)
1991 let source_info = self.source_info(guard_span);
1992 let guard_end = self.source_info(tcx.sess.source_map().end_point(guard_span));
1993 let guard_frame = self.guard_context.pop().unwrap();
1994 debug!("Exiting guard building context with locals: {:?}", guard_frame);
1996 for &(_, temp) in fake_borrows {
1997 let cause = FakeReadCause::ForMatchGuard;
1998 self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(temp));
2001 let otherwise_block = candidate.otherwise_block.unwrap_or_else(|| {
2002 let unreachable = self.cfg.start_new_block();
2003 self.cfg.terminate(unreachable, source_info, TerminatorKind::Unreachable);
2007 otherwise_post_guard_block,
2009 candidate.next_candidate_pre_binding_block,
2013 // We want to ensure that the matched candidates are bound
2014 // after we have confirmed this candidate *and* any
2015 // associated guard; Binding them on `block` is too soon,
2016 // because that would be before we've checked the result
2019 // But binding them on the arm is *too late*, because
2020 // then all of the candidates for a single arm would be
2021 // bound in the same place, that would cause a case like:
2025 // (mut x, 1) | (2, mut x) if { true } => { ... }
2026 // ... // ^^^^^^^ (this is `arm_block`)
2030 // would yield an `arm_block` something like:
2033 // StorageLive(_4); // _4 is `x`
2034 // _4 = &mut (_1.0: i32); // this is handling `(mut x, 1)` case
2035 // _4 = &mut (_1.1: i32); // this is handling `(2, mut x)` case
2038 // and that is clearly not correct.
2039 let by_value_bindings = parent_bindings
2041 .flat_map(|(bindings, _)| bindings)
2042 .chain(&candidate.bindings)
2043 .filter(|binding| matches!(binding.binding_mode, BindingMode::ByValue));
2044 // Read all of the by reference bindings to ensure that the
2045 // place they refer to can't be modified by the guard.
2046 for binding in by_value_bindings.clone() {
2047 let local_id = self.var_local_id(binding.var_id, RefWithinGuard);
2048 let cause = FakeReadCause::ForGuardBinding;
2049 self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(local_id));
2051 assert!(schedule_drops, "patterns with guards must schedule drops");
2052 self.bind_matched_candidate_for_arm_body(
2061 // (Here, it is not too early to bind the matched
2062 // candidate on `block`, because there is no guard result
2063 // that we have to inspect before we bind them.)
2064 self.bind_matched_candidate_for_arm_body(
2069 .flat_map(|(bindings, _)| bindings)
2070 .chain(&candidate.bindings),
2077 /// Append `AscribeUserType` statements onto the end of `block`
2078 /// for each ascription
2082 ascriptions: impl IntoIterator<Item = Ascription<'tcx>>,
2084 for ascription in ascriptions {
2085 let source_info = self.source_info(ascription.annotation.span);
2087 let base = self.canonical_user_type_annotations.push(ascription.annotation);
2092 kind: StatementKind::AscribeUserType(
2095 UserTypeProjection { base, projs: Vec::new() },
2097 ascription.variance,
2104 fn bind_matched_candidate_for_guard<'b>(
2107 schedule_drops: bool,
2108 bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
2112 debug!("bind_matched_candidate_for_guard(block={:?})", block);
2114 // Assign each of the bindings. Since we are binding for a
2115 // guard expression, this will never trigger moves out of the
2117 let re_erased = self.tcx.lifetimes.re_erased;
2118 for binding in bindings {
2119 debug!("bind_matched_candidate_for_guard(binding={:?})", binding);
2120 let source_info = self.source_info(binding.span);
2122 // For each pattern ident P of type T, `ref_for_guard` is
2123 // a reference R: &T pointing to the location matched by
2124 // the pattern, and every occurrence of P within a guard
2126 let ref_for_guard = self.storage_live_binding(
2133 match binding.binding_mode {
2134 BindingMode::ByValue => {
2135 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, binding.source);
2136 self.cfg.push_assign(block, source_info, ref_for_guard, rvalue);
2138 BindingMode::ByRef(borrow_kind) => {
2139 let value_for_arm = self.storage_live_binding(
2147 let rvalue = Rvalue::Ref(re_erased, borrow_kind, binding.source);
2148 self.cfg.push_assign(block, source_info, value_for_arm, rvalue);
2149 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, value_for_arm);
2150 self.cfg.push_assign(block, source_info, ref_for_guard, rvalue);
2156 fn bind_matched_candidate_for_arm_body<'b>(
2159 schedule_drops: bool,
2160 bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
2161 storages_alive: bool,
2165 debug!("bind_matched_candidate_for_arm_body(block={:?})", block);
2167 let re_erased = self.tcx.lifetimes.re_erased;
2168 // Assign each of the bindings. This may trigger moves out of the candidate.
2169 for binding in bindings {
2170 let source_info = self.source_info(binding.span);
2171 let local = if storages_alive {
2172 // Here storages are already alive, probably because this is a binding
2174 // We just need to schedule drop for the value.
2175 self.var_local_id(binding.var_id, OutsideGuard).into()
2177 self.storage_live_binding(
2186 self.schedule_drop_for_binding(binding.var_id, binding.span, OutsideGuard);
2188 let rvalue = match binding.binding_mode {
2189 BindingMode::ByValue => Rvalue::Use(self.consume_by_copy_or_move(binding.source)),
2190 BindingMode::ByRef(borrow_kind) => {
2191 Rvalue::Ref(re_erased, borrow_kind, binding.source)
2194 self.cfg.push_assign(block, source_info, local, rvalue);
2198 /// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where the bound
2199 /// `var` has type `T` in the arm body) in a pattern maps to 2 locals. The
2200 /// first local is a binding for occurrences of `var` in the guard, which
2201 /// will have type `&T`. The second local is a binding for occurrences of
2202 /// `var` in the arm body, which will have type `T`.
2203 #[instrument(skip(self), level = "debug")]
2206 source_info: SourceInfo,
2207 visibility_scope: SourceScope,
2208 mutability: Mutability,
2213 user_ty: UserTypeProjections,
2214 has_guard: ArmHasGuard,
2215 opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
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),
2224 let local = LocalDecl::<'tcx> {
2227 user_ty: if user_ty.is_empty() { None } else { Some(Box::new(user_ty)) },
2230 is_block_tail: None,
2231 local_info: Some(Box::new(LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
2234 // hypothetically, `visit_primary_bindings` could try to unzip
2235 // an outermost hir::Ty as we descend, matching up
2236 // idents in pat; but complex w/ unclear UI payoff.
2237 // Instead, just abandon providing diagnostic info.
2244 let for_arm_body = self.local_decls.push(local);
2245 self.var_debug_info.push(VarDebugInfo {
2247 source_info: debug_source_info,
2248 value: VarDebugInfoContents::Place(for_arm_body.into()),
2250 let locals = if has_guard.0 {
2251 let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
2252 // This variable isn't mutated but has a name, so has to be
2253 // immutable to avoid the unused mut lint.
2254 mutability: Mutability::Not,
2255 ty: tcx.mk_imm_ref(tcx.lifetimes.re_erased, var_ty),
2259 is_block_tail: None,
2260 local_info: Some(Box::new(LocalInfo::User(ClearCrossCrate::Set(
2261 BindingForm::RefForGuard,
2264 self.var_debug_info.push(VarDebugInfo {
2266 source_info: debug_source_info,
2267 value: VarDebugInfoContents::Place(ref_for_guard.into()),
2269 LocalsForNode::ForGuard { ref_for_guard, for_arm_body }
2271 LocalsForNode::One(for_arm_body)
2274 self.var_indices.insert(var_id, locals);
2277 pub(crate) fn ast_let_else(
2279 mut block: BasicBlock,
2281 initializer_span: Span,
2282 else_block: BlockId,
2283 let_else_scope: ®ion::Scope,
2284 pattern: &Pat<'tcx>,
2285 ) -> BlockAnd<BasicBlock> {
2286 let else_block_span = self.thir[else_block].span;
2287 let (matching, failure) = self.in_if_then_scope(*let_else_scope, else_block_span, |this| {
2288 let scrutinee = unpack!(block = this.lower_scrutinee(block, init, initializer_span));
2289 let pat = Pat { ty: init.ty, span: else_block_span, kind: PatKind::Wild };
2290 let mut wildcard = Candidate::new(scrutinee.clone(), &pat, false, this);
2291 let mut candidate = Candidate::new(scrutinee.clone(), pattern, false, this);
2292 let fake_borrow_temps = this.lower_match_tree(
2297 &mut [&mut candidate, &mut wildcard],
2299 // This block is for the matching case
2300 let matching = this.bind_pattern(
2301 this.source_info(pattern.span),
2308 // This block is for the failure case
2309 let failure = this.bind_pattern(
2310 this.source_info(else_block_span),
2317 this.break_for_else(failure, *let_else_scope, this.source_info(initializer_span));
2320 matching.and(failure)