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) =
224 scrutinee_place_builder.clone().try_upvars_resolved(self.tcx, self.typeck_results)
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);
230 block.and(scrutinee_place_builder)
233 /// Create the initial `Candidate`s for a `match` expression.
234 fn create_match_candidates<'pat>(
236 scrutinee: PlaceBuilder<'tcx>,
238 ) -> Vec<(&'pat Arm<'tcx>, Candidate<'pat, 'tcx>)>
242 // Assemble a list of candidates: there is one candidate per pattern,
243 // which means there may be more than one candidate *per 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);
255 /// Create the decision tree for the match expression, starting from `block`.
257 /// Modifies `candidates` to store the bindings and type ascriptions for
260 /// Returns the places that need fake borrows because we bind or test them.
261 fn lower_match_tree<'pat>(
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
272 let mut fake_borrows = match_has_guard.then(FxIndexSet::default);
274 let mut otherwise = None;
276 // This will generate code to test scrutinee_place and
277 // branch to the appropriate arm block
278 self.match_candidates(
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
291 let source_info = self.source_info(scrutinee_span);
292 self.cfg.terminate(otherwise_block, source_info, TerminatorKind::Unreachable);
295 // Link each leaf candidate to the `pre_binding_block` of the next one.
296 let mut previous_candidate: Option<&mut Candidate<'_, '_>> = None;
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;
303 previous_candidate = Some(leaf_candidate);
307 if let Some(ref borrows) = fake_borrows {
308 self.calculate_fake_borrows(borrows, scrutinee_span)
314 /// Lower the bindings, guards and arm bodies of a `match` expression.
316 /// The decision tree should have already been created
317 /// (by [Builder::lower_match_tree]).
319 /// `outer_source_info` is the SourceInfo for the whole match.
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)>,
329 let arm_end_blocks: Vec<_> = arm_candidates
331 .map(|(arm, candidate)| {
332 debug!("lowering arm {:?}\ncandidate = {:?}", arm, candidate);
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 (`_`).
346 // match foo { _ => () };
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
353 .try_upvars_resolved(this.tcx, this.typeck_results)
356 scrutinee_builder.into_place(this.tcx, this.typeck_results);
357 opt_scrutinee_place = Some((Some(&scrutinee_place), scrutinee_span));
359 let scope = this.declare_bindings(
363 ArmHasGuard(arm.guard.is_some()),
367 let arm_block = this.bind_pattern(
378 if let Some(source_scope) = scope {
379 this.source_scope = source_scope;
382 this.expr_into_dest(destination, arm_block, &&this.thir[arm.body])
387 // all the arm blocks will rejoin here
388 let end_block = self.cfg.start_new_block();
390 let end_brace = self.source_info(
391 outer_source_info.span.with_lo(outer_source_info.span.hi() - BytePos::from_usize(1)),
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);
397 self.cfg.goto(unpack!(arm_block), last_location.unwrap_or(end_brace), end_block);
400 self.source_scope = outer_source_info.scope;
405 /// Binds the variables and ascribes types for a given `match` arm or
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
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>,
422 if candidate.subcandidates.is_empty() {
423 // Avoid generating another `BasicBlock` when we only have one
425 self.bind_and_guard_matched_candidate(
436 // It's helpful to avoid scheduling drops multiple times to save
437 // drop elaboration from having to clean up the extra drops.
439 // If we are in a `let` then we only schedule drops for the first
442 // If we're in a `match` arm then we could have a case like so:
444 // Ok(x) | Err(x) if return => { /* ... */ }
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.
458 &mut |leaf_candidate, parent_bindings| {
459 if let Some(arm_scope) = arm_scope {
460 self.clear_top_scope(arm_scope);
462 let binding_end = self.bind_and_guard_matched_candidate(
472 if arm_scope.is_none() {
473 schedule_drops = false;
475 self.cfg.goto(binding_end, outer_source_info, target_block);
477 |inner_candidate, parent_bindings| {
478 parent_bindings.push((inner_candidate.bindings, inner_candidate.ascriptions));
479 inner_candidate.subcandidates.into_iter()
482 parent_bindings.pop();
490 pub(super) fn expr_into_pattern(
492 mut block: BasicBlock,
493 irrefutable_pat: &Pat<'tcx>,
494 initializer: &Expr<'tcx>,
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, .. } => {
500 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
501 unpack!(block = self.expr_into_dest(place, block, initializer));
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);
507 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
511 // Optimize the case of `let x: T = ...` to write directly
512 // into `x` and then require that `T == typeof(x)`.
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
519 PatKind::AscribeUserType {
524 mode: BindingMode::ByValue, var, subpattern: None, ..
528 ascription: thir::Ascription { ref annotation, variance: _ },
531 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
532 unpack!(block = self.expr_into_dest(place, block, initializer));
534 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
535 let pattern_source_info = self.source_info(irrefutable_pat.span);
536 let cause_let = FakeReadCause::ForLet(None);
537 self.cfg.push_fake_read(block, pattern_source_info, cause_let, place);
539 let ty_source_info = self.source_info(annotation.span);
541 let base = self.canonical_user_type_annotations.push(annotation.clone());
545 source_info: ty_source_info,
546 kind: StatementKind::AscribeUserType(
547 Box::new((place, UserTypeProjection { base, projs: Vec::new() })),
548 // We always use invariant as the variance here. This is because the
549 // variance field from the ascription refers to the variance to use
550 // when applying the type to the value being matched, but this
551 // ascription applies rather to the type of the binding. e.g., in this
558 // We are creating an ascription that defines the type of `x` to be
559 // exactly `T` (i.e., with invariance). The variance field, in
560 // contrast, is intended to be used to relate `T` to the type of
562 ty::Variance::Invariant,
567 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
572 let place_builder = unpack!(block = self.as_place_builder(block, initializer));
573 self.place_into_pattern(block, irrefutable_pat, place_builder, true)
578 pub(crate) fn place_into_pattern(
581 irrefutable_pat: &Pat<'tcx>,
582 initializer: PlaceBuilder<'tcx>,
583 set_match_place: bool,
585 let mut candidate = Candidate::new(initializer.clone(), &irrefutable_pat, false);
586 let fake_borrow_temps = self.lower_match_tree(
588 irrefutable_pat.span,
589 irrefutable_pat.span,
591 &mut [&mut candidate],
593 // For matches and function arguments, the place that is being matched
594 // can be set when creating the variables. But the place for
595 // let PATTERN = ... might not even exist until we do the assignment.
596 // so we set it here instead.
598 let mut candidate_ref = &candidate;
599 while let Some(next) = {
600 for binding in &candidate_ref.bindings {
601 let local = self.var_local_id(binding.var_id, OutsideGuard);
603 let Some(box LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
604 VarBindingForm { opt_match_place: Some((ref mut match_place, _)), .. },
605 )))) = self.local_decls[local].local_info else {
606 bug!("Let binding to non-user variable.")
608 // `try_upvars_resolved` may fail if it is unable to resolve the given
609 // `PlaceBuilder` inside a closure. In this case, we don't want to include
610 // a scrutinee place. `scrutinee_place_builder` will fail for destructured
611 // assignments. This is because a closure only captures the precise places
612 // that it will read and as a result a closure may not capture the entire
613 // tuple/struct and rather have individual places that will be read in the
619 // let (v1, v2) = foo;
622 if let Ok(match_pair_resolved) =
623 initializer.clone().try_upvars_resolved(self.tcx, self.typeck_results)
625 let place = match_pair_resolved.into_place(self.tcx, self.typeck_results);
626 *match_place = Some(place);
629 // All of the subcandidates should bind the same locals, so we
630 // only visit the first one.
631 candidate_ref.subcandidates.get(0)
633 candidate_ref = next;
638 self.source_info(irrefutable_pat.span),
642 irrefutable_pat.span,
650 /// Declares the bindings of the given patterns and returns the visibility
651 /// scope for the bindings in these patterns, if such a scope had to be
652 /// created. NOTE: Declaring the bindings should always be done in their
654 pub(crate) fn declare_bindings(
656 mut visibility_scope: Option<SourceScope>,
659 has_guard: ArmHasGuard,
660 opt_match_place: Option<(Option<&Place<'tcx>>, Span)>,
661 ) -> Option<SourceScope> {
662 debug!("declare_bindings: pattern={:?}", pattern);
663 self.visit_primary_bindings(
665 UserTypeProjections::none(),
666 &mut |this, mutability, name, mode, var, span, ty, user_ty| {
667 if visibility_scope.is_none() {
669 Some(this.new_source_scope(scope_span, LintLevel::Inherited, None));
671 let source_info = SourceInfo { span, scope: this.source_scope };
672 let visibility_scope = visibility_scope.unwrap();
673 this.declare_binding(
683 opt_match_place.map(|(x, y)| (x.cloned(), y)),
691 pub(crate) fn storage_live_binding(
699 let local_id = self.var_local_id(var, for_guard);
700 let source_info = self.source_info(span);
701 self.cfg.push(block, Statement { source_info, kind: StatementKind::StorageLive(local_id) });
702 // Although there is almost always scope for given variable in corner cases
703 // like #92893 we might get variable with no scope.
704 if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) && schedule_drop{
705 self.schedule_drop(span, region_scope, local_id, DropKind::Storage);
707 Place::from(local_id)
710 pub(crate) fn schedule_drop_for_binding(
716 let local_id = self.var_local_id(var, for_guard);
717 if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) {
718 self.schedule_drop(span, region_scope, local_id, DropKind::Value);
722 /// Visit all of the primary bindings in a patterns, that is, visit the
723 /// leftmost occurrence of each variable bound in a pattern. A variable
724 /// will occur more than once in an or-pattern.
725 pub(super) fn visit_primary_bindings(
728 pattern_user_ty: UserTypeProjections,
741 "visit_primary_bindings: pattern={:?} pattern_user_ty={:?}",
742 pattern, pattern_user_ty
756 f(self, mutability, name, mode, var, pattern.span, ty, pattern_user_ty.clone());
758 if let Some(subpattern) = subpattern.as_ref() {
759 self.visit_primary_bindings(subpattern, pattern_user_ty, f);
763 PatKind::Array { ref prefix, ref slice, ref suffix }
764 | PatKind::Slice { ref prefix, ref slice, ref suffix } => {
765 let from = u64::try_from(prefix.len()).unwrap();
766 let to = u64::try_from(suffix.len()).unwrap();
767 for subpattern in prefix.iter() {
768 self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
770 for subpattern in slice {
771 self.visit_primary_bindings(
773 pattern_user_ty.clone().subslice(from, to),
777 for subpattern in suffix.iter() {
778 self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
782 PatKind::Constant { .. } | PatKind::Range { .. } | PatKind::Wild => {}
784 PatKind::Deref { ref subpattern } => {
785 self.visit_primary_bindings(subpattern, pattern_user_ty.deref(), f);
788 PatKind::AscribeUserType {
790 ascription: thir::Ascription { ref annotation, variance: _ },
792 // This corresponds to something like
795 // let A::<'a>(_): A<'static> = ...;
798 // Note that the variance doesn't apply here, as we are tracking the effect
799 // of `user_ty` on any bindings contained with subpattern.
801 let projection = UserTypeProjection {
802 base: self.canonical_user_type_annotations.push(annotation.clone()),
805 let subpattern_user_ty =
806 pattern_user_ty.push_projection(&projection, annotation.span);
807 self.visit_primary_bindings(subpattern, subpattern_user_ty, f)
810 PatKind::Leaf { ref subpatterns } => {
811 for subpattern in subpatterns {
812 let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field);
813 debug!("visit_primary_bindings: subpattern_user_ty={:?}", subpattern_user_ty);
814 self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
818 PatKind::Variant { adt_def, substs: _, variant_index, ref subpatterns } => {
819 for subpattern in subpatterns {
820 let subpattern_user_ty =
821 pattern_user_ty.clone().variant(adt_def, variant_index, subpattern.field);
822 self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
825 PatKind::Or { ref pats } => {
826 // In cases where we recover from errors the primary bindings
827 // may not all be in the leftmost subpattern. For example in
828 // `let (x | y) = ...`, the primary binding of `y` occurs in
829 // the right subpattern
830 for subpattern in pats.iter() {
831 self.visit_primary_bindings(subpattern, pattern_user_ty.clone(), f);
839 struct Candidate<'pat, 'tcx> {
840 /// [`Span`] of the original pattern that gave rise to this candidate.
843 /// Whether this `Candidate` has a guard.
846 /// All of these must be satisfied...
847 match_pairs: SmallVec<[MatchPair<'pat, 'tcx>; 1]>,
849 /// ...these bindings established...
850 bindings: Vec<Binding<'tcx>>,
852 /// ...and these types asserted...
853 ascriptions: Vec<Ascription<'tcx>>,
855 /// ...and if this is non-empty, one of these subcandidates also has to match...
856 subcandidates: Vec<Candidate<'pat, 'tcx>>,
858 /// ...and the guard must be evaluated; if it's `false` then branch to `otherwise_block`.
859 otherwise_block: Option<BasicBlock>,
861 /// The block before the `bindings` have been established.
862 pre_binding_block: Option<BasicBlock>,
863 /// The pre-binding block of the next candidate.
864 next_candidate_pre_binding_block: Option<BasicBlock>,
867 impl<'tcx, 'pat> Candidate<'pat, 'tcx> {
868 fn new(place: PlaceBuilder<'tcx>, pattern: &'pat Pat<'tcx>, has_guard: bool) -> Self {
872 match_pairs: smallvec![MatchPair { place, pattern }],
873 bindings: Vec::new(),
874 ascriptions: Vec::new(),
875 subcandidates: Vec::new(),
876 otherwise_block: None,
877 pre_binding_block: None,
878 next_candidate_pre_binding_block: None,
882 /// Visit the leaf candidates (those with no subcandidates) contained in
884 fn visit_leaves<'a>(&'a mut self, mut visit_leaf: impl FnMut(&'a mut Self)) {
888 &mut move |c, _| visit_leaf(c),
889 move |c, _| c.subcandidates.iter_mut(),
895 /// A depth-first traversal of the `Candidate` and all of its recursive
897 fn traverse_candidate<'pat, 'tcx: 'pat, C, T, I>(
900 visit_leaf: &mut impl FnMut(C, &mut T),
901 get_children: impl Copy + Fn(C, &mut T) -> I,
902 complete_children: impl Copy + Fn(&mut T),
904 C: Borrow<Candidate<'pat, 'tcx>>,
905 I: Iterator<Item = C>,
907 if candidate.borrow().subcandidates.is_empty() {
908 visit_leaf(candidate, context)
910 for child in get_children(candidate, context) {
911 traverse_candidate(child, context, visit_leaf, get_children, complete_children);
913 complete_children(context)
917 #[derive(Clone, Debug)]
918 struct Binding<'tcx> {
922 binding_mode: BindingMode,
925 /// Indicates that the type of `source` must be a subtype of the
926 /// user-given type `user_ty`; this is basically a no-op but can
927 /// influence region inference.
928 #[derive(Clone, Debug)]
929 struct Ascription<'tcx> {
931 annotation: CanonicalUserTypeAnnotation<'tcx>,
932 variance: ty::Variance,
935 #[derive(Clone, Debug)]
936 pub(crate) struct MatchPair<'pat, 'tcx> {
938 place: PlaceBuilder<'tcx>,
940 // ... must match this pattern.
941 pattern: &'pat Pat<'tcx>,
944 /// See [`Test`] for more.
945 #[derive(Clone, Debug, PartialEq)]
946 enum TestKind<'tcx> {
947 /// Test what enum variant a value is.
949 /// The enum type being tested.
950 adt_def: ty::AdtDef<'tcx>,
951 /// The set of variants that we should create a branch for. We also
952 /// create an additional "otherwise" case.
953 variants: BitSet<VariantIdx>,
956 /// Test what value an integer, `bool`, or `char` has.
958 /// The type of the value that we're testing.
960 /// The (ordered) set of values that we test for.
962 /// For integers and `char`s we create a branch to each of the values in
963 /// `options`, as well as an "otherwise" branch for all other values, even
964 /// in the (rare) case that `options` is exhaustive.
966 /// For `bool` we always generate two edges, one for `true` and one for
968 options: FxIndexMap<ConstantKind<'tcx>, u128>,
971 /// Test for equality with value, possibly after an unsizing coercion to
974 value: ConstantKind<'tcx>,
975 // Integer types are handled by `SwitchInt`, and constants with ADT
976 // types are converted back into patterns, so this can only be `&str`,
977 // `&[T]`, `f32` or `f64`.
981 /// Test whether the value falls within an inclusive or exclusive range
982 Range(Box<PatRange<'tcx>>),
984 /// Test that the length of the slice is equal to `len`.
985 Len { len: u64, op: BinOp },
988 /// A test to perform to determine which [`Candidate`] matches a value.
990 /// [`Test`] is just the test to perform; it does not include the value
993 pub(crate) struct Test<'tcx> {
995 kind: TestKind<'tcx>,
998 /// `ArmHasGuard` is a wrapper around a boolean flag. It indicates whether
999 /// a match arm has a guard expression attached to it.
1000 #[derive(Copy, Clone, Debug)]
1001 pub(crate) struct ArmHasGuard(pub(crate) bool);
1003 ///////////////////////////////////////////////////////////////////////////
1004 // Main matching algorithm
1006 impl<'a, 'tcx> Builder<'a, 'tcx> {
1007 /// The main match algorithm. It begins with a set of candidates
1008 /// `candidates` and has the job of generating code to determine
1009 /// which of these candidates, if any, is the correct one. The
1010 /// candidates are sorted such that the first item in the list
1011 /// has the highest priority. When a candidate is found to match
1012 /// the value, we will set and generate a branch to the appropriate
1013 /// pre-binding block.
1015 /// If we find that *NONE* of the candidates apply, we branch to the
1016 /// `otherwise_block`, setting it to `Some` if required. In principle, this
1017 /// means that the input list was not exhaustive, though at present we
1018 /// sometimes are not smart enough to recognize all exhaustive inputs.
1020 /// It might be surprising that the input can be non-exhaustive.
1021 /// Indeed, initially, it is not, because all matches are
1022 /// exhaustive in Rust. But during processing we sometimes divide
1023 /// up the list of candidates and recurse with a non-exhaustive
1024 /// list. This is important to keep the size of the generated code
1025 /// under control. See [`Builder::test_candidates`] for more details.
1027 /// If `fake_borrows` is `Some`, then places which need fake borrows
1028 /// will be added to it.
1030 /// For an example of a case where we set `otherwise_block`, even for an
1031 /// exhaustive match, consider:
1034 /// # fn foo(x: (bool, bool)) {
1036 /// (true, true) => (),
1037 /// (_, false) => (),
1038 /// (false, true) => (),
1043 /// For this match, we check if `x.0` matches `true` (for the first
1044 /// arm). If it doesn't match, we check `x.1`. If `x.1` is `true` we check
1045 /// if `x.0` matches `false` (for the third arm). In the (impossible at
1046 /// runtime) case when `x.0` is now `true`, we branch to
1047 /// `otherwise_block`.
1048 fn match_candidates<'pat>(
1051 scrutinee_span: Span,
1052 start_block: BasicBlock,
1053 otherwise_block: &mut Option<BasicBlock>,
1054 candidates: &mut [&mut Candidate<'pat, 'tcx>],
1055 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1058 "matched_candidate(span={:?}, candidates={:?}, start_block={:?}, otherwise_block={:?})",
1059 span, candidates, start_block, otherwise_block,
1062 // Start by simplifying candidates. Once this process is complete, all
1063 // the match pairs which remain require some form of test, whether it
1064 // be a switch or pattern comparison.
1065 let mut split_or_candidate = false;
1066 for candidate in &mut *candidates {
1067 split_or_candidate |= self.simplify_candidate(candidate);
1070 ensure_sufficient_stack(|| {
1071 if split_or_candidate {
1072 // At least one of the candidates has been split into subcandidates.
1073 // We need to change the candidate list to include those.
1074 let mut new_candidates = Vec::new();
1076 for candidate in candidates {
1077 candidate.visit_leaves(|leaf_candidate| new_candidates.push(leaf_candidate));
1079 self.match_simplified_candidates(
1084 &mut *new_candidates,
1088 self.match_simplified_candidates(
1100 fn match_simplified_candidates(
1103 scrutinee_span: Span,
1104 start_block: BasicBlock,
1105 otherwise_block: &mut Option<BasicBlock>,
1106 candidates: &mut [&mut Candidate<'_, 'tcx>],
1107 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1109 // The candidates are sorted by priority. Check to see whether the
1110 // higher priority candidates (and hence at the front of the slice)
1111 // have satisfied all their match pairs.
1112 let fully_matched = candidates.iter().take_while(|c| c.match_pairs.is_empty()).count();
1113 debug!("match_candidates: {:?} candidates fully matched", fully_matched);
1114 let (matched_candidates, unmatched_candidates) = candidates.split_at_mut(fully_matched);
1116 let block = if !matched_candidates.is_empty() {
1117 let otherwise_block =
1118 self.select_matched_candidates(matched_candidates, start_block, fake_borrows);
1120 if let Some(last_otherwise_block) = otherwise_block {
1121 last_otherwise_block
1123 // Any remaining candidates are unreachable.
1124 if unmatched_candidates.is_empty() {
1127 self.cfg.start_new_block()
1133 // If there are no candidates that still need testing, we're
1134 // done. Since all matches are exhaustive, execution should
1135 // never reach this point.
1136 if unmatched_candidates.is_empty() {
1137 let source_info = self.source_info(span);
1138 if let Some(otherwise) = *otherwise_block {
1139 self.cfg.goto(block, source_info, otherwise);
1141 *otherwise_block = Some(block);
1146 // Test for the remaining candidates.
1147 self.test_candidates_with_or(
1150 unmatched_candidates,
1157 /// Link up matched candidates.
1159 /// For example, if we have something like this:
1161 /// ```ignore (illustrative)
1163 /// Some(x) if cond1 => ...
1165 /// Some(x) if cond2 => ...
1169 /// We generate real edges from:
1171 /// * `start_block` to the [pre-binding block] of the first pattern,
1172 /// * the [otherwise block] of the first pattern to the second pattern,
1173 /// * the [otherwise block] of the third pattern to a block with an
1174 /// [`Unreachable` terminator](TerminatorKind::Unreachable).
1176 /// In addition, we add fake edges from the otherwise blocks to the
1177 /// pre-binding block of the next candidate in the original set of
1180 /// [pre-binding block]: Candidate::pre_binding_block
1181 /// [otherwise block]: Candidate::otherwise_block
1182 fn select_matched_candidates(
1184 matched_candidates: &mut [&mut Candidate<'_, 'tcx>],
1185 start_block: BasicBlock,
1186 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1187 ) -> Option<BasicBlock> {
1189 !matched_candidates.is_empty(),
1190 "select_matched_candidates called with no candidates",
1193 matched_candidates.iter().all(|c| c.subcandidates.is_empty()),
1194 "subcandidates should be empty in select_matched_candidates",
1197 // Insert a borrows of prefixes of places that are bound and are
1198 // behind a dereference projection.
1200 // These borrows are taken to avoid situations like the following:
1203 // _ if { x = &[0]; false } => (),
1204 // y => (), // Out of bounds array access!
1208 // // y is bound by reference in the guard and then by copy in the
1209 // // arm, so y is 2 in the arm!
1210 // y if { y == 1 && (x = &2) == () } => y,
1213 if let Some(fake_borrows) = fake_borrows {
1214 for Binding { source, .. } in
1215 matched_candidates.iter().flat_map(|candidate| &candidate.bindings)
1218 source.projection.iter().rposition(|elem| elem == ProjectionElem::Deref)
1220 let proj_base = &source.projection[..i];
1222 fake_borrows.insert(Place {
1223 local: source.local,
1224 projection: self.tcx.intern_place_elems(proj_base),
1230 let fully_matched_with_guard = matched_candidates
1232 .position(|c| !c.has_guard)
1233 .unwrap_or(matched_candidates.len() - 1);
1235 let (reachable_candidates, unreachable_candidates) =
1236 matched_candidates.split_at_mut(fully_matched_with_guard + 1);
1238 let mut next_prebinding = start_block;
1240 for candidate in reachable_candidates.iter_mut() {
1241 assert!(candidate.otherwise_block.is_none());
1242 assert!(candidate.pre_binding_block.is_none());
1243 candidate.pre_binding_block = Some(next_prebinding);
1244 if candidate.has_guard {
1245 // Create the otherwise block for this candidate, which is the
1246 // pre-binding block for the next candidate.
1247 next_prebinding = self.cfg.start_new_block();
1248 candidate.otherwise_block = Some(next_prebinding);
1253 "match_candidates: add pre_binding_blocks for unreachable {:?}",
1254 unreachable_candidates,
1256 for candidate in unreachable_candidates {
1257 assert!(candidate.pre_binding_block.is_none());
1258 candidate.pre_binding_block = Some(self.cfg.start_new_block());
1261 reachable_candidates.last_mut().unwrap().otherwise_block
1264 /// Tests a candidate where there are only or-patterns left to test, or
1265 /// forwards to [Builder::test_candidates].
1267 /// Given a pattern `(P | Q, R | S)` we (in principle) generate a CFG like
1275 /// +----------------------------------------+------------------------------------+
1278 /// [ P matches ] [ Q matches ] [ otherwise ]
1281 /// [ match R, S ] [ match R, S ] |
1283 /// +--------------+------------+ +--------------+------------+ |
1286 /// [ R matches ] [ S matches ] [otherwise ] [ R matches ] [ S matches ] [otherwise ] |
1288 /// +--------------+------------|------------+--------------+ | |
1290 /// | +----------------------------------------+--------+
1293 /// [ Success ] [ Failure ]
1296 /// In practice there are some complications:
1298 /// * If there's a guard, then the otherwise branch of the first match on
1299 /// `R | S` goes to a test for whether `Q` matches, and the control flow
1300 /// doesn't merge into a single success block until after the guard is
1302 /// * If neither `P` or `Q` has any bindings or type ascriptions and there
1303 /// isn't a match guard, then we create a smaller CFG like:
1307 /// +---------------+------------+
1309 /// [ P matches ] [ Q matches ] [ otherwise ]
1311 /// +---------------+ |
1317 fn test_candidates_with_or(
1320 scrutinee_span: Span,
1321 candidates: &mut [&mut Candidate<'_, 'tcx>],
1323 otherwise_block: &mut Option<BasicBlock>,
1324 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1326 let (first_candidate, remaining_candidates) = candidates.split_first_mut().unwrap();
1328 // All of the or-patterns have been sorted to the end, so if the first
1329 // pattern is an or-pattern we only have or-patterns.
1330 match first_candidate.match_pairs[0].pattern.kind {
1331 PatKind::Or { .. } => (),
1333 self.test_candidates(
1345 let match_pairs = mem::take(&mut first_candidate.match_pairs);
1346 first_candidate.pre_binding_block = Some(block);
1348 let mut otherwise = None;
1349 for match_pair in match_pairs {
1350 let PatKind::Or { ref pats } = &match_pair.pattern.kind else {
1351 bug!("Or-patterns should have been sorted to the end");
1353 let or_span = match_pair.pattern.span;
1354 let place = match_pair.place;
1356 first_candidate.visit_leaves(|leaf_candidate| {
1357 self.test_or_pattern(
1368 let remainder_start = otherwise.unwrap_or_else(|| self.cfg.start_new_block());
1370 self.match_candidates(
1375 remaining_candidates,
1380 fn test_or_pattern<'pat>(
1382 candidate: &mut Candidate<'pat, 'tcx>,
1383 otherwise: &mut Option<BasicBlock>,
1384 pats: &'pat [Box<Pat<'tcx>>],
1386 place: PlaceBuilder<'tcx>,
1387 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1389 debug!("test_or_pattern:\ncandidate={:#?}\npats={:#?}", candidate, pats);
1390 let mut or_candidates: Vec<_> = pats
1392 .map(|pat| Candidate::new(place.clone(), pat, candidate.has_guard))
1394 let mut or_candidate_refs: Vec<_> = or_candidates.iter_mut().collect();
1395 let otherwise = if candidate.otherwise_block.is_some() {
1396 &mut candidate.otherwise_block
1400 self.match_candidates(
1403 candidate.pre_binding_block.unwrap(),
1405 &mut or_candidate_refs,
1408 candidate.subcandidates = or_candidates;
1409 self.merge_trivial_subcandidates(candidate, self.source_info(or_span));
1412 /// Try to merge all of the subcandidates of the given candidate into one.
1413 /// This avoids exponentially large CFGs in cases like `(1 | 2, 3 | 4, ...)`.
1414 fn merge_trivial_subcandidates(
1416 candidate: &mut Candidate<'_, 'tcx>,
1417 source_info: SourceInfo,
1419 if candidate.subcandidates.is_empty() || candidate.has_guard {
1420 // FIXME(or_patterns; matthewjasper) Don't give up if we have a guard.
1424 let mut can_merge = true;
1426 // Not `Iterator::all` because we don't want to short-circuit.
1427 for subcandidate in &mut candidate.subcandidates {
1428 self.merge_trivial_subcandidates(subcandidate, source_info);
1430 // FIXME(or_patterns; matthewjasper) Try to be more aggressive here.
1431 can_merge &= subcandidate.subcandidates.is_empty()
1432 && subcandidate.bindings.is_empty()
1433 && subcandidate.ascriptions.is_empty();
1437 let any_matches = self.cfg.start_new_block();
1438 for subcandidate in mem::take(&mut candidate.subcandidates) {
1439 let or_block = subcandidate.pre_binding_block.unwrap();
1440 self.cfg.goto(or_block, source_info, any_matches);
1442 candidate.pre_binding_block = Some(any_matches);
1446 /// This is the most subtle part of the matching algorithm. At
1447 /// this point, the input candidates have been fully simplified,
1448 /// and so we know that all remaining match-pairs require some
1449 /// sort of test. To decide what test to perform, we take the highest
1450 /// priority candidate (the first one in the list, as of January 2021)
1451 /// and extract the first match-pair from the list. From this we decide
1452 /// what kind of test is needed using [`Builder::test`], defined in the
1453 /// [`test` module](mod@test).
1455 /// *Note:* taking the first match pair is somewhat arbitrary, and
1456 /// we might do better here by choosing more carefully what to
1459 /// For example, consider the following possible match-pairs:
1461 /// 1. `x @ Some(P)` -- we will do a [`Switch`] to decide what variant `x` has
1462 /// 2. `x @ 22` -- we will do a [`SwitchInt`] to decide what value `x` has
1463 /// 3. `x @ 3..5` -- we will do a [`Range`] test to decide what range `x` falls in
1466 /// [`Switch`]: TestKind::Switch
1467 /// [`SwitchInt`]: TestKind::SwitchInt
1468 /// [`Range`]: TestKind::Range
1470 /// Once we know what sort of test we are going to perform, this
1471 /// test may also help us winnow down our candidates. So we walk over
1472 /// the candidates (from high to low priority) and check. This
1473 /// gives us, for each outcome of the test, a transformed list of
1474 /// candidates. For example, if we are testing `x.0`'s variant,
1475 /// and we have a candidate `(x.0 @ Some(v), x.1 @ 22)`,
1476 /// then we would have a resulting candidate of `((x.0 as Some).0 @ v, x.1 @ 22)`.
1477 /// Note that the first match-pair is now simpler (and, in fact, irrefutable).
1479 /// But there may also be candidates that the test just doesn't
1480 /// apply to. The classical example involves wildcards:
1483 /// # let (x, y, z) = (true, true, true);
1484 /// match (x, y, z) {
1485 /// (true , _ , true ) => true, // (0)
1486 /// (_ , true , _ ) => true, // (1)
1487 /// (false, false, _ ) => false, // (2)
1488 /// (true , _ , false) => false, // (3)
1493 /// In that case, after we test on `x`, there are 2 overlapping candidate
1496 /// - If the outcome is that `x` is true, candidates 0, 1, and 3
1497 /// - If the outcome is that `x` is false, candidates 1 and 2
1499 /// Here, the traditional "decision tree" method would generate 2
1500 /// separate code-paths for the 2 separate cases.
1502 /// In some cases, this duplication can create an exponential amount of
1503 /// code. This is most easily seen by noticing that this method terminates
1504 /// with precisely the reachable arms being reachable - but that problem
1505 /// is trivially NP-complete:
1507 /// ```ignore (illustrative)
1508 /// match (var0, var1, var2, var3, ...) {
1509 /// (true , _ , _ , false, true, ...) => false,
1510 /// (_ , true, true , false, _ , ...) => false,
1511 /// (false, _ , false, false, _ , ...) => false,
1517 /// Here the last arm is reachable only if there is an assignment to
1518 /// the variables that does not match any of the literals. Therefore,
1519 /// compilation would take an exponential amount of time in some cases.
1521 /// That kind of exponential worst-case might not occur in practice, but
1522 /// our simplistic treatment of constants and guards would make it occur
1523 /// in very common situations - for example [#29740]:
1525 /// ```ignore (illustrative)
1527 /// "foo" if foo_guard => ...,
1528 /// "bar" if bar_guard => ...,
1529 /// "baz" if baz_guard => ...,
1534 /// [#29740]: https://github.com/rust-lang/rust/issues/29740
1536 /// Here we first test the match-pair `x @ "foo"`, which is an [`Eq` test].
1538 /// [`Eq` test]: TestKind::Eq
1540 /// It might seem that we would end up with 2 disjoint candidate
1541 /// sets, consisting of the first candidate or the other two, but our
1542 /// algorithm doesn't reason about `"foo"` being distinct from the other
1543 /// constants; it considers the latter arms to potentially match after
1544 /// both outcomes, which obviously leads to an exponential number
1547 /// To avoid these kinds of problems, our algorithm tries to ensure
1548 /// the amount of generated tests is linear. When we do a k-way test,
1549 /// we return an additional "unmatched" set alongside the obvious `k`
1550 /// sets. When we encounter a candidate that would be present in more
1551 /// than one of the sets, we put it and all candidates below it into the
1552 /// "unmatched" set. This ensures these `k+1` sets are disjoint.
1554 /// After we perform our test, we branch into the appropriate candidate
1555 /// set and recurse with `match_candidates`. These sub-matches are
1556 /// obviously non-exhaustive - as we discarded our otherwise set - so
1557 /// we set their continuation to do `match_candidates` on the
1558 /// "unmatched" set (which is again non-exhaustive).
1560 /// If you apply this to the above test, you basically wind up
1561 /// with an if-else-if chain, testing each candidate in turn,
1562 /// which is precisely what we want.
1564 /// In addition to avoiding exponential-time blowups, this algorithm
1565 /// also has the nice property that each guard and arm is only generated
1567 fn test_candidates<'pat, 'b, 'c>(
1570 scrutinee_span: Span,
1571 mut candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>],
1573 otherwise_block: &mut Option<BasicBlock>,
1574 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1576 // extract the match-pair from the highest priority candidate
1577 let match_pair = &candidates.first().unwrap().match_pairs[0];
1578 let mut test = self.test(match_pair);
1579 let match_place = match_pair.place.clone();
1581 // most of the time, the test to perform is simply a function
1582 // of the main candidate; but for a test like SwitchInt, we
1583 // may want to add cases based on the candidates that are
1586 TestKind::SwitchInt { switch_ty, ref mut options } => {
1587 for candidate in candidates.iter() {
1588 if !self.add_cases_to_switch(&match_place, candidate, switch_ty, options) {
1593 TestKind::Switch { adt_def: _, ref mut variants } => {
1594 for candidate in candidates.iter() {
1595 if !self.add_variants_to_switch(&match_place, candidate, variants) {
1603 // Insert a Shallow borrow of any places that is switched on.
1604 if let Some(fb) = fake_borrows && let Ok(match_place_resolved) =
1605 match_place.clone().try_upvars_resolved(self.tcx, self.typeck_results)
1607 let resolved_place = match_place_resolved.into_place(self.tcx, self.typeck_results);
1608 fb.insert(resolved_place);
1611 // perform the test, branching to one of N blocks. For each of
1612 // those N possible outcomes, create a (initially empty)
1613 // vector of candidates. Those are the candidates that still
1614 // apply if the test has that particular outcome.
1615 debug!("test_candidates: test={:?} match_pair={:?}", test, match_pair);
1616 let mut target_candidates: Vec<Vec<&mut Candidate<'pat, 'tcx>>> = vec![];
1617 target_candidates.resize_with(test.targets(), Default::default);
1619 let total_candidate_count = candidates.len();
1621 // Sort the candidates into the appropriate vector in
1622 // `target_candidates`. Note that at some point we may
1623 // encounter a candidate where the test is not relevant; at
1624 // that point, we stop sorting.
1625 while let Some(candidate) = candidates.first_mut() {
1626 let Some(idx) = self.sort_candidate(&match_place.clone(), &test, candidate) else {
1629 let (candidate, rest) = candidates.split_first_mut().unwrap();
1630 target_candidates[idx].push(candidate);
1633 // at least the first candidate ought to be tested
1634 assert!(total_candidate_count > candidates.len());
1635 debug!("test_candidates: tested_candidates: {}", total_candidate_count - candidates.len());
1636 debug!("test_candidates: untested_candidates: {}", candidates.len());
1638 // HACK(matthewjasper) This is a closure so that we can let the test
1639 // create its blocks before the rest of the match. This currently
1640 // improves the speed of llvm when optimizing long string literal
1642 let make_target_blocks = move |this: &mut Self| -> Vec<BasicBlock> {
1643 // The block that we should branch to if none of the
1644 // `target_candidates` match. This is either the block where we
1645 // start matching the untested candidates if there are any,
1646 // otherwise it's the `otherwise_block`.
1647 let remainder_start = &mut None;
1648 let remainder_start =
1649 if candidates.is_empty() { &mut *otherwise_block } else { remainder_start };
1651 // For each outcome of test, process the candidates that still
1652 // apply. Collect a list of blocks where control flow will
1653 // branch if one of the `target_candidate` sets is not
1655 let target_blocks: Vec<_> = target_candidates
1657 .map(|mut candidates| {
1658 if !candidates.is_empty() {
1659 let candidate_start = this.cfg.start_new_block();
1660 this.match_candidates(
1670 *remainder_start.get_or_insert_with(|| this.cfg.start_new_block())
1675 if !candidates.is_empty() {
1676 let remainder_start = remainder_start.unwrap_or_else(|| this.cfg.start_new_block());
1677 this.match_candidates(
1690 self.perform_test(span, scrutinee_span, block, match_place, &test, make_target_blocks);
1693 /// Determine the fake borrows that are needed from a set of places that
1694 /// have to be stable across match guards.
1696 /// Returns a list of places that need a fake borrow and the temporary
1697 /// that's used to store the fake borrow.
1699 /// Match exhaustiveness checking is not able to handle the case where the
1700 /// place being matched on is mutated in the guards. We add "fake borrows"
1701 /// to the guards that prevent any mutation of the place being matched.
1702 /// There are a some subtleties:
1704 /// 1. Borrowing `*x` doesn't prevent assigning to `x`. If `x` is a shared
1705 /// reference, the borrow isn't even tracked. As such we have to add fake
1706 /// borrows of any prefixes of a place
1707 /// 2. We don't want `match x { _ => (), }` to conflict with mutable
1708 /// borrows of `x`, so we only add fake borrows for places which are
1709 /// bound or tested by the match.
1710 /// 3. We don't want the fake borrows to conflict with `ref mut` bindings,
1711 /// so we use a special BorrowKind for them.
1712 /// 4. The fake borrows may be of places in inactive variants, so it would
1713 /// be UB to generate code for them. They therefore have to be removed
1714 /// by a MIR pass run after borrow checking.
1715 fn calculate_fake_borrows<'b>(
1717 fake_borrows: &'b FxIndexSet<Place<'tcx>>,
1719 ) -> Vec<(Place<'tcx>, Local)> {
1722 debug!("add_fake_borrows fake_borrows = {:?}", fake_borrows);
1724 let mut all_fake_borrows = Vec::with_capacity(fake_borrows.len());
1726 // Insert a Shallow borrow of the prefixes of any fake borrows.
1727 for place in fake_borrows {
1728 let mut cursor = place.projection.as_ref();
1729 while let [proj_base @ .., elem] = cursor {
1732 if let ProjectionElem::Deref = elem {
1733 // Insert a shallow borrow after a deref. For other
1734 // projections the borrow of prefix_cursor will
1735 // conflict with any mutation of base.
1736 all_fake_borrows.push(PlaceRef { local: place.local, projection: proj_base });
1740 all_fake_borrows.push(place.as_ref());
1744 let mut dedup = FxHashSet::default();
1745 all_fake_borrows.retain(|b| dedup.insert(*b));
1747 debug!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows);
1751 .map(|matched_place_ref| {
1752 let matched_place = Place {
1753 local: matched_place_ref.local,
1754 projection: tcx.intern_place_elems(matched_place_ref.projection),
1756 let fake_borrow_deref_ty = matched_place.ty(&self.local_decls, tcx).ty;
1757 let fake_borrow_ty = tcx.mk_imm_ref(tcx.lifetimes.re_erased, fake_borrow_deref_ty);
1758 let fake_borrow_temp =
1759 self.local_decls.push(LocalDecl::new(fake_borrow_ty, temp_span));
1761 (matched_place, fake_borrow_temp)
1767 ///////////////////////////////////////////////////////////////////////////
1768 // Pat binding - used for `let` and function parameters as well.
1770 impl<'a, 'tcx> Builder<'a, 'tcx> {
1771 pub(crate) fn lower_let_expr(
1773 mut block: BasicBlock,
1776 else_target: region::Scope,
1777 source_scope: Option<SourceScope>,
1780 let expr_span = expr.span;
1781 let expr_place_builder = unpack!(block = self.lower_scrutinee(block, expr, expr_span));
1782 let wildcard = Pat::wildcard_from_ty(pat.ty);
1783 let mut guard_candidate = Candidate::new(expr_place_builder.clone(), &pat, false);
1784 let mut otherwise_candidate = Candidate::new(expr_place_builder.clone(), &wildcard, false);
1785 let fake_borrow_temps = self.lower_match_tree(
1790 &mut [&mut guard_candidate, &mut otherwise_candidate],
1792 let mut opt_expr_place: Option<(Option<&Place<'tcx>>, Span)> = None;
1793 let expr_place: Place<'tcx>;
1794 if let Ok(expr_builder) =
1795 expr_place_builder.try_upvars_resolved(self.tcx, self.typeck_results)
1797 expr_place = expr_builder.into_place(self.tcx, self.typeck_results);
1798 opt_expr_place = Some((Some(&expr_place), expr_span));
1800 let otherwise_post_guard_block = otherwise_candidate.pre_binding_block.unwrap();
1801 self.break_for_else(otherwise_post_guard_block, else_target, self.source_info(expr_span));
1803 self.declare_bindings(
1811 let post_guard_block = self.bind_pattern(
1812 self.source_info(pat.span),
1822 post_guard_block.unit()
1825 /// Initializes each of the bindings from the candidate by
1826 /// moving/copying/ref'ing the source as appropriate. Tests the guard, if
1827 /// any, and then branches to the arm. Returns the block for the case where
1828 /// the guard succeeds.
1830 /// Note: we do not check earlier that if there is a guard,
1831 /// there cannot be move bindings. We avoid a use-after-move by only
1832 /// moving the binding once the guard has evaluated to true (see below).
1833 fn bind_and_guard_matched_candidate<'pat>(
1835 candidate: Candidate<'pat, 'tcx>,
1836 parent_bindings: &[(Vec<Binding<'tcx>>, Vec<Ascription<'tcx>>)],
1837 guard: Option<&Guard<'tcx>>,
1838 fake_borrows: &[(Place<'tcx>, Local)],
1839 scrutinee_span: Span,
1840 arm_span: Option<Span>,
1841 match_scope: Option<region::Scope>,
1842 schedule_drops: bool,
1844 debug!("bind_and_guard_matched_candidate(candidate={:?})", candidate);
1846 debug_assert!(candidate.match_pairs.is_empty());
1848 let candidate_source_info = self.source_info(candidate.span);
1850 let mut block = candidate.pre_binding_block.unwrap();
1852 if candidate.next_candidate_pre_binding_block.is_some() {
1853 let fresh_block = self.cfg.start_new_block();
1857 candidate.next_candidate_pre_binding_block,
1858 candidate_source_info,
1860 block = fresh_block;
1867 .flat_map(|(_, ascriptions)| ascriptions)
1869 .chain(candidate.ascriptions),
1872 // rust-lang/rust#27282: The `autoref` business deserves some
1873 // explanation here.
1875 // The intent of the `autoref` flag is that when it is true,
1876 // then any pattern bindings of type T will map to a `&T`
1877 // within the context of the guard expression, but will
1878 // continue to map to a `T` in the context of the arm body. To
1879 // avoid surfacing this distinction in the user source code
1880 // (which would be a severe change to the language and require
1881 // far more revision to the compiler), when `autoref` is true,
1882 // then any occurrence of the identifier in the guard
1883 // expression will automatically get a deref op applied to it.
1885 // So an input like:
1888 // let place = Foo::new();
1889 // match place { foo if inspect(foo)
1890 // => feed(foo), ... }
1893 // will be treated as if it were really something like:
1896 // let place = Foo::new();
1897 // match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) }
1898 // => { let tmp2 = place; feed(tmp2) }, ... }
1900 // And an input like:
1903 // let place = Foo::new();
1904 // match place { ref mut foo if inspect(foo)
1905 // => feed(foo), ... }
1908 // will be treated as if it were really something like:
1911 // let place = Foo::new();
1912 // match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) }
1913 // => { let tmp2 = &mut place; feed(tmp2) }, ... }
1916 // In short, any pattern binding will always look like *some*
1917 // kind of `&T` within the guard at least in terms of how the
1918 // MIR-borrowck views it, and this will ensure that guard
1919 // expressions cannot mutate their the match inputs via such
1920 // bindings. (It also ensures that guard expressions can at
1921 // most *copy* values from such bindings; non-Copy things
1922 // cannot be moved via pattern bindings in guard expressions.)
1926 // Implementation notes (under assumption `autoref` is true).
1928 // To encode the distinction above, we must inject the
1929 // temporaries `tmp1` and `tmp2`.
1931 // There are two cases of interest: binding by-value, and binding by-ref.
1933 // 1. Binding by-value: Things are simple.
1935 // * Establishing `tmp1` creates a reference into the
1936 // matched place. This code is emitted by
1937 // bind_matched_candidate_for_guard.
1939 // * `tmp2` is only initialized "lazily", after we have
1940 // checked the guard. Thus, the code that can trigger
1941 // moves out of the candidate can only fire after the
1942 // guard evaluated to true. This initialization code is
1943 // emitted by bind_matched_candidate_for_arm.
1945 // 2. Binding by-reference: Things are tricky.
1947 // * Here, the guard expression wants a `&&` or `&&mut`
1948 // into the original input. This means we need to borrow
1949 // the reference that we create for the arm.
1950 // * So we eagerly create the reference for the arm and then take a
1951 // reference to that.
1952 if let Some(guard) = guard {
1954 let bindings = parent_bindings
1956 .flat_map(|(bindings, _)| bindings)
1957 .chain(&candidate.bindings);
1959 self.bind_matched_candidate_for_guard(block, schedule_drops, bindings.clone());
1960 let guard_frame = GuardFrame {
1961 locals: bindings.map(|b| GuardFrameLocal::new(b.var_id, b.binding_mode)).collect(),
1963 debug!("entering guard building context: {:?}", guard_frame);
1964 self.guard_context.push(guard_frame);
1966 let re_erased = tcx.lifetimes.re_erased;
1967 let scrutinee_source_info = self.source_info(scrutinee_span);
1968 for &(place, temp) in fake_borrows {
1969 let borrow = Rvalue::Ref(re_erased, BorrowKind::Shallow, place);
1970 self.cfg.push_assign(block, scrutinee_source_info, Place::from(temp), borrow);
1973 let arm_span = arm_span.unwrap();
1974 let match_scope = match_scope.unwrap();
1975 let mut guard_span = rustc_span::DUMMY_SP;
1977 let (post_guard_block, otherwise_post_guard_block) =
1978 self.in_if_then_scope(match_scope, |this| match *guard {
1980 let e = &this.thir[e];
1981 guard_span = e.span;
1982 this.then_else_break(
1987 this.source_info(arm_span),
1990 Guard::IfLet(ref pat, scrutinee) => {
1991 let s = &this.thir[scrutinee];
1992 guard_span = s.span;
1993 this.lower_let_expr(block, s, pat, match_scope, None, arm_span)
1997 let source_info = self.source_info(guard_span);
1998 let guard_end = self.source_info(tcx.sess.source_map().end_point(guard_span));
1999 let guard_frame = self.guard_context.pop().unwrap();
2000 debug!("Exiting guard building context with locals: {:?}", guard_frame);
2002 for &(_, temp) in fake_borrows {
2003 let cause = FakeReadCause::ForMatchGuard;
2004 self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(temp));
2007 let otherwise_block = candidate.otherwise_block.unwrap_or_else(|| {
2008 let unreachable = self.cfg.start_new_block();
2009 self.cfg.terminate(unreachable, source_info, TerminatorKind::Unreachable);
2013 otherwise_post_guard_block,
2015 candidate.next_candidate_pre_binding_block,
2019 // We want to ensure that the matched candidates are bound
2020 // after we have confirmed this candidate *and* any
2021 // associated guard; Binding them on `block` is too soon,
2022 // because that would be before we've checked the result
2025 // But binding them on the arm is *too late*, because
2026 // then all of the candidates for a single arm would be
2027 // bound in the same place, that would cause a case like:
2031 // (mut x, 1) | (2, mut x) if { true } => { ... }
2032 // ... // ^^^^^^^ (this is `arm_block`)
2036 // would yield an `arm_block` something like:
2039 // StorageLive(_4); // _4 is `x`
2040 // _4 = &mut (_1.0: i32); // this is handling `(mut x, 1)` case
2041 // _4 = &mut (_1.1: i32); // this is handling `(2, mut x)` case
2044 // and that is clearly not correct.
2045 let by_value_bindings = parent_bindings
2047 .flat_map(|(bindings, _)| bindings)
2048 .chain(&candidate.bindings)
2049 .filter(|binding| matches!(binding.binding_mode, BindingMode::ByValue));
2050 // Read all of the by reference bindings to ensure that the
2051 // place they refer to can't be modified by the guard.
2052 for binding in by_value_bindings.clone() {
2053 let local_id = self.var_local_id(binding.var_id, RefWithinGuard);
2054 let cause = FakeReadCause::ForGuardBinding;
2055 self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(local_id));
2057 assert!(schedule_drops, "patterns with guards must schedule drops");
2058 self.bind_matched_candidate_for_arm_body(post_guard_block, true, by_value_bindings);
2062 // (Here, it is not too early to bind the matched
2063 // candidate on `block`, because there is no guard result
2064 // that we have to inspect before we bind them.)
2065 self.bind_matched_candidate_for_arm_body(
2070 .flat_map(|(bindings, _)| bindings)
2071 .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>>,
2164 debug!("bind_matched_candidate_for_arm_body(block={:?})", block);
2166 let re_erased = self.tcx.lifetimes.re_erased;
2167 // Assign each of the bindings. This may trigger moves out of the candidate.
2168 for binding in bindings {
2169 let source_info = self.source_info(binding.span);
2170 let local = self.storage_live_binding(
2178 self.schedule_drop_for_binding(binding.var_id, binding.span, OutsideGuard);
2180 let rvalue = match binding.binding_mode {
2181 BindingMode::ByValue => Rvalue::Use(self.consume_by_copy_or_move(binding.source)),
2182 BindingMode::ByRef(borrow_kind) => {
2183 Rvalue::Ref(re_erased, borrow_kind, binding.source)
2186 self.cfg.push_assign(block, source_info, local, rvalue);
2190 /// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where the bound
2191 /// `var` has type `T` in the arm body) in a pattern maps to 2 locals. The
2192 /// first local is a binding for occurrences of `var` in the guard, which
2193 /// will have type `&T`. The second local is a binding for occurrences of
2194 /// `var` in the arm body, which will have type `T`.
2197 source_info: SourceInfo,
2198 visibility_scope: SourceScope,
2199 mutability: Mutability,
2204 user_ty: UserTypeProjections,
2205 has_guard: ArmHasGuard,
2206 opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
2210 "declare_binding(var_id={:?}, name={:?}, mode={:?}, var_ty={:?}, \
2211 visibility_scope={:?}, source_info={:?})",
2212 var_id, name, mode, var_ty, visibility_scope, source_info
2216 let debug_source_info = SourceInfo { span: source_info.span, scope: visibility_scope };
2217 let binding_mode = match mode {
2218 BindingMode::ByValue => ty::BindingMode::BindByValue(mutability),
2219 BindingMode::ByRef(_) => ty::BindingMode::BindByReference(mutability),
2221 debug!("declare_binding: user_ty={:?}", user_ty);
2222 let local = LocalDecl::<'tcx> {
2225 user_ty: if user_ty.is_empty() { None } else { Some(Box::new(user_ty)) },
2228 is_block_tail: None,
2229 local_info: Some(Box::new(LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
2232 // hypothetically, `visit_primary_bindings` could try to unzip
2233 // an outermost hir::Ty as we descend, matching up
2234 // idents in pat; but complex w/ unclear UI payoff.
2235 // Instead, just abandon providing diagnostic info.
2242 let for_arm_body = self.local_decls.push(local);
2243 self.var_debug_info.push(VarDebugInfo {
2245 source_info: debug_source_info,
2246 value: VarDebugInfoContents::Place(for_arm_body.into()),
2248 let locals = if has_guard.0 {
2249 let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
2250 // This variable isn't mutated but has a name, so has to be
2251 // immutable to avoid the unused mut lint.
2252 mutability: Mutability::Not,
2253 ty: tcx.mk_imm_ref(tcx.lifetimes.re_erased, var_ty),
2257 is_block_tail: None,
2258 local_info: Some(Box::new(LocalInfo::User(ClearCrossCrate::Set(
2259 BindingForm::RefForGuard,
2262 self.var_debug_info.push(VarDebugInfo {
2264 source_info: debug_source_info,
2265 value: VarDebugInfoContents::Place(ref_for_guard.into()),
2267 LocalsForNode::ForGuard { ref_for_guard, for_arm_body }
2269 LocalsForNode::One(for_arm_body)
2271 debug!("declare_binding: vars={:?}", locals);
2272 self.var_indices.insert(var_id, locals);
2275 pub(crate) fn ast_let_else(
2277 mut block: BasicBlock,
2279 initializer_span: Span,
2280 else_block: BlockId,
2281 visibility_scope: Option<SourceScope>,
2282 remainder_scope: region::Scope,
2283 remainder_span: Span,
2284 pattern: &Pat<'tcx>,
2286 let else_block_span = self.thir[else_block].span;
2287 let (matching, failure) = self.in_if_then_scope(remainder_scope, |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);
2291 this.declare_bindings(
2296 Some((None, initializer_span)),
2298 let mut candidate = Candidate::new(scrutinee.clone(), pattern, false);
2299 let fake_borrow_temps = this.lower_match_tree(
2304 &mut [&mut candidate, &mut wildcard],
2306 // This block is for the matching case
2307 let matching = this.bind_pattern(
2308 this.source_info(pattern.span),
2317 // This block is for the failure case
2318 let failure = this.bind_pattern(
2319 this.source_info(else_block_span),
2328 this.break_for_else(failure, remainder_scope, this.source_info(initializer_span));
2332 // This place is not really used because this destination place
2333 // should never be used to take values at the end of the failure
2335 let dummy_place = self.temp(self.tcx.types.never, else_block_span);
2338 failure_block = self.ast_block(
2342 self.source_info(else_block_span),
2347 self.source_info(else_block_span),
2348 TerminatorKind::Unreachable,