1 //! Code related to match expressions. These are sufficiently complex
2 //! to warrant their own module and submodules. :) This main module
3 //! includes the high-level algorithm, the submodules contain the
6 use crate::build::scope::{CachedBlock, DropKind};
7 use crate::build::ForGuard::{self, OutsideGuard, RefWithinGuard, ValWithinGuard};
8 use crate::build::{BlockAnd, BlockAndExtension, Builder};
9 use crate::build::{GuardFrame, GuardFrameLocal, LocalsForNode};
12 use rustc::ty::{self, CanonicalUserTypeAnnotation, Ty};
13 use rustc::ty::layout::VariantIdx;
14 use rustc_data_structures::bit_set::BitSet;
15 use rustc_data_structures::fx::FxHashMap;
16 use syntax::ast::{Name, NodeId};
19 // helper functions, broken out by category:
24 use std::convert::TryFrom;
26 /// ArmHasGuard is isomorphic to a boolean flag. It indicates whether
27 /// a match arm has a guard expression attached to it.
28 #[derive(Copy, Clone, Debug)]
29 pub(crate) struct ArmHasGuard(pub bool);
31 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
34 destination: &Place<'tcx>,
36 mut block: BasicBlock,
37 discriminant: ExprRef<'tcx>,
40 let tcx = self.hir.tcx();
41 let discriminant_span = discriminant.span();
42 let discriminant_place = unpack!(block = self.as_place(block, discriminant));
44 // Matching on a `discriminant_place` with an uninhabited type doesn't
45 // generate any memory reads by itself, and so if the place "expression"
46 // contains unsafe operations like raw pointer dereferences or union
47 // field projections, we wouldn't know to require an `unsafe` block
48 // around a `match` equivalent to `std::intrinsics::unreachable()`.
49 // See issue #47412 for this hole being discovered in the wild.
51 // HACK(eddyb) Work around the above issue by adding a dummy inspection
52 // of `discriminant_place`, specifically by applying `ReadForMatch`.
54 // NOTE: ReadForMatch also checks that the discriminant is initialized.
55 // This is currently needed to not allow matching on an uninitialized,
56 // uninhabited value. If we get never patterns, those will check that
57 // the place is initialized, and so this read would only be used to
60 let source_info = self.source_info(discriminant_span);
61 self.cfg.push(block, Statement {
63 kind: StatementKind::FakeRead(
64 FakeReadCause::ForMatchedPlace,
65 discriminant_place.clone(),
69 let mut arm_blocks = ArmBlocks {
70 blocks: arms.iter().map(|_| self.cfg.start_new_block()).collect(),
73 // Get the arm bodies and their scopes, while declaring bindings.
74 let arm_bodies: Vec<_> = arms.iter()
76 // BUG: use arm lint level
77 let body = self.hir.mirror(arm.body.clone());
78 let scope = self.declare_bindings(
83 ArmHasGuard(arm.guard.is_some()),
84 Some((Some(&discriminant_place), discriminant_span)),
86 (body, scope.unwrap_or(self.source_scope))
90 // create binding start block for link them by false edges
91 let candidate_count = arms.iter().map(|c| c.patterns.len()).sum::<usize>();
92 let pre_binding_blocks: Vec<_> = (0..=candidate_count)
93 .map(|_| self.cfg.start_new_block())
96 let mut has_guard = false;
98 // assemble a list of candidates: there is one candidate per
99 // pattern, which means there may be more than one candidate
100 // *per arm*. These candidates are kept sorted such that the
101 // highest priority candidate comes first in the list.
102 // (i.e., same order as in source)
104 let candidates: Vec<_> = arms.iter()
106 .flat_map(|(arm_index, arm)| {
110 .map(move |(pat_index, pat)| (arm_index, pat_index, pat, arm.guard.clone()))
115 .zip(pre_binding_blocks.iter().skip(1)),
119 (arm_index, pat_index, pattern, guard),
120 (pre_binding_block, next_candidate_pre_binding_block)
122 has_guard |= guard.is_some();
124 // One might ask: why not build up the match pair such that it
125 // matches via `borrowed_input_temp.deref()` instead of
126 // using the `discriminant_place` directly, as it is doing here?
128 // The basic answer is that if you do that, then you end up with
129 // accceses to a shared borrow of the input and that conflicts with
130 // any arms that look like e.g.
134 // ... /* mutate `foo` in arm body */ ...
138 // (Perhaps we could further revise the MIR
139 // construction here so that it only does a
140 // shared borrow at the outset and delays doing
141 // the mutable borrow until after the pattern is
142 // matched *and* the guard (if any) for the arm
147 match_pairs: vec![MatchPair::new(discriminant_place.clone(), pattern)],
153 pre_binding_block: *pre_binding_block,
154 next_candidate_pre_binding_block: *next_candidate_pre_binding_block,
160 let outer_source_info = self.source_info(span);
162 *pre_binding_blocks.last().unwrap(),
164 TerminatorKind::Unreachable,
167 // Maps a place to the kind of Fake borrow that we want to perform on
168 // it: either Shallow or Shared, depending on whether the place is
169 // bound in the match, or just switched on.
170 // If there are no match guards then we don't need any fake borrows,
171 // so don't track them.
172 let mut fake_borrows = if has_guard && tcx.generate_borrow_of_any_match_input() {
173 Some(FxHashMap::default())
178 let pre_binding_blocks: Vec<_> = candidates
180 .map(|cand| (cand.pre_binding_block, cand.span))
183 // this will generate code to test discriminant_place and
184 // branch to the appropriate arm block
185 let otherwise = self.match_candidates(
193 if !otherwise.is_empty() {
194 // All matches are exhaustive. However, because some matches
195 // only have exponentially-large exhaustive decision trees, we
196 // sometimes generate an inexhaustive decision tree.
198 // In that case, the inexhaustive tips of the decision tree
199 // can't be reached - terminate them with an `unreachable`.
200 let source_info = self.source_info(span);
202 let mut otherwise = otherwise;
204 otherwise.dedup(); // variant switches can introduce duplicate target blocks
205 for block in otherwise {
207 .terminate(block, source_info, TerminatorKind::Unreachable);
211 if let Some(fake_borrows) = fake_borrows {
212 self.add_fake_borrows(&pre_binding_blocks, fake_borrows, source_info, block);
215 // all the arm blocks will rejoin here
216 let end_block = self.cfg.start_new_block();
218 let outer_source_info = self.source_info(span);
219 for (arm_index, (body, source_scope)) in arm_bodies.into_iter().enumerate() {
220 let mut arm_block = arm_blocks.blocks[arm_index];
221 // Re-enter the source scope we created the bindings in.
222 self.source_scope = source_scope;
223 unpack!(arm_block = self.into(destination, arm_block, body));
227 TerminatorKind::Goto { target: end_block },
230 self.source_scope = outer_source_info.scope;
235 pub(super) fn expr_into_pattern(
237 mut block: BasicBlock,
238 irrefutable_pat: Pattern<'tcx>,
239 initializer: ExprRef<'tcx>,
241 match *irrefutable_pat.kind {
242 // Optimize the case of `let x = ...` to write directly into `x`
243 PatternKind::Binding {
244 mode: BindingMode::ByValue,
250 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard);
251 unpack!(block = self.into(&place, block, initializer));
254 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
255 let source_info = self.source_info(irrefutable_pat.span);
260 kind: StatementKind::FakeRead(FakeReadCause::ForLet, place),
264 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
268 // Optimize the case of `let x: T = ...` to write directly
269 // into `x` and then require that `T == typeof(x)`.
271 // Weirdly, this is needed to prevent the
272 // `intrinsic-move-val.rs` test case from crashing. That
273 // test works with uninitialized values in a rather
274 // dubious way, so it may be that the test is kind of
276 PatternKind::AscribeUserType {
277 subpattern: Pattern {
278 kind: box PatternKind::Binding {
279 mode: BindingMode::ByValue,
286 user_ty: pat_ascription_ty,
291 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard);
292 unpack!(block = self.into(&place, block, initializer));
294 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
295 let pattern_source_info = self.source_info(irrefutable_pat.span);
299 source_info: pattern_source_info,
300 kind: StatementKind::FakeRead(FakeReadCause::ForLet, place.clone()),
304 let ty_source_info = self.source_info(user_ty_span);
305 let user_ty = box pat_ascription_ty.user_ty(
306 &mut self.canonical_user_type_annotations,
307 place.ty(&self.local_decls, self.hir.tcx()).to_ty(self.hir.tcx()),
313 source_info: ty_source_info,
314 kind: StatementKind::AscribeUserType(
316 // We always use invariant as the variance here. This is because the
317 // variance field from the ascription refers to the variance to use
318 // when applying the type to the value being matched, but this
319 // ascription applies rather to the type of the binding. e.g., in this
326 // We are creating an ascription that defines the type of `x` to be
327 // exactly `T` (i.e., with invariance). The variance field, in
328 // contrast, is intended to be used to relate `T` to the type of
330 ty::Variance::Invariant,
336 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
340 let place = unpack!(block = self.as_place(block, initializer));
341 self.place_into_pattern(block, irrefutable_pat, &place, true)
346 pub fn place_into_pattern(
349 irrefutable_pat: Pattern<'tcx>,
350 initializer: &Place<'tcx>,
351 set_match_place: bool,
353 // create a dummy candidate
354 let mut candidate = Candidate {
355 span: irrefutable_pat.span,
356 match_pairs: vec![MatchPair::new(initializer.clone(), &irrefutable_pat)],
361 // since we don't call `match_candidates`, next fields is unused
364 pre_binding_block: block,
365 next_candidate_pre_binding_block: block,
368 // Simplify the candidate. Since the pattern is irrefutable, this should
369 // always convert all match-pairs into bindings.
370 self.simplify_candidate(&mut candidate);
372 if !candidate.match_pairs.is_empty() {
374 candidate.match_pairs[0].pattern.span,
375 "match pairs {:?} remaining after simplifying \
376 irrefutable pattern",
377 candidate.match_pairs
381 // for matches and function arguments, the place that is being matched
382 // can be set when creating the variables. But the place for
383 // let PATTERN = ... might not even exist until we do the assignment.
384 // so we set it here instead
386 for binding in &candidate.bindings {
387 let local = self.var_local_id(binding.var_id, OutsideGuard);
389 if let Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
390 opt_match_place: Some((ref mut match_place, _)),
392 }))) = self.local_decls[local].is_user_variable
394 *match_place = Some(initializer.clone());
396 bug!("Let binding to non-user variable.")
401 self.ascribe_types(block, &candidate.ascriptions);
403 // now apply the bindings, which will also declare the variables
404 self.bind_matched_candidate_for_arm_body(block, &candidate.bindings);
409 /// Declares the bindings of the given patterns and returns the visibility
410 /// scope for the bindings in these patterns, if such a scope had to be
411 /// created. NOTE: Declaring the bindings should always be done in their
413 pub fn declare_bindings(
415 mut visibility_scope: Option<SourceScope>,
417 lint_level: LintLevel,
418 patterns: &[Pattern<'tcx>],
419 has_guard: ArmHasGuard,
420 opt_match_place: Option<(Option<&Place<'tcx>>, Span)>,
421 ) -> Option<SourceScope> {
423 !(visibility_scope.is_some() && lint_level.is_explicit()),
424 "can't have both a visibility and a lint scope at the same time"
426 let mut scope = self.source_scope;
427 let num_patterns = patterns.len();
428 debug!("declare_bindings: patterns={:?}", patterns);
431 UserTypeProjections::none(),
432 &mut |this, mutability, name, mode, var, span, ty, user_ty| {
433 if visibility_scope.is_none() {
435 Some(this.new_source_scope(scope_span, LintLevel::Inherited, None));
436 // If we have lints, create a new source scope
437 // that marks the lints for the locals. See the comment
438 // on the `source_info` field for why this is needed.
439 if lint_level.is_explicit() {
440 scope = this.new_source_scope(scope_span, lint_level, None);
443 let source_info = SourceInfo { span, scope };
444 let visibility_scope = visibility_scope.unwrap();
445 this.declare_binding(
456 opt_match_place.map(|(x, y)| (x.cloned(), y)),
464 pub fn storage_live_binding(
471 let local_id = self.var_local_id(var, for_guard);
472 let source_info = self.source_info(span);
477 kind: StatementKind::StorageLive(local_id),
480 let place = Place::Local(local_id);
481 let var_ty = self.local_decls[local_id].ty;
482 let hir_id = self.hir.tcx().hir().node_to_hir_id(var);
483 let region_scope = self.hir.region_scope_tree.var_scope(hir_id.local_id);
484 self.schedule_drop(span, region_scope, &place, var_ty, DropKind::Storage);
488 pub fn schedule_drop_for_binding(&mut self, var: NodeId, span: Span, for_guard: ForGuard) {
489 let local_id = self.var_local_id(var, for_guard);
490 let var_ty = self.local_decls[local_id].ty;
491 let hir_id = self.hir.tcx().hir().node_to_hir_id(var);
492 let region_scope = self.hir.region_scope_tree.var_scope(hir_id.local_id);
496 &Place::Local(local_id),
499 cached_block: CachedBlock::default(),
504 pub(super) fn visit_bindings(
506 pattern: &Pattern<'tcx>,
507 pattern_user_ty: UserTypeProjections<'tcx>,
516 UserTypeProjections<'tcx>,
519 debug!("visit_bindings: pattern={:?} pattern_user_ty={:?}", pattern, pattern_user_ty);
520 match *pattern.kind {
521 PatternKind::Binding {
530 f(self, mutability, name, mode, var, pattern.span, ty, pattern_user_ty.clone());
531 if let Some(subpattern) = subpattern.as_ref() {
532 self.visit_bindings(subpattern, pattern_user_ty, f);
540 | PatternKind::Slice {
545 let from = u32::try_from(prefix.len()).unwrap();
546 let to = u32::try_from(suffix.len()).unwrap();
547 for subpattern in prefix {
548 self.visit_bindings(subpattern, pattern_user_ty.clone().index(), f);
550 for subpattern in slice {
551 self.visit_bindings(subpattern, pattern_user_ty.clone().subslice(from, to), f);
553 for subpattern in suffix {
554 self.visit_bindings(subpattern, pattern_user_ty.clone().index(), f);
557 PatternKind::Constant { .. } | PatternKind::Range { .. } | PatternKind::Wild => {}
558 PatternKind::Deref { ref subpattern } => {
559 self.visit_bindings(subpattern, pattern_user_ty.deref(), f);
561 PatternKind::AscribeUserType {
567 // This corresponds to something like
570 // let A::<'a>(_): A<'static> = ...;
573 // Note that the variance doesn't apply here, as we are tracking the effect
574 // of `user_ty` on any bindings contained with subpattern.
575 let annotation = CanonicalUserTypeAnnotation {
577 user_ty: user_ty.user_ty,
578 inferred_ty: subpattern.ty,
580 let projection = UserTypeProjection {
581 base: self.canonical_user_type_annotations.push(annotation),
584 let subpattern_user_ty = pattern_user_ty.push_projection(&projection, user_ty_span);
585 self.visit_bindings(subpattern, subpattern_user_ty, f)
588 PatternKind::Leaf { ref subpatterns } => {
589 for subpattern in subpatterns {
590 let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field);
591 debug!("visit_bindings: subpattern_user_ty={:?}", subpattern_user_ty);
592 self.visit_bindings(&subpattern.pattern, subpattern_user_ty, f);
596 PatternKind::Variant { adt_def, substs: _, variant_index, ref subpatterns } => {
597 for subpattern in subpatterns {
598 let subpattern_user_ty = pattern_user_ty.clone().variant(
599 adt_def, variant_index, subpattern.field);
600 self.visit_bindings(&subpattern.pattern, subpattern_user_ty, f);
607 /// List of blocks for each arm (and potentially other metadata in the
610 blocks: Vec<BasicBlock>,
613 #[derive(Clone, Debug)]
614 pub struct Candidate<'pat, 'tcx: 'pat> {
615 // span of the original pattern that gave rise to this candidate
618 // all of these must be satisfied...
619 match_pairs: Vec<MatchPair<'pat, 'tcx>>,
621 // ...these bindings established...
622 bindings: Vec<Binding<'tcx>>,
624 // ...these types asserted...
625 ascriptions: Vec<Ascription<'tcx>>,
627 // ...and the guard must be evaluated...
628 guard: Option<Guard<'tcx>>,
630 // ...and then we branch to arm with this index.
633 // ...and the blocks for add false edges between candidates
634 pre_binding_block: BasicBlock,
635 next_candidate_pre_binding_block: BasicBlock,
637 // This uniquely identifies this candidate *within* the arm.
641 #[derive(Clone, Debug)]
642 struct Binding<'tcx> {
648 mutability: Mutability,
649 binding_mode: BindingMode,
652 /// Indicates that the type of `source` must be a subtype of the
653 /// user-given type `user_ty`; this is basically a no-op but can
654 /// influence region inference.
655 #[derive(Clone, Debug)]
656 struct Ascription<'tcx> {
659 user_ty: PatternTypeProjection<'tcx>,
660 variance: ty::Variance,
663 #[derive(Clone, Debug)]
664 pub struct MatchPair<'pat, 'tcx: 'pat> {
668 // ... must match this pattern.
669 pattern: &'pat Pattern<'tcx>,
671 // HACK(eddyb) This is used to toggle whether a Slice pattern
672 // has had its length checked. This is only necessary because
673 // the "rest" part of the pattern right now has type &[T] and
674 // as such, it requires an Rvalue::Slice to be generated.
675 // See RFC 495 / issue #23121 for the eventual (proper) solution.
676 slice_len_checked: bool,
679 #[derive(Clone, Debug, PartialEq)]
680 enum TestKind<'tcx> {
681 // test the branches of enum
683 adt_def: &'tcx ty::AdtDef,
684 variants: BitSet<VariantIdx>,
687 // test the branches of enum
691 indices: FxHashMap<ty::Const<'tcx>, usize>,
696 value: ty::Const<'tcx>,
700 // test whether the value falls within an inclusive or exclusive range
701 Range(PatternRange<'tcx>),
703 // test length of the slice is equal to len
711 pub struct Test<'tcx> {
713 kind: TestKind<'tcx>,
716 ///////////////////////////////////////////////////////////////////////////
717 // Main matching algorithm
719 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
720 /// The main match algorithm. It begins with a set of candidates
721 /// `candidates` and has the job of generating code to determine
722 /// which of these candidates, if any, is the correct one. The
723 /// candidates are sorted such that the first item in the list
724 /// has the highest priority. When a candidate is found to match
725 /// the value, we will generate a branch to the appropriate
726 /// block found in `arm_blocks`.
728 /// The return value is a list of "otherwise" blocks. These are
729 /// points in execution where we found that *NONE* of the
730 /// candidates apply. In principle, this means that the input
731 /// list was not exhaustive, though at present we sometimes are
732 /// not smart enough to recognize all exhaustive inputs.
734 /// It might be surprising that the input can be inexhaustive.
735 /// Indeed, initially, it is not, because all matches are
736 /// exhaustive in Rust. But during processing we sometimes divide
737 /// up the list of candidates and recurse with a non-exhaustive
738 /// list. This is important to keep the size of the generated code
739 /// under control. See `test_candidates` for more details.
741 /// If `add_fake_borrows` is true, then places which need fake borrows
742 /// will be added to it.
743 fn match_candidates<'pat>(
746 arm_blocks: &mut ArmBlocks,
747 mut candidates: Vec<Candidate<'pat, 'tcx>>,
748 mut block: BasicBlock,
749 fake_borrows: &mut Option<FxHashMap<Place<'tcx>, BorrowKind>>,
750 ) -> Vec<BasicBlock> {
752 "matched_candidate(span={:?}, block={:?}, candidates={:?})",
753 span, block, candidates
756 // Start by simplifying candidates. Once this process is
757 // complete, all the match pairs which remain require some
758 // form of test, whether it be a switch or pattern comparison.
759 for candidate in &mut candidates {
760 self.simplify_candidate(candidate);
763 // The candidates are sorted by priority. Check to see
764 // whether the higher priority candidates (and hence at
765 // the front of the vec) have satisfied all their match
767 let fully_matched = candidates
769 .take_while(|c| c.match_pairs.is_empty())
772 "match_candidates: {:?} candidates fully matched",
775 let mut unmatched_candidates = candidates.split_off(fully_matched);
777 // Insert a *Shared* borrow of any places that are bound.
778 if let Some(fake_borrows) = fake_borrows {
779 for Binding { source, .. }
780 in candidates.iter().flat_map(|candidate| &candidate.bindings)
782 fake_borrows.insert(source.clone(), BorrowKind::Shared);
786 let fully_matched_with_guard = candidates.iter().take_while(|c| c.guard.is_some()).count();
788 let unreachable_candidates = if fully_matched_with_guard + 1 < candidates.len() {
789 candidates.split_off(fully_matched_with_guard + 1)
794 for candidate in candidates {
795 // If so, apply any bindings, test the guard (if any), and
796 // branch to the arm.
797 if let Some(b) = self.bind_and_guard_matched_candidate(block, arm_blocks, candidate) {
800 // if None is returned, then any remaining candidates
801 // are unreachable (at least not through this path).
802 // Link them with false edges.
804 "match_candidates: add false edges for unreachable {:?} and unmatched {:?}",
805 unreachable_candidates, unmatched_candidates
807 for candidate in unreachable_candidates {
808 let source_info = self.source_info(candidate.span);
809 let target = self.cfg.start_new_block();
810 if let Some(otherwise) =
811 self.bind_and_guard_matched_candidate(target, arm_blocks, candidate)
814 .terminate(otherwise, source_info, TerminatorKind::Unreachable);
818 if unmatched_candidates.is_empty() {
821 let target = self.cfg.start_new_block();
822 return self.match_candidates(
825 unmatched_candidates,
833 // If there are no candidates that still need testing, we're done.
834 // Since all matches are exhaustive, execution should never reach this point.
835 if unmatched_candidates.is_empty() {
839 // Test candidates where possible.
840 let (otherwise, tested_candidates) =
841 self.test_candidates(span, arm_blocks, &unmatched_candidates, block, fake_borrows);
843 // If the target candidates were exhaustive, then we are done.
844 // But for borrowck continue build decision tree.
846 // If all candidates were sorted into `target_candidates` somewhere, then
847 // the initial set was inexhaustive.
848 let untested_candidates = unmatched_candidates.split_off(tested_candidates);
849 if untested_candidates.len() == 0 {
853 // Otherwise, let's process those remaining candidates.
854 let join_block = self.join_otherwise_blocks(span, otherwise);
855 self.match_candidates(span, arm_blocks, untested_candidates, join_block, &mut None)
858 fn join_otherwise_blocks(&mut self, span: Span, mut otherwise: Vec<BasicBlock>) -> BasicBlock {
859 let source_info = self.source_info(span);
861 otherwise.dedup(); // variant switches can introduce duplicate target blocks
862 if otherwise.len() == 1 {
865 let join_block = self.cfg.start_new_block();
866 for block in otherwise {
870 TerminatorKind::Goto { target: join_block },
877 /// This is the most subtle part of the matching algorithm. At
878 /// this point, the input candidates have been fully simplified,
879 /// and so we know that all remaining match-pairs require some
880 /// sort of test. To decide what test to do, we take the highest
881 /// priority candidate (last one in the list) and extract the
882 /// first match-pair from the list. From this we decide what kind
883 /// of test is needed using `test`, defined in the `test` module.
885 /// *Note:* taking the first match pair is somewhat arbitrary, and
886 /// we might do better here by choosing more carefully what to
889 /// For example, consider the following possible match-pairs:
891 /// 1. `x @ Some(P)` -- we will do a `Switch` to decide what variant `x` has
892 /// 2. `x @ 22` -- we will do a `SwitchInt`
893 /// 3. `x @ 3..5` -- we will do a range test
896 /// Once we know what sort of test we are going to perform, this
897 /// Tests may also help us with other candidates. So we walk over
898 /// the candidates (from high to low priority) and check. This
899 /// gives us, for each outcome of the test, a transformed list of
900 /// candidates. For example, if we are testing the current
901 /// variant of `x.0`, and we have a candidate `{x.0 @ Some(v), x.1
902 /// @ 22}`, then we would have a resulting candidate of `{(x.0 as
903 /// Some).0 @ v, x.1 @ 22}`. Note that the first match-pair is now
904 /// simpler (and, in fact, irrefutable).
906 /// But there may also be candidates that the test just doesn't
907 /// apply to. The classical example involves wildcards:
910 /// # let (x, y, z) = (true, true, true);
911 /// match (x, y, z) {
912 /// (true, _, true) => true, // (0)
913 /// (_, true, _) => true, // (1)
914 /// (false, false, _) => false, // (2)
915 /// (true, _, false) => false, // (3)
919 /// In that case, after we test on `x`, there are 2 overlapping candidate
922 /// - If the outcome is that `x` is true, candidates 0, 1, and 3
923 /// - If the outcome is that `x` is false, candidates 1 and 2
925 /// Here, the traditional "decision tree" method would generate 2
926 /// separate code-paths for the 2 separate cases.
928 /// In some cases, this duplication can create an exponential amount of
929 /// code. This is most easily seen by noticing that this method terminates
930 /// with precisely the reachable arms being reachable - but that problem
931 /// is trivially NP-complete:
934 /// match (var0, var1, var2, var3, ..) {
935 /// (true, _, _, false, true, ...) => false,
936 /// (_, true, true, false, _, ...) => false,
937 /// (false, _, false, false, _, ...) => false,
943 /// Here the last arm is reachable only if there is an assignment to
944 /// the variables that does not match any of the literals. Therefore,
945 /// compilation would take an exponential amount of time in some cases.
947 /// That kind of exponential worst-case might not occur in practice, but
948 /// our simplistic treatment of constants and guards would make it occur
949 /// in very common situations - for example #29740:
953 /// "foo" if foo_guard => ...,
954 /// "bar" if bar_guard => ...,
955 /// "baz" if baz_guard => ...,
960 /// Here we first test the match-pair `x @ "foo"`, which is an `Eq` test.
962 /// It might seem that we would end up with 2 disjoint candidate
963 /// sets, consisting of the first candidate or the other 3, but our
964 /// algorithm doesn't reason about "foo" being distinct from the other
965 /// constants; it considers the latter arms to potentially match after
966 /// both outcomes, which obviously leads to an exponential amount
969 /// To avoid these kinds of problems, our algorithm tries to ensure
970 /// the amount of generated tests is linear. When we do a k-way test,
971 /// we return an additional "unmatched" set alongside the obvious `k`
972 /// sets. When we encounter a candidate that would be present in more
973 /// than one of the sets, we put it and all candidates below it into the
974 /// "unmatched" set. This ensures these `k+1` sets are disjoint.
976 /// After we perform our test, we branch into the appropriate candidate
977 /// set and recurse with `match_candidates`. These sub-matches are
978 /// obviously inexhaustive - as we discarded our otherwise set - so
979 /// we set their continuation to do `match_candidates` on the
980 /// "unmatched" set (which is again inexhaustive).
982 /// If you apply this to the above test, you basically wind up
983 /// with an if-else-if chain, testing each candidate in turn,
984 /// which is precisely what we want.
986 /// In addition to avoiding exponential-time blowups, this algorithm
987 /// also has nice property that each guard and arm is only generated
989 fn test_candidates<'pat>(
992 arm_blocks: &mut ArmBlocks,
993 candidates: &[Candidate<'pat, 'tcx>],
995 fake_borrows: &mut Option<FxHashMap<Place<'tcx>, BorrowKind>>,
996 ) -> (Vec<BasicBlock>, usize) {
997 // extract the match-pair from the highest priority candidate
998 let match_pair = &candidates.first().unwrap().match_pairs[0];
999 let mut test = self.test(match_pair);
1001 // most of the time, the test to perform is simply a function
1002 // of the main candidate; but for a test like SwitchInt, we
1003 // may want to add cases based on the candidates that are
1006 TestKind::SwitchInt {
1011 for candidate in candidates.iter() {
1012 if !self.add_cases_to_switch(
1027 for candidate in candidates.iter() {
1028 if !self.add_variants_to_switch(&match_pair.place, candidate, variants) {
1036 // Insert a Shallow borrow of any places that is switched on.
1037 fake_borrows.as_mut().map(|fb| {
1038 fb.entry(match_pair.place.clone()).or_insert(BorrowKind::Shallow)
1041 // perform the test, branching to one of N blocks. For each of
1042 // those N possible outcomes, create a (initially empty)
1043 // vector of candidates. Those are the candidates that still
1044 // apply if the test has that particular outcome.
1046 "match_candidates: test={:?} match_pair={:?}",
1049 let target_blocks = self.perform_test(block, &match_pair.place, &test);
1050 let mut target_candidates = vec![vec![]; target_blocks.len()];
1052 // Sort the candidates into the appropriate vector in
1053 // `target_candidates`. Note that at some point we may
1054 // encounter a candidate where the test is not relevant; at
1055 // that point, we stop sorting.
1056 let tested_candidates = candidates
1059 self.sort_candidate(&match_pair.place, &test, c, &mut target_candidates)
1062 assert!(tested_candidates > 0); // at least the last candidate ought to be tested
1063 debug!("tested_candidates: {}", tested_candidates);
1065 "untested_candidates: {}",
1066 candidates.len() - tested_candidates
1069 // For each outcome of test, process the candidates that still
1070 // apply. Collect a list of blocks where control flow will
1071 // branch if one of the `target_candidate` sets is not
1073 let otherwise: Vec<_> = target_blocks
1075 .zip(target_candidates)
1076 .flat_map(|(target_block, target_candidates)| {
1077 self.match_candidates(
1087 (otherwise, tested_candidates)
1090 /// Initializes each of the bindings from the candidate by
1091 /// moving/copying/ref'ing the source as appropriate. Tests the
1092 /// guard, if any, and then branches to the arm. Returns the block
1093 /// for the case where the guard fails.
1095 /// Note: we check earlier that if there is a guard, there cannot
1096 /// be move bindings. This isn't really important for the
1097 /// self-consistency of this fn, but the reason for it should be
1098 /// clear: after we've done the assignments, if there were move
1099 /// bindings, further tests would be a use-after-move (which would
1100 /// in turn be detected by the borrowck code that runs on the
1102 fn bind_and_guard_matched_candidate<'pat>(
1104 mut block: BasicBlock,
1105 arm_blocks: &mut ArmBlocks,
1106 candidate: Candidate<'pat, 'tcx>,
1107 ) -> Option<BasicBlock> {
1109 "bind_and_guard_matched_candidate(block={:?}, candidate={:?})",
1113 debug_assert!(candidate.match_pairs.is_empty());
1115 self.ascribe_types(block, &candidate.ascriptions);
1117 let arm_block = arm_blocks.blocks[candidate.arm_index];
1118 let candidate_source_info = self.source_info(candidate.span);
1122 candidate_source_info,
1123 TerminatorKind::Goto {
1124 target: candidate.pre_binding_block,
1128 block = self.cfg.start_new_block();
1130 candidate.pre_binding_block,
1131 candidate_source_info,
1132 TerminatorKind::FalseEdges {
1134 imaginary_targets: vec![candidate.next_candidate_pre_binding_block],
1138 // rust-lang/rust#27282: The `autoref` business deserves some
1139 // explanation here.
1141 // The intent of the `autoref` flag is that when it is true,
1142 // then any pattern bindings of type T will map to a `&T`
1143 // within the context of the guard expression, but will
1144 // continue to map to a `T` in the context of the arm body. To
1145 // avoid surfacing this distinction in the user source code
1146 // (which would be a severe change to the language and require
1147 // far more revision to the compiler), when `autoref` is true,
1148 // then any occurrence of the identifier in the guard
1149 // expression will automatically get a deref op applied to it.
1151 // So an input like:
1154 // let place = Foo::new();
1155 // match place { foo if inspect(foo)
1156 // => feed(foo), ... }
1159 // will be treated as if it were really something like:
1162 // let place = Foo::new();
1163 // match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) }
1164 // => { let tmp2 = place; feed(tmp2) }, ... }
1166 // And an input like:
1169 // let place = Foo::new();
1170 // match place { ref mut foo if inspect(foo)
1171 // => feed(foo), ... }
1174 // will be treated as if it were really something like:
1177 // let place = Foo::new();
1178 // match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) }
1179 // => { let tmp2 = &mut place; feed(tmp2) }, ... }
1182 // In short, any pattern binding will always look like *some*
1183 // kind of `&T` within the guard at least in terms of how the
1184 // MIR-borrowck views it, and this will ensure that guard
1185 // expressions cannot mutate their the match inputs via such
1186 // bindings. (It also ensures that guard expressions can at
1187 // most *copy* values from such bindings; non-Copy things
1188 // cannot be moved via pattern bindings in guard expressions.)
1192 // Implementation notes (under assumption `autoref` is true).
1194 // To encode the distinction above, we must inject the
1195 // temporaries `tmp1` and `tmp2`.
1197 // There are two cases of interest: binding by-value, and binding by-ref.
1199 // 1. Binding by-value: Things are simple.
1201 // * Establishing `tmp1` creates a reference into the
1202 // matched place. This code is emitted by
1203 // bind_matched_candidate_for_guard.
1205 // * `tmp2` is only initialized "lazily", after we have
1206 // checked the guard. Thus, the code that can trigger
1207 // moves out of the candidate can only fire after the
1208 // guard evaluated to true. This initialization code is
1209 // emitted by bind_matched_candidate_for_arm.
1211 // 2. Binding by-reference: Things are tricky.
1213 // * Here, the guard expression wants a `&&` or `&&mut`
1214 // into the original input. This means we need to borrow
1215 // a reference that we do not immediately have at hand
1216 // (because all we have is the places associated with the
1217 // match input itself; it is up to us to create a place
1218 // holding a `&` or `&mut` that we can then borrow).
1220 let autoref = self.hir
1222 .all_pat_vars_are_implicit_refs_within_guards();
1223 if let Some(guard) = candidate.guard {
1225 self.bind_matched_candidate_for_guard(
1227 candidate.pat_index,
1228 &candidate.bindings,
1230 let guard_frame = GuardFrame {
1234 .map(|b| GuardFrameLocal::new(b.var_id, b.binding_mode))
1237 debug!("Entering guard building context: {:?}", guard_frame);
1238 self.guard_context.push(guard_frame);
1240 self.bind_matched_candidate_for_arm_body(block, &candidate.bindings);
1243 // the block to branch to if the guard fails; if there is no
1244 // guard, this block is simply unreachable
1245 let guard = match guard {
1246 Guard::If(e) => self.hir.mirror(e),
1248 let source_info = self.source_info(guard.span);
1249 let cond = unpack!(block = self.as_local_operand(block, guard));
1251 let guard_frame = self.guard_context.pop().unwrap();
1253 "Exiting guard building context with locals: {:?}",
1258 let false_edge_block = self.cfg.start_new_block();
1260 // We want to ensure that the matched candidates are bound
1261 // after we have confirmed this candidate *and* any
1262 // associated guard; Binding them on `block` is too soon,
1263 // because that would be before we've checked the result
1266 // But binding them on `arm_block` is *too late*, because
1267 // then all of the candidates for a single arm would be
1268 // bound in the same place, that would cause a case like:
1272 // (mut x, 1) | (2, mut x) if { true } => { ... }
1273 // ... // ^^^^^^^ (this is `arm_block`)
1277 // would yield a `arm_block` something like:
1280 // StorageLive(_4); // _4 is `x`
1281 // _4 = &mut (_1.0: i32); // this is handling `(mut x, 1)` case
1282 // _4 = &mut (_1.1: i32); // this is handling `(2, mut x)` case
1285 // and that is clearly not correct.
1286 let post_guard_block = self.cfg.start_new_block();
1290 TerminatorKind::if_(self.hir.tcx(), cond, post_guard_block, false_edge_block),
1294 self.bind_matched_candidate_for_arm_body(post_guard_block, &candidate.bindings);
1300 TerminatorKind::Goto { target: arm_block },
1303 let otherwise = self.cfg.start_new_block();
1308 TerminatorKind::FalseEdges {
1309 real_target: otherwise,
1310 imaginary_targets: vec![candidate.next_candidate_pre_binding_block],
1315 // (Here, it is not too early to bind the matched
1316 // candidate on `block`, because there is no guard result
1317 // that we have to inspect before we bind them.)
1318 self.bind_matched_candidate_for_arm_body(block, &candidate.bindings);
1321 candidate_source_info,
1322 TerminatorKind::Goto { target: arm_block },
1328 /// Append `AscribeUserType` statements onto the end of `block`
1329 /// for each ascription
1330 fn ascribe_types<'pat>(
1333 ascriptions: &[Ascription<'tcx>],
1335 for ascription in ascriptions {
1336 let source_info = self.source_info(ascription.span);
1339 "adding user ascription at span {:?} of place {:?} and {:?}",
1345 let user_ty = box ascription.user_ty.clone().user_ty(
1346 &mut self.canonical_user_type_annotations,
1347 ascription.source.ty(&self.local_decls, self.hir.tcx()).to_ty(self.hir.tcx()),
1354 kind: StatementKind::AscribeUserType(
1355 ascription.source.clone(),
1356 ascription.variance,
1364 // Only called when all_pat_vars_are_implicit_refs_within_guards,
1365 // and thus all code/comments assume we are in that context.
1366 fn bind_matched_candidate_for_guard(
1370 bindings: &[Binding<'tcx>],
1373 "bind_matched_candidate_for_guard(block={:?}, pat_index={:?}, bindings={:?})",
1374 block, pat_index, bindings
1377 // Assign each of the bindings. Since we are binding for a
1378 // guard expression, this will never trigger moves out of the
1380 let re_erased = self.hir.tcx().types.re_erased;
1381 for binding in bindings {
1382 let source_info = self.source_info(binding.span);
1384 // For each pattern ident P of type T, `ref_for_guard` is
1385 // a reference R: &T pointing to the location matched by
1386 // the pattern, and every occurrence of P within a guard
1389 self.storage_live_binding(block, binding.var_id, binding.span, RefWithinGuard);
1390 // Question: Why schedule drops if bindings are all
1391 // shared-&'s? Answer: Because schedule_drop_for_binding
1392 // also emits StorageDead's for those locals.
1393 self.schedule_drop_for_binding(binding.var_id, binding.span, RefWithinGuard);
1394 match binding.binding_mode {
1395 BindingMode::ByValue => {
1396 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, binding.source.clone());
1398 .push_assign(block, source_info, &ref_for_guard, rvalue);
1400 BindingMode::ByRef(borrow_kind) => {
1401 // Tricky business: For `ref id` and `ref mut id`
1402 // patterns, we want `id` within the guard to
1403 // correspond to a temp of type `& &T` or `& &mut
1404 // T` (i.e., a "borrow of a borrow") that is
1405 // implicitly dereferenced.
1407 // To borrow a borrow, we need that inner borrow
1408 // to point to. So, create a temp for the inner
1409 // borrow, and then take a reference to it.
1411 // Note: the temp created here is *not* the one
1412 // used by the arm body itself. This eases
1413 // observing two-phase borrow restrictions.
1414 let val_for_guard = self.storage_live_binding(
1418 ValWithinGuard(pat_index),
1420 self.schedule_drop_for_binding(
1423 ValWithinGuard(pat_index),
1426 // rust-lang/rust#27282: We reuse the two-phase
1427 // borrow infrastructure so that the mutable
1428 // borrow (whose mutabilty is *unusable* within
1429 // the guard) does not conflict with the implicit
1430 // borrow of the whole match input. See additional
1431 // discussion on rust-lang/rust#49870.
1432 let borrow_kind = match borrow_kind {
1434 | BorrowKind::Shallow
1435 | BorrowKind::Unique => borrow_kind,
1436 BorrowKind::Mut { .. } => BorrowKind::Mut {
1437 allow_two_phase_borrow: true,
1440 let rvalue = Rvalue::Ref(re_erased, borrow_kind, binding.source.clone());
1442 .push_assign(block, source_info, &val_for_guard, rvalue);
1443 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, val_for_guard);
1445 .push_assign(block, source_info, &ref_for_guard, rvalue);
1451 fn bind_matched_candidate_for_arm_body(
1454 bindings: &[Binding<'tcx>],
1457 "bind_matched_candidate_for_arm_body(block={:?}, bindings={:?}",
1462 let re_erased = self.hir.tcx().types.re_erased;
1463 // Assign each of the bindings. This may trigger moves out of the candidate.
1464 for binding in bindings {
1465 let source_info = self.source_info(binding.span);
1467 self.storage_live_binding(block, binding.var_id, binding.span, OutsideGuard);
1468 self.schedule_drop_for_binding(binding.var_id, binding.span, OutsideGuard);
1469 let rvalue = match binding.binding_mode {
1470 BindingMode::ByValue => {
1471 Rvalue::Use(self.consume_by_copy_or_move(binding.source.clone()))
1473 BindingMode::ByRef(borrow_kind) => {
1474 Rvalue::Ref(re_erased, borrow_kind, binding.source.clone())
1477 self.cfg.push_assign(block, source_info, &local, rvalue);
1481 /// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where
1482 /// the bound `var` has type `T` in the arm body) in a pattern
1483 /// maps to `2+N` locals. The first local is a binding for
1484 /// occurrences of `var` in the guard, which will all have type
1485 /// `&T`. The N locals are bindings for the `T` that is referenced
1486 /// by the first local; they are not used outside of the
1487 /// guard. The last local is a binding for occurrences of `var` in
1488 /// the arm body, which will have type `T`.
1490 /// The reason we have N locals rather than just 1 is to
1491 /// accommodate rust-lang/rust#51348: If the arm has N candidate
1492 /// patterns, then in general they can correspond to distinct
1493 /// parts of the matched data, and we want them to be distinct
1494 /// temps in order to simplify checks performed by our internal
1495 /// leveraging of two-phase borrows).
1498 source_info: SourceInfo,
1499 visibility_scope: SourceScope,
1500 mutability: Mutability,
1503 num_patterns: usize,
1506 user_ty: UserTypeProjections<'tcx>,
1507 has_guard: ArmHasGuard,
1508 opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
1512 "declare_binding(var_id={:?}, name={:?}, mode={:?}, var_ty={:?}, \
1513 visibility_scope={:?}, source_info={:?})",
1514 var_id, name, mode, var_ty, visibility_scope, source_info
1517 let tcx = self.hir.tcx();
1518 let binding_mode = match mode {
1519 BindingMode::ByValue => ty::BindingMode::BindByValue(mutability.into()),
1520 BindingMode::ByRef(_) => ty::BindingMode::BindByReference(mutability.into()),
1522 debug!("declare_binding: user_ty={:?}", user_ty);
1523 let local = LocalDecl::<'tcx> {
1531 is_block_tail: None,
1532 is_user_variable: Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
1534 // hypothetically, `visit_bindings` could try to unzip
1535 // an outermost hir::Ty as we descend, matching up
1536 // idents in pat; but complex w/ unclear UI payoff.
1537 // Instead, just abandon providing diagnostic info.
1543 let for_arm_body = self.local_decls.push(local.clone());
1544 let locals = if has_guard.0 && tcx.all_pat_vars_are_implicit_refs_within_guards() {
1545 let mut vals_for_guard = Vec::with_capacity(num_patterns);
1546 for _ in 0..num_patterns {
1547 let val_for_guard_idx = self.local_decls.push(LocalDecl {
1548 // This variable isn't mutated but has a name, so has to be
1549 // immutable to avoid the unused mut lint.
1550 mutability: Mutability::Not,
1553 vals_for_guard.push(val_for_guard_idx);
1555 let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
1556 // See previous comment.
1557 mutability: Mutability::Not,
1558 ty: tcx.mk_imm_ref(tcx.types.re_erased, var_ty),
1559 user_ty: UserTypeProjections::none(),
1563 // FIXME: should these secretly injected ref_for_guard's be marked as `internal`?
1565 is_block_tail: None,
1566 is_user_variable: Some(ClearCrossCrate::Set(BindingForm::RefForGuard)),
1568 LocalsForNode::ForGuard {
1574 LocalsForNode::One(for_arm_body)
1576 debug!("declare_binding: vars={:?}", locals);
1577 self.var_indices.insert(var_id, locals);
1580 // Determine the fake borrows that are needed to ensure that the place
1581 // will evaluate to the same thing until an arm has been chosen.
1582 fn add_fake_borrows<'pat>(
1584 pre_binding_blocks: &[(BasicBlock, Span)],
1585 fake_borrows: FxHashMap<Place<'tcx>, BorrowKind>,
1586 source_info: SourceInfo,
1587 start_block: BasicBlock,
1589 let tcx = self.hir.tcx();
1591 debug!("add_fake_borrows pre_binding_blocks = {:?}, fake_borrows = {:?}",
1592 pre_binding_blocks, fake_borrows);
1594 let mut all_fake_borrows = Vec::with_capacity(fake_borrows.len());
1596 // Insert a Shallow borrow of the prefixes of any fake borrows.
1597 for (place, borrow_kind) in fake_borrows
1600 let mut prefix_cursor = &place;
1601 while let Place::Projection(box Projection { base, elem }) = prefix_cursor {
1602 if let ProjectionElem::Deref = elem {
1603 // Insert a shallow borrow after a deref. For other
1604 // projections the borrow of prefix_cursor will
1605 // conflict with any mutation of base.
1606 all_fake_borrows.push((base.clone(), BorrowKind::Shallow));
1608 prefix_cursor = base;
1612 all_fake_borrows.push((place, borrow_kind));
1615 // Deduplicate and ensure a deterministic order.
1616 all_fake_borrows.sort();
1617 all_fake_borrows.dedup();
1619 debug!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows);
1621 // Add fake borrows to the start of the match and reads of them before
1622 // the start of each arm.
1623 let mut borrowed_input_temps = Vec::with_capacity(all_fake_borrows.len());
1625 for (matched_place, borrow_kind) in all_fake_borrows {
1626 let borrowed_input =
1627 Rvalue::Ref(tcx.types.re_erased, borrow_kind, matched_place.clone());
1628 let borrowed_input_ty = borrowed_input.ty(&self.local_decls, tcx);
1629 let borrowed_input_temp = self.temp(borrowed_input_ty, source_info.span);
1630 self.cfg.push_assign(
1633 &borrowed_input_temp,
1636 borrowed_input_temps.push(borrowed_input_temp);
1639 // FIXME: This could be a lot of reads (#fake borrows * #patterns).
1640 // The false edges that we currently generate would allow us to only do
1641 // this on the last Candidate, but it's possible that there might not be
1642 // so many false edges in the future, so we read for all Candidates for
1644 // Another option would be to make our own block and add our own false
1646 if tcx.emit_read_for_match() {
1647 for &(pre_binding_block, span) in pre_binding_blocks {
1648 let pattern_source_info = self.source_info(span);
1649 for temp in &borrowed_input_temps {
1650 self.cfg.push(pre_binding_block, Statement {
1651 source_info: pattern_source_info,
1652 kind: StatementKind::FakeRead(
1653 FakeReadCause::ForMatchGuard,