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 build::scope::{CachedBlock, DropKind};
7 use build::ForGuard::{self, OutsideGuard, RefWithinGuard, ValWithinGuard};
8 use build::{BlockAnd, BlockAndExtension, Builder};
9 use build::{GuardFrame, GuardFrameLocal, LocalsForNode};
12 use rustc::ty::{self, 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, ty_source_info.span
311 source_info: ty_source_info,
312 kind: StatementKind::AscribeUserType(
314 // We always use invariant as the variance here. This is because the
315 // variance field from the ascription refers to the variance to use
316 // when applying the type to the value being matched, but this
317 // ascription applies rather to the type of the binding. e.g., in this
324 // We are creating an ascription that defines the type of `x` to be
325 // exactly `T` (i.e., with invariance). The variance field, in
326 // contrast, is intended to be used to relate `T` to the type of
328 ty::Variance::Invariant,
334 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
338 let place = unpack!(block = self.as_place(block, initializer));
339 self.place_into_pattern(block, irrefutable_pat, &place, true)
344 pub fn place_into_pattern(
347 irrefutable_pat: Pattern<'tcx>,
348 initializer: &Place<'tcx>,
349 set_match_place: bool,
351 // create a dummy candidate
352 let mut candidate = Candidate {
353 span: irrefutable_pat.span,
354 match_pairs: vec![MatchPair::new(initializer.clone(), &irrefutable_pat)],
359 // since we don't call `match_candidates`, next fields is unused
362 pre_binding_block: block,
363 next_candidate_pre_binding_block: block,
366 // Simplify the candidate. Since the pattern is irrefutable, this should
367 // always convert all match-pairs into bindings.
368 self.simplify_candidate(&mut candidate);
370 if !candidate.match_pairs.is_empty() {
372 candidate.match_pairs[0].pattern.span,
373 "match pairs {:?} remaining after simplifying \
374 irrefutable pattern",
375 candidate.match_pairs
379 // for matches and function arguments, the place that is being matched
380 // can be set when creating the variables. But the place for
381 // let PATTERN = ... might not even exist until we do the assignment.
382 // so we set it here instead
384 for binding in &candidate.bindings {
385 let local = self.var_local_id(binding.var_id, OutsideGuard);
387 if let Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
388 opt_match_place: Some((ref mut match_place, _)),
390 }))) = self.local_decls[local].is_user_variable
392 *match_place = Some(initializer.clone());
394 bug!("Let binding to non-user variable.")
399 self.ascribe_types(block, &candidate.ascriptions);
401 // now apply the bindings, which will also declare the variables
402 self.bind_matched_candidate_for_arm_body(block, &candidate.bindings);
407 /// Declares the bindings of the given patterns and returns the visibility
408 /// scope for the bindings in these patterns, if such a scope had to be
409 /// created. NOTE: Declaring the bindings should always be done in their
411 pub fn declare_bindings(
413 mut visibility_scope: Option<SourceScope>,
415 lint_level: LintLevel,
416 patterns: &[Pattern<'tcx>],
417 has_guard: ArmHasGuard,
418 opt_match_place: Option<(Option<&Place<'tcx>>, Span)>,
419 ) -> Option<SourceScope> {
421 !(visibility_scope.is_some() && lint_level.is_explicit()),
422 "can't have both a visibility and a lint scope at the same time"
424 let mut scope = self.source_scope;
425 let num_patterns = patterns.len();
426 debug!("declare_bindings: patterns={:?}", patterns);
429 UserTypeProjections::none(),
430 &mut |this, mutability, name, mode, var, span, ty, user_ty| {
431 if visibility_scope.is_none() {
433 Some(this.new_source_scope(scope_span, LintLevel::Inherited, None));
434 // If we have lints, create a new source scope
435 // that marks the lints for the locals. See the comment
436 // on the `source_info` field for why this is needed.
437 if lint_level.is_explicit() {
438 scope = this.new_source_scope(scope_span, lint_level, None);
441 let source_info = SourceInfo { span, scope };
442 let visibility_scope = visibility_scope.unwrap();
443 this.declare_binding(
454 opt_match_place.map(|(x, y)| (x.cloned(), y)),
462 pub fn storage_live_binding(
469 let local_id = self.var_local_id(var, for_guard);
470 let source_info = self.source_info(span);
475 kind: StatementKind::StorageLive(local_id),
478 let place = Place::Local(local_id);
479 let var_ty = self.local_decls[local_id].ty;
480 let hir_id = self.hir.tcx().hir().node_to_hir_id(var);
481 let region_scope = self.hir.region_scope_tree.var_scope(hir_id.local_id);
482 self.schedule_drop(span, region_scope, &place, var_ty, DropKind::Storage);
486 pub fn schedule_drop_for_binding(&mut self, var: NodeId, span: Span, for_guard: ForGuard) {
487 let local_id = self.var_local_id(var, for_guard);
488 let var_ty = self.local_decls[local_id].ty;
489 let hir_id = self.hir.tcx().hir().node_to_hir_id(var);
490 let region_scope = self.hir.region_scope_tree.var_scope(hir_id.local_id);
494 &Place::Local(local_id),
497 cached_block: CachedBlock::default(),
502 pub(super) fn visit_bindings(
504 pattern: &Pattern<'tcx>,
505 pattern_user_ty: UserTypeProjections<'tcx>,
514 UserTypeProjections<'tcx>,
517 debug!("visit_bindings: pattern={:?} pattern_user_ty={:?}", pattern, pattern_user_ty);
518 match *pattern.kind {
519 PatternKind::Binding {
528 f(self, mutability, name, mode, var, pattern.span, ty, pattern_user_ty.clone());
529 if let Some(subpattern) = subpattern.as_ref() {
530 self.visit_bindings(subpattern, pattern_user_ty, f);
538 | PatternKind::Slice {
543 let from = u32::try_from(prefix.len()).unwrap();
544 let to = u32::try_from(suffix.len()).unwrap();
545 for subpattern in prefix {
546 self.visit_bindings(subpattern, pattern_user_ty.clone().index(), f);
548 for subpattern in slice {
549 self.visit_bindings(subpattern, pattern_user_ty.clone().subslice(from, to), f);
551 for subpattern in suffix {
552 self.visit_bindings(subpattern, pattern_user_ty.clone().index(), f);
555 PatternKind::Constant { .. } | PatternKind::Range { .. } | PatternKind::Wild => {}
556 PatternKind::Deref { ref subpattern } => {
557 self.visit_bindings(subpattern, pattern_user_ty.deref(), f);
559 PatternKind::AscribeUserType {
565 // This corresponds to something like
568 // let A::<'a>(_): A<'static> = ...;
571 // Note that the variance doesn't apply here, as we are tracking the effect
572 // of `user_ty` on any bindings contained with subpattern.
573 let annotation = (user_ty_span, user_ty.base);
574 let projection = UserTypeProjection {
575 base: self.canonical_user_type_annotations.push(annotation),
576 projs: user_ty.projs.clone(),
578 let subpattern_user_ty = pattern_user_ty.push_projection(&projection, user_ty_span);
579 self.visit_bindings(subpattern, subpattern_user_ty, f)
582 PatternKind::Leaf { ref subpatterns } => {
583 for subpattern in subpatterns {
584 let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field);
585 debug!("visit_bindings: subpattern_user_ty={:?}", subpattern_user_ty);
586 self.visit_bindings(&subpattern.pattern, subpattern_user_ty, f);
590 PatternKind::Variant { adt_def, substs: _, variant_index, ref subpatterns } => {
591 for subpattern in subpatterns {
592 let subpattern_user_ty = pattern_user_ty.clone().variant(
593 adt_def, variant_index, subpattern.field);
594 self.visit_bindings(&subpattern.pattern, subpattern_user_ty, f);
601 /// List of blocks for each arm (and potentially other metadata in the
604 blocks: Vec<BasicBlock>,
607 #[derive(Clone, Debug)]
608 pub struct Candidate<'pat, 'tcx: 'pat> {
609 // span of the original pattern that gave rise to this candidate
612 // all of these must be satisfied...
613 match_pairs: Vec<MatchPair<'pat, 'tcx>>,
615 // ...these bindings established...
616 bindings: Vec<Binding<'tcx>>,
618 // ...these types asserted...
619 ascriptions: Vec<Ascription<'tcx>>,
621 // ...and the guard must be evaluated...
622 guard: Option<Guard<'tcx>>,
624 // ...and then we branch to arm with this index.
627 // ...and the blocks for add false edges between candidates
628 pre_binding_block: BasicBlock,
629 next_candidate_pre_binding_block: BasicBlock,
631 // This uniquely identifies this candidate *within* the arm.
635 #[derive(Clone, Debug)]
636 struct Binding<'tcx> {
642 mutability: Mutability,
643 binding_mode: BindingMode,
646 /// Indicates that the type of `source` must be a subtype of the
647 /// user-given type `user_ty`; this is basically a no-op but can
648 /// influence region inference.
649 #[derive(Clone, Debug)]
650 struct Ascription<'tcx> {
653 user_ty: PatternTypeProjection<'tcx>,
654 variance: ty::Variance,
657 #[derive(Clone, Debug)]
658 pub struct MatchPair<'pat, 'tcx: 'pat> {
662 // ... must match this pattern.
663 pattern: &'pat Pattern<'tcx>,
665 // HACK(eddyb) This is used to toggle whether a Slice pattern
666 // has had its length checked. This is only necessary because
667 // the "rest" part of the pattern right now has type &[T] and
668 // as such, it requires an Rvalue::Slice to be generated.
669 // See RFC 495 / issue #23121 for the eventual (proper) solution.
670 slice_len_checked: bool,
673 #[derive(Clone, Debug, PartialEq)]
674 enum TestKind<'tcx> {
675 // test the branches of enum
677 adt_def: &'tcx ty::AdtDef,
678 variants: BitSet<VariantIdx>,
681 // test the branches of enum
685 indices: FxHashMap<ty::Const<'tcx>, usize>,
690 value: ty::Const<'tcx>,
694 // test whether the value falls within an inclusive or exclusive range
695 Range(PatternRange<'tcx>),
697 // test length of the slice is equal to len
705 pub struct Test<'tcx> {
707 kind: TestKind<'tcx>,
710 ///////////////////////////////////////////////////////////////////////////
711 // Main matching algorithm
713 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
714 /// The main match algorithm. It begins with a set of candidates
715 /// `candidates` and has the job of generating code to determine
716 /// which of these candidates, if any, is the correct one. The
717 /// candidates are sorted such that the first item in the list
718 /// has the highest priority. When a candidate is found to match
719 /// the value, we will generate a branch to the appropriate
720 /// block found in `arm_blocks`.
722 /// The return value is a list of "otherwise" blocks. These are
723 /// points in execution where we found that *NONE* of the
724 /// candidates apply. In principle, this means that the input
725 /// list was not exhaustive, though at present we sometimes are
726 /// not smart enough to recognize all exhaustive inputs.
728 /// It might be surprising that the input can be inexhaustive.
729 /// Indeed, initially, it is not, because all matches are
730 /// exhaustive in Rust. But during processing we sometimes divide
731 /// up the list of candidates and recurse with a non-exhaustive
732 /// list. This is important to keep the size of the generated code
733 /// under control. See `test_candidates` for more details.
735 /// If `add_fake_borrows` is true, then places which need fake borrows
736 /// will be added to it.
737 fn match_candidates<'pat>(
740 arm_blocks: &mut ArmBlocks,
741 mut candidates: Vec<Candidate<'pat, 'tcx>>,
742 mut block: BasicBlock,
743 fake_borrows: &mut Option<FxHashMap<Place<'tcx>, BorrowKind>>,
744 ) -> Vec<BasicBlock> {
746 "matched_candidate(span={:?}, block={:?}, candidates={:?})",
747 span, block, candidates
750 // Start by simplifying candidates. Once this process is
751 // complete, all the match pairs which remain require some
752 // form of test, whether it be a switch or pattern comparison.
753 for candidate in &mut candidates {
754 self.simplify_candidate(candidate);
757 // The candidates are sorted by priority. Check to see
758 // whether the higher priority candidates (and hence at
759 // the front of the vec) have satisfied all their match
761 let fully_matched = candidates
763 .take_while(|c| c.match_pairs.is_empty())
766 "match_candidates: {:?} candidates fully matched",
769 let mut unmatched_candidates = candidates.split_off(fully_matched);
771 // Insert a *Shared* borrow of any places that are bound.
772 if let Some(fake_borrows) = fake_borrows {
773 for Binding { source, .. }
774 in candidates.iter().flat_map(|candidate| &candidate.bindings)
776 fake_borrows.insert(source.clone(), BorrowKind::Shared);
780 let fully_matched_with_guard = candidates.iter().take_while(|c| c.guard.is_some()).count();
782 let unreachable_candidates = if fully_matched_with_guard + 1 < candidates.len() {
783 candidates.split_off(fully_matched_with_guard + 1)
788 for candidate in candidates {
789 // If so, apply any bindings, test the guard (if any), and
790 // branch to the arm.
791 if let Some(b) = self.bind_and_guard_matched_candidate(block, arm_blocks, candidate) {
794 // if None is returned, then any remaining candidates
795 // are unreachable (at least not through this path).
796 // Link them with false edges.
798 "match_candidates: add false edges for unreachable {:?} and unmatched {:?}",
799 unreachable_candidates, unmatched_candidates
801 for candidate in unreachable_candidates {
802 let source_info = self.source_info(candidate.span);
803 let target = self.cfg.start_new_block();
804 if let Some(otherwise) =
805 self.bind_and_guard_matched_candidate(target, arm_blocks, candidate)
808 .terminate(otherwise, source_info, TerminatorKind::Unreachable);
812 if unmatched_candidates.is_empty() {
815 let target = self.cfg.start_new_block();
816 return self.match_candidates(
819 unmatched_candidates,
827 // If there are no candidates that still need testing, we're done.
828 // Since all matches are exhaustive, execution should never reach this point.
829 if unmatched_candidates.is_empty() {
833 // Test candidates where possible.
834 let (otherwise, tested_candidates) =
835 self.test_candidates(span, arm_blocks, &unmatched_candidates, block, fake_borrows);
837 // If the target candidates were exhaustive, then we are done.
838 // But for borrowck continue build decision tree.
840 // If all candidates were sorted into `target_candidates` somewhere, then
841 // the initial set was inexhaustive.
842 let untested_candidates = unmatched_candidates.split_off(tested_candidates);
843 if untested_candidates.len() == 0 {
847 // Otherwise, let's process those remaining candidates.
848 let join_block = self.join_otherwise_blocks(span, otherwise);
849 self.match_candidates(span, arm_blocks, untested_candidates, join_block, &mut None)
852 fn join_otherwise_blocks(&mut self, span: Span, mut otherwise: Vec<BasicBlock>) -> BasicBlock {
853 let source_info = self.source_info(span);
855 otherwise.dedup(); // variant switches can introduce duplicate target blocks
856 if otherwise.len() == 1 {
859 let join_block = self.cfg.start_new_block();
860 for block in otherwise {
864 TerminatorKind::Goto { target: join_block },
871 /// This is the most subtle part of the matching algorithm. At
872 /// this point, the input candidates have been fully simplified,
873 /// and so we know that all remaining match-pairs require some
874 /// sort of test. To decide what test to do, we take the highest
875 /// priority candidate (last one in the list) and extract the
876 /// first match-pair from the list. From this we decide what kind
877 /// of test is needed using `test`, defined in the `test` module.
879 /// *Note:* taking the first match pair is somewhat arbitrary, and
880 /// we might do better here by choosing more carefully what to
883 /// For example, consider the following possible match-pairs:
885 /// 1. `x @ Some(P)` -- we will do a `Switch` to decide what variant `x` has
886 /// 2. `x @ 22` -- we will do a `SwitchInt`
887 /// 3. `x @ 3..5` -- we will do a range test
890 /// Once we know what sort of test we are going to perform, this
891 /// test may also help us with other candidates. So we walk over
892 /// the candidates (from high to low priority) and check. This
893 /// gives us, for each outcome of the test, a transformed list of
894 /// candidates. For example, if we are testing the current
895 /// variant of `x.0`, and we have a candidate `{x.0 @ Some(v), x.1
896 /// @ 22}`, then we would have a resulting candidate of `{(x.0 as
897 /// Some).0 @ v, x.1 @ 22}`. Note that the first match-pair is now
898 /// simpler (and, in fact, irrefutable).
900 /// But there may also be candidates that the test just doesn't
901 /// apply to. The classical example involves wildcards:
904 /// # let (x, y, z) = (true, true, true);
905 /// match (x, y, z) {
906 /// (true, _, true) => true, // (0)
907 /// (_, true, _) => true, // (1)
908 /// (false, false, _) => false, // (2)
909 /// (true, _, false) => false, // (3)
913 /// In that case, after we test on `x`, there are 2 overlapping candidate
916 /// - If the outcome is that `x` is true, candidates 0, 1, and 3
917 /// - If the outcome is that `x` is false, candidates 1 and 2
919 /// Here, the traditional "decision tree" method would generate 2
920 /// separate code-paths for the 2 separate cases.
922 /// In some cases, this duplication can create an exponential amount of
923 /// code. This is most easily seen by noticing that this method terminates
924 /// with precisely the reachable arms being reachable - but that problem
925 /// is trivially NP-complete:
928 /// match (var0, var1, var2, var3, ..) {
929 /// (true, _, _, false, true, ...) => false,
930 /// (_, true, true, false, _, ...) => false,
931 /// (false, _, false, false, _, ...) => false,
937 /// Here the last arm is reachable only if there is an assignment to
938 /// the variables that does not match any of the literals. Therefore,
939 /// compilation would take an exponential amount of time in some cases.
941 /// That kind of exponential worst-case might not occur in practice, but
942 /// our simplistic treatment of constants and guards would make it occur
943 /// in very common situations - for example #29740:
947 /// "foo" if foo_guard => ...,
948 /// "bar" if bar_guard => ...,
949 /// "baz" if baz_guard => ...,
954 /// Here we first test the match-pair `x @ "foo"`, which is an `Eq` test.
956 /// It might seem that we would end up with 2 disjoint candidate
957 /// sets, consisting of the first candidate or the other 3, but our
958 /// algorithm doesn't reason about "foo" being distinct from the other
959 /// constants; it considers the latter arms to potentially match after
960 /// both outcomes, which obviously leads to an exponential amount
963 /// To avoid these kinds of problems, our algorithm tries to ensure
964 /// the amount of generated tests is linear. When we do a k-way test,
965 /// we return an additional "unmatched" set alongside the obvious `k`
966 /// sets. When we encounter a candidate that would be present in more
967 /// than one of the sets, we put it and all candidates below it into the
968 /// "unmatched" set. This ensures these `k+1` sets are disjoint.
970 /// After we perform our test, we branch into the appropriate candidate
971 /// set and recurse with `match_candidates`. These sub-matches are
972 /// obviously inexhaustive - as we discarded our otherwise set - so
973 /// we set their continuation to do `match_candidates` on the
974 /// "unmatched" set (which is again inexhaustive).
976 /// If you apply this to the above test, you basically wind up
977 /// with an if-else-if chain, testing each candidate in turn,
978 /// which is precisely what we want.
980 /// In addition to avoiding exponential-time blowups, this algorithm
981 /// also has nice property that each guard and arm is only generated
983 fn test_candidates<'pat>(
986 arm_blocks: &mut ArmBlocks,
987 candidates: &[Candidate<'pat, 'tcx>],
989 fake_borrows: &mut Option<FxHashMap<Place<'tcx>, BorrowKind>>,
990 ) -> (Vec<BasicBlock>, usize) {
991 // extract the match-pair from the highest priority candidate
992 let match_pair = &candidates.first().unwrap().match_pairs[0];
993 let mut test = self.test(match_pair);
995 // most of the time, the test to perform is simply a function
996 // of the main candidate; but for a test like SwitchInt, we
997 // may want to add cases based on the candidates that are
1000 TestKind::SwitchInt {
1005 for candidate in candidates.iter() {
1006 if !self.add_cases_to_switch(
1021 for candidate in candidates.iter() {
1022 if !self.add_variants_to_switch(&match_pair.place, candidate, variants) {
1030 // Insert a Shallow borrow of any places that is switched on.
1031 fake_borrows.as_mut().map(|fb| {
1032 fb.entry(match_pair.place.clone()).or_insert(BorrowKind::Shallow)
1035 // perform the test, branching to one of N blocks. For each of
1036 // those N possible outcomes, create a (initially empty)
1037 // vector of candidates. Those are the candidates that still
1038 // apply if the test has that particular outcome.
1040 "match_candidates: test={:?} match_pair={:?}",
1043 let target_blocks = self.perform_test(block, &match_pair.place, &test);
1044 let mut target_candidates = vec![vec![]; target_blocks.len()];
1046 // Sort the candidates into the appropriate vector in
1047 // `target_candidates`. Note that at some point we may
1048 // encounter a candidate where the test is not relevant; at
1049 // that point, we stop sorting.
1050 let tested_candidates = candidates
1053 self.sort_candidate(&match_pair.place, &test, c, &mut target_candidates)
1056 assert!(tested_candidates > 0); // at least the last candidate ought to be tested
1057 debug!("tested_candidates: {}", tested_candidates);
1059 "untested_candidates: {}",
1060 candidates.len() - tested_candidates
1063 // For each outcome of test, process the candidates that still
1064 // apply. Collect a list of blocks where control flow will
1065 // branch if one of the `target_candidate` sets is not
1067 let otherwise: Vec<_> = target_blocks
1069 .zip(target_candidates)
1070 .flat_map(|(target_block, target_candidates)| {
1071 self.match_candidates(
1081 (otherwise, tested_candidates)
1084 /// Initializes each of the bindings from the candidate by
1085 /// moving/copying/ref'ing the source as appropriate. Tests the
1086 /// guard, if any, and then branches to the arm. Returns the block
1087 /// for the case where the guard fails.
1089 /// Note: we check earlier that if there is a guard, there cannot
1090 /// be move bindings. This isn't really important for the
1091 /// self-consistency of this fn, but the reason for it should be
1092 /// clear: after we've done the assignments, if there were move
1093 /// bindings, further tests would be a use-after-move (which would
1094 /// in turn be detected by the borrowck code that runs on the
1096 fn bind_and_guard_matched_candidate<'pat>(
1098 mut block: BasicBlock,
1099 arm_blocks: &mut ArmBlocks,
1100 candidate: Candidate<'pat, 'tcx>,
1101 ) -> Option<BasicBlock> {
1103 "bind_and_guard_matched_candidate(block={:?}, candidate={:?})",
1107 debug_assert!(candidate.match_pairs.is_empty());
1109 self.ascribe_types(block, &candidate.ascriptions);
1111 let arm_block = arm_blocks.blocks[candidate.arm_index];
1112 let candidate_source_info = self.source_info(candidate.span);
1116 candidate_source_info,
1117 TerminatorKind::Goto {
1118 target: candidate.pre_binding_block,
1122 block = self.cfg.start_new_block();
1124 candidate.pre_binding_block,
1125 candidate_source_info,
1126 TerminatorKind::FalseEdges {
1128 imaginary_targets: vec![candidate.next_candidate_pre_binding_block],
1132 // rust-lang/rust#27282: The `autoref` business deserves some
1133 // explanation here.
1135 // The intent of the `autoref` flag is that when it is true,
1136 // then any pattern bindings of type T will map to a `&T`
1137 // within the context of the guard expression, but will
1138 // continue to map to a `T` in the context of the arm body. To
1139 // avoid surfacing this distinction in the user source code
1140 // (which would be a severe change to the language and require
1141 // far more revision to the compiler), when `autoref` is true,
1142 // then any occurrence of the identifier in the guard
1143 // expression will automatically get a deref op applied to it.
1145 // So an input like:
1148 // let place = Foo::new();
1149 // match place { foo if inspect(foo)
1150 // => feed(foo), ... }
1153 // will be treated as if it were really something like:
1156 // let place = Foo::new();
1157 // match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) }
1158 // => { let tmp2 = place; feed(tmp2) }, ... }
1160 // And an input like:
1163 // let place = Foo::new();
1164 // match place { ref mut foo if inspect(foo)
1165 // => feed(foo), ... }
1168 // will be treated as if it were really something like:
1171 // let place = Foo::new();
1172 // match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) }
1173 // => { let tmp2 = &mut place; feed(tmp2) }, ... }
1176 // In short, any pattern binding will always look like *some*
1177 // kind of `&T` within the guard at least in terms of how the
1178 // MIR-borrowck views it, and this will ensure that guard
1179 // expressions cannot mutate their the match inputs via such
1180 // bindings. (It also ensures that guard expressions can at
1181 // most *copy* values from such bindings; non-Copy things
1182 // cannot be moved via pattern bindings in guard expressions.)
1186 // Implementation notes (under assumption `autoref` is true).
1188 // To encode the distinction above, we must inject the
1189 // temporaries `tmp1` and `tmp2`.
1191 // There are two cases of interest: binding by-value, and binding by-ref.
1193 // 1. Binding by-value: Things are simple.
1195 // * Establishing `tmp1` creates a reference into the
1196 // matched place. This code is emitted by
1197 // bind_matched_candidate_for_guard.
1199 // * `tmp2` is only initialized "lazily", after we have
1200 // checked the guard. Thus, the code that can trigger
1201 // moves out of the candidate can only fire after the
1202 // guard evaluated to true. This initialization code is
1203 // emitted by bind_matched_candidate_for_arm.
1205 // 2. Binding by-reference: Things are tricky.
1207 // * Here, the guard expression wants a `&&` or `&&mut`
1208 // into the original input. This means we need to borrow
1209 // a reference that we do not immediately have at hand
1210 // (because all we have is the places associated with the
1211 // match input itself; it is up to us to create a place
1212 // holding a `&` or `&mut` that we can then borrow).
1214 let autoref = self.hir
1216 .all_pat_vars_are_implicit_refs_within_guards();
1217 if let Some(guard) = candidate.guard {
1219 self.bind_matched_candidate_for_guard(
1221 candidate.pat_index,
1222 &candidate.bindings,
1224 let guard_frame = GuardFrame {
1228 .map(|b| GuardFrameLocal::new(b.var_id, b.binding_mode))
1231 debug!("Entering guard building context: {:?}", guard_frame);
1232 self.guard_context.push(guard_frame);
1234 self.bind_matched_candidate_for_arm_body(block, &candidate.bindings);
1237 // the block to branch to if the guard fails; if there is no
1238 // guard, this block is simply unreachable
1239 let guard = match guard {
1240 Guard::If(e) => self.hir.mirror(e),
1242 let source_info = self.source_info(guard.span);
1243 let cond = unpack!(block = self.as_local_operand(block, guard));
1245 let guard_frame = self.guard_context.pop().unwrap();
1247 "Exiting guard building context with locals: {:?}",
1252 let false_edge_block = self.cfg.start_new_block();
1254 // We want to ensure that the matched candidates are bound
1255 // after we have confirmed this candidate *and* any
1256 // associated guard; Binding them on `block` is too soon,
1257 // because that would be before we've checked the result
1260 // But binding them on `arm_block` is *too late*, because
1261 // then all of the candidates for a single arm would be
1262 // bound in the same place, that would cause a case like:
1266 // (mut x, 1) | (2, mut x) if { true } => { ... }
1267 // ... // ^^^^^^^ (this is `arm_block`)
1271 // would yield a `arm_block` something like:
1274 // StorageLive(_4); // _4 is `x`
1275 // _4 = &mut (_1.0: i32); // this is handling `(mut x, 1)` case
1276 // _4 = &mut (_1.1: i32); // this is handling `(2, mut x)` case
1279 // and that is clearly not correct.
1280 let post_guard_block = self.cfg.start_new_block();
1284 TerminatorKind::if_(self.hir.tcx(), cond, post_guard_block, false_edge_block),
1288 self.bind_matched_candidate_for_arm_body(post_guard_block, &candidate.bindings);
1294 TerminatorKind::Goto { target: arm_block },
1297 let otherwise = self.cfg.start_new_block();
1302 TerminatorKind::FalseEdges {
1303 real_target: otherwise,
1304 imaginary_targets: vec![candidate.next_candidate_pre_binding_block],
1309 // (Here, it is not too early to bind the matched
1310 // candidate on `block`, because there is no guard result
1311 // that we have to inspect before we bind them.)
1312 self.bind_matched_candidate_for_arm_body(block, &candidate.bindings);
1315 candidate_source_info,
1316 TerminatorKind::Goto { target: arm_block },
1322 /// Append `AscribeUserType` statements onto the end of `block`
1323 /// for each ascription
1324 fn ascribe_types<'pat>(
1327 ascriptions: &[Ascription<'tcx>],
1329 for ascription in ascriptions {
1330 let source_info = self.source_info(ascription.span);
1333 "adding user ascription at span {:?} of place {:?} and {:?}",
1339 let user_ty = box ascription.user_ty.clone().user_ty(
1340 &mut self.canonical_user_type_annotations, source_info.span
1346 kind: StatementKind::AscribeUserType(
1347 ascription.source.clone(),
1348 ascription.variance,
1356 // Only called when all_pat_vars_are_implicit_refs_within_guards,
1357 // and thus all code/comments assume we are in that context.
1358 fn bind_matched_candidate_for_guard(
1362 bindings: &[Binding<'tcx>],
1365 "bind_matched_candidate_for_guard(block={:?}, pat_index={:?}, bindings={:?})",
1366 block, pat_index, bindings
1369 // Assign each of the bindings. Since we are binding for a
1370 // guard expression, this will never trigger moves out of the
1372 let re_erased = self.hir.tcx().types.re_erased;
1373 for binding in bindings {
1374 let source_info = self.source_info(binding.span);
1376 // For each pattern ident P of type T, `ref_for_guard` is
1377 // a reference R: &T pointing to the location matched by
1378 // the pattern, and every occurrence of P within a guard
1381 self.storage_live_binding(block, binding.var_id, binding.span, RefWithinGuard);
1382 // Question: Why schedule drops if bindings are all
1383 // shared-&'s? Answer: Because schedule_drop_for_binding
1384 // also emits StorageDead's for those locals.
1385 self.schedule_drop_for_binding(binding.var_id, binding.span, RefWithinGuard);
1386 match binding.binding_mode {
1387 BindingMode::ByValue => {
1388 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, binding.source.clone());
1390 .push_assign(block, source_info, &ref_for_guard, rvalue);
1392 BindingMode::ByRef(borrow_kind) => {
1393 // Tricky business: For `ref id` and `ref mut id`
1394 // patterns, we want `id` within the guard to
1395 // correspond to a temp of type `& &T` or `& &mut
1396 // T` (i.e., a "borrow of a borrow") that is
1397 // implicitly dereferenced.
1399 // To borrow a borrow, we need that inner borrow
1400 // to point to. So, create a temp for the inner
1401 // borrow, and then take a reference to it.
1403 // Note: the temp created here is *not* the one
1404 // used by the arm body itself. This eases
1405 // observing two-phase borrow restrictions.
1406 let val_for_guard = self.storage_live_binding(
1410 ValWithinGuard(pat_index),
1412 self.schedule_drop_for_binding(
1415 ValWithinGuard(pat_index),
1418 // rust-lang/rust#27282: We reuse the two-phase
1419 // borrow infrastructure so that the mutable
1420 // borrow (whose mutabilty is *unusable* within
1421 // the guard) does not conflict with the implicit
1422 // borrow of the whole match input. See additional
1423 // discussion on rust-lang/rust#49870.
1424 let borrow_kind = match borrow_kind {
1426 | BorrowKind::Shallow
1427 | BorrowKind::Unique => borrow_kind,
1428 BorrowKind::Mut { .. } => BorrowKind::Mut {
1429 allow_two_phase_borrow: true,
1432 let rvalue = Rvalue::Ref(re_erased, borrow_kind, binding.source.clone());
1434 .push_assign(block, source_info, &val_for_guard, rvalue);
1435 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, val_for_guard);
1437 .push_assign(block, source_info, &ref_for_guard, rvalue);
1443 fn bind_matched_candidate_for_arm_body(
1446 bindings: &[Binding<'tcx>],
1449 "bind_matched_candidate_for_arm_body(block={:?}, bindings={:?}",
1454 let re_erased = self.hir.tcx().types.re_erased;
1455 // Assign each of the bindings. This may trigger moves out of the candidate.
1456 for binding in bindings {
1457 let source_info = self.source_info(binding.span);
1459 self.storage_live_binding(block, binding.var_id, binding.span, OutsideGuard);
1460 self.schedule_drop_for_binding(binding.var_id, binding.span, OutsideGuard);
1461 let rvalue = match binding.binding_mode {
1462 BindingMode::ByValue => {
1463 Rvalue::Use(self.consume_by_copy_or_move(binding.source.clone()))
1465 BindingMode::ByRef(borrow_kind) => {
1466 Rvalue::Ref(re_erased, borrow_kind, binding.source.clone())
1469 self.cfg.push_assign(block, source_info, &local, rvalue);
1473 /// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where
1474 /// the bound `var` has type `T` in the arm body) in a pattern
1475 /// maps to `2+N` locals. The first local is a binding for
1476 /// occurrences of `var` in the guard, which will all have type
1477 /// `&T`. The N locals are bindings for the `T` that is referenced
1478 /// by the first local; they are not used outside of the
1479 /// guard. The last local is a binding for occurrences of `var` in
1480 /// the arm body, which will have type `T`.
1482 /// The reason we have N locals rather than just 1 is to
1483 /// accommodate rust-lang/rust#51348: If the arm has N candidate
1484 /// patterns, then in general they can correspond to distinct
1485 /// parts of the matched data, and we want them to be distinct
1486 /// temps in order to simplify checks performed by our internal
1487 /// leveraging of two-phase borrows).
1490 source_info: SourceInfo,
1491 visibility_scope: SourceScope,
1492 mutability: Mutability,
1495 num_patterns: usize,
1498 user_ty: UserTypeProjections<'tcx>,
1499 has_guard: ArmHasGuard,
1500 opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
1504 "declare_binding(var_id={:?}, name={:?}, mode={:?}, var_ty={:?}, \
1505 visibility_scope={:?}, source_info={:?})",
1506 var_id, name, mode, var_ty, visibility_scope, source_info
1509 let tcx = self.hir.tcx();
1510 let binding_mode = match mode {
1511 BindingMode::ByValue => ty::BindingMode::BindByValue(mutability.into()),
1512 BindingMode::ByRef(_) => ty::BindingMode::BindByReference(mutability.into()),
1514 debug!("declare_binding: user_ty={:?}", user_ty);
1515 let local = LocalDecl::<'tcx> {
1523 is_block_tail: None,
1524 is_user_variable: Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
1526 // hypothetically, `visit_bindings` could try to unzip
1527 // an outermost hir::Ty as we descend, matching up
1528 // idents in pat; but complex w/ unclear UI payoff.
1529 // Instead, just abandon providing diagnostic info.
1535 let for_arm_body = self.local_decls.push(local.clone());
1536 let locals = if has_guard.0 && tcx.all_pat_vars_are_implicit_refs_within_guards() {
1537 let mut vals_for_guard = Vec::with_capacity(num_patterns);
1538 for _ in 0..num_patterns {
1539 let val_for_guard_idx = self.local_decls.push(LocalDecl {
1540 // This variable isn't mutated but has a name, so has to be
1541 // immutable to avoid the unused mut lint.
1542 mutability: Mutability::Not,
1545 vals_for_guard.push(val_for_guard_idx);
1547 let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
1548 // See previous comment.
1549 mutability: Mutability::Not,
1550 ty: tcx.mk_imm_ref(tcx.types.re_erased, var_ty),
1551 user_ty: UserTypeProjections::none(),
1555 // FIXME: should these secretly injected ref_for_guard's be marked as `internal`?
1557 is_block_tail: None,
1558 is_user_variable: Some(ClearCrossCrate::Set(BindingForm::RefForGuard)),
1560 LocalsForNode::ForGuard {
1566 LocalsForNode::One(for_arm_body)
1568 debug!("declare_binding: vars={:?}", locals);
1569 self.var_indices.insert(var_id, locals);
1572 // Determine the fake borrows that are needed to ensure that the place
1573 // will evaluate to the same thing until an arm has been chosen.
1574 fn add_fake_borrows<'pat>(
1576 pre_binding_blocks: &[(BasicBlock, Span)],
1577 fake_borrows: FxHashMap<Place<'tcx>, BorrowKind>,
1578 source_info: SourceInfo,
1579 start_block: BasicBlock,
1581 let tcx = self.hir.tcx();
1583 debug!("add_fake_borrows pre_binding_blocks = {:?}, fake_borrows = {:?}",
1584 pre_binding_blocks, fake_borrows);
1586 let mut all_fake_borrows = Vec::with_capacity(fake_borrows.len());
1588 // Insert a Shallow borrow of the prefixes of any fake borrows.
1589 for (place, borrow_kind) in fake_borrows
1592 let mut prefix_cursor = &place;
1593 while let Place::Projection(box Projection { base, elem }) = prefix_cursor {
1594 if let ProjectionElem::Deref = elem {
1595 // Insert a shallow borrow after a deref. For other
1596 // projections the borrow of prefix_cursor will
1597 // conflict with any mutation of base.
1598 all_fake_borrows.push((base.clone(), BorrowKind::Shallow));
1600 prefix_cursor = base;
1604 all_fake_borrows.push((place, borrow_kind));
1607 // Deduplicate and ensure a deterministic order.
1608 all_fake_borrows.sort();
1609 all_fake_borrows.dedup();
1611 debug!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows);
1613 // Add fake borrows to the start of the match and reads of them before
1614 // the start of each arm.
1615 let mut borrowed_input_temps = Vec::with_capacity(all_fake_borrows.len());
1617 for (matched_place, borrow_kind) in all_fake_borrows {
1618 let borrowed_input =
1619 Rvalue::Ref(tcx.types.re_erased, borrow_kind, matched_place.clone());
1620 let borrowed_input_ty = borrowed_input.ty(&self.local_decls, tcx);
1621 let borrowed_input_temp = self.temp(borrowed_input_ty, source_info.span);
1622 self.cfg.push_assign(
1625 &borrowed_input_temp,
1628 borrowed_input_temps.push(borrowed_input_temp);
1631 // FIXME: This could be a lot of reads (#fake borrows * #patterns).
1632 // The false edges that we currently generate would allow us to only do
1633 // this on the last Candidate, but it's possible that there might not be
1634 // so many false edges in the future, so we read for all Candidates for
1636 // Another option would be to make our own block and add our own false
1638 if tcx.emit_read_for_match() {
1639 for &(pre_binding_block, span) in pre_binding_blocks {
1640 let pattern_source_info = self.source_info(span);
1641 for temp in &borrowed_input_temps {
1642 self.cfg.push(pre_binding_block, Statement {
1643 source_info: pattern_source_info,
1644 kind: StatementKind::FakeRead(
1645 FakeReadCause::ForMatchGuard,