1 //! Code related to match expressions. These are sufficiently complex to
2 //! warrant their own module and submodules. :) This main module includes the
3 //! high-level algorithm, the submodules contain the details.
5 //! This also includes code for pattern bindings in `let` statements and
6 //! function parameters.
8 use crate::build::scope::DropKind;
9 use crate::build::ForGuard::{self, OutsideGuard, RefWithinGuard};
10 use crate::build::{BlockAnd, BlockAndExtension, Builder};
11 use crate::build::{GuardFrame, GuardFrameLocal, LocalsForNode};
12 use crate::hair::{self, *};
13 use rustc::hir::HirId;
15 use rustc::middle::region;
16 use rustc::ty::{self, CanonicalUserTypeAnnotation, Ty};
17 use rustc::ty::layout::VariantIdx;
18 use rustc_data_structures::bit_set::BitSet;
19 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
20 use syntax::ast::Name;
23 // helper functions, broken out by category:
28 use std::convert::TryFrom;
30 impl<'a, 'tcx> Builder<'a, 'tcx> {
31 /// Generates MIR for a `match` expression.
33 /// The MIR that we generate for a match looks like this.
38 /// [ 1. Evaluate Scrutinee (expression being matched on) ]
39 /// [ (fake read of scrutinee) ]
41 /// [ 2. Decision tree -- check discriminants ] <--------+
43 /// | (once a specific arm is chosen) |
45 /// [pre_binding_block] [otherwise_block]
47 /// [ 3. Create "guard bindings" for arm ] |
48 /// [ (create fake borrows) ] |
50 /// [ 4. Execute guard code ] |
51 /// [ (read fake borrows) ] --(guard is false)-----------+
53 /// | (guard results in true)
55 /// [ 5. Create real bindings and execute arm ]
60 /// All of the different arms have been stacked on top of each other to
61 /// simplify the diagram. For an arm with no guard the blocks marked 3 and
62 /// 4 and the fake borrows are omitted.
64 /// We generate MIR in the following steps:
66 /// 1. Evaluate the scrutinee and add the fake read of it.
67 /// 2. Create the prebinding and otherwise blocks.
68 /// 3. Create the decision tree and record the places that we bind or test.
69 /// 4. Determine the fake borrows that are needed from the above places.
70 /// Create the required temporaries for them.
71 /// 5. Create everything else: Create everything else: the guards and the
74 /// ## Fake Reads and borrows
76 /// Match exhaustiveness checking is not able to handle the case where the
77 /// place being matched on is mutated in the guards. There is an AST check
78 /// that tries to stop this but it is buggy and overly restrictive. Instead
79 /// we add "fake borrows" to the guards that prevent any mutation of the
80 /// place being matched. There are a some subtleties:
82 /// 1. Borrowing `*x` doesn't prevent assigning to `x`. If `x` is a shared
83 /// refence, the borrow isn't even tracked. As such we have to add fake
84 /// borrows of any prefixes of a place
85 /// 2. We don't want `match x { _ => (), }` to conflict with mutable
86 /// borrows of `x`, so we only add fake borrows for places which are
87 /// bound or tested by the match.
88 /// 3. We don't want the fake borrows to conflict with `ref mut` bindings,
89 /// so we use a special BorrowKind for them.
90 /// 4. The fake borrows may be of places in inactive variants, so it would
91 /// be UB to generate code for them. They therefore have to be removed
92 /// by a MIR pass run after borrow checking.
96 /// We don't want to have the exact structure of the decision tree be
97 /// visible through borrow checking. False edges ensure that the CFG as
98 /// seen by borrow checking doesn't encode this. False edges are added:
100 /// * From each prebinding block to the next prebinding block.
101 /// * From each otherwise block to the next prebinding block.
104 destination: &Place<'tcx>,
106 mut block: BasicBlock,
107 scrutinee: ExprRef<'tcx>,
108 arms: Vec<Arm<'tcx>>,
110 let tcx = self.hir.tcx();
112 // Step 1. Evaluate the scrutinee and add the fake read of it.
114 let scrutinee_span = scrutinee.span();
115 let scrutinee_place = unpack!(block = self.as_place(block, scrutinee));
117 // Matching on a `scrutinee_place` with an uninhabited type doesn't
118 // generate any memory reads by itself, and so if the place "expression"
119 // contains unsafe operations like raw pointer dereferences or union
120 // field projections, we wouldn't know to require an `unsafe` block
121 // around a `match` equivalent to `std::intrinsics::unreachable()`.
122 // See issue #47412 for this hole being discovered in the wild.
124 // HACK(eddyb) Work around the above issue by adding a dummy inspection
125 // of `scrutinee_place`, specifically by applying `ReadForMatch`.
127 // NOTE: ReadForMatch also checks that the scrutinee is initialized.
128 // This is currently needed to not allow matching on an uninitialized,
129 // uninhabited value. If we get never patterns, those will check that
130 // the place is initialized, and so this read would only be used to
133 let source_info = self.source_info(scrutinee_span);
134 self.cfg.push(block, Statement {
136 kind: StatementKind::FakeRead(
137 FakeReadCause::ForMatchedPlace,
138 scrutinee_place.clone(),
142 // Step 2. Create the otherwise and prebinding blocks.
144 // create binding start block for link them by false edges
145 let candidate_count = arms.iter().map(|c| c.patterns.len()).sum::<usize>();
146 let pre_binding_blocks: Vec<_> = (0..candidate_count)
147 .map(|_| self.cfg.start_new_block())
150 let mut match_has_guard = false;
152 let mut candidate_pre_binding_blocks = pre_binding_blocks.iter();
153 let mut next_candidate_pre_binding_blocks = pre_binding_blocks.iter().skip(1);
155 // Assemble a list of candidates: there is one candidate per pattern,
156 // which means there may be more than one candidate *per arm*.
157 let mut arm_candidates: Vec<_> = arms
160 let arm_has_guard = arm.guard.is_some();
161 match_has_guard |= arm_has_guard;
162 let arm_candidates: Vec<_> = arm.patterns
164 .zip(candidate_pre_binding_blocks.by_ref())
166 |(pattern, pre_binding_block)| {
170 MatchPair::new(scrutinee_place.clone(), pattern),
174 otherwise_block: if arm_has_guard {
175 Some(self.cfg.start_new_block())
179 pre_binding_block: *pre_binding_block,
180 next_candidate_pre_binding_block:
181 next_candidate_pre_binding_blocks.next().copied(),
186 (arm, arm_candidates)
190 // Step 3. Create the decision tree and record the places that we bind or test.
192 // The set of places that we are creating fake borrows of. If there are
193 // no match guards then we don't need any fake borrows, so don't track
195 let mut fake_borrows = if match_has_guard && tcx.generate_borrow_of_any_match_input() {
196 Some(FxHashSet::default())
201 // These candidates are kept sorted such that the highest priority
202 // candidate comes first in the list. (i.e., same order as in source)
203 // As we gnerate the decision tree,
204 let candidates = &mut arm_candidates
206 .flat_map(|(_, candidates)| candidates)
207 .collect::<Vec<_>>();
209 let outer_source_info = self.source_info(span);
211 // this will generate code to test scrutinee_place and
212 // branch to the appropriate arm block
213 self.match_candidates(
221 // Step 4. Determine the fake borrows that are needed from the above
222 // places. Create the required temporaries for them.
224 let fake_borrow_temps = if let Some(ref borrows) = fake_borrows {
225 self.calculate_fake_borrows(borrows, scrutinee_span)
230 // Step 5. Create everything else: the guards and the arms.
231 let match_scope = self.scopes.topmost();
233 let arm_end_blocks: Vec<_> = arm_candidates.into_iter().map(|(arm, mut candidates)| {
234 let arm_source_info = self.source_info(arm.span);
235 let arm_scope = (arm.scope, arm_source_info);
236 self.in_scope(arm_scope, arm.lint_level, |this| {
237 let body = this.hir.mirror(arm.body.clone());
238 let scope = this.declare_bindings(
242 ArmHasGuard(arm.guard.is_some()),
243 Some((Some(&scrutinee_place), scrutinee_span)),
247 if candidates.len() == 1 {
248 arm_block = this.bind_and_guard_matched_candidate(
249 candidates.pop().unwrap(),
256 arm_block = this.cfg.start_new_block();
257 for candidate in candidates {
258 this.clear_top_scope(arm.scope);
259 let binding_end = this.bind_and_guard_matched_candidate(
269 TerminatorKind::Goto { target: arm_block },
274 if let Some(source_scope) = scope {
275 this.source_scope = source_scope;
278 this.into(destination, arm_block, body)
282 // all the arm blocks will rejoin here
283 let end_block = self.cfg.start_new_block();
285 for arm_block in arm_end_blocks {
289 TerminatorKind::Goto { target: end_block },
293 self.source_scope = outer_source_info.scope;
298 pub(super) fn expr_into_pattern(
300 mut block: BasicBlock,
301 irrefutable_pat: Pattern<'tcx>,
302 initializer: ExprRef<'tcx>,
304 match *irrefutable_pat.kind {
305 // Optimize the case of `let x = ...` to write directly into `x`
306 PatternKind::Binding {
307 mode: BindingMode::ByValue,
313 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard);
314 unpack!(block = self.into(&place, block, initializer));
317 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
318 let source_info = self.source_info(irrefutable_pat.span);
323 kind: StatementKind::FakeRead(FakeReadCause::ForLet, place),
327 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
331 // Optimize the case of `let x: T = ...` to write directly
332 // into `x` and then require that `T == typeof(x)`.
334 // Weirdly, this is needed to prevent the
335 // `intrinsic-move-val.rs` test case from crashing. That
336 // test works with uninitialized values in a rather
337 // dubious way, so it may be that the test is kind of
339 PatternKind::AscribeUserType {
340 subpattern: Pattern {
341 kind: box PatternKind::Binding {
342 mode: BindingMode::ByValue,
349 ascription: hair::pattern::Ascription {
350 user_ty: pat_ascription_ty,
356 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard);
357 unpack!(block = self.into(&place, block, initializer));
359 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
360 let pattern_source_info = self.source_info(irrefutable_pat.span);
364 source_info: pattern_source_info,
365 kind: StatementKind::FakeRead(FakeReadCause::ForLet, place.clone()),
369 let ty_source_info = self.source_info(user_ty_span);
370 let user_ty = box pat_ascription_ty.user_ty(
371 &mut self.canonical_user_type_annotations,
372 place.ty(&self.local_decls, self.hir.tcx()).ty,
378 source_info: ty_source_info,
379 kind: StatementKind::AscribeUserType(
381 // We always use invariant as the variance here. This is because the
382 // variance field from the ascription refers to the variance to use
383 // when applying the type to the value being matched, but this
384 // ascription applies rather to the type of the binding. e.g., in this
391 // We are creating an ascription that defines the type of `x` to be
392 // exactly `T` (i.e., with invariance). The variance field, in
393 // contrast, is intended to be used to relate `T` to the type of
395 ty::Variance::Invariant,
401 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
406 let place = unpack!(block = self.as_place(block, initializer));
407 self.place_into_pattern(block, irrefutable_pat, &place, true)
412 pub fn place_into_pattern(
415 irrefutable_pat: Pattern<'tcx>,
416 initializer: &Place<'tcx>,
417 set_match_place: bool,
419 // create a dummy candidate
420 let mut candidate = Candidate {
421 span: irrefutable_pat.span,
422 match_pairs: vec![MatchPair::new(initializer.clone(), &irrefutable_pat)],
426 // since we don't call `match_candidates`, next fields are unused
427 otherwise_block: None,
428 pre_binding_block: block,
429 next_candidate_pre_binding_block: None,
432 // Simplify the candidate. Since the pattern is irrefutable, this should
433 // always convert all match-pairs into bindings.
434 self.simplify_candidate(&mut candidate);
436 if !candidate.match_pairs.is_empty() {
437 // ICE if no other errors have been emitted. This used to be a hard error that wouldn't
438 // be reached because `hair::pattern::check_match::check_match` wouldn't have let the
439 // compiler continue. In our tests this is only ever hit by
440 // `ui/consts/const-match-check.rs` with `--cfg eval1`, and that file already generates
441 // a different error before hand.
442 self.hir.tcx().sess.delay_span_bug(
443 candidate.match_pairs[0].pattern.span,
445 "match pairs {:?} remaining after simplifying irrefutable pattern",
446 candidate.match_pairs,
451 // for matches and function arguments, the place that is being matched
452 // can be set when creating the variables. But the place for
453 // let PATTERN = ... might not even exist until we do the assignment.
454 // so we set it here instead
456 for binding in &candidate.bindings {
457 let local = self.var_local_id(binding.var_id, OutsideGuard);
459 if let Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
460 opt_match_place: Some((ref mut match_place, _)),
462 }))) = self.local_decls[local].is_user_variable
464 *match_place = Some(initializer.clone());
466 bug!("Let binding to non-user variable.")
471 self.ascribe_types(block, &candidate.ascriptions);
473 // now apply the bindings, which will also declare the variables
474 self.bind_matched_candidate_for_arm_body(block, &candidate.bindings);
479 /// Declares the bindings of the given patterns and returns the visibility
480 /// scope for the bindings in these patterns, if such a scope had to be
481 /// created. NOTE: Declaring the bindings should always be done in their
483 pub fn declare_bindings(
485 mut visibility_scope: Option<SourceScope>,
487 pattern: &Pattern<'tcx>,
488 has_guard: ArmHasGuard,
489 opt_match_place: Option<(Option<&Place<'tcx>>, Span)>,
490 ) -> Option<SourceScope> {
491 debug!("declare_bindings: pattern={:?}", pattern);
494 UserTypeProjections::none(),
495 &mut |this, mutability, name, mode, var, span, ty, user_ty| {
496 if visibility_scope.is_none() {
498 Some(this.new_source_scope(scope_span, LintLevel::Inherited, None));
500 let source_info = SourceInfo { span, scope: this.source_scope };
501 let visibility_scope = visibility_scope.unwrap();
502 this.declare_binding(
512 opt_match_place.map(|(x, y)| (x.cloned(), y)),
520 pub fn storage_live_binding(
527 let local_id = self.var_local_id(var, for_guard);
528 let source_info = self.source_info(span);
533 kind: StatementKind::StorageLive(local_id),
536 let var_ty = self.local_decls[local_id].ty;
537 let region_scope = self.hir.region_scope_tree.var_scope(var.local_id);
538 self.schedule_drop(span, region_scope, local_id, var_ty, DropKind::Storage);
539 Place::from(local_id)
542 pub fn schedule_drop_for_binding(&mut self, var: HirId, span: Span, for_guard: ForGuard) {
543 let local_id = self.var_local_id(var, for_guard);
544 let var_ty = self.local_decls[local_id].ty;
545 let region_scope = self.hir.region_scope_tree.var_scope(var.local_id);
555 pub(super) fn visit_bindings(
557 pattern: &Pattern<'tcx>,
558 pattern_user_ty: UserTypeProjections,
570 debug!("visit_bindings: pattern={:?} pattern_user_ty={:?}", pattern, pattern_user_ty);
571 match *pattern.kind {
572 PatternKind::Binding {
581 f(self, mutability, name, mode, var, pattern.span, ty, pattern_user_ty.clone());
582 if let Some(subpattern) = subpattern.as_ref() {
583 self.visit_bindings(subpattern, pattern_user_ty, f);
592 | PatternKind::Slice {
597 let from = u32::try_from(prefix.len()).unwrap();
598 let to = u32::try_from(suffix.len()).unwrap();
599 for subpattern in prefix {
600 self.visit_bindings(subpattern, pattern_user_ty.clone().index(), f);
602 for subpattern in slice {
603 self.visit_bindings(subpattern, pattern_user_ty.clone().subslice(from, to), f);
605 for subpattern in suffix {
606 self.visit_bindings(subpattern, pattern_user_ty.clone().index(), f);
610 PatternKind::Constant { .. } | PatternKind::Range { .. } | PatternKind::Wild => {}
612 PatternKind::Deref { ref subpattern } => {
613 self.visit_bindings(subpattern, pattern_user_ty.deref(), f);
616 PatternKind::AscribeUserType {
618 ascription: hair::pattern::Ascription {
624 // This corresponds to something like
627 // let A::<'a>(_): A<'static> = ...;
630 // Note that the variance doesn't apply here, as we are tracking the effect
631 // of `user_ty` on any bindings contained with subpattern.
632 let annotation = CanonicalUserTypeAnnotation {
634 user_ty: user_ty.user_ty,
635 inferred_ty: subpattern.ty,
637 let projection = UserTypeProjection {
638 base: self.canonical_user_type_annotations.push(annotation),
641 let subpattern_user_ty = pattern_user_ty.push_projection(&projection, user_ty_span);
642 self.visit_bindings(subpattern, subpattern_user_ty, f)
645 PatternKind::Leaf { ref subpatterns } => {
646 for subpattern in subpatterns {
647 let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field);
648 debug!("visit_bindings: subpattern_user_ty={:?}", subpattern_user_ty);
649 self.visit_bindings(&subpattern.pattern, subpattern_user_ty, f);
653 PatternKind::Variant { adt_def, substs: _, variant_index, ref subpatterns } => {
654 for subpattern in subpatterns {
655 let subpattern_user_ty = pattern_user_ty.clone().variant(
656 adt_def, variant_index, subpattern.field);
657 self.visit_bindings(&subpattern.pattern, subpattern_user_ty, f);
660 PatternKind::Or { ref pats } => {
662 self.visit_bindings(&pat, pattern_user_ty.clone(), f);
670 pub struct Candidate<'pat, 'tcx> {
671 // span of the original pattern that gave rise to this candidate
674 // all of these must be satisfied...
675 match_pairs: Vec<MatchPair<'pat, 'tcx>>,
677 // ...these bindings established...
678 bindings: Vec<Binding<'tcx>>,
680 // ...and these types asserted...
681 ascriptions: Vec<Ascription<'tcx>>,
683 // ...and the guard must be evaluated, if false branch to Block...
684 otherwise_block: Option<BasicBlock>,
686 // ...and the blocks for add false edges between candidates
687 pre_binding_block: BasicBlock,
688 next_candidate_pre_binding_block: Option<BasicBlock>,
691 #[derive(Clone, Debug)]
692 struct Binding<'tcx> {
698 mutability: Mutability,
699 binding_mode: BindingMode,
702 /// Indicates that the type of `source` must be a subtype of the
703 /// user-given type `user_ty`; this is basically a no-op but can
704 /// influence region inference.
705 #[derive(Clone, Debug)]
706 struct Ascription<'tcx> {
709 user_ty: PatternTypeProjection<'tcx>,
710 variance: ty::Variance,
713 #[derive(Clone, Debug)]
714 pub struct MatchPair<'pat, 'tcx> {
718 // ... must match this pattern.
719 pattern: &'pat Pattern<'tcx>,
722 #[derive(Clone, Debug, PartialEq)]
723 enum TestKind<'tcx> {
724 /// Test the branches of enum.
726 /// The enum being tested
727 adt_def: &'tcx ty::AdtDef,
728 /// The set of variants that we should create a branch for. We also
729 /// create an additional "otherwise" case.
730 variants: BitSet<VariantIdx>,
733 /// Test what value an `integer`, `bool` or `char` has.
735 /// The type of the value that we're testing.
737 /// The (ordered) set of values that we test for.
739 /// For integers and `char`s we create a branch to each of the values in
740 /// `options`, as well as an "otherwise" branch for all other values, even
741 /// in the (rare) case that options is exhaustive.
743 /// For `bool` we always generate two edges, one for `true` and one for
746 /// Reverse map used to ensure that the values in `options` are unique.
747 indices: FxHashMap<&'tcx ty::Const<'tcx>, usize>,
750 /// Test for equality with value, possibly after an unsizing coercion to
753 value: &'tcx ty::Const<'tcx>,
754 // Integer types are handled by `SwitchInt`, and constants with ADT
755 // types are converted back into patterns, so this can only be `&str`,
756 // `&[T]`, `f32` or `f64`.
760 /// Test whether the value falls within an inclusive or exclusive range
761 Range(PatternRange<'tcx>),
763 /// Test length of the slice is equal to len
771 pub struct Test<'tcx> {
773 kind: TestKind<'tcx>,
776 /// ArmHasGuard is isomorphic to a boolean flag. It indicates whether
777 /// a match arm has a guard expression attached to it.
778 #[derive(Copy, Clone, Debug)]
779 pub(crate) struct ArmHasGuard(pub bool);
781 ///////////////////////////////////////////////////////////////////////////
782 // Main matching algorithm
784 impl<'a, 'tcx> Builder<'a, 'tcx> {
785 /// The main match algorithm. It begins with a set of candidates
786 /// `candidates` and has the job of generating code to determine
787 /// which of these candidates, if any, is the correct one. The
788 /// candidates are sorted such that the first item in the list
789 /// has the highest priority. When a candidate is found to match
790 /// the value, we will generate a branch to the appropriate
791 /// prebinding block.
793 /// If we find that *NONE* of the candidates apply, we branch to the
794 /// `otherwise_block`. In principle, this means that the input list was not
795 /// exhaustive, though at present we sometimes are not smart enough to
796 /// recognize all exhaustive inputs.
798 /// It might be surprising that the input can be inexhaustive.
799 /// Indeed, initially, it is not, because all matches are
800 /// exhaustive in Rust. But during processing we sometimes divide
801 /// up the list of candidates and recurse with a non-exhaustive
802 /// list. This is important to keep the size of the generated code
803 /// under control. See `test_candidates` for more details.
805 /// If `fake_borrows` is Some, then places which need fake borrows
806 /// will be added to it.
807 fn match_candidates<'pat>(
810 start_block: &mut Option<BasicBlock>,
811 otherwise_block: Option<BasicBlock>,
812 candidates: &mut [&mut Candidate<'pat, 'tcx>],
813 fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>,
816 "matched_candidate(span={:?}, candidates={:?}, start_block={:?}, otherwise_block={:?})",
823 // Start by simplifying candidates. Once this process is complete, all
824 // the match pairs which remain require some form of test, whether it
825 // be a switch or pattern comparison.
826 for candidate in &mut *candidates {
827 self.simplify_candidate(candidate);
830 // The candidates are sorted by priority. Check to see whether the
831 // higher priority candidates (and hence at the front of the slice)
832 // have satisfied all their match pairs.
833 let fully_matched = candidates
835 .take_while(|c| c.match_pairs.is_empty())
838 "match_candidates: {:?} candidates fully matched",
841 let (matched_candidates, unmatched_candidates) = candidates.split_at_mut(fully_matched);
843 let block: BasicBlock;
845 if !matched_candidates.is_empty() {
846 let otherwise_block = self.select_matched_candidates(
852 if let Some(last_otherwise_block) = otherwise_block {
853 block = last_otherwise_block
855 // Any remaining candidates are unreachable.
856 if unmatched_candidates.is_empty() {
859 block = self.cfg.start_new_block();
862 block = *start_block.get_or_insert_with(|| self.cfg.start_new_block());
865 // If there are no candidates that still need testing, we're
866 // done. Since all matches are exhaustive, execution should
867 // never reach this point.
868 if unmatched_candidates.is_empty() {
869 let source_info = self.source_info(span);
870 if let Some(otherwise) = otherwise_block {
874 TerminatorKind::Goto { target: otherwise },
880 TerminatorKind::Unreachable,
886 // Test for the remaining candidates.
887 self.test_candidates(
889 unmatched_candidates,
896 /// Link up matched candidates. For example, if we have something like
900 /// Some(x) if cond => ...
902 /// Some(x) if cond => ...
905 /// We generate real edges from:
906 /// * `block` to the prebinding_block of the first pattern,
907 /// * the otherwise block of the first pattern to the second pattern,
908 /// * the otherwise block of the third pattern to the a block with an
909 /// Unreachable terminator.
911 /// As well as that we add fake edges from the otherwise blocks to the
912 /// prebinding block of the next candidate in the original set of
914 fn select_matched_candidates(
916 matched_candidates: &mut [&mut Candidate<'_, 'tcx>],
917 start_block: &mut Option<BasicBlock>,
918 fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>,
919 ) -> Option<BasicBlock> {
921 !matched_candidates.is_empty(),
922 "select_matched_candidates called with no candidates",
925 // Insert a borrows of prefixes of places that are bound and are
926 // behind a dereference projection.
928 // These borrows are taken to avoid situations like the following:
931 // _ if { x = &[0]; false } => (),
932 // y => (), // Out of bounds array access!
936 // // y is bound by reference in the guard and then by copy in the
937 // // arm, so y is 2 in the arm!
938 // y if { y == 1 && (x = &2) == () } => y,
941 if let Some(fake_borrows) = fake_borrows {
942 for Binding { source, .. }
943 in matched_candidates.iter().flat_map(|candidate| &candidate.bindings)
946 source.projection.iter().rposition(|elem| *elem == ProjectionElem::Deref)
948 let proj_base = &source.projection[..i];
950 fake_borrows.insert(Place {
951 base: source.base.clone(),
952 projection: proj_base.to_vec().into_boxed_slice(),
958 let fully_matched_with_guard = matched_candidates
960 .position(|c| c.otherwise_block.is_none())
961 .unwrap_or(matched_candidates.len() - 1);
963 let (reachable_candidates, unreachable_candidates)
964 = matched_candidates.split_at_mut(fully_matched_with_guard + 1);
966 let first_candidate = &reachable_candidates[0];
967 let first_prebinding_block = first_candidate.pre_binding_block;
969 if let Some(start_block) = *start_block {
970 let source_info = self.source_info(first_candidate.span);
974 TerminatorKind::Goto { target: first_prebinding_block },
977 *start_block = Some(first_prebinding_block);
980 for window in reachable_candidates.windows(2) {
981 if let [first_candidate, second_candidate] = window {
982 let source_info = self.source_info(first_candidate.span);
983 if let Some(otherwise_block) = first_candidate.otherwise_block {
986 second_candidate.pre_binding_block,
987 first_candidate.next_candidate_pre_binding_block,
991 bug!("candidate other than the last has no guard");
994 bug!("<[_]>::windows returned incorrectly sized window");
998 debug!("match_candidates: add false edges for unreachable {:?}", unreachable_candidates);
999 for candidate in unreachable_candidates {
1000 if let Some(otherwise) = candidate.otherwise_block {
1001 let source_info = self.source_info(candidate.span);
1002 let unreachable = self.cfg.start_new_block();
1006 candidate.next_candidate_pre_binding_block,
1009 self.cfg.terminate(unreachable, source_info, TerminatorKind::Unreachable);
1013 let last_candidate = reachable_candidates.last().unwrap();
1015 if let Some(otherwise) = last_candidate.otherwise_block {
1016 let source_info = self.source_info(last_candidate.span);
1017 let block = self.cfg.start_new_block();
1021 last_candidate.next_candidate_pre_binding_block,
1030 /// This is the most subtle part of the matching algorithm. At
1031 /// this point, the input candidates have been fully simplified,
1032 /// and so we know that all remaining match-pairs require some
1033 /// sort of test. To decide what test to do, we take the highest
1034 /// priority candidate (last one in the list) and extract the
1035 /// first match-pair from the list. From this we decide what kind
1036 /// of test is needed using `test`, defined in the `test` module.
1038 /// *Note:* taking the first match pair is somewhat arbitrary, and
1039 /// we might do better here by choosing more carefully what to
1042 /// For example, consider the following possible match-pairs:
1044 /// 1. `x @ Some(P)` -- we will do a `Switch` to decide what variant `x` has
1045 /// 2. `x @ 22` -- we will do a `SwitchInt`
1046 /// 3. `x @ 3..5` -- we will do a range test
1049 /// Once we know what sort of test we are going to perform, this
1050 /// Tests may also help us with other candidates. So we walk over
1051 /// the candidates (from high to low priority) and check. This
1052 /// gives us, for each outcome of the test, a transformed list of
1053 /// candidates. For example, if we are testing the current
1054 /// variant of `x.0`, and we have a candidate `{x.0 @ Some(v), x.1
1055 /// @ 22}`, then we would have a resulting candidate of `{(x.0 as
1056 /// Some).0 @ v, x.1 @ 22}`. Note that the first match-pair is now
1057 /// simpler (and, in fact, irrefutable).
1059 /// But there may also be candidates that the test just doesn't
1060 /// apply to. The classical example involves wildcards:
1063 /// # let (x, y, z) = (true, true, true);
1064 /// match (x, y, z) {
1065 /// (true, _, true) => true, // (0)
1066 /// (_, true, _) => true, // (1)
1067 /// (false, false, _) => false, // (2)
1068 /// (true, _, false) => false, // (3)
1072 /// In that case, after we test on `x`, there are 2 overlapping candidate
1075 /// - If the outcome is that `x` is true, candidates 0, 1, and 3
1076 /// - If the outcome is that `x` is false, candidates 1 and 2
1078 /// Here, the traditional "decision tree" method would generate 2
1079 /// separate code-paths for the 2 separate cases.
1081 /// In some cases, this duplication can create an exponential amount of
1082 /// code. This is most easily seen by noticing that this method terminates
1083 /// with precisely the reachable arms being reachable - but that problem
1084 /// is trivially NP-complete:
1087 /// match (var0, var1, var2, var3, ..) {
1088 /// (true, _, _, false, true, ...) => false,
1089 /// (_, true, true, false, _, ...) => false,
1090 /// (false, _, false, false, _, ...) => false,
1096 /// Here the last arm is reachable only if there is an assignment to
1097 /// the variables that does not match any of the literals. Therefore,
1098 /// compilation would take an exponential amount of time in some cases.
1100 /// That kind of exponential worst-case might not occur in practice, but
1101 /// our simplistic treatment of constants and guards would make it occur
1102 /// in very common situations - for example #29740:
1106 /// "foo" if foo_guard => ...,
1107 /// "bar" if bar_guard => ...,
1108 /// "baz" if baz_guard => ...,
1113 /// Here we first test the match-pair `x @ "foo"`, which is an `Eq` test.
1115 /// It might seem that we would end up with 2 disjoint candidate
1116 /// sets, consisting of the first candidate or the other 3, but our
1117 /// algorithm doesn't reason about "foo" being distinct from the other
1118 /// constants; it considers the latter arms to potentially match after
1119 /// both outcomes, which obviously leads to an exponential amount
1122 /// To avoid these kinds of problems, our algorithm tries to ensure
1123 /// the amount of generated tests is linear. When we do a k-way test,
1124 /// we return an additional "unmatched" set alongside the obvious `k`
1125 /// sets. When we encounter a candidate that would be present in more
1126 /// than one of the sets, we put it and all candidates below it into the
1127 /// "unmatched" set. This ensures these `k+1` sets are disjoint.
1129 /// After we perform our test, we branch into the appropriate candidate
1130 /// set and recurse with `match_candidates`. These sub-matches are
1131 /// obviously inexhaustive - as we discarded our otherwise set - so
1132 /// we set their continuation to do `match_candidates` on the
1133 /// "unmatched" set (which is again inexhaustive).
1135 /// If you apply this to the above test, you basically wind up
1136 /// with an if-else-if chain, testing each candidate in turn,
1137 /// which is precisely what we want.
1139 /// In addition to avoiding exponential-time blowups, this algorithm
1140 /// also has nice property that each guard and arm is only generated
1142 fn test_candidates<'pat, 'b, 'c>(
1145 mut candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>],
1147 mut otherwise_block: Option<BasicBlock>,
1148 fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>,
1150 // extract the match-pair from the highest priority candidate
1151 let match_pair = &candidates.first().unwrap().match_pairs[0];
1152 let mut test = self.test(match_pair);
1153 let match_place = match_pair.place.clone();
1155 // most of the time, the test to perform is simply a function
1156 // of the main candidate; but for a test like SwitchInt, we
1157 // may want to add cases based on the candidates that are
1160 TestKind::SwitchInt {
1165 for candidate in candidates.iter() {
1166 if !self.add_cases_to_switch(
1181 for candidate in candidates.iter() {
1182 if !self.add_variants_to_switch(&match_place, candidate, variants) {
1190 // Insert a Shallow borrow of any places that is switched on.
1191 fake_borrows.as_mut().map(|fb| {
1192 fb.insert(match_place.clone())
1195 // perform the test, branching to one of N blocks. For each of
1196 // those N possible outcomes, create a (initially empty)
1197 // vector of candidates. Those are the candidates that still
1198 // apply if the test has that particular outcome.
1200 "match_candidates: test={:?} match_pair={:?}",
1203 let mut target_candidates: Vec<Vec<&mut Candidate<'pat, 'tcx>>> = vec![];
1204 target_candidates.resize_with(test.targets(), Default::default);
1206 let total_candidate_count = candidates.len();
1208 // Sort the candidates into the appropriate vector in
1209 // `target_candidates`. Note that at some point we may
1210 // encounter a candidate where the test is not relevant; at
1211 // that point, we stop sorting.
1212 while let Some(candidate) = candidates.first_mut() {
1213 if let Some(idx) = self.sort_candidate(&match_place, &test, candidate) {
1214 let (candidate, rest) = candidates.split_first_mut().unwrap();
1215 target_candidates[idx].push(candidate);
1221 // at least the first candidate ought to be tested
1222 assert!(total_candidate_count > candidates.len());
1223 debug!("tested_candidates: {}", total_candidate_count - candidates.len());
1224 debug!("untested_candidates: {}", candidates.len());
1226 // HACK(matthewjasper) This is a closure so that we can let the test
1227 // create its blocks before the rest of the match. This currently
1228 // improves the speed of llvm when optimizing long string literal
1230 let make_target_blocks = move |this: &mut Self| -> Vec<BasicBlock> {
1231 // For each outcome of test, process the candidates that still
1232 // apply. Collect a list of blocks where control flow will
1233 // branch if one of the `target_candidate` sets is not
1235 if !candidates.is_empty() {
1236 let remainder_start = &mut None;
1237 this.match_candidates(
1244 otherwise_block = Some(remainder_start.unwrap());
1247 target_candidates.into_iter().map(|mut candidates| {
1248 if candidates.len() != 0 {
1249 let candidate_start = &mut None;
1250 this.match_candidates(
1257 candidate_start.unwrap()
1259 *otherwise_block.get_or_insert_with(|| {
1260 let unreachable = this.cfg.start_new_block();
1261 let source_info = this.source_info(span);
1265 TerminatorKind::Unreachable,
1281 // Determine the fake borrows that are needed to ensure that the place
1282 // will evaluate to the same thing until an arm has been chosen.
1283 fn calculate_fake_borrows<'b>(
1285 fake_borrows: &'b FxHashSet<Place<'tcx>>,
1287 ) -> Vec<(PlaceRef<'b, 'tcx>, Local)> {
1288 let tcx = self.hir.tcx();
1290 debug!("add_fake_borrows fake_borrows = {:?}", fake_borrows);
1292 let mut all_fake_borrows = Vec::with_capacity(fake_borrows.len());
1294 // Insert a Shallow borrow of the prefixes of any fake borrows.
1295 for place in fake_borrows
1297 for (i, elem) in place.projection.iter().enumerate().rev() {
1298 let proj_base = &place.projection[..i];
1300 if let ProjectionElem::Deref = elem {
1301 // Insert a shallow borrow after a deref. For other
1302 // projections the borrow of prefix_cursor will
1303 // conflict with any mutation of base.
1304 all_fake_borrows.push(PlaceRef {
1306 projection: proj_base,
1311 all_fake_borrows.push(place.as_ref());
1314 // Deduplicate and ensure a deterministic order.
1315 all_fake_borrows.sort();
1316 all_fake_borrows.dedup();
1318 debug!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows);
1320 all_fake_borrows.into_iter().map(|matched_place| {
1321 let fake_borrow_deref_ty = Place::ty_from(
1323 matched_place.projection,
1328 let fake_borrow_ty = tcx.mk_imm_ref(tcx.lifetimes.re_erased, fake_borrow_deref_ty);
1329 let fake_borrow_temp = self.local_decls.push(
1330 LocalDecl::new_temp(fake_borrow_ty, temp_span)
1333 (matched_place, fake_borrow_temp)
1338 ///////////////////////////////////////////////////////////////////////////
1339 // Pattern binding - used for `let` and function parameters as well.
1341 impl<'a, 'tcx> Builder<'a, 'tcx> {
1342 /// Initializes each of the bindings from the candidate by
1343 /// moving/copying/ref'ing the source as appropriate. Tests the guard, if
1344 /// any, and then branches to the arm. Returns the block for the case where
1345 /// the guard fails.
1347 /// Note: we check earlier that if there is a guard, there cannot be move
1348 /// bindings (unless feature(bind_by_move_pattern_guards) is used). This
1349 /// isn't really important for the self-consistency of this fn, but the
1350 /// reason for it should be clear: after we've done the assignments, if
1351 /// there were move bindings, further tests would be a use-after-move.
1352 /// bind_by_move_pattern_guards avoids this by only moving the binding once
1353 /// the guard has evaluated to true (see below).
1354 fn bind_and_guard_matched_candidate<'pat>(
1356 candidate: Candidate<'pat, 'tcx>,
1357 guard: Option<Guard<'tcx>>,
1358 fake_borrows: &Vec<(PlaceRef<'_, 'tcx>, Local)>,
1359 scrutinee_span: Span,
1360 region_scope: region::Scope,
1362 debug!("bind_and_guard_matched_candidate(candidate={:?})", candidate);
1364 debug_assert!(candidate.match_pairs.is_empty());
1366 let candidate_source_info = self.source_info(candidate.span);
1368 let mut block = candidate.pre_binding_block;
1370 // If we are adding our own statements, then we need a fresh block.
1371 let create_fresh_block = candidate.next_candidate_pre_binding_block.is_some()
1372 || !candidate.bindings.is_empty()
1373 || !candidate.ascriptions.is_empty()
1376 if create_fresh_block {
1377 let fresh_block = self.cfg.start_new_block();
1381 candidate.next_candidate_pre_binding_block,
1382 candidate_source_info,
1384 block = fresh_block;
1385 self.ascribe_types(block, &candidate.ascriptions);
1390 // rust-lang/rust#27282: The `autoref` business deserves some
1391 // explanation here.
1393 // The intent of the `autoref` flag is that when it is true,
1394 // then any pattern bindings of type T will map to a `&T`
1395 // within the context of the guard expression, but will
1396 // continue to map to a `T` in the context of the arm body. To
1397 // avoid surfacing this distinction in the user source code
1398 // (which would be a severe change to the language and require
1399 // far more revision to the compiler), when `autoref` is true,
1400 // then any occurrence of the identifier in the guard
1401 // expression will automatically get a deref op applied to it.
1403 // So an input like:
1406 // let place = Foo::new();
1407 // match place { foo if inspect(foo)
1408 // => feed(foo), ... }
1411 // will be treated as if it were really something like:
1414 // let place = Foo::new();
1415 // match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) }
1416 // => { let tmp2 = place; feed(tmp2) }, ... }
1418 // And an input like:
1421 // let place = Foo::new();
1422 // match place { ref mut foo if inspect(foo)
1423 // => feed(foo), ... }
1426 // will be treated as if it were really something like:
1429 // let place = Foo::new();
1430 // match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) }
1431 // => { let tmp2 = &mut place; feed(tmp2) }, ... }
1434 // In short, any pattern binding will always look like *some*
1435 // kind of `&T` within the guard at least in terms of how the
1436 // MIR-borrowck views it, and this will ensure that guard
1437 // expressions cannot mutate their the match inputs via such
1438 // bindings. (It also ensures that guard expressions can at
1439 // most *copy* values from such bindings; non-Copy things
1440 // cannot be moved via pattern bindings in guard expressions.)
1444 // Implementation notes (under assumption `autoref` is true).
1446 // To encode the distinction above, we must inject the
1447 // temporaries `tmp1` and `tmp2`.
1449 // There are two cases of interest: binding by-value, and binding by-ref.
1451 // 1. Binding by-value: Things are simple.
1453 // * Establishing `tmp1` creates a reference into the
1454 // matched place. This code is emitted by
1455 // bind_matched_candidate_for_guard.
1457 // * `tmp2` is only initialized "lazily", after we have
1458 // checked the guard. Thus, the code that can trigger
1459 // moves out of the candidate can only fire after the
1460 // guard evaluated to true. This initialization code is
1461 // emitted by bind_matched_candidate_for_arm.
1463 // 2. Binding by-reference: Things are tricky.
1465 // * Here, the guard expression wants a `&&` or `&&mut`
1466 // into the original input. This means we need to borrow
1467 // the reference that we create for the arm.
1468 // * So we eagerly create the reference for the arm and then take a
1469 // reference to that.
1470 if let Some(guard) = guard {
1471 let tcx = self.hir.tcx();
1473 self.bind_matched_candidate_for_guard(
1475 &candidate.bindings,
1477 let guard_frame = GuardFrame {
1481 .map(|b| GuardFrameLocal::new(b.var_id, b.binding_mode))
1484 debug!("entering guard building context: {:?}", guard_frame);
1485 self.guard_context.push(guard_frame);
1487 let re_erased = tcx.lifetimes.re_erased;
1488 let scrutinee_source_info = self.source_info(scrutinee_span);
1489 for (place, temp) in fake_borrows {
1490 let borrow = Rvalue::Ref(
1492 BorrowKind::Shallow,
1494 base: place.base.clone(),
1495 projection: place.projection.to_vec().into_boxed_slice(),
1498 self.cfg.push_assign(
1500 scrutinee_source_info,
1501 &Place::from(*temp),
1506 // the block to branch to if the guard fails; if there is no
1507 // guard, this block is simply unreachable
1508 let guard = match guard {
1509 Guard::If(e) => self.hir.mirror(e),
1511 let source_info = self.source_info(guard.span);
1512 let guard_end = self.source_info(tcx.sess.source_map().end_point(guard.span));
1513 let (post_guard_block, otherwise_post_guard_block)
1514 = self.test_bool(block, guard, source_info);
1515 let guard_frame = self.guard_context.pop().unwrap();
1517 "Exiting guard building context with locals: {:?}",
1521 for &(_, temp) in fake_borrows {
1522 self.cfg.push(post_guard_block, Statement {
1523 source_info: guard_end,
1524 kind: StatementKind::FakeRead(
1525 FakeReadCause::ForMatchGuard,
1534 otherwise_post_guard_block,
1535 candidate.otherwise_block.unwrap(),
1538 // We want to ensure that the matched candidates are bound
1539 // after we have confirmed this candidate *and* any
1540 // associated guard; Binding them on `block` is too soon,
1541 // because that would be before we've checked the result
1544 // But binding them on the arm is *too late*, because
1545 // then all of the candidates for a single arm would be
1546 // bound in the same place, that would cause a case like:
1550 // (mut x, 1) | (2, mut x) if { true } => { ... }
1551 // ... // ^^^^^^^ (this is `arm_block`)
1555 // would yield a `arm_block` something like:
1558 // StorageLive(_4); // _4 is `x`
1559 // _4 = &mut (_1.0: i32); // this is handling `(mut x, 1)` case
1560 // _4 = &mut (_1.1: i32); // this is handling `(2, mut x)` case
1563 // and that is clearly not correct.
1564 let by_value_bindings = candidate.bindings.iter().filter(|binding| {
1565 if let BindingMode::ByValue = binding.binding_mode { true } else { false }
1567 // Read all of the by reference bindings to ensure that the
1568 // place they refer to can't be modified by the guard.
1569 for binding in by_value_bindings.clone() {
1570 let local_id = self.var_local_id(binding.var_id, RefWithinGuard);
1571 let place = Place::from(local_id);
1575 source_info: guard_end,
1576 kind: StatementKind::FakeRead(FakeReadCause::ForGuardBinding, place),
1580 self.bind_matched_candidate_for_arm_body(
1587 assert!(candidate.otherwise_block.is_none());
1588 // (Here, it is not too early to bind the matched
1589 // candidate on `block`, because there is no guard result
1590 // that we have to inspect before we bind them.)
1591 self.bind_matched_candidate_for_arm_body(block, &candidate.bindings);
1596 /// Append `AscribeUserType` statements onto the end of `block`
1597 /// for each ascription
1598 fn ascribe_types(&mut self, block: BasicBlock, ascriptions: &[Ascription<'tcx>]) {
1599 for ascription in ascriptions {
1600 let source_info = self.source_info(ascription.span);
1603 "adding user ascription at span {:?} of place {:?} and {:?}",
1609 let user_ty = box ascription.user_ty.clone().user_ty(
1610 &mut self.canonical_user_type_annotations,
1611 ascription.source.ty(&self.local_decls, self.hir.tcx()).ty,
1618 kind: StatementKind::AscribeUserType(
1619 ascription.source.clone(),
1620 ascription.variance,
1628 fn bind_matched_candidate_for_guard(
1631 bindings: &[Binding<'tcx>],
1633 debug!("bind_matched_candidate_for_guard(block={:?}, bindings={:?})", block, bindings);
1635 // Assign each of the bindings. Since we are binding for a
1636 // guard expression, this will never trigger moves out of the
1638 let re_erased = self.hir.tcx().lifetimes.re_erased;
1639 for binding in bindings {
1640 let source_info = self.source_info(binding.span);
1642 // For each pattern ident P of type T, `ref_for_guard` is
1643 // a reference R: &T pointing to the location matched by
1644 // the pattern, and every occurrence of P within a guard
1647 self.storage_live_binding(block, binding.var_id, binding.span, RefWithinGuard);
1648 match binding.binding_mode {
1649 BindingMode::ByValue => {
1650 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, binding.source.clone());
1652 .push_assign(block, source_info, &ref_for_guard, rvalue);
1654 BindingMode::ByRef(borrow_kind) => {
1655 let value_for_arm = self.storage_live_binding(
1662 let rvalue = Rvalue::Ref(re_erased, borrow_kind, binding.source.clone());
1664 .push_assign(block, source_info, &value_for_arm, rvalue);
1665 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, value_for_arm);
1667 .push_assign(block, source_info, &ref_for_guard, rvalue);
1673 fn bind_matched_candidate_for_arm_body<'b>(
1676 bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
1678 debug!("bind_matched_candidate_for_arm_body(block={:?})", block);
1680 let re_erased = self.hir.tcx().lifetimes.re_erased;
1681 // Assign each of the bindings. This may trigger moves out of the candidate.
1682 for binding in bindings {
1683 let source_info = self.source_info(binding.span);
1685 self.storage_live_binding(block, binding.var_id, binding.span, OutsideGuard);
1686 self.schedule_drop_for_binding(binding.var_id, binding.span, OutsideGuard);
1687 let rvalue = match binding.binding_mode {
1688 BindingMode::ByValue => {
1689 Rvalue::Use(self.consume_by_copy_or_move(binding.source.clone()))
1691 BindingMode::ByRef(borrow_kind) => {
1692 Rvalue::Ref(re_erased, borrow_kind, binding.source.clone())
1695 self.cfg.push_assign(block, source_info, &local, rvalue);
1699 /// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where the bound
1700 /// `var` has type `T` in the arm body) in a pattern maps to 2 locals. The
1701 /// first local is a binding for occurrences of `var` in the guard, which
1702 /// will have type `&T`. The second local is a binding for occurrences of
1703 /// `var` in the arm body, which will have type `T`.
1706 source_info: SourceInfo,
1707 visibility_scope: SourceScope,
1708 mutability: Mutability,
1713 user_ty: UserTypeProjections,
1714 has_guard: ArmHasGuard,
1715 opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
1719 "declare_binding(var_id={:?}, name={:?}, mode={:?}, var_ty={:?}, \
1720 visibility_scope={:?}, source_info={:?})",
1721 var_id, name, mode, var_ty, visibility_scope, source_info
1724 let tcx = self.hir.tcx();
1725 let binding_mode = match mode {
1726 BindingMode::ByValue => ty::BindingMode::BindByValue(mutability.into()),
1727 BindingMode::ByRef(_) => ty::BindingMode::BindByReference(mutability.into()),
1729 debug!("declare_binding: user_ty={:?}", user_ty);
1730 let local = LocalDecl::<'tcx> {
1738 is_block_tail: None,
1739 is_user_variable: Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
1741 // hypothetically, `visit_bindings` could try to unzip
1742 // an outermost hir::Ty as we descend, matching up
1743 // idents in pat; but complex w/ unclear UI payoff.
1744 // Instead, just abandon providing diagnostic info.
1750 let for_arm_body = self.local_decls.push(local);
1751 let locals = if has_guard.0 {
1752 let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
1753 // This variable isn't mutated but has a name, so has to be
1754 // immutable to avoid the unused mut lint.
1755 mutability: Mutability::Not,
1756 ty: tcx.mk_imm_ref(tcx.lifetimes.re_erased, var_ty),
1757 user_ty: UserTypeProjections::none(),
1762 is_block_tail: None,
1763 is_user_variable: Some(ClearCrossCrate::Set(BindingForm::RefForGuard)),
1765 LocalsForNode::ForGuard {
1770 LocalsForNode::One(for_arm_body)
1772 debug!("declare_binding: vars={:?}", locals);
1773 self.var_indices.insert(var_id, locals);