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, 'gcx, 'tcx> Builder<'a, 'gcx, '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 // There's one more pre_binding block than there are candidates so that
151 // every candidate can have a `next_candidate_pre_binding_block`.
152 let outer_source_info = self.source_info(span);
154 *pre_binding_blocks.last().unwrap(),
156 TerminatorKind::Unreachable,
159 let mut match_has_guard = false;
161 let mut candidate_pre_binding_blocks = pre_binding_blocks.iter();
162 let mut next_candidate_pre_binding_blocks = pre_binding_blocks.iter().skip(1);
164 // Assemble a list of candidates: there is one candidate per pattern,
165 // which means there may be more than one candidate *per arm*.
166 let mut arm_candidates: Vec<_> = arms
169 let arm_has_guard = arm.guard.is_some();
170 match_has_guard |= arm_has_guard;
171 let arm_candidates: Vec<_> = arm.patterns
173 .zip(candidate_pre_binding_blocks.by_ref())
174 .zip(next_candidate_pre_binding_blocks.by_ref())
176 |((pattern, pre_binding_block), next_candidate_pre_binding_block)| {
180 MatchPair::new(scrutinee_place.clone(), pattern),
184 otherwise_block: if arm_has_guard {
185 Some(self.cfg.start_new_block())
189 pre_binding_block: *pre_binding_block,
190 next_candidate_pre_binding_block:
191 *next_candidate_pre_binding_block,
196 (arm, arm_candidates)
200 // Step 3. Create the decision tree and record the places that we bind or test.
202 // The set of places that we are creating fake borrows of. If there are
203 // no match guards then we don't need any fake borrows, so don't track
205 let mut fake_borrows = if match_has_guard && tcx.generate_borrow_of_any_match_input() {
206 Some(FxHashSet::default())
211 // These candidates are kept sorted such that the highest priority
212 // candidate comes first in the list. (i.e., same order as in source)
213 // As we gnerate the decision tree,
214 let candidates = &mut arm_candidates
216 .flat_map(|(_, candidates)| candidates)
217 .collect::<Vec<_>>();
219 // this will generate code to test scrutinee_place and
220 // branch to the appropriate arm block
221 let otherwise = self.match_candidates(
228 if !otherwise.is_empty() {
229 // All matches are exhaustive. However, because some matches
230 // only have exponentially-large exhaustive decision trees, we
231 // sometimes generate an inexhaustive decision tree.
233 // In that case, the inexhaustive tips of the decision tree
234 // can't be reached - terminate them with an `unreachable`.
235 let mut otherwise = otherwise;
237 otherwise.dedup(); // variant switches can introduce duplicate target blocks
238 for block in otherwise {
240 .terminate(block, outer_source_info, TerminatorKind::Unreachable);
244 // Step 4. Determine the fake borrows that are needed from the above
245 // places. Create the required temporaries for them.
247 let fake_borrow_temps = if let Some(ref borrows) = fake_borrows {
248 self.calculate_fake_borrows(borrows, scrutinee_span)
253 // Step 5. Create everything else: the guards and the arms.
255 let arm_end_blocks: Vec<_> = arm_candidates.into_iter().map(|(arm, candidates)| {
256 let arm_source_info = self.source_info(arm.span);
257 let region_scope = (arm.scope, arm_source_info);
258 self.in_scope(region_scope, arm.lint_level, |this| {
259 let arm_block = this.cfg.start_new_block();
261 let body = this.hir.mirror(arm.body.clone());
262 let scope = this.declare_bindings(
266 ArmHasGuard(arm.guard.is_some()),
267 Some((Some(&scrutinee_place), scrutinee_span)),
270 if let Some(source_scope) = scope {
271 this.source_scope = source_scope;
274 for candidate in candidates {
275 this.clear_top_scope(arm.scope);
276 this.bind_and_guard_matched_candidate(
286 this.into(destination, arm_block, body)
290 // all the arm blocks will rejoin here
291 let end_block = self.cfg.start_new_block();
293 for arm_block in arm_end_blocks {
297 TerminatorKind::Goto { target: end_block },
301 self.source_scope = outer_source_info.scope;
306 pub(super) fn expr_into_pattern(
308 mut block: BasicBlock,
309 irrefutable_pat: Pattern<'tcx>,
310 initializer: ExprRef<'tcx>,
312 match *irrefutable_pat.kind {
313 // Optimize the case of `let x = ...` to write directly into `x`
314 PatternKind::Binding {
315 mode: BindingMode::ByValue,
321 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard);
322 unpack!(block = self.into(&place, block, initializer));
325 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
326 let source_info = self.source_info(irrefutable_pat.span);
331 kind: StatementKind::FakeRead(FakeReadCause::ForLet, place),
335 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
339 // Optimize the case of `let x: T = ...` to write directly
340 // into `x` and then require that `T == typeof(x)`.
342 // Weirdly, this is needed to prevent the
343 // `intrinsic-move-val.rs` test case from crashing. That
344 // test works with uninitialized values in a rather
345 // dubious way, so it may be that the test is kind of
347 PatternKind::AscribeUserType {
348 subpattern: Pattern {
349 kind: box PatternKind::Binding {
350 mode: BindingMode::ByValue,
357 ascription: hair::pattern::Ascription {
358 user_ty: pat_ascription_ty,
364 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard);
365 unpack!(block = self.into(&place, block, initializer));
367 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
368 let pattern_source_info = self.source_info(irrefutable_pat.span);
372 source_info: pattern_source_info,
373 kind: StatementKind::FakeRead(FakeReadCause::ForLet, place.clone()),
377 let ty_source_info = self.source_info(user_ty_span);
378 let user_ty = box pat_ascription_ty.user_ty(
379 &mut self.canonical_user_type_annotations,
380 place.ty(&self.local_decls, self.hir.tcx()).ty,
386 source_info: ty_source_info,
387 kind: StatementKind::AscribeUserType(
389 // We always use invariant as the variance here. This is because the
390 // variance field from the ascription refers to the variance to use
391 // when applying the type to the value being matched, but this
392 // ascription applies rather to the type of the binding. e.g., in this
399 // We are creating an ascription that defines the type of `x` to be
400 // exactly `T` (i.e., with invariance). The variance field, in
401 // contrast, is intended to be used to relate `T` to the type of
403 ty::Variance::Invariant,
409 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
414 let place = unpack!(block = self.as_place(block, initializer));
415 self.place_into_pattern(block, irrefutable_pat, &place, true)
420 pub fn place_into_pattern(
423 irrefutable_pat: Pattern<'tcx>,
424 initializer: &Place<'tcx>,
425 set_match_place: bool,
427 // create a dummy candidate
428 let mut candidate = Candidate {
429 span: irrefutable_pat.span,
430 match_pairs: vec![MatchPair::new(initializer.clone(), &irrefutable_pat)],
434 // since we don't call `match_candidates`, next fields are unused
435 otherwise_block: None,
436 pre_binding_block: block,
437 next_candidate_pre_binding_block: block,
440 // Simplify the candidate. Since the pattern is irrefutable, this should
441 // always convert all match-pairs into bindings.
442 self.simplify_candidate(&mut candidate);
444 if !candidate.match_pairs.is_empty() {
445 // ICE if no other errors have been emitted. This used to be a hard error that wouldn't
446 // be reached because `hair::pattern::check_match::check_match` wouldn't have let the
447 // compiler continue. In our tests this is only ever hit by
448 // `ui/consts/const-match-check.rs` with `--cfg eval1`, and that file already generates
449 // a different error before hand.
450 self.hir.tcx().sess.delay_span_bug(
451 candidate.match_pairs[0].pattern.span,
453 "match pairs {:?} remaining after simplifying irrefutable pattern",
454 candidate.match_pairs,
459 // for matches and function arguments, the place that is being matched
460 // can be set when creating the variables. But the place for
461 // let PATTERN = ... might not even exist until we do the assignment.
462 // so we set it here instead
464 for binding in &candidate.bindings {
465 let local = self.var_local_id(binding.var_id, OutsideGuard);
467 if let Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
468 opt_match_place: Some((ref mut match_place, _)),
470 }))) = self.local_decls[local].is_user_variable
472 *match_place = Some(initializer.clone());
474 bug!("Let binding to non-user variable.")
479 self.ascribe_types(block, &candidate.ascriptions);
481 // now apply the bindings, which will also declare the variables
482 self.bind_matched_candidate_for_arm_body(block, &candidate.bindings);
487 /// Declares the bindings of the given patterns and returns the visibility
488 /// scope for the bindings in these patterns, if such a scope had to be
489 /// created. NOTE: Declaring the bindings should always be done in their
491 pub fn declare_bindings(
493 mut visibility_scope: Option<SourceScope>,
495 pattern: &Pattern<'tcx>,
496 has_guard: ArmHasGuard,
497 opt_match_place: Option<(Option<&Place<'tcx>>, Span)>,
498 ) -> Option<SourceScope> {
499 debug!("declare_bindings: pattern={:?}", pattern);
502 UserTypeProjections::none(),
503 &mut |this, mutability, name, mode, var, span, ty, user_ty| {
504 if visibility_scope.is_none() {
506 Some(this.new_source_scope(scope_span, LintLevel::Inherited, None));
508 let source_info = SourceInfo { span, scope: this.source_scope };
509 let visibility_scope = visibility_scope.unwrap();
510 this.declare_binding(
520 opt_match_place.map(|(x, y)| (x.cloned(), y)),
528 pub fn storage_live_binding(
535 let local_id = self.var_local_id(var, for_guard);
536 let source_info = self.source_info(span);
541 kind: StatementKind::StorageLive(local_id),
544 let place = Place::Base(PlaceBase::Local(local_id));
545 let var_ty = self.local_decls[local_id].ty;
546 let region_scope = self.hir.region_scope_tree.var_scope(var.local_id);
547 self.schedule_drop(span, region_scope, &place, var_ty, DropKind::Storage);
551 pub fn schedule_drop_for_binding(&mut self, var: HirId, span: Span, for_guard: ForGuard) {
552 let local_id = self.var_local_id(var, for_guard);
553 let var_ty = self.local_decls[local_id].ty;
554 let region_scope = self.hir.region_scope_tree.var_scope(var.local_id);
558 &Place::Base(PlaceBase::Local(local_id)),
564 pub(super) fn visit_bindings(
566 pattern: &Pattern<'tcx>,
567 pattern_user_ty: UserTypeProjections,
579 debug!("visit_bindings: pattern={:?} pattern_user_ty={:?}", pattern, pattern_user_ty);
580 match *pattern.kind {
581 PatternKind::Binding {
590 f(self, mutability, name, mode, var, pattern.span, ty, pattern_user_ty.clone());
591 if let Some(subpattern) = subpattern.as_ref() {
592 self.visit_bindings(subpattern, pattern_user_ty, f);
601 | PatternKind::Slice {
606 let from = u32::try_from(prefix.len()).unwrap();
607 let to = u32::try_from(suffix.len()).unwrap();
608 for subpattern in prefix {
609 self.visit_bindings(subpattern, pattern_user_ty.clone().index(), f);
611 for subpattern in slice {
612 self.visit_bindings(subpattern, pattern_user_ty.clone().subslice(from, to), f);
614 for subpattern in suffix {
615 self.visit_bindings(subpattern, pattern_user_ty.clone().index(), f);
619 PatternKind::Constant { .. } | PatternKind::Range { .. } | PatternKind::Wild => {}
621 PatternKind::Deref { ref subpattern } => {
622 self.visit_bindings(subpattern, pattern_user_ty.deref(), f);
625 PatternKind::AscribeUserType {
627 ascription: hair::pattern::Ascription {
633 // This corresponds to something like
636 // let A::<'a>(_): A<'static> = ...;
639 // Note that the variance doesn't apply here, as we are tracking the effect
640 // of `user_ty` on any bindings contained with subpattern.
641 let annotation = CanonicalUserTypeAnnotation {
643 user_ty: user_ty.user_ty,
644 inferred_ty: subpattern.ty,
646 let projection = UserTypeProjection {
647 base: self.canonical_user_type_annotations.push(annotation),
650 let subpattern_user_ty = pattern_user_ty.push_projection(&projection, user_ty_span);
651 self.visit_bindings(subpattern, subpattern_user_ty, f)
654 PatternKind::Leaf { ref subpatterns } => {
655 for subpattern in subpatterns {
656 let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field);
657 debug!("visit_bindings: subpattern_user_ty={:?}", subpattern_user_ty);
658 self.visit_bindings(&subpattern.pattern, subpattern_user_ty, f);
662 PatternKind::Variant { adt_def, substs: _, variant_index, ref subpatterns } => {
663 for subpattern in subpatterns {
664 let subpattern_user_ty = pattern_user_ty.clone().variant(
665 adt_def, variant_index, subpattern.field);
666 self.visit_bindings(&subpattern.pattern, subpattern_user_ty, f);
674 pub struct Candidate<'pat, 'tcx: 'pat> {
675 // span of the original pattern that gave rise to this candidate
678 // all of these must be satisfied...
679 match_pairs: Vec<MatchPair<'pat, 'tcx>>,
681 // ...these bindings established...
682 bindings: Vec<Binding<'tcx>>,
684 // ...and these types asserted...
685 ascriptions: Vec<Ascription<'tcx>>,
687 // ...and the guard must be evaluated, if false branch to Block...
688 otherwise_block: Option<BasicBlock>,
690 // ...and the blocks for add false edges between candidates
691 pre_binding_block: BasicBlock,
692 next_candidate_pre_binding_block: BasicBlock,
695 #[derive(Clone, Debug)]
696 struct Binding<'tcx> {
702 mutability: Mutability,
703 binding_mode: BindingMode,
706 /// Indicates that the type of `source` must be a subtype of the
707 /// user-given type `user_ty`; this is basically a no-op but can
708 /// influence region inference.
709 #[derive(Clone, Debug)]
710 struct Ascription<'tcx> {
713 user_ty: PatternTypeProjection<'tcx>,
714 variance: ty::Variance,
717 #[derive(Clone, Debug)]
718 pub struct MatchPair<'pat, 'tcx: 'pat> {
722 // ... must match this pattern.
723 pattern: &'pat Pattern<'tcx>,
726 #[derive(Clone, Debug, PartialEq)]
727 enum TestKind<'tcx> {
728 // test the branches of enum
730 adt_def: &'tcx ty::AdtDef,
731 variants: BitSet<VariantIdx>,
734 // test the branches of enum
738 indices: FxHashMap<&'tcx ty::Const<'tcx>, usize>,
743 value: &'tcx ty::Const<'tcx>,
747 // test whether the value falls within an inclusive or exclusive range
748 Range(PatternRange<'tcx>),
750 // test length of the slice is equal to len
758 pub struct Test<'tcx> {
760 kind: TestKind<'tcx>,
763 /// ArmHasGuard is isomorphic to a boolean flag. It indicates whether
764 /// a match arm has a guard expression attached to it.
765 #[derive(Copy, Clone, Debug)]
766 pub(crate) struct ArmHasGuard(pub bool);
768 ///////////////////////////////////////////////////////////////////////////
769 // Main matching algorithm
771 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
772 /// The main match algorithm. It begins with a set of candidates
773 /// `candidates` and has the job of generating code to determine
774 /// which of these candidates, if any, is the correct one. The
775 /// candidates are sorted such that the first item in the list
776 /// has the highest priority. When a candidate is found to match
777 /// the value, we will generate a branch to the appropriate
778 /// prebinding block.
780 /// The return value is a list of "otherwise" blocks. These are
781 /// points in execution where we found that *NONE* of the
782 /// candidates apply. In principle, this means that the input
783 /// list was not exhaustive, though at present we sometimes are
784 /// not smart enough to recognize all exhaustive inputs.
786 /// It might be surprising that the input can be inexhaustive.
787 /// Indeed, initially, it is not, because all matches are
788 /// exhaustive in Rust. But during processing we sometimes divide
789 /// up the list of candidates and recurse with a non-exhaustive
790 /// list. This is important to keep the size of the generated code
791 /// under control. See `test_candidates` for more details.
793 /// If `fake_borrows` is Some, then places which need fake borrows
794 /// will be added to it.
795 fn match_candidates<'pat>(
798 candidates: &mut [&mut Candidate<'pat, 'tcx>],
799 mut block: BasicBlock,
800 fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>,
801 ) -> Vec<BasicBlock> {
803 "matched_candidate(span={:?}, block={:?}, candidates={:?})",
804 span, block, candidates
807 // Start by simplifying candidates. Once this process is complete, all
808 // the match pairs which remain require some form of test, whether it
809 // be a switch or pattern comparison.
810 for candidate in &mut *candidates {
811 self.simplify_candidate(candidate);
814 // The candidates are sorted by priority. Check to see whether the
815 // higher priority candidates (and hence at the front of the slice)
816 // have satisfied all their match pairs.
817 let fully_matched = candidates
819 .take_while(|c| c.match_pairs.is_empty())
822 "match_candidates: {:?} candidates fully matched",
825 let (matched_candidates, unmatched_candidates) = candidates.split_at_mut(fully_matched);
827 if !matched_candidates.is_empty() {
828 block = if let Some(last_otherwise_block) = self.select_matched_candidates(
835 // Any remaining candidates are unreachable.
836 if unmatched_candidates.is_empty() {
839 self.cfg.start_new_block()
844 // If there are no candidates that still need testing, we're
845 // done. Since all matches are exhaustive, execution should
846 // never reach this point.
847 if unmatched_candidates.is_empty() {
851 // Test candidates where possible.
852 let (otherwise, untested_candidates) = self.test_candidates(
854 unmatched_candidates,
859 // If the target candidates were exhaustive, then we are done.
860 // But for borrowck continue build decision tree.
861 if untested_candidates.is_empty() {
865 // Otherwise, let's process those remaining candidates.
866 let join_block = self.join_otherwise_blocks(span, otherwise);
867 self.match_candidates(
875 /// Link up matched candidates. For example, if we have something like
879 /// Some(x) if cond => ...
881 /// Some(x) if cond => ...
884 /// We generate real edges from:
885 /// * `block` to the prebinding_block of the first pattern,
886 /// * the otherwise block of the first pattern to the second pattern,
887 /// * the otherwise block of the third pattern to the a block with an
888 /// Unreachable terminator.
890 /// As well as that we add fake edges from the otherwise blocks to the
891 /// prebinding block of the next candidate in the original set of
893 fn select_matched_candidates(
895 matched_candidates: &mut [&mut Candidate<'_, 'tcx>],
897 fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>,
898 ) -> Option<BasicBlock> {
900 !matched_candidates.is_empty(),
901 "select_matched_candidates called with no candidates",
904 // Insert a borrows of prefixes of places that are bound and are
905 // behind a dereference projection.
907 // These borrows are taken to avoid situations like the following:
910 // _ if { x = &[0]; false } => (),
911 // y => (), // Out of bounds array access!
915 // // y is bound by reference in the guard and then by copy in the
916 // // arm, so y is 2 in the arm!
917 // y if { y == 1 && (x = &2) == () } => y,
920 if let Some(fake_borrows) = fake_borrows {
921 for Binding { source, .. }
922 in matched_candidates.iter().flat_map(|candidate| &candidate.bindings)
924 let mut cursor = source;
925 while let Place::Projection(box Projection { base, elem }) = cursor {
927 if let ProjectionElem::Deref = elem {
928 fake_borrows.insert(cursor.clone());
935 let fully_matched_with_guard = matched_candidates
937 .position(|c| c.otherwise_block.is_none())
938 .unwrap_or(matched_candidates.len() - 1);
940 let (reachable_candidates, unreachable_candidates)
941 = matched_candidates.split_at_mut(fully_matched_with_guard + 1);
943 let first_candidate = &reachable_candidates[0];
945 let candidate_source_info = self.source_info(first_candidate.span);
949 candidate_source_info,
950 TerminatorKind::Goto {
951 target: first_candidate.pre_binding_block,
955 for window in reachable_candidates.windows(2) {
956 if let [first_candidate, second_candidate] = window {
957 let source_info = self.source_info(first_candidate.span);
958 if let Some(otherwise_block) = first_candidate.otherwise_block {
962 TerminatorKind::FalseEdges {
963 real_target: second_candidate.pre_binding_block,
964 imaginary_targets: vec![
965 first_candidate.next_candidate_pre_binding_block
970 bug!("candidate other than the last has no guard");
973 bug!("<[_]>::windows returned incorrectly sized window");
977 debug!("match_candidates: add false edges for unreachable {:?}", unreachable_candidates);
978 for candidate in unreachable_candidates {
979 if let Some(otherwise) = candidate.otherwise_block {
980 let source_info = self.source_info(candidate.span);
981 let unreachable = self.cfg.start_new_block();
985 TerminatorKind::FalseEdges {
986 real_target: unreachable,
987 imaginary_targets: vec![candidate.next_candidate_pre_binding_block],
990 self.cfg.terminate(unreachable, source_info, TerminatorKind::Unreachable);
994 let last_candidate = reachable_candidates.last().unwrap();
996 if let Some(otherwise) = last_candidate.otherwise_block {
997 let source_info = self.source_info(last_candidate.span);
998 let block = self.cfg.start_new_block();
1002 TerminatorKind::FalseEdges {
1004 imaginary_targets: vec![last_candidate.next_candidate_pre_binding_block]
1013 fn join_otherwise_blocks(&mut self, span: Span, mut otherwise: Vec<BasicBlock>) -> BasicBlock {
1014 let source_info = self.source_info(span);
1016 otherwise.dedup(); // variant switches can introduce duplicate target blocks
1017 if otherwise.len() == 1 {
1020 let join_block = self.cfg.start_new_block();
1021 for block in otherwise {
1025 TerminatorKind::Goto { target: join_block },
1032 /// This is the most subtle part of the matching algorithm. At
1033 /// this point, the input candidates have been fully simplified,
1034 /// and so we know that all remaining match-pairs require some
1035 /// sort of test. To decide what test to do, we take the highest
1036 /// priority candidate (last one in the list) and extract the
1037 /// first match-pair from the list. From this we decide what kind
1038 /// of test is needed using `test`, defined in the `test` module.
1040 /// *Note:* taking the first match pair is somewhat arbitrary, and
1041 /// we might do better here by choosing more carefully what to
1044 /// For example, consider the following possible match-pairs:
1046 /// 1. `x @ Some(P)` -- we will do a `Switch` to decide what variant `x` has
1047 /// 2. `x @ 22` -- we will do a `SwitchInt`
1048 /// 3. `x @ 3..5` -- we will do a range test
1051 /// Once we know what sort of test we are going to perform, this
1052 /// Tests may also help us with other candidates. So we walk over
1053 /// the candidates (from high to low priority) and check. This
1054 /// gives us, for each outcome of the test, a transformed list of
1055 /// candidates. For example, if we are testing the current
1056 /// variant of `x.0`, and we have a candidate `{x.0 @ Some(v), x.1
1057 /// @ 22}`, then we would have a resulting candidate of `{(x.0 as
1058 /// Some).0 @ v, x.1 @ 22}`. Note that the first match-pair is now
1059 /// simpler (and, in fact, irrefutable).
1061 /// But there may also be candidates that the test just doesn't
1062 /// apply to. The classical example involves wildcards:
1065 /// # let (x, y, z) = (true, true, true);
1066 /// match (x, y, z) {
1067 /// (true, _, true) => true, // (0)
1068 /// (_, true, _) => true, // (1)
1069 /// (false, false, _) => false, // (2)
1070 /// (true, _, false) => false, // (3)
1074 /// In that case, after we test on `x`, there are 2 overlapping candidate
1077 /// - If the outcome is that `x` is true, candidates 0, 1, and 3
1078 /// - If the outcome is that `x` is false, candidates 1 and 2
1080 /// Here, the traditional "decision tree" method would generate 2
1081 /// separate code-paths for the 2 separate cases.
1083 /// In some cases, this duplication can create an exponential amount of
1084 /// code. This is most easily seen by noticing that this method terminates
1085 /// with precisely the reachable arms being reachable - but that problem
1086 /// is trivially NP-complete:
1089 /// match (var0, var1, var2, var3, ..) {
1090 /// (true, _, _, false, true, ...) => false,
1091 /// (_, true, true, false, _, ...) => false,
1092 /// (false, _, false, false, _, ...) => false,
1098 /// Here the last arm is reachable only if there is an assignment to
1099 /// the variables that does not match any of the literals. Therefore,
1100 /// compilation would take an exponential amount of time in some cases.
1102 /// That kind of exponential worst-case might not occur in practice, but
1103 /// our simplistic treatment of constants and guards would make it occur
1104 /// in very common situations - for example #29740:
1108 /// "foo" if foo_guard => ...,
1109 /// "bar" if bar_guard => ...,
1110 /// "baz" if baz_guard => ...,
1115 /// Here we first test the match-pair `x @ "foo"`, which is an `Eq` test.
1117 /// It might seem that we would end up with 2 disjoint candidate
1118 /// sets, consisting of the first candidate or the other 3, but our
1119 /// algorithm doesn't reason about "foo" being distinct from the other
1120 /// constants; it considers the latter arms to potentially match after
1121 /// both outcomes, which obviously leads to an exponential amount
1124 /// To avoid these kinds of problems, our algorithm tries to ensure
1125 /// the amount of generated tests is linear. When we do a k-way test,
1126 /// we return an additional "unmatched" set alongside the obvious `k`
1127 /// sets. When we encounter a candidate that would be present in more
1128 /// than one of the sets, we put it and all candidates below it into the
1129 /// "unmatched" set. This ensures these `k+1` sets are disjoint.
1131 /// After we perform our test, we branch into the appropriate candidate
1132 /// set and recurse with `match_candidates`. These sub-matches are
1133 /// obviously inexhaustive - as we discarded our otherwise set - so
1134 /// we set their continuation to do `match_candidates` on the
1135 /// "unmatched" set (which is again inexhaustive).
1137 /// If you apply this to the above test, you basically wind up
1138 /// with an if-else-if chain, testing each candidate in turn,
1139 /// which is precisely what we want.
1141 /// In addition to avoiding exponential-time blowups, this algorithm
1142 /// also has nice property that each guard and arm is only generated
1144 fn test_candidates<'pat, 'b, 'c>(
1147 mut candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>],
1149 fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>,
1150 ) -> (Vec<BasicBlock>, &'b mut [&'c mut Candidate<'pat, 'tcx>]) {
1151 // extract the match-pair from the highest priority candidate
1152 let match_pair = &candidates.first().unwrap().match_pairs[0];
1153 let mut test = self.test(match_pair);
1154 let match_place = match_pair.place.clone();
1156 // most of the time, the test to perform is simply a function
1157 // of the main candidate; but for a test like SwitchInt, we
1158 // may want to add cases based on the candidates that are
1161 TestKind::SwitchInt {
1166 for candidate in candidates.iter() {
1167 if !self.add_cases_to_switch(
1182 for candidate in candidates.iter() {
1183 if !self.add_variants_to_switch(&match_place, candidate, variants) {
1191 // Insert a Shallow borrow of any places that is switched on.
1192 fake_borrows.as_mut().map(|fb| {
1193 fb.insert(match_place.clone())
1196 // perform the test, branching to one of N blocks. For each of
1197 // those N possible outcomes, create a (initially empty)
1198 // vector of candidates. Those are the candidates that still
1199 // apply if the test has that particular outcome.
1201 "match_candidates: test={:?} match_pair={:?}",
1204 let target_blocks = self.perform_test(block, &match_place, &test);
1205 let mut target_candidates: Vec<Vec<&mut Candidate<'pat, 'tcx>>> = vec![];
1206 target_candidates.resize_with(target_blocks.len(), Default::default);
1208 let total_candidate_count = candidates.len();
1210 // Sort the candidates into the appropriate vector in
1211 // `target_candidates`. Note that at some point we may
1212 // encounter a candidate where the test is not relevant; at
1213 // that point, we stop sorting.
1214 while let Some(candidate) = candidates.first_mut() {
1215 if let Some(idx) = self.sort_candidate(&match_place, &test, candidate) {
1216 let (candidate, rest) = candidates.split_first_mut().unwrap();
1217 target_candidates[idx].push(candidate);
1223 // at least the first candidate ought to be tested
1224 assert!(total_candidate_count > candidates.len());
1225 debug!("tested_candidates: {}", total_candidate_count - candidates.len());
1226 debug!("untested_candidates: {}", candidates.len());
1228 // For each outcome of test, process the candidates that still
1229 // apply. Collect a list of blocks where control flow will
1230 // branch if one of the `target_candidate` sets is not
1232 let otherwise: Vec<_> = target_blocks
1234 .zip(target_candidates)
1235 .flat_map(|(target_block, mut target_candidates)| {
1236 self.match_candidates(
1238 &mut *target_candidates,
1245 (otherwise, candidates)
1248 // Determine the fake borrows that are needed to ensure that the place
1249 // will evaluate to the same thing until an arm has been chosen.
1250 fn calculate_fake_borrows<'b>(
1252 fake_borrows: &'b FxHashSet<Place<'tcx>>,
1254 ) -> Vec<(&'b Place<'tcx>, Local)> {
1255 let tcx = self.hir.tcx();
1257 debug!("add_fake_borrows fake_borrows = {:?}", fake_borrows);
1259 let mut all_fake_borrows = Vec::with_capacity(fake_borrows.len());
1261 // Insert a Shallow borrow of the prefixes of any fake borrows.
1262 for place in fake_borrows
1264 let mut prefix_cursor = place;
1265 while let Place::Projection(box Projection { base, elem }) = prefix_cursor {
1266 if let ProjectionElem::Deref = elem {
1267 // Insert a shallow borrow after a deref. For other
1268 // projections the borrow of prefix_cursor will
1269 // conflict with any mutation of base.
1270 all_fake_borrows.push(base);
1272 prefix_cursor = base;
1275 all_fake_borrows.push(place);
1278 // Deduplicate and ensure a deterministic order.
1279 all_fake_borrows.sort();
1280 all_fake_borrows.dedup();
1282 debug!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows);
1284 all_fake_borrows.into_iter().map(|matched_place| {
1285 let fake_borrow_deref_ty = matched_place.ty(&self.local_decls, tcx).ty;
1286 let fake_borrow_ty = tcx.mk_imm_ref(tcx.lifetimes.re_erased, fake_borrow_deref_ty);
1287 let fake_borrow_temp = self.local_decls.push(
1288 LocalDecl::new_temp(fake_borrow_ty, temp_span)
1291 (matched_place, fake_borrow_temp)
1296 ///////////////////////////////////////////////////////////////////////////
1297 // Pattern binding - used for `let` and function parameters as well.
1299 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
1300 /// Initializes each of the bindings from the candidate by
1301 /// moving/copying/ref'ing the source as appropriate. Tests the guard, if
1302 /// any, and then branches to the arm. Returns the block for the case where
1303 /// the guard fails.
1305 /// Note: we check earlier that if there is a guard, there cannot be move
1306 /// bindings (unless feature(bind_by_move_pattern_guards) is used). This
1307 /// isn't really important for the self-consistency of this fn, but the
1308 /// reason for it should be clear: after we've done the assignments, if
1309 /// there were move bindings, further tests would be a use-after-move.
1310 /// bind_by_move_pattern_guards avoids this by only moving the binding once
1311 /// the guard has evaluated to true (see below).
1312 fn bind_and_guard_matched_candidate<'pat>(
1314 candidate: Candidate<'pat, 'tcx>,
1315 guard: Option<Guard<'tcx>>,
1316 arm_block: BasicBlock,
1317 fake_borrows: &Vec<(&Place<'tcx>, Local)>,
1318 scrutinee_span: Span,
1319 region_scope: (region::Scope, SourceInfo),
1321 debug!("bind_and_guard_matched_candidate(candidate={:?})", candidate);
1323 debug_assert!(candidate.match_pairs.is_empty());
1325 let candidate_source_info = self.source_info(candidate.span);
1327 let mut block = self.cfg.start_new_block();
1329 candidate.pre_binding_block,
1330 candidate_source_info,
1331 TerminatorKind::FalseEdges {
1333 imaginary_targets: vec![candidate.next_candidate_pre_binding_block],
1336 self.ascribe_types(block, &candidate.ascriptions);
1338 // rust-lang/rust#27282: The `autoref` business deserves some
1339 // explanation here.
1341 // The intent of the `autoref` flag is that when it is true,
1342 // then any pattern bindings of type T will map to a `&T`
1343 // within the context of the guard expression, but will
1344 // continue to map to a `T` in the context of the arm body. To
1345 // avoid surfacing this distinction in the user source code
1346 // (which would be a severe change to the language and require
1347 // far more revision to the compiler), when `autoref` is true,
1348 // then any occurrence of the identifier in the guard
1349 // expression will automatically get a deref op applied to it.
1351 // So an input like:
1354 // let place = Foo::new();
1355 // match place { foo if inspect(foo)
1356 // => feed(foo), ... }
1359 // will be treated as if it were really something like:
1362 // let place = Foo::new();
1363 // match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) }
1364 // => { let tmp2 = place; feed(tmp2) }, ... }
1366 // And an input like:
1369 // let place = Foo::new();
1370 // match place { ref mut foo if inspect(foo)
1371 // => feed(foo), ... }
1374 // will be treated as if it were really something like:
1377 // let place = Foo::new();
1378 // match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) }
1379 // => { let tmp2 = &mut place; feed(tmp2) }, ... }
1382 // In short, any pattern binding will always look like *some*
1383 // kind of `&T` within the guard at least in terms of how the
1384 // MIR-borrowck views it, and this will ensure that guard
1385 // expressions cannot mutate their the match inputs via such
1386 // bindings. (It also ensures that guard expressions can at
1387 // most *copy* values from such bindings; non-Copy things
1388 // cannot be moved via pattern bindings in guard expressions.)
1392 // Implementation notes (under assumption `autoref` is true).
1394 // To encode the distinction above, we must inject the
1395 // temporaries `tmp1` and `tmp2`.
1397 // There are two cases of interest: binding by-value, and binding by-ref.
1399 // 1. Binding by-value: Things are simple.
1401 // * Establishing `tmp1` creates a reference into the
1402 // matched place. This code is emitted by
1403 // bind_matched_candidate_for_guard.
1405 // * `tmp2` is only initialized "lazily", after we have
1406 // checked the guard. Thus, the code that can trigger
1407 // moves out of the candidate can only fire after the
1408 // guard evaluated to true. This initialization code is
1409 // emitted by bind_matched_candidate_for_arm.
1411 // 2. Binding by-reference: Things are tricky.
1413 // * Here, the guard expression wants a `&&` or `&&mut`
1414 // into the original input. This means we need to borrow
1415 // the reference that we create for the arm.
1416 // * So we eagerly create the reference for the arm and then take a
1417 // reference to that.
1418 if let Some(guard) = guard {
1419 let tcx = self.hir.tcx();
1421 self.bind_matched_candidate_for_guard(
1423 &candidate.bindings,
1425 let guard_frame = GuardFrame {
1429 .map(|b| GuardFrameLocal::new(b.var_id, b.binding_mode))
1432 debug!("Entering guard building context: {:?}", guard_frame);
1433 self.guard_context.push(guard_frame);
1435 let re_erased = tcx.lifetimes.re_erased;
1436 let scrutinee_source_info = self.source_info(scrutinee_span);
1437 for &(place, temp) in fake_borrows {
1438 let borrow = Rvalue::Ref(
1440 BorrowKind::Shallow,
1443 self.cfg.push_assign(
1445 scrutinee_source_info,
1446 &Place::Base(PlaceBase::Local(temp)),
1451 // the block to branch to if the guard fails; if there is no
1452 // guard, this block is simply unreachable
1453 let guard = match guard {
1454 Guard::If(e) => self.hir.mirror(e),
1456 let source_info = self.source_info(guard.span);
1457 let guard_end = self.source_info(tcx.sess.source_map().end_point(guard.span));
1458 let cond = unpack!(block = self.as_local_operand(block, guard));
1459 let guard_frame = self.guard_context.pop().unwrap();
1461 "Exiting guard building context with locals: {:?}",
1465 for &(_, temp) in fake_borrows {
1466 self.cfg.push(block, Statement {
1467 source_info: guard_end,
1468 kind: StatementKind::FakeRead(
1469 FakeReadCause::ForMatchGuard,
1470 Place::Base(PlaceBase::Local(temp)),
1475 // We want to ensure that the matched candidates are bound
1476 // after we have confirmed this candidate *and* any
1477 // associated guard; Binding them on `block` is too soon,
1478 // because that would be before we've checked the result
1481 // But binding them on `arm_block` is *too late*, because
1482 // then all of the candidates for a single arm would be
1483 // bound in the same place, that would cause a case like:
1487 // (mut x, 1) | (2, mut x) if { true } => { ... }
1488 // ... // ^^^^^^^ (this is `arm_block`)
1492 // would yield a `arm_block` something like:
1495 // StorageLive(_4); // _4 is `x`
1496 // _4 = &mut (_1.0: i32); // this is handling `(mut x, 1)` case
1497 // _4 = &mut (_1.1: i32); // this is handling `(2, mut x)` case
1500 // and that is clearly not correct.
1501 let post_guard_block = self.cfg.start_new_block();
1502 let otherwise_post_guard_block = self.cfg.start_new_block();
1506 TerminatorKind::if_(
1510 otherwise_post_guard_block,
1517 otherwise_post_guard_block,
1518 candidate.otherwise_block.unwrap(),
1521 if let Operand::Copy(cond_place) | Operand::Move(cond_place) = cond {
1522 if let Place::Base(PlaceBase::Local(cond_temp)) = cond_place {
1523 // We will call `clear_top_scope` if there's another guard. So
1524 // we have to drop this variable now or it will be "storage
1532 bug!("Expected as_local_operand to produce a temporary");
1536 let by_value_bindings = candidate.bindings.iter().filter(|binding| {
1537 if let BindingMode::ByValue = binding.binding_mode { true } else { false }
1539 // Read all of the by reference bindings to ensure that the
1540 // place they refer to can't be modified by the guard.
1541 for binding in by_value_bindings.clone() {
1542 let local_id = self.var_local_id(binding.var_id, RefWithinGuard);
1543 let place = Place::Base(PlaceBase::Local(local_id));
1547 source_info: guard_end,
1548 kind: StatementKind::FakeRead(FakeReadCause::ForGuardBinding, place),
1552 self.bind_matched_candidate_for_arm_body(
1560 TerminatorKind::Goto { target: arm_block },
1563 assert!(candidate.otherwise_block.is_none());
1564 // (Here, it is not too early to bind the matched
1565 // candidate on `block`, because there is no guard result
1566 // that we have to inspect before we bind them.)
1567 self.bind_matched_candidate_for_arm_body(block, &candidate.bindings);
1570 candidate_source_info,
1571 TerminatorKind::Goto { target: arm_block },
1576 /// Append `AscribeUserType` statements onto the end of `block`
1577 /// for each ascription
1578 fn ascribe_types(&mut self, block: BasicBlock, ascriptions: &[Ascription<'tcx>]) {
1579 for ascription in ascriptions {
1580 let source_info = self.source_info(ascription.span);
1583 "adding user ascription at span {:?} of place {:?} and {:?}",
1589 let user_ty = box ascription.user_ty.clone().user_ty(
1590 &mut self.canonical_user_type_annotations,
1591 ascription.source.ty(&self.local_decls, self.hir.tcx()).ty,
1598 kind: StatementKind::AscribeUserType(
1599 ascription.source.clone(),
1600 ascription.variance,
1608 fn bind_matched_candidate_for_guard(
1611 bindings: &[Binding<'tcx>],
1613 debug!("bind_matched_candidate_for_guard(block={:?}, bindings={:?})", block, bindings);
1615 // Assign each of the bindings. Since we are binding for a
1616 // guard expression, this will never trigger moves out of the
1618 let re_erased = self.hir.tcx().lifetimes.re_erased;
1619 for binding in bindings {
1620 let source_info = self.source_info(binding.span);
1622 // For each pattern ident P of type T, `ref_for_guard` is
1623 // a reference R: &T pointing to the location matched by
1624 // the pattern, and every occurrence of P within a guard
1627 self.storage_live_binding(block, binding.var_id, binding.span, RefWithinGuard);
1628 // Question: Why schedule drops if bindings are all
1630 // Answer: Because schedule_drop_for_binding also emits
1631 // StorageDead's for those locals.
1632 self.schedule_drop_for_binding(binding.var_id, binding.span, RefWithinGuard);
1633 match binding.binding_mode {
1634 BindingMode::ByValue => {
1635 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, binding.source.clone());
1637 .push_assign(block, source_info, &ref_for_guard, rvalue);
1639 BindingMode::ByRef(borrow_kind) => {
1640 let value_for_arm = self.storage_live_binding(
1646 self.schedule_drop_for_binding(
1652 let rvalue = Rvalue::Ref(re_erased, borrow_kind, binding.source.clone());
1654 .push_assign(block, source_info, &value_for_arm, rvalue);
1655 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, value_for_arm);
1657 .push_assign(block, source_info, &ref_for_guard, rvalue);
1663 fn bind_matched_candidate_for_arm_body<'b>(
1666 bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
1668 debug!("bind_matched_candidate_for_arm_body(block={:?})", block);
1670 let re_erased = self.hir.tcx().lifetimes.re_erased;
1671 // Assign each of the bindings. This may trigger moves out of the candidate.
1672 for binding in bindings {
1673 let source_info = self.source_info(binding.span);
1675 self.storage_live_binding(block, binding.var_id, binding.span, OutsideGuard);
1676 self.schedule_drop_for_binding(binding.var_id, binding.span, OutsideGuard);
1677 let rvalue = match binding.binding_mode {
1678 BindingMode::ByValue => {
1679 Rvalue::Use(self.consume_by_copy_or_move(binding.source.clone()))
1681 BindingMode::ByRef(borrow_kind) => {
1682 Rvalue::Ref(re_erased, borrow_kind, binding.source.clone())
1685 self.cfg.push_assign(block, source_info, &local, rvalue);
1689 /// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where the bound
1690 /// `var` has type `T` in the arm body) in a pattern maps to 2 locals. The
1691 /// first local is a binding for occurrences of `var` in the guard, which
1692 /// will have type `&T`. The second local is a binding for occurrences of
1693 /// `var` in the arm body, which will have type `T`.
1696 source_info: SourceInfo,
1697 visibility_scope: SourceScope,
1698 mutability: Mutability,
1703 user_ty: UserTypeProjections,
1704 has_guard: ArmHasGuard,
1705 opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
1709 "declare_binding(var_id={:?}, name={:?}, mode={:?}, var_ty={:?}, \
1710 visibility_scope={:?}, source_info={:?})",
1711 var_id, name, mode, var_ty, visibility_scope, source_info
1714 let tcx = self.hir.tcx();
1715 let binding_mode = match mode {
1716 BindingMode::ByValue => ty::BindingMode::BindByValue(mutability.into()),
1717 BindingMode::ByRef(_) => ty::BindingMode::BindByReference(mutability.into()),
1719 debug!("declare_binding: user_ty={:?}", user_ty);
1720 let local = LocalDecl::<'tcx> {
1728 is_block_tail: None,
1729 is_user_variable: Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
1731 // hypothetically, `visit_bindings` could try to unzip
1732 // an outermost hir::Ty as we descend, matching up
1733 // idents in pat; but complex w/ unclear UI payoff.
1734 // Instead, just abandon providing diagnostic info.
1740 let for_arm_body = self.local_decls.push(local.clone());
1741 let locals = if has_guard.0 {
1742 let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
1743 // This variable isn't mutated but has a name, so has to be
1744 // immutable to avoid the unused mut lint.
1745 mutability: Mutability::Not,
1746 ty: tcx.mk_imm_ref(tcx.lifetimes.re_erased, var_ty),
1747 user_ty: UserTypeProjections::none(),
1752 is_block_tail: None,
1753 is_user_variable: Some(ClearCrossCrate::Set(BindingForm::RefForGuard)),
1755 LocalsForNode::ForGuard {
1760 LocalsForNode::One(for_arm_body)
1762 debug!("declare_binding: vars={:?}", locals);
1763 self.var_indices.insert(var_id, locals);