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
::{CachedBlock, 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, *}
;
14 use rustc
::ty
::{self, CanonicalUserTypeAnnotation, Ty}
;
15 use rustc
::ty
::layout
::VariantIdx
;
16 use rustc_data_structures
::bit_set
::BitSet
;
17 use rustc_data_structures
::fx
::{FxHashMap, FxHashSet}
;
18 use syntax
::ast
::{Name, NodeId}
;
21 // helper functions, broken out by category:
26 use std
::convert
::TryFrom
;
28 impl<'a
, 'gcx
, 'tcx
> Builder
<'a
, 'gcx
, 'tcx
> {
29 /// Generates MIR for a `match` expression.
31 /// The MIR that we generate for a match looks like this.
36 /// [ 1. Evaluate Scrutinee (expression being matched on) ]
37 /// [ (fake read of scrutinee) ]
39 /// [ 2. Decision tree -- check discriminants ] <--------+
41 /// | (once a specific arm is chosen) |
43 /// [pre_binding_block] [otherwise_block]
45 /// [ 3. Create "guard bindings" for arm ] |
46 /// [ (create fake borrows) ] |
48 /// [ 4. Execute guard code ] |
49 /// [ (read fake borrows) ] --(guard is false)-----------+
51 /// | (guard results in true)
53 /// [ 5. Create real bindings and execute arm ]
58 /// All of the different arms have been stacked on top of each other to
59 /// simplify the diagram. For an arm with no guard the blocks marked 3 and
60 /// 4 and the fake borrows are omitted.
62 /// We generate MIR in the following steps:
64 /// 1. Evaluate the scrutinee and add the fake read of it.
65 /// 2. Create the prebinding and otherwise blocks.
66 /// 3. Create the decision tree and record the places that we bind or test.
67 /// 4. Determine the fake borrows that are needed from the above places.
68 /// Create the required temporaries for them.
69 /// 5. Create everything else: Create everything else: the guards and the
72 /// ## Fake Reads and borrows
74 /// Match exhaustiveness checking is not able to handle the case where the
75 /// place being matched on is mutated in the guards. There is an AST check
76 /// that tries to stop this but it is buggy and overly restrictive. Instead
77 /// we add "fake borrows" to the guards that prevent any mutation of the
78 /// place being matched. There are a some subtleties:
80 /// 1. Borrowing `*x` doesn't prevent assigning to `x`. If `x` is a shared
81 /// refence, the borrow isn't even tracked. As such we have to add fake
82 /// borrows of any prefixes of a place
83 /// 2. We don't want `match x { _ => (), }` to conflict with mutable
84 /// borrows of `x`, so we only add fake borrows for places which are
85 /// bound or tested by the match.
86 /// 3. We don't want the fake borrows to conflict with `ref mut` bindings,
87 /// so we use a special BorrowKind for them.
88 /// 4. The fake borrows may be of places in inactive variants, so it would
89 /// be UB to generate code for them. They therefore have to be removed
90 /// by a MIR pass run after borrow checking.
94 /// We don't want to have the exact structure of the decision tree be
95 /// visible through borrow checking. False edges ensure that the CFG as
96 /// seen by borrow checking doesn't encode this. False edges are added:
98 /// * From each prebinding block to the next prebinding block.
99 /// * From each otherwise block to the next prebinding block.
102 destination
: &Place
<'tcx
>,
104 mut block
: BasicBlock
,
105 scrutinee
: ExprRef
<'tcx
>,
106 arms
: Vec
<Arm
<'tcx
>>,
108 let tcx
= self.hir
.tcx();
110 // Step 1. Evaluate the scrutinee and add the fake read of it.
112 let scrutinee_span
= scrutinee
.span();
113 let scrutinee_place
= unpack
!(block
= self.as_place(block
, scrutinee
));
115 // Matching on a `scrutinee_place` with an uninhabited type doesn't
116 // generate any memory reads by itself, and so if the place "expression"
117 // contains unsafe operations like raw pointer dereferences or union
118 // field projections, we wouldn't know to require an `unsafe` block
119 // around a `match` equivalent to `std::intrinsics::unreachable()`.
120 // See issue #47412 for this hole being discovered in the wild.
122 // HACK(eddyb) Work around the above issue by adding a dummy inspection
123 // of `scrutinee_place`, specifically by applying `ReadForMatch`.
125 // NOTE: ReadForMatch also checks that the scrutinee is initialized.
126 // This is currently needed to not allow matching on an uninitialized,
127 // uninhabited value. If we get never patterns, those will check that
128 // the place is initialized, and so this read would only be used to
131 let source_info
= self.source_info(scrutinee_span
);
132 self.cfg
.push(block
, Statement
{
134 kind
: StatementKind
::FakeRead(
135 FakeReadCause
::ForMatchedPlace
,
136 scrutinee_place
.clone(),
140 // Step 2. Create the otherwise and prebinding blocks.
142 // create binding start block for link them by false edges
143 let candidate_count
= arms
.iter().map(|c
| c
.patterns
.len()).sum
::<usize>();
144 let pre_binding_blocks
: Vec
<_
> = (0..=candidate_count
)
145 .map(|_
| self.cfg
.start_new_block())
148 // There's one more pre_binding block than there are candidates so that
149 // every candidate can have a `next_candidate_pre_binding_block`.
150 let outer_source_info
= self.source_info(span
);
152 *pre_binding_blocks
.last().unwrap(),
154 TerminatorKind
::Unreachable
,
157 let mut match_has_guard
= false;
159 let mut candidate_pre_binding_blocks
= pre_binding_blocks
.iter();
160 let mut next_candidate_pre_binding_blocks
= pre_binding_blocks
.iter().skip(1);
162 // Assemble a list of candidates: there is one candidate per pattern,
163 // which means there may be more than one candidate *per arm*.
164 let mut arm_candidates
: Vec
<_
> = arms
167 let arm_has_guard
= arm
.guard
.is_some();
168 match_has_guard
|= arm_has_guard
;
169 let arm_candidates
: Vec
<_
> = arm
.patterns
171 .zip(candidate_pre_binding_blocks
.by_ref())
172 .zip(next_candidate_pre_binding_blocks
.by_ref())
174 |((pattern
, pre_binding_block
), next_candidate_pre_binding_block
)| {
178 MatchPair
::new(scrutinee_place
.clone(), pattern
),
182 otherwise_block
: if arm_has_guard
{
183 Some(self.cfg
.start_new_block())
187 pre_binding_block
: *pre_binding_block
,
188 next_candidate_pre_binding_block
:
189 *next_candidate_pre_binding_block
,
194 (arm
, arm_candidates
)
198 // Step 3. Create the decision tree and record the places that we bind or test.
200 // The set of places that we are creating fake borrows of. If there are
201 // no match guards then we don't need any fake borrows, so don't track
203 let mut fake_borrows
= if match_has_guard
&& tcx
.generate_borrow_of_any_match_input() {
204 Some(FxHashSet
::default())
209 // These candidates are kept sorted such that the highest priority
210 // candidate comes first in the list. (i.e., same order as in source)
211 // As we gnerate the decision tree,
212 let candidates
= &mut arm_candidates
214 .flat_map(|(_
, candidates
)| candidates
)
215 .collect
::<Vec
<_
>>();
217 // this will generate code to test scrutinee_place and
218 // branch to the appropriate arm block
219 let otherwise
= self.match_candidates(
226 if !otherwise
.is_empty() {
227 // All matches are exhaustive. However, because some matches
228 // only have exponentially-large exhaustive decision trees, we
229 // sometimes generate an inexhaustive decision tree.
231 // In that case, the inexhaustive tips of the decision tree
232 // can't be reached - terminate them with an `unreachable`.
233 let mut otherwise
= otherwise
;
235 otherwise
.dedup(); // variant switches can introduce duplicate target blocks
236 for block
in otherwise
{
238 .terminate(block
, outer_source_info
, TerminatorKind
::Unreachable
);
242 // Step 4. Determine the fake borrows that are needed from the above
243 // places. Create the required temporaries for them.
245 let fake_borrow_temps
= if let Some(ref borrows
) = fake_borrows
{
246 self.calculate_fake_borrows(borrows
, scrutinee_span
)
251 // Step 5. Create everything else: the guards and the arms.
253 let outer_source_info
= self.source_info(span
);
254 let arm_end_blocks
: Vec
<_
> = arm_candidates
.into_iter().map(|(arm
, candidates
)| {
255 let mut arm_block
= self.cfg
.start_new_block();
257 let body
= self.hir
.mirror(arm
.body
.clone());
258 let scope
= self.declare_bindings(
261 LintLevel
::Inherited
,
263 ArmHasGuard(arm
.guard
.is_some()),
264 Some((Some(&scrutinee_place
), scrutinee_span
)),
267 for candidate
in candidates
{
268 self.bind_and_guard_matched_candidate(
277 if let Some(source_scope
) = scope
{
278 self.source_scope
= source_scope
;
281 unpack
!(arm_block
= self.into(destination
, arm_block
, body
));
286 // all the arm blocks will rejoin here
287 let end_block
= self.cfg
.start_new_block();
289 for arm_block
in arm_end_blocks
{
293 TerminatorKind
::Goto { target: end_block }
,
297 self.source_scope
= outer_source_info
.scope
;
302 pub(super) fn expr_into_pattern(
304 mut block
: BasicBlock
,
305 irrefutable_pat
: Pattern
<'tcx
>,
306 initializer
: ExprRef
<'tcx
>,
308 match *irrefutable_pat
.kind
{
309 // Optimize the case of `let x = ...` to write directly into `x`
310 PatternKind
::Binding
{
311 mode
: BindingMode
::ByValue
,
317 self.storage_live_binding(block
, var
, irrefutable_pat
.span
, OutsideGuard
);
318 unpack
!(block
= self.into(&place
, block
, initializer
));
321 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
322 let source_info
= self.source_info(irrefutable_pat
.span
);
327 kind
: StatementKind
::FakeRead(FakeReadCause
::ForLet
, place
),
331 self.schedule_drop_for_binding(var
, irrefutable_pat
.span
, OutsideGuard
);
335 // Optimize the case of `let x: T = ...` to write directly
336 // into `x` and then require that `T == typeof(x)`.
338 // Weirdly, this is needed to prevent the
339 // `intrinsic-move-val.rs` test case from crashing. That
340 // test works with uninitialized values in a rather
341 // dubious way, so it may be that the test is kind of
343 PatternKind
::AscribeUserType
{
344 subpattern
: Pattern
{
345 kind
: box PatternKind
::Binding
{
346 mode
: BindingMode
::ByValue
,
353 ascription
: hair
::pattern
::Ascription
{
354 user_ty
: pat_ascription_ty
,
360 self.storage_live_binding(block
, var
, irrefutable_pat
.span
, OutsideGuard
);
361 unpack
!(block
= self.into(&place
, block
, initializer
));
363 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
364 let pattern_source_info
= self.source_info(irrefutable_pat
.span
);
368 source_info
: pattern_source_info
,
369 kind
: StatementKind
::FakeRead(FakeReadCause
::ForLet
, place
.clone()),
373 let ty_source_info
= self.source_info(user_ty_span
);
374 let user_ty
= box pat_ascription_ty
.user_ty(
375 &mut self.canonical_user_type_annotations
,
376 place
.ty(&self.local_decls
, self.hir
.tcx()).to_ty(self.hir
.tcx()),
382 source_info
: ty_source_info
,
383 kind
: StatementKind
::AscribeUserType(
385 // We always use invariant as the variance here. This is because the
386 // variance field from the ascription refers to the variance to use
387 // when applying the type to the value being matched, but this
388 // ascription applies rather to the type of the binding. e.g., in this
395 // We are creating an ascription that defines the type of `x` to be
396 // exactly `T` (i.e., with invariance). The variance field, in
397 // contrast, is intended to be used to relate `T` to the type of
399 ty
::Variance
::Invariant
,
405 self.schedule_drop_for_binding(var
, irrefutable_pat
.span
, OutsideGuard
);
410 let place
= unpack
!(block
= self.as_place(block
, initializer
));
411 self.place_into_pattern(block
, irrefutable_pat
, &place
, true)
416 pub fn place_into_pattern(
419 irrefutable_pat
: Pattern
<'tcx
>,
420 initializer
: &Place
<'tcx
>,
421 set_match_place
: bool
,
423 // create a dummy candidate
424 let mut candidate
= Candidate
{
425 span
: irrefutable_pat
.span
,
426 match_pairs
: vec
![MatchPair
::new(initializer
.clone(), &irrefutable_pat
)],
430 // since we don't call `match_candidates`, next fields are unused
431 otherwise_block
: None
,
432 pre_binding_block
: block
,
433 next_candidate_pre_binding_block
: block
,
436 // Simplify the candidate. Since the pattern is irrefutable, this should
437 // always convert all match-pairs into bindings.
438 self.simplify_candidate(&mut candidate
);
440 if !candidate
.match_pairs
.is_empty() {
442 candidate
.match_pairs
[0].pattern
.span
,
443 "match pairs {:?} remaining after simplifying \
444 irrefutable pattern",
445 candidate
.match_pairs
449 // for matches and function arguments, the place that is being matched
450 // can be set when creating the variables. But the place for
451 // let PATTERN = ... might not even exist until we do the assignment.
452 // so we set it here instead
454 for binding
in &candidate
.bindings
{
455 let local
= self.var_local_id(binding
.var_id
, OutsideGuard
);
457 if let Some(ClearCrossCrate
::Set(BindingForm
::Var(VarBindingForm
{
458 opt_match_place
: Some((ref mut match_place
, _
)),
460 }))) = self.local_decls
[local
].is_user_variable
462 *match_place
= Some(initializer
.clone());
464 bug
!("Let binding to non-user variable.")
469 self.ascribe_types(block
, &candidate
.ascriptions
);
471 // now apply the bindings, which will also declare the variables
472 self.bind_matched_candidate_for_arm_body(block
, &candidate
.bindings
);
477 /// Declares the bindings of the given patterns and returns the visibility
478 /// scope for the bindings in these patterns, if such a scope had to be
479 /// created. NOTE: Declaring the bindings should always be done in their
481 pub fn declare_bindings(
483 mut visibility_scope
: Option
<SourceScope
>,
485 lint_level
: LintLevel
,
486 pattern
: &Pattern
<'tcx
>,
487 has_guard
: ArmHasGuard
,
488 opt_match_place
: Option
<(Option
<&Place
<'tcx
>>, Span
)>,
489 ) -> Option
<SourceScope
> {
491 !(visibility_scope
.is_some() && lint_level
.is_explicit()),
492 "can't have both a visibility and a lint scope at the same time"
494 let mut scope
= self.source_scope
;
495 debug
!("declare_bindings: pattern={:?}", pattern
);
498 UserTypeProjections
::none(),
499 &mut |this
, mutability
, name
, mode
, var
, span
, ty
, user_ty
| {
500 if visibility_scope
.is_none() {
502 Some(this
.new_source_scope(scope_span
, LintLevel
::Inherited
, None
));
503 // If we have lints, create a new source scope
504 // that marks the lints for the locals. See the comment
505 // on the `source_info` field for why this is needed.
506 if lint_level
.is_explicit() {
507 scope
= this
.new_source_scope(scope_span
, lint_level
, None
);
510 let source_info
= SourceInfo { span, scope }
;
511 let visibility_scope
= visibility_scope
.unwrap();
512 this
.declare_binding(
522 opt_match_place
.map(|(x
, y
)| (x
.cloned(), y
)),
530 pub fn storage_live_binding(
537 let local_id
= self.var_local_id(var
, for_guard
);
538 let source_info
= self.source_info(span
);
543 kind
: StatementKind
::StorageLive(local_id
),
546 let place
= Place
::Local(local_id
);
547 let var_ty
= self.local_decls
[local_id
].ty
;
548 let hir_id
= self.hir
.tcx().hir().node_to_hir_id(var
);
549 let region_scope
= self.hir
.region_scope_tree
.var_scope(hir_id
.local_id
);
550 self.schedule_drop(span
, region_scope
, &place
, var_ty
, DropKind
::Storage
);
554 pub fn schedule_drop_for_binding(&mut self, var
: NodeId
, span
: Span
, for_guard
: ForGuard
) {
555 let local_id
= self.var_local_id(var
, for_guard
);
556 let var_ty
= self.local_decls
[local_id
].ty
;
557 let hir_id
= self.hir
.tcx().hir().node_to_hir_id(var
);
558 let region_scope
= self.hir
.region_scope_tree
.var_scope(hir_id
.local_id
);
562 &Place
::Local(local_id
),
565 cached_block
: CachedBlock
::default(),
570 pub(super) fn visit_bindings(
572 pattern
: &Pattern
<'tcx
>,
573 pattern_user_ty
: UserTypeProjections
<'tcx
>,
582 UserTypeProjections
<'tcx
>,
585 debug
!("visit_bindings: pattern={:?} pattern_user_ty={:?}", pattern
, pattern_user_ty
);
586 match *pattern
.kind
{
587 PatternKind
::Binding
{
596 f(self, mutability
, name
, mode
, var
, pattern
.span
, ty
, pattern_user_ty
.clone());
597 if let Some(subpattern
) = subpattern
.as_ref() {
598 self.visit_bindings(subpattern
, pattern_user_ty
, f
);
607 | PatternKind
::Slice
{
612 let from
= u32::try_from(prefix
.len()).unwrap();
613 let to
= u32::try_from(suffix
.len()).unwrap();
614 for subpattern
in prefix
{
615 self.visit_bindings(subpattern
, pattern_user_ty
.clone().index(), f
);
617 for subpattern
in slice
{
618 self.visit_bindings(subpattern
, pattern_user_ty
.clone().subslice(from
, to
), f
);
620 for subpattern
in suffix
{
621 self.visit_bindings(subpattern
, pattern_user_ty
.clone().index(), f
);
625 PatternKind
::Constant { .. }
| PatternKind
::Range { .. }
| PatternKind
::Wild
=> {}
627 PatternKind
::Deref { ref subpattern }
=> {
628 self.visit_bindings(subpattern
, pattern_user_ty
.deref(), f
);
631 PatternKind
::AscribeUserType
{
633 ascription
: hair
::pattern
::Ascription
{
639 // This corresponds to something like
642 // let A::<'a>(_): A<'static> = ...;
645 // Note that the variance doesn't apply here, as we are tracking the effect
646 // of `user_ty` on any bindings contained with subpattern.
647 let annotation
= CanonicalUserTypeAnnotation
{
649 user_ty
: user_ty
.user_ty
,
650 inferred_ty
: subpattern
.ty
,
652 let projection
= UserTypeProjection
{
653 base
: self.canonical_user_type_annotations
.push(annotation
),
656 let subpattern_user_ty
= pattern_user_ty
.push_projection(&projection
, user_ty_span
);
657 self.visit_bindings(subpattern
, subpattern_user_ty
, f
)
660 PatternKind
::Leaf { ref subpatterns }
=> {
661 for subpattern
in subpatterns
{
662 let subpattern_user_ty
= pattern_user_ty
.clone().leaf(subpattern
.field
);
663 debug
!("visit_bindings: subpattern_user_ty={:?}", subpattern_user_ty
);
664 self.visit_bindings(&subpattern
.pattern
, subpattern_user_ty
, f
);
668 PatternKind
::Variant { adt_def, substs: _, variant_index, ref subpatterns }
=> {
669 for subpattern
in subpatterns
{
670 let subpattern_user_ty
= pattern_user_ty
.clone().variant(
671 adt_def
, variant_index
, subpattern
.field
);
672 self.visit_bindings(&subpattern
.pattern
, subpattern_user_ty
, f
);
680 pub struct Candidate
<'pat
, 'tcx
: 'pat
> {
681 // span of the original pattern that gave rise to this candidate
684 // all of these must be satisfied...
685 match_pairs
: Vec
<MatchPair
<'pat
, 'tcx
>>,
687 // ...these bindings established...
688 bindings
: Vec
<Binding
<'tcx
>>,
690 // ...and these types asserted...
691 ascriptions
: Vec
<Ascription
<'tcx
>>,
693 // ...and the guard must be evaluated, if false branch to Block...
694 otherwise_block
: Option
<BasicBlock
>,
696 // ...and the blocks for add false edges between candidates
697 pre_binding_block
: BasicBlock
,
698 next_candidate_pre_binding_block
: BasicBlock
,
701 #[derive(Clone, Debug)]
702 struct Binding
<'tcx
> {
708 mutability
: Mutability
,
709 binding_mode
: BindingMode
,
712 /// Indicates that the type of `source` must be a subtype of the
713 /// user-given type `user_ty`; this is basically a no-op but can
714 /// influence region inference.
715 #[derive(Clone, Debug)]
716 struct Ascription
<'tcx
> {
719 user_ty
: PatternTypeProjection
<'tcx
>,
720 variance
: ty
::Variance
,
723 #[derive(Clone, Debug)]
724 pub struct MatchPair
<'pat
, 'tcx
: 'pat
> {
728 // ... must match this pattern.
729 pattern
: &'pat Pattern
<'tcx
>,
732 #[derive(Clone, Debug, PartialEq)]
733 enum TestKind
<'tcx
> {
734 // test the branches of enum
736 adt_def
: &'tcx ty
::AdtDef
,
737 variants
: BitSet
<VariantIdx
>,
740 // test the branches of enum
744 indices
: FxHashMap
<ty
::Const
<'tcx
>, usize>,
749 value
: ty
::Const
<'tcx
>,
753 // test whether the value falls within an inclusive or exclusive range
754 Range(PatternRange
<'tcx
>),
756 // test length of the slice is equal to len
764 pub struct Test
<'tcx
> {
766 kind
: TestKind
<'tcx
>,
769 /// ArmHasGuard is isomorphic to a boolean flag. It indicates whether
770 /// a match arm has a guard expression attached to it.
771 #[derive(Copy, Clone, Debug)]
772 pub(crate) struct ArmHasGuard(pub bool
);
774 ///////////////////////////////////////////////////////////////////////////
775 // Main matching algorithm
777 impl<'a
, 'gcx
, 'tcx
> Builder
<'a
, 'gcx
, 'tcx
> {
778 /// The main match algorithm. It begins with a set of candidates
779 /// `candidates` and has the job of generating code to determine
780 /// which of these candidates, if any, is the correct one. The
781 /// candidates are sorted such that the first item in the list
782 /// has the highest priority. When a candidate is found to match
783 /// the value, we will generate a branch to the appropriate
784 /// prebinding block.
786 /// The return value is a list of "otherwise" blocks. These are
787 /// points in execution where we found that *NONE* of the
788 /// candidates apply. In principle, this means that the input
789 /// list was not exhaustive, though at present we sometimes are
790 /// not smart enough to recognize all exhaustive inputs.
792 /// It might be surprising that the input can be inexhaustive.
793 /// Indeed, initially, it is not, because all matches are
794 /// exhaustive in Rust. But during processing we sometimes divide
795 /// up the list of candidates and recurse with a non-exhaustive
796 /// list. This is important to keep the size of the generated code
797 /// under control. See `test_candidates` for more details.
799 /// If `fake_borrows` is Some, then places which need fake borrows
800 /// will be added to it.
801 fn match_candidates
<'pat
>(
804 candidates
: &mut [&mut Candidate
<'pat
, 'tcx
>],
805 mut block
: BasicBlock
,
806 fake_borrows
: &mut Option
<FxHashSet
<Place
<'tcx
>>>,
807 ) -> Vec
<BasicBlock
> {
809 "matched_candidate(span={:?}, block={:?}, candidates={:?})",
810 span
, block
, candidates
813 // Start by simplifying candidates. Once this process is complete, all
814 // the match pairs which remain require some form of test, whether it
815 // be a switch or pattern comparison.
816 for candidate
in &mut *candidates
{
817 self.simplify_candidate(candidate
);
820 // The candidates are sorted by priority. Check to see whether the
821 // higher priority candidates (and hence at the front of the slice)
822 // have satisfied all their match pairs.
823 let fully_matched
= candidates
825 .take_while(|c
| c
.match_pairs
.is_empty())
828 "match_candidates: {:?} candidates fully matched",
831 let (matched_candidates
, unmatched_candidates
) = candidates
.split_at_mut(fully_matched
);
833 if !matched_candidates
.is_empty() {
834 block
= if let Some(last_otherwise_block
) = self.select_matched_candidates(
841 // Any remaining candidates are unreachable.
842 if unmatched_candidates
.is_empty() {
845 self.cfg
.start_new_block()
850 // If there are no candidates that still need testing, we're
851 // done. Since all matches are exhaustive, execution should
852 // never reach this point.
853 if unmatched_candidates
.is_empty() {
857 // Test candidates where possible.
858 let (otherwise
, untested_candidates
) = self.test_candidates(
860 unmatched_candidates
,
865 // If the target candidates were exhaustive, then we are done.
866 // But for borrowck continue build decision tree.
867 if untested_candidates
.is_empty() {
871 // Otherwise, let's process those remaining candidates.
872 let join_block
= self.join_otherwise_blocks(span
, otherwise
);
873 self.match_candidates(
881 /// Link up matched candidates. For example, if we have something like
885 /// Some(x) if cond => ...
887 /// Some(x) if cond => ...
890 /// We generate real edges from:
891 /// * `block` to the prebinding_block of the first pattern,
892 /// * the otherwise block of the first pattern to the second pattern,
893 /// * the otherwise block of the third pattern to the a block with an
894 /// Unreachable terminator.
896 /// As well as that we add fake edges from the otherwise blocks to the
897 /// prebinding block of the next candidate in the original set of
899 fn select_matched_candidates(
901 matched_candidates
: &mut [&mut Candidate
<'_
, 'tcx
>],
903 fake_borrows
: &mut Option
<FxHashSet
<Place
<'tcx
>>>,
904 ) -> Option
<BasicBlock
> {
906 !matched_candidates
.is_empty(),
907 "select_matched_candidates called with no candidates",
910 // Insert a borrows of prefixes of places that are bound and are
911 // behind a dereference projection.
913 // These borrows are taken to avoid situations like the following:
916 // _ if { x = &[0]; false } => (),
917 // y => (), // Out of bounds array access!
921 // // y is bound by reference in the guard and then by copy in the
922 // // arm, so y is 2 in the arm!
923 // y if { y == 1 && (x = &2) == () } => y,
926 if let Some(fake_borrows
) = fake_borrows
{
927 for Binding { source, .. }
928 in matched_candidates
.iter().flat_map(|candidate
| &candidate
.bindings
)
930 let mut cursor
= source
;
931 while let Place
::Projection(box Projection { base, elem }
) = cursor
{
933 if let ProjectionElem
::Deref
= elem
{
934 fake_borrows
.insert(cursor
.clone());
941 let fully_matched_with_guard
= matched_candidates
943 .position(|c
| c
.otherwise_block
.is_none())
944 .unwrap_or(matched_candidates
.len() - 1);
946 let (reachable_candidates
, unreachable_candidates
)
947 = matched_candidates
.split_at_mut(fully_matched_with_guard
+ 1);
949 let first_candidate
= &reachable_candidates
[0];
951 let candidate_source_info
= self.source_info(first_candidate
.span
);
955 candidate_source_info
,
956 TerminatorKind
::Goto
{
957 target
: first_candidate
.pre_binding_block
,
961 for window
in reachable_candidates
.windows(2) {
962 if let [first_candidate
, second_candidate
] = window
{
963 let source_info
= self.source_info(first_candidate
.span
);
964 if let Some(otherwise_block
) = first_candidate
.otherwise_block
{
968 TerminatorKind
::FalseEdges
{
969 real_target
: second_candidate
.pre_binding_block
,
970 imaginary_targets
: vec
![
971 first_candidate
.next_candidate_pre_binding_block
976 bug
!("candidate other than the last has no guard");
979 bug
!("<[_]>::windows returned incorrectly sized window");
983 debug
!("match_candidates: add false edges for unreachable {:?}", unreachable_candidates
);
984 for candidate
in unreachable_candidates
{
985 if let Some(otherwise
) = candidate
.otherwise_block
{
986 let source_info
= self.source_info(candidate
.span
);
987 let unreachable
= self.cfg
.start_new_block();
991 TerminatorKind
::FalseEdges
{
992 real_target
: unreachable
,
993 imaginary_targets
: vec
![candidate
.next_candidate_pre_binding_block
],
996 self.cfg
.terminate(unreachable
, source_info
, TerminatorKind
::Unreachable
);
1000 let last_candidate
= reachable_candidates
.last().unwrap();
1002 if let Some(otherwise
) = last_candidate
.otherwise_block
{
1003 let source_info
= self.source_info(last_candidate
.span
);
1004 let block
= self.cfg
.start_new_block();
1008 TerminatorKind
::FalseEdges
{
1010 imaginary_targets
: vec
![last_candidate
.next_candidate_pre_binding_block
]
1019 fn join_otherwise_blocks(&mut self, span
: Span
, mut otherwise
: Vec
<BasicBlock
>) -> BasicBlock
{
1020 let source_info
= self.source_info(span
);
1022 otherwise
.dedup(); // variant switches can introduce duplicate target blocks
1023 if otherwise
.len() == 1 {
1026 let join_block
= self.cfg
.start_new_block();
1027 for block
in otherwise
{
1031 TerminatorKind
::Goto { target: join_block }
,
1038 /// This is the most subtle part of the matching algorithm. At
1039 /// this point, the input candidates have been fully simplified,
1040 /// and so we know that all remaining match-pairs require some
1041 /// sort of test. To decide what test to do, we take the highest
1042 /// priority candidate (last one in the list) and extract the
1043 /// first match-pair from the list. From this we decide what kind
1044 /// of test is needed using `test`, defined in the `test` module.
1046 /// *Note:* taking the first match pair is somewhat arbitrary, and
1047 /// we might do better here by choosing more carefully what to
1050 /// For example, consider the following possible match-pairs:
1052 /// 1. `x @ Some(P)` -- we will do a `Switch` to decide what variant `x` has
1053 /// 2. `x @ 22` -- we will do a `SwitchInt`
1054 /// 3. `x @ 3..5` -- we will do a range test
1057 /// Once we know what sort of test we are going to perform, this
1058 /// Tests may also help us with other candidates. So we walk over
1059 /// the candidates (from high to low priority) and check. This
1060 /// gives us, for each outcome of the test, a transformed list of
1061 /// candidates. For example, if we are testing the current
1062 /// variant of `x.0`, and we have a candidate `{x.0 @ Some(v), x.1
1063 /// @ 22}`, then we would have a resulting candidate of `{(x.0 as
1064 /// Some).0 @ v, x.1 @ 22}`. Note that the first match-pair is now
1065 /// simpler (and, in fact, irrefutable).
1067 /// But there may also be candidates that the test just doesn't
1068 /// apply to. The classical example involves wildcards:
1071 /// # let (x, y, z) = (true, true, true);
1072 /// match (x, y, z) {
1073 /// (true, _, true) => true, // (0)
1074 /// (_, true, _) => true, // (1)
1075 /// (false, false, _) => false, // (2)
1076 /// (true, _, false) => false, // (3)
1080 /// In that case, after we test on `x`, there are 2 overlapping candidate
1083 /// - If the outcome is that `x` is true, candidates 0, 1, and 3
1084 /// - If the outcome is that `x` is false, candidates 1 and 2
1086 /// Here, the traditional "decision tree" method would generate 2
1087 /// separate code-paths for the 2 separate cases.
1089 /// In some cases, this duplication can create an exponential amount of
1090 /// code. This is most easily seen by noticing that this method terminates
1091 /// with precisely the reachable arms being reachable - but that problem
1092 /// is trivially NP-complete:
1095 /// match (var0, var1, var2, var3, ..) {
1096 /// (true, _, _, false, true, ...) => false,
1097 /// (_, true, true, false, _, ...) => false,
1098 /// (false, _, false, false, _, ...) => false,
1104 /// Here the last arm is reachable only if there is an assignment to
1105 /// the variables that does not match any of the literals. Therefore,
1106 /// compilation would take an exponential amount of time in some cases.
1108 /// That kind of exponential worst-case might not occur in practice, but
1109 /// our simplistic treatment of constants and guards would make it occur
1110 /// in very common situations - for example #29740:
1114 /// "foo" if foo_guard => ...,
1115 /// "bar" if bar_guard => ...,
1116 /// "baz" if baz_guard => ...,
1121 /// Here we first test the match-pair `x @ "foo"`, which is an `Eq` test.
1123 /// It might seem that we would end up with 2 disjoint candidate
1124 /// sets, consisting of the first candidate or the other 3, but our
1125 /// algorithm doesn't reason about "foo" being distinct from the other
1126 /// constants; it considers the latter arms to potentially match after
1127 /// both outcomes, which obviously leads to an exponential amount
1130 /// To avoid these kinds of problems, our algorithm tries to ensure
1131 /// the amount of generated tests is linear. When we do a k-way test,
1132 /// we return an additional "unmatched" set alongside the obvious `k`
1133 /// sets. When we encounter a candidate that would be present in more
1134 /// than one of the sets, we put it and all candidates below it into the
1135 /// "unmatched" set. This ensures these `k+1` sets are disjoint.
1137 /// After we perform our test, we branch into the appropriate candidate
1138 /// set and recurse with `match_candidates`. These sub-matches are
1139 /// obviously inexhaustive - as we discarded our otherwise set - so
1140 /// we set their continuation to do `match_candidates` on the
1141 /// "unmatched" set (which is again inexhaustive).
1143 /// If you apply this to the above test, you basically wind up
1144 /// with an if-else-if chain, testing each candidate in turn,
1145 /// which is precisely what we want.
1147 /// In addition to avoiding exponential-time blowups, this algorithm
1148 /// also has nice property that each guard and arm is only generated
1150 fn test_candidates
<'pat
, 'b
, 'c
>(
1153 mut candidates
: &'b
mut [&'c
mut Candidate
<'pat
, 'tcx
>],
1155 fake_borrows
: &mut Option
<FxHashSet
<Place
<'tcx
>>>,
1156 ) -> (Vec
<BasicBlock
>, &'b
mut [&'c
mut Candidate
<'pat
, 'tcx
>]) {
1157 // extract the match-pair from the highest priority candidate
1158 let match_pair
= &candidates
.first().unwrap().match_pairs
[0];
1159 let mut test
= self.test(match_pair
);
1160 let match_place
= match_pair
.place
.clone();
1162 // most of the time, the test to perform is simply a function
1163 // of the main candidate; but for a test like SwitchInt, we
1164 // may want to add cases based on the candidates that are
1167 TestKind
::SwitchInt
{
1172 for candidate
in candidates
.iter() {
1173 if !self.add_cases_to_switch(
1188 for candidate
in candidates
.iter() {
1189 if !self.add_variants_to_switch(&match_place
, candidate
, variants
) {
1197 // Insert a Shallow borrow of any places that is switched on.
1198 fake_borrows
.as_mut().map(|fb
| {
1199 fb
.insert(match_place
.clone())
1202 // perform the test, branching to one of N blocks. For each of
1203 // those N possible outcomes, create a (initially empty)
1204 // vector of candidates. Those are the candidates that still
1205 // apply if the test has that particular outcome.
1207 "match_candidates: test={:?} match_pair={:?}",
1210 let target_blocks
= self.perform_test(block
, &match_place
, &test
);
1211 let mut target_candidates
: Vec
<Vec
<&mut Candidate
<'pat
, 'tcx
>>> = vec
![];
1212 target_candidates
.resize_with(target_blocks
.len(), Default
::default);
1214 let total_candidate_count
= candidates
.len();
1216 // Sort the candidates into the appropriate vector in
1217 // `target_candidates`. Note that at some point we may
1218 // encounter a candidate where the test is not relevant; at
1219 // that point, we stop sorting.
1220 while let Some(candidate
) = candidates
.first_mut() {
1221 if let Some(idx
) = self.sort_candidate(&match_place
, &test
, candidate
) {
1222 let (candidate
, rest
) = candidates
.split_first_mut().unwrap();
1223 target_candidates
[idx
].push(candidate
);
1229 // at least the first candidate ought to be tested
1230 assert
!(total_candidate_count
> candidates
.len());
1231 debug
!("tested_candidates: {}", total_candidate_count
- candidates
.len());
1232 debug
!("untested_candidates: {}", candidates
.len());
1234 // For each outcome of test, process the candidates that still
1235 // apply. Collect a list of blocks where control flow will
1236 // branch if one of the `target_candidate` sets is not
1238 let otherwise
: Vec
<_
> = target_blocks
1240 .zip(target_candidates
)
1241 .flat_map(|(target_block
, mut target_candidates
)| {
1242 self.match_candidates(
1244 &mut *target_candidates
,
1251 (otherwise
, candidates
)
1254 // Determine the fake borrows that are needed to ensure that the place
1255 // will evaluate to the same thing until an arm has been chosen.
1256 fn calculate_fake_borrows
<'b
>(
1258 fake_borrows
: &'b FxHashSet
<Place
<'tcx
>>,
1260 ) -> Vec
<(&'b Place
<'tcx
>, Local
)> {
1261 let tcx
= self.hir
.tcx();
1263 debug
!("add_fake_borrows fake_borrows = {:?}", fake_borrows
);
1265 let mut all_fake_borrows
= Vec
::with_capacity(fake_borrows
.len());
1267 // Insert a Shallow borrow of the prefixes of any fake borrows.
1268 for place
in fake_borrows
1270 let mut prefix_cursor
= place
;
1271 while let Place
::Projection(box Projection { base, elem }
) = prefix_cursor
{
1272 if let ProjectionElem
::Deref
= elem
{
1273 // Insert a shallow borrow after a deref. For other
1274 // projections the borrow of prefix_cursor will
1275 // conflict with any mutation of base.
1276 all_fake_borrows
.push(base
);
1278 prefix_cursor
= base
;
1281 all_fake_borrows
.push(place
);
1284 // Deduplicate and ensure a deterministic order.
1285 all_fake_borrows
.sort();
1286 all_fake_borrows
.dedup();
1288 debug
!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows
);
1290 all_fake_borrows
.into_iter().map(|matched_place
| {
1291 let fake_borrow_deref_ty
= matched_place
.ty(&self.local_decls
, tcx
).to_ty(tcx
);
1292 let fake_borrow_ty
= tcx
.mk_imm_ref(tcx
.types
.re_erased
, fake_borrow_deref_ty
);
1293 let fake_borrow_temp
= self.local_decls
.push(
1294 LocalDecl
::new_temp(fake_borrow_ty
, temp_span
)
1297 (matched_place
, fake_borrow_temp
)
1302 ///////////////////////////////////////////////////////////////////////////
1303 // Pattern binding - used for `let` and function parameters as well.
1305 impl<'a
, 'gcx
, 'tcx
> Builder
<'a
, 'gcx
, 'tcx
> {
1306 /// Initializes each of the bindings from the candidate by
1307 /// moving/copying/ref'ing the source as appropriate. Tests the guard, if
1308 /// any, and then branches to the arm. Returns the block for the case where
1309 /// the guard fails.
1311 /// Note: we check earlier that if there is a guard, there cannot be move
1312 /// bindings (unless feature(bind_by_move_pattern_guards) is used). This
1313 /// isn't really important for the self-consistency of this fn, but the
1314 /// reason for it should be clear: after we've done the assignments, if
1315 /// there were move bindings, further tests would be a use-after-move.
1316 /// bind_by_move_pattern_guards avoids this by only moving the binding once
1317 /// the guard has evaluated to true (see below).
1318 fn bind_and_guard_matched_candidate
<'pat
>(
1320 candidate
: Candidate
<'pat
, 'tcx
>,
1321 guard
: Option
<Guard
<'tcx
>>,
1322 arm_block
: BasicBlock
,
1323 fake_borrows
: &Vec
<(&Place
<'tcx
>, Local
)>,
1324 scrutinee_span
: Span
,
1326 debug
!("bind_and_guard_matched_candidate(candidate={:?})", candidate
);
1328 debug_assert
!(candidate
.match_pairs
.is_empty());
1330 let candidate_source_info
= self.source_info(candidate
.span
);
1332 let mut block
= self.cfg
.start_new_block();
1334 candidate
.pre_binding_block
,
1335 candidate_source_info
,
1336 TerminatorKind
::FalseEdges
{
1338 imaginary_targets
: vec
![candidate
.next_candidate_pre_binding_block
],
1341 self.ascribe_types(block
, &candidate
.ascriptions
);
1343 // rust-lang/rust#27282: The `autoref` business deserves some
1344 // explanation here.
1346 // The intent of the `autoref` flag is that when it is true,
1347 // then any pattern bindings of type T will map to a `&T`
1348 // within the context of the guard expression, but will
1349 // continue to map to a `T` in the context of the arm body. To
1350 // avoid surfacing this distinction in the user source code
1351 // (which would be a severe change to the language and require
1352 // far more revision to the compiler), when `autoref` is true,
1353 // then any occurrence of the identifier in the guard
1354 // expression will automatically get a deref op applied to it.
1356 // So an input like:
1359 // let place = Foo::new();
1360 // match place { foo if inspect(foo)
1361 // => feed(foo), ... }
1364 // will be treated as if it were really something like:
1367 // let place = Foo::new();
1368 // match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) }
1369 // => { let tmp2 = place; feed(tmp2) }, ... }
1371 // And an input like:
1374 // let place = Foo::new();
1375 // match place { ref mut foo if inspect(foo)
1376 // => feed(foo), ... }
1379 // will be treated as if it were really something like:
1382 // let place = Foo::new();
1383 // match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) }
1384 // => { let tmp2 = &mut place; feed(tmp2) }, ... }
1387 // In short, any pattern binding will always look like *some*
1388 // kind of `&T` within the guard at least in terms of how the
1389 // MIR-borrowck views it, and this will ensure that guard
1390 // expressions cannot mutate their the match inputs via such
1391 // bindings. (It also ensures that guard expressions can at
1392 // most *copy* values from such bindings; non-Copy things
1393 // cannot be moved via pattern bindings in guard expressions.)
1397 // Implementation notes (under assumption `autoref` is true).
1399 // To encode the distinction above, we must inject the
1400 // temporaries `tmp1` and `tmp2`.
1402 // There are two cases of interest: binding by-value, and binding by-ref.
1404 // 1. Binding by-value: Things are simple.
1406 // * Establishing `tmp1` creates a reference into the
1407 // matched place. This code is emitted by
1408 // bind_matched_candidate_for_guard.
1410 // * `tmp2` is only initialized "lazily", after we have
1411 // checked the guard. Thus, the code that can trigger
1412 // moves out of the candidate can only fire after the
1413 // guard evaluated to true. This initialization code is
1414 // emitted by bind_matched_candidate_for_arm.
1416 // 2. Binding by-reference: Things are tricky.
1418 // * Here, the guard expression wants a `&&` or `&&mut`
1419 // into the original input. This means we need to borrow
1420 // the reference that we create for the arm.
1421 // * So we eagerly create the reference for the arm and then take a
1422 // reference to that.
1423 let tcx
= self.hir
.tcx();
1424 let autoref
= tcx
.all_pat_vars_are_implicit_refs_within_guards();
1425 if let Some(guard
) = guard
{
1427 self.bind_matched_candidate_for_guard(
1429 &candidate
.bindings
,
1431 let guard_frame
= GuardFrame
{
1435 .map(|b
| GuardFrameLocal
::new(b
.var_id
, b
.binding_mode
))
1438 debug
!("Entering guard building context: {:?}", guard_frame
);
1439 self.guard_context
.push(guard_frame
);
1441 self.bind_matched_candidate_for_arm_body(block
, &candidate
.bindings
);
1444 let re_erased
= tcx
.types
.re_erased
;
1445 let scrutinee_source_info
= self.source_info(scrutinee_span
);
1446 for &(place
, temp
) in fake_borrows
{
1447 let borrow
= Rvalue
::Ref(
1449 BorrowKind
::Shallow
,
1452 self.cfg
.push_assign(
1454 scrutinee_source_info
,
1455 &Place
::Local(temp
),
1460 // the block to branch to if the guard fails; if there is no
1461 // guard, this block is simply unreachable
1462 let guard
= match guard
{
1463 Guard
::If(e
) => self.hir
.mirror(e
),
1465 let source_info
= self.source_info(guard
.span
);
1466 let guard_end
= self.source_info(tcx
.sess
.source_map().end_point(guard
.span
));
1467 let cond
= unpack
!(block
= self.as_local_operand(block
, guard
));
1469 let guard_frame
= self.guard_context
.pop().unwrap();
1471 "Exiting guard building context with locals: {:?}",
1476 for &(_
, temp
) in fake_borrows
{
1477 self.cfg
.push(block
, Statement
{
1478 source_info
: guard_end
,
1479 kind
: StatementKind
::FakeRead(
1480 FakeReadCause
::ForMatchGuard
,
1486 // We want to ensure that the matched candidates are bound
1487 // after we have confirmed this candidate *and* any
1488 // associated guard; Binding them on `block` is too soon,
1489 // because that would be before we've checked the result
1492 // But binding them on `arm_block` is *too late*, because
1493 // then all of the candidates for a single arm would be
1494 // bound in the same place, that would cause a case like:
1498 // (mut x, 1) | (2, mut x) if { true } => { ... }
1499 // ... // ^^^^^^^ (this is `arm_block`)
1503 // would yield a `arm_block` something like:
1506 // StorageLive(_4); // _4 is `x`
1507 // _4 = &mut (_1.0: i32); // this is handling `(mut x, 1)` case
1508 // _4 = &mut (_1.1: i32); // this is handling `(2, mut x)` case
1511 // and that is clearly not correct.
1512 let post_guard_block
= self.cfg
.start_new_block();
1516 TerminatorKind
::if_(
1520 candidate
.otherwise_block
.unwrap()
1525 let by_value_bindings
= candidate
.bindings
.iter().filter(|binding
| {
1526 if let BindingMode
::ByValue
= binding
.binding_mode { true }
else { false }
1528 // Read all of the by reference bindings to ensure that the
1529 // place they refer to can't be modified by the guard.
1530 for binding
in by_value_bindings
.clone() {
1531 let local_id
= self.var_local_id(binding
.var_id
, RefWithinGuard
);
1532 let place
= Place
::Local(local_id
);
1536 source_info
: guard_end
,
1537 kind
: StatementKind
::FakeRead(FakeReadCause
::ForGuardBinding
, place
),
1541 self.bind_matched_candidate_for_arm_body(
1550 TerminatorKind
::Goto { target: arm_block }
,
1553 assert
!(candidate
.otherwise_block
.is_none());
1554 // (Here, it is not too early to bind the matched
1555 // candidate on `block`, because there is no guard result
1556 // that we have to inspect before we bind them.)
1557 self.bind_matched_candidate_for_arm_body(block
, &candidate
.bindings
);
1560 candidate_source_info
,
1561 TerminatorKind
::Goto { target: arm_block }
,
1566 /// Append `AscribeUserType` statements onto the end of `block`
1567 /// for each ascription
1568 fn ascribe_types
<'pat
>(
1571 ascriptions
: &[Ascription
<'tcx
>],
1573 for ascription
in ascriptions
{
1574 let source_info
= self.source_info(ascription
.span
);
1577 "adding user ascription at span {:?} of place {:?} and {:?}",
1583 let user_ty
= box ascription
.user_ty
.clone().user_ty(
1584 &mut self.canonical_user_type_annotations
,
1585 ascription
.source
.ty(&self.local_decls
, self.hir
.tcx()).to_ty(self.hir
.tcx()),
1592 kind
: StatementKind
::AscribeUserType(
1593 ascription
.source
.clone(),
1594 ascription
.variance
,
1602 // Only called when all_pat_vars_are_implicit_refs_within_guards,
1603 // and thus all code/comments assume we are in that context.
1604 fn bind_matched_candidate_for_guard(
1607 bindings
: &[Binding
<'tcx
>],
1609 debug
!("bind_matched_candidate_for_guard(block={:?}, bindings={:?})", block
, bindings
);
1611 // Assign each of the bindings. Since we are binding for a
1612 // guard expression, this will never trigger moves out of the
1614 let re_erased
= self.hir
.tcx().types
.re_erased
;
1615 for binding
in bindings
{
1616 let source_info
= self.source_info(binding
.span
);
1618 // For each pattern ident P of type T, `ref_for_guard` is
1619 // a reference R: &T pointing to the location matched by
1620 // the pattern, and every occurrence of P within a guard
1623 self.storage_live_binding(block
, binding
.var_id
, binding
.span
, RefWithinGuard
);
1624 // Question: Why schedule drops if bindings are all
1626 // Answer: Because schedule_drop_for_binding also emits
1627 // StorageDead's for those locals.
1628 self.schedule_drop_for_binding(binding
.var_id
, binding
.span
, RefWithinGuard
);
1629 match binding
.binding_mode
{
1630 BindingMode
::ByValue
=> {
1631 let rvalue
= Rvalue
::Ref(re_erased
, BorrowKind
::Shared
, binding
.source
.clone());
1633 .push_assign(block
, source_info
, &ref_for_guard
, rvalue
);
1635 BindingMode
::ByRef(borrow_kind
) => {
1636 let value_for_arm
= self.storage_live_binding(
1642 self.schedule_drop_for_binding(
1648 let rvalue
= Rvalue
::Ref(re_erased
, borrow_kind
, binding
.source
.clone());
1650 .push_assign(block
, source_info
, &value_for_arm
, rvalue
);
1651 let rvalue
= Rvalue
::Ref(re_erased
, BorrowKind
::Shared
, value_for_arm
);
1653 .push_assign(block
, source_info
, &ref_for_guard
, rvalue
);
1659 fn bind_matched_candidate_for_arm_body
<'b
>(
1662 bindings
: impl IntoIterator
<Item
= &'b Binding
<'tcx
>>,
1664 debug
!("bind_matched_candidate_for_arm_body(block={:?})", block
);
1666 let re_erased
= self.hir
.tcx().types
.re_erased
;
1667 // Assign each of the bindings. This may trigger moves out of the candidate.
1668 for binding
in bindings
{
1669 let source_info
= self.source_info(binding
.span
);
1671 self.storage_live_binding(block
, binding
.var_id
, binding
.span
, OutsideGuard
);
1672 self.schedule_drop_for_binding(binding
.var_id
, binding
.span
, OutsideGuard
);
1673 let rvalue
= match binding
.binding_mode
{
1674 BindingMode
::ByValue
=> {
1675 Rvalue
::Use(self.consume_by_copy_or_move(binding
.source
.clone()))
1677 BindingMode
::ByRef(borrow_kind
) => {
1678 Rvalue
::Ref(re_erased
, borrow_kind
, binding
.source
.clone())
1681 self.cfg
.push_assign(block
, source_info
, &local
, rvalue
);
1685 /// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where the bound
1686 /// `var` has type `T` in the arm body) in a pattern maps to 2 locals. The
1687 /// first local is a binding for occurrences of `var` in the guard, which
1688 /// will have type `&T`. The second local is a binding for occurrences of
1689 /// `var` in the arm body, which will have type `T`.
1692 source_info
: SourceInfo
,
1693 visibility_scope
: SourceScope
,
1694 mutability
: Mutability
,
1699 user_ty
: UserTypeProjections
<'tcx
>,
1700 has_guard
: ArmHasGuard
,
1701 opt_match_place
: Option
<(Option
<Place
<'tcx
>>, Span
)>,
1705 "declare_binding(var_id={:?}, name={:?}, mode={:?}, var_ty={:?}, \
1706 visibility_scope={:?}, source_info={:?})",
1707 var_id
, name
, mode
, var_ty
, visibility_scope
, source_info
1710 let tcx
= self.hir
.tcx();
1711 let binding_mode
= match mode
{
1712 BindingMode
::ByValue
=> ty
::BindingMode
::BindByValue(mutability
.into()),
1713 BindingMode
::ByRef(_
) => ty
::BindingMode
::BindByReference(mutability
.into()),
1715 debug
!("declare_binding: user_ty={:?}", user_ty
);
1716 let local
= LocalDecl
::<'tcx
> {
1724 is_block_tail
: None
,
1725 is_user_variable
: Some(ClearCrossCrate
::Set(BindingForm
::Var(VarBindingForm
{
1727 // hypothetically, `visit_bindings` could try to unzip
1728 // an outermost hir::Ty as we descend, matching up
1729 // idents in pat; but complex w/ unclear UI payoff.
1730 // Instead, just abandon providing diagnostic info.
1736 let for_arm_body
= self.local_decls
.push(local
.clone());
1737 let locals
= if has_guard
.0 && tcx
.all_pat_vars_are_implicit_refs_within_guards() {
1738 let ref_for_guard
= self.local_decls
.push(LocalDecl
::<'tcx
> {
1739 // This variable isn't mutated but has a name, so has to be
1740 // immutable to avoid the unused mut lint.
1741 mutability
: Mutability
::Not
,
1742 ty
: tcx
.mk_imm_ref(tcx
.types
.re_erased
, var_ty
),
1743 user_ty
: UserTypeProjections
::none(),
1748 is_block_tail
: None
,
1749 is_user_variable
: Some(ClearCrossCrate
::Set(BindingForm
::RefForGuard
)),
1751 LocalsForNode
::ForGuard
{
1756 LocalsForNode
::One(for_arm_body
)
1758 debug
!("declare_binding: vars={:?}", locals
);
1759 self.var_indices
.insert(var_id
, locals
);