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::expr::as_place::PlaceBuilder;
9 use crate::build::scope::DropKind;
10 use crate::build::ForGuard::{self, OutsideGuard, RefWithinGuard};
11 use crate::build::{BlockAnd, BlockAndExtension, Builder};
12 use crate::build::{GuardFrame, GuardFrameLocal, LocalsForNode};
13 use rustc_data_structures::{
14 fx::{FxHashSet, FxIndexMap, FxIndexSet},
15 stack::ensure_sufficient_stack,
17 use rustc_index::bit_set::BitSet;
18 use rustc_middle::middle::region;
19 use rustc_middle::mir::*;
20 use rustc_middle::thir::{self, *};
21 use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty};
22 use rustc_span::symbol::Symbol;
23 use rustc_span::{BytePos, Pos, Span};
24 use rustc_target::abi::VariantIdx;
25 use smallvec::{smallvec, SmallVec};
27 // helper functions, broken out by category:
32 use std::borrow::Borrow;
33 use std::convert::TryFrom;
36 impl<'a, 'tcx> Builder<'a, 'tcx> {
37 pub(crate) fn then_else_break(
39 mut block: BasicBlock,
41 temp_scope_override: Option<region::Scope>,
42 break_scope: region::Scope,
43 variable_source_info: SourceInfo,
46 let expr_span = expr.span;
49 ExprKind::LogicalOp { op: LogicalOp::And, lhs, rhs } => {
50 let lhs_then_block = unpack!(this.then_else_break(
58 let rhs_then_block = unpack!(this.then_else_break(
68 ExprKind::Scope { region_scope, lint_level, value } => {
69 let region_scope = (region_scope, this.source_info(expr_span));
70 this.in_scope(region_scope, lint_level, |this| {
80 ExprKind::Let { expr, ref pat } => this.lower_let_expr(
85 Some(variable_source_info.scope),
86 variable_source_info.span,
89 let temp_scope = temp_scope_override.unwrap_or_else(|| this.local_scope());
90 let mutability = Mutability::Mut;
92 unpack!(block = this.as_temp(block, Some(temp_scope), expr, mutability));
93 let operand = Operand::Move(Place::from(place));
95 let then_block = this.cfg.start_new_block();
96 let else_block = this.cfg.start_new_block();
97 let term = TerminatorKind::if_(this.tcx, operand, then_block, else_block);
99 let source_info = this.source_info(expr_span);
100 this.cfg.terminate(block, source_info, term);
101 this.break_for_else(else_block, break_scope, source_info);
108 /// Generates MIR for a `match` expression.
110 /// The MIR that we generate for a match looks like this.
115 /// [ 1. Evaluate Scrutinee (expression being matched on) ]
116 /// [ (fake read of scrutinee) ]
118 /// [ 2. Decision tree -- check discriminants ] <--------+
120 /// | (once a specific arm is chosen) |
122 /// [pre_binding_block] [otherwise_block]
124 /// [ 3. Create "guard bindings" for arm ] |
125 /// [ (create fake borrows) ] |
127 /// [ 4. Execute guard code ] |
128 /// [ (read fake borrows) ] --(guard is false)-----------+
130 /// | (guard results in true)
132 /// [ 5. Create real bindings and execute arm ]
137 /// All of the different arms have been stacked on top of each other to
138 /// simplify the diagram. For an arm with no guard the blocks marked 3 and
139 /// 4 and the fake borrows are omitted.
141 /// We generate MIR in the following steps:
143 /// 1. Evaluate the scrutinee and add the fake read of it ([Builder::lower_scrutinee]).
144 /// 2. Create the decision tree ([Builder::lower_match_tree]).
145 /// 3. Determine the fake borrows that are needed from the places that were
146 /// matched against and create the required temporaries for them
147 /// ([Builder::calculate_fake_borrows]).
148 /// 4. Create everything else: the guards and the arms ([Builder::lower_match_arms]).
152 /// We don't want to have the exact structure of the decision tree be
153 /// visible through borrow checking. False edges ensure that the CFG as
154 /// seen by borrow checking doesn't encode this. False edges are added:
156 /// * From each pre-binding block to the next pre-binding block.
157 /// * From each otherwise block to the next pre-binding block.
158 #[instrument(level = "debug", skip(self, arms))]
159 pub(crate) fn match_expr(
161 destination: Place<'tcx>,
163 mut block: BasicBlock,
164 scrutinee: &Expr<'tcx>,
167 let scrutinee_span = scrutinee.span;
168 let scrutinee_place =
169 unpack!(block = self.lower_scrutinee(block, scrutinee, scrutinee_span,));
171 let mut arm_candidates = self.create_match_candidates(scrutinee_place.clone(), &arms);
173 let match_has_guard = arm_candidates.iter().any(|(_, candidate)| candidate.has_guard);
175 arm_candidates.iter_mut().map(|(_, candidate)| candidate).collect::<Vec<_>>();
177 let match_start_span = span.shrink_to_lo().to(scrutinee.span);
179 let fake_borrow_temps = self.lower_match_tree(
187 self.lower_match_arms(
192 self.source_info(span),
197 /// Evaluate the scrutinee and add the fake read of it.
200 mut block: BasicBlock,
201 scrutinee: &Expr<'tcx>,
202 scrutinee_span: Span,
203 ) -> BlockAnd<PlaceBuilder<'tcx>> {
204 let scrutinee_place_builder = unpack!(block = self.as_place_builder(block, scrutinee));
205 // Matching on a `scrutinee_place` with an uninhabited type doesn't
206 // generate any memory reads by itself, and so if the place "expression"
207 // contains unsafe operations like raw pointer dereferences or union
208 // field projections, we wouldn't know to require an `unsafe` block
209 // around a `match` equivalent to `std::intrinsics::unreachable()`.
210 // See issue #47412 for this hole being discovered in the wild.
212 // HACK(eddyb) Work around the above issue by adding a dummy inspection
213 // of `scrutinee_place`, specifically by applying `ReadForMatch`.
215 // NOTE: ReadForMatch also checks that the scrutinee is initialized.
216 // This is currently needed to not allow matching on an uninitialized,
217 // uninhabited value. If we get never patterns, those will check that
218 // the place is initialized, and so this read would only be used to
220 let cause_matched_place = FakeReadCause::ForMatchedPlace(None);
221 let source_info = self.source_info(scrutinee_span);
223 if let Some(scrutinee_place) = scrutinee_place_builder.try_to_place(self) {
224 self.cfg.push_fake_read(block, source_info, cause_matched_place, scrutinee_place);
227 block.and(scrutinee_place_builder)
230 /// Create the initial `Candidate`s for a `match` expression.
231 fn create_match_candidates<'pat>(
233 scrutinee: PlaceBuilder<'tcx>,
235 ) -> Vec<(&'pat Arm<'tcx>, Candidate<'pat, 'tcx>)>
239 // Assemble a list of candidates: there is one candidate per pattern,
240 // which means there may be more than one candidate *per arm*.
244 let arm = &self.thir[arm];
245 let arm_has_guard = arm.guard.is_some();
247 Candidate::new(scrutinee.clone(), &arm.pattern, arm_has_guard, self);
253 /// Create the decision tree for the match expression, starting from `block`.
255 /// Modifies `candidates` to store the bindings and type ascriptions for
258 /// Returns the places that need fake borrows because we bind or test them.
259 fn lower_match_tree<'pat>(
262 scrutinee_span: Span,
263 match_start_span: Span,
264 match_has_guard: bool,
265 candidates: &mut [&mut Candidate<'pat, 'tcx>],
266 ) -> Vec<(Place<'tcx>, Local)> {
267 // The set of places that we are creating fake borrows of. If there are
268 // no match guards then we don't need any fake borrows, so don't track
270 let mut fake_borrows = match_has_guard.then(FxIndexSet::default);
272 let mut otherwise = None;
274 // This will generate code to test scrutinee_place and
275 // branch to the appropriate arm block
276 self.match_candidates(
285 if let Some(otherwise_block) = otherwise {
286 // See the doc comment on `match_candidates` for why we may have an
287 // otherwise block. Match checking will ensure this is actually
289 let source_info = self.source_info(scrutinee_span);
290 self.cfg.terminate(otherwise_block, source_info, TerminatorKind::Unreachable);
293 // Link each leaf candidate to the `pre_binding_block` of the next one.
294 let mut previous_candidate: Option<&mut Candidate<'_, '_>> = None;
296 for candidate in candidates {
297 candidate.visit_leaves(|leaf_candidate| {
298 if let Some(ref mut prev) = previous_candidate {
299 prev.next_candidate_pre_binding_block = leaf_candidate.pre_binding_block;
301 previous_candidate = Some(leaf_candidate);
305 if let Some(ref borrows) = fake_borrows {
306 self.calculate_fake_borrows(borrows, scrutinee_span)
312 /// Lower the bindings, guards and arm bodies of a `match` expression.
314 /// The decision tree should have already been created
315 /// (by [Builder::lower_match_tree]).
317 /// `outer_source_info` is the SourceInfo for the whole match.
320 destination: Place<'tcx>,
321 scrutinee_place_builder: PlaceBuilder<'tcx>,
322 scrutinee_span: Span,
323 arm_candidates: Vec<(&'_ Arm<'tcx>, Candidate<'_, 'tcx>)>,
324 outer_source_info: SourceInfo,
325 fake_borrow_temps: Vec<(Place<'tcx>, Local)>,
327 let arm_end_blocks: Vec<_> = arm_candidates
329 .map(|(arm, candidate)| {
330 debug!("lowering arm {:?}\ncandidate = {:?}", arm, candidate);
332 let arm_source_info = self.source_info(arm.span);
333 let arm_scope = (arm.scope, arm_source_info);
334 let match_scope = self.local_scope();
335 self.in_scope(arm_scope, arm.lint_level, |this| {
336 // `try_to_place` may fail if it is unable to resolve the given
337 // `PlaceBuilder` inside a closure. In this case, we don't want to include
338 // a scrutinee place. `scrutinee_place_builder` will fail to be resolved
339 // if the only match arm is a wildcard (`_`).
344 // match foo { _ => () };
347 let scrutinee_place = scrutinee_place_builder.try_to_place(this);
348 let opt_scrutinee_place =
349 scrutinee_place.as_ref().map(|place| (Some(place), scrutinee_span));
350 let scope = this.declare_bindings(
354 ArmHasGuard(arm.guard.is_some()),
358 let arm_block = this.bind_pattern(
363 Some((arm, match_scope)),
367 if let Some(source_scope) = scope {
368 this.source_scope = source_scope;
371 this.expr_into_dest(destination, arm_block, &&this.thir[arm.body])
376 // all the arm blocks will rejoin here
377 let end_block = self.cfg.start_new_block();
379 let end_brace = self.source_info(
380 outer_source_info.span.with_lo(outer_source_info.span.hi() - BytePos::from_usize(1)),
382 for arm_block in arm_end_blocks {
383 let block = &self.cfg.basic_blocks[arm_block.0];
384 let last_location = block.statements.last().map(|s| s.source_info);
386 self.cfg.goto(unpack!(arm_block), last_location.unwrap_or(end_brace), end_block);
389 self.source_scope = outer_source_info.scope;
394 /// Binds the variables and ascribes types for a given `match` arm or
397 /// Also check if the guard matches, if it's provided.
398 /// `arm_scope` should be `Some` if and only if this is called for a
402 outer_source_info: SourceInfo,
403 candidate: Candidate<'_, 'tcx>,
404 fake_borrow_temps: &[(Place<'tcx>, Local)],
405 scrutinee_span: Span,
406 arm_match_scope: Option<(&Arm<'tcx>, region::Scope)>,
407 storages_alive: bool,
409 if candidate.subcandidates.is_empty() {
410 // Avoid generating another `BasicBlock` when we only have one
412 self.bind_and_guard_matched_candidate(
422 // It's helpful to avoid scheduling drops multiple times to save
423 // drop elaboration from having to clean up the extra drops.
425 // If we are in a `let` then we only schedule drops for the first
428 // If we're in a `match` arm then we could have a case like so:
430 // Ok(x) | Err(x) if return => { /* ... */ }
432 // In this case we don't want a drop of `x` scheduled when we
433 // return: it isn't bound by move until right before enter the arm.
434 // To handle this we instead unschedule it's drop after each time
435 // we lower the guard.
436 let target_block = self.cfg.start_new_block();
437 let mut schedule_drops = true;
438 let arm = arm_match_scope.unzip().0;
439 // We keep a stack of all of the bindings and type ascriptions
440 // from the parent candidates that we visit, that also need to
441 // be bound for each candidate.
445 &mut |leaf_candidate, parent_bindings| {
446 if let Some(arm) = arm {
447 self.clear_top_scope(arm.scope);
449 let binding_end = self.bind_and_guard_matched_candidate(
459 schedule_drops = false;
461 self.cfg.goto(binding_end, outer_source_info, target_block);
463 |inner_candidate, parent_bindings| {
464 parent_bindings.push((inner_candidate.bindings, inner_candidate.ascriptions));
465 inner_candidate.subcandidates.into_iter()
468 parent_bindings.pop();
476 pub(super) fn expr_into_pattern(
478 mut block: BasicBlock,
479 irrefutable_pat: &Pat<'tcx>,
480 initializer: &Expr<'tcx>,
482 match irrefutable_pat.kind {
483 // Optimize the case of `let x = ...` to write directly into `x`
484 PatKind::Binding { mode: BindingMode::ByValue, var, subpattern: None, .. } => {
486 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
487 unpack!(block = self.expr_into_dest(place, block, initializer));
489 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
490 let source_info = self.source_info(irrefutable_pat.span);
491 self.cfg.push_fake_read(block, source_info, FakeReadCause::ForLet(None), place);
493 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
497 // Optimize the case of `let x: T = ...` to write directly
498 // into `x` and then require that `T == typeof(x)`.
500 // Weirdly, this is needed to prevent the
501 // `intrinsic-move-val.rs` test case from crashing. That
502 // test works with uninitialized values in a rather
503 // dubious way, so it may be that the test is kind of
505 PatKind::AscribeUserType {
510 mode: BindingMode::ByValue, var, subpattern: None, ..
514 ascription: thir::Ascription { ref annotation, variance: _ },
517 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
518 unpack!(block = self.expr_into_dest(place, block, initializer));
520 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
521 let pattern_source_info = self.source_info(irrefutable_pat.span);
522 let cause_let = FakeReadCause::ForLet(None);
523 self.cfg.push_fake_read(block, pattern_source_info, cause_let, place);
525 let ty_source_info = self.source_info(annotation.span);
527 let base = self.canonical_user_type_annotations.push(annotation.clone());
531 source_info: ty_source_info,
532 kind: StatementKind::AscribeUserType(
533 Box::new((place, UserTypeProjection { base, projs: Vec::new() })),
534 // We always use invariant as the variance here. This is because the
535 // variance field from the ascription refers to the variance to use
536 // when applying the type to the value being matched, but this
537 // ascription applies rather to the type of the binding. e.g., in this
544 // We are creating an ascription that defines the type of `x` to be
545 // exactly `T` (i.e., with invariance). The variance field, in
546 // contrast, is intended to be used to relate `T` to the type of
548 ty::Variance::Invariant,
553 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
558 let place_builder = unpack!(block = self.as_place_builder(block, initializer));
559 self.place_into_pattern(block, &irrefutable_pat, place_builder, true)
564 pub(crate) fn place_into_pattern(
567 irrefutable_pat: &Pat<'tcx>,
568 initializer: PlaceBuilder<'tcx>,
569 set_match_place: bool,
571 let mut candidate = Candidate::new(initializer.clone(), &irrefutable_pat, false, self);
572 let fake_borrow_temps = self.lower_match_tree(
574 irrefutable_pat.span,
575 irrefutable_pat.span,
577 &mut [&mut candidate],
579 // For matches and function arguments, the place that is being matched
580 // can be set when creating the variables. But the place for
581 // let PATTERN = ... might not even exist until we do the assignment.
582 // so we set it here instead.
584 let mut candidate_ref = &candidate;
585 while let Some(next) = {
586 for binding in &candidate_ref.bindings {
587 let local = self.var_local_id(binding.var_id, OutsideGuard);
588 // `try_to_place` may fail if it is unable to resolve the given
589 // `PlaceBuilder` inside a closure. In this case, we don't want to include
590 // a scrutinee place. `scrutinee_place_builder` will fail for destructured
591 // assignments. This is because a closure only captures the precise places
592 // that it will read and as a result a closure may not capture the entire
593 // tuple/struct and rather have individual places that will be read in the
599 // let (v1, v2) = foo;
602 if let Some(place) = initializer.try_to_place(self) {
603 let Some(box LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
604 VarBindingForm { opt_match_place: Some((ref mut match_place, _)), .. },
605 )))) = self.local_decls[local].local_info else {
606 bug!("Let binding to non-user variable.")
608 *match_place = Some(place);
611 // All of the subcandidates should bind the same locals, so we
612 // only visit the first one.
613 candidate_ref.subcandidates.get(0)
615 candidate_ref = next;
620 self.source_info(irrefutable_pat.span),
623 irrefutable_pat.span,
630 /// Declares the bindings of the given patterns and returns the visibility
631 /// scope for the bindings in these patterns, if such a scope had to be
632 /// created. NOTE: Declaring the bindings should always be done in their
634 #[instrument(skip(self), level = "debug")]
635 pub(crate) fn declare_bindings(
637 mut visibility_scope: Option<SourceScope>,
640 has_guard: ArmHasGuard,
641 opt_match_place: Option<(Option<&Place<'tcx>>, Span)>,
642 ) -> Option<SourceScope> {
643 self.visit_primary_bindings(
645 UserTypeProjections::none(),
646 &mut |this, mutability, name, mode, var, span, ty, user_ty| {
647 if visibility_scope.is_none() {
649 Some(this.new_source_scope(scope_span, LintLevel::Inherited, None));
651 let source_info = SourceInfo { span, scope: this.source_scope };
652 let visibility_scope = visibility_scope.unwrap();
653 this.declare_binding(
663 opt_match_place.map(|(x, y)| (x.cloned(), y)),
671 pub(crate) fn storage_live_binding(
679 let local_id = self.var_local_id(var, for_guard);
680 let source_info = self.source_info(span);
681 self.cfg.push(block, Statement { source_info, kind: StatementKind::StorageLive(local_id) });
682 // Although there is almost always scope for given variable in corner cases
683 // like #92893 we might get variable with no scope.
684 if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) && schedule_drop {
685 self.schedule_drop(span, region_scope, local_id, DropKind::Storage);
687 Place::from(local_id)
690 pub(crate) fn schedule_drop_for_binding(
696 let local_id = self.var_local_id(var, for_guard);
697 if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) {
698 self.schedule_drop(span, region_scope, local_id, DropKind::Value);
702 /// Visit all of the primary bindings in a patterns, that is, visit the
703 /// leftmost occurrence of each variable bound in a pattern. A variable
704 /// will occur more than once in an or-pattern.
705 pub(super) fn visit_primary_bindings(
708 pattern_user_ty: UserTypeProjections,
721 "visit_primary_bindings: pattern={:?} pattern_user_ty={:?}",
722 pattern, pattern_user_ty
736 f(self, mutability, name, mode, var, pattern.span, ty, pattern_user_ty.clone());
738 if let Some(subpattern) = subpattern.as_ref() {
739 self.visit_primary_bindings(subpattern, pattern_user_ty, f);
743 PatKind::Array { ref prefix, ref slice, ref suffix }
744 | PatKind::Slice { ref prefix, ref slice, ref suffix } => {
745 let from = u64::try_from(prefix.len()).unwrap();
746 let to = u64::try_from(suffix.len()).unwrap();
747 for subpattern in prefix.iter() {
748 self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
750 for subpattern in slice {
751 self.visit_primary_bindings(
753 pattern_user_ty.clone().subslice(from, to),
757 for subpattern in suffix.iter() {
758 self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
762 PatKind::Constant { .. } | PatKind::Range { .. } | PatKind::Wild => {}
764 PatKind::Deref { ref subpattern } => {
765 self.visit_primary_bindings(subpattern, pattern_user_ty.deref(), f);
768 PatKind::AscribeUserType {
770 ascription: thir::Ascription { ref annotation, variance: _ },
772 // This corresponds to something like
775 // let A::<'a>(_): A<'static> = ...;
778 // Note that the variance doesn't apply here, as we are tracking the effect
779 // of `user_ty` on any bindings contained with subpattern.
781 let projection = UserTypeProjection {
782 base: self.canonical_user_type_annotations.push(annotation.clone()),
785 let subpattern_user_ty =
786 pattern_user_ty.push_projection(&projection, annotation.span);
787 self.visit_primary_bindings(subpattern, subpattern_user_ty, f)
790 PatKind::Leaf { ref subpatterns } => {
791 for subpattern in subpatterns {
792 let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field);
793 debug!("visit_primary_bindings: subpattern_user_ty={:?}", subpattern_user_ty);
794 self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
798 PatKind::Variant { adt_def, substs: _, variant_index, ref subpatterns } => {
799 for subpattern in subpatterns {
800 let subpattern_user_ty =
801 pattern_user_ty.clone().variant(adt_def, variant_index, subpattern.field);
802 self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
805 PatKind::Or { ref pats } => {
806 // In cases where we recover from errors the primary bindings
807 // may not all be in the leftmost subpattern. For example in
808 // `let (x | y) = ...`, the primary binding of `y` occurs in
809 // the right subpattern
810 for subpattern in pats.iter() {
811 self.visit_primary_bindings(subpattern, pattern_user_ty.clone(), f);
819 struct Candidate<'pat, 'tcx> {
820 /// [`Span`] of the original pattern that gave rise to this candidate.
823 /// Whether this `Candidate` has a guard.
826 /// All of these must be satisfied...
827 match_pairs: SmallVec<[MatchPair<'pat, 'tcx>; 1]>,
829 /// ...these bindings established...
830 bindings: Vec<Binding<'tcx>>,
832 /// ...and these types asserted...
833 ascriptions: Vec<Ascription<'tcx>>,
835 /// ...and if this is non-empty, one of these subcandidates also has to match...
836 subcandidates: Vec<Candidate<'pat, 'tcx>>,
838 /// ...and the guard must be evaluated; if it's `false` then branch to `otherwise_block`.
839 otherwise_block: Option<BasicBlock>,
841 /// The block before the `bindings` have been established.
842 pre_binding_block: Option<BasicBlock>,
843 /// The pre-binding block of the next candidate.
844 next_candidate_pre_binding_block: Option<BasicBlock>,
847 impl<'tcx, 'pat> Candidate<'pat, 'tcx> {
849 place: PlaceBuilder<'tcx>,
850 pattern: &'pat Pat<'tcx>,
852 cx: &Builder<'_, 'tcx>,
857 match_pairs: smallvec![MatchPair::new(place, pattern, cx)],
858 bindings: Vec::new(),
859 ascriptions: Vec::new(),
860 subcandidates: Vec::new(),
861 otherwise_block: None,
862 pre_binding_block: None,
863 next_candidate_pre_binding_block: None,
867 /// Visit the leaf candidates (those with no subcandidates) contained in
869 fn visit_leaves<'a>(&'a mut self, mut visit_leaf: impl FnMut(&'a mut Self)) {
873 &mut move |c, _| visit_leaf(c),
874 move |c, _| c.subcandidates.iter_mut(),
880 /// A depth-first traversal of the `Candidate` and all of its recursive
882 fn traverse_candidate<'pat, 'tcx: 'pat, C, T, I>(
885 visit_leaf: &mut impl FnMut(C, &mut T),
886 get_children: impl Copy + Fn(C, &mut T) -> I,
887 complete_children: impl Copy + Fn(&mut T),
889 C: Borrow<Candidate<'pat, 'tcx>>,
890 I: Iterator<Item = C>,
892 if candidate.borrow().subcandidates.is_empty() {
893 visit_leaf(candidate, context)
895 for child in get_children(candidate, context) {
896 traverse_candidate(child, context, visit_leaf, get_children, complete_children);
898 complete_children(context)
902 #[derive(Clone, Debug)]
903 struct Binding<'tcx> {
907 binding_mode: BindingMode,
910 /// Indicates that the type of `source` must be a subtype of the
911 /// user-given type `user_ty`; this is basically a no-op but can
912 /// influence region inference.
913 #[derive(Clone, Debug)]
914 struct Ascription<'tcx> {
916 annotation: CanonicalUserTypeAnnotation<'tcx>,
917 variance: ty::Variance,
920 #[derive(Clone, Debug)]
921 pub(crate) struct MatchPair<'pat, 'tcx> {
923 place: PlaceBuilder<'tcx>,
925 // ... must match this pattern.
926 pattern: &'pat Pat<'tcx>,
929 /// See [`Test`] for more.
930 #[derive(Clone, Debug, PartialEq)]
931 enum TestKind<'tcx> {
932 /// Test what enum variant a value is.
934 /// The enum type being tested.
935 adt_def: ty::AdtDef<'tcx>,
936 /// The set of variants that we should create a branch for. We also
937 /// create an additional "otherwise" case.
938 variants: BitSet<VariantIdx>,
941 /// Test what value an integer, `bool`, or `char` has.
943 /// The type of the value that we're testing.
945 /// The (ordered) set of values that we test for.
947 /// For integers and `char`s we create a branch to each of the values in
948 /// `options`, as well as an "otherwise" branch for all other values, even
949 /// in the (rare) case that `options` is exhaustive.
951 /// For `bool` we always generate two edges, one for `true` and one for
953 options: FxIndexMap<ConstantKind<'tcx>, u128>,
956 /// Test for equality with value, possibly after an unsizing coercion to
959 value: ConstantKind<'tcx>,
960 // Integer types are handled by `SwitchInt`, and constants with ADT
961 // types are converted back into patterns, so this can only be `&str`,
962 // `&[T]`, `f32` or `f64`.
966 /// Test whether the value falls within an inclusive or exclusive range
967 Range(Box<PatRange<'tcx>>),
969 /// Test that the length of the slice is equal to `len`.
970 Len { len: u64, op: BinOp },
973 /// A test to perform to determine which [`Candidate`] matches a value.
975 /// [`Test`] is just the test to perform; it does not include the value
978 pub(crate) struct Test<'tcx> {
980 kind: TestKind<'tcx>,
983 /// `ArmHasGuard` is a wrapper around a boolean flag. It indicates whether
984 /// a match arm has a guard expression attached to it.
985 #[derive(Copy, Clone, Debug)]
986 pub(crate) struct ArmHasGuard(pub(crate) bool);
988 ///////////////////////////////////////////////////////////////////////////
989 // Main matching algorithm
991 impl<'a, 'tcx> Builder<'a, 'tcx> {
992 /// The main match algorithm. It begins with a set of candidates
993 /// `candidates` and has the job of generating code to determine
994 /// which of these candidates, if any, is the correct one. The
995 /// candidates are sorted such that the first item in the list
996 /// has the highest priority. When a candidate is found to match
997 /// the value, we will set and generate a branch to the appropriate
998 /// pre-binding block.
1000 /// If we find that *NONE* of the candidates apply, we branch to the
1001 /// `otherwise_block`, setting it to `Some` if required. In principle, this
1002 /// means that the input list was not exhaustive, though at present we
1003 /// sometimes are not smart enough to recognize all exhaustive inputs.
1005 /// It might be surprising that the input can be non-exhaustive.
1006 /// Indeed, initially, it is not, because all matches are
1007 /// exhaustive in Rust. But during processing we sometimes divide
1008 /// up the list of candidates and recurse with a non-exhaustive
1009 /// list. This is important to keep the size of the generated code
1010 /// under control. See [`Builder::test_candidates`] for more details.
1012 /// If `fake_borrows` is `Some`, then places which need fake borrows
1013 /// will be added to it.
1015 /// For an example of a case where we set `otherwise_block`, even for an
1016 /// exhaustive match, consider:
1019 /// # fn foo(x: (bool, bool)) {
1021 /// (true, true) => (),
1022 /// (_, false) => (),
1023 /// (false, true) => (),
1028 /// For this match, we check if `x.0` matches `true` (for the first
1029 /// arm). If it doesn't match, we check `x.1`. If `x.1` is `true` we check
1030 /// if `x.0` matches `false` (for the third arm). In the (impossible at
1031 /// runtime) case when `x.0` is now `true`, we branch to
1032 /// `otherwise_block`.
1033 #[instrument(skip(self, fake_borrows), level = "debug")]
1034 fn match_candidates<'pat>(
1037 scrutinee_span: Span,
1038 start_block: BasicBlock,
1039 otherwise_block: &mut Option<BasicBlock>,
1040 candidates: &mut [&mut Candidate<'pat, 'tcx>],
1041 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1043 // Start by simplifying candidates. Once this process is complete, all
1044 // the match pairs which remain require some form of test, whether it
1045 // be a switch or pattern comparison.
1046 let mut split_or_candidate = false;
1047 for candidate in &mut *candidates {
1048 split_or_candidate |= self.simplify_candidate(candidate);
1051 ensure_sufficient_stack(|| {
1052 if split_or_candidate {
1053 // At least one of the candidates has been split into subcandidates.
1054 // We need to change the candidate list to include those.
1055 let mut new_candidates = Vec::new();
1057 for candidate in candidates {
1058 candidate.visit_leaves(|leaf_candidate| new_candidates.push(leaf_candidate));
1060 self.match_simplified_candidates(
1065 &mut *new_candidates,
1069 self.match_simplified_candidates(
1081 fn match_simplified_candidates(
1084 scrutinee_span: Span,
1085 start_block: BasicBlock,
1086 otherwise_block: &mut Option<BasicBlock>,
1087 candidates: &mut [&mut Candidate<'_, 'tcx>],
1088 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1090 // The candidates are sorted by priority. Check to see whether the
1091 // higher priority candidates (and hence at the front of the slice)
1092 // have satisfied all their match pairs.
1093 let fully_matched = candidates.iter().take_while(|c| c.match_pairs.is_empty()).count();
1094 debug!("match_candidates: {:?} candidates fully matched", fully_matched);
1095 let (matched_candidates, unmatched_candidates) = candidates.split_at_mut(fully_matched);
1097 let block = if !matched_candidates.is_empty() {
1098 let otherwise_block =
1099 self.select_matched_candidates(matched_candidates, start_block, fake_borrows);
1101 if let Some(last_otherwise_block) = otherwise_block {
1102 last_otherwise_block
1104 // Any remaining candidates are unreachable.
1105 if unmatched_candidates.is_empty() {
1108 self.cfg.start_new_block()
1114 // If there are no candidates that still need testing, we're
1115 // done. Since all matches are exhaustive, execution should
1116 // never reach this point.
1117 if unmatched_candidates.is_empty() {
1118 let source_info = self.source_info(span);
1119 if let Some(otherwise) = *otherwise_block {
1120 self.cfg.goto(block, source_info, otherwise);
1122 *otherwise_block = Some(block);
1127 // Test for the remaining candidates.
1128 self.test_candidates_with_or(
1131 unmatched_candidates,
1138 /// Link up matched candidates.
1140 /// For example, if we have something like this:
1142 /// ```ignore (illustrative)
1144 /// Some(x) if cond1 => ...
1146 /// Some(x) if cond2 => ...
1150 /// We generate real edges from:
1152 /// * `start_block` to the [pre-binding block] of the first pattern,
1153 /// * the [otherwise block] of the first pattern to the second pattern,
1154 /// * the [otherwise block] of the third pattern to a block with an
1155 /// [`Unreachable` terminator](TerminatorKind::Unreachable).
1157 /// In addition, we add fake edges from the otherwise blocks to the
1158 /// pre-binding block of the next candidate in the original set of
1161 /// [pre-binding block]: Candidate::pre_binding_block
1162 /// [otherwise block]: Candidate::otherwise_block
1163 fn select_matched_candidates(
1165 matched_candidates: &mut [&mut Candidate<'_, 'tcx>],
1166 start_block: BasicBlock,
1167 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1168 ) -> Option<BasicBlock> {
1170 !matched_candidates.is_empty(),
1171 "select_matched_candidates called with no candidates",
1174 matched_candidates.iter().all(|c| c.subcandidates.is_empty()),
1175 "subcandidates should be empty in select_matched_candidates",
1178 // Insert a borrows of prefixes of places that are bound and are
1179 // behind a dereference projection.
1181 // These borrows are taken to avoid situations like the following:
1184 // _ if { x = &[0]; false } => (),
1185 // y => (), // Out of bounds array access!
1189 // // y is bound by reference in the guard and then by copy in the
1190 // // arm, so y is 2 in the arm!
1191 // y if { y == 1 && (x = &2) == () } => y,
1194 if let Some(fake_borrows) = fake_borrows {
1195 for Binding { source, .. } in
1196 matched_candidates.iter().flat_map(|candidate| &candidate.bindings)
1199 source.projection.iter().rposition(|elem| elem == ProjectionElem::Deref)
1201 let proj_base = &source.projection[..i];
1203 fake_borrows.insert(Place {
1204 local: source.local,
1205 projection: self.tcx.intern_place_elems(proj_base),
1211 let fully_matched_with_guard = matched_candidates
1213 .position(|c| !c.has_guard)
1214 .unwrap_or(matched_candidates.len() - 1);
1216 let (reachable_candidates, unreachable_candidates) =
1217 matched_candidates.split_at_mut(fully_matched_with_guard + 1);
1219 let mut next_prebinding = start_block;
1221 for candidate in reachable_candidates.iter_mut() {
1222 assert!(candidate.otherwise_block.is_none());
1223 assert!(candidate.pre_binding_block.is_none());
1224 candidate.pre_binding_block = Some(next_prebinding);
1225 if candidate.has_guard {
1226 // Create the otherwise block for this candidate, which is the
1227 // pre-binding block for the next candidate.
1228 next_prebinding = self.cfg.start_new_block();
1229 candidate.otherwise_block = Some(next_prebinding);
1234 "match_candidates: add pre_binding_blocks for unreachable {:?}",
1235 unreachable_candidates,
1237 for candidate in unreachable_candidates {
1238 assert!(candidate.pre_binding_block.is_none());
1239 candidate.pre_binding_block = Some(self.cfg.start_new_block());
1242 reachable_candidates.last_mut().unwrap().otherwise_block
1245 /// Tests a candidate where there are only or-patterns left to test, or
1246 /// forwards to [Builder::test_candidates].
1248 /// Given a pattern `(P | Q, R | S)` we (in principle) generate a CFG like
1256 /// +----------------------------------------+------------------------------------+
1259 /// [ P matches ] [ Q matches ] [ otherwise ]
1262 /// [ match R, S ] [ match R, S ] |
1264 /// +--------------+------------+ +--------------+------------+ |
1267 /// [ R matches ] [ S matches ] [otherwise ] [ R matches ] [ S matches ] [otherwise ] |
1269 /// +--------------+------------|------------+--------------+ | |
1271 /// | +----------------------------------------+--------+
1274 /// [ Success ] [ Failure ]
1277 /// In practice there are some complications:
1279 /// * If there's a guard, then the otherwise branch of the first match on
1280 /// `R | S` goes to a test for whether `Q` matches, and the control flow
1281 /// doesn't merge into a single success block until after the guard is
1283 /// * If neither `P` or `Q` has any bindings or type ascriptions and there
1284 /// isn't a match guard, then we create a smaller CFG like:
1288 /// +---------------+------------+
1290 /// [ P matches ] [ Q matches ] [ otherwise ]
1292 /// +---------------+ |
1298 fn test_candidates_with_or(
1301 scrutinee_span: Span,
1302 candidates: &mut [&mut Candidate<'_, 'tcx>],
1304 otherwise_block: &mut Option<BasicBlock>,
1305 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1307 let (first_candidate, remaining_candidates) = candidates.split_first_mut().unwrap();
1309 // All of the or-patterns have been sorted to the end, so if the first
1310 // pattern is an or-pattern we only have or-patterns.
1311 match first_candidate.match_pairs[0].pattern.kind {
1312 PatKind::Or { .. } => (),
1314 self.test_candidates(
1326 let match_pairs = mem::take(&mut first_candidate.match_pairs);
1327 first_candidate.pre_binding_block = Some(block);
1329 let mut otherwise = None;
1330 for match_pair in match_pairs {
1331 let PatKind::Or { ref pats } = &match_pair.pattern.kind else {
1332 bug!("Or-patterns should have been sorted to the end");
1334 let or_span = match_pair.pattern.span;
1335 let place = match_pair.place;
1337 first_candidate.visit_leaves(|leaf_candidate| {
1338 self.test_or_pattern(
1349 let remainder_start = otherwise.unwrap_or_else(|| self.cfg.start_new_block());
1351 self.match_candidates(
1356 remaining_candidates,
1362 skip(self, otherwise, or_span, place, fake_borrows, candidate, pats),
1365 fn test_or_pattern<'pat>(
1367 candidate: &mut Candidate<'pat, 'tcx>,
1368 otherwise: &mut Option<BasicBlock>,
1369 pats: &'pat [Box<Pat<'tcx>>],
1371 place: PlaceBuilder<'tcx>,
1372 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1374 debug!("candidate={:#?}\npats={:#?}", candidate, pats);
1375 let mut or_candidates: Vec<_> = pats
1377 .map(|pat| Candidate::new(place.clone(), pat, candidate.has_guard, self))
1379 let mut or_candidate_refs: Vec<_> = or_candidates.iter_mut().collect();
1380 let otherwise = if candidate.otherwise_block.is_some() {
1381 &mut candidate.otherwise_block
1385 self.match_candidates(
1388 candidate.pre_binding_block.unwrap(),
1390 &mut or_candidate_refs,
1393 candidate.subcandidates = or_candidates;
1394 self.merge_trivial_subcandidates(candidate, self.source_info(or_span));
1397 /// Try to merge all of the subcandidates of the given candidate into one.
1398 /// This avoids exponentially large CFGs in cases like `(1 | 2, 3 | 4, ...)`.
1399 fn merge_trivial_subcandidates(
1401 candidate: &mut Candidate<'_, 'tcx>,
1402 source_info: SourceInfo,
1404 if candidate.subcandidates.is_empty() || candidate.has_guard {
1405 // FIXME(or_patterns; matthewjasper) Don't give up if we have a guard.
1409 let mut can_merge = true;
1411 // Not `Iterator::all` because we don't want to short-circuit.
1412 for subcandidate in &mut candidate.subcandidates {
1413 self.merge_trivial_subcandidates(subcandidate, source_info);
1415 // FIXME(or_patterns; matthewjasper) Try to be more aggressive here.
1416 can_merge &= subcandidate.subcandidates.is_empty()
1417 && subcandidate.bindings.is_empty()
1418 && subcandidate.ascriptions.is_empty();
1422 let any_matches = self.cfg.start_new_block();
1423 for subcandidate in mem::take(&mut candidate.subcandidates) {
1424 let or_block = subcandidate.pre_binding_block.unwrap();
1425 self.cfg.goto(or_block, source_info, any_matches);
1427 candidate.pre_binding_block = Some(any_matches);
1431 /// This is the most subtle part of the matching algorithm. At
1432 /// this point, the input candidates have been fully simplified,
1433 /// and so we know that all remaining match-pairs require some
1434 /// sort of test. To decide what test to perform, we take the highest
1435 /// priority candidate (the first one in the list, as of January 2021)
1436 /// and extract the first match-pair from the list. From this we decide
1437 /// what kind of test is needed using [`Builder::test`], defined in the
1438 /// [`test` module](mod@test).
1440 /// *Note:* taking the first match pair is somewhat arbitrary, and
1441 /// we might do better here by choosing more carefully what to
1444 /// For example, consider the following possible match-pairs:
1446 /// 1. `x @ Some(P)` -- we will do a [`Switch`] to decide what variant `x` has
1447 /// 2. `x @ 22` -- we will do a [`SwitchInt`] to decide what value `x` has
1448 /// 3. `x @ 3..5` -- we will do a [`Range`] test to decide what range `x` falls in
1451 /// [`Switch`]: TestKind::Switch
1452 /// [`SwitchInt`]: TestKind::SwitchInt
1453 /// [`Range`]: TestKind::Range
1455 /// Once we know what sort of test we are going to perform, this
1456 /// test may also help us winnow down our candidates. So we walk over
1457 /// the candidates (from high to low priority) and check. This
1458 /// gives us, for each outcome of the test, a transformed list of
1459 /// candidates. For example, if we are testing `x.0`'s variant,
1460 /// and we have a candidate `(x.0 @ Some(v), x.1 @ 22)`,
1461 /// then we would have a resulting candidate of `((x.0 as Some).0 @ v, x.1 @ 22)`.
1462 /// Note that the first match-pair is now simpler (and, in fact, irrefutable).
1464 /// But there may also be candidates that the test just doesn't
1465 /// apply to. The classical example involves wildcards:
1468 /// # let (x, y, z) = (true, true, true);
1469 /// match (x, y, z) {
1470 /// (true , _ , true ) => true, // (0)
1471 /// (_ , true , _ ) => true, // (1)
1472 /// (false, false, _ ) => false, // (2)
1473 /// (true , _ , false) => false, // (3)
1478 /// In that case, after we test on `x`, there are 2 overlapping candidate
1481 /// - If the outcome is that `x` is true, candidates 0, 1, and 3
1482 /// - If the outcome is that `x` is false, candidates 1 and 2
1484 /// Here, the traditional "decision tree" method would generate 2
1485 /// separate code-paths for the 2 separate cases.
1487 /// In some cases, this duplication can create an exponential amount of
1488 /// code. This is most easily seen by noticing that this method terminates
1489 /// with precisely the reachable arms being reachable - but that problem
1490 /// is trivially NP-complete:
1492 /// ```ignore (illustrative)
1493 /// match (var0, var1, var2, var3, ...) {
1494 /// (true , _ , _ , false, true, ...) => false,
1495 /// (_ , true, true , false, _ , ...) => false,
1496 /// (false, _ , false, false, _ , ...) => false,
1502 /// Here the last arm is reachable only if there is an assignment to
1503 /// the variables that does not match any of the literals. Therefore,
1504 /// compilation would take an exponential amount of time in some cases.
1506 /// That kind of exponential worst-case might not occur in practice, but
1507 /// our simplistic treatment of constants and guards would make it occur
1508 /// in very common situations - for example [#29740]:
1510 /// ```ignore (illustrative)
1512 /// "foo" if foo_guard => ...,
1513 /// "bar" if bar_guard => ...,
1514 /// "baz" if baz_guard => ...,
1519 /// [#29740]: https://github.com/rust-lang/rust/issues/29740
1521 /// Here we first test the match-pair `x @ "foo"`, which is an [`Eq` test].
1523 /// [`Eq` test]: TestKind::Eq
1525 /// It might seem that we would end up with 2 disjoint candidate
1526 /// sets, consisting of the first candidate or the other two, but our
1527 /// algorithm doesn't reason about `"foo"` being distinct from the other
1528 /// constants; it considers the latter arms to potentially match after
1529 /// both outcomes, which obviously leads to an exponential number
1532 /// To avoid these kinds of problems, our algorithm tries to ensure
1533 /// the amount of generated tests is linear. When we do a k-way test,
1534 /// we return an additional "unmatched" set alongside the obvious `k`
1535 /// sets. When we encounter a candidate that would be present in more
1536 /// than one of the sets, we put it and all candidates below it into the
1537 /// "unmatched" set. This ensures these `k+1` sets are disjoint.
1539 /// After we perform our test, we branch into the appropriate candidate
1540 /// set and recurse with `match_candidates`. These sub-matches are
1541 /// obviously non-exhaustive - as we discarded our otherwise set - so
1542 /// we set their continuation to do `match_candidates` on the
1543 /// "unmatched" set (which is again non-exhaustive).
1545 /// If you apply this to the above test, you basically wind up
1546 /// with an if-else-if chain, testing each candidate in turn,
1547 /// which is precisely what we want.
1549 /// In addition to avoiding exponential-time blowups, this algorithm
1550 /// also has the nice property that each guard and arm is only generated
1552 fn test_candidates<'pat, 'b, 'c>(
1555 scrutinee_span: Span,
1556 mut candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>],
1558 otherwise_block: &mut Option<BasicBlock>,
1559 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1561 // extract the match-pair from the highest priority candidate
1562 let match_pair = &candidates.first().unwrap().match_pairs[0];
1563 let mut test = self.test(match_pair);
1564 let match_place = match_pair.place.clone();
1566 // most of the time, the test to perform is simply a function
1567 // of the main candidate; but for a test like SwitchInt, we
1568 // may want to add cases based on the candidates that are
1571 TestKind::SwitchInt { switch_ty, ref mut options } => {
1572 for candidate in candidates.iter() {
1573 if !self.add_cases_to_switch(&match_place, candidate, switch_ty, options) {
1578 TestKind::Switch { adt_def: _, ref mut variants } => {
1579 for candidate in candidates.iter() {
1580 if !self.add_variants_to_switch(&match_place, candidate, variants) {
1588 // Insert a Shallow borrow of any places that is switched on.
1589 if let Some(fb) = fake_borrows
1590 && let Some(resolved_place) = match_place.try_to_place(self)
1592 fb.insert(resolved_place);
1595 // perform the test, branching to one of N blocks. For each of
1596 // those N possible outcomes, create a (initially empty)
1597 // vector of candidates. Those are the candidates that still
1598 // apply if the test has that particular outcome.
1599 debug!("test_candidates: test={:?} match_pair={:?}", test, match_pair);
1600 let mut target_candidates: Vec<Vec<&mut Candidate<'pat, 'tcx>>> = vec![];
1601 target_candidates.resize_with(test.targets(), Default::default);
1603 let total_candidate_count = candidates.len();
1605 // Sort the candidates into the appropriate vector in
1606 // `target_candidates`. Note that at some point we may
1607 // encounter a candidate where the test is not relevant; at
1608 // that point, we stop sorting.
1609 while let Some(candidate) = candidates.first_mut() {
1610 let Some(idx) = self.sort_candidate(&match_place.clone(), &test, candidate) else {
1613 let (candidate, rest) = candidates.split_first_mut().unwrap();
1614 target_candidates[idx].push(candidate);
1617 // at least the first candidate ought to be tested
1619 total_candidate_count > candidates.len(),
1621 total_candidate_count,
1624 debug!("tested_candidates: {}", total_candidate_count - candidates.len());
1625 debug!("untested_candidates: {}", candidates.len());
1627 // HACK(matthewjasper) This is a closure so that we can let the test
1628 // create its blocks before the rest of the match. This currently
1629 // improves the speed of llvm when optimizing long string literal
1631 let make_target_blocks = move |this: &mut Self| -> Vec<BasicBlock> {
1632 // The block that we should branch to if none of the
1633 // `target_candidates` match. This is either the block where we
1634 // start matching the untested candidates if there are any,
1635 // otherwise it's the `otherwise_block`.
1636 let remainder_start = &mut None;
1637 let remainder_start =
1638 if candidates.is_empty() { &mut *otherwise_block } else { remainder_start };
1640 // For each outcome of test, process the candidates that still
1641 // apply. Collect a list of blocks where control flow will
1642 // branch if one of the `target_candidate` sets is not
1644 let target_blocks: Vec<_> = target_candidates
1646 .map(|mut candidates| {
1647 if !candidates.is_empty() {
1648 let candidate_start = this.cfg.start_new_block();
1649 this.match_candidates(
1659 *remainder_start.get_or_insert_with(|| this.cfg.start_new_block())
1664 if !candidates.is_empty() {
1665 let remainder_start = remainder_start.unwrap_or_else(|| this.cfg.start_new_block());
1666 this.match_candidates(
1679 self.perform_test(span, scrutinee_span, block, match_place, &test, make_target_blocks);
1682 /// Determine the fake borrows that are needed from a set of places that
1683 /// have to be stable across match guards.
1685 /// Returns a list of places that need a fake borrow and the temporary
1686 /// that's used to store the fake borrow.
1688 /// Match exhaustiveness checking is not able to handle the case where the
1689 /// place being matched on is mutated in the guards. We add "fake borrows"
1690 /// to the guards that prevent any mutation of the place being matched.
1691 /// There are a some subtleties:
1693 /// 1. Borrowing `*x` doesn't prevent assigning to `x`. If `x` is a shared
1694 /// reference, the borrow isn't even tracked. As such we have to add fake
1695 /// borrows of any prefixes of a place
1696 /// 2. We don't want `match x { _ => (), }` to conflict with mutable
1697 /// borrows of `x`, so we only add fake borrows for places which are
1698 /// bound or tested by the match.
1699 /// 3. We don't want the fake borrows to conflict with `ref mut` bindings,
1700 /// so we use a special BorrowKind for them.
1701 /// 4. The fake borrows may be of places in inactive variants, so it would
1702 /// be UB to generate code for them. They therefore have to be removed
1703 /// by a MIR pass run after borrow checking.
1704 fn calculate_fake_borrows<'b>(
1706 fake_borrows: &'b FxIndexSet<Place<'tcx>>,
1708 ) -> Vec<(Place<'tcx>, Local)> {
1711 debug!("add_fake_borrows fake_borrows = {:?}", fake_borrows);
1713 let mut all_fake_borrows = Vec::with_capacity(fake_borrows.len());
1715 // Insert a Shallow borrow of the prefixes of any fake borrows.
1716 for place in fake_borrows {
1717 let mut cursor = place.projection.as_ref();
1718 while let [proj_base @ .., elem] = cursor {
1721 if let ProjectionElem::Deref = elem {
1722 // Insert a shallow borrow after a deref. For other
1723 // projections the borrow of prefix_cursor will
1724 // conflict with any mutation of base.
1725 all_fake_borrows.push(PlaceRef { local: place.local, projection: proj_base });
1729 all_fake_borrows.push(place.as_ref());
1733 let mut dedup = FxHashSet::default();
1734 all_fake_borrows.retain(|b| dedup.insert(*b));
1736 debug!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows);
1740 .map(|matched_place_ref| {
1741 let matched_place = Place {
1742 local: matched_place_ref.local,
1743 projection: tcx.intern_place_elems(matched_place_ref.projection),
1745 let fake_borrow_deref_ty = matched_place.ty(&self.local_decls, tcx).ty;
1746 let fake_borrow_ty = tcx.mk_imm_ref(tcx.lifetimes.re_erased, fake_borrow_deref_ty);
1747 let fake_borrow_temp =
1748 self.local_decls.push(LocalDecl::new(fake_borrow_ty, temp_span));
1750 (matched_place, fake_borrow_temp)
1756 ///////////////////////////////////////////////////////////////////////////
1757 // Pat binding - used for `let` and function parameters as well.
1759 impl<'a, 'tcx> Builder<'a, 'tcx> {
1760 pub(crate) fn lower_let_expr(
1762 mut block: BasicBlock,
1765 else_target: region::Scope,
1766 source_scope: Option<SourceScope>,
1769 let expr_span = expr.span;
1770 let expr_place_builder = unpack!(block = self.lower_scrutinee(block, expr, expr_span));
1771 let wildcard = Pat::wildcard_from_ty(pat.ty);
1772 let mut guard_candidate = Candidate::new(expr_place_builder.clone(), &pat, false, self);
1773 let mut otherwise_candidate =
1774 Candidate::new(expr_place_builder.clone(), &wildcard, false, self);
1775 let fake_borrow_temps = self.lower_match_tree(
1780 &mut [&mut guard_candidate, &mut otherwise_candidate],
1782 let expr_place = expr_place_builder.try_to_place(self);
1783 let opt_expr_place = expr_place.as_ref().map(|place| (Some(place), expr_span));
1784 let otherwise_post_guard_block = otherwise_candidate.pre_binding_block.unwrap();
1785 self.break_for_else(otherwise_post_guard_block, else_target, self.source_info(expr_span));
1787 self.declare_bindings(
1795 let post_guard_block = self.bind_pattern(
1796 self.source_info(pat.span),
1804 post_guard_block.unit()
1807 /// Initializes each of the bindings from the candidate by
1808 /// moving/copying/ref'ing the source as appropriate. Tests the guard, if
1809 /// any, and then branches to the arm. Returns the block for the case where
1810 /// the guard succeeds.
1812 /// Note: we do not check earlier that if there is a guard,
1813 /// there cannot be move bindings. We avoid a use-after-move by only
1814 /// moving the binding once the guard has evaluated to true (see below).
1815 fn bind_and_guard_matched_candidate<'pat>(
1817 candidate: Candidate<'pat, 'tcx>,
1818 parent_bindings: &[(Vec<Binding<'tcx>>, Vec<Ascription<'tcx>>)],
1819 fake_borrows: &[(Place<'tcx>, Local)],
1820 scrutinee_span: Span,
1821 arm_match_scope: Option<(&Arm<'tcx>, region::Scope)>,
1822 schedule_drops: bool,
1823 storages_alive: bool,
1825 debug!("bind_and_guard_matched_candidate(candidate={:?})", candidate);
1827 debug_assert!(candidate.match_pairs.is_empty());
1829 let candidate_source_info = self.source_info(candidate.span);
1831 let mut block = candidate.pre_binding_block.unwrap();
1833 if candidate.next_candidate_pre_binding_block.is_some() {
1834 let fresh_block = self.cfg.start_new_block();
1838 candidate.next_candidate_pre_binding_block,
1839 candidate_source_info,
1841 block = fresh_block;
1848 .flat_map(|(_, ascriptions)| ascriptions)
1850 .chain(candidate.ascriptions),
1853 // rust-lang/rust#27282: The `autoref` business deserves some
1854 // explanation here.
1856 // The intent of the `autoref` flag is that when it is true,
1857 // then any pattern bindings of type T will map to a `&T`
1858 // within the context of the guard expression, but will
1859 // continue to map to a `T` in the context of the arm body. To
1860 // avoid surfacing this distinction in the user source code
1861 // (which would be a severe change to the language and require
1862 // far more revision to the compiler), when `autoref` is true,
1863 // then any occurrence of the identifier in the guard
1864 // expression will automatically get a deref op applied to it.
1866 // So an input like:
1869 // let place = Foo::new();
1870 // match place { foo if inspect(foo)
1871 // => feed(foo), ... }
1874 // will be treated as if it were really something like:
1877 // let place = Foo::new();
1878 // match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) }
1879 // => { let tmp2 = place; feed(tmp2) }, ... }
1881 // And an input like:
1884 // let place = Foo::new();
1885 // match place { ref mut foo if inspect(foo)
1886 // => feed(foo), ... }
1889 // will be treated as if it were really something like:
1892 // let place = Foo::new();
1893 // match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) }
1894 // => { let tmp2 = &mut place; feed(tmp2) }, ... }
1897 // In short, any pattern binding will always look like *some*
1898 // kind of `&T` within the guard at least in terms of how the
1899 // MIR-borrowck views it, and this will ensure that guard
1900 // expressions cannot mutate their the match inputs via such
1901 // bindings. (It also ensures that guard expressions can at
1902 // most *copy* values from such bindings; non-Copy things
1903 // cannot be moved via pattern bindings in guard expressions.)
1907 // Implementation notes (under assumption `autoref` is true).
1909 // To encode the distinction above, we must inject the
1910 // temporaries `tmp1` and `tmp2`.
1912 // There are two cases of interest: binding by-value, and binding by-ref.
1914 // 1. Binding by-value: Things are simple.
1916 // * Establishing `tmp1` creates a reference into the
1917 // matched place. This code is emitted by
1918 // bind_matched_candidate_for_guard.
1920 // * `tmp2` is only initialized "lazily", after we have
1921 // checked the guard. Thus, the code that can trigger
1922 // moves out of the candidate can only fire after the
1923 // guard evaluated to true. This initialization code is
1924 // emitted by bind_matched_candidate_for_arm.
1926 // 2. Binding by-reference: Things are tricky.
1928 // * Here, the guard expression wants a `&&` or `&&mut`
1929 // into the original input. This means we need to borrow
1930 // the reference that we create for the arm.
1931 // * So we eagerly create the reference for the arm and then take a
1932 // reference to that.
1933 if let Some((arm, match_scope)) = arm_match_scope
1934 && let Some(guard) = &arm.guard
1937 let bindings = parent_bindings
1939 .flat_map(|(bindings, _)| bindings)
1940 .chain(&candidate.bindings);
1942 self.bind_matched_candidate_for_guard(block, schedule_drops, bindings.clone());
1943 let guard_frame = GuardFrame {
1944 locals: bindings.map(|b| GuardFrameLocal::new(b.var_id, b.binding_mode)).collect(),
1946 debug!("entering guard building context: {:?}", guard_frame);
1947 self.guard_context.push(guard_frame);
1949 let re_erased = tcx.lifetimes.re_erased;
1950 let scrutinee_source_info = self.source_info(scrutinee_span);
1951 for &(place, temp) in fake_borrows {
1952 let borrow = Rvalue::Ref(re_erased, BorrowKind::Shallow, place);
1953 self.cfg.push_assign(block, scrutinee_source_info, Place::from(temp), borrow);
1956 let mut guard_span = rustc_span::DUMMY_SP;
1958 let (post_guard_block, otherwise_post_guard_block) =
1959 self.in_if_then_scope(match_scope, guard_span, |this| match *guard {
1961 let e = &this.thir[e];
1962 guard_span = e.span;
1963 this.then_else_break(
1968 this.source_info(arm.span),
1971 Guard::IfLet(ref pat, scrutinee) => {
1972 let s = &this.thir[scrutinee];
1973 guard_span = s.span;
1974 this.lower_let_expr(block, s, pat, match_scope, None, arm.span)
1978 let source_info = self.source_info(guard_span);
1979 let guard_end = self.source_info(tcx.sess.source_map().end_point(guard_span));
1980 let guard_frame = self.guard_context.pop().unwrap();
1981 debug!("Exiting guard building context with locals: {:?}", guard_frame);
1983 for &(_, temp) in fake_borrows {
1984 let cause = FakeReadCause::ForMatchGuard;
1985 self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(temp));
1988 let otherwise_block = candidate.otherwise_block.unwrap_or_else(|| {
1989 let unreachable = self.cfg.start_new_block();
1990 self.cfg.terminate(unreachable, source_info, TerminatorKind::Unreachable);
1994 otherwise_post_guard_block,
1996 candidate.next_candidate_pre_binding_block,
2000 // We want to ensure that the matched candidates are bound
2001 // after we have confirmed this candidate *and* any
2002 // associated guard; Binding them on `block` is too soon,
2003 // because that would be before we've checked the result
2006 // But binding them on the arm is *too late*, because
2007 // then all of the candidates for a single arm would be
2008 // bound in the same place, that would cause a case like:
2012 // (mut x, 1) | (2, mut x) if { true } => { ... }
2013 // ... // ^^^^^^^ (this is `arm_block`)
2017 // would yield an `arm_block` something like:
2020 // StorageLive(_4); // _4 is `x`
2021 // _4 = &mut (_1.0: i32); // this is handling `(mut x, 1)` case
2022 // _4 = &mut (_1.1: i32); // this is handling `(2, mut x)` case
2025 // and that is clearly not correct.
2026 let by_value_bindings = parent_bindings
2028 .flat_map(|(bindings, _)| bindings)
2029 .chain(&candidate.bindings)
2030 .filter(|binding| matches!(binding.binding_mode, BindingMode::ByValue));
2031 // Read all of the by reference bindings to ensure that the
2032 // place they refer to can't be modified by the guard.
2033 for binding in by_value_bindings.clone() {
2034 let local_id = self.var_local_id(binding.var_id, RefWithinGuard);
2035 let cause = FakeReadCause::ForGuardBinding;
2036 self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(local_id));
2038 assert!(schedule_drops, "patterns with guards must schedule drops");
2039 self.bind_matched_candidate_for_arm_body(
2048 // (Here, it is not too early to bind the matched
2049 // candidate on `block`, because there is no guard result
2050 // that we have to inspect before we bind them.)
2051 self.bind_matched_candidate_for_arm_body(
2056 .flat_map(|(bindings, _)| bindings)
2057 .chain(&candidate.bindings),
2064 /// Append `AscribeUserType` statements onto the end of `block`
2065 /// for each ascription
2069 ascriptions: impl IntoIterator<Item = Ascription<'tcx>>,
2071 for ascription in ascriptions {
2072 let source_info = self.source_info(ascription.annotation.span);
2074 let base = self.canonical_user_type_annotations.push(ascription.annotation);
2079 kind: StatementKind::AscribeUserType(
2082 UserTypeProjection { base, projs: Vec::new() },
2084 ascription.variance,
2091 fn bind_matched_candidate_for_guard<'b>(
2094 schedule_drops: bool,
2095 bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
2099 debug!("bind_matched_candidate_for_guard(block={:?})", block);
2101 // Assign each of the bindings. Since we are binding for a
2102 // guard expression, this will never trigger moves out of the
2104 let re_erased = self.tcx.lifetimes.re_erased;
2105 for binding in bindings {
2106 debug!("bind_matched_candidate_for_guard(binding={:?})", binding);
2107 let source_info = self.source_info(binding.span);
2109 // For each pattern ident P of type T, `ref_for_guard` is
2110 // a reference R: &T pointing to the location matched by
2111 // the pattern, and every occurrence of P within a guard
2113 let ref_for_guard = self.storage_live_binding(
2120 match binding.binding_mode {
2121 BindingMode::ByValue => {
2122 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, binding.source);
2123 self.cfg.push_assign(block, source_info, ref_for_guard, rvalue);
2125 BindingMode::ByRef(borrow_kind) => {
2126 let value_for_arm = self.storage_live_binding(
2134 let rvalue = Rvalue::Ref(re_erased, borrow_kind, binding.source);
2135 self.cfg.push_assign(block, source_info, value_for_arm, rvalue);
2136 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, value_for_arm);
2137 self.cfg.push_assign(block, source_info, ref_for_guard, rvalue);
2143 fn bind_matched_candidate_for_arm_body<'b>(
2146 schedule_drops: bool,
2147 bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
2148 storages_alive: bool,
2152 debug!("bind_matched_candidate_for_arm_body(block={:?})", block);
2154 let re_erased = self.tcx.lifetimes.re_erased;
2155 // Assign each of the bindings. This may trigger moves out of the candidate.
2156 for binding in bindings {
2157 let source_info = self.source_info(binding.span);
2158 let local = if storages_alive {
2159 // Here storages are already alive, probably because this is a binding
2161 // We just need to schedule drop for the value.
2162 self.var_local_id(binding.var_id, OutsideGuard).into()
2164 self.storage_live_binding(
2173 self.schedule_drop_for_binding(binding.var_id, binding.span, OutsideGuard);
2175 let rvalue = match binding.binding_mode {
2176 BindingMode::ByValue => Rvalue::Use(self.consume_by_copy_or_move(binding.source)),
2177 BindingMode::ByRef(borrow_kind) => {
2178 Rvalue::Ref(re_erased, borrow_kind, binding.source)
2181 self.cfg.push_assign(block, source_info, local, rvalue);
2185 /// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where the bound
2186 /// `var` has type `T` in the arm body) in a pattern maps to 2 locals. The
2187 /// first local is a binding for occurrences of `var` in the guard, which
2188 /// will have type `&T`. The second local is a binding for occurrences of
2189 /// `var` in the arm body, which will have type `T`.
2190 #[instrument(skip(self), level = "debug")]
2193 source_info: SourceInfo,
2194 visibility_scope: SourceScope,
2195 mutability: Mutability,
2200 user_ty: UserTypeProjections,
2201 has_guard: ArmHasGuard,
2202 opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
2206 let debug_source_info = SourceInfo { span: source_info.span, scope: visibility_scope };
2207 let binding_mode = match mode {
2208 BindingMode::ByValue => ty::BindingMode::BindByValue(mutability),
2209 BindingMode::ByRef(_) => ty::BindingMode::BindByReference(mutability),
2211 let local = LocalDecl::<'tcx> {
2214 user_ty: if user_ty.is_empty() { None } else { Some(Box::new(user_ty)) },
2217 is_block_tail: None,
2218 local_info: Some(Box::new(LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
2221 // hypothetically, `visit_primary_bindings` could try to unzip
2222 // an outermost hir::Ty as we descend, matching up
2223 // idents in pat; but complex w/ unclear UI payoff.
2224 // Instead, just abandon providing diagnostic info.
2231 let for_arm_body = self.local_decls.push(local);
2232 self.var_debug_info.push(VarDebugInfo {
2234 source_info: debug_source_info,
2235 value: VarDebugInfoContents::Place(for_arm_body.into()),
2237 let locals = if has_guard.0 {
2238 let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
2239 // This variable isn't mutated but has a name, so has to be
2240 // immutable to avoid the unused mut lint.
2241 mutability: Mutability::Not,
2242 ty: tcx.mk_imm_ref(tcx.lifetimes.re_erased, var_ty),
2246 is_block_tail: None,
2247 local_info: Some(Box::new(LocalInfo::User(ClearCrossCrate::Set(
2248 BindingForm::RefForGuard,
2251 self.var_debug_info.push(VarDebugInfo {
2253 source_info: debug_source_info,
2254 value: VarDebugInfoContents::Place(ref_for_guard.into()),
2256 LocalsForNode::ForGuard { ref_for_guard, for_arm_body }
2258 LocalsForNode::One(for_arm_body)
2261 self.var_indices.insert(var_id, locals);
2264 pub(crate) fn ast_let_else(
2266 mut block: BasicBlock,
2268 initializer_span: Span,
2269 else_block: BlockId,
2270 let_else_scope: ®ion::Scope,
2271 pattern: &Pat<'tcx>,
2272 ) -> BlockAnd<BasicBlock> {
2273 let else_block_span = self.thir[else_block].span;
2274 let (matching, failure) = self.in_if_then_scope(*let_else_scope, else_block_span, |this| {
2275 let scrutinee = unpack!(block = this.lower_scrutinee(block, init, initializer_span));
2276 let pat = Pat { ty: init.ty, span: else_block_span, kind: PatKind::Wild };
2277 let mut wildcard = Candidate::new(scrutinee.clone(), &pat, false, this);
2278 let mut candidate = Candidate::new(scrutinee.clone(), pattern, false, this);
2279 let fake_borrow_temps = this.lower_match_tree(
2284 &mut [&mut candidate, &mut wildcard],
2286 // This block is for the matching case
2287 let matching = this.bind_pattern(
2288 this.source_info(pattern.span),
2295 // This block is for the failure case
2296 let failure = this.bind_pattern(
2297 this.source_info(else_block_span),
2304 this.break_for_else(failure, *let_else_scope, this.source_info(initializer_span));
2307 matching.and(failure)