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,
90 let temp_scope = temp_scope_override.unwrap_or_else(|| this.local_scope());
91 let mutability = Mutability::Mut;
93 unpack!(block = this.as_temp(block, Some(temp_scope), expr, mutability));
94 let operand = Operand::Move(Place::from(place));
96 let then_block = this.cfg.start_new_block();
97 let else_block = this.cfg.start_new_block();
98 let term = TerminatorKind::if_(operand, then_block, else_block);
100 let source_info = this.source_info(expr_span);
101 this.cfg.terminate(block, source_info, term);
102 this.break_for_else(else_block, break_scope, source_info);
109 /// Generates MIR for a `match` expression.
111 /// The MIR that we generate for a match looks like this.
116 /// [ 1. Evaluate Scrutinee (expression being matched on) ]
117 /// [ (fake read of scrutinee) ]
119 /// [ 2. Decision tree -- check discriminants ] <--------+
121 /// | (once a specific arm is chosen) |
123 /// [pre_binding_block] [otherwise_block]
125 /// [ 3. Create "guard bindings" for arm ] |
126 /// [ (create fake borrows) ] |
128 /// [ 4. Execute guard code ] |
129 /// [ (read fake borrows) ] --(guard is false)-----------+
131 /// | (guard results in true)
133 /// [ 5. Create real bindings and execute arm ]
138 /// All of the different arms have been stacked on top of each other to
139 /// simplify the diagram. For an arm with no guard the blocks marked 3 and
140 /// 4 and the fake borrows are omitted.
142 /// We generate MIR in the following steps:
144 /// 1. Evaluate the scrutinee and add the fake read of it ([Builder::lower_scrutinee]).
145 /// 2. Create the decision tree ([Builder::lower_match_tree]).
146 /// 3. Determine the fake borrows that are needed from the places that were
147 /// matched against and create the required temporaries for them
148 /// ([Builder::calculate_fake_borrows]).
149 /// 4. Create everything else: the guards and the arms ([Builder::lower_match_arms]).
153 /// We don't want to have the exact structure of the decision tree be
154 /// visible through borrow checking. False edges ensure that the CFG as
155 /// seen by borrow checking doesn't encode this. False edges are added:
157 /// * From each pre-binding block to the next pre-binding block.
158 /// * From each otherwise block to the next pre-binding block.
159 #[instrument(level = "debug", skip(self, arms))]
160 pub(crate) fn match_expr(
162 destination: Place<'tcx>,
164 mut block: BasicBlock,
165 scrutinee: &Expr<'tcx>,
168 let scrutinee_span = scrutinee.span;
169 let scrutinee_place =
170 unpack!(block = self.lower_scrutinee(block, scrutinee, scrutinee_span,));
172 let mut arm_candidates = self.create_match_candidates(&scrutinee_place, &arms);
174 let match_has_guard = arm_candidates.iter().any(|(_, candidate)| candidate.has_guard);
176 arm_candidates.iter_mut().map(|(_, candidate)| candidate).collect::<Vec<_>>();
178 let match_start_span = span.shrink_to_lo().to(scrutinee.span);
180 let fake_borrow_temps = self.lower_match_tree(
188 self.lower_match_arms(
193 self.source_info(span),
198 /// Evaluate the scrutinee and add the fake read of it.
201 mut block: BasicBlock,
202 scrutinee: &Expr<'tcx>,
203 scrutinee_span: Span,
204 ) -> BlockAnd<PlaceBuilder<'tcx>> {
205 let scrutinee_place_builder = unpack!(block = self.as_place_builder(block, scrutinee));
206 // Matching on a `scrutinee_place` with an uninhabited type doesn't
207 // generate any memory reads by itself, and so if the place "expression"
208 // contains unsafe operations like raw pointer dereferences or union
209 // field projections, we wouldn't know to require an `unsafe` block
210 // around a `match` equivalent to `std::intrinsics::unreachable()`.
211 // See issue #47412 for this hole being discovered in the wild.
213 // HACK(eddyb) Work around the above issue by adding a dummy inspection
214 // of `scrutinee_place`, specifically by applying `ReadForMatch`.
216 // NOTE: ReadForMatch also checks that the scrutinee is initialized.
217 // This is currently needed to not allow matching on an uninitialized,
218 // uninhabited value. If we get never patterns, those will check that
219 // the place is initialized, and so this read would only be used to
221 let cause_matched_place = FakeReadCause::ForMatchedPlace(None);
222 let source_info = self.source_info(scrutinee_span);
224 if let Some(scrutinee_place) = scrutinee_place_builder.try_to_place(self) {
225 self.cfg.push_fake_read(block, source_info, cause_matched_place, scrutinee_place);
228 block.and(scrutinee_place_builder)
231 /// Create the initial `Candidate`s for a `match` expression.
232 fn create_match_candidates<'pat>(
234 scrutinee: &PlaceBuilder<'tcx>,
236 ) -> Vec<(&'pat Arm<'tcx>, Candidate<'pat, 'tcx>)>
240 // Assemble a list of candidates: there is one candidate per pattern,
241 // which means there may be more than one candidate *per arm*.
245 let arm = &self.thir[arm];
246 let arm_has_guard = arm.guard.is_some();
248 Candidate::new(scrutinee.clone(), &arm.pattern, arm_has_guard, self);
254 /// Create the decision tree for the match expression, starting from `block`.
256 /// Modifies `candidates` to store the bindings and type ascriptions for
259 /// Returns the places that need fake borrows because we bind or test them.
260 fn lower_match_tree<'pat>(
263 scrutinee_span: Span,
264 match_start_span: Span,
265 match_has_guard: bool,
266 candidates: &mut [&mut Candidate<'pat, 'tcx>],
267 ) -> Vec<(Place<'tcx>, Local)> {
268 // The set of places that we are creating fake borrows of. If there are
269 // no match guards then we don't need any fake borrows, so don't track
271 let mut fake_borrows = match_has_guard.then(FxIndexSet::default);
273 let mut otherwise = None;
275 // This will generate code to test scrutinee_place and
276 // branch to the appropriate arm block
277 self.match_candidates(
286 if let Some(otherwise_block) = otherwise {
287 // See the doc comment on `match_candidates` for why we may have an
288 // otherwise block. Match checking will ensure this is actually
290 let source_info = self.source_info(scrutinee_span);
291 self.cfg.terminate(otherwise_block, source_info, TerminatorKind::Unreachable);
294 // Link each leaf candidate to the `pre_binding_block` of the next one.
295 let mut previous_candidate: Option<&mut Candidate<'_, '_>> = None;
297 for candidate in candidates {
298 candidate.visit_leaves(|leaf_candidate| {
299 if let Some(ref mut prev) = previous_candidate {
300 prev.next_candidate_pre_binding_block = leaf_candidate.pre_binding_block;
302 previous_candidate = Some(leaf_candidate);
306 if let Some(ref borrows) = fake_borrows {
307 self.calculate_fake_borrows(borrows, scrutinee_span)
313 /// Lower the bindings, guards and arm bodies of a `match` expression.
315 /// The decision tree should have already been created
316 /// (by [Builder::lower_match_tree]).
318 /// `outer_source_info` is the SourceInfo for the whole match.
321 destination: Place<'tcx>,
322 scrutinee_place_builder: PlaceBuilder<'tcx>,
323 scrutinee_span: Span,
324 arm_candidates: Vec<(&'_ Arm<'tcx>, Candidate<'_, 'tcx>)>,
325 outer_source_info: SourceInfo,
326 fake_borrow_temps: Vec<(Place<'tcx>, Local)>,
328 let arm_end_blocks: Vec<_> = arm_candidates
330 .map(|(arm, candidate)| {
331 debug!("lowering arm {:?}\ncandidate = {:?}", arm, candidate);
333 let arm_source_info = self.source_info(arm.span);
334 let arm_scope = (arm.scope, arm_source_info);
335 let match_scope = self.local_scope();
336 self.in_scope(arm_scope, arm.lint_level, |this| {
337 // `try_to_place` may fail if it is unable to resolve the given
338 // `PlaceBuilder` inside a closure. In this case, we don't want to include
339 // a scrutinee place. `scrutinee_place_builder` will fail to be resolved
340 // if the only match arm is a wildcard (`_`).
345 // match foo { _ => () };
348 let scrutinee_place = scrutinee_place_builder.try_to_place(this);
349 let opt_scrutinee_place =
350 scrutinee_place.as_ref().map(|place| (Some(place), scrutinee_span));
351 let scope = this.declare_bindings(
359 let arm_block = this.bind_pattern(
364 Some((arm, match_scope)),
368 if let Some(source_scope) = scope {
369 this.source_scope = source_scope;
372 this.expr_into_dest(destination, arm_block, &&this.thir[arm.body])
377 // all the arm blocks will rejoin here
378 let end_block = self.cfg.start_new_block();
380 let end_brace = self.source_info(
381 outer_source_info.span.with_lo(outer_source_info.span.hi() - BytePos::from_usize(1)),
383 for arm_block in arm_end_blocks {
384 let block = &self.cfg.basic_blocks[arm_block.0];
385 let last_location = block.statements.last().map(|s| s.source_info);
387 self.cfg.goto(unpack!(arm_block), last_location.unwrap_or(end_brace), end_block);
390 self.source_scope = outer_source_info.scope;
395 /// Binds the variables and ascribes types for a given `match` arm or
398 /// Also check if the guard matches, if it's provided.
399 /// `arm_scope` should be `Some` if and only if this is called for a
403 outer_source_info: SourceInfo,
404 candidate: Candidate<'_, 'tcx>,
405 fake_borrow_temps: &[(Place<'tcx>, Local)],
406 scrutinee_span: Span,
407 arm_match_scope: Option<(&Arm<'tcx>, region::Scope)>,
408 storages_alive: bool,
410 if candidate.subcandidates.is_empty() {
411 // Avoid generating another `BasicBlock` when we only have one
413 self.bind_and_guard_matched_candidate(
423 // It's helpful to avoid scheduling drops multiple times to save
424 // drop elaboration from having to clean up the extra drops.
426 // If we are in a `let` then we only schedule drops for the first
429 // If we're in a `match` arm then we could have a case like so:
431 // Ok(x) | Err(x) if return => { /* ... */ }
433 // In this case we don't want a drop of `x` scheduled when we
434 // return: it isn't bound by move until right before enter the arm.
435 // To handle this we instead unschedule it's drop after each time
436 // we lower the guard.
437 let target_block = self.cfg.start_new_block();
438 let mut schedule_drops = true;
439 let arm = arm_match_scope.unzip().0;
440 // We keep a stack of all of the bindings and type ascriptions
441 // from the parent candidates that we visit, that also need to
442 // be bound for each candidate.
446 &mut |leaf_candidate, parent_bindings| {
447 if let Some(arm) = arm {
448 self.clear_top_scope(arm.scope);
450 let binding_end = self.bind_and_guard_matched_candidate(
460 schedule_drops = false;
462 self.cfg.goto(binding_end, outer_source_info, target_block);
464 |inner_candidate, parent_bindings| {
465 parent_bindings.push((inner_candidate.bindings, inner_candidate.ascriptions));
466 inner_candidate.subcandidates.into_iter()
469 parent_bindings.pop();
477 pub(super) fn expr_into_pattern(
479 mut block: BasicBlock,
480 irrefutable_pat: &Pat<'tcx>,
481 initializer: &Expr<'tcx>,
483 match irrefutable_pat.kind {
484 // Optimize the case of `let x = ...` to write directly into `x`
485 PatKind::Binding { mode: BindingMode::ByValue, var, subpattern: None, .. } => {
487 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
488 unpack!(block = self.expr_into_dest(place, block, initializer));
490 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
491 let source_info = self.source_info(irrefutable_pat.span);
492 self.cfg.push_fake_read(block, source_info, FakeReadCause::ForLet(None), place);
494 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
498 // Optimize the case of `let x: T = ...` to write directly
499 // into `x` and then require that `T == typeof(x)`.
501 // Weirdly, this is needed to prevent the
502 // `intrinsic-move-val.rs` test case from crashing. That
503 // test works with uninitialized values in a rather
504 // dubious way, so it may be that the test is kind of
506 PatKind::AscribeUserType {
511 mode: BindingMode::ByValue, var, subpattern: None, ..
515 ascription: thir::Ascription { ref annotation, variance: _ },
518 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
519 unpack!(block = self.expr_into_dest(place, block, initializer));
521 // Inject a fake read, see comments on `FakeReadCause::ForLet`.
522 let pattern_source_info = self.source_info(irrefutable_pat.span);
523 let cause_let = FakeReadCause::ForLet(None);
524 self.cfg.push_fake_read(block, pattern_source_info, cause_let, place);
526 let ty_source_info = self.source_info(annotation.span);
528 let base = self.canonical_user_type_annotations.push(annotation.clone());
532 source_info: ty_source_info,
533 kind: StatementKind::AscribeUserType(
534 Box::new((place, UserTypeProjection { base, projs: Vec::new() })),
535 // We always use invariant as the variance here. This is because the
536 // variance field from the ascription refers to the variance to use
537 // when applying the type to the value being matched, but this
538 // ascription applies rather to the type of the binding. e.g., in this
545 // We are creating an ascription that defines the type of `x` to be
546 // exactly `T` (i.e., with invariance). The variance field, in
547 // contrast, is intended to be used to relate `T` to the type of
549 ty::Variance::Invariant,
554 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
559 let place_builder = unpack!(block = self.as_place_builder(block, initializer));
560 self.place_into_pattern(block, &irrefutable_pat, place_builder, true)
565 pub(crate) fn place_into_pattern(
568 irrefutable_pat: &Pat<'tcx>,
569 initializer: PlaceBuilder<'tcx>,
570 set_match_place: bool,
572 let mut candidate = Candidate::new(initializer.clone(), &irrefutable_pat, false, self);
573 let fake_borrow_temps = self.lower_match_tree(
575 irrefutable_pat.span,
576 irrefutable_pat.span,
578 &mut [&mut candidate],
580 // For matches and function arguments, the place that is being matched
581 // can be set when creating the variables. But the place for
582 // let PATTERN = ... might not even exist until we do the assignment.
583 // so we set it here instead.
585 let mut candidate_ref = &candidate;
586 while let Some(next) = {
587 for binding in &candidate_ref.bindings {
588 let local = self.var_local_id(binding.var_id, OutsideGuard);
589 // `try_to_place` may fail if it is unable to resolve the given
590 // `PlaceBuilder` inside a closure. In this case, we don't want to include
591 // a scrutinee place. `scrutinee_place_builder` will fail for destructured
592 // assignments. This is because a closure only captures the precise places
593 // that it will read and as a result a closure may not capture the entire
594 // tuple/struct and rather have individual places that will be read in the
600 // let (v1, v2) = foo;
603 if let Some(place) = initializer.try_to_place(self) {
604 let Some(box LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
605 VarBindingForm { opt_match_place: Some((ref mut match_place, _)), .. },
606 )))) = self.local_decls[local].local_info else {
607 bug!("Let binding to non-user variable.")
609 *match_place = Some(place);
612 // All of the subcandidates should bind the same locals, so we
613 // only visit the first one.
614 candidate_ref.subcandidates.get(0)
616 candidate_ref = next;
621 self.source_info(irrefutable_pat.span),
624 irrefutable_pat.span,
631 /// Declares the bindings of the given patterns and returns the visibility
632 /// scope for the bindings in these patterns, if such a scope had to be
633 /// created. NOTE: Declaring the bindings should always be done in their
635 #[instrument(skip(self), level = "debug")]
636 pub(crate) fn declare_bindings(
638 mut visibility_scope: Option<SourceScope>,
641 guard: Option<&Guard<'tcx>>,
642 opt_match_place: Option<(Option<&Place<'tcx>>, Span)>,
643 ) -> Option<SourceScope> {
644 self.visit_primary_bindings(
646 UserTypeProjections::none(),
647 &mut |this, mutability, name, mode, var, span, ty, user_ty| {
648 if visibility_scope.is_none() {
650 Some(this.new_source_scope(scope_span, LintLevel::Inherited, None));
652 let source_info = SourceInfo { span, scope: this.source_scope };
653 let visibility_scope = visibility_scope.unwrap();
654 this.declare_binding(
663 ArmHasGuard(guard.is_some()),
664 opt_match_place.map(|(x, y)| (x.cloned(), y)),
669 if let Some(Guard::IfLet(guard_pat, _)) = guard {
670 // FIXME: pass a proper `opt_match_place`
671 self.declare_bindings(visibility_scope, scope_span, guard_pat, None, None);
676 pub(crate) fn storage_live_binding(
684 let local_id = self.var_local_id(var, for_guard);
685 let source_info = self.source_info(span);
686 self.cfg.push(block, Statement { source_info, kind: StatementKind::StorageLive(local_id) });
687 // Although there is almost always scope for given variable in corner cases
688 // like #92893 we might get variable with no scope.
689 if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) && schedule_drop {
690 self.schedule_drop(span, region_scope, local_id, DropKind::Storage);
692 Place::from(local_id)
695 pub(crate) fn schedule_drop_for_binding(
701 let local_id = self.var_local_id(var, for_guard);
702 if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) {
703 self.schedule_drop(span, region_scope, local_id, DropKind::Value);
707 /// Visit all of the primary bindings in a patterns, that is, visit the
708 /// leftmost occurrence of each variable bound in a pattern. A variable
709 /// will occur more than once in an or-pattern.
710 pub(super) fn visit_primary_bindings(
713 pattern_user_ty: UserTypeProjections,
726 "visit_primary_bindings: pattern={:?} pattern_user_ty={:?}",
727 pattern, pattern_user_ty
741 f(self, mutability, name, mode, var, pattern.span, ty, pattern_user_ty.clone());
743 if let Some(subpattern) = subpattern.as_ref() {
744 self.visit_primary_bindings(subpattern, pattern_user_ty, f);
748 PatKind::Array { ref prefix, ref slice, ref suffix }
749 | PatKind::Slice { ref prefix, ref slice, ref suffix } => {
750 let from = u64::try_from(prefix.len()).unwrap();
751 let to = u64::try_from(suffix.len()).unwrap();
752 for subpattern in prefix.iter() {
753 self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
755 for subpattern in slice {
756 self.visit_primary_bindings(
758 pattern_user_ty.clone().subslice(from, to),
762 for subpattern in suffix.iter() {
763 self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
767 PatKind::Constant { .. } | PatKind::Range { .. } | PatKind::Wild => {}
769 PatKind::Deref { ref subpattern } => {
770 self.visit_primary_bindings(subpattern, pattern_user_ty.deref(), f);
773 PatKind::AscribeUserType {
775 ascription: thir::Ascription { ref annotation, variance: _ },
777 // This corresponds to something like
780 // let A::<'a>(_): A<'static> = ...;
783 // Note that the variance doesn't apply here, as we are tracking the effect
784 // of `user_ty` on any bindings contained with subpattern.
786 let projection = UserTypeProjection {
787 base: self.canonical_user_type_annotations.push(annotation.clone()),
790 let subpattern_user_ty =
791 pattern_user_ty.push_projection(&projection, annotation.span);
792 self.visit_primary_bindings(subpattern, subpattern_user_ty, f)
795 PatKind::Leaf { ref subpatterns } => {
796 for subpattern in subpatterns {
797 let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field);
798 debug!("visit_primary_bindings: subpattern_user_ty={:?}", subpattern_user_ty);
799 self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
803 PatKind::Variant { adt_def, substs: _, variant_index, ref subpatterns } => {
804 for subpattern in subpatterns {
805 let subpattern_user_ty =
806 pattern_user_ty.clone().variant(adt_def, variant_index, subpattern.field);
807 self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
810 PatKind::Or { ref pats } => {
811 // In cases where we recover from errors the primary bindings
812 // may not all be in the leftmost subpattern. For example in
813 // `let (x | y) = ...`, the primary binding of `y` occurs in
814 // the right subpattern
815 for subpattern in pats.iter() {
816 self.visit_primary_bindings(subpattern, pattern_user_ty.clone(), f);
824 struct Candidate<'pat, 'tcx> {
825 /// [`Span`] of the original pattern that gave rise to this candidate.
828 /// Whether this `Candidate` has a guard.
831 /// All of these must be satisfied...
832 match_pairs: SmallVec<[MatchPair<'pat, 'tcx>; 1]>,
834 /// ...these bindings established...
835 bindings: Vec<Binding<'tcx>>,
837 /// ...and these types asserted...
838 ascriptions: Vec<Ascription<'tcx>>,
840 /// ...and if this is non-empty, one of these subcandidates also has to match...
841 subcandidates: Vec<Candidate<'pat, 'tcx>>,
843 /// ...and the guard must be evaluated; if it's `false` then branch to `otherwise_block`.
844 otherwise_block: Option<BasicBlock>,
846 /// The block before the `bindings` have been established.
847 pre_binding_block: Option<BasicBlock>,
848 /// The pre-binding block of the next candidate.
849 next_candidate_pre_binding_block: Option<BasicBlock>,
852 impl<'tcx, 'pat> Candidate<'pat, 'tcx> {
854 place: PlaceBuilder<'tcx>,
855 pattern: &'pat Pat<'tcx>,
857 cx: &Builder<'_, 'tcx>,
862 match_pairs: smallvec![MatchPair::new(place, pattern, cx)],
863 bindings: Vec::new(),
864 ascriptions: Vec::new(),
865 subcandidates: Vec::new(),
866 otherwise_block: None,
867 pre_binding_block: None,
868 next_candidate_pre_binding_block: None,
872 /// Visit the leaf candidates (those with no subcandidates) contained in
874 fn visit_leaves<'a>(&'a mut self, mut visit_leaf: impl FnMut(&'a mut Self)) {
878 &mut move |c, _| visit_leaf(c),
879 move |c, _| c.subcandidates.iter_mut(),
885 /// A depth-first traversal of the `Candidate` and all of its recursive
887 fn traverse_candidate<'pat, 'tcx: 'pat, C, T, I>(
890 visit_leaf: &mut impl FnMut(C, &mut T),
891 get_children: impl Copy + Fn(C, &mut T) -> I,
892 complete_children: impl Copy + Fn(&mut T),
894 C: Borrow<Candidate<'pat, 'tcx>>,
895 I: Iterator<Item = C>,
897 if candidate.borrow().subcandidates.is_empty() {
898 visit_leaf(candidate, context)
900 for child in get_children(candidate, context) {
901 traverse_candidate(child, context, visit_leaf, get_children, complete_children);
903 complete_children(context)
907 #[derive(Clone, Debug)]
908 struct Binding<'tcx> {
912 binding_mode: BindingMode,
915 /// Indicates that the type of `source` must be a subtype of the
916 /// user-given type `user_ty`; this is basically a no-op but can
917 /// influence region inference.
918 #[derive(Clone, Debug)]
919 struct Ascription<'tcx> {
921 annotation: CanonicalUserTypeAnnotation<'tcx>,
922 variance: ty::Variance,
925 #[derive(Clone, Debug)]
926 pub(crate) struct MatchPair<'pat, 'tcx> {
928 place: PlaceBuilder<'tcx>,
930 // ... must match this pattern.
931 pattern: &'pat Pat<'tcx>,
934 /// See [`Test`] for more.
935 #[derive(Clone, Debug, PartialEq)]
936 enum TestKind<'tcx> {
937 /// Test what enum variant a value is.
939 /// The enum type being tested.
940 adt_def: ty::AdtDef<'tcx>,
941 /// The set of variants that we should create a branch for. We also
942 /// create an additional "otherwise" case.
943 variants: BitSet<VariantIdx>,
946 /// Test what value an integer, `bool`, or `char` has.
948 /// The type of the value that we're testing.
950 /// The (ordered) set of values that we test for.
952 /// For integers and `char`s we create a branch to each of the values in
953 /// `options`, as well as an "otherwise" branch for all other values, even
954 /// in the (rare) case that `options` is exhaustive.
956 /// For `bool` we always generate two edges, one for `true` and one for
958 options: FxIndexMap<ConstantKind<'tcx>, u128>,
961 /// Test for equality with value, possibly after an unsizing coercion to
964 value: ConstantKind<'tcx>,
965 // Integer types are handled by `SwitchInt`, and constants with ADT
966 // types are converted back into patterns, so this can only be `&str`,
967 // `&[T]`, `f32` or `f64`.
971 /// Test whether the value falls within an inclusive or exclusive range
972 Range(Box<PatRange<'tcx>>),
974 /// Test that the length of the slice is equal to `len`.
975 Len { len: u64, op: BinOp },
978 /// A test to perform to determine which [`Candidate`] matches a value.
980 /// [`Test`] is just the test to perform; it does not include the value
983 pub(crate) struct Test<'tcx> {
985 kind: TestKind<'tcx>,
988 /// `ArmHasGuard` is a wrapper around a boolean flag. It indicates whether
989 /// a match arm has a guard expression attached to it.
990 #[derive(Copy, Clone, Debug)]
991 pub(crate) struct ArmHasGuard(pub(crate) bool);
993 ///////////////////////////////////////////////////////////////////////////
994 // Main matching algorithm
996 impl<'a, 'tcx> Builder<'a, 'tcx> {
997 /// The main match algorithm. It begins with a set of candidates
998 /// `candidates` and has the job of generating code to determine
999 /// which of these candidates, if any, is the correct one. The
1000 /// candidates are sorted such that the first item in the list
1001 /// has the highest priority. When a candidate is found to match
1002 /// the value, we will set and generate a branch to the appropriate
1003 /// pre-binding block.
1005 /// If we find that *NONE* of the candidates apply, we branch to the
1006 /// `otherwise_block`, setting it to `Some` if required. In principle, this
1007 /// means that the input list was not exhaustive, though at present we
1008 /// sometimes are not smart enough to recognize all exhaustive inputs.
1010 /// It might be surprising that the input can be non-exhaustive.
1011 /// Indeed, initially, it is not, because all matches are
1012 /// exhaustive in Rust. But during processing we sometimes divide
1013 /// up the list of candidates and recurse with a non-exhaustive
1014 /// list. This is important to keep the size of the generated code
1015 /// under control. See [`Builder::test_candidates`] for more details.
1017 /// If `fake_borrows` is `Some`, then places which need fake borrows
1018 /// will be added to it.
1020 /// For an example of a case where we set `otherwise_block`, even for an
1021 /// exhaustive match, consider:
1024 /// # fn foo(x: (bool, bool)) {
1026 /// (true, true) => (),
1027 /// (_, false) => (),
1028 /// (false, true) => (),
1033 /// For this match, we check if `x.0` matches `true` (for the first
1034 /// arm). If it doesn't match, we check `x.1`. If `x.1` is `true` we check
1035 /// if `x.0` matches `false` (for the third arm). In the (impossible at
1036 /// runtime) case when `x.0` is now `true`, we branch to
1037 /// `otherwise_block`.
1038 #[instrument(skip(self, fake_borrows), level = "debug")]
1039 fn match_candidates<'pat>(
1042 scrutinee_span: Span,
1043 start_block: BasicBlock,
1044 otherwise_block: &mut Option<BasicBlock>,
1045 candidates: &mut [&mut Candidate<'pat, 'tcx>],
1046 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1048 // Start by simplifying candidates. Once this process is complete, all
1049 // the match pairs which remain require some form of test, whether it
1050 // be a switch or pattern comparison.
1051 let mut split_or_candidate = false;
1052 for candidate in &mut *candidates {
1053 split_or_candidate |= self.simplify_candidate(candidate);
1056 ensure_sufficient_stack(|| {
1057 if split_or_candidate {
1058 // At least one of the candidates has been split into subcandidates.
1059 // We need to change the candidate list to include those.
1060 let mut new_candidates = Vec::new();
1062 for candidate in candidates {
1063 candidate.visit_leaves(|leaf_candidate| new_candidates.push(leaf_candidate));
1065 self.match_simplified_candidates(
1070 &mut *new_candidates,
1074 self.match_simplified_candidates(
1086 fn match_simplified_candidates(
1089 scrutinee_span: Span,
1090 start_block: BasicBlock,
1091 otherwise_block: &mut Option<BasicBlock>,
1092 candidates: &mut [&mut Candidate<'_, 'tcx>],
1093 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1095 // The candidates are sorted by priority. Check to see whether the
1096 // higher priority candidates (and hence at the front of the slice)
1097 // have satisfied all their match pairs.
1098 let fully_matched = candidates.iter().take_while(|c| c.match_pairs.is_empty()).count();
1099 debug!("match_candidates: {:?} candidates fully matched", fully_matched);
1100 let (matched_candidates, unmatched_candidates) = candidates.split_at_mut(fully_matched);
1102 let block = if !matched_candidates.is_empty() {
1103 let otherwise_block =
1104 self.select_matched_candidates(matched_candidates, start_block, fake_borrows);
1106 if let Some(last_otherwise_block) = otherwise_block {
1107 last_otherwise_block
1109 // Any remaining candidates are unreachable.
1110 if unmatched_candidates.is_empty() {
1113 self.cfg.start_new_block()
1119 // If there are no candidates that still need testing, we're
1120 // done. Since all matches are exhaustive, execution should
1121 // never reach this point.
1122 if unmatched_candidates.is_empty() {
1123 let source_info = self.source_info(span);
1124 if let Some(otherwise) = *otherwise_block {
1125 self.cfg.goto(block, source_info, otherwise);
1127 *otherwise_block = Some(block);
1132 // Test for the remaining candidates.
1133 self.test_candidates_with_or(
1136 unmatched_candidates,
1143 /// Link up matched candidates.
1145 /// For example, if we have something like this:
1147 /// ```ignore (illustrative)
1149 /// Some(x) if cond1 => ...
1151 /// Some(x) if cond2 => ...
1155 /// We generate real edges from:
1157 /// * `start_block` to the [pre-binding block] of the first pattern,
1158 /// * the [otherwise block] of the first pattern to the second pattern,
1159 /// * the [otherwise block] of the third pattern to a block with an
1160 /// [`Unreachable` terminator](TerminatorKind::Unreachable).
1162 /// In addition, we add fake edges from the otherwise blocks to the
1163 /// pre-binding block of the next candidate in the original set of
1166 /// [pre-binding block]: Candidate::pre_binding_block
1167 /// [otherwise block]: Candidate::otherwise_block
1168 fn select_matched_candidates(
1170 matched_candidates: &mut [&mut Candidate<'_, 'tcx>],
1171 start_block: BasicBlock,
1172 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1173 ) -> Option<BasicBlock> {
1175 !matched_candidates.is_empty(),
1176 "select_matched_candidates called with no candidates",
1179 matched_candidates.iter().all(|c| c.subcandidates.is_empty()),
1180 "subcandidates should be empty in select_matched_candidates",
1183 // Insert a borrows of prefixes of places that are bound and are
1184 // behind a dereference projection.
1186 // These borrows are taken to avoid situations like the following:
1189 // _ if { x = &[0]; false } => (),
1190 // y => (), // Out of bounds array access!
1194 // // y is bound by reference in the guard and then by copy in the
1195 // // arm, so y is 2 in the arm!
1196 // y if { y == 1 && (x = &2) == () } => y,
1199 if let Some(fake_borrows) = fake_borrows {
1200 for Binding { source, .. } in
1201 matched_candidates.iter().flat_map(|candidate| &candidate.bindings)
1204 source.projection.iter().rposition(|elem| elem == ProjectionElem::Deref)
1206 let proj_base = &source.projection[..i];
1208 fake_borrows.insert(Place {
1209 local: source.local,
1210 projection: self.tcx.intern_place_elems(proj_base),
1216 let fully_matched_with_guard = matched_candidates
1218 .position(|c| !c.has_guard)
1219 .unwrap_or(matched_candidates.len() - 1);
1221 let (reachable_candidates, unreachable_candidates) =
1222 matched_candidates.split_at_mut(fully_matched_with_guard + 1);
1224 let mut next_prebinding = start_block;
1226 for candidate in reachable_candidates.iter_mut() {
1227 assert!(candidate.otherwise_block.is_none());
1228 assert!(candidate.pre_binding_block.is_none());
1229 candidate.pre_binding_block = Some(next_prebinding);
1230 if candidate.has_guard {
1231 // Create the otherwise block for this candidate, which is the
1232 // pre-binding block for the next candidate.
1233 next_prebinding = self.cfg.start_new_block();
1234 candidate.otherwise_block = Some(next_prebinding);
1239 "match_candidates: add pre_binding_blocks for unreachable {:?}",
1240 unreachable_candidates,
1242 for candidate in unreachable_candidates {
1243 assert!(candidate.pre_binding_block.is_none());
1244 candidate.pre_binding_block = Some(self.cfg.start_new_block());
1247 reachable_candidates.last_mut().unwrap().otherwise_block
1250 /// Tests a candidate where there are only or-patterns left to test, or
1251 /// forwards to [Builder::test_candidates].
1253 /// Given a pattern `(P | Q, R | S)` we (in principle) generate a CFG like
1261 /// +----------------------------------------+------------------------------------+
1264 /// [ P matches ] [ Q matches ] [ otherwise ]
1267 /// [ match R, S ] [ match R, S ] |
1269 /// +--------------+------------+ +--------------+------------+ |
1272 /// [ R matches ] [ S matches ] [otherwise ] [ R matches ] [ S matches ] [otherwise ] |
1274 /// +--------------+------------|------------+--------------+ | |
1276 /// | +----------------------------------------+--------+
1279 /// [ Success ] [ Failure ]
1282 /// In practice there are some complications:
1284 /// * If there's a guard, then the otherwise branch of the first match on
1285 /// `R | S` goes to a test for whether `Q` matches, and the control flow
1286 /// doesn't merge into a single success block until after the guard is
1288 /// * If neither `P` or `Q` has any bindings or type ascriptions and there
1289 /// isn't a match guard, then we create a smaller CFG like:
1293 /// +---------------+------------+
1295 /// [ P matches ] [ Q matches ] [ otherwise ]
1297 /// +---------------+ |
1303 fn test_candidates_with_or(
1306 scrutinee_span: Span,
1307 candidates: &mut [&mut Candidate<'_, 'tcx>],
1309 otherwise_block: &mut Option<BasicBlock>,
1310 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1312 let (first_candidate, remaining_candidates) = candidates.split_first_mut().unwrap();
1314 // All of the or-patterns have been sorted to the end, so if the first
1315 // pattern is an or-pattern we only have or-patterns.
1316 match first_candidate.match_pairs[0].pattern.kind {
1317 PatKind::Or { .. } => (),
1319 self.test_candidates(
1331 let match_pairs = mem::take(&mut first_candidate.match_pairs);
1332 first_candidate.pre_binding_block = Some(block);
1334 let mut otherwise = None;
1335 for match_pair in match_pairs {
1336 let PatKind::Or { ref pats } = &match_pair.pattern.kind else {
1337 bug!("Or-patterns should have been sorted to the end");
1339 let or_span = match_pair.pattern.span;
1341 first_candidate.visit_leaves(|leaf_candidate| {
1342 self.test_or_pattern(
1353 let remainder_start = otherwise.unwrap_or_else(|| self.cfg.start_new_block());
1355 self.match_candidates(
1360 remaining_candidates,
1366 skip(self, otherwise, or_span, place, fake_borrows, candidate, pats),
1369 fn test_or_pattern<'pat>(
1371 candidate: &mut Candidate<'pat, 'tcx>,
1372 otherwise: &mut Option<BasicBlock>,
1373 pats: &'pat [Box<Pat<'tcx>>],
1375 place: &PlaceBuilder<'tcx>,
1376 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1378 debug!("candidate={:#?}\npats={:#?}", candidate, pats);
1379 let mut or_candidates: Vec<_> = pats
1381 .map(|pat| Candidate::new(place.clone(), pat, candidate.has_guard, self))
1383 let mut or_candidate_refs: Vec<_> = or_candidates.iter_mut().collect();
1384 let otherwise = if candidate.otherwise_block.is_some() {
1385 &mut candidate.otherwise_block
1389 self.match_candidates(
1392 candidate.pre_binding_block.unwrap(),
1394 &mut or_candidate_refs,
1397 candidate.subcandidates = or_candidates;
1398 self.merge_trivial_subcandidates(candidate, self.source_info(or_span));
1401 /// Try to merge all of the subcandidates of the given candidate into one.
1402 /// This avoids exponentially large CFGs in cases like `(1 | 2, 3 | 4, ...)`.
1403 fn merge_trivial_subcandidates(
1405 candidate: &mut Candidate<'_, 'tcx>,
1406 source_info: SourceInfo,
1408 if candidate.subcandidates.is_empty() || candidate.has_guard {
1409 // FIXME(or_patterns; matthewjasper) Don't give up if we have a guard.
1413 let mut can_merge = true;
1415 // Not `Iterator::all` because we don't want to short-circuit.
1416 for subcandidate in &mut candidate.subcandidates {
1417 self.merge_trivial_subcandidates(subcandidate, source_info);
1419 // FIXME(or_patterns; matthewjasper) Try to be more aggressive here.
1420 can_merge &= subcandidate.subcandidates.is_empty()
1421 && subcandidate.bindings.is_empty()
1422 && subcandidate.ascriptions.is_empty();
1426 let any_matches = self.cfg.start_new_block();
1427 for subcandidate in mem::take(&mut candidate.subcandidates) {
1428 let or_block = subcandidate.pre_binding_block.unwrap();
1429 self.cfg.goto(or_block, source_info, any_matches);
1431 candidate.pre_binding_block = Some(any_matches);
1435 /// This is the most subtle part of the matching algorithm. At
1436 /// this point, the input candidates have been fully simplified,
1437 /// and so we know that all remaining match-pairs require some
1438 /// sort of test. To decide what test to perform, we take the highest
1439 /// priority candidate (the first one in the list, as of January 2021)
1440 /// and extract the first match-pair from the list. From this we decide
1441 /// what kind of test is needed using [`Builder::test`], defined in the
1442 /// [`test` module](mod@test).
1444 /// *Note:* taking the first match pair is somewhat arbitrary, and
1445 /// we might do better here by choosing more carefully what to
1448 /// For example, consider the following possible match-pairs:
1450 /// 1. `x @ Some(P)` -- we will do a [`Switch`] to decide what variant `x` has
1451 /// 2. `x @ 22` -- we will do a [`SwitchInt`] to decide what value `x` has
1452 /// 3. `x @ 3..5` -- we will do a [`Range`] test to decide what range `x` falls in
1455 /// [`Switch`]: TestKind::Switch
1456 /// [`SwitchInt`]: TestKind::SwitchInt
1457 /// [`Range`]: TestKind::Range
1459 /// Once we know what sort of test we are going to perform, this
1460 /// test may also help us winnow down our candidates. So we walk over
1461 /// the candidates (from high to low priority) and check. This
1462 /// gives us, for each outcome of the test, a transformed list of
1463 /// candidates. For example, if we are testing `x.0`'s variant,
1464 /// and we have a candidate `(x.0 @ Some(v), x.1 @ 22)`,
1465 /// then we would have a resulting candidate of `((x.0 as Some).0 @ v, x.1 @ 22)`.
1466 /// Note that the first match-pair is now simpler (and, in fact, irrefutable).
1468 /// But there may also be candidates that the test just doesn't
1469 /// apply to. The classical example involves wildcards:
1472 /// # let (x, y, z) = (true, true, true);
1473 /// match (x, y, z) {
1474 /// (true , _ , true ) => true, // (0)
1475 /// (_ , true , _ ) => true, // (1)
1476 /// (false, false, _ ) => false, // (2)
1477 /// (true , _ , false) => false, // (3)
1482 /// In that case, after we test on `x`, there are 2 overlapping candidate
1485 /// - If the outcome is that `x` is true, candidates 0, 1, and 3
1486 /// - If the outcome is that `x` is false, candidates 1 and 2
1488 /// Here, the traditional "decision tree" method would generate 2
1489 /// separate code-paths for the 2 separate cases.
1491 /// In some cases, this duplication can create an exponential amount of
1492 /// code. This is most easily seen by noticing that this method terminates
1493 /// with precisely the reachable arms being reachable - but that problem
1494 /// is trivially NP-complete:
1496 /// ```ignore (illustrative)
1497 /// match (var0, var1, var2, var3, ...) {
1498 /// (true , _ , _ , false, true, ...) => false,
1499 /// (_ , true, true , false, _ , ...) => false,
1500 /// (false, _ , false, false, _ , ...) => false,
1506 /// Here the last arm is reachable only if there is an assignment to
1507 /// the variables that does not match any of the literals. Therefore,
1508 /// compilation would take an exponential amount of time in some cases.
1510 /// That kind of exponential worst-case might not occur in practice, but
1511 /// our simplistic treatment of constants and guards would make it occur
1512 /// in very common situations - for example [#29740]:
1514 /// ```ignore (illustrative)
1516 /// "foo" if foo_guard => ...,
1517 /// "bar" if bar_guard => ...,
1518 /// "baz" if baz_guard => ...,
1523 /// [#29740]: https://github.com/rust-lang/rust/issues/29740
1525 /// Here we first test the match-pair `x @ "foo"`, which is an [`Eq` test].
1527 /// [`Eq` test]: TestKind::Eq
1529 /// It might seem that we would end up with 2 disjoint candidate
1530 /// sets, consisting of the first candidate or the other two, but our
1531 /// algorithm doesn't reason about `"foo"` being distinct from the other
1532 /// constants; it considers the latter arms to potentially match after
1533 /// both outcomes, which obviously leads to an exponential number
1536 /// To avoid these kinds of problems, our algorithm tries to ensure
1537 /// the amount of generated tests is linear. When we do a k-way test,
1538 /// we return an additional "unmatched" set alongside the obvious `k`
1539 /// sets. When we encounter a candidate that would be present in more
1540 /// than one of the sets, we put it and all candidates below it into the
1541 /// "unmatched" set. This ensures these `k+1` sets are disjoint.
1543 /// After we perform our test, we branch into the appropriate candidate
1544 /// set and recurse with `match_candidates`. These sub-matches are
1545 /// obviously non-exhaustive - as we discarded our otherwise set - so
1546 /// we set their continuation to do `match_candidates` on the
1547 /// "unmatched" set (which is again non-exhaustive).
1549 /// If you apply this to the above test, you basically wind up
1550 /// with an if-else-if chain, testing each candidate in turn,
1551 /// which is precisely what we want.
1553 /// In addition to avoiding exponential-time blowups, this algorithm
1554 /// also has the nice property that each guard and arm is only generated
1556 fn test_candidates<'pat, 'b, 'c>(
1559 scrutinee_span: Span,
1560 mut candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>],
1562 otherwise_block: &mut Option<BasicBlock>,
1563 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1565 // extract the match-pair from the highest priority candidate
1566 let match_pair = &candidates.first().unwrap().match_pairs[0];
1567 let mut test = self.test(match_pair);
1568 let match_place = match_pair.place.clone();
1570 // most of the time, the test to perform is simply a function
1571 // of the main candidate; but for a test like SwitchInt, we
1572 // may want to add cases based on the candidates that are
1575 TestKind::SwitchInt { switch_ty, ref mut options } => {
1576 for candidate in candidates.iter() {
1577 if !self.add_cases_to_switch(&match_place, candidate, switch_ty, options) {
1582 TestKind::Switch { adt_def: _, ref mut variants } => {
1583 for candidate in candidates.iter() {
1584 if !self.add_variants_to_switch(&match_place, candidate, variants) {
1592 // Insert a Shallow borrow of any places that is switched on.
1593 if let Some(fb) = fake_borrows
1594 && let Some(resolved_place) = match_place.try_to_place(self)
1596 fb.insert(resolved_place);
1599 // perform the test, branching to one of N blocks. For each of
1600 // those N possible outcomes, create a (initially empty)
1601 // vector of candidates. Those are the candidates that still
1602 // apply if the test has that particular outcome.
1603 debug!("test_candidates: test={:?} match_pair={:?}", test, match_pair);
1604 let mut target_candidates: Vec<Vec<&mut Candidate<'pat, 'tcx>>> = vec![];
1605 target_candidates.resize_with(test.targets(), Default::default);
1607 let total_candidate_count = candidates.len();
1609 // Sort the candidates into the appropriate vector in
1610 // `target_candidates`. Note that at some point we may
1611 // encounter a candidate where the test is not relevant; at
1612 // that point, we stop sorting.
1613 while let Some(candidate) = candidates.first_mut() {
1614 let Some(idx) = self.sort_candidate(&match_place, &test, candidate) else {
1617 let (candidate, rest) = candidates.split_first_mut().unwrap();
1618 target_candidates[idx].push(candidate);
1621 // at least the first candidate ought to be tested
1623 total_candidate_count > candidates.len(),
1625 total_candidate_count,
1628 debug!("tested_candidates: {}", total_candidate_count - candidates.len());
1629 debug!("untested_candidates: {}", candidates.len());
1631 // HACK(matthewjasper) This is a closure so that we can let the test
1632 // create its blocks before the rest of the match. This currently
1633 // improves the speed of llvm when optimizing long string literal
1635 let make_target_blocks = move |this: &mut Self| -> Vec<BasicBlock> {
1636 // The block that we should branch to if none of the
1637 // `target_candidates` match. This is either the block where we
1638 // start matching the untested candidates if there are any,
1639 // otherwise it's the `otherwise_block`.
1640 let remainder_start = &mut None;
1641 let remainder_start =
1642 if candidates.is_empty() { &mut *otherwise_block } else { remainder_start };
1644 // For each outcome of test, process the candidates that still
1645 // apply. Collect a list of blocks where control flow will
1646 // branch if one of the `target_candidate` sets is not
1648 let target_blocks: Vec<_> = target_candidates
1650 .map(|mut candidates| {
1651 if !candidates.is_empty() {
1652 let candidate_start = this.cfg.start_new_block();
1653 this.match_candidates(
1663 *remainder_start.get_or_insert_with(|| this.cfg.start_new_block())
1668 if !candidates.is_empty() {
1669 let remainder_start = remainder_start.unwrap_or_else(|| this.cfg.start_new_block());
1670 this.match_candidates(
1683 self.perform_test(span, scrutinee_span, block, &match_place, &test, make_target_blocks);
1686 /// Determine the fake borrows that are needed from a set of places that
1687 /// have to be stable across match guards.
1689 /// Returns a list of places that need a fake borrow and the temporary
1690 /// that's used to store the fake borrow.
1692 /// Match exhaustiveness checking is not able to handle the case where the
1693 /// place being matched on is mutated in the guards. We add "fake borrows"
1694 /// to the guards that prevent any mutation of the place being matched.
1695 /// There are a some subtleties:
1697 /// 1. Borrowing `*x` doesn't prevent assigning to `x`. If `x` is a shared
1698 /// reference, the borrow isn't even tracked. As such we have to add fake
1699 /// borrows of any prefixes of a place
1700 /// 2. We don't want `match x { _ => (), }` to conflict with mutable
1701 /// borrows of `x`, so we only add fake borrows for places which are
1702 /// bound or tested by the match.
1703 /// 3. We don't want the fake borrows to conflict with `ref mut` bindings,
1704 /// so we use a special BorrowKind for them.
1705 /// 4. The fake borrows may be of places in inactive variants, so it would
1706 /// be UB to generate code for them. They therefore have to be removed
1707 /// by a MIR pass run after borrow checking.
1708 fn calculate_fake_borrows<'b>(
1710 fake_borrows: &'b FxIndexSet<Place<'tcx>>,
1712 ) -> Vec<(Place<'tcx>, Local)> {
1715 debug!("add_fake_borrows fake_borrows = {:?}", fake_borrows);
1717 let mut all_fake_borrows = Vec::with_capacity(fake_borrows.len());
1719 // Insert a Shallow borrow of the prefixes of any fake borrows.
1720 for place in fake_borrows {
1721 let mut cursor = place.projection.as_ref();
1722 while let [proj_base @ .., elem] = cursor {
1725 if let ProjectionElem::Deref = elem {
1726 // Insert a shallow borrow after a deref. For other
1727 // projections the borrow of prefix_cursor will
1728 // conflict with any mutation of base.
1729 all_fake_borrows.push(PlaceRef { local: place.local, projection: proj_base });
1733 all_fake_borrows.push(place.as_ref());
1737 let mut dedup = FxHashSet::default();
1738 all_fake_borrows.retain(|b| dedup.insert(*b));
1740 debug!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows);
1744 .map(|matched_place_ref| {
1745 let matched_place = Place {
1746 local: matched_place_ref.local,
1747 projection: tcx.intern_place_elems(matched_place_ref.projection),
1749 let fake_borrow_deref_ty = matched_place.ty(&self.local_decls, tcx).ty;
1750 let fake_borrow_ty = tcx.mk_imm_ref(tcx.lifetimes.re_erased, fake_borrow_deref_ty);
1751 let fake_borrow_temp =
1752 self.local_decls.push(LocalDecl::new(fake_borrow_ty, temp_span));
1754 (matched_place, fake_borrow_temp)
1760 ///////////////////////////////////////////////////////////////////////////
1761 // Pat binding - used for `let` and function parameters as well.
1763 impl<'a, 'tcx> Builder<'a, 'tcx> {
1764 /// If the bindings have already been declared, set `declare_bindings` to
1765 /// `false` to avoid duplicated bindings declaration. Used for if-let guards.
1766 pub(crate) fn lower_let_expr(
1768 mut block: BasicBlock,
1771 else_target: region::Scope,
1772 source_scope: Option<SourceScope>,
1774 declare_bindings: bool,
1776 let expr_span = expr.span;
1777 let expr_place_builder = unpack!(block = self.lower_scrutinee(block, expr, expr_span));
1778 let wildcard = Pat::wildcard_from_ty(pat.ty);
1779 let mut guard_candidate = Candidate::new(expr_place_builder.clone(), &pat, false, self);
1780 let mut otherwise_candidate =
1781 Candidate::new(expr_place_builder.clone(), &wildcard, false, self);
1782 let fake_borrow_temps = self.lower_match_tree(
1787 &mut [&mut guard_candidate, &mut otherwise_candidate],
1789 let expr_place = expr_place_builder.try_to_place(self);
1790 let opt_expr_place = expr_place.as_ref().map(|place| (Some(place), expr_span));
1791 let otherwise_post_guard_block = otherwise_candidate.pre_binding_block.unwrap();
1792 self.break_for_else(otherwise_post_guard_block, else_target, self.source_info(expr_span));
1794 if declare_bindings {
1795 self.declare_bindings(source_scope, pat.span.to(span), pat, None, opt_expr_place);
1798 let post_guard_block = self.bind_pattern(
1799 self.source_info(pat.span),
1807 post_guard_block.unit()
1810 /// Initializes each of the bindings from the candidate by
1811 /// moving/copying/ref'ing the source as appropriate. Tests the guard, if
1812 /// any, and then branches to the arm. Returns the block for the case where
1813 /// the guard succeeds.
1815 /// Note: we do not check earlier that if there is a guard,
1816 /// there cannot be move bindings. We avoid a use-after-move by only
1817 /// moving the binding once the guard has evaluated to true (see below).
1818 fn bind_and_guard_matched_candidate<'pat>(
1820 candidate: Candidate<'pat, 'tcx>,
1821 parent_bindings: &[(Vec<Binding<'tcx>>, Vec<Ascription<'tcx>>)],
1822 fake_borrows: &[(Place<'tcx>, Local)],
1823 scrutinee_span: Span,
1824 arm_match_scope: Option<(&Arm<'tcx>, region::Scope)>,
1825 schedule_drops: bool,
1826 storages_alive: bool,
1828 debug!("bind_and_guard_matched_candidate(candidate={:?})", candidate);
1830 debug_assert!(candidate.match_pairs.is_empty());
1832 let candidate_source_info = self.source_info(candidate.span);
1834 let mut block = candidate.pre_binding_block.unwrap();
1836 if candidate.next_candidate_pre_binding_block.is_some() {
1837 let fresh_block = self.cfg.start_new_block();
1841 candidate.next_candidate_pre_binding_block,
1842 candidate_source_info,
1844 block = fresh_block;
1851 .flat_map(|(_, ascriptions)| ascriptions)
1853 .chain(candidate.ascriptions),
1856 // rust-lang/rust#27282: The `autoref` business deserves some
1857 // explanation here.
1859 // The intent of the `autoref` flag is that when it is true,
1860 // then any pattern bindings of type T will map to a `&T`
1861 // within the context of the guard expression, but will
1862 // continue to map to a `T` in the context of the arm body. To
1863 // avoid surfacing this distinction in the user source code
1864 // (which would be a severe change to the language and require
1865 // far more revision to the compiler), when `autoref` is true,
1866 // then any occurrence of the identifier in the guard
1867 // expression will automatically get a deref op applied to it.
1869 // So an input like:
1872 // let place = Foo::new();
1873 // match place { foo if inspect(foo)
1874 // => feed(foo), ... }
1877 // will be treated as if it were really something like:
1880 // let place = Foo::new();
1881 // match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) }
1882 // => { let tmp2 = place; feed(tmp2) }, ... }
1884 // And an input like:
1887 // let place = Foo::new();
1888 // match place { ref mut foo if inspect(foo)
1889 // => feed(foo), ... }
1892 // will be treated as if it were really something like:
1895 // let place = Foo::new();
1896 // match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) }
1897 // => { let tmp2 = &mut place; feed(tmp2) }, ... }
1900 // In short, any pattern binding will always look like *some*
1901 // kind of `&T` within the guard at least in terms of how the
1902 // MIR-borrowck views it, and this will ensure that guard
1903 // expressions cannot mutate their the match inputs via such
1904 // bindings. (It also ensures that guard expressions can at
1905 // most *copy* values from such bindings; non-Copy things
1906 // cannot be moved via pattern bindings in guard expressions.)
1910 // Implementation notes (under assumption `autoref` is true).
1912 // To encode the distinction above, we must inject the
1913 // temporaries `tmp1` and `tmp2`.
1915 // There are two cases of interest: binding by-value, and binding by-ref.
1917 // 1. Binding by-value: Things are simple.
1919 // * Establishing `tmp1` creates a reference into the
1920 // matched place. This code is emitted by
1921 // bind_matched_candidate_for_guard.
1923 // * `tmp2` is only initialized "lazily", after we have
1924 // checked the guard. Thus, the code that can trigger
1925 // moves out of the candidate can only fire after the
1926 // guard evaluated to true. This initialization code is
1927 // emitted by bind_matched_candidate_for_arm.
1929 // 2. Binding by-reference: Things are tricky.
1931 // * Here, the guard expression wants a `&&` or `&&mut`
1932 // into the original input. This means we need to borrow
1933 // the reference that we create for the arm.
1934 // * So we eagerly create the reference for the arm and then take a
1935 // reference to that.
1936 if let Some((arm, match_scope)) = arm_match_scope
1937 && let Some(guard) = &arm.guard
1940 let bindings = parent_bindings
1942 .flat_map(|(bindings, _)| bindings)
1943 .chain(&candidate.bindings);
1945 self.bind_matched_candidate_for_guard(block, schedule_drops, bindings.clone());
1946 let guard_frame = GuardFrame {
1947 locals: bindings.map(|b| GuardFrameLocal::new(b.var_id, b.binding_mode)).collect(),
1949 debug!("entering guard building context: {:?}", guard_frame);
1950 self.guard_context.push(guard_frame);
1952 let re_erased = tcx.lifetimes.re_erased;
1953 let scrutinee_source_info = self.source_info(scrutinee_span);
1954 for &(place, temp) in fake_borrows {
1955 let borrow = Rvalue::Ref(re_erased, BorrowKind::Shallow, place);
1956 self.cfg.push_assign(block, scrutinee_source_info, Place::from(temp), borrow);
1959 let mut guard_span = rustc_span::DUMMY_SP;
1961 let (post_guard_block, otherwise_post_guard_block) =
1962 self.in_if_then_scope(match_scope, guard_span, |this| match *guard {
1964 let e = &this.thir[e];
1965 guard_span = e.span;
1966 this.then_else_break(
1971 this.source_info(arm.span),
1974 Guard::IfLet(ref pat, scrutinee) => {
1975 let s = &this.thir[scrutinee];
1976 guard_span = s.span;
1977 this.lower_let_expr(block, s, pat, match_scope, None, arm.span, false)
1981 let source_info = self.source_info(guard_span);
1982 let guard_end = self.source_info(tcx.sess.source_map().end_point(guard_span));
1983 let guard_frame = self.guard_context.pop().unwrap();
1984 debug!("Exiting guard building context with locals: {:?}", guard_frame);
1986 for &(_, temp) in fake_borrows {
1987 let cause = FakeReadCause::ForMatchGuard;
1988 self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(temp));
1991 let otherwise_block = candidate.otherwise_block.unwrap_or_else(|| {
1992 let unreachable = self.cfg.start_new_block();
1993 self.cfg.terminate(unreachable, source_info, TerminatorKind::Unreachable);
1997 otherwise_post_guard_block,
1999 candidate.next_candidate_pre_binding_block,
2003 // We want to ensure that the matched candidates are bound
2004 // after we have confirmed this candidate *and* any
2005 // associated guard; Binding them on `block` is too soon,
2006 // because that would be before we've checked the result
2009 // But binding them on the arm is *too late*, because
2010 // then all of the candidates for a single arm would be
2011 // bound in the same place, that would cause a case like:
2015 // (mut x, 1) | (2, mut x) if { true } => { ... }
2016 // ... // ^^^^^^^ (this is `arm_block`)
2020 // would yield an `arm_block` something like:
2023 // StorageLive(_4); // _4 is `x`
2024 // _4 = &mut (_1.0: i32); // this is handling `(mut x, 1)` case
2025 // _4 = &mut (_1.1: i32); // this is handling `(2, mut x)` case
2028 // and that is clearly not correct.
2029 let by_value_bindings = parent_bindings
2031 .flat_map(|(bindings, _)| bindings)
2032 .chain(&candidate.bindings)
2033 .filter(|binding| matches!(binding.binding_mode, BindingMode::ByValue));
2034 // Read all of the by reference bindings to ensure that the
2035 // place they refer to can't be modified by the guard.
2036 for binding in by_value_bindings.clone() {
2037 let local_id = self.var_local_id(binding.var_id, RefWithinGuard);
2038 let cause = FakeReadCause::ForGuardBinding;
2039 self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(local_id));
2041 assert!(schedule_drops, "patterns with guards must schedule drops");
2042 self.bind_matched_candidate_for_arm_body(
2051 // (Here, it is not too early to bind the matched
2052 // candidate on `block`, because there is no guard result
2053 // that we have to inspect before we bind them.)
2054 self.bind_matched_candidate_for_arm_body(
2059 .flat_map(|(bindings, _)| bindings)
2060 .chain(&candidate.bindings),
2067 /// Append `AscribeUserType` statements onto the end of `block`
2068 /// for each ascription
2072 ascriptions: impl IntoIterator<Item = Ascription<'tcx>>,
2074 for ascription in ascriptions {
2075 let source_info = self.source_info(ascription.annotation.span);
2077 let base = self.canonical_user_type_annotations.push(ascription.annotation);
2082 kind: StatementKind::AscribeUserType(
2085 UserTypeProjection { base, projs: Vec::new() },
2087 ascription.variance,
2094 fn bind_matched_candidate_for_guard<'b>(
2097 schedule_drops: bool,
2098 bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
2102 debug!("bind_matched_candidate_for_guard(block={:?})", block);
2104 // Assign each of the bindings. Since we are binding for a
2105 // guard expression, this will never trigger moves out of the
2107 let re_erased = self.tcx.lifetimes.re_erased;
2108 for binding in bindings {
2109 debug!("bind_matched_candidate_for_guard(binding={:?})", binding);
2110 let source_info = self.source_info(binding.span);
2112 // For each pattern ident P of type T, `ref_for_guard` is
2113 // a reference R: &T pointing to the location matched by
2114 // the pattern, and every occurrence of P within a guard
2116 let ref_for_guard = self.storage_live_binding(
2123 match binding.binding_mode {
2124 BindingMode::ByValue => {
2125 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, binding.source);
2126 self.cfg.push_assign(block, source_info, ref_for_guard, rvalue);
2128 BindingMode::ByRef(borrow_kind) => {
2129 let value_for_arm = self.storage_live_binding(
2137 let rvalue = Rvalue::Ref(re_erased, borrow_kind, binding.source);
2138 self.cfg.push_assign(block, source_info, value_for_arm, rvalue);
2139 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, value_for_arm);
2140 self.cfg.push_assign(block, source_info, ref_for_guard, rvalue);
2146 fn bind_matched_candidate_for_arm_body<'b>(
2149 schedule_drops: bool,
2150 bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
2151 storages_alive: bool,
2155 debug!("bind_matched_candidate_for_arm_body(block={:?})", block);
2157 let re_erased = self.tcx.lifetimes.re_erased;
2158 // Assign each of the bindings. This may trigger moves out of the candidate.
2159 for binding in bindings {
2160 let source_info = self.source_info(binding.span);
2161 let local = if storages_alive {
2162 // Here storages are already alive, probably because this is a binding
2164 // We just need to schedule drop for the value.
2165 self.var_local_id(binding.var_id, OutsideGuard).into()
2167 self.storage_live_binding(
2176 self.schedule_drop_for_binding(binding.var_id, binding.span, OutsideGuard);
2178 let rvalue = match binding.binding_mode {
2179 BindingMode::ByValue => Rvalue::Use(self.consume_by_copy_or_move(binding.source)),
2180 BindingMode::ByRef(borrow_kind) => {
2181 Rvalue::Ref(re_erased, borrow_kind, binding.source)
2184 self.cfg.push_assign(block, source_info, local, rvalue);
2188 /// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where the bound
2189 /// `var` has type `T` in the arm body) in a pattern maps to 2 locals. The
2190 /// first local is a binding for occurrences of `var` in the guard, which
2191 /// will have type `&T`. The second local is a binding for occurrences of
2192 /// `var` in the arm body, which will have type `T`.
2193 #[instrument(skip(self), level = "debug")]
2196 source_info: SourceInfo,
2197 visibility_scope: SourceScope,
2198 mutability: Mutability,
2203 user_ty: UserTypeProjections,
2204 has_guard: ArmHasGuard,
2205 opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
2209 let debug_source_info = SourceInfo { span: source_info.span, scope: visibility_scope };
2210 let binding_mode = match mode {
2211 BindingMode::ByValue => ty::BindingMode::BindByValue(mutability),
2212 BindingMode::ByRef(_) => ty::BindingMode::BindByReference(mutability),
2214 let local = LocalDecl::<'tcx> {
2217 user_ty: if user_ty.is_empty() { None } else { Some(Box::new(user_ty)) },
2220 is_block_tail: None,
2221 local_info: Some(Box::new(LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
2224 // hypothetically, `visit_primary_bindings` could try to unzip
2225 // an outermost hir::Ty as we descend, matching up
2226 // idents in pat; but complex w/ unclear UI payoff.
2227 // Instead, just abandon providing diagnostic info.
2234 let for_arm_body = self.local_decls.push(local);
2235 self.var_debug_info.push(VarDebugInfo {
2237 source_info: debug_source_info,
2238 value: VarDebugInfoContents::Place(for_arm_body.into()),
2240 let locals = if has_guard.0 {
2241 let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
2242 // This variable isn't mutated but has a name, so has to be
2243 // immutable to avoid the unused mut lint.
2244 mutability: Mutability::Not,
2245 ty: tcx.mk_imm_ref(tcx.lifetimes.re_erased, var_ty),
2249 is_block_tail: None,
2250 local_info: Some(Box::new(LocalInfo::User(ClearCrossCrate::Set(
2251 BindingForm::RefForGuard,
2254 self.var_debug_info.push(VarDebugInfo {
2256 source_info: debug_source_info,
2257 value: VarDebugInfoContents::Place(ref_for_guard.into()),
2259 LocalsForNode::ForGuard { ref_for_guard, for_arm_body }
2261 LocalsForNode::One(for_arm_body)
2264 self.var_indices.insert(var_id, locals);
2267 pub(crate) fn ast_let_else(
2269 mut block: BasicBlock,
2271 initializer_span: Span,
2272 else_block: BlockId,
2273 let_else_scope: ®ion::Scope,
2274 pattern: &Pat<'tcx>,
2275 ) -> BlockAnd<BasicBlock> {
2276 let else_block_span = self.thir[else_block].span;
2277 let (matching, failure) = self.in_if_then_scope(*let_else_scope, else_block_span, |this| {
2278 let scrutinee = unpack!(block = this.lower_scrutinee(block, init, initializer_span));
2279 let pat = Pat { ty: init.ty, span: else_block_span, kind: PatKind::Wild };
2280 let mut wildcard = Candidate::new(scrutinee.clone(), &pat, false, this);
2281 let mut candidate = Candidate::new(scrutinee.clone(), pattern, false, this);
2282 let fake_borrow_temps = this.lower_match_tree(
2287 &mut [&mut candidate, &mut wildcard],
2289 // This block is for the matching case
2290 let matching = this.bind_pattern(
2291 this.source_info(pattern.span),
2298 // This block is for the failure case
2299 let failure = this.bind_pattern(
2300 this.source_info(else_block_span),
2307 this.break_for_else(failure, *let_else_scope, this.source_info(initializer_span));
2310 matching.and(failure)