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.
4 //!
5 //! This also includes code for pattern bindings in `let` statements and
6 //! function parameters.
7
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,
16 };
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};
26
27 // helper functions, broken out by category:
28 mod simplify;
29 mod test;
30 mod util;
31
32 use std::borrow::Borrow;
33 use std::mem;
34
35 impl<'a, 'tcx> Builder<'a, 'tcx> {
then_else_break( &mut self, mut block: BasicBlock, expr: &Expr<'tcx>, temp_scope_override: Option<region::Scope>, break_scope: region::Scope, variable_source_info: SourceInfo, ) -> BlockAnd<()>36 pub(crate) fn then_else_break(
37 &mut self,
38 mut block: BasicBlock,
39 expr: &Expr<'tcx>,
40 temp_scope_override: Option<region::Scope>,
41 break_scope: region::Scope,
42 variable_source_info: SourceInfo,
43 ) -> BlockAnd<()> {
44 let this = self;
45 let expr_span = expr.span;
46
47 match expr.kind {
48 ExprKind::LogicalOp { op: LogicalOp::And, lhs, rhs } => {
49 let lhs_then_block = unpack!(this.then_else_break(
50 block,
51 &this.thir[lhs],
52 temp_scope_override,
53 break_scope,
54 variable_source_info,
55 ));
56
57 let rhs_then_block = unpack!(this.then_else_break(
58 lhs_then_block,
59 &this.thir[rhs],
60 temp_scope_override,
61 break_scope,
62 variable_source_info,
63 ));
64
65 rhs_then_block.unit()
66 }
67 ExprKind::Scope { region_scope, lint_level, value } => {
68 let region_scope = (region_scope, this.source_info(expr_span));
69 this.in_scope(region_scope, lint_level, |this| {
70 this.then_else_break(
71 block,
72 &this.thir[value],
73 temp_scope_override,
74 break_scope,
75 variable_source_info,
76 )
77 })
78 }
79 ExprKind::Let { expr, ref pat } => this.lower_let_expr(
80 block,
81 &this.thir[expr],
82 pat,
83 break_scope,
84 Some(variable_source_info.scope),
85 variable_source_info.span,
86 true,
87 ),
88 _ => {
89 let temp_scope = temp_scope_override.unwrap_or_else(|| this.local_scope());
90 let mutability = Mutability::Mut;
91 let place =
92 unpack!(block = this.as_temp(block, Some(temp_scope), expr, mutability));
93 let operand = Operand::Move(Place::from(place));
94
95 let then_block = this.cfg.start_new_block();
96 let else_block = this.cfg.start_new_block();
97 let term = TerminatorKind::if_(operand, then_block, else_block);
98
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);
102
103 then_block.unit()
104 }
105 }
106 }
107
108 /// Generates MIR for a `match` expression.
109 ///
110 /// The MIR that we generate for a match looks like this.
111 ///
112 /// ```text
113 /// [ 0. Pre-match ]
114 /// |
115 /// [ 1. Evaluate Scrutinee (expression being matched on) ]
116 /// [ (fake read of scrutinee) ]
117 /// |
118 /// [ 2. Decision tree -- check discriminants ] <--------+
119 /// | |
120 /// | (once a specific arm is chosen) |
121 /// | |
122 /// [pre_binding_block] [otherwise_block]
123 /// | |
124 /// [ 3. Create "guard bindings" for arm ] |
125 /// [ (create fake borrows) ] |
126 /// | |
127 /// [ 4. Execute guard code ] |
128 /// [ (read fake borrows) ] --(guard is false)-----------+
129 /// |
130 /// | (guard results in true)
131 /// |
132 /// [ 5. Create real bindings and execute arm ]
133 /// |
134 /// [ Exit match ]
135 /// ```
136 ///
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.
140 ///
141 /// We generate MIR in the following steps:
142 ///
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]).
149 ///
150 /// ## False edges
151 ///
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:
155 ///
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))]
match_expr( &mut self, destination: Place<'tcx>, span: Span, mut block: BasicBlock, scrutinee: &Expr<'tcx>, arms: &[ArmId], ) -> BlockAnd<()>159 pub(crate) fn match_expr(
160 &mut self,
161 destination: Place<'tcx>,
162 span: Span,
163 mut block: BasicBlock,
164 scrutinee: &Expr<'tcx>,
165 arms: &[ArmId],
166 ) -> BlockAnd<()> {
167 let scrutinee_span = scrutinee.span;
168 let scrutinee_place =
169 unpack!(block = self.lower_scrutinee(block, scrutinee, scrutinee_span,));
170
171 let mut arm_candidates = self.create_match_candidates(&scrutinee_place, &arms);
172
173 let match_has_guard = arm_candidates.iter().any(|(_, candidate)| candidate.has_guard);
174 let mut candidates =
175 arm_candidates.iter_mut().map(|(_, candidate)| candidate).collect::<Vec<_>>();
176
177 let match_start_span = span.shrink_to_lo().to(scrutinee.span);
178
179 let fake_borrow_temps = self.lower_match_tree(
180 block,
181 scrutinee_span,
182 match_start_span,
183 match_has_guard,
184 &mut candidates,
185 );
186
187 self.lower_match_arms(
188 destination,
189 scrutinee_place,
190 scrutinee_span,
191 arm_candidates,
192 self.source_info(span),
193 fake_borrow_temps,
194 )
195 }
196
197 /// Evaluate the scrutinee and add the fake read of it.
lower_scrutinee( &mut self, mut block: BasicBlock, scrutinee: &Expr<'tcx>, scrutinee_span: Span, ) -> BlockAnd<PlaceBuilder<'tcx>>198 fn lower_scrutinee(
199 &mut self,
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.
211 //
212 // HACK(eddyb) Work around the above issue by adding a dummy inspection
213 // of `scrutinee_place`, specifically by applying `ReadForMatch`.
214 //
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
219 // check safety.
220 let cause_matched_place = FakeReadCause::ForMatchedPlace(None);
221 let source_info = self.source_info(scrutinee_span);
222
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);
225 }
226
227 block.and(scrutinee_place_builder)
228 }
229
230 /// Create the initial `Candidate`s for a `match` expression.
create_match_candidates<'pat>( &mut self, scrutinee: &PlaceBuilder<'tcx>, arms: &'pat [ArmId], ) -> Vec<(&'pat Arm<'tcx>, Candidate<'pat, 'tcx>)> where 'a: 'pat,231 fn create_match_candidates<'pat>(
232 &mut self,
233 scrutinee: &PlaceBuilder<'tcx>,
234 arms: &'pat [ArmId],
235 ) -> Vec<(&'pat Arm<'tcx>, Candidate<'pat, 'tcx>)>
236 where
237 'a: 'pat,
238 {
239 // Assemble a list of candidates: there is one candidate per pattern,
240 // which means there may be more than one candidate *per arm*.
241 arms.iter()
242 .copied()
243 .map(|arm| {
244 let arm = &self.thir[arm];
245 let arm_has_guard = arm.guard.is_some();
246 let arm_candidate =
247 Candidate::new(scrutinee.clone(), &arm.pattern, arm_has_guard, self);
248 (arm, arm_candidate)
249 })
250 .collect()
251 }
252
253 /// Create the decision tree for the match expression, starting from `block`.
254 ///
255 /// Modifies `candidates` to store the bindings and type ascriptions for
256 /// that candidate.
257 ///
258 /// Returns the places that need fake borrows because we bind or test them.
lower_match_tree<'pat>( &mut self, block: BasicBlock, scrutinee_span: Span, match_start_span: Span, match_has_guard: bool, candidates: &mut [&mut Candidate<'pat, 'tcx>], ) -> Vec<(Place<'tcx>, Local)>259 fn lower_match_tree<'pat>(
260 &mut self,
261 block: BasicBlock,
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
269 // them.
270 let mut fake_borrows = match_has_guard.then(FxIndexSet::default);
271
272 let mut otherwise = None;
273
274 // This will generate code to test scrutinee_place and
275 // branch to the appropriate arm block
276 self.match_candidates(
277 match_start_span,
278 scrutinee_span,
279 block,
280 &mut otherwise,
281 candidates,
282 &mut fake_borrows,
283 );
284
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
288 // unreachable.
289 let source_info = self.source_info(scrutinee_span);
290 self.cfg.terminate(otherwise_block, source_info, TerminatorKind::Unreachable);
291 }
292
293 // Link each leaf candidate to the `pre_binding_block` of the next one.
294 let mut previous_candidate: Option<&mut Candidate<'_, '_>> = None;
295
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;
300 }
301 previous_candidate = Some(leaf_candidate);
302 });
303 }
304
305 if let Some(ref borrows) = fake_borrows {
306 self.calculate_fake_borrows(borrows, scrutinee_span)
307 } else {
308 Vec::new()
309 }
310 }
311
312 /// Lower the bindings, guards and arm bodies of a `match` expression.
313 ///
314 /// The decision tree should have already been created
315 /// (by [Builder::lower_match_tree]).
316 ///
317 /// `outer_source_info` is the SourceInfo for the whole match.
lower_match_arms( &mut self, destination: Place<'tcx>, scrutinee_place_builder: PlaceBuilder<'tcx>, scrutinee_span: Span, arm_candidates: Vec<(&'_ Arm<'tcx>, Candidate<'_, 'tcx>)>, outer_source_info: SourceInfo, fake_borrow_temps: Vec<(Place<'tcx>, Local)>, ) -> BlockAnd<()>318 fn lower_match_arms(
319 &mut self,
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)>,
326 ) -> BlockAnd<()> {
327 let arm_end_blocks: Vec<_> = arm_candidates
328 .into_iter()
329 .map(|(arm, candidate)| {
330 debug!("lowering arm {:?}\ncandidate = {:?}", arm, candidate);
331
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 (`_`).
340 // Example:
341 // ```
342 // let foo = (0, 1);
343 // let c = || {
344 // match foo { _ => () };
345 // };
346 // ```
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(
351 None,
352 arm.span,
353 &arm.pattern,
354 arm.guard.as_ref(),
355 opt_scrutinee_place,
356 );
357
358 let arm_block = this.bind_pattern(
359 outer_source_info,
360 candidate,
361 &fake_borrow_temps,
362 scrutinee_span,
363 Some((arm, match_scope)),
364 false,
365 );
366
367 if let Some(source_scope) = scope {
368 this.source_scope = source_scope;
369 }
370
371 this.expr_into_dest(destination, arm_block, &&this.thir[arm.body])
372 })
373 })
374 .collect();
375
376 // all the arm blocks will rejoin here
377 let end_block = self.cfg.start_new_block();
378
379 let end_brace = self.source_info(
380 outer_source_info.span.with_lo(outer_source_info.span.hi() - BytePos::from_usize(1)),
381 );
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);
385
386 self.cfg.goto(unpack!(arm_block), last_location.unwrap_or(end_brace), end_block);
387 }
388
389 self.source_scope = outer_source_info.scope;
390
391 end_block.unit()
392 }
393
394 /// Binds the variables and ascribes types for a given `match` arm or
395 /// `let` binding.
396 ///
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
399 /// `match` arm.
bind_pattern( &mut self, outer_source_info: SourceInfo, candidate: Candidate<'_, 'tcx>, fake_borrow_temps: &[(Place<'tcx>, Local)], scrutinee_span: Span, arm_match_scope: Option<(&Arm<'tcx>, region::Scope)>, storages_alive: bool, ) -> BasicBlock400 fn bind_pattern(
401 &mut self,
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,
408 ) -> BasicBlock {
409 if candidate.subcandidates.is_empty() {
410 // Avoid generating another `BasicBlock` when we only have one
411 // candidate.
412 self.bind_and_guard_matched_candidate(
413 candidate,
414 &[],
415 fake_borrow_temps,
416 scrutinee_span,
417 arm_match_scope,
418 true,
419 storages_alive,
420 )
421 } else {
422 // It's helpful to avoid scheduling drops multiple times to save
423 // drop elaboration from having to clean up the extra drops.
424 //
425 // If we are in a `let` then we only schedule drops for the first
426 // candidate.
427 //
428 // If we're in a `match` arm then we could have a case like so:
429 //
430 // Ok(x) | Err(x) if return => { /* ... */ }
431 //
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.
442 traverse_candidate(
443 candidate,
444 &mut Vec::new(),
445 &mut |leaf_candidate, parent_bindings| {
446 if let Some(arm) = arm {
447 self.clear_top_scope(arm.scope);
448 }
449 let binding_end = self.bind_and_guard_matched_candidate(
450 leaf_candidate,
451 parent_bindings,
452 &fake_borrow_temps,
453 scrutinee_span,
454 arm_match_scope,
455 schedule_drops,
456 storages_alive,
457 );
458 if arm.is_none() {
459 schedule_drops = false;
460 }
461 self.cfg.goto(binding_end, outer_source_info, target_block);
462 },
463 |inner_candidate, parent_bindings| {
464 parent_bindings.push((inner_candidate.bindings, inner_candidate.ascriptions));
465 inner_candidate.subcandidates.into_iter()
466 },
467 |parent_bindings| {
468 parent_bindings.pop();
469 },
470 );
471
472 target_block
473 }
474 }
475
expr_into_pattern( &mut self, mut block: BasicBlock, irrefutable_pat: &Pat<'tcx>, initializer: &Expr<'tcx>, ) -> BlockAnd<()>476 pub(super) fn expr_into_pattern(
477 &mut self,
478 mut block: BasicBlock,
479 irrefutable_pat: &Pat<'tcx>,
480 initializer: &Expr<'tcx>,
481 ) -> BlockAnd<()> {
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, .. } => {
485 let place =
486 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
487 unpack!(block = self.expr_into_dest(place, block, initializer));
488
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);
492
493 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
494 block.unit()
495 }
496
497 // Optimize the case of `let x: T = ...` to write directly
498 // into `x` and then require that `T == typeof(x)`.
499 //
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
504 // broken.
505 PatKind::AscribeUserType {
506 subpattern:
507 box Pat {
508 kind:
509 PatKind::Binding {
510 mode: BindingMode::ByValue, var, subpattern: None, ..
511 },
512 ..
513 },
514 ascription: thir::Ascription { ref annotation, variance: _ },
515 } => {
516 let place =
517 self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
518 unpack!(block = self.expr_into_dest(place, block, initializer));
519
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);
524
525 let ty_source_info = self.source_info(annotation.span);
526
527 let base = self.canonical_user_type_annotations.push(annotation.clone());
528 self.cfg.push(
529 block,
530 Statement {
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
538 // example:
539 //
540 // ```
541 // let x: T = <expr>
542 // ```
543 //
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
547 // `<expr>`.
548 ty::Variance::Invariant,
549 ),
550 },
551 );
552
553 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
554 block.unit()
555 }
556
557 _ => {
558 let place_builder = unpack!(block = self.as_place_builder(block, initializer));
559
560 if let Some(place) = place_builder.try_to_place(self) {
561 let source_info = self.source_info(initializer.span);
562 self.cfg.push_place_mention(block, source_info, place);
563 }
564
565 self.place_into_pattern(block, &irrefutable_pat, place_builder, true)
566 }
567 }
568 }
569
place_into_pattern( &mut self, block: BasicBlock, irrefutable_pat: &Pat<'tcx>, initializer: PlaceBuilder<'tcx>, set_match_place: bool, ) -> BlockAnd<()>570 pub(crate) fn place_into_pattern(
571 &mut self,
572 block: BasicBlock,
573 irrefutable_pat: &Pat<'tcx>,
574 initializer: PlaceBuilder<'tcx>,
575 set_match_place: bool,
576 ) -> BlockAnd<()> {
577 let mut candidate = Candidate::new(initializer.clone(), &irrefutable_pat, false, self);
578 let fake_borrow_temps = self.lower_match_tree(
579 block,
580 irrefutable_pat.span,
581 irrefutable_pat.span,
582 false,
583 &mut [&mut candidate],
584 );
585
586 // For matches and function arguments, the place that is being matched
587 // can be set when creating the variables. But the place for
588 // let PATTERN = ... might not even exist until we do the assignment.
589 // so we set it here instead.
590 if set_match_place {
591 let mut next = Some(&candidate);
592 while let Some(candidate_ref) = next.take() {
593 for binding in &candidate_ref.bindings {
594 let local = self.var_local_id(binding.var_id, OutsideGuard);
595 // `try_to_place` may fail if it is unable to resolve the given
596 // `PlaceBuilder` inside a closure. In this case, we don't want to include
597 // a scrutinee place. `scrutinee_place_builder` will fail for destructured
598 // assignments. This is because a closure only captures the precise places
599 // that it will read and as a result a closure may not capture the entire
600 // tuple/struct and rather have individual places that will be read in the
601 // final MIR.
602 // Example:
603 // ```
604 // let foo = (0, 1);
605 // let c = || {
606 // let (v1, v2) = foo;
607 // };
608 // ```
609 if let Some(place) = initializer.try_to_place(self) {
610 let LocalInfo::User(BindingForm::Var(
611 VarBindingForm { opt_match_place: Some((ref mut match_place, _)), .. },
612 )) = **self.local_decls[local].local_info.as_mut().assert_crate_local() else {
613 bug!("Let binding to non-user variable.")
614 };
615 *match_place = Some(place);
616 }
617 }
618 // All of the subcandidates should bind the same locals, so we
619 // only visit the first one.
620 next = candidate_ref.subcandidates.get(0)
621 }
622 }
623
624 self.bind_pattern(
625 self.source_info(irrefutable_pat.span),
626 candidate,
627 &fake_borrow_temps,
628 irrefutable_pat.span,
629 None,
630 false,
631 )
632 .unit()
633 }
634
635 /// Declares the bindings of the given patterns and returns the visibility
636 /// scope for the bindings in these patterns, if such a scope had to be
637 /// created. NOTE: Declaring the bindings should always be done in their
638 /// drop scope.
639 #[instrument(skip(self), level = "debug")]
declare_bindings( &mut self, mut visibility_scope: Option<SourceScope>, scope_span: Span, pattern: &Pat<'tcx>, guard: Option<&Guard<'tcx>>, opt_match_place: Option<(Option<&Place<'tcx>>, Span)>, ) -> Option<SourceScope>640 pub(crate) fn declare_bindings(
641 &mut self,
642 mut visibility_scope: Option<SourceScope>,
643 scope_span: Span,
644 pattern: &Pat<'tcx>,
645 guard: Option<&Guard<'tcx>>,
646 opt_match_place: Option<(Option<&Place<'tcx>>, Span)>,
647 ) -> Option<SourceScope> {
648 self.visit_primary_bindings(
649 &pattern,
650 UserTypeProjections::none(),
651 &mut |this, mutability, name, mode, var, span, ty, user_ty| {
652 if visibility_scope.is_none() {
653 visibility_scope =
654 Some(this.new_source_scope(scope_span, LintLevel::Inherited, None));
655 }
656 let source_info = SourceInfo { span, scope: this.source_scope };
657 let visibility_scope = visibility_scope.unwrap();
658 this.declare_binding(
659 source_info,
660 visibility_scope,
661 mutability,
662 name,
663 mode,
664 var,
665 ty,
666 user_ty,
667 ArmHasGuard(guard.is_some()),
668 opt_match_place.map(|(x, y)| (x.cloned(), y)),
669 pattern.span,
670 );
671 },
672 );
673 if let Some(Guard::IfLet(guard_pat, _)) = guard {
674 // FIXME: pass a proper `opt_match_place`
675 self.declare_bindings(visibility_scope, scope_span, guard_pat, None, None);
676 }
677 visibility_scope
678 }
679
storage_live_binding( &mut self, block: BasicBlock, var: LocalVarId, span: Span, for_guard: ForGuard, schedule_drop: bool, ) -> Place<'tcx>680 pub(crate) fn storage_live_binding(
681 &mut self,
682 block: BasicBlock,
683 var: LocalVarId,
684 span: Span,
685 for_guard: ForGuard,
686 schedule_drop: bool,
687 ) -> Place<'tcx> {
688 let local_id = self.var_local_id(var, for_guard);
689 let source_info = self.source_info(span);
690 self.cfg.push(block, Statement { source_info, kind: StatementKind::StorageLive(local_id) });
691 // Although there is almost always scope for given variable in corner cases
692 // like #92893 we might get variable with no scope.
693 if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) && schedule_drop {
694 self.schedule_drop(span, region_scope, local_id, DropKind::Storage);
695 }
696 Place::from(local_id)
697 }
698
schedule_drop_for_binding( &mut self, var: LocalVarId, span: Span, for_guard: ForGuard, )699 pub(crate) fn schedule_drop_for_binding(
700 &mut self,
701 var: LocalVarId,
702 span: Span,
703 for_guard: ForGuard,
704 ) {
705 let local_id = self.var_local_id(var, for_guard);
706 if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) {
707 self.schedule_drop(span, region_scope, local_id, DropKind::Value);
708 }
709 }
710
711 /// Visit all of the primary bindings in a patterns, that is, visit the
712 /// leftmost occurrence of each variable bound in a pattern. A variable
713 /// will occur more than once in an or-pattern.
visit_primary_bindings( &mut self, pattern: &Pat<'tcx>, pattern_user_ty: UserTypeProjections, f: &mut impl FnMut( &mut Self, Mutability, Symbol, BindingMode, LocalVarId, Span, Ty<'tcx>, UserTypeProjections, ), )714 pub(super) fn visit_primary_bindings(
715 &mut self,
716 pattern: &Pat<'tcx>,
717 pattern_user_ty: UserTypeProjections,
718 f: &mut impl FnMut(
719 &mut Self,
720 Mutability,
721 Symbol,
722 BindingMode,
723 LocalVarId,
724 Span,
725 Ty<'tcx>,
726 UserTypeProjections,
727 ),
728 ) {
729 debug!(
730 "visit_primary_bindings: pattern={:?} pattern_user_ty={:?}",
731 pattern, pattern_user_ty
732 );
733 match pattern.kind {
734 PatKind::Binding {
735 mutability,
736 name,
737 mode,
738 var,
739 ty,
740 ref subpattern,
741 is_primary,
742 ..
743 } => {
744 if is_primary {
745 f(self, mutability, name, mode, var, pattern.span, ty, pattern_user_ty.clone());
746 }
747 if let Some(subpattern) = subpattern.as_ref() {
748 self.visit_primary_bindings(subpattern, pattern_user_ty, f);
749 }
750 }
751
752 PatKind::Array { ref prefix, ref slice, ref suffix }
753 | PatKind::Slice { ref prefix, ref slice, ref suffix } => {
754 let from = u64::try_from(prefix.len()).unwrap();
755 let to = u64::try_from(suffix.len()).unwrap();
756 for subpattern in prefix.iter() {
757 self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
758 }
759 for subpattern in slice {
760 self.visit_primary_bindings(
761 subpattern,
762 pattern_user_ty.clone().subslice(from, to),
763 f,
764 );
765 }
766 for subpattern in suffix.iter() {
767 self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
768 }
769 }
770
771 PatKind::Constant { .. } | PatKind::Range { .. } | PatKind::Wild => {}
772
773 PatKind::Deref { ref subpattern } => {
774 self.visit_primary_bindings(subpattern, pattern_user_ty.deref(), f);
775 }
776
777 PatKind::AscribeUserType {
778 ref subpattern,
779 ascription: thir::Ascription { ref annotation, variance: _ },
780 } => {
781 // This corresponds to something like
782 //
783 // ```
784 // let A::<'a>(_): A<'static> = ...;
785 // ```
786 //
787 // Note that the variance doesn't apply here, as we are tracking the effect
788 // of `user_ty` on any bindings contained with subpattern.
789
790 let projection = UserTypeProjection {
791 base: self.canonical_user_type_annotations.push(annotation.clone()),
792 projs: Vec::new(),
793 };
794 let subpattern_user_ty =
795 pattern_user_ty.push_projection(&projection, annotation.span);
796 self.visit_primary_bindings(subpattern, subpattern_user_ty, f)
797 }
798
799 PatKind::Leaf { ref subpatterns } => {
800 for subpattern in subpatterns {
801 let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field);
802 debug!("visit_primary_bindings: subpattern_user_ty={:?}", subpattern_user_ty);
803 self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
804 }
805 }
806
807 PatKind::Variant { adt_def, substs: _, variant_index, ref subpatterns } => {
808 for subpattern in subpatterns {
809 let subpattern_user_ty =
810 pattern_user_ty.clone().variant(adt_def, variant_index, subpattern.field);
811 self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
812 }
813 }
814 PatKind::Or { ref pats } => {
815 // In cases where we recover from errors the primary bindings
816 // may not all be in the leftmost subpattern. For example in
817 // `let (x | y) = ...`, the primary binding of `y` occurs in
818 // the right subpattern
819 for subpattern in pats.iter() {
820 self.visit_primary_bindings(subpattern, pattern_user_ty.clone(), f);
821 }
822 }
823 }
824 }
825 }
826
827 #[derive(Debug)]
828 struct Candidate<'pat, 'tcx> {
829 /// [`Span`] of the original pattern that gave rise to this candidate.
830 span: Span,
831
832 /// Whether this `Candidate` has a guard.
833 has_guard: bool,
834
835 /// All of these must be satisfied...
836 match_pairs: SmallVec<[MatchPair<'pat, 'tcx>; 1]>,
837
838 /// ...these bindings established...
839 bindings: Vec<Binding<'tcx>>,
840
841 /// ...and these types asserted...
842 ascriptions: Vec<Ascription<'tcx>>,
843
844 /// ...and if this is non-empty, one of these subcandidates also has to match...
845 subcandidates: Vec<Candidate<'pat, 'tcx>>,
846
847 /// ...and the guard must be evaluated; if it's `false` then branch to `otherwise_block`.
848 otherwise_block: Option<BasicBlock>,
849
850 /// The block before the `bindings` have been established.
851 pre_binding_block: Option<BasicBlock>,
852 /// The pre-binding block of the next candidate.
853 next_candidate_pre_binding_block: Option<BasicBlock>,
854 }
855
856 impl<'tcx, 'pat> Candidate<'pat, 'tcx> {
new( place: PlaceBuilder<'tcx>, pattern: &'pat Pat<'tcx>, has_guard: bool, cx: &Builder<'_, 'tcx>, ) -> Self857 fn new(
858 place: PlaceBuilder<'tcx>,
859 pattern: &'pat Pat<'tcx>,
860 has_guard: bool,
861 cx: &Builder<'_, 'tcx>,
862 ) -> Self {
863 Candidate {
864 span: pattern.span,
865 has_guard,
866 match_pairs: smallvec![MatchPair::new(place, pattern, cx)],
867 bindings: Vec::new(),
868 ascriptions: Vec::new(),
869 subcandidates: Vec::new(),
870 otherwise_block: None,
871 pre_binding_block: None,
872 next_candidate_pre_binding_block: None,
873 }
874 }
875
876 /// Visit the leaf candidates (those with no subcandidates) contained in
877 /// this candidate.
visit_leaves<'a>(&'a mut self, mut visit_leaf: impl FnMut(&'a mut Self))878 fn visit_leaves<'a>(&'a mut self, mut visit_leaf: impl FnMut(&'a mut Self)) {
879 traverse_candidate(
880 self,
881 &mut (),
882 &mut move |c, _| visit_leaf(c),
883 move |c, _| c.subcandidates.iter_mut(),
884 |_| {},
885 );
886 }
887 }
888
889 /// A depth-first traversal of the `Candidate` and all of its recursive
890 /// subcandidates.
traverse_candidate<'pat, 'tcx: 'pat, C, T, I>( candidate: C, context: &mut T, visit_leaf: &mut impl FnMut(C, &mut T), get_children: impl Copy + Fn(C, &mut T) -> I, complete_children: impl Copy + Fn(&mut T), ) where C: Borrow<Candidate<'pat, 'tcx>>, I: Iterator<Item = C>,891 fn traverse_candidate<'pat, 'tcx: 'pat, C, T, I>(
892 candidate: C,
893 context: &mut T,
894 visit_leaf: &mut impl FnMut(C, &mut T),
895 get_children: impl Copy + Fn(C, &mut T) -> I,
896 complete_children: impl Copy + Fn(&mut T),
897 ) where
898 C: Borrow<Candidate<'pat, 'tcx>>,
899 I: Iterator<Item = C>,
900 {
901 if candidate.borrow().subcandidates.is_empty() {
902 visit_leaf(candidate, context)
903 } else {
904 for child in get_children(candidate, context) {
905 traverse_candidate(child, context, visit_leaf, get_children, complete_children);
906 }
907 complete_children(context)
908 }
909 }
910
911 #[derive(Clone, Debug)]
912 struct Binding<'tcx> {
913 span: Span,
914 source: Place<'tcx>,
915 var_id: LocalVarId,
916 binding_mode: BindingMode,
917 }
918
919 /// Indicates that the type of `source` must be a subtype of the
920 /// user-given type `user_ty`; this is basically a no-op but can
921 /// influence region inference.
922 #[derive(Clone, Debug)]
923 struct Ascription<'tcx> {
924 source: Place<'tcx>,
925 annotation: CanonicalUserTypeAnnotation<'tcx>,
926 variance: ty::Variance,
927 }
928
929 #[derive(Clone, Debug)]
930 pub(crate) struct MatchPair<'pat, 'tcx> {
931 // this place...
932 place: PlaceBuilder<'tcx>,
933
934 // ... must match this pattern.
935 pattern: &'pat Pat<'tcx>,
936 }
937
938 /// See [`Test`] for more.
939 #[derive(Clone, Debug, PartialEq)]
940 enum TestKind<'tcx> {
941 /// Test what enum variant a value is.
942 Switch {
943 /// The enum type being tested.
944 adt_def: ty::AdtDef<'tcx>,
945 /// The set of variants that we should create a branch for. We also
946 /// create an additional "otherwise" case.
947 variants: BitSet<VariantIdx>,
948 },
949
950 /// Test what value an integer, `bool`, or `char` has.
951 SwitchInt {
952 /// The type of the value that we're testing.
953 switch_ty: Ty<'tcx>,
954 /// The (ordered) set of values that we test for.
955 ///
956 /// For integers and `char`s we create a branch to each of the values in
957 /// `options`, as well as an "otherwise" branch for all other values, even
958 /// in the (rare) case that `options` is exhaustive.
959 ///
960 /// For `bool` we always generate two edges, one for `true` and one for
961 /// `false`.
962 options: FxIndexMap<ConstantKind<'tcx>, u128>,
963 },
964
965 /// Test for equality with value, possibly after an unsizing coercion to
966 /// `ty`,
967 Eq {
968 value: ConstantKind<'tcx>,
969 // Integer types are handled by `SwitchInt`, and constants with ADT
970 // types are converted back into patterns, so this can only be `&str`,
971 // `&[T]`, `f32` or `f64`.
972 ty: Ty<'tcx>,
973 },
974
975 /// Test whether the value falls within an inclusive or exclusive range
976 Range(Box<PatRange<'tcx>>),
977
978 /// Test that the length of the slice is equal to `len`.
979 Len { len: u64, op: BinOp },
980 }
981
982 /// A test to perform to determine which [`Candidate`] matches a value.
983 ///
984 /// [`Test`] is just the test to perform; it does not include the value
985 /// to be tested.
986 #[derive(Debug)]
987 pub(crate) struct Test<'tcx> {
988 span: Span,
989 kind: TestKind<'tcx>,
990 }
991
992 /// `ArmHasGuard` is a wrapper around a boolean flag. It indicates whether
993 /// a match arm has a guard expression attached to it.
994 #[derive(Copy, Clone, Debug)]
995 pub(crate) struct ArmHasGuard(pub(crate) bool);
996
997 ///////////////////////////////////////////////////////////////////////////
998 // Main matching algorithm
999
1000 impl<'a, 'tcx> Builder<'a, 'tcx> {
1001 /// The main match algorithm. It begins with a set of candidates
1002 /// `candidates` and has the job of generating code to determine
1003 /// which of these candidates, if any, is the correct one. The
1004 /// candidates are sorted such that the first item in the list
1005 /// has the highest priority. When a candidate is found to match
1006 /// the value, we will set and generate a branch to the appropriate
1007 /// pre-binding block.
1008 ///
1009 /// If we find that *NONE* of the candidates apply, we branch to the
1010 /// `otherwise_block`, setting it to `Some` if required. In principle, this
1011 /// means that the input list was not exhaustive, though at present we
1012 /// sometimes are not smart enough to recognize all exhaustive inputs.
1013 ///
1014 /// It might be surprising that the input can be non-exhaustive.
1015 /// Indeed, initially, it is not, because all matches are
1016 /// exhaustive in Rust. But during processing we sometimes divide
1017 /// up the list of candidates and recurse with a non-exhaustive
1018 /// list. This is important to keep the size of the generated code
1019 /// under control. See [`Builder::test_candidates`] for more details.
1020 ///
1021 /// If `fake_borrows` is `Some`, then places which need fake borrows
1022 /// will be added to it.
1023 ///
1024 /// For an example of a case where we set `otherwise_block`, even for an
1025 /// exhaustive match, consider:
1026 ///
1027 /// ```
1028 /// # fn foo(x: (bool, bool)) {
1029 /// match x {
1030 /// (true, true) => (),
1031 /// (_, false) => (),
1032 /// (false, true) => (),
1033 /// }
1034 /// # }
1035 /// ```
1036 ///
1037 /// For this match, we check if `x.0` matches `true` (for the first
1038 /// arm). If it doesn't match, we check `x.1`. If `x.1` is `true` we check
1039 /// if `x.0` matches `false` (for the third arm). In the (impossible at
1040 /// runtime) case when `x.0` is now `true`, we branch to
1041 /// `otherwise_block`.
1042 #[instrument(skip(self, fake_borrows), level = "debug")]
match_candidates<'pat>( &mut self, span: Span, scrutinee_span: Span, start_block: BasicBlock, otherwise_block: &mut Option<BasicBlock>, candidates: &mut [&mut Candidate<'pat, 'tcx>], fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>, )1043 fn match_candidates<'pat>(
1044 &mut self,
1045 span: Span,
1046 scrutinee_span: Span,
1047 start_block: BasicBlock,
1048 otherwise_block: &mut Option<BasicBlock>,
1049 candidates: &mut [&mut Candidate<'pat, 'tcx>],
1050 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1051 ) {
1052 // Start by simplifying candidates. Once this process is complete, all
1053 // the match pairs which remain require some form of test, whether it
1054 // be a switch or pattern comparison.
1055 let mut split_or_candidate = false;
1056 for candidate in &mut *candidates {
1057 split_or_candidate |= self.simplify_candidate(candidate);
1058 }
1059
1060 ensure_sufficient_stack(|| {
1061 if split_or_candidate {
1062 // At least one of the candidates has been split into subcandidates.
1063 // We need to change the candidate list to include those.
1064 let mut new_candidates = Vec::new();
1065
1066 for candidate in candidates {
1067 candidate.visit_leaves(|leaf_candidate| new_candidates.push(leaf_candidate));
1068 }
1069 self.match_simplified_candidates(
1070 span,
1071 scrutinee_span,
1072 start_block,
1073 otherwise_block,
1074 &mut *new_candidates,
1075 fake_borrows,
1076 );
1077 } else {
1078 self.match_simplified_candidates(
1079 span,
1080 scrutinee_span,
1081 start_block,
1082 otherwise_block,
1083 candidates,
1084 fake_borrows,
1085 );
1086 }
1087 });
1088 }
1089
match_simplified_candidates( &mut self, span: Span, scrutinee_span: Span, start_block: BasicBlock, otherwise_block: &mut Option<BasicBlock>, candidates: &mut [&mut Candidate<'_, 'tcx>], fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>, )1090 fn match_simplified_candidates(
1091 &mut self,
1092 span: Span,
1093 scrutinee_span: Span,
1094 start_block: BasicBlock,
1095 otherwise_block: &mut Option<BasicBlock>,
1096 candidates: &mut [&mut Candidate<'_, 'tcx>],
1097 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1098 ) {
1099 // The candidates are sorted by priority. Check to see whether the
1100 // higher priority candidates (and hence at the front of the slice)
1101 // have satisfied all their match pairs.
1102 let fully_matched = candidates.iter().take_while(|c| c.match_pairs.is_empty()).count();
1103 debug!("match_candidates: {:?} candidates fully matched", fully_matched);
1104 let (matched_candidates, unmatched_candidates) = candidates.split_at_mut(fully_matched);
1105
1106 let block = if !matched_candidates.is_empty() {
1107 let otherwise_block =
1108 self.select_matched_candidates(matched_candidates, start_block, fake_borrows);
1109
1110 if let Some(last_otherwise_block) = otherwise_block {
1111 last_otherwise_block
1112 } else {
1113 // Any remaining candidates are unreachable.
1114 if unmatched_candidates.is_empty() {
1115 return;
1116 }
1117 self.cfg.start_new_block()
1118 }
1119 } else {
1120 start_block
1121 };
1122
1123 // If there are no candidates that still need testing, we're
1124 // done. Since all matches are exhaustive, execution should
1125 // never reach this point.
1126 if unmatched_candidates.is_empty() {
1127 let source_info = self.source_info(span);
1128 if let Some(otherwise) = *otherwise_block {
1129 self.cfg.goto(block, source_info, otherwise);
1130 } else {
1131 *otherwise_block = Some(block);
1132 }
1133 return;
1134 }
1135
1136 // Test for the remaining candidates.
1137 self.test_candidates_with_or(
1138 span,
1139 scrutinee_span,
1140 unmatched_candidates,
1141 block,
1142 otherwise_block,
1143 fake_borrows,
1144 );
1145 }
1146
1147 /// Link up matched candidates.
1148 ///
1149 /// For example, if we have something like this:
1150 ///
1151 /// ```ignore (illustrative)
1152 /// ...
1153 /// Some(x) if cond1 => ...
1154 /// Some(x) => ...
1155 /// Some(x) if cond2 => ...
1156 /// ...
1157 /// ```
1158 ///
1159 /// We generate real edges from:
1160 ///
1161 /// * `start_block` to the [pre-binding block] of the first pattern,
1162 /// * the [otherwise block] of the first pattern to the second pattern,
1163 /// * the [otherwise block] of the third pattern to a block with an
1164 /// [`Unreachable` terminator](TerminatorKind::Unreachable).
1165 ///
1166 /// In addition, we add fake edges from the otherwise blocks to the
1167 /// pre-binding block of the next candidate in the original set of
1168 /// candidates.
1169 ///
1170 /// [pre-binding block]: Candidate::pre_binding_block
1171 /// [otherwise block]: Candidate::otherwise_block
select_matched_candidates( &mut self, matched_candidates: &mut [&mut Candidate<'_, 'tcx>], start_block: BasicBlock, fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>, ) -> Option<BasicBlock>1172 fn select_matched_candidates(
1173 &mut self,
1174 matched_candidates: &mut [&mut Candidate<'_, 'tcx>],
1175 start_block: BasicBlock,
1176 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1177 ) -> Option<BasicBlock> {
1178 debug_assert!(
1179 !matched_candidates.is_empty(),
1180 "select_matched_candidates called with no candidates",
1181 );
1182 debug_assert!(
1183 matched_candidates.iter().all(|c| c.subcandidates.is_empty()),
1184 "subcandidates should be empty in select_matched_candidates",
1185 );
1186
1187 // Insert a borrows of prefixes of places that are bound and are
1188 // behind a dereference projection.
1189 //
1190 // These borrows are taken to avoid situations like the following:
1191 //
1192 // match x[10] {
1193 // _ if { x = &[0]; false } => (),
1194 // y => (), // Out of bounds array access!
1195 // }
1196 //
1197 // match *x {
1198 // // y is bound by reference in the guard and then by copy in the
1199 // // arm, so y is 2 in the arm!
1200 // y if { y == 1 && (x = &2) == () } => y,
1201 // _ => 3,
1202 // }
1203 if let Some(fake_borrows) = fake_borrows {
1204 for Binding { source, .. } in
1205 matched_candidates.iter().flat_map(|candidate| &candidate.bindings)
1206 {
1207 if let Some(i) =
1208 source.projection.iter().rposition(|elem| elem == ProjectionElem::Deref)
1209 {
1210 let proj_base = &source.projection[..i];
1211
1212 fake_borrows.insert(Place {
1213 local: source.local,
1214 projection: self.tcx.mk_place_elems(proj_base),
1215 });
1216 }
1217 }
1218 }
1219
1220 let fully_matched_with_guard = matched_candidates
1221 .iter()
1222 .position(|c| !c.has_guard)
1223 .unwrap_or(matched_candidates.len() - 1);
1224
1225 let (reachable_candidates, unreachable_candidates) =
1226 matched_candidates.split_at_mut(fully_matched_with_guard + 1);
1227
1228 let mut next_prebinding = start_block;
1229
1230 for candidate in reachable_candidates.iter_mut() {
1231 assert!(candidate.otherwise_block.is_none());
1232 assert!(candidate.pre_binding_block.is_none());
1233 candidate.pre_binding_block = Some(next_prebinding);
1234 if candidate.has_guard {
1235 // Create the otherwise block for this candidate, which is the
1236 // pre-binding block for the next candidate.
1237 next_prebinding = self.cfg.start_new_block();
1238 candidate.otherwise_block = Some(next_prebinding);
1239 }
1240 }
1241
1242 debug!(
1243 "match_candidates: add pre_binding_blocks for unreachable {:?}",
1244 unreachable_candidates,
1245 );
1246 for candidate in unreachable_candidates {
1247 assert!(candidate.pre_binding_block.is_none());
1248 candidate.pre_binding_block = Some(self.cfg.start_new_block());
1249 }
1250
1251 reachable_candidates.last_mut().unwrap().otherwise_block
1252 }
1253
1254 /// Tests a candidate where there are only or-patterns left to test, or
1255 /// forwards to [Builder::test_candidates].
1256 ///
1257 /// Given a pattern `(P | Q, R | S)` we (in principle) generate a CFG like
1258 /// so:
1259 ///
1260 /// ```text
1261 /// [ start ]
1262 /// |
1263 /// [ match P, Q ]
1264 /// |
1265 /// +----------------------------------------+------------------------------------+
1266 /// | | |
1267 /// V V V
1268 /// [ P matches ] [ Q matches ] [ otherwise ]
1269 /// | | |
1270 /// V V |
1271 /// [ match R, S ] [ match R, S ] |
1272 /// | | |
1273 /// +--------------+------------+ +--------------+------------+ |
1274 /// | | | | | | |
1275 /// V V V V V V |
1276 /// [ R matches ] [ S matches ] [otherwise ] [ R matches ] [ S matches ] [otherwise ] |
1277 /// | | | | | | |
1278 /// +--------------+------------|------------+--------------+ | |
1279 /// | | | |
1280 /// | +----------------------------------------+--------+
1281 /// | |
1282 /// V V
1283 /// [ Success ] [ Failure ]
1284 /// ```
1285 ///
1286 /// In practice there are some complications:
1287 ///
1288 /// * If there's a guard, then the otherwise branch of the first match on
1289 /// `R | S` goes to a test for whether `Q` matches, and the control flow
1290 /// doesn't merge into a single success block until after the guard is
1291 /// tested.
1292 /// * If neither `P` or `Q` has any bindings or type ascriptions and there
1293 /// isn't a match guard, then we create a smaller CFG like:
1294 ///
1295 /// ```text
1296 /// ...
1297 /// +---------------+------------+
1298 /// | | |
1299 /// [ P matches ] [ Q matches ] [ otherwise ]
1300 /// | | |
1301 /// +---------------+ |
1302 /// | ...
1303 /// [ match R, S ]
1304 /// |
1305 /// ...
1306 /// ```
test_candidates_with_or( &mut self, span: Span, scrutinee_span: Span, candidates: &mut [&mut Candidate<'_, 'tcx>], block: BasicBlock, otherwise_block: &mut Option<BasicBlock>, fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>, )1307 fn test_candidates_with_or(
1308 &mut self,
1309 span: Span,
1310 scrutinee_span: Span,
1311 candidates: &mut [&mut Candidate<'_, 'tcx>],
1312 block: BasicBlock,
1313 otherwise_block: &mut Option<BasicBlock>,
1314 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1315 ) {
1316 let (first_candidate, remaining_candidates) = candidates.split_first_mut().unwrap();
1317
1318 // All of the or-patterns have been sorted to the end, so if the first
1319 // pattern is an or-pattern we only have or-patterns.
1320 match first_candidate.match_pairs[0].pattern.kind {
1321 PatKind::Or { .. } => (),
1322 _ => {
1323 self.test_candidates(
1324 span,
1325 scrutinee_span,
1326 candidates,
1327 block,
1328 otherwise_block,
1329 fake_borrows,
1330 );
1331 return;
1332 }
1333 }
1334
1335 let match_pairs = mem::take(&mut first_candidate.match_pairs);
1336 first_candidate.pre_binding_block = Some(block);
1337
1338 let mut otherwise = None;
1339 for match_pair in match_pairs {
1340 let PatKind::Or { ref pats } = &match_pair.pattern.kind else {
1341 bug!("Or-patterns should have been sorted to the end");
1342 };
1343 let or_span = match_pair.pattern.span;
1344
1345 first_candidate.visit_leaves(|leaf_candidate| {
1346 self.test_or_pattern(
1347 leaf_candidate,
1348 &mut otherwise,
1349 pats,
1350 or_span,
1351 &match_pair.place,
1352 fake_borrows,
1353 );
1354 });
1355 }
1356
1357 let remainder_start = otherwise.unwrap_or_else(|| self.cfg.start_new_block());
1358
1359 self.match_candidates(
1360 span,
1361 scrutinee_span,
1362 remainder_start,
1363 otherwise_block,
1364 remaining_candidates,
1365 fake_borrows,
1366 )
1367 }
1368
1369 #[instrument(
1370 skip(self, otherwise, or_span, place, fake_borrows, candidate, pats),
1371 level = "debug"
1372 )]
test_or_pattern<'pat>( &mut self, candidate: &mut Candidate<'pat, 'tcx>, otherwise: &mut Option<BasicBlock>, pats: &'pat [Box<Pat<'tcx>>], or_span: Span, place: &PlaceBuilder<'tcx>, fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>, )1373 fn test_or_pattern<'pat>(
1374 &mut self,
1375 candidate: &mut Candidate<'pat, 'tcx>,
1376 otherwise: &mut Option<BasicBlock>,
1377 pats: &'pat [Box<Pat<'tcx>>],
1378 or_span: Span,
1379 place: &PlaceBuilder<'tcx>,
1380 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1381 ) {
1382 debug!("candidate={:#?}\npats={:#?}", candidate, pats);
1383 let mut or_candidates: Vec<_> = pats
1384 .iter()
1385 .map(|pat| Candidate::new(place.clone(), pat, candidate.has_guard, self))
1386 .collect();
1387 let mut or_candidate_refs: Vec<_> = or_candidates.iter_mut().collect();
1388 let otherwise = if candidate.otherwise_block.is_some() {
1389 &mut candidate.otherwise_block
1390 } else {
1391 otherwise
1392 };
1393 self.match_candidates(
1394 or_span,
1395 or_span,
1396 candidate.pre_binding_block.unwrap(),
1397 otherwise,
1398 &mut or_candidate_refs,
1399 fake_borrows,
1400 );
1401 candidate.subcandidates = or_candidates;
1402 self.merge_trivial_subcandidates(candidate, self.source_info(or_span));
1403 }
1404
1405 /// Try to merge all of the subcandidates of the given candidate into one.
1406 /// This avoids exponentially large CFGs in cases like `(1 | 2, 3 | 4, ...)`.
merge_trivial_subcandidates( &mut self, candidate: &mut Candidate<'_, 'tcx>, source_info: SourceInfo, )1407 fn merge_trivial_subcandidates(
1408 &mut self,
1409 candidate: &mut Candidate<'_, 'tcx>,
1410 source_info: SourceInfo,
1411 ) {
1412 if candidate.subcandidates.is_empty() || candidate.has_guard {
1413 // FIXME(or_patterns; matthewjasper) Don't give up if we have a guard.
1414 return;
1415 }
1416
1417 let mut can_merge = true;
1418
1419 // Not `Iterator::all` because we don't want to short-circuit.
1420 for subcandidate in &mut candidate.subcandidates {
1421 self.merge_trivial_subcandidates(subcandidate, source_info);
1422
1423 // FIXME(or_patterns; matthewjasper) Try to be more aggressive here.
1424 can_merge &= subcandidate.subcandidates.is_empty()
1425 && subcandidate.bindings.is_empty()
1426 && subcandidate.ascriptions.is_empty();
1427 }
1428
1429 if can_merge {
1430 let any_matches = self.cfg.start_new_block();
1431 for subcandidate in mem::take(&mut candidate.subcandidates) {
1432 let or_block = subcandidate.pre_binding_block.unwrap();
1433 self.cfg.goto(or_block, source_info, any_matches);
1434 }
1435 candidate.pre_binding_block = Some(any_matches);
1436 }
1437 }
1438
1439 /// This is the most subtle part of the matching algorithm. At
1440 /// this point, the input candidates have been fully simplified,
1441 /// and so we know that all remaining match-pairs require some
1442 /// sort of test. To decide what test to perform, we take the highest
1443 /// priority candidate (the first one in the list, as of January 2021)
1444 /// and extract the first match-pair from the list. From this we decide
1445 /// what kind of test is needed using [`Builder::test`], defined in the
1446 /// [`test` module](mod@test).
1447 ///
1448 /// *Note:* taking the first match pair is somewhat arbitrary, and
1449 /// we might do better here by choosing more carefully what to
1450 /// test.
1451 ///
1452 /// For example, consider the following possible match-pairs:
1453 ///
1454 /// 1. `x @ Some(P)` -- we will do a [`Switch`] to decide what variant `x` has
1455 /// 2. `x @ 22` -- we will do a [`SwitchInt`] to decide what value `x` has
1456 /// 3. `x @ 3..5` -- we will do a [`Range`] test to decide what range `x` falls in
1457 /// 4. etc.
1458 ///
1459 /// [`Switch`]: TestKind::Switch
1460 /// [`SwitchInt`]: TestKind::SwitchInt
1461 /// [`Range`]: TestKind::Range
1462 ///
1463 /// Once we know what sort of test we are going to perform, this
1464 /// test may also help us winnow down our candidates. So we walk over
1465 /// the candidates (from high to low priority) and check. This
1466 /// gives us, for each outcome of the test, a transformed list of
1467 /// candidates. For example, if we are testing `x.0`'s variant,
1468 /// and we have a candidate `(x.0 @ Some(v), x.1 @ 22)`,
1469 /// then we would have a resulting candidate of `((x.0 as Some).0 @ v, x.1 @ 22)`.
1470 /// Note that the first match-pair is now simpler (and, in fact, irrefutable).
1471 ///
1472 /// But there may also be candidates that the test just doesn't
1473 /// apply to. The classical example involves wildcards:
1474 ///
1475 /// ```
1476 /// # let (x, y, z) = (true, true, true);
1477 /// match (x, y, z) {
1478 /// (true , _ , true ) => true, // (0)
1479 /// (_ , true , _ ) => true, // (1)
1480 /// (false, false, _ ) => false, // (2)
1481 /// (true , _ , false) => false, // (3)
1482 /// }
1483 /// # ;
1484 /// ```
1485 ///
1486 /// In that case, after we test on `x`, there are 2 overlapping candidate
1487 /// sets:
1488 ///
1489 /// - If the outcome is that `x` is true, candidates 0, 1, and 3
1490 /// - If the outcome is that `x` is false, candidates 1 and 2
1491 ///
1492 /// Here, the traditional "decision tree" method would generate 2
1493 /// separate code-paths for the 2 separate cases.
1494 ///
1495 /// In some cases, this duplication can create an exponential amount of
1496 /// code. This is most easily seen by noticing that this method terminates
1497 /// with precisely the reachable arms being reachable - but that problem
1498 /// is trivially NP-complete:
1499 ///
1500 /// ```ignore (illustrative)
1501 /// match (var0, var1, var2, var3, ...) {
1502 /// (true , _ , _ , false, true, ...) => false,
1503 /// (_ , true, true , false, _ , ...) => false,
1504 /// (false, _ , false, false, _ , ...) => false,
1505 /// ...
1506 /// _ => true
1507 /// }
1508 /// ```
1509 ///
1510 /// Here the last arm is reachable only if there is an assignment to
1511 /// the variables that does not match any of the literals. Therefore,
1512 /// compilation would take an exponential amount of time in some cases.
1513 ///
1514 /// That kind of exponential worst-case might not occur in practice, but
1515 /// our simplistic treatment of constants and guards would make it occur
1516 /// in very common situations - for example [#29740]:
1517 ///
1518 /// ```ignore (illustrative)
1519 /// match x {
1520 /// "foo" if foo_guard => ...,
1521 /// "bar" if bar_guard => ...,
1522 /// "baz" if baz_guard => ...,
1523 /// ...
1524 /// }
1525 /// ```
1526 ///
1527 /// [#29740]: https://github.com/rust-lang/rust/issues/29740
1528 ///
1529 /// Here we first test the match-pair `x @ "foo"`, which is an [`Eq` test].
1530 ///
1531 /// [`Eq` test]: TestKind::Eq
1532 ///
1533 /// It might seem that we would end up with 2 disjoint candidate
1534 /// sets, consisting of the first candidate or the other two, but our
1535 /// algorithm doesn't reason about `"foo"` being distinct from the other
1536 /// constants; it considers the latter arms to potentially match after
1537 /// both outcomes, which obviously leads to an exponential number
1538 /// of tests.
1539 ///
1540 /// To avoid these kinds of problems, our algorithm tries to ensure
1541 /// the amount of generated tests is linear. When we do a k-way test,
1542 /// we return an additional "unmatched" set alongside the obvious `k`
1543 /// sets. When we encounter a candidate that would be present in more
1544 /// than one of the sets, we put it and all candidates below it into the
1545 /// "unmatched" set. This ensures these `k+1` sets are disjoint.
1546 ///
1547 /// After we perform our test, we branch into the appropriate candidate
1548 /// set and recurse with `match_candidates`. These sub-matches are
1549 /// obviously non-exhaustive - as we discarded our otherwise set - so
1550 /// we set their continuation to do `match_candidates` on the
1551 /// "unmatched" set (which is again non-exhaustive).
1552 ///
1553 /// If you apply this to the above test, you basically wind up
1554 /// with an if-else-if chain, testing each candidate in turn,
1555 /// which is precisely what we want.
1556 ///
1557 /// In addition to avoiding exponential-time blowups, this algorithm
1558 /// also has the nice property that each guard and arm is only generated
1559 /// once.
test_candidates<'pat, 'b, 'c>( &mut self, span: Span, scrutinee_span: Span, mut candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>], block: BasicBlock, otherwise_block: &mut Option<BasicBlock>, fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>, )1560 fn test_candidates<'pat, 'b, 'c>(
1561 &mut self,
1562 span: Span,
1563 scrutinee_span: Span,
1564 mut candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>],
1565 block: BasicBlock,
1566 otherwise_block: &mut Option<BasicBlock>,
1567 fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
1568 ) {
1569 // extract the match-pair from the highest priority candidate
1570 let match_pair = &candidates.first().unwrap().match_pairs[0];
1571 let mut test = self.test(match_pair);
1572 let match_place = match_pair.place.clone();
1573
1574 // most of the time, the test to perform is simply a function
1575 // of the main candidate; but for a test like SwitchInt, we
1576 // may want to add cases based on the candidates that are
1577 // available
1578 match test.kind {
1579 TestKind::SwitchInt { switch_ty, ref mut options } => {
1580 for candidate in candidates.iter() {
1581 if !self.add_cases_to_switch(&match_place, candidate, switch_ty, options) {
1582 break;
1583 }
1584 }
1585 }
1586 TestKind::Switch { adt_def: _, ref mut variants } => {
1587 for candidate in candidates.iter() {
1588 if !self.add_variants_to_switch(&match_place, candidate, variants) {
1589 break;
1590 }
1591 }
1592 }
1593 _ => {}
1594 }
1595
1596 // Insert a Shallow borrow of any places that is switched on.
1597 if let Some(fb) = fake_borrows
1598 && let Some(resolved_place) = match_place.try_to_place(self)
1599 {
1600 fb.insert(resolved_place);
1601 }
1602
1603 // perform the test, branching to one of N blocks. For each of
1604 // those N possible outcomes, create a (initially empty)
1605 // vector of candidates. Those are the candidates that still
1606 // apply if the test has that particular outcome.
1607 debug!("test_candidates: test={:?} match_pair={:?}", test, match_pair);
1608 let mut target_candidates: Vec<Vec<&mut Candidate<'pat, 'tcx>>> = vec![];
1609 target_candidates.resize_with(test.targets(), Default::default);
1610
1611 let total_candidate_count = candidates.len();
1612
1613 // Sort the candidates into the appropriate vector in
1614 // `target_candidates`. Note that at some point we may
1615 // encounter a candidate where the test is not relevant; at
1616 // that point, we stop sorting.
1617 while let Some(candidate) = candidates.first_mut() {
1618 let Some(idx) = self.sort_candidate(&match_place, &test, candidate) else {
1619 break;
1620 };
1621 let (candidate, rest) = candidates.split_first_mut().unwrap();
1622 target_candidates[idx].push(candidate);
1623 candidates = rest;
1624 }
1625 // at least the first candidate ought to be tested
1626 assert!(
1627 total_candidate_count > candidates.len(),
1628 "{}, {:#?}",
1629 total_candidate_count,
1630 candidates
1631 );
1632 debug!("tested_candidates: {}", total_candidate_count - candidates.len());
1633 debug!("untested_candidates: {}", candidates.len());
1634
1635 // HACK(matthewjasper) This is a closure so that we can let the test
1636 // create its blocks before the rest of the match. This currently
1637 // improves the speed of llvm when optimizing long string literal
1638 // matches
1639 let make_target_blocks = move |this: &mut Self| -> Vec<BasicBlock> {
1640 // The block that we should branch to if none of the
1641 // `target_candidates` match. This is either the block where we
1642 // start matching the untested candidates if there are any,
1643 // otherwise it's the `otherwise_block`.
1644 let remainder_start = &mut None;
1645 let remainder_start =
1646 if candidates.is_empty() { &mut *otherwise_block } else { remainder_start };
1647
1648 // For each outcome of test, process the candidates that still
1649 // apply. Collect a list of blocks where control flow will
1650 // branch if one of the `target_candidate` sets is not
1651 // exhaustive.
1652 let target_blocks: Vec<_> = target_candidates
1653 .into_iter()
1654 .map(|mut candidates| {
1655 if !candidates.is_empty() {
1656 let candidate_start = this.cfg.start_new_block();
1657 this.match_candidates(
1658 span,
1659 scrutinee_span,
1660 candidate_start,
1661 remainder_start,
1662 &mut *candidates,
1663 fake_borrows,
1664 );
1665 candidate_start
1666 } else {
1667 *remainder_start.get_or_insert_with(|| this.cfg.start_new_block())
1668 }
1669 })
1670 .collect();
1671
1672 if !candidates.is_empty() {
1673 let remainder_start = remainder_start.unwrap_or_else(|| this.cfg.start_new_block());
1674 this.match_candidates(
1675 span,
1676 scrutinee_span,
1677 remainder_start,
1678 otherwise_block,
1679 candidates,
1680 fake_borrows,
1681 );
1682 };
1683
1684 target_blocks
1685 };
1686
1687 self.perform_test(span, scrutinee_span, block, &match_place, &test, make_target_blocks);
1688 }
1689
1690 /// Determine the fake borrows that are needed from a set of places that
1691 /// have to be stable across match guards.
1692 ///
1693 /// Returns a list of places that need a fake borrow and the temporary
1694 /// that's used to store the fake borrow.
1695 ///
1696 /// Match exhaustiveness checking is not able to handle the case where the
1697 /// place being matched on is mutated in the guards. We add "fake borrows"
1698 /// to the guards that prevent any mutation of the place being matched.
1699 /// There are a some subtleties:
1700 ///
1701 /// 1. Borrowing `*x` doesn't prevent assigning to `x`. If `x` is a shared
1702 /// reference, the borrow isn't even tracked. As such we have to add fake
1703 /// borrows of any prefixes of a place
1704 /// 2. We don't want `match x { _ => (), }` to conflict with mutable
1705 /// borrows of `x`, so we only add fake borrows for places which are
1706 /// bound or tested by the match.
1707 /// 3. We don't want the fake borrows to conflict with `ref mut` bindings,
1708 /// so we use a special BorrowKind for them.
1709 /// 4. The fake borrows may be of places in inactive variants, so it would
1710 /// be UB to generate code for them. They therefore have to be removed
1711 /// by a MIR pass run after borrow checking.
calculate_fake_borrows<'b>( &mut self, fake_borrows: &'b FxIndexSet<Place<'tcx>>, temp_span: Span, ) -> Vec<(Place<'tcx>, Local)>1712 fn calculate_fake_borrows<'b>(
1713 &mut self,
1714 fake_borrows: &'b FxIndexSet<Place<'tcx>>,
1715 temp_span: Span,
1716 ) -> Vec<(Place<'tcx>, Local)> {
1717 let tcx = self.tcx;
1718
1719 debug!("add_fake_borrows fake_borrows = {:?}", fake_borrows);
1720
1721 let mut all_fake_borrows = Vec::with_capacity(fake_borrows.len());
1722
1723 // Insert a Shallow borrow of the prefixes of any fake borrows.
1724 for place in fake_borrows {
1725 let mut cursor = place.projection.as_ref();
1726 while let [proj_base @ .., elem] = cursor {
1727 cursor = proj_base;
1728
1729 if let ProjectionElem::Deref = elem {
1730 // Insert a shallow borrow after a deref. For other
1731 // projections the borrow of prefix_cursor will
1732 // conflict with any mutation of base.
1733 all_fake_borrows.push(PlaceRef { local: place.local, projection: proj_base });
1734 }
1735 }
1736
1737 all_fake_borrows.push(place.as_ref());
1738 }
1739
1740 // Deduplicate
1741 let mut dedup = FxHashSet::default();
1742 all_fake_borrows.retain(|b| dedup.insert(*b));
1743
1744 debug!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows);
1745
1746 all_fake_borrows
1747 .into_iter()
1748 .map(|matched_place_ref| {
1749 let matched_place = Place {
1750 local: matched_place_ref.local,
1751 projection: tcx.mk_place_elems(matched_place_ref.projection),
1752 };
1753 let fake_borrow_deref_ty = matched_place.ty(&self.local_decls, tcx).ty;
1754 let fake_borrow_ty =
1755 Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, fake_borrow_deref_ty);
1756 let mut fake_borrow_temp = LocalDecl::new(fake_borrow_ty, temp_span);
1757 fake_borrow_temp.internal = self.local_decls[matched_place.local].internal;
1758 fake_borrow_temp.local_info = ClearCrossCrate::Set(Box::new(LocalInfo::FakeBorrow));
1759 let fake_borrow_temp = self.local_decls.push(fake_borrow_temp);
1760
1761 (matched_place, fake_borrow_temp)
1762 })
1763 .collect()
1764 }
1765 }
1766
1767 ///////////////////////////////////////////////////////////////////////////
1768 // Pat binding - used for `let` and function parameters as well.
1769
1770 impl<'a, 'tcx> Builder<'a, 'tcx> {
1771 /// If the bindings have already been declared, set `declare_bindings` to
1772 /// `false` to avoid duplicated bindings declaration. Used for if-let guards.
lower_let_expr( &mut self, mut block: BasicBlock, expr: &Expr<'tcx>, pat: &Pat<'tcx>, else_target: region::Scope, source_scope: Option<SourceScope>, span: Span, declare_bindings: bool, ) -> BlockAnd<()>1773 pub(crate) fn lower_let_expr(
1774 &mut self,
1775 mut block: BasicBlock,
1776 expr: &Expr<'tcx>,
1777 pat: &Pat<'tcx>,
1778 else_target: region::Scope,
1779 source_scope: Option<SourceScope>,
1780 span: Span,
1781 declare_bindings: bool,
1782 ) -> BlockAnd<()> {
1783 let expr_span = expr.span;
1784 let expr_place_builder = unpack!(block = self.lower_scrutinee(block, expr, expr_span));
1785 let wildcard = Pat::wildcard_from_ty(pat.ty);
1786 let mut guard_candidate = Candidate::new(expr_place_builder.clone(), &pat, false, self);
1787 let mut otherwise_candidate =
1788 Candidate::new(expr_place_builder.clone(), &wildcard, false, self);
1789 let fake_borrow_temps = self.lower_match_tree(
1790 block,
1791 pat.span,
1792 pat.span,
1793 false,
1794 &mut [&mut guard_candidate, &mut otherwise_candidate],
1795 );
1796 let expr_place = expr_place_builder.try_to_place(self);
1797 let opt_expr_place = expr_place.as_ref().map(|place| (Some(place), expr_span));
1798 let otherwise_post_guard_block = otherwise_candidate.pre_binding_block.unwrap();
1799 self.break_for_else(otherwise_post_guard_block, else_target, self.source_info(expr_span));
1800
1801 if declare_bindings {
1802 self.declare_bindings(source_scope, pat.span.to(span), pat, None, opt_expr_place);
1803 }
1804
1805 let post_guard_block = self.bind_pattern(
1806 self.source_info(pat.span),
1807 guard_candidate,
1808 &fake_borrow_temps,
1809 expr.span,
1810 None,
1811 false,
1812 );
1813
1814 post_guard_block.unit()
1815 }
1816
1817 /// Initializes each of the bindings from the candidate by
1818 /// moving/copying/ref'ing the source as appropriate. Tests the guard, if
1819 /// any, and then branches to the arm. Returns the block for the case where
1820 /// the guard succeeds.
1821 ///
1822 /// Note: we do not check earlier that if there is a guard,
1823 /// there cannot be move bindings. We avoid a use-after-move by only
1824 /// moving the binding once the guard has evaluated to true (see below).
bind_and_guard_matched_candidate<'pat>( &mut self, candidate: Candidate<'pat, 'tcx>, parent_bindings: &[(Vec<Binding<'tcx>>, Vec<Ascription<'tcx>>)], fake_borrows: &[(Place<'tcx>, Local)], scrutinee_span: Span, arm_match_scope: Option<(&Arm<'tcx>, region::Scope)>, schedule_drops: bool, storages_alive: bool, ) -> BasicBlock1825 fn bind_and_guard_matched_candidate<'pat>(
1826 &mut self,
1827 candidate: Candidate<'pat, 'tcx>,
1828 parent_bindings: &[(Vec<Binding<'tcx>>, Vec<Ascription<'tcx>>)],
1829 fake_borrows: &[(Place<'tcx>, Local)],
1830 scrutinee_span: Span,
1831 arm_match_scope: Option<(&Arm<'tcx>, region::Scope)>,
1832 schedule_drops: bool,
1833 storages_alive: bool,
1834 ) -> BasicBlock {
1835 debug!("bind_and_guard_matched_candidate(candidate={:?})", candidate);
1836
1837 debug_assert!(candidate.match_pairs.is_empty());
1838
1839 let candidate_source_info = self.source_info(candidate.span);
1840
1841 let mut block = candidate.pre_binding_block.unwrap();
1842
1843 if candidate.next_candidate_pre_binding_block.is_some() {
1844 let fresh_block = self.cfg.start_new_block();
1845 self.false_edges(
1846 block,
1847 fresh_block,
1848 candidate.next_candidate_pre_binding_block,
1849 candidate_source_info,
1850 );
1851 block = fresh_block;
1852 }
1853
1854 self.ascribe_types(
1855 block,
1856 parent_bindings
1857 .iter()
1858 .flat_map(|(_, ascriptions)| ascriptions)
1859 .cloned()
1860 .chain(candidate.ascriptions),
1861 );
1862
1863 // rust-lang/rust#27282: The `autoref` business deserves some
1864 // explanation here.
1865 //
1866 // The intent of the `autoref` flag is that when it is true,
1867 // then any pattern bindings of type T will map to a `&T`
1868 // within the context of the guard expression, but will
1869 // continue to map to a `T` in the context of the arm body. To
1870 // avoid surfacing this distinction in the user source code
1871 // (which would be a severe change to the language and require
1872 // far more revision to the compiler), when `autoref` is true,
1873 // then any occurrence of the identifier in the guard
1874 // expression will automatically get a deref op applied to it.
1875 //
1876 // So an input like:
1877 //
1878 // ```
1879 // let place = Foo::new();
1880 // match place { foo if inspect(foo)
1881 // => feed(foo), ... }
1882 // ```
1883 //
1884 // will be treated as if it were really something like:
1885 //
1886 // ```
1887 // let place = Foo::new();
1888 // match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) }
1889 // => { let tmp2 = place; feed(tmp2) }, ... }
1890 // ```
1891 //
1892 // And an input like:
1893 //
1894 // ```
1895 // let place = Foo::new();
1896 // match place { ref mut foo if inspect(foo)
1897 // => feed(foo), ... }
1898 // ```
1899 //
1900 // will be treated as if it were really something like:
1901 //
1902 // ```
1903 // let place = Foo::new();
1904 // match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) }
1905 // => { let tmp2 = &mut place; feed(tmp2) }, ... }
1906 // ```
1907 //
1908 // In short, any pattern binding will always look like *some*
1909 // kind of `&T` within the guard at least in terms of how the
1910 // MIR-borrowck views it, and this will ensure that guard
1911 // expressions cannot mutate their the match inputs via such
1912 // bindings. (It also ensures that guard expressions can at
1913 // most *copy* values from such bindings; non-Copy things
1914 // cannot be moved via pattern bindings in guard expressions.)
1915 //
1916 // ----
1917 //
1918 // Implementation notes (under assumption `autoref` is true).
1919 //
1920 // To encode the distinction above, we must inject the
1921 // temporaries `tmp1` and `tmp2`.
1922 //
1923 // There are two cases of interest: binding by-value, and binding by-ref.
1924 //
1925 // 1. Binding by-value: Things are simple.
1926 //
1927 // * Establishing `tmp1` creates a reference into the
1928 // matched place. This code is emitted by
1929 // bind_matched_candidate_for_guard.
1930 //
1931 // * `tmp2` is only initialized "lazily", after we have
1932 // checked the guard. Thus, the code that can trigger
1933 // moves out of the candidate can only fire after the
1934 // guard evaluated to true. This initialization code is
1935 // emitted by bind_matched_candidate_for_arm.
1936 //
1937 // 2. Binding by-reference: Things are tricky.
1938 //
1939 // * Here, the guard expression wants a `&&` or `&&mut`
1940 // into the original input. This means we need to borrow
1941 // the reference that we create for the arm.
1942 // * So we eagerly create the reference for the arm and then take a
1943 // reference to that.
1944 if let Some((arm, match_scope)) = arm_match_scope
1945 && let Some(guard) = &arm.guard
1946 {
1947 let tcx = self.tcx;
1948 let bindings = parent_bindings
1949 .iter()
1950 .flat_map(|(bindings, _)| bindings)
1951 .chain(&candidate.bindings);
1952
1953 self.bind_matched_candidate_for_guard(block, schedule_drops, bindings.clone());
1954 let guard_frame = GuardFrame {
1955 locals: bindings.map(|b| GuardFrameLocal::new(b.var_id, b.binding_mode)).collect(),
1956 };
1957 debug!("entering guard building context: {:?}", guard_frame);
1958 self.guard_context.push(guard_frame);
1959
1960 let re_erased = tcx.lifetimes.re_erased;
1961 let scrutinee_source_info = self.source_info(scrutinee_span);
1962 for &(place, temp) in fake_borrows {
1963 let borrow = Rvalue::Ref(re_erased, BorrowKind::Shallow, place);
1964 self.cfg.push_assign(block, scrutinee_source_info, Place::from(temp), borrow);
1965 }
1966
1967 let mut guard_span = rustc_span::DUMMY_SP;
1968
1969 let (post_guard_block, otherwise_post_guard_block) =
1970 self.in_if_then_scope(match_scope, guard_span, |this| match *guard {
1971 Guard::If(e) => {
1972 let e = &this.thir[e];
1973 guard_span = e.span;
1974 this.then_else_break(
1975 block,
1976 e,
1977 None,
1978 match_scope,
1979 this.source_info(arm.span),
1980 )
1981 }
1982 Guard::IfLet(ref pat, scrutinee) => {
1983 let s = &this.thir[scrutinee];
1984 guard_span = s.span;
1985 this.lower_let_expr(block, s, pat, match_scope, None, arm.span, false)
1986 }
1987 });
1988
1989 let source_info = self.source_info(guard_span);
1990 let guard_end = self.source_info(tcx.sess.source_map().end_point(guard_span));
1991 let guard_frame = self.guard_context.pop().unwrap();
1992 debug!("Exiting guard building context with locals: {:?}", guard_frame);
1993
1994 for &(_, temp) in fake_borrows {
1995 let cause = FakeReadCause::ForMatchGuard;
1996 self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(temp));
1997 }
1998
1999 let otherwise_block = candidate.otherwise_block.unwrap_or_else(|| {
2000 let unreachable = self.cfg.start_new_block();
2001 self.cfg.terminate(unreachable, source_info, TerminatorKind::Unreachable);
2002 unreachable
2003 });
2004 self.false_edges(
2005 otherwise_post_guard_block,
2006 otherwise_block,
2007 candidate.next_candidate_pre_binding_block,
2008 source_info,
2009 );
2010
2011 // We want to ensure that the matched candidates are bound
2012 // after we have confirmed this candidate *and* any
2013 // associated guard; Binding them on `block` is too soon,
2014 // because that would be before we've checked the result
2015 // from the guard.
2016 //
2017 // But binding them on the arm is *too late*, because
2018 // then all of the candidates for a single arm would be
2019 // bound in the same place, that would cause a case like:
2020 //
2021 // ```rust
2022 // match (30, 2) {
2023 // (mut x, 1) | (2, mut x) if { true } => { ... }
2024 // ... // ^^^^^^^ (this is `arm_block`)
2025 // }
2026 // ```
2027 //
2028 // would yield an `arm_block` something like:
2029 //
2030 // ```
2031 // StorageLive(_4); // _4 is `x`
2032 // _4 = &mut (_1.0: i32); // this is handling `(mut x, 1)` case
2033 // _4 = &mut (_1.1: i32); // this is handling `(2, mut x)` case
2034 // ```
2035 //
2036 // and that is clearly not correct.
2037 let by_value_bindings = parent_bindings
2038 .iter()
2039 .flat_map(|(bindings, _)| bindings)
2040 .chain(&candidate.bindings)
2041 .filter(|binding| matches!(binding.binding_mode, BindingMode::ByValue));
2042 // Read all of the by reference bindings to ensure that the
2043 // place they refer to can't be modified by the guard.
2044 for binding in by_value_bindings.clone() {
2045 let local_id = self.var_local_id(binding.var_id, RefWithinGuard);
2046 let cause = FakeReadCause::ForGuardBinding;
2047 self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(local_id));
2048 }
2049 assert!(schedule_drops, "patterns with guards must schedule drops");
2050 self.bind_matched_candidate_for_arm_body(
2051 post_guard_block,
2052 true,
2053 by_value_bindings,
2054 storages_alive,
2055 );
2056
2057 post_guard_block
2058 } else {
2059 // (Here, it is not too early to bind the matched
2060 // candidate on `block`, because there is no guard result
2061 // that we have to inspect before we bind them.)
2062 self.bind_matched_candidate_for_arm_body(
2063 block,
2064 schedule_drops,
2065 parent_bindings
2066 .iter()
2067 .flat_map(|(bindings, _)| bindings)
2068 .chain(&candidate.bindings),
2069 storages_alive,
2070 );
2071 block
2072 }
2073 }
2074
2075 /// Append `AscribeUserType` statements onto the end of `block`
2076 /// for each ascription
ascribe_types( &mut self, block: BasicBlock, ascriptions: impl IntoIterator<Item = Ascription<'tcx>>, )2077 fn ascribe_types(
2078 &mut self,
2079 block: BasicBlock,
2080 ascriptions: impl IntoIterator<Item = Ascription<'tcx>>,
2081 ) {
2082 for ascription in ascriptions {
2083 let source_info = self.source_info(ascription.annotation.span);
2084
2085 let base = self.canonical_user_type_annotations.push(ascription.annotation);
2086 self.cfg.push(
2087 block,
2088 Statement {
2089 source_info,
2090 kind: StatementKind::AscribeUserType(
2091 Box::new((
2092 ascription.source,
2093 UserTypeProjection { base, projs: Vec::new() },
2094 )),
2095 ascription.variance,
2096 ),
2097 },
2098 );
2099 }
2100 }
2101
bind_matched_candidate_for_guard<'b>( &mut self, block: BasicBlock, schedule_drops: bool, bindings: impl IntoIterator<Item = &'b Binding<'tcx>>, ) where 'tcx: 'b,2102 fn bind_matched_candidate_for_guard<'b>(
2103 &mut self,
2104 block: BasicBlock,
2105 schedule_drops: bool,
2106 bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
2107 ) where
2108 'tcx: 'b,
2109 {
2110 debug!("bind_matched_candidate_for_guard(block={:?})", block);
2111
2112 // Assign each of the bindings. Since we are binding for a
2113 // guard expression, this will never trigger moves out of the
2114 // candidate.
2115 let re_erased = self.tcx.lifetimes.re_erased;
2116 for binding in bindings {
2117 debug!("bind_matched_candidate_for_guard(binding={:?})", binding);
2118 let source_info = self.source_info(binding.span);
2119
2120 // For each pattern ident P of type T, `ref_for_guard` is
2121 // a reference R: &T pointing to the location matched by
2122 // the pattern, and every occurrence of P within a guard
2123 // denotes *R.
2124 let ref_for_guard = self.storage_live_binding(
2125 block,
2126 binding.var_id,
2127 binding.span,
2128 RefWithinGuard,
2129 schedule_drops,
2130 );
2131 match binding.binding_mode {
2132 BindingMode::ByValue => {
2133 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, binding.source);
2134 self.cfg.push_assign(block, source_info, ref_for_guard, rvalue);
2135 }
2136 BindingMode::ByRef(borrow_kind) => {
2137 let value_for_arm = self.storage_live_binding(
2138 block,
2139 binding.var_id,
2140 binding.span,
2141 OutsideGuard,
2142 schedule_drops,
2143 );
2144
2145 let rvalue = Rvalue::Ref(re_erased, borrow_kind, binding.source);
2146 self.cfg.push_assign(block, source_info, value_for_arm, rvalue);
2147 let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, value_for_arm);
2148 self.cfg.push_assign(block, source_info, ref_for_guard, rvalue);
2149 }
2150 }
2151 }
2152 }
2153
bind_matched_candidate_for_arm_body<'b>( &mut self, block: BasicBlock, schedule_drops: bool, bindings: impl IntoIterator<Item = &'b Binding<'tcx>>, storages_alive: bool, ) where 'tcx: 'b,2154 fn bind_matched_candidate_for_arm_body<'b>(
2155 &mut self,
2156 block: BasicBlock,
2157 schedule_drops: bool,
2158 bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
2159 storages_alive: bool,
2160 ) where
2161 'tcx: 'b,
2162 {
2163 debug!("bind_matched_candidate_for_arm_body(block={:?})", block);
2164
2165 let re_erased = self.tcx.lifetimes.re_erased;
2166 // Assign each of the bindings. This may trigger moves out of the candidate.
2167 for binding in bindings {
2168 let source_info = self.source_info(binding.span);
2169 let local = if storages_alive {
2170 // Here storages are already alive, probably because this is a binding
2171 // from let-else.
2172 // We just need to schedule drop for the value.
2173 self.var_local_id(binding.var_id, OutsideGuard).into()
2174 } else {
2175 self.storage_live_binding(
2176 block,
2177 binding.var_id,
2178 binding.span,
2179 OutsideGuard,
2180 schedule_drops,
2181 )
2182 };
2183 if schedule_drops {
2184 self.schedule_drop_for_binding(binding.var_id, binding.span, OutsideGuard);
2185 }
2186 let rvalue = match binding.binding_mode {
2187 BindingMode::ByValue => Rvalue::Use(self.consume_by_copy_or_move(binding.source)),
2188 BindingMode::ByRef(borrow_kind) => {
2189 Rvalue::Ref(re_erased, borrow_kind, binding.source)
2190 }
2191 };
2192 self.cfg.push_assign(block, source_info, local, rvalue);
2193 }
2194 }
2195
2196 /// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where the bound
2197 /// `var` has type `T` in the arm body) in a pattern maps to 2 locals. The
2198 /// first local is a binding for occurrences of `var` in the guard, which
2199 /// will have type `&T`. The second local is a binding for occurrences of
2200 /// `var` in the arm body, which will have type `T`.
2201 #[instrument(skip(self), level = "debug")]
declare_binding( &mut self, source_info: SourceInfo, visibility_scope: SourceScope, mutability: Mutability, name: Symbol, mode: BindingMode, var_id: LocalVarId, var_ty: Ty<'tcx>, user_ty: UserTypeProjections, has_guard: ArmHasGuard, opt_match_place: Option<(Option<Place<'tcx>>, Span)>, pat_span: Span, )2202 fn declare_binding(
2203 &mut self,
2204 source_info: SourceInfo,
2205 visibility_scope: SourceScope,
2206 mutability: Mutability,
2207 name: Symbol,
2208 mode: BindingMode,
2209 var_id: LocalVarId,
2210 var_ty: Ty<'tcx>,
2211 user_ty: UserTypeProjections,
2212 has_guard: ArmHasGuard,
2213 opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
2214 pat_span: Span,
2215 ) {
2216 let tcx = self.tcx;
2217 let debug_source_info = SourceInfo { span: source_info.span, scope: visibility_scope };
2218 let binding_mode = match mode {
2219 BindingMode::ByValue => ty::BindingMode::BindByValue(mutability),
2220 BindingMode::ByRef(_) => ty::BindingMode::BindByReference(mutability),
2221 };
2222 let local = LocalDecl {
2223 mutability,
2224 ty: var_ty,
2225 user_ty: if user_ty.is_empty() { None } else { Some(Box::new(user_ty)) },
2226 source_info,
2227 internal: false,
2228 local_info: ClearCrossCrate::Set(Box::new(LocalInfo::User(BindingForm::Var(
2229 VarBindingForm {
2230 binding_mode,
2231 // hypothetically, `visit_primary_bindings` could try to unzip
2232 // an outermost hir::Ty as we descend, matching up
2233 // idents in pat; but complex w/ unclear UI payoff.
2234 // Instead, just abandon providing diagnostic info.
2235 opt_ty_info: None,
2236 opt_match_place,
2237 pat_span,
2238 },
2239 )))),
2240 };
2241 let for_arm_body = self.local_decls.push(local);
2242 self.var_debug_info.push(VarDebugInfo {
2243 name,
2244 source_info: debug_source_info,
2245 references: 0,
2246 value: VarDebugInfoContents::Place(for_arm_body.into()),
2247 argument_index: None,
2248 });
2249 let locals = if has_guard.0 {
2250 let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
2251 // This variable isn't mutated but has a name, so has to be
2252 // immutable to avoid the unused mut lint.
2253 mutability: Mutability::Not,
2254 ty: Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, var_ty),
2255 user_ty: None,
2256 source_info,
2257 internal: false,
2258 local_info: ClearCrossCrate::Set(Box::new(LocalInfo::User(
2259 BindingForm::RefForGuard,
2260 ))),
2261 });
2262 self.var_debug_info.push(VarDebugInfo {
2263 name,
2264 source_info: debug_source_info,
2265 references: 0,
2266 value: VarDebugInfoContents::Place(ref_for_guard.into()),
2267 argument_index: None,
2268 });
2269 LocalsForNode::ForGuard { ref_for_guard, for_arm_body }
2270 } else {
2271 LocalsForNode::One(for_arm_body)
2272 };
2273 debug!(?locals);
2274 self.var_indices.insert(var_id, locals);
2275 }
2276
ast_let_else( &mut self, mut block: BasicBlock, init: &Expr<'tcx>, initializer_span: Span, else_block: BlockId, let_else_scope: ®ion::Scope, pattern: &Pat<'tcx>, ) -> BlockAnd<BasicBlock>2277 pub(crate) fn ast_let_else(
2278 &mut self,
2279 mut block: BasicBlock,
2280 init: &Expr<'tcx>,
2281 initializer_span: Span,
2282 else_block: BlockId,
2283 let_else_scope: ®ion::Scope,
2284 pattern: &Pat<'tcx>,
2285 ) -> BlockAnd<BasicBlock> {
2286 let else_block_span = self.thir[else_block].span;
2287 let (matching, failure) = self.in_if_then_scope(*let_else_scope, else_block_span, |this| {
2288 let scrutinee = unpack!(block = this.lower_scrutinee(block, init, initializer_span));
2289 let pat = Pat { ty: init.ty, span: else_block_span, kind: PatKind::Wild };
2290 let mut wildcard = Candidate::new(scrutinee.clone(), &pat, false, this);
2291 let mut candidate = Candidate::new(scrutinee.clone(), pattern, false, this);
2292 let fake_borrow_temps = this.lower_match_tree(
2293 block,
2294 initializer_span,
2295 pattern.span,
2296 false,
2297 &mut [&mut candidate, &mut wildcard],
2298 );
2299 // This block is for the matching case
2300 let matching = this.bind_pattern(
2301 this.source_info(pattern.span),
2302 candidate,
2303 &fake_borrow_temps,
2304 initializer_span,
2305 None,
2306 true,
2307 );
2308 // This block is for the failure case
2309 let failure = this.bind_pattern(
2310 this.source_info(else_block_span),
2311 wildcard,
2312 &fake_borrow_temps,
2313 initializer_span,
2314 None,
2315 true,
2316 );
2317 this.break_for_else(failure, *let_else_scope, this.source_info(initializer_span));
2318 matching.unit()
2319 });
2320 matching.and(failure)
2321 }
2322 }
2323