rustc_borrowck/region_infer/mod.rs
1use std::collections::VecDeque;
2use std::rc::Rc;
3
4use rustc_data_structures::frozen::Frozen;
5use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
6use rustc_data_structures::graph::scc::Sccs;
7use rustc_errors::Diag;
8use rustc_hir::def_id::CRATE_DEF_ID;
9use rustc_index::IndexVec;
10use rustc_infer::infer::outlives::test_type_match;
11use rustc_infer::infer::region_constraints::{GenericKind, VerifyBound, VerifyIfEq};
12use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin};
13use rustc_middle::bug;
14use rustc_middle::mir::{
15 AnnotationSource, BasicBlock, Body, ConstraintCategory, Local, Location, ReturnConstraint,
16 TerminatorKind,
17};
18use rustc_middle::traits::{ObligationCause, ObligationCauseCode};
19use rustc_middle::ty::{self, RegionVid, Ty, TyCtxt, TypeFoldable, UniverseIndex, fold_regions};
20use rustc_mir_dataflow::points::DenseLocationMap;
21use rustc_span::hygiene::DesugaringKind;
22use rustc_span::{DUMMY_SP, Span};
23use tracing::{Level, debug, enabled, instrument, trace};
24
25use crate::constraints::graph::NormalConstraintGraph;
26use crate::constraints::{ConstraintSccIndex, OutlivesConstraint, OutlivesConstraintSet};
27use crate::dataflow::BorrowIndex;
28use crate::diagnostics::{RegionErrorKind, RegionErrors, UniverseInfo};
29use crate::handle_placeholders::{LoweredConstraints, RegionTracker};
30use crate::polonius::LiveLoans;
31use crate::polonius::legacy::PoloniusOutput;
32use crate::region_infer::values::{LivenessValues, RegionElement, RegionValues, ToElementIndex};
33use crate::type_check::Locations;
34use crate::type_check::free_region_relations::UniversalRegionRelations;
35use crate::universal_regions::UniversalRegions;
36use crate::{
37 BorrowckInferCtxt, ClosureOutlivesRequirement, ClosureOutlivesSubject,
38 ClosureOutlivesSubjectTy, ClosureRegionRequirements,
39};
40
41mod dump_mir;
42mod graphviz;
43pub(crate) mod opaque_types;
44mod reverse_sccs;
45
46pub(crate) mod values;
47
48/// The representative region variable for an SCC, tagged by its origin.
49/// We prefer placeholders over existentially quantified variables, otherwise
50/// it's the one with the smallest Region Variable ID. In other words,
51/// the order of this enumeration really matters!
52#[derive(Copy, Debug, Clone, PartialEq, PartialOrd, Eq, Ord)]
53pub(crate) enum Representative {
54 FreeRegion(RegionVid),
55 Placeholder(RegionVid),
56 Existential(RegionVid),
57}
58
59impl Representative {
60 pub(crate) fn rvid(self) -> RegionVid {
61 match self {
62 Representative::FreeRegion(region_vid)
63 | Representative::Placeholder(region_vid)
64 | Representative::Existential(region_vid) => region_vid,
65 }
66 }
67
68 pub(crate) fn new(r: RegionVid, definition: &RegionDefinition<'_>) -> Self {
69 match definition.origin {
70 NllRegionVariableOrigin::FreeRegion => Representative::FreeRegion(r),
71 NllRegionVariableOrigin::Placeholder(_) => Representative::Placeholder(r),
72 NllRegionVariableOrigin::Existential { .. } => Representative::Existential(r),
73 }
74 }
75}
76
77pub(crate) type ConstraintSccs = Sccs<RegionVid, ConstraintSccIndex>;
78
79pub struct RegionInferenceContext<'tcx> {
80 /// Contains the definition for every region variable. Region
81 /// variables are identified by their index (`RegionVid`). The
82 /// definition contains information about where the region came
83 /// from as well as its final inferred value.
84 pub(crate) definitions: Frozen<IndexVec<RegionVid, RegionDefinition<'tcx>>>,
85
86 /// The liveness constraints added to each region. For most
87 /// regions, these start out empty and steadily grow, though for
88 /// each universally quantified region R they start out containing
89 /// the entire CFG and `end(R)`.
90 liveness_constraints: LivenessValues,
91
92 /// The outlives constraints computed by the type-check.
93 constraints: Frozen<OutlivesConstraintSet<'tcx>>,
94
95 /// The constraint-set, but in graph form, making it easy to traverse
96 /// the constraints adjacent to a particular region. Used to construct
97 /// the SCC (see `constraint_sccs`) and for error reporting.
98 constraint_graph: Frozen<NormalConstraintGraph>,
99
100 /// The SCC computed from `constraints` and the constraint
101 /// graph. We have an edge from SCC A to SCC B if `A: B`. Used to
102 /// compute the values of each region.
103 constraint_sccs: ConstraintSccs,
104
105 scc_annotations: IndexVec<ConstraintSccIndex, RegionTracker>,
106
107 /// Map universe indexes to information on why we created it.
108 universe_causes: FxIndexMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
109
110 /// The final inferred values of the region variables; we compute
111 /// one value per SCC. To get the value for any given *region*,
112 /// you first find which scc it is a part of.
113 scc_values: RegionValues<ConstraintSccIndex>,
114
115 /// Type constraints that we check after solving.
116 type_tests: Vec<TypeTest<'tcx>>,
117
118 /// Information about how the universally quantified regions in
119 /// scope on this function relate to one another.
120 universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
121}
122
123#[derive(Debug)]
124pub(crate) struct RegionDefinition<'tcx> {
125 /// What kind of variable is this -- a free region? existential
126 /// variable? etc. (See the `NllRegionVariableOrigin` for more
127 /// info.)
128 pub(crate) origin: NllRegionVariableOrigin,
129
130 /// Which universe is this region variable defined in? This is
131 /// most often `ty::UniverseIndex::ROOT`, but when we encounter
132 /// forall-quantifiers like `for<'a> { 'a = 'b }`, we would create
133 /// the variable for `'a` in a fresh universe that extends ROOT.
134 pub(crate) universe: ty::UniverseIndex,
135
136 /// If this is 'static or an early-bound region, then this is
137 /// `Some(X)` where `X` is the name of the region.
138 pub(crate) external_name: Option<ty::Region<'tcx>>,
139}
140
141/// N.B., the variants in `Cause` are intentionally ordered. Lower
142/// values are preferred when it comes to error messages. Do not
143/// reorder willy nilly.
144#[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
145pub(crate) enum Cause {
146 /// point inserted because Local was live at the given Location
147 LiveVar(Local, Location),
148
149 /// point inserted because Local was dropped at the given Location
150 DropVar(Local, Location),
151}
152
153/// A "type test" corresponds to an outlives constraint between a type
154/// and a lifetime, like `T: 'x` or `<T as Foo>::Bar: 'x`. They are
155/// translated from the `Verify` region constraints in the ordinary
156/// inference context.
157///
158/// These sorts of constraints are handled differently than ordinary
159/// constraints, at least at present. During type checking, the
160/// `InferCtxt::process_registered_region_obligations` method will
161/// attempt to convert a type test like `T: 'x` into an ordinary
162/// outlives constraint when possible (for example, `&'a T: 'b` will
163/// be converted into `'a: 'b` and registered as a `Constraint`).
164///
165/// In some cases, however, there are outlives relationships that are
166/// not converted into a region constraint, but rather into one of
167/// these "type tests". The distinction is that a type test does not
168/// influence the inference result, but instead just examines the
169/// values that we ultimately inferred for each region variable and
170/// checks that they meet certain extra criteria. If not, an error
171/// can be issued.
172///
173/// One reason for this is that these type tests typically boil down
174/// to a check like `'a: 'x` where `'a` is a universally quantified
175/// region -- and therefore not one whose value is really meant to be
176/// *inferred*, precisely (this is not always the case: one can have a
177/// type test like `<Foo as Trait<'?0>>::Bar: 'x`, where `'?0` is an
178/// inference variable). Another reason is that these type tests can
179/// involve *disjunction* -- that is, they can be satisfied in more
180/// than one way.
181///
182/// For more information about this translation, see
183/// `InferCtxt::process_registered_region_obligations` and
184/// `InferCtxt::type_must_outlive` in `rustc_infer::infer::InferCtxt`.
185#[derive(Clone, Debug)]
186pub(crate) struct TypeTest<'tcx> {
187 /// The type `T` that must outlive the region.
188 pub generic_kind: GenericKind<'tcx>,
189
190 /// The region `'x` that the type must outlive.
191 pub lower_bound: RegionVid,
192
193 /// The span to blame.
194 pub span: Span,
195
196 /// A test which, if met by the region `'x`, proves that this type
197 /// constraint is satisfied.
198 pub verify_bound: VerifyBound<'tcx>,
199}
200
201/// When we have an unmet lifetime constraint, we try to propagate it outward (e.g. to a closure
202/// environment). If we can't, it is an error.
203#[derive(Clone, Copy, Debug, Eq, PartialEq)]
204enum RegionRelationCheckResult {
205 Ok,
206 Propagated,
207 Error,
208}
209
210#[derive(Clone, PartialEq, Eq, Debug)]
211enum Trace<'a, 'tcx> {
212 StartRegion,
213 FromGraph(&'a OutlivesConstraint<'tcx>),
214 FromStatic(RegionVid),
215 NotVisited,
216}
217
218#[instrument(skip(infcx, sccs), level = "debug")]
219fn sccs_info<'tcx>(infcx: &BorrowckInferCtxt<'tcx>, sccs: &ConstraintSccs) {
220 use crate::renumber::RegionCtxt;
221
222 let var_to_origin = infcx.reg_var_to_origin.borrow();
223
224 let mut var_to_origin_sorted = var_to_origin.clone().into_iter().collect::<Vec<_>>();
225 var_to_origin_sorted.sort_by_key(|vto| vto.0);
226
227 if enabled!(Level::DEBUG) {
228 let mut reg_vars_to_origins_str = "region variables to origins:\n".to_string();
229 for (reg_var, origin) in var_to_origin_sorted.into_iter() {
230 reg_vars_to_origins_str.push_str(&format!("{reg_var:?}: {origin:?}\n"));
231 }
232 debug!("{}", reg_vars_to_origins_str);
233 }
234
235 let num_components = sccs.num_sccs();
236 let mut components = vec![FxIndexSet::default(); num_components];
237
238 for (reg_var, scc_idx) in sccs.scc_indices().iter_enumerated() {
239 let origin = var_to_origin.get(®_var).unwrap_or(&RegionCtxt::Unknown);
240 components[scc_idx.as_usize()].insert((reg_var, *origin));
241 }
242
243 if enabled!(Level::DEBUG) {
244 let mut components_str = "strongly connected components:".to_string();
245 for (scc_idx, reg_vars_origins) in components.iter().enumerate() {
246 let regions_info = reg_vars_origins.clone().into_iter().collect::<Vec<_>>();
247 components_str.push_str(&format!(
248 "{:?}: {:?},\n)",
249 ConstraintSccIndex::from_usize(scc_idx),
250 regions_info,
251 ))
252 }
253 debug!("{}", components_str);
254 }
255
256 // calculate the best representative for each component
257 let components_representatives = components
258 .into_iter()
259 .enumerate()
260 .map(|(scc_idx, region_ctxts)| {
261 let repr = region_ctxts
262 .into_iter()
263 .map(|reg_var_origin| reg_var_origin.1)
264 .max_by(|x, y| x.preference_value().cmp(&y.preference_value()))
265 .unwrap();
266
267 (ConstraintSccIndex::from_usize(scc_idx), repr)
268 })
269 .collect::<FxIndexMap<_, _>>();
270
271 let mut scc_node_to_edges = FxIndexMap::default();
272 for (scc_idx, repr) in components_representatives.iter() {
273 let edge_representatives = sccs
274 .successors(*scc_idx)
275 .iter()
276 .map(|scc_idx| components_representatives[scc_idx])
277 .collect::<Vec<_>>();
278 scc_node_to_edges.insert((scc_idx, repr), edge_representatives);
279 }
280
281 debug!("SCC edges {:#?}", scc_node_to_edges);
282}
283
284impl<'tcx> RegionInferenceContext<'tcx> {
285 /// Creates a new region inference context with a total of
286 /// `num_region_variables` valid inference variables; the first N
287 /// of those will be constant regions representing the free
288 /// regions defined in `universal_regions`.
289 ///
290 /// The `outlives_constraints` and `type_tests` are an initial set
291 /// of constraints produced by the MIR type check.
292 pub(crate) fn new(
293 infcx: &BorrowckInferCtxt<'tcx>,
294 lowered_constraints: LoweredConstraints<'tcx>,
295 universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
296 location_map: Rc<DenseLocationMap>,
297 ) -> Self {
298 let universal_regions = &universal_region_relations.universal_regions;
299
300 let LoweredConstraints {
301 constraint_sccs,
302 definitions,
303 outlives_constraints,
304 scc_annotations,
305 type_tests,
306 liveness_constraints,
307 universe_causes,
308 placeholder_indices,
309 } = lowered_constraints;
310
311 debug!("universal_regions: {:#?}", universal_region_relations.universal_regions);
312 debug!("outlives constraints: {:#?}", outlives_constraints);
313 debug!("placeholder_indices: {:#?}", placeholder_indices);
314 debug!("type tests: {:#?}", type_tests);
315
316 let constraint_graph = Frozen::freeze(outlives_constraints.graph(definitions.len()));
317
318 if cfg!(debug_assertions) {
319 sccs_info(infcx, &constraint_sccs);
320 }
321
322 let mut scc_values =
323 RegionValues::new(location_map, universal_regions.len(), placeholder_indices);
324
325 for region in liveness_constraints.regions() {
326 let scc = constraint_sccs.scc(region);
327 scc_values.merge_liveness(scc, region, &liveness_constraints);
328 }
329
330 let mut result = Self {
331 definitions,
332 liveness_constraints,
333 constraints: outlives_constraints,
334 constraint_graph,
335 constraint_sccs,
336 scc_annotations,
337 universe_causes,
338 scc_values,
339 type_tests,
340 universal_region_relations,
341 };
342
343 result.init_free_and_bound_regions();
344
345 result
346 }
347
348 /// Initializes the region variables for each universally
349 /// quantified region (lifetime parameter). The first N variables
350 /// always correspond to the regions appearing in the function
351 /// signature (both named and anonymous) and where-clauses. This
352 /// function iterates over those regions and initializes them with
353 /// minimum values.
354 ///
355 /// For example:
356 /// ```ignore (illustrative)
357 /// fn foo<'a, 'b>( /* ... */ ) where 'a: 'b { /* ... */ }
358 /// ```
359 /// would initialize two variables like so:
360 /// ```ignore (illustrative)
361 /// R0 = { CFG, R0 } // 'a
362 /// R1 = { CFG, R0, R1 } // 'b
363 /// ```
364 /// Here, R0 represents `'a`, and it contains (a) the entire CFG
365 /// and (b) any universally quantified regions that it outlives,
366 /// which in this case is just itself. R1 (`'b`) in contrast also
367 /// outlives `'a` and hence contains R0 and R1.
368 ///
369 /// This bit of logic also handles invalid universe relations
370 /// for higher-kinded types.
371 ///
372 /// We Walk each SCC `A` and `B` such that `A: B`
373 /// and ensure that universe(A) can see universe(B).
374 ///
375 /// This serves to enforce the 'empty/placeholder' hierarchy
376 /// (described in more detail on `RegionKind`):
377 ///
378 /// ```ignore (illustrative)
379 /// static -----+
380 /// | |
381 /// empty(U0) placeholder(U1)
382 /// | /
383 /// empty(U1)
384 /// ```
385 ///
386 /// In particular, imagine we have variables R0 in U0 and R1
387 /// created in U1, and constraints like this;
388 ///
389 /// ```ignore (illustrative)
390 /// R1: !1 // R1 outlives the placeholder in U1
391 /// R1: R0 // R1 outlives R0
392 /// ```
393 ///
394 /// Here, we wish for R1 to be `'static`, because it
395 /// cannot outlive `placeholder(U1)` and `empty(U0)` any other way.
396 ///
397 /// Thanks to this loop, what happens is that the `R1: R0`
398 /// constraint has lowered the universe of `R1` to `U0`, which in turn
399 /// means that the `R1: !1` constraint here will cause
400 /// `R1` to become `'static`.
401 fn init_free_and_bound_regions(&mut self) {
402 for variable in self.definitions.indices() {
403 let scc = self.constraint_sccs.scc(variable);
404
405 match self.definitions[variable].origin {
406 NllRegionVariableOrigin::FreeRegion => {
407 // For each free, universally quantified region X:
408
409 // Add all nodes in the CFG to liveness constraints
410 self.liveness_constraints.add_all_points(variable);
411 self.scc_values.add_all_points(scc);
412
413 // Add `end(X)` into the set for X.
414 self.scc_values.add_element(scc, variable);
415 }
416
417 NllRegionVariableOrigin::Placeholder(placeholder) => {
418 self.scc_values.add_element(scc, placeholder);
419 }
420
421 NllRegionVariableOrigin::Existential { .. } => {
422 // For existential, regions, nothing to do.
423 }
424 }
425 }
426 }
427
428 /// Returns an iterator over all the region indices.
429 pub(crate) fn regions(&self) -> impl Iterator<Item = RegionVid> + 'tcx {
430 self.definitions.indices()
431 }
432
433 /// Given a universal region in scope on the MIR, returns the
434 /// corresponding index.
435 ///
436 /// Panics if `r` is not a registered universal region, most notably
437 /// if it is a placeholder. Handling placeholders requires access to the
438 /// `MirTypeckRegionConstraints`.
439 pub(crate) fn to_region_vid(&self, r: ty::Region<'tcx>) -> RegionVid {
440 self.universal_regions().to_region_vid(r)
441 }
442
443 /// Returns an iterator over all the outlives constraints.
444 pub(crate) fn outlives_constraints(&self) -> impl Iterator<Item = OutlivesConstraint<'tcx>> {
445 self.constraints.outlives().iter().copied()
446 }
447
448 /// Adds annotations for `#[rustc_regions]`; see `UniversalRegions::annotate`.
449 pub(crate) fn annotate(&self, tcx: TyCtxt<'tcx>, err: &mut Diag<'_, ()>) {
450 self.universal_regions().annotate(tcx, err)
451 }
452
453 /// Returns `true` if the region `r` contains the point `p`.
454 ///
455 /// Panics if called before `solve()` executes,
456 pub(crate) fn region_contains(&self, r: RegionVid, p: impl ToElementIndex) -> bool {
457 let scc = self.constraint_sccs.scc(r);
458 self.scc_values.contains(scc, p)
459 }
460
461 /// Returns the lowest statement index in `start..=end` which is not contained by `r`.
462 ///
463 /// Panics if called before `solve()` executes.
464 pub(crate) fn first_non_contained_inclusive(
465 &self,
466 r: RegionVid,
467 block: BasicBlock,
468 start: usize,
469 end: usize,
470 ) -> Option<usize> {
471 let scc = self.constraint_sccs.scc(r);
472 self.scc_values.first_non_contained_inclusive(scc, block, start, end)
473 }
474
475 /// Returns access to the value of `r` for debugging purposes.
476 pub(crate) fn region_value_str(&self, r: RegionVid) -> String {
477 let scc = self.constraint_sccs.scc(r);
478 self.scc_values.region_value_str(scc)
479 }
480
481 pub(crate) fn placeholders_contained_in(
482 &self,
483 r: RegionVid,
484 ) -> impl Iterator<Item = ty::PlaceholderRegion> {
485 let scc = self.constraint_sccs.scc(r);
486 self.scc_values.placeholders_contained_in(scc)
487 }
488
489 /// Performs region inference and report errors if we see any
490 /// unsatisfiable constraints. If this is a closure, returns the
491 /// region requirements to propagate to our creator, if any.
492 #[instrument(skip(self, infcx, body, polonius_output), level = "debug")]
493 pub(super) fn solve(
494 &mut self,
495 infcx: &InferCtxt<'tcx>,
496 body: &Body<'tcx>,
497 polonius_output: Option<Box<PoloniusOutput>>,
498 ) -> (Option<ClosureRegionRequirements<'tcx>>, RegionErrors<'tcx>) {
499 let mir_def_id = body.source.def_id();
500 self.propagate_constraints();
501
502 let mut errors_buffer = RegionErrors::new(infcx.tcx);
503
504 // If this is a closure, we can propagate unsatisfied
505 // `outlives_requirements` to our creator, so create a vector
506 // to store those. Otherwise, we'll pass in `None` to the
507 // functions below, which will trigger them to report errors
508 // eagerly.
509 let mut outlives_requirements = infcx.tcx.is_typeck_child(mir_def_id).then(Vec::new);
510
511 self.check_type_tests(infcx, outlives_requirements.as_mut(), &mut errors_buffer);
512
513 debug!(?errors_buffer);
514 debug!(?outlives_requirements);
515
516 // In Polonius mode, the errors about missing universal region relations are in the output
517 // and need to be emitted or propagated. Otherwise, we need to check whether the
518 // constraints were too strong, and if so, emit or propagate those errors.
519 if infcx.tcx.sess.opts.unstable_opts.polonius.is_legacy_enabled() {
520 self.check_polonius_subset_errors(
521 outlives_requirements.as_mut(),
522 &mut errors_buffer,
523 polonius_output
524 .as_ref()
525 .expect("Polonius output is unavailable despite `-Z polonius`"),
526 );
527 } else {
528 self.check_universal_regions(outlives_requirements.as_mut(), &mut errors_buffer);
529 }
530
531 debug!(?errors_buffer);
532
533 let outlives_requirements = outlives_requirements.unwrap_or_default();
534
535 if outlives_requirements.is_empty() {
536 (None, errors_buffer)
537 } else {
538 let num_external_vids = self.universal_regions().num_global_and_external_regions();
539 (
540 Some(ClosureRegionRequirements { num_external_vids, outlives_requirements }),
541 errors_buffer,
542 )
543 }
544 }
545
546 /// Propagate the region constraints: this will grow the values
547 /// for each region variable until all the constraints are
548 /// satisfied. Note that some values may grow **too** large to be
549 /// feasible, but we check this later.
550 #[instrument(skip(self), level = "debug")]
551 fn propagate_constraints(&mut self) {
552 debug!("constraints={:#?}", {
553 let mut constraints: Vec<_> = self.outlives_constraints().collect();
554 constraints.sort_by_key(|c| (c.sup, c.sub));
555 constraints
556 .into_iter()
557 .map(|c| (c, self.constraint_sccs.scc(c.sup), self.constraint_sccs.scc(c.sub)))
558 .collect::<Vec<_>>()
559 });
560
561 // To propagate constraints, we walk the DAG induced by the
562 // SCC. For each SCC `A`, we visit its successors and compute
563 // their values, then we union all those values to get our
564 // own.
565 for scc_a in self.constraint_sccs.all_sccs() {
566 // Walk each SCC `B` such that `A: B`...
567 for &scc_b in self.constraint_sccs.successors(scc_a) {
568 debug!(?scc_b);
569 self.scc_values.add_region(scc_a, scc_b);
570 }
571 }
572 }
573
574 /// Returns `true` if all the placeholders in the value of `scc_b` are nameable
575 /// in `scc_a`. Used during constraint propagation, and only once
576 /// the value of `scc_b` has been computed.
577 fn can_name_all_placeholders(
578 &self,
579 scc_a: ConstraintSccIndex,
580 scc_b: ConstraintSccIndex,
581 ) -> bool {
582 self.scc_annotations[scc_a].can_name_all_placeholders(self.scc_annotations[scc_b])
583 }
584
585 /// Once regions have been propagated, this method is used to see
586 /// whether the "type tests" produced by typeck were satisfied;
587 /// type tests encode type-outlives relationships like `T:
588 /// 'a`. See `TypeTest` for more details.
589 fn check_type_tests(
590 &self,
591 infcx: &InferCtxt<'tcx>,
592 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
593 errors_buffer: &mut RegionErrors<'tcx>,
594 ) {
595 let tcx = infcx.tcx;
596
597 // Sometimes we register equivalent type-tests that would
598 // result in basically the exact same error being reported to
599 // the user. Avoid that.
600 let mut deduplicate_errors = FxIndexSet::default();
601
602 for type_test in &self.type_tests {
603 debug!("check_type_test: {:?}", type_test);
604
605 let generic_ty = type_test.generic_kind.to_ty(tcx);
606 if self.eval_verify_bound(
607 infcx,
608 generic_ty,
609 type_test.lower_bound,
610 &type_test.verify_bound,
611 ) {
612 continue;
613 }
614
615 if let Some(propagated_outlives_requirements) = &mut propagated_outlives_requirements
616 && self.try_promote_type_test(infcx, type_test, propagated_outlives_requirements)
617 {
618 continue;
619 }
620
621 // Type-test failed. Report the error.
622 let erased_generic_kind = infcx.tcx.erase_and_anonymize_regions(type_test.generic_kind);
623
624 // Skip duplicate-ish errors.
625 if deduplicate_errors.insert((
626 erased_generic_kind,
627 type_test.lower_bound,
628 type_test.span,
629 )) {
630 debug!(
631 "check_type_test: reporting error for erased_generic_kind={:?}, \
632 lower_bound_region={:?}, \
633 type_test.span={:?}",
634 erased_generic_kind, type_test.lower_bound, type_test.span,
635 );
636
637 errors_buffer.push(RegionErrorKind::TypeTestError { type_test: type_test.clone() });
638 }
639 }
640 }
641
642 /// Invoked when we have some type-test (e.g., `T: 'X`) that we cannot
643 /// prove to be satisfied. If this is a closure, we will attempt to
644 /// "promote" this type-test into our `ClosureRegionRequirements` and
645 /// hence pass it up the creator. To do this, we have to phrase the
646 /// type-test in terms of external free regions, as local free
647 /// regions are not nameable by the closure's creator.
648 ///
649 /// Promotion works as follows: we first check that the type `T`
650 /// contains only regions that the creator knows about. If this is
651 /// true, then -- as a consequence -- we know that all regions in
652 /// the type `T` are free regions that outlive the closure body. If
653 /// false, then promotion fails.
654 ///
655 /// Once we've promoted T, we have to "promote" `'X` to some region
656 /// that is "external" to the closure. Generally speaking, a region
657 /// may be the union of some points in the closure body as well as
658 /// various free lifetimes. We can ignore the points in the closure
659 /// body: if the type T can be expressed in terms of external regions,
660 /// we know it outlives the points in the closure body. That
661 /// just leaves the free regions.
662 ///
663 /// The idea then is to lower the `T: 'X` constraint into multiple
664 /// bounds -- e.g., if `'X` is the union of two free lifetimes,
665 /// `'1` and `'2`, then we would create `T: '1` and `T: '2`.
666 #[instrument(level = "debug", skip(self, infcx, propagated_outlives_requirements))]
667 fn try_promote_type_test(
668 &self,
669 infcx: &InferCtxt<'tcx>,
670 type_test: &TypeTest<'tcx>,
671 propagated_outlives_requirements: &mut Vec<ClosureOutlivesRequirement<'tcx>>,
672 ) -> bool {
673 let tcx = infcx.tcx;
674 let TypeTest { generic_kind, lower_bound, span: blame_span, verify_bound: _ } = *type_test;
675
676 let generic_ty = generic_kind.to_ty(tcx);
677 let Some(subject) = self.try_promote_type_test_subject(infcx, generic_ty) else {
678 return false;
679 };
680
681 let r_scc = self.constraint_sccs.scc(lower_bound);
682 debug!(
683 "lower_bound = {:?} r_scc={:?} universe={:?}",
684 lower_bound,
685 r_scc,
686 self.max_nameable_universe(r_scc)
687 );
688 // If the type test requires that `T: 'a` where `'a` is a
689 // placeholder from another universe, that effectively requires
690 // `T: 'static`, so we have to propagate that requirement.
691 //
692 // It doesn't matter *what* universe because the promoted `T` will
693 // always be in the root universe.
694 if let Some(p) = self.scc_values.placeholders_contained_in(r_scc).next() {
695 debug!("encountered placeholder in higher universe: {:?}, requiring 'static", p);
696 let static_r = self.universal_regions().fr_static;
697 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
698 subject,
699 outlived_free_region: static_r,
700 blame_span,
701 category: ConstraintCategory::Boring,
702 });
703
704 // we can return here -- the code below might push add'l constraints
705 // but they would all be weaker than this one.
706 return true;
707 }
708
709 // For each region outlived by lower_bound find a non-local,
710 // universal region (it may be the same region) and add it to
711 // `ClosureOutlivesRequirement`.
712 let mut found_outlived_universal_region = false;
713 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
714 found_outlived_universal_region = true;
715 debug!("universal_region_outlived_by ur={:?}", ur);
716 let non_local_ub = self.universal_region_relations.non_local_upper_bounds(ur);
717 debug!(?non_local_ub);
718
719 // This is slightly too conservative. To show T: '1, given `'2: '1`
720 // and `'3: '1` we only need to prove that T: '2 *or* T: '3, but to
721 // avoid potential non-determinism we approximate this by requiring
722 // T: '1 and T: '2.
723 for upper_bound in non_local_ub {
724 debug_assert!(self.universal_regions().is_universal_region(upper_bound));
725 debug_assert!(!self.universal_regions().is_local_free_region(upper_bound));
726
727 let requirement = ClosureOutlivesRequirement {
728 subject,
729 outlived_free_region: upper_bound,
730 blame_span,
731 category: ConstraintCategory::Boring,
732 };
733 debug!(?requirement, "adding closure requirement");
734 propagated_outlives_requirements.push(requirement);
735 }
736 }
737 // If we succeed to promote the subject, i.e. it only contains non-local regions,
738 // and fail to prove the type test inside of the closure, the `lower_bound` has to
739 // also be at least as large as some universal region, as the type test is otherwise
740 // trivial.
741 assert!(found_outlived_universal_region);
742 true
743 }
744
745 /// When we promote a type test `T: 'r`, we have to replace all region
746 /// variables in the type `T` with an equal universal region from the
747 /// closure signature.
748 /// This is not always possible, so this is a fallible process.
749 #[instrument(level = "debug", skip(self, infcx), ret)]
750 fn try_promote_type_test_subject(
751 &self,
752 infcx: &InferCtxt<'tcx>,
753 ty: Ty<'tcx>,
754 ) -> Option<ClosureOutlivesSubject<'tcx>> {
755 let tcx = infcx.tcx;
756 let mut failed = false;
757 let ty = fold_regions(tcx, ty, |r, _depth| {
758 let r_vid = self.to_region_vid(r);
759 let r_scc = self.constraint_sccs.scc(r_vid);
760
761 // The challenge is this. We have some region variable `r`
762 // whose value is a set of CFG points and universal
763 // regions. We want to find if that set is *equivalent* to
764 // any of the named regions found in the closure.
765 // To do so, we simply check every candidate `u_r` for equality.
766 self.scc_values
767 .universal_regions_outlived_by(r_scc)
768 .filter(|&u_r| !self.universal_regions().is_local_free_region(u_r))
769 .find(|&u_r| self.eval_equal(u_r, r_vid))
770 .map(|u_r| ty::Region::new_var(tcx, u_r))
771 // In case we could not find a named region to map to,
772 // we will return `None` below.
773 .unwrap_or_else(|| {
774 failed = true;
775 r
776 })
777 });
778
779 debug!("try_promote_type_test_subject: folded ty = {:?}", ty);
780
781 // This will be true if we failed to promote some region.
782 if failed {
783 return None;
784 }
785
786 Some(ClosureOutlivesSubject::Ty(ClosureOutlivesSubjectTy::bind(tcx, ty)))
787 }
788
789 /// Like `universal_upper_bound`, but returns an approximation more suitable
790 /// for diagnostics. If `r` contains multiple disjoint universal regions
791 /// (e.g. 'a and 'b in `fn foo<'a, 'b> { ... }`, we pick the lower-numbered region.
792 /// This corresponds to picking named regions over unnamed regions
793 /// (e.g. picking early-bound regions over a closure late-bound region).
794 ///
795 /// This means that the returned value may not be a true upper bound, since
796 /// only 'static is known to outlive disjoint universal regions.
797 /// Therefore, this method should only be used in diagnostic code,
798 /// where displaying *some* named universal region is better than
799 /// falling back to 'static.
800 #[instrument(level = "debug", skip(self))]
801 pub(crate) fn approx_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
802 debug!("{}", self.region_value_str(r));
803
804 // Find the smallest universal region that contains all other
805 // universal regions within `region`.
806 let mut lub = self.universal_regions().fr_fn_body;
807 let r_scc = self.constraint_sccs.scc(r);
808 let static_r = self.universal_regions().fr_static;
809 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
810 let new_lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
811 debug!(?ur, ?lub, ?new_lub);
812 // The upper bound of two non-static regions is static: this
813 // means we know nothing about the relationship between these
814 // two regions. Pick a 'better' one to use when constructing
815 // a diagnostic
816 if ur != static_r && lub != static_r && new_lub == static_r {
817 // Prefer the region with an `external_name` - this
818 // indicates that the region is early-bound, so working with
819 // it can produce a nicer error.
820 if self.region_definition(ur).external_name.is_some() {
821 lub = ur;
822 } else if self.region_definition(lub).external_name.is_some() {
823 // Leave lub unchanged
824 } else {
825 // If we get here, we don't have any reason to prefer
826 // one region over the other. Just pick the
827 // one with the lower index for now.
828 lub = std::cmp::min(ur, lub);
829 }
830 } else {
831 lub = new_lub;
832 }
833 }
834
835 debug!(?r, ?lub);
836
837 lub
838 }
839
840 /// Tests if `test` is true when applied to `lower_bound` at
841 /// `point`.
842 fn eval_verify_bound(
843 &self,
844 infcx: &InferCtxt<'tcx>,
845 generic_ty: Ty<'tcx>,
846 lower_bound: RegionVid,
847 verify_bound: &VerifyBound<'tcx>,
848 ) -> bool {
849 debug!("eval_verify_bound(lower_bound={:?}, verify_bound={:?})", lower_bound, verify_bound);
850
851 match verify_bound {
852 VerifyBound::IfEq(verify_if_eq_b) => {
853 self.eval_if_eq(infcx, generic_ty, lower_bound, *verify_if_eq_b)
854 }
855
856 VerifyBound::IsEmpty => {
857 let lower_bound_scc = self.constraint_sccs.scc(lower_bound);
858 self.scc_values.elements_contained_in(lower_bound_scc).next().is_none()
859 }
860
861 VerifyBound::OutlivedBy(r) => {
862 let r_vid = self.to_region_vid(*r);
863 self.eval_outlives(r_vid, lower_bound)
864 }
865
866 VerifyBound::AnyBound(verify_bounds) => verify_bounds.iter().any(|verify_bound| {
867 self.eval_verify_bound(infcx, generic_ty, lower_bound, verify_bound)
868 }),
869
870 VerifyBound::AllBounds(verify_bounds) => verify_bounds.iter().all(|verify_bound| {
871 self.eval_verify_bound(infcx, generic_ty, lower_bound, verify_bound)
872 }),
873 }
874 }
875
876 fn eval_if_eq(
877 &self,
878 infcx: &InferCtxt<'tcx>,
879 generic_ty: Ty<'tcx>,
880 lower_bound: RegionVid,
881 verify_if_eq_b: ty::Binder<'tcx, VerifyIfEq<'tcx>>,
882 ) -> bool {
883 let generic_ty = self.normalize_to_scc_representatives(infcx.tcx, generic_ty);
884 let verify_if_eq_b = self.normalize_to_scc_representatives(infcx.tcx, verify_if_eq_b);
885 match test_type_match::extract_verify_if_eq(infcx.tcx, &verify_if_eq_b, generic_ty) {
886 Some(r) => {
887 let r_vid = self.to_region_vid(r);
888 self.eval_outlives(r_vid, lower_bound)
889 }
890 None => false,
891 }
892 }
893
894 /// This is a conservative normalization procedure. It takes every
895 /// free region in `value` and replaces it with the
896 /// "representative" of its SCC (see `scc_representatives` field).
897 /// We are guaranteed that if two values normalize to the same
898 /// thing, then they are equal; this is a conservative check in
899 /// that they could still be equal even if they normalize to
900 /// different results. (For example, there might be two regions
901 /// with the same value that are not in the same SCC).
902 ///
903 /// N.B., this is not an ideal approach and I would like to revisit
904 /// it. However, it works pretty well in practice. In particular,
905 /// this is needed to deal with projection outlives bounds like
906 ///
907 /// ```text
908 /// <T as Foo<'0>>::Item: '1
909 /// ```
910 ///
911 /// In particular, this routine winds up being important when
912 /// there are bounds like `where <T as Foo<'a>>::Item: 'b` in the
913 /// environment. In this case, if we can show that `'0 == 'a`,
914 /// and that `'b: '1`, then we know that the clause is
915 /// satisfied. In such cases, particularly due to limitations of
916 /// the trait solver =), we usually wind up with a where-clause like
917 /// `T: Foo<'a>` in scope, which thus forces `'0 == 'a` to be added as
918 /// a constraint, and thus ensures that they are in the same SCC.
919 ///
920 /// So why can't we do a more correct routine? Well, we could
921 /// *almost* use the `relate_tys` code, but the way it is
922 /// currently setup it creates inference variables to deal with
923 /// higher-ranked things and so forth, and right now the inference
924 /// context is not permitted to make more inference variables. So
925 /// we use this kind of hacky solution.
926 fn normalize_to_scc_representatives<T>(&self, tcx: TyCtxt<'tcx>, value: T) -> T
927 where
928 T: TypeFoldable<TyCtxt<'tcx>>,
929 {
930 fold_regions(tcx, value, |r, _db| {
931 let vid = self.to_region_vid(r);
932 let scc = self.constraint_sccs.scc(vid);
933 let repr = self.scc_representative(scc);
934 ty::Region::new_var(tcx, repr)
935 })
936 }
937
938 /// Evaluate whether `sup_region == sub_region`.
939 ///
940 /// Panics if called before `solve()` executes,
941 // This is `pub` because it's used by unstable external borrowck data users, see `consumers.rs`.
942 pub fn eval_equal(&self, r1: RegionVid, r2: RegionVid) -> bool {
943 self.eval_outlives(r1, r2) && self.eval_outlives(r2, r1)
944 }
945
946 /// Evaluate whether `sup_region: sub_region`.
947 ///
948 /// Panics if called before `solve()` executes,
949 // This is `pub` because it's used by unstable external borrowck data users, see `consumers.rs`.
950 #[instrument(skip(self), level = "debug", ret)]
951 pub fn eval_outlives(&self, sup_region: RegionVid, sub_region: RegionVid) -> bool {
952 debug!(
953 "sup_region's value = {:?} universal={:?}",
954 self.region_value_str(sup_region),
955 self.universal_regions().is_universal_region(sup_region),
956 );
957 debug!(
958 "sub_region's value = {:?} universal={:?}",
959 self.region_value_str(sub_region),
960 self.universal_regions().is_universal_region(sub_region),
961 );
962
963 let sub_region_scc = self.constraint_sccs.scc(sub_region);
964 let sup_region_scc = self.constraint_sccs.scc(sup_region);
965
966 if sub_region_scc == sup_region_scc {
967 debug!("{sup_region:?}: {sub_region:?} holds trivially; they are in the same SCC");
968 return true;
969 }
970
971 let fr_static = self.universal_regions().fr_static;
972
973 // If we are checking that `'sup: 'sub`, and `'sub` contains
974 // some placeholder that `'sup` cannot name, then this is only
975 // true if `'sup` outlives static.
976 //
977 // Avoid infinite recursion if `sub_region` is already `'static`
978 if sub_region != fr_static
979 && !self.can_name_all_placeholders(sup_region_scc, sub_region_scc)
980 {
981 debug!(
982 "sub universe `{sub_region_scc:?}` is not nameable \
983 by super `{sup_region_scc:?}`, promoting to static",
984 );
985
986 return self.eval_outlives(sup_region, fr_static);
987 }
988
989 // Both the `sub_region` and `sup_region` consist of the union
990 // of some number of universal regions (along with the union
991 // of various points in the CFG; ignore those points for
992 // now). Therefore, the sup-region outlives the sub-region if,
993 // for each universal region R1 in the sub-region, there
994 // exists some region R2 in the sup-region that outlives R1.
995 let universal_outlives =
996 self.scc_values.universal_regions_outlived_by(sub_region_scc).all(|r1| {
997 self.scc_values
998 .universal_regions_outlived_by(sup_region_scc)
999 .any(|r2| self.universal_region_relations.outlives(r2, r1))
1000 });
1001
1002 if !universal_outlives {
1003 debug!("sub region contains a universal region not present in super");
1004 return false;
1005 }
1006
1007 // Now we have to compare all the points in the sub region and make
1008 // sure they exist in the sup region.
1009
1010 if self.universal_regions().is_universal_region(sup_region) {
1011 // Micro-opt: universal regions contain all points.
1012 debug!("super is universal and hence contains all points");
1013 return true;
1014 }
1015
1016 debug!("comparison between points in sup/sub");
1017
1018 self.scc_values.contains_points(sup_region_scc, sub_region_scc)
1019 }
1020
1021 /// Once regions have been propagated, this method is used to see
1022 /// whether any of the constraints were too strong. In particular,
1023 /// we want to check for a case where a universally quantified
1024 /// region exceeded its bounds. Consider:
1025 /// ```compile_fail
1026 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1027 /// ```
1028 /// In this case, returning `x` requires `&'a u32 <: &'b u32`
1029 /// and hence we establish (transitively) a constraint that
1030 /// `'a: 'b`. The `propagate_constraints` code above will
1031 /// therefore add `end('a)` into the region for `'b` -- but we
1032 /// have no evidence that `'b` outlives `'a`, so we want to report
1033 /// an error.
1034 ///
1035 /// If `propagated_outlives_requirements` is `Some`, then we will
1036 /// push unsatisfied obligations into there. Otherwise, we'll
1037 /// report them as errors.
1038 fn check_universal_regions(
1039 &self,
1040 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1041 errors_buffer: &mut RegionErrors<'tcx>,
1042 ) {
1043 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1044 debug!(?fr, ?fr_definition);
1045 match fr_definition.origin {
1046 NllRegionVariableOrigin::FreeRegion => {
1047 // Go through each of the universal regions `fr` and check that
1048 // they did not grow too large, accumulating any requirements
1049 // for our caller into the `outlives_requirements` vector.
1050 self.check_universal_region(
1051 fr,
1052 &mut propagated_outlives_requirements,
1053 errors_buffer,
1054 );
1055 }
1056
1057 NllRegionVariableOrigin::Placeholder(placeholder) => {
1058 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1059 }
1060
1061 NllRegionVariableOrigin::Existential { .. } => {
1062 // nothing to check here
1063 }
1064 }
1065 }
1066 }
1067
1068 /// Checks if Polonius has found any unexpected free region relations.
1069 ///
1070 /// In Polonius terms, a "subset error" (or "illegal subset relation error") is the equivalent
1071 /// of NLL's "checking if any region constraints were too strong": a placeholder origin `'a`
1072 /// was unexpectedly found to be a subset of another placeholder origin `'b`, and means in NLL
1073 /// terms that the "longer free region" `'a` outlived the "shorter free region" `'b`.
1074 ///
1075 /// More details can be found in this blog post by Niko:
1076 /// <https://smallcultfollowing.com/babysteps/blog/2019/01/17/polonius-and-region-errors/>
1077 ///
1078 /// In the canonical example
1079 /// ```compile_fail
1080 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1081 /// ```
1082 /// returning `x` requires `&'a u32 <: &'b u32` and hence we establish (transitively) a
1083 /// constraint that `'a: 'b`. It is an error that we have no evidence that this
1084 /// constraint holds.
1085 ///
1086 /// If `propagated_outlives_requirements` is `Some`, then we will
1087 /// push unsatisfied obligations into there. Otherwise, we'll
1088 /// report them as errors.
1089 fn check_polonius_subset_errors(
1090 &self,
1091 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1092 errors_buffer: &mut RegionErrors<'tcx>,
1093 polonius_output: &PoloniusOutput,
1094 ) {
1095 debug!(
1096 "check_polonius_subset_errors: {} subset_errors",
1097 polonius_output.subset_errors.len()
1098 );
1099
1100 // Similarly to `check_universal_regions`: a free region relation, which was not explicitly
1101 // declared ("known") was found by Polonius, so emit an error, or propagate the
1102 // requirements for our caller into the `propagated_outlives_requirements` vector.
1103 //
1104 // Polonius doesn't model regions ("origins") as CFG-subsets or durations, but the
1105 // `longer_fr` and `shorter_fr` terminology will still be used here, for consistency with
1106 // the rest of the NLL infrastructure. The "subset origin" is the "longer free region",
1107 // and the "superset origin" is the outlived "shorter free region".
1108 //
1109 // Note: Polonius will produce a subset error at every point where the unexpected
1110 // `longer_fr`'s "placeholder loan" is contained in the `shorter_fr`. This can be helpful
1111 // for diagnostics in the future, e.g. to point more precisely at the key locations
1112 // requiring this constraint to hold. However, the error and diagnostics code downstream
1113 // expects that these errors are not duplicated (and that they are in a certain order).
1114 // Otherwise, diagnostics messages such as the ones giving names like `'1` to elided or
1115 // anonymous lifetimes for example, could give these names differently, while others like
1116 // the outlives suggestions or the debug output from `#[rustc_regions]` would be
1117 // duplicated. The polonius subset errors are deduplicated here, while keeping the
1118 // CFG-location ordering.
1119 // We can iterate the HashMap here because the result is sorted afterwards.
1120 #[allow(rustc::potential_query_instability)]
1121 let mut subset_errors: Vec<_> = polonius_output
1122 .subset_errors
1123 .iter()
1124 .flat_map(|(_location, subset_errors)| subset_errors.iter())
1125 .collect();
1126 subset_errors.sort();
1127 subset_errors.dedup();
1128
1129 for &(longer_fr, shorter_fr) in subset_errors.into_iter() {
1130 debug!(
1131 "check_polonius_subset_errors: subset_error longer_fr={:?},\
1132 shorter_fr={:?}",
1133 longer_fr, shorter_fr
1134 );
1135
1136 let propagated = self.try_propagate_universal_region_error(
1137 longer_fr.into(),
1138 shorter_fr.into(),
1139 &mut propagated_outlives_requirements,
1140 );
1141 if propagated == RegionRelationCheckResult::Error {
1142 errors_buffer.push(RegionErrorKind::RegionError {
1143 longer_fr: longer_fr.into(),
1144 shorter_fr: shorter_fr.into(),
1145 fr_origin: NllRegionVariableOrigin::FreeRegion,
1146 is_reported: true,
1147 });
1148 }
1149 }
1150
1151 // Handle the placeholder errors as usual, until the chalk-rustc-polonius triumvirate has
1152 // a more complete picture on how to separate this responsibility.
1153 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1154 match fr_definition.origin {
1155 NllRegionVariableOrigin::FreeRegion => {
1156 // handled by polonius above
1157 }
1158
1159 NllRegionVariableOrigin::Placeholder(placeholder) => {
1160 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1161 }
1162
1163 NllRegionVariableOrigin::Existential { .. } => {
1164 // nothing to check here
1165 }
1166 }
1167 }
1168 }
1169
1170 /// The largest universe of any region nameable from this SCC.
1171 fn max_nameable_universe(&self, scc: ConstraintSccIndex) -> UniverseIndex {
1172 self.scc_annotations[scc].max_nameable_universe()
1173 }
1174
1175 /// Checks the final value for the free region `fr` to see if it
1176 /// grew too large. In particular, examine what `end(X)` points
1177 /// wound up in `fr`'s final value; for each `end(X)` where `X !=
1178 /// fr`, we want to check that `fr: X`. If not, that's either an
1179 /// error, or something we have to propagate to our creator.
1180 ///
1181 /// Things that are to be propagated are accumulated into the
1182 /// `outlives_requirements` vector.
1183 #[instrument(skip(self, propagated_outlives_requirements, errors_buffer), level = "debug")]
1184 fn check_universal_region(
1185 &self,
1186 longer_fr: RegionVid,
1187 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1188 errors_buffer: &mut RegionErrors<'tcx>,
1189 ) {
1190 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1191
1192 // Because this free region must be in the ROOT universe, we
1193 // know it cannot contain any bound universes.
1194 assert!(self.max_nameable_universe(longer_fr_scc).is_root());
1195
1196 // Only check all of the relations for the main representative of each
1197 // SCC, otherwise just check that we outlive said representative. This
1198 // reduces the number of redundant relations propagated out of
1199 // closures.
1200 // Note that the representative will be a universal region if there is
1201 // one in this SCC, so we will always check the representative here.
1202 let representative = self.scc_representative(longer_fr_scc);
1203 if representative != longer_fr {
1204 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1205 longer_fr,
1206 representative,
1207 propagated_outlives_requirements,
1208 ) {
1209 errors_buffer.push(RegionErrorKind::RegionError {
1210 longer_fr,
1211 shorter_fr: representative,
1212 fr_origin: NllRegionVariableOrigin::FreeRegion,
1213 is_reported: true,
1214 });
1215 }
1216 return;
1217 }
1218
1219 // Find every region `o` such that `fr: o`
1220 // (because `fr` includes `end(o)`).
1221 let mut error_reported = false;
1222 for shorter_fr in self.scc_values.universal_regions_outlived_by(longer_fr_scc) {
1223 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1224 longer_fr,
1225 shorter_fr,
1226 propagated_outlives_requirements,
1227 ) {
1228 // We only report the first region error. Subsequent errors are hidden so as
1229 // not to overwhelm the user, but we do record them so as to potentially print
1230 // better diagnostics elsewhere...
1231 errors_buffer.push(RegionErrorKind::RegionError {
1232 longer_fr,
1233 shorter_fr,
1234 fr_origin: NllRegionVariableOrigin::FreeRegion,
1235 is_reported: !error_reported,
1236 });
1237
1238 error_reported = true;
1239 }
1240 }
1241 }
1242
1243 /// Checks that we can prove that `longer_fr: shorter_fr`. If we can't we attempt to propagate
1244 /// the constraint outward (e.g. to a closure environment), but if that fails, there is an
1245 /// error.
1246 fn check_universal_region_relation(
1247 &self,
1248 longer_fr: RegionVid,
1249 shorter_fr: RegionVid,
1250 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1251 ) -> RegionRelationCheckResult {
1252 // If it is known that `fr: o`, carry on.
1253 if self.universal_region_relations.outlives(longer_fr, shorter_fr) {
1254 RegionRelationCheckResult::Ok
1255 } else {
1256 // If we are not in a context where we can't propagate errors, or we
1257 // could not shrink `fr` to something smaller, then just report an
1258 // error.
1259 //
1260 // Note: in this case, we use the unapproximated regions to report the
1261 // error. This gives better error messages in some cases.
1262 self.try_propagate_universal_region_error(
1263 longer_fr,
1264 shorter_fr,
1265 propagated_outlives_requirements,
1266 )
1267 }
1268 }
1269
1270 /// Attempt to propagate a region error (e.g. `'a: 'b`) that is not met to a closure's
1271 /// creator. If we cannot, then the caller should report an error to the user.
1272 fn try_propagate_universal_region_error(
1273 &self,
1274 longer_fr: RegionVid,
1275 shorter_fr: RegionVid,
1276 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1277 ) -> RegionRelationCheckResult {
1278 if let Some(propagated_outlives_requirements) = propagated_outlives_requirements
1279 // Shrink `longer_fr` until we find a non-local region (if we do).
1280 // We'll call it `fr-` -- it's ever so slightly smaller than
1281 // `longer_fr`.
1282 && let Some(fr_minus) = self.universal_region_relations.non_local_lower_bound(longer_fr)
1283 {
1284 debug!("try_propagate_universal_region_error: fr_minus={:?}", fr_minus);
1285
1286 let blame_constraint = self
1287 .best_blame_constraint(longer_fr, NllRegionVariableOrigin::FreeRegion, shorter_fr)
1288 .0;
1289
1290 // Grow `shorter_fr` until we find some non-local regions. (We
1291 // always will.) We'll call them `shorter_fr+` -- they're ever
1292 // so slightly larger than `shorter_fr`.
1293 let shorter_fr_plus =
1294 self.universal_region_relations.non_local_upper_bounds(shorter_fr);
1295 debug!("try_propagate_universal_region_error: shorter_fr_plus={:?}", shorter_fr_plus);
1296 for fr in shorter_fr_plus {
1297 // Push the constraint `fr-: shorter_fr+`
1298 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
1299 subject: ClosureOutlivesSubject::Region(fr_minus),
1300 outlived_free_region: fr,
1301 blame_span: blame_constraint.cause.span,
1302 category: blame_constraint.category,
1303 });
1304 }
1305 return RegionRelationCheckResult::Propagated;
1306 }
1307
1308 RegionRelationCheckResult::Error
1309 }
1310
1311 fn check_bound_universal_region(
1312 &self,
1313 longer_fr: RegionVid,
1314 placeholder: ty::PlaceholderRegion,
1315 errors_buffer: &mut RegionErrors<'tcx>,
1316 ) {
1317 debug!("check_bound_universal_region(fr={:?}, placeholder={:?})", longer_fr, placeholder,);
1318
1319 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1320 debug!("check_bound_universal_region: longer_fr_scc={:?}", longer_fr_scc,);
1321
1322 // If we have some bound universal region `'a`, then the only
1323 // elements it can contain is itself -- we don't know anything
1324 // else about it!
1325 if let Some(error_element) = self
1326 .scc_values
1327 .elements_contained_in(longer_fr_scc)
1328 .find(|e| *e != RegionElement::PlaceholderRegion(placeholder))
1329 {
1330 // Stop after the first error, it gets too noisy otherwise, and does not provide more information.
1331 errors_buffer.push(RegionErrorKind::BoundUniversalRegionError {
1332 longer_fr,
1333 error_element,
1334 placeholder,
1335 });
1336 } else {
1337 debug!("check_bound_universal_region: all bounds satisfied");
1338 }
1339 }
1340
1341 pub(crate) fn constraint_path_between_regions(
1342 &self,
1343 from_region: RegionVid,
1344 to_region: RegionVid,
1345 ) -> Option<Vec<OutlivesConstraint<'tcx>>> {
1346 if from_region == to_region {
1347 bug!("Tried to find a path between {from_region:?} and itself!");
1348 }
1349 self.constraint_path_to(from_region, |to| to == to_region, true).map(|o| o.0)
1350 }
1351
1352 /// Walks the graph of constraints (where `'a: 'b` is considered
1353 /// an edge `'a -> 'b`) to find a path from `from_region` to
1354 /// `to_region`.
1355 ///
1356 /// Returns: a series of constraints as well as the region `R`
1357 /// that passed the target test.
1358 /// If `include_static_outlives_all` is `true`, then the synthetic
1359 /// outlives constraints `'static -> a` for every region `a` are
1360 /// considered in the search, otherwise they are ignored.
1361 #[instrument(skip(self, target_test), ret)]
1362 pub(crate) fn constraint_path_to(
1363 &self,
1364 from_region: RegionVid,
1365 target_test: impl Fn(RegionVid) -> bool,
1366 include_placeholder_static: bool,
1367 ) -> Option<(Vec<OutlivesConstraint<'tcx>>, RegionVid)> {
1368 self.find_constraint_path_between_regions_inner(
1369 true,
1370 from_region,
1371 &target_test,
1372 include_placeholder_static,
1373 )
1374 .or_else(|| {
1375 self.find_constraint_path_between_regions_inner(
1376 false,
1377 from_region,
1378 &target_test,
1379 include_placeholder_static,
1380 )
1381 })
1382 }
1383
1384 /// The constraints we get from equating the hidden type of each use of an opaque
1385 /// with its final concrete type may end up getting preferred over other, potentially
1386 /// longer constraint paths.
1387 ///
1388 /// Given that we compute the final concrete type by relying on this existing constraint
1389 /// path, this can easily end up hiding the actual reason for why we require these regions
1390 /// to be equal.
1391 ///
1392 /// To handle this, we first look at the path while ignoring these constraints and then
1393 /// retry while considering them. This is not perfect, as the `from_region` may have already
1394 /// been partially related to its argument region, so while we rely on a member constraint
1395 /// to get a complete path, the most relevant step of that path already existed before then.
1396 fn find_constraint_path_between_regions_inner(
1397 &self,
1398 ignore_opaque_type_constraints: bool,
1399 from_region: RegionVid,
1400 target_test: impl Fn(RegionVid) -> bool,
1401 include_placeholder_static: bool,
1402 ) -> Option<(Vec<OutlivesConstraint<'tcx>>, RegionVid)> {
1403 let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions);
1404 context[from_region] = Trace::StartRegion;
1405
1406 let fr_static = self.universal_regions().fr_static;
1407
1408 // Use a deque so that we do a breadth-first search. We will
1409 // stop at the first match, which ought to be the shortest
1410 // path (fewest constraints).
1411 let mut deque = VecDeque::new();
1412 deque.push_back(from_region);
1413
1414 while let Some(r) = deque.pop_front() {
1415 debug!(
1416 "constraint_path_to: from_region={:?} r={:?} value={}",
1417 from_region,
1418 r,
1419 self.region_value_str(r),
1420 );
1421
1422 // Check if we reached the region we were looking for. If so,
1423 // we can reconstruct the path that led to it and return it.
1424 if target_test(r) {
1425 let mut result = vec![];
1426 let mut p = r;
1427 // This loop is cold and runs at the end, which is why we delay
1428 // `OutlivesConstraint` construction until now.
1429 loop {
1430 match context[p] {
1431 Trace::FromGraph(c) => {
1432 p = c.sup;
1433 result.push(*c);
1434 }
1435
1436 Trace::FromStatic(sub) => {
1437 let c = OutlivesConstraint {
1438 sup: fr_static,
1439 sub,
1440 locations: Locations::All(DUMMY_SP),
1441 span: DUMMY_SP,
1442 category: ConstraintCategory::Internal,
1443 variance_info: ty::VarianceDiagInfo::default(),
1444 from_closure: false,
1445 };
1446 p = c.sup;
1447 result.push(c);
1448 }
1449
1450 Trace::StartRegion => {
1451 result.reverse();
1452 return Some((result, r));
1453 }
1454
1455 Trace::NotVisited => {
1456 bug!("found unvisited region {:?} on path to {:?}", p, r)
1457 }
1458 }
1459 }
1460 }
1461
1462 // Otherwise, walk over the outgoing constraints and
1463 // enqueue any regions we find, keeping track of how we
1464 // reached them.
1465
1466 // A constraint like `'r: 'x` can come from our constraint
1467 // graph.
1468
1469 // Always inline this closure because it can be hot.
1470 let mut handle_trace = #[inline(always)]
1471 |sub, trace| {
1472 if let Trace::NotVisited = context[sub] {
1473 context[sub] = trace;
1474 deque.push_back(sub);
1475 }
1476 };
1477
1478 // If this is the `'static` region and the graph's direction is normal, then set up the
1479 // Edges iterator to return all regions (#53178).
1480 if r == fr_static && self.constraint_graph.is_normal() {
1481 for sub in self.constraint_graph.outgoing_edges_from_static() {
1482 handle_trace(sub, Trace::FromStatic(sub));
1483 }
1484 } else {
1485 let edges = self.constraint_graph.outgoing_edges_from_graph(r, &self.constraints);
1486 // This loop can be hot.
1487 for constraint in edges {
1488 match constraint.category {
1489 ConstraintCategory::OutlivesUnnameablePlaceholder(_)
1490 if !include_placeholder_static =>
1491 {
1492 debug!("Ignoring illegal placeholder constraint: {constraint:?}");
1493 continue;
1494 }
1495 ConstraintCategory::OpaqueType if ignore_opaque_type_constraints => {
1496 debug!("Ignoring member constraint: {constraint:?}");
1497 continue;
1498 }
1499 _ => {}
1500 }
1501
1502 debug_assert_eq!(constraint.sup, r);
1503 handle_trace(constraint.sub, Trace::FromGraph(constraint));
1504 }
1505 }
1506 }
1507
1508 None
1509 }
1510
1511 /// Finds some region R such that `fr1: R` and `R` is live at `location`.
1512 #[instrument(skip(self), level = "trace", ret)]
1513 pub(crate) fn find_sub_region_live_at(&self, fr1: RegionVid, location: Location) -> RegionVid {
1514 trace!(scc = ?self.constraint_sccs.scc(fr1));
1515 trace!(universe = ?self.max_nameable_universe(self.constraint_sccs.scc(fr1)));
1516 self.constraint_path_to(fr1, |r| {
1517 trace!(?r, liveness_constraints=?self.liveness_constraints.pretty_print_live_points(r));
1518 self.liveness_constraints.is_live_at(r, location)
1519 }, true).unwrap().1
1520 }
1521
1522 /// Get the region outlived by `longer_fr` and live at `element`.
1523 pub(crate) fn region_from_element(
1524 &self,
1525 longer_fr: RegionVid,
1526 element: &RegionElement,
1527 ) -> RegionVid {
1528 match *element {
1529 RegionElement::Location(l) => self.find_sub_region_live_at(longer_fr, l),
1530 RegionElement::RootUniversalRegion(r) => r,
1531 RegionElement::PlaceholderRegion(error_placeholder) => self
1532 .definitions
1533 .iter_enumerated()
1534 .find_map(|(r, definition)| match definition.origin {
1535 NllRegionVariableOrigin::Placeholder(p) if p == error_placeholder => Some(r),
1536 _ => None,
1537 })
1538 .unwrap(),
1539 }
1540 }
1541
1542 /// Get the region definition of `r`.
1543 pub(crate) fn region_definition(&self, r: RegionVid) -> &RegionDefinition<'tcx> {
1544 &self.definitions[r]
1545 }
1546
1547 /// Check if the SCC of `r` contains `upper`.
1548 pub(crate) fn upper_bound_in_region_scc(&self, r: RegionVid, upper: RegionVid) -> bool {
1549 let r_scc = self.constraint_sccs.scc(r);
1550 self.scc_values.contains(r_scc, upper)
1551 }
1552
1553 pub(crate) fn universal_regions(&self) -> &UniversalRegions<'tcx> {
1554 &self.universal_region_relations.universal_regions
1555 }
1556
1557 /// Tries to find the best constraint to blame for the fact that
1558 /// `R: from_region`, where `R` is some region that meets
1559 /// `target_test`. This works by following the constraint graph,
1560 /// creating a constraint path that forces `R` to outlive
1561 /// `from_region`, and then finding the best choices within that
1562 /// path to blame.
1563 #[instrument(level = "debug", skip(self))]
1564 pub(crate) fn best_blame_constraint(
1565 &self,
1566 from_region: RegionVid,
1567 from_region_origin: NllRegionVariableOrigin,
1568 to_region: RegionVid,
1569 ) -> (BlameConstraint<'tcx>, Vec<OutlivesConstraint<'tcx>>) {
1570 assert!(from_region != to_region, "Trying to blame a region for itself!");
1571
1572 let path = self.constraint_path_between_regions(from_region, to_region).unwrap();
1573
1574 // If we are passing through a constraint added because we reached an unnameable placeholder `'unnameable`,
1575 // redirect search towards `'unnameable`.
1576 let due_to_placeholder_outlives = path.iter().find_map(|c| {
1577 if let ConstraintCategory::OutlivesUnnameablePlaceholder(unnameable) = c.category {
1578 Some(unnameable)
1579 } else {
1580 None
1581 }
1582 });
1583
1584 // Edge case: it's possible that `'from_region` is an unnameable placeholder.
1585 let path = if let Some(unnameable) = due_to_placeholder_outlives
1586 && unnameable != from_region
1587 {
1588 // We ignore the extra edges due to unnameable placeholders to get
1589 // an explanation that was present in the original constraint graph.
1590 self.constraint_path_to(from_region, |r| r == unnameable, false).unwrap().0
1591 } else {
1592 path
1593 };
1594
1595 debug!(
1596 "path={:#?}",
1597 path.iter()
1598 .map(|c| format!(
1599 "{:?} ({:?}: {:?})",
1600 c,
1601 self.constraint_sccs.scc(c.sup),
1602 self.constraint_sccs.scc(c.sub),
1603 ))
1604 .collect::<Vec<_>>()
1605 );
1606
1607 // We try to avoid reporting a `ConstraintCategory::Predicate` as our best constraint.
1608 // Instead, we use it to produce an improved `ObligationCauseCode`.
1609 // FIXME - determine what we should do if we encounter multiple
1610 // `ConstraintCategory::Predicate` constraints. Currently, we just pick the first one.
1611 let cause_code = path
1612 .iter()
1613 .find_map(|constraint| {
1614 if let ConstraintCategory::Predicate(predicate_span) = constraint.category {
1615 // We currently do not store the `DefId` in the `ConstraintCategory`
1616 // for performances reasons. The error reporting code used by NLL only
1617 // uses the span, so this doesn't cause any problems at the moment.
1618 Some(ObligationCauseCode::WhereClause(CRATE_DEF_ID.to_def_id(), predicate_span))
1619 } else {
1620 None
1621 }
1622 })
1623 .unwrap_or_else(|| ObligationCauseCode::Misc);
1624
1625 // When reporting an error, there is typically a chain of constraints leading from some
1626 // "source" region which must outlive some "target" region.
1627 // In most cases, we prefer to "blame" the constraints closer to the target --
1628 // but there is one exception. When constraints arise from higher-ranked subtyping,
1629 // we generally prefer to blame the source value,
1630 // as the "target" in this case tends to be some type annotation that the user gave.
1631 // Therefore, if we find that the region origin is some instantiation
1632 // of a higher-ranked region, we start our search from the "source" point
1633 // rather than the "target", and we also tweak a few other things.
1634 //
1635 // An example might be this bit of Rust code:
1636 //
1637 // ```rust
1638 // let x: fn(&'static ()) = |_| {};
1639 // let y: for<'a> fn(&'a ()) = x;
1640 // ```
1641 //
1642 // In MIR, this will be converted into a combination of assignments and type ascriptions.
1643 // In particular, the 'static is imposed through a type ascription:
1644 //
1645 // ```rust
1646 // x = ...;
1647 // AscribeUserType(x, fn(&'static ())
1648 // y = x;
1649 // ```
1650 //
1651 // We wind up ultimately with constraints like
1652 //
1653 // ```rust
1654 // !a: 'temp1 // from the `y = x` statement
1655 // 'temp1: 'temp2
1656 // 'temp2: 'static // from the AscribeUserType
1657 // ```
1658 //
1659 // and here we prefer to blame the source (the y = x statement).
1660 let blame_source = match from_region_origin {
1661 NllRegionVariableOrigin::FreeRegion => true,
1662 NllRegionVariableOrigin::Placeholder(_) => false,
1663 // `'existential: 'whatever` never results in a region error by itself.
1664 // We may always infer it to `'static` afterall. This means while an error
1665 // path may go through an existential, these existentials are never the
1666 // `from_region`.
1667 NllRegionVariableOrigin::Existential { name: _ } => {
1668 unreachable!("existentials can outlive everything")
1669 }
1670 };
1671
1672 // To pick a constraint to blame, we organize constraints by how interesting we expect them
1673 // to be in diagnostics, then pick the most interesting one closest to either the source or
1674 // the target on our constraint path.
1675 let constraint_interest = |constraint: &OutlivesConstraint<'tcx>| {
1676 // Try to avoid blaming constraints from desugarings, since they may not clearly match
1677 // match what users have written. As an exception, allow blaming returns generated by
1678 // `?` desugaring, since the correspondence is fairly clear.
1679 let category = if let Some(kind) = constraint.span.desugaring_kind()
1680 && (kind != DesugaringKind::QuestionMark
1681 || !matches!(constraint.category, ConstraintCategory::Return(_)))
1682 {
1683 ConstraintCategory::Boring
1684 } else {
1685 constraint.category
1686 };
1687
1688 let interest = match category {
1689 // Returns usually provide a type to blame and have specially written diagnostics,
1690 // so prioritize them.
1691 ConstraintCategory::Return(_) => 0,
1692 // Unsizing coercions are interesting, since we have a note for that:
1693 // `BorrowExplanation::add_object_lifetime_default_note`.
1694 // FIXME(dianne): That note shouldn't depend on a coercion being blamed; see issue
1695 // #131008 for an example of where we currently don't emit it but should.
1696 // Once the note is handled properly, this case should be removed. Until then, it
1697 // should be as limited as possible; the note is prone to false positives and this
1698 // constraint usually isn't best to blame.
1699 ConstraintCategory::Cast {
1700 unsize_to: Some(unsize_ty),
1701 is_implicit_coercion: true,
1702 } if to_region == self.universal_regions().fr_static
1703 // Mirror the note's condition, to minimize how often this diverts blame.
1704 && let ty::Adt(_, args) = unsize_ty.kind()
1705 && args.iter().any(|arg| arg.as_type().is_some_and(|ty| ty.is_trait()))
1706 // Mimic old logic for this, to minimize false positives in tests.
1707 && !path
1708 .iter()
1709 .any(|c| matches!(c.category, ConstraintCategory::TypeAnnotation(_))) =>
1710 {
1711 1
1712 }
1713 // Between other interesting constraints, order by their position on the `path`.
1714 ConstraintCategory::Yield
1715 | ConstraintCategory::UseAsConst
1716 | ConstraintCategory::UseAsStatic
1717 | ConstraintCategory::TypeAnnotation(
1718 AnnotationSource::Ascription
1719 | AnnotationSource::Declaration
1720 | AnnotationSource::OpaqueCast,
1721 )
1722 | ConstraintCategory::Cast { .. }
1723 | ConstraintCategory::CallArgument(_)
1724 | ConstraintCategory::CopyBound
1725 | ConstraintCategory::SizedBound
1726 | ConstraintCategory::Assignment
1727 | ConstraintCategory::Usage
1728 | ConstraintCategory::ClosureUpvar(_) => 2,
1729 // Generic arguments are unlikely to be what relates regions together
1730 ConstraintCategory::TypeAnnotation(AnnotationSource::GenericArg) => 3,
1731 // We handle predicates and opaque types specially; don't prioritize them here.
1732 ConstraintCategory::Predicate(_) | ConstraintCategory::OpaqueType => 4,
1733 // `Boring` constraints can correspond to user-written code and have useful spans,
1734 // but don't provide any other useful information for diagnostics.
1735 ConstraintCategory::Boring => 5,
1736 // `BoringNoLocation` constraints can point to user-written code, but are less
1737 // specific, and are not used for relations that would make sense to blame.
1738 ConstraintCategory::BoringNoLocation => 6,
1739 // Do not blame internal constraints.
1740 ConstraintCategory::OutlivesUnnameablePlaceholder(_) => 7,
1741 ConstraintCategory::Internal => 8,
1742 };
1743
1744 debug!("constraint {constraint:?} category: {category:?}, interest: {interest:?}");
1745
1746 interest
1747 };
1748
1749 let best_choice = if blame_source {
1750 path.iter().enumerate().rev().min_by_key(|(_, c)| constraint_interest(c)).unwrap().0
1751 } else {
1752 path.iter().enumerate().min_by_key(|(_, c)| constraint_interest(c)).unwrap().0
1753 };
1754
1755 debug!(?best_choice, ?blame_source);
1756
1757 let best_constraint = if let Some(next) = path.get(best_choice + 1)
1758 && matches!(path[best_choice].category, ConstraintCategory::Return(_))
1759 && next.category == ConstraintCategory::OpaqueType
1760 {
1761 // The return expression is being influenced by the return type being
1762 // impl Trait, point at the return type and not the return expr.
1763 *next
1764 } else if path[best_choice].category == ConstraintCategory::Return(ReturnConstraint::Normal)
1765 && let Some(field) = path.iter().find_map(|p| {
1766 if let ConstraintCategory::ClosureUpvar(f) = p.category { Some(f) } else { None }
1767 })
1768 {
1769 OutlivesConstraint {
1770 category: ConstraintCategory::Return(ReturnConstraint::ClosureUpvar(field)),
1771 ..path[best_choice]
1772 }
1773 } else {
1774 path[best_choice]
1775 };
1776
1777 assert!(
1778 !matches!(
1779 best_constraint.category,
1780 ConstraintCategory::OutlivesUnnameablePlaceholder(_)
1781 ),
1782 "Illegal placeholder constraint blamed; should have redirected to other region relation"
1783 );
1784
1785 let blame_constraint = BlameConstraint {
1786 category: best_constraint.category,
1787 from_closure: best_constraint.from_closure,
1788 cause: ObligationCause::new(best_constraint.span, CRATE_DEF_ID, cause_code.clone()),
1789 variance_info: best_constraint.variance_info,
1790 };
1791 (blame_constraint, path)
1792 }
1793
1794 pub(crate) fn universe_info(&self, universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
1795 // Query canonicalization can create local superuniverses (for example in
1796 // `InferCtx::query_response_instantiation_guess`), but they don't have an associated
1797 // `UniverseInfo` explaining why they were created.
1798 // This can cause ICEs if these causes are accessed in diagnostics, for example in issue
1799 // #114907 where this happens via liveness and dropck outlives results.
1800 // Therefore, we return a default value in case that happens, which should at worst emit a
1801 // suboptimal error, instead of the ICE.
1802 self.universe_causes.get(&universe).cloned().unwrap_or_else(UniverseInfo::other)
1803 }
1804
1805 /// Tries to find the terminator of the loop in which the region 'r' resides.
1806 /// Returns the location of the terminator if found.
1807 pub(crate) fn find_loop_terminator_location(
1808 &self,
1809 r: RegionVid,
1810 body: &Body<'_>,
1811 ) -> Option<Location> {
1812 let scc = self.constraint_sccs.scc(r);
1813 let locations = self.scc_values.locations_outlived_by(scc);
1814 for location in locations {
1815 let bb = &body[location.block];
1816 if let Some(terminator) = &bb.terminator
1817 // terminator of a loop should be TerminatorKind::FalseUnwind
1818 && let TerminatorKind::FalseUnwind { .. } = terminator.kind
1819 {
1820 return Some(location);
1821 }
1822 }
1823 None
1824 }
1825
1826 /// Access to the SCC constraint graph.
1827 /// This can be used to quickly under-approximate the regions which are equal to each other
1828 /// and their relative orderings.
1829 // This is `pub` because it's used by unstable external borrowck data users, see `consumers.rs`.
1830 pub fn constraint_sccs(&self) -> &ConstraintSccs {
1831 &self.constraint_sccs
1832 }
1833
1834 /// Returns the representative `RegionVid` for a given SCC.
1835 /// See `RegionTracker` for how a region variable ID is chosen.
1836 ///
1837 /// It is a hacky way to manage checking regions for equality,
1838 /// since we can 'canonicalize' each region to the representative
1839 /// of its SCC and be sure that -- if they have the same repr --
1840 /// they *must* be equal (though not having the same repr does not
1841 /// mean they are unequal).
1842 fn scc_representative(&self, scc: ConstraintSccIndex) -> RegionVid {
1843 self.scc_annotations[scc].representative.rvid()
1844 }
1845
1846 pub(crate) fn liveness_constraints(&self) -> &LivenessValues {
1847 &self.liveness_constraints
1848 }
1849
1850 /// When using `-Zpolonius=next`, records the given live loans for the loan scopes and active
1851 /// loans dataflow computations.
1852 pub(crate) fn record_live_loans(&mut self, live_loans: LiveLoans) {
1853 self.liveness_constraints.record_live_loans(live_loans);
1854 }
1855
1856 /// Returns whether the `loan_idx` is live at the given `location`: whether its issuing
1857 /// region is contained within the type of a variable that is live at this point.
1858 /// Note: for now, the sets of live loans is only available when using `-Zpolonius=next`.
1859 pub(crate) fn is_loan_live_at(&self, loan_idx: BorrowIndex, location: Location) -> bool {
1860 let point = self.liveness_constraints.point_from_location(location);
1861 self.liveness_constraints.is_loan_live_at(loan_idx, point)
1862 }
1863}
1864
1865#[derive(Clone, Debug)]
1866pub(crate) struct BlameConstraint<'tcx> {
1867 pub category: ConstraintCategory<'tcx>,
1868 pub from_closure: bool,
1869 pub cause: ObligationCause<'tcx>,
1870 pub variance_info: ty::VarianceDiagInfo<TyCtxt<'tcx>>,
1871}