rustc_borrowck/region_infer/mod.rs
1use std::cell::OnceCell;
2use std::collections::VecDeque;
3use std::rc::Rc;
4
5use rustc_data_structures::binary_search_util;
6use rustc_data_structures::frozen::Frozen;
7use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
8use rustc_data_structures::graph::scc::{self, Sccs};
9use rustc_errors::Diag;
10use rustc_hir::def_id::CRATE_DEF_ID;
11use rustc_index::IndexVec;
12use rustc_infer::infer::outlives::test_type_match;
13use rustc_infer::infer::region_constraints::{GenericKind, VerifyBound, VerifyIfEq};
14use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin};
15use rustc_middle::bug;
16use rustc_middle::mir::{
17 AnnotationSource, BasicBlock, Body, ConstraintCategory, Local, Location, ReturnConstraint,
18 TerminatorKind,
19};
20use rustc_middle::traits::{ObligationCause, ObligationCauseCode};
21use rustc_middle::ty::{self, RegionVid, Ty, TyCtxt, TypeFoldable, UniverseIndex, fold_regions};
22use rustc_mir_dataflow::points::DenseLocationMap;
23use rustc_span::hygiene::DesugaringKind;
24use rustc_span::{DUMMY_SP, Span};
25use tracing::{Level, debug, enabled, instrument, trace};
26
27use crate::constraints::graph::{self, NormalConstraintGraph, RegionGraph};
28use crate::constraints::{ConstraintSccIndex, OutlivesConstraint, OutlivesConstraintSet};
29use crate::dataflow::BorrowIndex;
30use crate::diagnostics::{RegionErrorKind, RegionErrors, UniverseInfo};
31use crate::member_constraints::{MemberConstraintSet, NllMemberConstraintIndex};
32use crate::polonius::LiveLoans;
33use crate::polonius::legacy::PoloniusOutput;
34use crate::region_infer::reverse_sccs::ReverseSccGraph;
35use crate::region_infer::values::{LivenessValues, RegionElement, RegionValues, ToElementIndex};
36use crate::type_check::free_region_relations::UniversalRegionRelations;
37use crate::type_check::{Locations, MirTypeckRegionConstraints};
38use crate::universal_regions::UniversalRegions;
39use crate::{
40 BorrowckInferCtxt, ClosureOutlivesRequirement, ClosureOutlivesSubject,
41 ClosureOutlivesSubjectTy, ClosureRegionRequirements,
42};
43
44mod dump_mir;
45mod graphviz;
46mod opaque_types;
47mod reverse_sccs;
48
49pub(crate) mod values;
50
51pub(crate) type ConstraintSccs = Sccs<RegionVid, ConstraintSccIndex>;
52pub(crate) type AnnotatedSccs = (ConstraintSccs, IndexVec<ConstraintSccIndex, RegionTracker>);
53
54/// An annotation for region graph SCCs that tracks
55/// the values of its elements. This annotates a single SCC.
56#[derive(Copy, Debug, Clone)]
57pub(crate) struct RegionTracker {
58 /// The largest universe of a placeholder reached from this SCC.
59 /// This includes placeholders within this SCC.
60 max_placeholder_universe_reached: UniverseIndex,
61
62 /// The smallest universe index reachable form the nodes of this SCC.
63 min_reachable_universe: UniverseIndex,
64
65 /// The representative Region Variable Id for this SCC. We prefer
66 /// placeholders over existentially quantified variables, otherwise
67 /// it's the one with the smallest Region Variable ID.
68 pub(crate) representative: RegionVid,
69
70 /// Is the current representative a placeholder?
71 representative_is_placeholder: bool,
72
73 /// Is the current representative existentially quantified?
74 representative_is_existential: bool,
75}
76
77impl scc::Annotation for RegionTracker {
78 fn merge_scc(mut self, mut other: Self) -> Self {
79 // Prefer any placeholder over any existential
80 if other.representative_is_placeholder && self.representative_is_existential {
81 other.merge_min_max_seen(&self);
82 return other;
83 }
84
85 if self.representative_is_placeholder && other.representative_is_existential
86 || (self.representative <= other.representative)
87 {
88 self.merge_min_max_seen(&other);
89 return self;
90 }
91 other.merge_min_max_seen(&self);
92 other
93 }
94
95 fn merge_reached(mut self, other: Self) -> Self {
96 // No update to in-component values, only add seen values.
97 self.merge_min_max_seen(&other);
98 self
99 }
100}
101
102/// A Visitor for SCC annotation construction.
103pub(crate) struct SccAnnotations<'d, 'tcx, A: scc::Annotation> {
104 pub(crate) scc_to_annotation: IndexVec<ConstraintSccIndex, A>,
105 definitions: &'d IndexVec<RegionVid, RegionDefinition<'tcx>>,
106}
107
108impl<'d, 'tcx, A: scc::Annotation> SccAnnotations<'d, 'tcx, A> {
109 pub(crate) fn new(definitions: &'d IndexVec<RegionVid, RegionDefinition<'tcx>>) -> Self {
110 Self { scc_to_annotation: IndexVec::new(), definitions }
111 }
112}
113
114impl scc::Annotations<RegionVid> for SccAnnotations<'_, '_, RegionTracker> {
115 fn new(&self, element: RegionVid) -> RegionTracker {
116 RegionTracker::new(element, &self.definitions[element])
117 }
118
119 fn annotate_scc(&mut self, scc: ConstraintSccIndex, annotation: RegionTracker) {
120 let idx = self.scc_to_annotation.push(annotation);
121 assert!(idx == scc);
122 }
123
124 type Ann = RegionTracker;
125 type SccIdx = ConstraintSccIndex;
126}
127
128impl RegionTracker {
129 pub(crate) fn new(rvid: RegionVid, definition: &RegionDefinition<'_>) -> Self {
130 let (representative_is_placeholder, representative_is_existential) = match definition.origin
131 {
132 NllRegionVariableOrigin::FreeRegion => (false, false),
133 NllRegionVariableOrigin::Placeholder(_) => (true, false),
134 NllRegionVariableOrigin::Existential { .. } => (false, true),
135 };
136
137 let placeholder_universe =
138 if representative_is_placeholder { definition.universe } else { UniverseIndex::ROOT };
139
140 Self {
141 max_placeholder_universe_reached: placeholder_universe,
142 min_reachable_universe: definition.universe,
143 representative: rvid,
144 representative_is_placeholder,
145 representative_is_existential,
146 }
147 }
148
149 /// The smallest-indexed universe reachable from and/or in this SCC.
150 fn min_universe(self) -> UniverseIndex {
151 self.min_reachable_universe
152 }
153
154 fn merge_min_max_seen(&mut self, other: &Self) {
155 self.max_placeholder_universe_reached = std::cmp::max(
156 self.max_placeholder_universe_reached,
157 other.max_placeholder_universe_reached,
158 );
159
160 self.min_reachable_universe =
161 std::cmp::min(self.min_reachable_universe, other.min_reachable_universe);
162 }
163
164 /// Returns `true` if during the annotated SCC reaches a placeholder
165 /// with a universe larger than the smallest reachable one, `false` otherwise.
166 pub(crate) fn has_incompatible_universes(&self) -> bool {
167 self.min_universe().cannot_name(self.max_placeholder_universe_reached)
168 }
169}
170
171pub struct RegionInferenceContext<'tcx> {
172 /// Contains the definition for every region variable. Region
173 /// variables are identified by their index (`RegionVid`). The
174 /// definition contains information about where the region came
175 /// from as well as its final inferred value.
176 pub(crate) definitions: Frozen<IndexVec<RegionVid, RegionDefinition<'tcx>>>,
177
178 /// The liveness constraints added to each region. For most
179 /// regions, these start out empty and steadily grow, though for
180 /// each universally quantified region R they start out containing
181 /// the entire CFG and `end(R)`.
182 liveness_constraints: LivenessValues,
183
184 /// The outlives constraints computed by the type-check.
185 constraints: Frozen<OutlivesConstraintSet<'tcx>>,
186
187 /// The constraint-set, but in graph form, making it easy to traverse
188 /// the constraints adjacent to a particular region. Used to construct
189 /// the SCC (see `constraint_sccs`) and for error reporting.
190 constraint_graph: Frozen<NormalConstraintGraph>,
191
192 /// The SCC computed from `constraints` and the constraint
193 /// graph. We have an edge from SCC A to SCC B if `A: B`. Used to
194 /// compute the values of each region.
195 constraint_sccs: ConstraintSccs,
196
197 scc_annotations: IndexVec<ConstraintSccIndex, RegionTracker>,
198
199 /// Reverse of the SCC constraint graph -- i.e., an edge `A -> B` exists if
200 /// `B: A`. This is used to compute the universal regions that are required
201 /// to outlive a given SCC.
202 rev_scc_graph: OnceCell<ReverseSccGraph>,
203
204 /// The "R0 member of [R1..Rn]" constraints, indexed by SCC.
205 member_constraints: Rc<MemberConstraintSet<'tcx, ConstraintSccIndex>>,
206
207 /// Records the member constraints that we applied to each scc.
208 /// This is useful for error reporting. Once constraint
209 /// propagation is done, this vector is sorted according to
210 /// `member_region_scc`.
211 member_constraints_applied: Vec<AppliedMemberConstraint>,
212
213 /// Map universe indexes to information on why we created it.
214 universe_causes: FxIndexMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
215
216 /// The final inferred values of the region variables; we compute
217 /// one value per SCC. To get the value for any given *region*,
218 /// you first find which scc it is a part of.
219 scc_values: RegionValues<ConstraintSccIndex>,
220
221 /// Type constraints that we check after solving.
222 type_tests: Vec<TypeTest<'tcx>>,
223
224 /// Information about how the universally quantified regions in
225 /// scope on this function relate to one another.
226 universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
227}
228
229/// Each time that `apply_member_constraint` is successful, it appends
230/// one of these structs to the `member_constraints_applied` field.
231/// This is used in error reporting to trace out what happened.
232///
233/// The way that `apply_member_constraint` works is that it effectively
234/// adds a new lower bound to the SCC it is analyzing: so you wind up
235/// with `'R: 'O` where `'R` is the pick-region and `'O` is the
236/// minimal viable option.
237#[derive(Debug)]
238pub(crate) struct AppliedMemberConstraint {
239 /// The SCC that was affected. (The "member region".)
240 ///
241 /// The vector if `AppliedMemberConstraint` elements is kept sorted
242 /// by this field.
243 pub(crate) member_region_scc: ConstraintSccIndex,
244
245 /// The "best option" that `apply_member_constraint` found -- this was
246 /// added as an "ad-hoc" lower-bound to `member_region_scc`.
247 pub(crate) min_choice: ty::RegionVid,
248
249 /// The "member constraint index" -- we can find out details about
250 /// the constraint from
251 /// `set.member_constraints[member_constraint_index]`.
252 pub(crate) member_constraint_index: NllMemberConstraintIndex,
253}
254
255#[derive(Debug)]
256pub(crate) struct RegionDefinition<'tcx> {
257 /// What kind of variable is this -- a free region? existential
258 /// variable? etc. (See the `NllRegionVariableOrigin` for more
259 /// info.)
260 pub(crate) origin: NllRegionVariableOrigin,
261
262 /// Which universe is this region variable defined in? This is
263 /// most often `ty::UniverseIndex::ROOT`, but when we encounter
264 /// forall-quantifiers like `for<'a> { 'a = 'b }`, we would create
265 /// the variable for `'a` in a fresh universe that extends ROOT.
266 pub(crate) universe: ty::UniverseIndex,
267
268 /// If this is 'static or an early-bound region, then this is
269 /// `Some(X)` where `X` is the name of the region.
270 pub(crate) external_name: Option<ty::Region<'tcx>>,
271}
272
273/// N.B., the variants in `Cause` are intentionally ordered. Lower
274/// values are preferred when it comes to error messages. Do not
275/// reorder willy nilly.
276#[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
277pub(crate) enum Cause {
278 /// point inserted because Local was live at the given Location
279 LiveVar(Local, Location),
280
281 /// point inserted because Local was dropped at the given Location
282 DropVar(Local, Location),
283}
284
285/// A "type test" corresponds to an outlives constraint between a type
286/// and a lifetime, like `T: 'x` or `<T as Foo>::Bar: 'x`. They are
287/// translated from the `Verify` region constraints in the ordinary
288/// inference context.
289///
290/// These sorts of constraints are handled differently than ordinary
291/// constraints, at least at present. During type checking, the
292/// `InferCtxt::process_registered_region_obligations` method will
293/// attempt to convert a type test like `T: 'x` into an ordinary
294/// outlives constraint when possible (for example, `&'a T: 'b` will
295/// be converted into `'a: 'b` and registered as a `Constraint`).
296///
297/// In some cases, however, there are outlives relationships that are
298/// not converted into a region constraint, but rather into one of
299/// these "type tests". The distinction is that a type test does not
300/// influence the inference result, but instead just examines the
301/// values that we ultimately inferred for each region variable and
302/// checks that they meet certain extra criteria. If not, an error
303/// can be issued.
304///
305/// One reason for this is that these type tests typically boil down
306/// to a check like `'a: 'x` where `'a` is a universally quantified
307/// region -- and therefore not one whose value is really meant to be
308/// *inferred*, precisely (this is not always the case: one can have a
309/// type test like `<Foo as Trait<'?0>>::Bar: 'x`, where `'?0` is an
310/// inference variable). Another reason is that these type tests can
311/// involve *disjunction* -- that is, they can be satisfied in more
312/// than one way.
313///
314/// For more information about this translation, see
315/// `InferCtxt::process_registered_region_obligations` and
316/// `InferCtxt::type_must_outlive` in `rustc_infer::infer::InferCtxt`.
317#[derive(Clone, Debug)]
318pub(crate) struct TypeTest<'tcx> {
319 /// The type `T` that must outlive the region.
320 pub generic_kind: GenericKind<'tcx>,
321
322 /// The region `'x` that the type must outlive.
323 pub lower_bound: RegionVid,
324
325 /// The span to blame.
326 pub span: Span,
327
328 /// A test which, if met by the region `'x`, proves that this type
329 /// constraint is satisfied.
330 pub verify_bound: VerifyBound<'tcx>,
331}
332
333/// When we have an unmet lifetime constraint, we try to propagate it outward (e.g. to a closure
334/// environment). If we can't, it is an error.
335#[derive(Clone, Copy, Debug, Eq, PartialEq)]
336enum RegionRelationCheckResult {
337 Ok,
338 Propagated,
339 Error,
340}
341
342#[derive(Clone, PartialEq, Eq, Debug)]
343enum Trace<'a, 'tcx> {
344 StartRegion,
345 FromGraph(&'a OutlivesConstraint<'tcx>),
346 FromStatic(RegionVid),
347 FromMember(RegionVid, RegionVid, Span),
348 NotVisited,
349}
350
351#[instrument(skip(infcx, sccs), level = "debug")]
352fn sccs_info<'tcx>(infcx: &BorrowckInferCtxt<'tcx>, sccs: &ConstraintSccs) {
353 use crate::renumber::RegionCtxt;
354
355 let var_to_origin = infcx.reg_var_to_origin.borrow();
356
357 let mut var_to_origin_sorted = var_to_origin.clone().into_iter().collect::<Vec<_>>();
358 var_to_origin_sorted.sort_by_key(|vto| vto.0);
359
360 if enabled!(Level::DEBUG) {
361 let mut reg_vars_to_origins_str = "region variables to origins:\n".to_string();
362 for (reg_var, origin) in var_to_origin_sorted.into_iter() {
363 reg_vars_to_origins_str.push_str(&format!("{reg_var:?}: {origin:?}\n"));
364 }
365 debug!("{}", reg_vars_to_origins_str);
366 }
367
368 let num_components = sccs.num_sccs();
369 let mut components = vec![FxIndexSet::default(); num_components];
370
371 for (reg_var, scc_idx) in sccs.scc_indices().iter_enumerated() {
372 let origin = var_to_origin.get(®_var).unwrap_or(&RegionCtxt::Unknown);
373 components[scc_idx.as_usize()].insert((reg_var, *origin));
374 }
375
376 if enabled!(Level::DEBUG) {
377 let mut components_str = "strongly connected components:".to_string();
378 for (scc_idx, reg_vars_origins) in components.iter().enumerate() {
379 let regions_info = reg_vars_origins.clone().into_iter().collect::<Vec<_>>();
380 components_str.push_str(&format!(
381 "{:?}: {:?},\n)",
382 ConstraintSccIndex::from_usize(scc_idx),
383 regions_info,
384 ))
385 }
386 debug!("{}", components_str);
387 }
388
389 // calculate the best representative for each component
390 let components_representatives = components
391 .into_iter()
392 .enumerate()
393 .map(|(scc_idx, region_ctxts)| {
394 let repr = region_ctxts
395 .into_iter()
396 .map(|reg_var_origin| reg_var_origin.1)
397 .max_by(|x, y| x.preference_value().cmp(&y.preference_value()))
398 .unwrap();
399
400 (ConstraintSccIndex::from_usize(scc_idx), repr)
401 })
402 .collect::<FxIndexMap<_, _>>();
403
404 let mut scc_node_to_edges = FxIndexMap::default();
405 for (scc_idx, repr) in components_representatives.iter() {
406 let edge_representatives = sccs
407 .successors(*scc_idx)
408 .iter()
409 .map(|scc_idx| components_representatives[scc_idx])
410 .collect::<Vec<_>>();
411 scc_node_to_edges.insert((scc_idx, repr), edge_representatives);
412 }
413
414 debug!("SCC edges {:#?}", scc_node_to_edges);
415}
416
417fn create_definitions<'tcx>(
418 infcx: &BorrowckInferCtxt<'tcx>,
419 universal_regions: &UniversalRegions<'tcx>,
420) -> Frozen<IndexVec<RegionVid, RegionDefinition<'tcx>>> {
421 // Create a RegionDefinition for each inference variable.
422 let mut definitions: IndexVec<_, _> = infcx
423 .get_region_var_infos()
424 .iter()
425 .map(|info| RegionDefinition::new(info.universe, info.origin))
426 .collect();
427
428 // Add the external name for all universal regions.
429 for (external_name, variable) in universal_regions.named_universal_regions_iter() {
430 debug!("region {variable:?} has external name {external_name:?}");
431 definitions[variable].external_name = Some(external_name);
432 }
433
434 Frozen::freeze(definitions)
435}
436
437impl<'tcx> RegionInferenceContext<'tcx> {
438 /// Creates a new region inference context with a total of
439 /// `num_region_variables` valid inference variables; the first N
440 /// of those will be constant regions representing the free
441 /// regions defined in `universal_regions`.
442 ///
443 /// The `outlives_constraints` and `type_tests` are an initial set
444 /// of constraints produced by the MIR type check.
445 pub(crate) fn new(
446 infcx: &BorrowckInferCtxt<'tcx>,
447 constraints: MirTypeckRegionConstraints<'tcx>,
448 universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
449 location_map: Rc<DenseLocationMap>,
450 ) -> Self {
451 let universal_regions = &universal_region_relations.universal_regions;
452 let MirTypeckRegionConstraints {
453 placeholder_indices,
454 placeholder_index_to_region: _,
455 liveness_constraints,
456 mut outlives_constraints,
457 mut member_constraints,
458 universe_causes,
459 type_tests,
460 } = constraints;
461
462 debug!("universal_regions: {:#?}", universal_region_relations.universal_regions);
463 debug!("outlives constraints: {:#?}", outlives_constraints);
464 debug!("placeholder_indices: {:#?}", placeholder_indices);
465 debug!("type tests: {:#?}", type_tests);
466
467 if let Some(guar) = universal_region_relations.universal_regions.tainted_by_errors() {
468 // Suppress unhelpful extra errors in `infer_opaque_types` by clearing out all
469 // outlives bounds that we may end up checking.
470 outlives_constraints = Default::default();
471 member_constraints = Default::default();
472
473 // Also taint the entire scope.
474 infcx.set_tainted_by_errors(guar);
475 }
476
477 let definitions = create_definitions(infcx, &universal_regions);
478
479 let (constraint_sccs, scc_annotations) =
480 outlives_constraints.add_outlives_static(&universal_regions, &definitions);
481 let constraints = Frozen::freeze(outlives_constraints);
482 let constraint_graph = Frozen::freeze(constraints.graph(definitions.len()));
483
484 if cfg!(debug_assertions) {
485 sccs_info(infcx, &constraint_sccs);
486 }
487
488 let mut scc_values =
489 RegionValues::new(location_map, universal_regions.len(), placeholder_indices);
490
491 for region in liveness_constraints.regions() {
492 let scc = constraint_sccs.scc(region);
493 scc_values.merge_liveness(scc, region, &liveness_constraints);
494 }
495
496 let member_constraints =
497 Rc::new(member_constraints.into_mapped(|r| constraint_sccs.scc(r)));
498
499 let mut result = Self {
500 definitions,
501 liveness_constraints,
502 constraints,
503 constraint_graph,
504 constraint_sccs,
505 scc_annotations,
506 rev_scc_graph: OnceCell::new(),
507 member_constraints,
508 member_constraints_applied: Vec::new(),
509 universe_causes,
510 scc_values,
511 type_tests,
512 universal_region_relations,
513 };
514
515 result.init_free_and_bound_regions();
516
517 result
518 }
519
520 /// Initializes the region variables for each universally
521 /// quantified region (lifetime parameter). The first N variables
522 /// always correspond to the regions appearing in the function
523 /// signature (both named and anonymous) and where-clauses. This
524 /// function iterates over those regions and initializes them with
525 /// minimum values.
526 ///
527 /// For example:
528 /// ```
529 /// fn foo<'a, 'b>( /* ... */ ) where 'a: 'b { /* ... */ }
530 /// ```
531 /// would initialize two variables like so:
532 /// ```ignore (illustrative)
533 /// R0 = { CFG, R0 } // 'a
534 /// R1 = { CFG, R0, R1 } // 'b
535 /// ```
536 /// Here, R0 represents `'a`, and it contains (a) the entire CFG
537 /// and (b) any universally quantified regions that it outlives,
538 /// which in this case is just itself. R1 (`'b`) in contrast also
539 /// outlives `'a` and hence contains R0 and R1.
540 ///
541 /// This bit of logic also handles invalid universe relations
542 /// for higher-kinded types.
543 ///
544 /// We Walk each SCC `A` and `B` such that `A: B`
545 /// and ensure that universe(A) can see universe(B).
546 ///
547 /// This serves to enforce the 'empty/placeholder' hierarchy
548 /// (described in more detail on `RegionKind`):
549 ///
550 /// ```ignore (illustrative)
551 /// static -----+
552 /// | |
553 /// empty(U0) placeholder(U1)
554 /// | /
555 /// empty(U1)
556 /// ```
557 ///
558 /// In particular, imagine we have variables R0 in U0 and R1
559 /// created in U1, and constraints like this;
560 ///
561 /// ```ignore (illustrative)
562 /// R1: !1 // R1 outlives the placeholder in U1
563 /// R1: R0 // R1 outlives R0
564 /// ```
565 ///
566 /// Here, we wish for R1 to be `'static`, because it
567 /// cannot outlive `placeholder(U1)` and `empty(U0)` any other way.
568 ///
569 /// Thanks to this loop, what happens is that the `R1: R0`
570 /// constraint has lowered the universe of `R1` to `U0`, which in turn
571 /// means that the `R1: !1` constraint here will cause
572 /// `R1` to become `'static`.
573 fn init_free_and_bound_regions(&mut self) {
574 for variable in self.definitions.indices() {
575 let scc = self.constraint_sccs.scc(variable);
576
577 match self.definitions[variable].origin {
578 NllRegionVariableOrigin::FreeRegion => {
579 // For each free, universally quantified region X:
580
581 // Add all nodes in the CFG to liveness constraints
582 self.liveness_constraints.add_all_points(variable);
583 self.scc_values.add_all_points(scc);
584
585 // Add `end(X)` into the set for X.
586 self.scc_values.add_element(scc, variable);
587 }
588
589 NllRegionVariableOrigin::Placeholder(placeholder) => {
590 self.scc_values.add_element(scc, placeholder);
591 }
592
593 NllRegionVariableOrigin::Existential { .. } => {
594 // For existential, regions, nothing to do.
595 }
596 }
597 }
598 }
599
600 /// Returns an iterator over all the region indices.
601 pub(crate) fn regions(&self) -> impl Iterator<Item = RegionVid> + 'tcx {
602 self.definitions.indices()
603 }
604
605 /// Given a universal region in scope on the MIR, returns the
606 /// corresponding index.
607 ///
608 /// Panics if `r` is not a registered universal region, most notably
609 /// if it is a placeholder. Handling placeholders requires access to the
610 /// `MirTypeckRegionConstraints`.
611 pub(crate) fn to_region_vid(&self, r: ty::Region<'tcx>) -> RegionVid {
612 self.universal_regions().to_region_vid(r)
613 }
614
615 /// Returns an iterator over all the outlives constraints.
616 pub(crate) fn outlives_constraints(&self) -> impl Iterator<Item = OutlivesConstraint<'tcx>> {
617 self.constraints.outlives().iter().copied()
618 }
619
620 /// Adds annotations for `#[rustc_regions]`; see `UniversalRegions::annotate`.
621 pub(crate) fn annotate(&self, tcx: TyCtxt<'tcx>, err: &mut Diag<'_, ()>) {
622 self.universal_regions().annotate(tcx, err)
623 }
624
625 /// Returns `true` if the region `r` contains the point `p`.
626 ///
627 /// Panics if called before `solve()` executes,
628 pub(crate) fn region_contains(&self, r: RegionVid, p: impl ToElementIndex) -> bool {
629 let scc = self.constraint_sccs.scc(r);
630 self.scc_values.contains(scc, p)
631 }
632
633 /// Returns the lowest statement index in `start..=end` which is not contained by `r`.
634 ///
635 /// Panics if called before `solve()` executes.
636 pub(crate) fn first_non_contained_inclusive(
637 &self,
638 r: RegionVid,
639 block: BasicBlock,
640 start: usize,
641 end: usize,
642 ) -> Option<usize> {
643 let scc = self.constraint_sccs.scc(r);
644 self.scc_values.first_non_contained_inclusive(scc, block, start, end)
645 }
646
647 /// Returns access to the value of `r` for debugging purposes.
648 pub(crate) fn region_value_str(&self, r: RegionVid) -> String {
649 let scc = self.constraint_sccs.scc(r);
650 self.scc_values.region_value_str(scc)
651 }
652
653 pub(crate) fn placeholders_contained_in(
654 &self,
655 r: RegionVid,
656 ) -> impl Iterator<Item = ty::PlaceholderRegion> {
657 let scc = self.constraint_sccs.scc(r);
658 self.scc_values.placeholders_contained_in(scc)
659 }
660
661 /// Returns access to the value of `r` for debugging purposes.
662 pub(crate) fn region_universe(&self, r: RegionVid) -> ty::UniverseIndex {
663 self.scc_universe(self.constraint_sccs.scc(r))
664 }
665
666 /// Once region solving has completed, this function will return the member constraints that
667 /// were applied to the value of a given SCC `scc`. See `AppliedMemberConstraint`.
668 pub(crate) fn applied_member_constraints(
669 &self,
670 scc: ConstraintSccIndex,
671 ) -> &[AppliedMemberConstraint] {
672 binary_search_util::binary_search_slice(
673 &self.member_constraints_applied,
674 |applied| applied.member_region_scc,
675 &scc,
676 )
677 }
678
679 /// Performs region inference and report errors if we see any
680 /// unsatisfiable constraints. If this is a closure, returns the
681 /// region requirements to propagate to our creator, if any.
682 #[instrument(skip(self, infcx, body, polonius_output), level = "debug")]
683 pub(super) fn solve(
684 &mut self,
685 infcx: &InferCtxt<'tcx>,
686 body: &Body<'tcx>,
687 polonius_output: Option<Box<PoloniusOutput>>,
688 ) -> (Option<ClosureRegionRequirements<'tcx>>, RegionErrors<'tcx>) {
689 let mir_def_id = body.source.def_id();
690 self.propagate_constraints();
691
692 let mut errors_buffer = RegionErrors::new(infcx.tcx);
693
694 // If this is a closure, we can propagate unsatisfied
695 // `outlives_requirements` to our creator, so create a vector
696 // to store those. Otherwise, we'll pass in `None` to the
697 // functions below, which will trigger them to report errors
698 // eagerly.
699 let mut outlives_requirements = infcx.tcx.is_typeck_child(mir_def_id).then(Vec::new);
700
701 self.check_type_tests(infcx, outlives_requirements.as_mut(), &mut errors_buffer);
702
703 debug!(?errors_buffer);
704 debug!(?outlives_requirements);
705
706 // In Polonius mode, the errors about missing universal region relations are in the output
707 // and need to be emitted or propagated. Otherwise, we need to check whether the
708 // constraints were too strong, and if so, emit or propagate those errors.
709 if infcx.tcx.sess.opts.unstable_opts.polonius.is_legacy_enabled() {
710 self.check_polonius_subset_errors(
711 outlives_requirements.as_mut(),
712 &mut errors_buffer,
713 polonius_output
714 .as_ref()
715 .expect("Polonius output is unavailable despite `-Z polonius`"),
716 );
717 } else {
718 self.check_universal_regions(outlives_requirements.as_mut(), &mut errors_buffer);
719 }
720
721 debug!(?errors_buffer);
722
723 if errors_buffer.is_empty() {
724 self.check_member_constraints(infcx, &mut errors_buffer);
725 }
726
727 debug!(?errors_buffer);
728
729 let outlives_requirements = outlives_requirements.unwrap_or_default();
730
731 if outlives_requirements.is_empty() {
732 (None, errors_buffer)
733 } else {
734 let num_external_vids = self.universal_regions().num_global_and_external_regions();
735 (
736 Some(ClosureRegionRequirements { num_external_vids, outlives_requirements }),
737 errors_buffer,
738 )
739 }
740 }
741
742 /// Propagate the region constraints: this will grow the values
743 /// for each region variable until all the constraints are
744 /// satisfied. Note that some values may grow **too** large to be
745 /// feasible, but we check this later.
746 #[instrument(skip(self), level = "debug")]
747 fn propagate_constraints(&mut self) {
748 debug!("constraints={:#?}", {
749 let mut constraints: Vec<_> = self.outlives_constraints().collect();
750 constraints.sort_by_key(|c| (c.sup, c.sub));
751 constraints
752 .into_iter()
753 .map(|c| (c, self.constraint_sccs.scc(c.sup), self.constraint_sccs.scc(c.sub)))
754 .collect::<Vec<_>>()
755 });
756
757 // To propagate constraints, we walk the DAG induced by the
758 // SCC. For each SCC, we visit its successors and compute
759 // their values, then we union all those values to get our
760 // own.
761 for scc in self.constraint_sccs.all_sccs() {
762 self.compute_value_for_scc(scc);
763 }
764
765 // Sort the applied member constraints so we can binary search
766 // through them later.
767 self.member_constraints_applied.sort_by_key(|applied| applied.member_region_scc);
768 }
769
770 /// Computes the value of the SCC `scc_a`, which has not yet been
771 /// computed, by unioning the values of its successors.
772 /// Assumes that all successors have been computed already
773 /// (which is assured by iterating over SCCs in dependency order).
774 #[instrument(skip(self), level = "debug")]
775 fn compute_value_for_scc(&mut self, scc_a: ConstraintSccIndex) {
776 // Walk each SCC `B` such that `A: B`...
777 for &scc_b in self.constraint_sccs.successors(scc_a) {
778 debug!(?scc_b);
779 self.scc_values.add_region(scc_a, scc_b);
780 }
781
782 // Now take member constraints into account.
783 let member_constraints = Rc::clone(&self.member_constraints);
784 for m_c_i in member_constraints.indices(scc_a) {
785 self.apply_member_constraint(scc_a, m_c_i, member_constraints.choice_regions(m_c_i));
786 }
787
788 debug!(value = ?self.scc_values.region_value_str(scc_a));
789 }
790
791 /// Invoked for each `R0 member of [R1..Rn]` constraint.
792 ///
793 /// `scc` is the SCC containing R0, and `choice_regions` are the
794 /// `R1..Rn` regions -- they are always known to be universal
795 /// regions (and if that's not true, we just don't attempt to
796 /// enforce the constraint).
797 ///
798 /// The current value of `scc` at the time the method is invoked
799 /// is considered a *lower bound*. If possible, we will modify
800 /// the constraint to set it equal to one of the option regions.
801 /// If we make any changes, returns true, else false.
802 ///
803 /// This function only adds the member constraints to the region graph,
804 /// it does not check them. They are later checked in
805 /// `check_member_constraints` after the region graph has been computed.
806 #[instrument(skip(self, member_constraint_index), level = "debug")]
807 fn apply_member_constraint(
808 &mut self,
809 scc: ConstraintSccIndex,
810 member_constraint_index: NllMemberConstraintIndex,
811 choice_regions: &[ty::RegionVid],
812 ) {
813 // Create a mutable vector of the options. We'll try to winnow
814 // them down.
815 let mut choice_regions: Vec<ty::RegionVid> = choice_regions.to_vec();
816
817 // Convert to the SCC representative: sometimes we have inference
818 // variables in the member constraint that wind up equated with
819 // universal regions. The scc representative is the minimal numbered
820 // one from the corresponding scc so it will be the universal region
821 // if one exists.
822 for c_r in &mut choice_regions {
823 let scc = self.constraint_sccs.scc(*c_r);
824 *c_r = self.scc_representative(scc);
825 }
826
827 // If the member region lives in a higher universe, we currently choose
828 // the most conservative option by leaving it unchanged.
829 if !self.scc_universe(scc).is_root() {
830 return;
831 }
832
833 // The existing value for `scc` is a lower-bound. This will
834 // consist of some set `{P} + {LB}` of points `{P}` and
835 // lower-bound free regions `{LB}`. As each choice region `O`
836 // is a free region, it will outlive the points. But we can
837 // only consider the option `O` if `O: LB`.
838 choice_regions.retain(|&o_r| {
839 self.scc_values
840 .universal_regions_outlived_by(scc)
841 .all(|lb| self.universal_region_relations.outlives(o_r, lb))
842 });
843 debug!(?choice_regions, "after lb");
844
845 // Now find all the *upper bounds* -- that is, each UB is a
846 // free region that must outlive the member region `R0` (`UB:
847 // R0`). Therefore, we need only keep an option `O` if `UB: O`
848 // for all UB.
849 let universal_region_relations = &self.universal_region_relations;
850 for ub in self.reverse_scc_graph().upper_bounds(scc) {
851 debug!(?ub);
852 choice_regions.retain(|&o_r| universal_region_relations.outlives(ub, o_r));
853 }
854 debug!(?choice_regions, "after ub");
855
856 // At this point we can pick any member of `choice_regions` and would like to choose
857 // it to be a small as possible. To avoid potential non-determinism we will pick the
858 // smallest such choice.
859 //
860 // Because universal regions are only partially ordered (i.e, not every two regions are
861 // comparable), we will ignore any region that doesn't compare to all others when picking
862 // the minimum choice.
863 //
864 // For example, consider `choice_regions = ['static, 'a, 'b, 'c, 'd, 'e]`, where
865 // `'static: 'a, 'static: 'b, 'a: 'c, 'b: 'c, 'c: 'd, 'c: 'e`.
866 // `['d, 'e]` are ignored because they do not compare - the same goes for `['a, 'b]`.
867 let totally_ordered_subset = choice_regions.iter().copied().filter(|&r1| {
868 choice_regions.iter().all(|&r2| {
869 self.universal_region_relations.outlives(r1, r2)
870 || self.universal_region_relations.outlives(r2, r1)
871 })
872 });
873 // Now we're left with `['static, 'c]`. Pick `'c` as the minimum!
874 let Some(min_choice) = totally_ordered_subset.reduce(|r1, r2| {
875 let r1_outlives_r2 = self.universal_region_relations.outlives(r1, r2);
876 let r2_outlives_r1 = self.universal_region_relations.outlives(r2, r1);
877 match (r1_outlives_r2, r2_outlives_r1) {
878 (true, true) => r1.min(r2),
879 (true, false) => r2,
880 (false, true) => r1,
881 (false, false) => bug!("incomparable regions in total order"),
882 }
883 }) else {
884 debug!("no unique minimum choice");
885 return;
886 };
887
888 // As we require `'scc: 'min_choice`, we have definitely already computed
889 // its `scc_values` at this point.
890 let min_choice_scc = self.constraint_sccs.scc(min_choice);
891 debug!(?min_choice, ?min_choice_scc);
892 if self.scc_values.add_region(scc, min_choice_scc) {
893 self.member_constraints_applied.push(AppliedMemberConstraint {
894 member_region_scc: scc,
895 min_choice,
896 member_constraint_index,
897 });
898 }
899 }
900
901 /// Returns `true` if all the elements in the value of `scc_b` are nameable
902 /// in `scc_a`. Used during constraint propagation, and only once
903 /// the value of `scc_b` has been computed.
904 fn universe_compatible(&self, scc_b: ConstraintSccIndex, scc_a: ConstraintSccIndex) -> bool {
905 let a_annotation = self.scc_annotations[scc_a];
906 let b_annotation = self.scc_annotations[scc_b];
907 let a_universe = a_annotation.min_universe();
908
909 // If scc_b's declared universe is a subset of
910 // scc_a's declared universe (typically, both are ROOT), then
911 // it cannot contain any problematic universe elements.
912 if a_universe.can_name(b_annotation.min_universe()) {
913 return true;
914 }
915
916 // Otherwise, there can be no placeholder in `b` with a too high
917 // universe index to name from `a`.
918 a_universe.can_name(b_annotation.max_placeholder_universe_reached)
919 }
920
921 /// Once regions have been propagated, this method is used to see
922 /// whether the "type tests" produced by typeck were satisfied;
923 /// type tests encode type-outlives relationships like `T:
924 /// 'a`. See `TypeTest` for more details.
925 fn check_type_tests(
926 &self,
927 infcx: &InferCtxt<'tcx>,
928 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
929 errors_buffer: &mut RegionErrors<'tcx>,
930 ) {
931 let tcx = infcx.tcx;
932
933 // Sometimes we register equivalent type-tests that would
934 // result in basically the exact same error being reported to
935 // the user. Avoid that.
936 let mut deduplicate_errors = FxIndexSet::default();
937
938 for type_test in &self.type_tests {
939 debug!("check_type_test: {:?}", type_test);
940
941 let generic_ty = type_test.generic_kind.to_ty(tcx);
942 if self.eval_verify_bound(
943 infcx,
944 generic_ty,
945 type_test.lower_bound,
946 &type_test.verify_bound,
947 ) {
948 continue;
949 }
950
951 if let Some(propagated_outlives_requirements) = &mut propagated_outlives_requirements {
952 if self.try_promote_type_test(infcx, type_test, propagated_outlives_requirements) {
953 continue;
954 }
955 }
956
957 // Type-test failed. Report the error.
958 let erased_generic_kind = infcx.tcx.erase_regions(type_test.generic_kind);
959
960 // Skip duplicate-ish errors.
961 if deduplicate_errors.insert((
962 erased_generic_kind,
963 type_test.lower_bound,
964 type_test.span,
965 )) {
966 debug!(
967 "check_type_test: reporting error for erased_generic_kind={:?}, \
968 lower_bound_region={:?}, \
969 type_test.span={:?}",
970 erased_generic_kind, type_test.lower_bound, type_test.span,
971 );
972
973 errors_buffer.push(RegionErrorKind::TypeTestError { type_test: type_test.clone() });
974 }
975 }
976 }
977
978 /// Invoked when we have some type-test (e.g., `T: 'X`) that we cannot
979 /// prove to be satisfied. If this is a closure, we will attempt to
980 /// "promote" this type-test into our `ClosureRegionRequirements` and
981 /// hence pass it up the creator. To do this, we have to phrase the
982 /// type-test in terms of external free regions, as local free
983 /// regions are not nameable by the closure's creator.
984 ///
985 /// Promotion works as follows: we first check that the type `T`
986 /// contains only regions that the creator knows about. If this is
987 /// true, then -- as a consequence -- we know that all regions in
988 /// the type `T` are free regions that outlive the closure body. If
989 /// false, then promotion fails.
990 ///
991 /// Once we've promoted T, we have to "promote" `'X` to some region
992 /// that is "external" to the closure. Generally speaking, a region
993 /// may be the union of some points in the closure body as well as
994 /// various free lifetimes. We can ignore the points in the closure
995 /// body: if the type T can be expressed in terms of external regions,
996 /// we know it outlives the points in the closure body. That
997 /// just leaves the free regions.
998 ///
999 /// The idea then is to lower the `T: 'X` constraint into multiple
1000 /// bounds -- e.g., if `'X` is the union of two free lifetimes,
1001 /// `'1` and `'2`, then we would create `T: '1` and `T: '2`.
1002 #[instrument(level = "debug", skip(self, infcx, propagated_outlives_requirements))]
1003 fn try_promote_type_test(
1004 &self,
1005 infcx: &InferCtxt<'tcx>,
1006 type_test: &TypeTest<'tcx>,
1007 propagated_outlives_requirements: &mut Vec<ClosureOutlivesRequirement<'tcx>>,
1008 ) -> bool {
1009 let tcx = infcx.tcx;
1010 let TypeTest { generic_kind, lower_bound, span: blame_span, verify_bound: _ } = *type_test;
1011
1012 let generic_ty = generic_kind.to_ty(tcx);
1013 let Some(subject) = self.try_promote_type_test_subject(infcx, generic_ty) else {
1014 return false;
1015 };
1016
1017 let r_scc = self.constraint_sccs.scc(lower_bound);
1018 debug!(
1019 "lower_bound = {:?} r_scc={:?} universe={:?}",
1020 lower_bound,
1021 r_scc,
1022 self.scc_universe(r_scc)
1023 );
1024 // If the type test requires that `T: 'a` where `'a` is a
1025 // placeholder from another universe, that effectively requires
1026 // `T: 'static`, so we have to propagate that requirement.
1027 //
1028 // It doesn't matter *what* universe because the promoted `T` will
1029 // always be in the root universe.
1030 if let Some(p) = self.scc_values.placeholders_contained_in(r_scc).next() {
1031 debug!("encountered placeholder in higher universe: {:?}, requiring 'static", p);
1032 let static_r = self.universal_regions().fr_static;
1033 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
1034 subject,
1035 outlived_free_region: static_r,
1036 blame_span,
1037 category: ConstraintCategory::Boring,
1038 });
1039
1040 // we can return here -- the code below might push add'l constraints
1041 // but they would all be weaker than this one.
1042 return true;
1043 }
1044
1045 // For each region outlived by lower_bound find a non-local,
1046 // universal region (it may be the same region) and add it to
1047 // `ClosureOutlivesRequirement`.
1048 let mut found_outlived_universal_region = false;
1049 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1050 found_outlived_universal_region = true;
1051 debug!("universal_region_outlived_by ur={:?}", ur);
1052 let non_local_ub = self.universal_region_relations.non_local_upper_bounds(ur);
1053 debug!(?non_local_ub);
1054
1055 // This is slightly too conservative. To show T: '1, given `'2: '1`
1056 // and `'3: '1` we only need to prove that T: '2 *or* T: '3, but to
1057 // avoid potential non-determinism we approximate this by requiring
1058 // T: '1 and T: '2.
1059 for upper_bound in non_local_ub {
1060 debug_assert!(self.universal_regions().is_universal_region(upper_bound));
1061 debug_assert!(!self.universal_regions().is_local_free_region(upper_bound));
1062
1063 let requirement = ClosureOutlivesRequirement {
1064 subject,
1065 outlived_free_region: upper_bound,
1066 blame_span,
1067 category: ConstraintCategory::Boring,
1068 };
1069 debug!(?requirement, "adding closure requirement");
1070 propagated_outlives_requirements.push(requirement);
1071 }
1072 }
1073 // If we succeed to promote the subject, i.e. it only contains non-local regions,
1074 // and fail to prove the type test inside of the closure, the `lower_bound` has to
1075 // also be at least as large as some universal region, as the type test is otherwise
1076 // trivial.
1077 assert!(found_outlived_universal_region);
1078 true
1079 }
1080
1081 /// When we promote a type test `T: 'r`, we have to replace all region
1082 /// variables in the type `T` with an equal universal region from the
1083 /// closure signature.
1084 /// This is not always possible, so this is a fallible process.
1085 #[instrument(level = "debug", skip(self, infcx), ret)]
1086 fn try_promote_type_test_subject(
1087 &self,
1088 infcx: &InferCtxt<'tcx>,
1089 ty: Ty<'tcx>,
1090 ) -> Option<ClosureOutlivesSubject<'tcx>> {
1091 let tcx = infcx.tcx;
1092 let mut failed = false;
1093 let ty = fold_regions(tcx, ty, |r, _depth| {
1094 let r_vid = self.to_region_vid(r);
1095 let r_scc = self.constraint_sccs.scc(r_vid);
1096
1097 // The challenge is this. We have some region variable `r`
1098 // whose value is a set of CFG points and universal
1099 // regions. We want to find if that set is *equivalent* to
1100 // any of the named regions found in the closure.
1101 // To do so, we simply check every candidate `u_r` for equality.
1102 self.scc_values
1103 .universal_regions_outlived_by(r_scc)
1104 .filter(|&u_r| !self.universal_regions().is_local_free_region(u_r))
1105 .find(|&u_r| self.eval_equal(u_r, r_vid))
1106 .map(|u_r| ty::Region::new_var(tcx, u_r))
1107 // In case we could not find a named region to map to,
1108 // we will return `None` below.
1109 .unwrap_or_else(|| {
1110 failed = true;
1111 r
1112 })
1113 });
1114
1115 debug!("try_promote_type_test_subject: folded ty = {:?}", ty);
1116
1117 // This will be true if we failed to promote some region.
1118 if failed {
1119 return None;
1120 }
1121
1122 Some(ClosureOutlivesSubject::Ty(ClosureOutlivesSubjectTy::bind(tcx, ty)))
1123 }
1124
1125 /// Like `universal_upper_bound`, but returns an approximation more suitable
1126 /// for diagnostics. If `r` contains multiple disjoint universal regions
1127 /// (e.g. 'a and 'b in `fn foo<'a, 'b> { ... }`, we pick the lower-numbered region.
1128 /// This corresponds to picking named regions over unnamed regions
1129 /// (e.g. picking early-bound regions over a closure late-bound region).
1130 ///
1131 /// This means that the returned value may not be a true upper bound, since
1132 /// only 'static is known to outlive disjoint universal regions.
1133 /// Therefore, this method should only be used in diagnostic code,
1134 /// where displaying *some* named universal region is better than
1135 /// falling back to 'static.
1136 #[instrument(level = "debug", skip(self))]
1137 pub(crate) fn approx_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1138 debug!("{}", self.region_value_str(r));
1139
1140 // Find the smallest universal region that contains all other
1141 // universal regions within `region`.
1142 let mut lub = self.universal_regions().fr_fn_body;
1143 let r_scc = self.constraint_sccs.scc(r);
1144 let static_r = self.universal_regions().fr_static;
1145 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1146 let new_lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1147 debug!(?ur, ?lub, ?new_lub);
1148 // The upper bound of two non-static regions is static: this
1149 // means we know nothing about the relationship between these
1150 // two regions. Pick a 'better' one to use when constructing
1151 // a diagnostic
1152 if ur != static_r && lub != static_r && new_lub == static_r {
1153 // Prefer the region with an `external_name` - this
1154 // indicates that the region is early-bound, so working with
1155 // it can produce a nicer error.
1156 if self.region_definition(ur).external_name.is_some() {
1157 lub = ur;
1158 } else if self.region_definition(lub).external_name.is_some() {
1159 // Leave lub unchanged
1160 } else {
1161 // If we get here, we don't have any reason to prefer
1162 // one region over the other. Just pick the
1163 // one with the lower index for now.
1164 lub = std::cmp::min(ur, lub);
1165 }
1166 } else {
1167 lub = new_lub;
1168 }
1169 }
1170
1171 debug!(?r, ?lub);
1172
1173 lub
1174 }
1175
1176 /// Tests if `test` is true when applied to `lower_bound` at
1177 /// `point`.
1178 fn eval_verify_bound(
1179 &self,
1180 infcx: &InferCtxt<'tcx>,
1181 generic_ty: Ty<'tcx>,
1182 lower_bound: RegionVid,
1183 verify_bound: &VerifyBound<'tcx>,
1184 ) -> bool {
1185 debug!("eval_verify_bound(lower_bound={:?}, verify_bound={:?})", lower_bound, verify_bound);
1186
1187 match verify_bound {
1188 VerifyBound::IfEq(verify_if_eq_b) => {
1189 self.eval_if_eq(infcx, generic_ty, lower_bound, *verify_if_eq_b)
1190 }
1191
1192 VerifyBound::IsEmpty => {
1193 let lower_bound_scc = self.constraint_sccs.scc(lower_bound);
1194 self.scc_values.elements_contained_in(lower_bound_scc).next().is_none()
1195 }
1196
1197 VerifyBound::OutlivedBy(r) => {
1198 let r_vid = self.to_region_vid(*r);
1199 self.eval_outlives(r_vid, lower_bound)
1200 }
1201
1202 VerifyBound::AnyBound(verify_bounds) => verify_bounds.iter().any(|verify_bound| {
1203 self.eval_verify_bound(infcx, generic_ty, lower_bound, verify_bound)
1204 }),
1205
1206 VerifyBound::AllBounds(verify_bounds) => verify_bounds.iter().all(|verify_bound| {
1207 self.eval_verify_bound(infcx, generic_ty, lower_bound, verify_bound)
1208 }),
1209 }
1210 }
1211
1212 fn eval_if_eq(
1213 &self,
1214 infcx: &InferCtxt<'tcx>,
1215 generic_ty: Ty<'tcx>,
1216 lower_bound: RegionVid,
1217 verify_if_eq_b: ty::Binder<'tcx, VerifyIfEq<'tcx>>,
1218 ) -> bool {
1219 let generic_ty = self.normalize_to_scc_representatives(infcx.tcx, generic_ty);
1220 let verify_if_eq_b = self.normalize_to_scc_representatives(infcx.tcx, verify_if_eq_b);
1221 match test_type_match::extract_verify_if_eq(infcx.tcx, &verify_if_eq_b, generic_ty) {
1222 Some(r) => {
1223 let r_vid = self.to_region_vid(r);
1224 self.eval_outlives(r_vid, lower_bound)
1225 }
1226 None => false,
1227 }
1228 }
1229
1230 /// This is a conservative normalization procedure. It takes every
1231 /// free region in `value` and replaces it with the
1232 /// "representative" of its SCC (see `scc_representatives` field).
1233 /// We are guaranteed that if two values normalize to the same
1234 /// thing, then they are equal; this is a conservative check in
1235 /// that they could still be equal even if they normalize to
1236 /// different results. (For example, there might be two regions
1237 /// with the same value that are not in the same SCC).
1238 ///
1239 /// N.B., this is not an ideal approach and I would like to revisit
1240 /// it. However, it works pretty well in practice. In particular,
1241 /// this is needed to deal with projection outlives bounds like
1242 ///
1243 /// ```text
1244 /// <T as Foo<'0>>::Item: '1
1245 /// ```
1246 ///
1247 /// In particular, this routine winds up being important when
1248 /// there are bounds like `where <T as Foo<'a>>::Item: 'b` in the
1249 /// environment. In this case, if we can show that `'0 == 'a`,
1250 /// and that `'b: '1`, then we know that the clause is
1251 /// satisfied. In such cases, particularly due to limitations of
1252 /// the trait solver =), we usually wind up with a where-clause like
1253 /// `T: Foo<'a>` in scope, which thus forces `'0 == 'a` to be added as
1254 /// a constraint, and thus ensures that they are in the same SCC.
1255 ///
1256 /// So why can't we do a more correct routine? Well, we could
1257 /// *almost* use the `relate_tys` code, but the way it is
1258 /// currently setup it creates inference variables to deal with
1259 /// higher-ranked things and so forth, and right now the inference
1260 /// context is not permitted to make more inference variables. So
1261 /// we use this kind of hacky solution.
1262 fn normalize_to_scc_representatives<T>(&self, tcx: TyCtxt<'tcx>, value: T) -> T
1263 where
1264 T: TypeFoldable<TyCtxt<'tcx>>,
1265 {
1266 fold_regions(tcx, value, |r, _db| {
1267 let vid = self.to_region_vid(r);
1268 let scc = self.constraint_sccs.scc(vid);
1269 let repr = self.scc_representative(scc);
1270 ty::Region::new_var(tcx, repr)
1271 })
1272 }
1273
1274 /// Evaluate whether `sup_region == sub_region`.
1275 ///
1276 /// Panics if called before `solve()` executes,
1277 // This is `pub` because it's used by unstable external borrowck data users, see `consumers.rs`.
1278 pub fn eval_equal(&self, r1: RegionVid, r2: RegionVid) -> bool {
1279 self.eval_outlives(r1, r2) && self.eval_outlives(r2, r1)
1280 }
1281
1282 /// Evaluate whether `sup_region: sub_region`.
1283 ///
1284 /// Panics if called before `solve()` executes,
1285 // This is `pub` because it's used by unstable external borrowck data users, see `consumers.rs`.
1286 #[instrument(skip(self), level = "debug", ret)]
1287 pub fn eval_outlives(&self, sup_region: RegionVid, sub_region: RegionVid) -> bool {
1288 debug!(
1289 "sup_region's value = {:?} universal={:?}",
1290 self.region_value_str(sup_region),
1291 self.universal_regions().is_universal_region(sup_region),
1292 );
1293 debug!(
1294 "sub_region's value = {:?} universal={:?}",
1295 self.region_value_str(sub_region),
1296 self.universal_regions().is_universal_region(sub_region),
1297 );
1298
1299 let sub_region_scc = self.constraint_sccs.scc(sub_region);
1300 let sup_region_scc = self.constraint_sccs.scc(sup_region);
1301
1302 if sub_region_scc == sup_region_scc {
1303 debug!("{sup_region:?}: {sub_region:?} holds trivially; they are in the same SCC");
1304 return true;
1305 }
1306
1307 // If we are checking that `'sup: 'sub`, and `'sub` contains
1308 // some placeholder that `'sup` cannot name, then this is only
1309 // true if `'sup` outlives static.
1310 if !self.universe_compatible(sub_region_scc, sup_region_scc) {
1311 debug!(
1312 "sub universe `{sub_region_scc:?}` is not nameable \
1313 by super `{sup_region_scc:?}`, promoting to static",
1314 );
1315
1316 return self.eval_outlives(sup_region, self.universal_regions().fr_static);
1317 }
1318
1319 // Both the `sub_region` and `sup_region` consist of the union
1320 // of some number of universal regions (along with the union
1321 // of various points in the CFG; ignore those points for
1322 // now). Therefore, the sup-region outlives the sub-region if,
1323 // for each universal region R1 in the sub-region, there
1324 // exists some region R2 in the sup-region that outlives R1.
1325 let universal_outlives =
1326 self.scc_values.universal_regions_outlived_by(sub_region_scc).all(|r1| {
1327 self.scc_values
1328 .universal_regions_outlived_by(sup_region_scc)
1329 .any(|r2| self.universal_region_relations.outlives(r2, r1))
1330 });
1331
1332 if !universal_outlives {
1333 debug!("sub region contains a universal region not present in super");
1334 return false;
1335 }
1336
1337 // Now we have to compare all the points in the sub region and make
1338 // sure they exist in the sup region.
1339
1340 if self.universal_regions().is_universal_region(sup_region) {
1341 // Micro-opt: universal regions contain all points.
1342 debug!("super is universal and hence contains all points");
1343 return true;
1344 }
1345
1346 debug!("comparison between points in sup/sub");
1347
1348 self.scc_values.contains_points(sup_region_scc, sub_region_scc)
1349 }
1350
1351 /// Once regions have been propagated, this method is used to see
1352 /// whether any of the constraints were too strong. In particular,
1353 /// we want to check for a case where a universally quantified
1354 /// region exceeded its bounds. Consider:
1355 /// ```compile_fail
1356 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1357 /// ```
1358 /// In this case, returning `x` requires `&'a u32 <: &'b u32`
1359 /// and hence we establish (transitively) a constraint that
1360 /// `'a: 'b`. The `propagate_constraints` code above will
1361 /// therefore add `end('a)` into the region for `'b` -- but we
1362 /// have no evidence that `'b` outlives `'a`, so we want to report
1363 /// an error.
1364 ///
1365 /// If `propagated_outlives_requirements` is `Some`, then we will
1366 /// push unsatisfied obligations into there. Otherwise, we'll
1367 /// report them as errors.
1368 fn check_universal_regions(
1369 &self,
1370 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1371 errors_buffer: &mut RegionErrors<'tcx>,
1372 ) {
1373 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1374 debug!(?fr, ?fr_definition);
1375 match fr_definition.origin {
1376 NllRegionVariableOrigin::FreeRegion => {
1377 // Go through each of the universal regions `fr` and check that
1378 // they did not grow too large, accumulating any requirements
1379 // for our caller into the `outlives_requirements` vector.
1380 self.check_universal_region(
1381 fr,
1382 &mut propagated_outlives_requirements,
1383 errors_buffer,
1384 );
1385 }
1386
1387 NllRegionVariableOrigin::Placeholder(placeholder) => {
1388 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1389 }
1390
1391 NllRegionVariableOrigin::Existential { .. } => {
1392 // nothing to check here
1393 }
1394 }
1395 }
1396 }
1397
1398 /// Checks if Polonius has found any unexpected free region relations.
1399 ///
1400 /// In Polonius terms, a "subset error" (or "illegal subset relation error") is the equivalent
1401 /// of NLL's "checking if any region constraints were too strong": a placeholder origin `'a`
1402 /// was unexpectedly found to be a subset of another placeholder origin `'b`, and means in NLL
1403 /// terms that the "longer free region" `'a` outlived the "shorter free region" `'b`.
1404 ///
1405 /// More details can be found in this blog post by Niko:
1406 /// <https://smallcultfollowing.com/babysteps/blog/2019/01/17/polonius-and-region-errors/>
1407 ///
1408 /// In the canonical example
1409 /// ```compile_fail
1410 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1411 /// ```
1412 /// returning `x` requires `&'a u32 <: &'b u32` and hence we establish (transitively) a
1413 /// constraint that `'a: 'b`. It is an error that we have no evidence that this
1414 /// constraint holds.
1415 ///
1416 /// If `propagated_outlives_requirements` is `Some`, then we will
1417 /// push unsatisfied obligations into there. Otherwise, we'll
1418 /// report them as errors.
1419 fn check_polonius_subset_errors(
1420 &self,
1421 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1422 errors_buffer: &mut RegionErrors<'tcx>,
1423 polonius_output: &PoloniusOutput,
1424 ) {
1425 debug!(
1426 "check_polonius_subset_errors: {} subset_errors",
1427 polonius_output.subset_errors.len()
1428 );
1429
1430 // Similarly to `check_universal_regions`: a free region relation, which was not explicitly
1431 // declared ("known") was found by Polonius, so emit an error, or propagate the
1432 // requirements for our caller into the `propagated_outlives_requirements` vector.
1433 //
1434 // Polonius doesn't model regions ("origins") as CFG-subsets or durations, but the
1435 // `longer_fr` and `shorter_fr` terminology will still be used here, for consistency with
1436 // the rest of the NLL infrastructure. The "subset origin" is the "longer free region",
1437 // and the "superset origin" is the outlived "shorter free region".
1438 //
1439 // Note: Polonius will produce a subset error at every point where the unexpected
1440 // `longer_fr`'s "placeholder loan" is contained in the `shorter_fr`. This can be helpful
1441 // for diagnostics in the future, e.g. to point more precisely at the key locations
1442 // requiring this constraint to hold. However, the error and diagnostics code downstream
1443 // expects that these errors are not duplicated (and that they are in a certain order).
1444 // Otherwise, diagnostics messages such as the ones giving names like `'1` to elided or
1445 // anonymous lifetimes for example, could give these names differently, while others like
1446 // the outlives suggestions or the debug output from `#[rustc_regions]` would be
1447 // duplicated. The polonius subset errors are deduplicated here, while keeping the
1448 // CFG-location ordering.
1449 // We can iterate the HashMap here because the result is sorted afterwards.
1450 #[allow(rustc::potential_query_instability)]
1451 let mut subset_errors: Vec<_> = polonius_output
1452 .subset_errors
1453 .iter()
1454 .flat_map(|(_location, subset_errors)| subset_errors.iter())
1455 .collect();
1456 subset_errors.sort();
1457 subset_errors.dedup();
1458
1459 for &(longer_fr, shorter_fr) in subset_errors.into_iter() {
1460 debug!(
1461 "check_polonius_subset_errors: subset_error longer_fr={:?},\
1462 shorter_fr={:?}",
1463 longer_fr, shorter_fr
1464 );
1465
1466 let propagated = self.try_propagate_universal_region_error(
1467 longer_fr.into(),
1468 shorter_fr.into(),
1469 &mut propagated_outlives_requirements,
1470 );
1471 if propagated == RegionRelationCheckResult::Error {
1472 errors_buffer.push(RegionErrorKind::RegionError {
1473 longer_fr: longer_fr.into(),
1474 shorter_fr: shorter_fr.into(),
1475 fr_origin: NllRegionVariableOrigin::FreeRegion,
1476 is_reported: true,
1477 });
1478 }
1479 }
1480
1481 // Handle the placeholder errors as usual, until the chalk-rustc-polonius triumvirate has
1482 // a more complete picture on how to separate this responsibility.
1483 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1484 match fr_definition.origin {
1485 NllRegionVariableOrigin::FreeRegion => {
1486 // handled by polonius above
1487 }
1488
1489 NllRegionVariableOrigin::Placeholder(placeholder) => {
1490 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1491 }
1492
1493 NllRegionVariableOrigin::Existential { .. } => {
1494 // nothing to check here
1495 }
1496 }
1497 }
1498 }
1499
1500 /// The minimum universe of any variable reachable from this
1501 /// SCC, inside or outside of it.
1502 fn scc_universe(&self, scc: ConstraintSccIndex) -> UniverseIndex {
1503 self.scc_annotations[scc].min_universe()
1504 }
1505
1506 /// Checks the final value for the free region `fr` to see if it
1507 /// grew too large. In particular, examine what `end(X)` points
1508 /// wound up in `fr`'s final value; for each `end(X)` where `X !=
1509 /// fr`, we want to check that `fr: X`. If not, that's either an
1510 /// error, or something we have to propagate to our creator.
1511 ///
1512 /// Things that are to be propagated are accumulated into the
1513 /// `outlives_requirements` vector.
1514 #[instrument(skip(self, propagated_outlives_requirements, errors_buffer), level = "debug")]
1515 fn check_universal_region(
1516 &self,
1517 longer_fr: RegionVid,
1518 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1519 errors_buffer: &mut RegionErrors<'tcx>,
1520 ) {
1521 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1522
1523 // Because this free region must be in the ROOT universe, we
1524 // know it cannot contain any bound universes.
1525 assert!(self.scc_universe(longer_fr_scc).is_root());
1526
1527 // Only check all of the relations for the main representative of each
1528 // SCC, otherwise just check that we outlive said representative. This
1529 // reduces the number of redundant relations propagated out of
1530 // closures.
1531 // Note that the representative will be a universal region if there is
1532 // one in this SCC, so we will always check the representative here.
1533 let representative = self.scc_representative(longer_fr_scc);
1534 if representative != longer_fr {
1535 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1536 longer_fr,
1537 representative,
1538 propagated_outlives_requirements,
1539 ) {
1540 errors_buffer.push(RegionErrorKind::RegionError {
1541 longer_fr,
1542 shorter_fr: representative,
1543 fr_origin: NllRegionVariableOrigin::FreeRegion,
1544 is_reported: true,
1545 });
1546 }
1547 return;
1548 }
1549
1550 // Find every region `o` such that `fr: o`
1551 // (because `fr` includes `end(o)`).
1552 let mut error_reported = false;
1553 for shorter_fr in self.scc_values.universal_regions_outlived_by(longer_fr_scc) {
1554 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1555 longer_fr,
1556 shorter_fr,
1557 propagated_outlives_requirements,
1558 ) {
1559 // We only report the first region error. Subsequent errors are hidden so as
1560 // not to overwhelm the user, but we do record them so as to potentially print
1561 // better diagnostics elsewhere...
1562 errors_buffer.push(RegionErrorKind::RegionError {
1563 longer_fr,
1564 shorter_fr,
1565 fr_origin: NllRegionVariableOrigin::FreeRegion,
1566 is_reported: !error_reported,
1567 });
1568
1569 error_reported = true;
1570 }
1571 }
1572 }
1573
1574 /// Checks that we can prove that `longer_fr: shorter_fr`. If we can't we attempt to propagate
1575 /// the constraint outward (e.g. to a closure environment), but if that fails, there is an
1576 /// error.
1577 fn check_universal_region_relation(
1578 &self,
1579 longer_fr: RegionVid,
1580 shorter_fr: RegionVid,
1581 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1582 ) -> RegionRelationCheckResult {
1583 // If it is known that `fr: o`, carry on.
1584 if self.universal_region_relations.outlives(longer_fr, shorter_fr) {
1585 RegionRelationCheckResult::Ok
1586 } else {
1587 // If we are not in a context where we can't propagate errors, or we
1588 // could not shrink `fr` to something smaller, then just report an
1589 // error.
1590 //
1591 // Note: in this case, we use the unapproximated regions to report the
1592 // error. This gives better error messages in some cases.
1593 self.try_propagate_universal_region_error(
1594 longer_fr,
1595 shorter_fr,
1596 propagated_outlives_requirements,
1597 )
1598 }
1599 }
1600
1601 /// Attempt to propagate a region error (e.g. `'a: 'b`) that is not met to a closure's
1602 /// creator. If we cannot, then the caller should report an error to the user.
1603 fn try_propagate_universal_region_error(
1604 &self,
1605 longer_fr: RegionVid,
1606 shorter_fr: RegionVid,
1607 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1608 ) -> RegionRelationCheckResult {
1609 if let Some(propagated_outlives_requirements) = propagated_outlives_requirements {
1610 // Shrink `longer_fr` until we find a non-local region (if we do).
1611 // We'll call it `fr-` -- it's ever so slightly smaller than
1612 // `longer_fr`.
1613 if let Some(fr_minus) = self.universal_region_relations.non_local_lower_bound(longer_fr)
1614 {
1615 debug!("try_propagate_universal_region_error: fr_minus={:?}", fr_minus);
1616
1617 let blame_span_category = self.find_outlives_blame_span(
1618 longer_fr,
1619 NllRegionVariableOrigin::FreeRegion,
1620 shorter_fr,
1621 );
1622
1623 // Grow `shorter_fr` until we find some non-local regions. (We
1624 // always will.) We'll call them `shorter_fr+` -- they're ever
1625 // so slightly larger than `shorter_fr`.
1626 let shorter_fr_plus =
1627 self.universal_region_relations.non_local_upper_bounds(shorter_fr);
1628 debug!(
1629 "try_propagate_universal_region_error: shorter_fr_plus={:?}",
1630 shorter_fr_plus
1631 );
1632 for fr in shorter_fr_plus {
1633 // Push the constraint `fr-: shorter_fr+`
1634 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
1635 subject: ClosureOutlivesSubject::Region(fr_minus),
1636 outlived_free_region: fr,
1637 blame_span: blame_span_category.1.span,
1638 category: blame_span_category.0,
1639 });
1640 }
1641 return RegionRelationCheckResult::Propagated;
1642 }
1643 }
1644
1645 RegionRelationCheckResult::Error
1646 }
1647
1648 fn check_bound_universal_region(
1649 &self,
1650 longer_fr: RegionVid,
1651 placeholder: ty::PlaceholderRegion,
1652 errors_buffer: &mut RegionErrors<'tcx>,
1653 ) {
1654 debug!("check_bound_universal_region(fr={:?}, placeholder={:?})", longer_fr, placeholder,);
1655
1656 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1657 debug!("check_bound_universal_region: longer_fr_scc={:?}", longer_fr_scc,);
1658
1659 // If we have some bound universal region `'a`, then the only
1660 // elements it can contain is itself -- we don't know anything
1661 // else about it!
1662 if let Some(error_element) = self
1663 .scc_values
1664 .elements_contained_in(longer_fr_scc)
1665 .find(|e| *e != RegionElement::PlaceholderRegion(placeholder))
1666 {
1667 // Stop after the first error, it gets too noisy otherwise, and does not provide more information.
1668 errors_buffer.push(RegionErrorKind::BoundUniversalRegionError {
1669 longer_fr,
1670 error_element,
1671 placeholder,
1672 });
1673 } else {
1674 debug!("check_bound_universal_region: all bounds satisfied");
1675 }
1676 }
1677
1678 #[instrument(level = "debug", skip(self, infcx, errors_buffer))]
1679 fn check_member_constraints(
1680 &self,
1681 infcx: &InferCtxt<'tcx>,
1682 errors_buffer: &mut RegionErrors<'tcx>,
1683 ) {
1684 let member_constraints = Rc::clone(&self.member_constraints);
1685 for m_c_i in member_constraints.all_indices() {
1686 debug!(?m_c_i);
1687 let m_c = &member_constraints[m_c_i];
1688 let member_region_vid = m_c.member_region_vid;
1689 debug!(
1690 ?member_region_vid,
1691 value = ?self.region_value_str(member_region_vid),
1692 );
1693 let choice_regions = member_constraints.choice_regions(m_c_i);
1694 debug!(?choice_regions);
1695
1696 // Did the member region wind up equal to any of the option regions?
1697 if let Some(o) =
1698 choice_regions.iter().find(|&&o_r| self.eval_equal(o_r, m_c.member_region_vid))
1699 {
1700 debug!("evaluated as equal to {:?}", o);
1701 continue;
1702 }
1703
1704 // If not, report an error.
1705 let member_region = ty::Region::new_var(infcx.tcx, member_region_vid);
1706 errors_buffer.push(RegionErrorKind::UnexpectedHiddenRegion {
1707 span: m_c.definition_span,
1708 hidden_ty: m_c.hidden_ty,
1709 key: m_c.key,
1710 member_region,
1711 });
1712 }
1713 }
1714
1715 /// We have a constraint `fr1: fr2` that is not satisfied, where
1716 /// `fr2` represents some universal region. Here, `r` is some
1717 /// region where we know that `fr1: r` and this function has the
1718 /// job of determining whether `r` is "to blame" for the fact that
1719 /// `fr1: fr2` is required.
1720 ///
1721 /// This is true under two conditions:
1722 ///
1723 /// - `r == fr2`
1724 /// - `fr2` is `'static` and `r` is some placeholder in a universe
1725 /// that cannot be named by `fr1`; in that case, we will require
1726 /// that `fr1: 'static` because it is the only way to `fr1: r` to
1727 /// be satisfied. (See `add_incompatible_universe`.)
1728 pub(crate) fn provides_universal_region(
1729 &self,
1730 r: RegionVid,
1731 fr1: RegionVid,
1732 fr2: RegionVid,
1733 ) -> bool {
1734 debug!("provides_universal_region(r={:?}, fr1={:?}, fr2={:?})", r, fr1, fr2);
1735 let result = {
1736 r == fr2 || {
1737 fr2 == self.universal_regions().fr_static && self.cannot_name_placeholder(fr1, r)
1738 }
1739 };
1740 debug!("provides_universal_region: result = {:?}", result);
1741 result
1742 }
1743
1744 /// If `r2` represents a placeholder region, then this returns
1745 /// `true` if `r1` cannot name that placeholder in its
1746 /// value; otherwise, returns `false`.
1747 pub(crate) fn cannot_name_placeholder(&self, r1: RegionVid, r2: RegionVid) -> bool {
1748 match self.definitions[r2].origin {
1749 NllRegionVariableOrigin::Placeholder(placeholder) => {
1750 let r1_universe = self.definitions[r1].universe;
1751 debug!(
1752 "cannot_name_value_of: universe1={r1_universe:?} placeholder={:?}",
1753 placeholder
1754 );
1755 r1_universe.cannot_name(placeholder.universe)
1756 }
1757
1758 NllRegionVariableOrigin::FreeRegion | NllRegionVariableOrigin::Existential { .. } => {
1759 false
1760 }
1761 }
1762 }
1763
1764 /// Finds a good `ObligationCause` to blame for the fact that `fr1` outlives `fr2`.
1765 pub(crate) fn find_outlives_blame_span(
1766 &self,
1767 fr1: RegionVid,
1768 fr1_origin: NllRegionVariableOrigin,
1769 fr2: RegionVid,
1770 ) -> (ConstraintCategory<'tcx>, ObligationCause<'tcx>) {
1771 let BlameConstraint { category, cause, .. } = self
1772 .best_blame_constraint(fr1, fr1_origin, |r| self.provides_universal_region(r, fr1, fr2))
1773 .0;
1774 (category, cause)
1775 }
1776
1777 /// Walks the graph of constraints (where `'a: 'b` is considered
1778 /// an edge `'a -> 'b`) to find all paths from `from_region` to
1779 /// `to_region`. The paths are accumulated into the vector
1780 /// `results`. The paths are stored as a series of
1781 /// `ConstraintIndex` values -- in other words, a list of *edges*.
1782 ///
1783 /// Returns: a series of constraints as well as the region `R`
1784 /// that passed the target test.
1785 #[instrument(skip(self, target_test), ret)]
1786 pub(crate) fn find_constraint_paths_between_regions(
1787 &self,
1788 from_region: RegionVid,
1789 target_test: impl Fn(RegionVid) -> bool,
1790 ) -> Option<(Vec<OutlivesConstraint<'tcx>>, RegionVid)> {
1791 let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions);
1792 context[from_region] = Trace::StartRegion;
1793
1794 let fr_static = self.universal_regions().fr_static;
1795
1796 // Use a deque so that we do a breadth-first search. We will
1797 // stop at the first match, which ought to be the shortest
1798 // path (fewest constraints).
1799 let mut deque = VecDeque::new();
1800 deque.push_back(from_region);
1801
1802 while let Some(r) = deque.pop_front() {
1803 debug!(
1804 "find_constraint_paths_between_regions: from_region={:?} r={:?} value={}",
1805 from_region,
1806 r,
1807 self.region_value_str(r),
1808 );
1809
1810 // Check if we reached the region we were looking for. If so,
1811 // we can reconstruct the path that led to it and return it.
1812 if target_test(r) {
1813 let mut result = vec![];
1814 let mut p = r;
1815 // This loop is cold and runs at the end, which is why we delay
1816 // `OutlivesConstraint` construction until now.
1817 loop {
1818 match context[p] {
1819 Trace::FromGraph(c) => {
1820 p = c.sup;
1821 result.push(*c);
1822 }
1823
1824 Trace::FromStatic(sub) => {
1825 let c = OutlivesConstraint {
1826 sup: fr_static,
1827 sub,
1828 locations: Locations::All(DUMMY_SP),
1829 span: DUMMY_SP,
1830 category: ConstraintCategory::Internal,
1831 variance_info: ty::VarianceDiagInfo::default(),
1832 from_closure: false,
1833 };
1834 p = c.sup;
1835 result.push(c);
1836 }
1837
1838 Trace::FromMember(sup, sub, span) => {
1839 let c = OutlivesConstraint {
1840 sup,
1841 sub,
1842 locations: Locations::All(span),
1843 span,
1844 category: ConstraintCategory::OpaqueType,
1845 variance_info: ty::VarianceDiagInfo::default(),
1846 from_closure: false,
1847 };
1848 p = c.sup;
1849 result.push(c);
1850 }
1851
1852 Trace::StartRegion => {
1853 result.reverse();
1854 return Some((result, r));
1855 }
1856
1857 Trace::NotVisited => {
1858 bug!("found unvisited region {:?} on path to {:?}", p, r)
1859 }
1860 }
1861 }
1862 }
1863
1864 // Otherwise, walk over the outgoing constraints and
1865 // enqueue any regions we find, keeping track of how we
1866 // reached them.
1867
1868 // A constraint like `'r: 'x` can come from our constraint
1869 // graph.
1870
1871 // Always inline this closure because it can be hot.
1872 let mut handle_trace = #[inline(always)]
1873 |sub, trace| {
1874 if let Trace::NotVisited = context[sub] {
1875 context[sub] = trace;
1876 deque.push_back(sub);
1877 }
1878 };
1879
1880 // If this is the `'static` region and the graph's direction is normal, then set up the
1881 // Edges iterator to return all regions (#53178).
1882 if r == fr_static && self.constraint_graph.is_normal() {
1883 for sub in self.constraint_graph.outgoing_edges_from_static() {
1884 handle_trace(sub, Trace::FromStatic(sub));
1885 }
1886 } else {
1887 let edges = self.constraint_graph.outgoing_edges_from_graph(r, &self.constraints);
1888 // This loop can be hot.
1889 for constraint in edges {
1890 if matches!(constraint.category, ConstraintCategory::IllegalUniverse) {
1891 debug!("Ignoring illegal universe constraint: {constraint:?}");
1892 continue;
1893 }
1894 debug_assert_eq!(constraint.sup, r);
1895 handle_trace(constraint.sub, Trace::FromGraph(constraint));
1896 }
1897 }
1898
1899 // Member constraints can also give rise to `'r: 'x` edges that
1900 // were not part of the graph initially, so watch out for those.
1901 // (But they are extremely rare; this loop is very cold.)
1902 for constraint in self.applied_member_constraints(self.constraint_sccs.scc(r)) {
1903 let sub = constraint.min_choice;
1904 let p_c = &self.member_constraints[constraint.member_constraint_index];
1905 handle_trace(sub, Trace::FromMember(r, sub, p_c.definition_span));
1906 }
1907 }
1908
1909 None
1910 }
1911
1912 /// Finds some region R such that `fr1: R` and `R` is live at `location`.
1913 #[instrument(skip(self), level = "trace", ret)]
1914 pub(crate) fn find_sub_region_live_at(&self, fr1: RegionVid, location: Location) -> RegionVid {
1915 trace!(scc = ?self.constraint_sccs.scc(fr1));
1916 trace!(universe = ?self.region_universe(fr1));
1917 self.find_constraint_paths_between_regions(fr1, |r| {
1918 // First look for some `r` such that `fr1: r` and `r` is live at `location`
1919 trace!(?r, liveness_constraints=?self.liveness_constraints.pretty_print_live_points(r));
1920 self.liveness_constraints.is_live_at(r, location)
1921 })
1922 .or_else(|| {
1923 // If we fail to find that, we may find some `r` such that
1924 // `fr1: r` and `r` is a placeholder from some universe
1925 // `fr1` cannot name. This would force `fr1` to be
1926 // `'static`.
1927 self.find_constraint_paths_between_regions(fr1, |r| {
1928 self.cannot_name_placeholder(fr1, r)
1929 })
1930 })
1931 .or_else(|| {
1932 // If we fail to find THAT, it may be that `fr1` is a
1933 // placeholder that cannot "fit" into its SCC. In that
1934 // case, there should be some `r` where `fr1: r` and `fr1` is a
1935 // placeholder that `r` cannot name. We can blame that
1936 // edge.
1937 //
1938 // Remember that if `R1: R2`, then the universe of R1
1939 // must be able to name the universe of R2, because R2 will
1940 // be at least `'empty(Universe(R2))`, and `R1` must be at
1941 // larger than that.
1942 self.find_constraint_paths_between_regions(fr1, |r| {
1943 self.cannot_name_placeholder(r, fr1)
1944 })
1945 })
1946 .map(|(_path, r)| r)
1947 .unwrap()
1948 }
1949
1950 /// Get the region outlived by `longer_fr` and live at `element`.
1951 pub(crate) fn region_from_element(
1952 &self,
1953 longer_fr: RegionVid,
1954 element: &RegionElement,
1955 ) -> RegionVid {
1956 match *element {
1957 RegionElement::Location(l) => self.find_sub_region_live_at(longer_fr, l),
1958 RegionElement::RootUniversalRegion(r) => r,
1959 RegionElement::PlaceholderRegion(error_placeholder) => self
1960 .definitions
1961 .iter_enumerated()
1962 .find_map(|(r, definition)| match definition.origin {
1963 NllRegionVariableOrigin::Placeholder(p) if p == error_placeholder => Some(r),
1964 _ => None,
1965 })
1966 .unwrap(),
1967 }
1968 }
1969
1970 /// Get the region definition of `r`.
1971 pub(crate) fn region_definition(&self, r: RegionVid) -> &RegionDefinition<'tcx> {
1972 &self.definitions[r]
1973 }
1974
1975 /// Check if the SCC of `r` contains `upper`.
1976 pub(crate) fn upper_bound_in_region_scc(&self, r: RegionVid, upper: RegionVid) -> bool {
1977 let r_scc = self.constraint_sccs.scc(r);
1978 self.scc_values.contains(r_scc, upper)
1979 }
1980
1981 pub(crate) fn universal_regions(&self) -> &UniversalRegions<'tcx> {
1982 &self.universal_region_relations.universal_regions
1983 }
1984
1985 /// Tries to find the best constraint to blame for the fact that
1986 /// `R: from_region`, where `R` is some region that meets
1987 /// `target_test`. This works by following the constraint graph,
1988 /// creating a constraint path that forces `R` to outlive
1989 /// `from_region`, and then finding the best choices within that
1990 /// path to blame.
1991 #[instrument(level = "debug", skip(self, target_test))]
1992 pub(crate) fn best_blame_constraint(
1993 &self,
1994 from_region: RegionVid,
1995 from_region_origin: NllRegionVariableOrigin,
1996 target_test: impl Fn(RegionVid) -> bool,
1997 ) -> (BlameConstraint<'tcx>, Vec<OutlivesConstraint<'tcx>>) {
1998 // Find all paths
1999 let (path, target_region) = self
2000 .find_constraint_paths_between_regions(from_region, target_test)
2001 .or_else(|| {
2002 self.find_constraint_paths_between_regions(from_region, |r| {
2003 self.cannot_name_placeholder(from_region, r)
2004 })
2005 })
2006 .unwrap();
2007 debug!(
2008 "path={:#?}",
2009 path.iter()
2010 .map(|c| format!(
2011 "{:?} ({:?}: {:?})",
2012 c,
2013 self.constraint_sccs.scc(c.sup),
2014 self.constraint_sccs.scc(c.sub),
2015 ))
2016 .collect::<Vec<_>>()
2017 );
2018
2019 // We try to avoid reporting a `ConstraintCategory::Predicate` as our best constraint.
2020 // Instead, we use it to produce an improved `ObligationCauseCode`.
2021 // FIXME - determine what we should do if we encounter multiple
2022 // `ConstraintCategory::Predicate` constraints. Currently, we just pick the first one.
2023 let cause_code = path
2024 .iter()
2025 .find_map(|constraint| {
2026 if let ConstraintCategory::Predicate(predicate_span) = constraint.category {
2027 // We currently do not store the `DefId` in the `ConstraintCategory`
2028 // for performances reasons. The error reporting code used by NLL only
2029 // uses the span, so this doesn't cause any problems at the moment.
2030 Some(ObligationCauseCode::WhereClause(CRATE_DEF_ID.to_def_id(), predicate_span))
2031 } else {
2032 None
2033 }
2034 })
2035 .unwrap_or_else(|| ObligationCauseCode::Misc);
2036
2037 // When reporting an error, there is typically a chain of constraints leading from some
2038 // "source" region which must outlive some "target" region.
2039 // In most cases, we prefer to "blame" the constraints closer to the target --
2040 // but there is one exception. When constraints arise from higher-ranked subtyping,
2041 // we generally prefer to blame the source value,
2042 // as the "target" in this case tends to be some type annotation that the user gave.
2043 // Therefore, if we find that the region origin is some instantiation
2044 // of a higher-ranked region, we start our search from the "source" point
2045 // rather than the "target", and we also tweak a few other things.
2046 //
2047 // An example might be this bit of Rust code:
2048 //
2049 // ```rust
2050 // let x: fn(&'static ()) = |_| {};
2051 // let y: for<'a> fn(&'a ()) = x;
2052 // ```
2053 //
2054 // In MIR, this will be converted into a combination of assignments and type ascriptions.
2055 // In particular, the 'static is imposed through a type ascription:
2056 //
2057 // ```rust
2058 // x = ...;
2059 // AscribeUserType(x, fn(&'static ())
2060 // y = x;
2061 // ```
2062 //
2063 // We wind up ultimately with constraints like
2064 //
2065 // ```rust
2066 // !a: 'temp1 // from the `y = x` statement
2067 // 'temp1: 'temp2
2068 // 'temp2: 'static // from the AscribeUserType
2069 // ```
2070 //
2071 // and here we prefer to blame the source (the y = x statement).
2072 let blame_source = match from_region_origin {
2073 NllRegionVariableOrigin::FreeRegion
2074 | NllRegionVariableOrigin::Existential { from_forall: false } => true,
2075 NllRegionVariableOrigin::Placeholder(_)
2076 | NllRegionVariableOrigin::Existential { from_forall: true } => false,
2077 };
2078
2079 // To pick a constraint to blame, we organize constraints by how interesting we expect them
2080 // to be in diagnostics, then pick the most interesting one closest to either the source or
2081 // the target on our constraint path.
2082 let constraint_interest = |constraint: &OutlivesConstraint<'tcx>| {
2083 // Try to avoid blaming constraints from desugarings, since they may not clearly match
2084 // match what users have written. As an exception, allow blaming returns generated by
2085 // `?` desugaring, since the correspondence is fairly clear.
2086 let category = if let Some(kind) = constraint.span.desugaring_kind()
2087 && (kind != DesugaringKind::QuestionMark
2088 || !matches!(constraint.category, ConstraintCategory::Return(_)))
2089 {
2090 ConstraintCategory::Boring
2091 } else {
2092 constraint.category
2093 };
2094
2095 let interest = match category {
2096 // Returns usually provide a type to blame and have specially written diagnostics,
2097 // so prioritize them.
2098 ConstraintCategory::Return(_) => 0,
2099 // Unsizing coercions are interesting, since we have a note for that:
2100 // `BorrowExplanation::add_object_lifetime_default_note`.
2101 // FIXME(dianne): That note shouldn't depend on a coercion being blamed; see issue
2102 // #131008 for an example of where we currently don't emit it but should.
2103 // Once the note is handled properly, this case should be removed. Until then, it
2104 // should be as limited as possible; the note is prone to false positives and this
2105 // constraint usually isn't best to blame.
2106 ConstraintCategory::Cast {
2107 unsize_to: Some(unsize_ty),
2108 is_implicit_coercion: true,
2109 } if target_region == self.universal_regions().fr_static
2110 // Mirror the note's condition, to minimize how often this diverts blame.
2111 && let ty::Adt(_, args) = unsize_ty.kind()
2112 && args.iter().any(|arg| arg.as_type().is_some_and(|ty| ty.is_trait()))
2113 // Mimic old logic for this, to minimize false positives in tests.
2114 && !path
2115 .iter()
2116 .any(|c| matches!(c.category, ConstraintCategory::TypeAnnotation(_))) =>
2117 {
2118 1
2119 }
2120 // Between other interesting constraints, order by their position on the `path`.
2121 ConstraintCategory::Yield
2122 | ConstraintCategory::UseAsConst
2123 | ConstraintCategory::UseAsStatic
2124 | ConstraintCategory::TypeAnnotation(
2125 AnnotationSource::Ascription
2126 | AnnotationSource::Declaration
2127 | AnnotationSource::OpaqueCast,
2128 )
2129 | ConstraintCategory::Cast { .. }
2130 | ConstraintCategory::CallArgument(_)
2131 | ConstraintCategory::CopyBound
2132 | ConstraintCategory::SizedBound
2133 | ConstraintCategory::Assignment
2134 | ConstraintCategory::Usage
2135 | ConstraintCategory::ClosureUpvar(_) => 2,
2136 // Generic arguments are unlikely to be what relates regions together
2137 ConstraintCategory::TypeAnnotation(AnnotationSource::GenericArg) => 3,
2138 // We handle predicates and opaque types specially; don't prioritize them here.
2139 ConstraintCategory::Predicate(_) | ConstraintCategory::OpaqueType => 4,
2140 // `Boring` constraints can correspond to user-written code and have useful spans,
2141 // but don't provide any other useful information for diagnostics.
2142 ConstraintCategory::Boring => 5,
2143 // `BoringNoLocation` constraints can point to user-written code, but are less
2144 // specific, and are not used for relations that would make sense to blame.
2145 ConstraintCategory::BoringNoLocation => 6,
2146 // Do not blame internal constraints.
2147 ConstraintCategory::IllegalUniverse => 7,
2148 ConstraintCategory::Internal => 8,
2149 };
2150
2151 debug!("constraint {constraint:?} category: {category:?}, interest: {interest:?}");
2152
2153 interest
2154 };
2155
2156 let best_choice = if blame_source {
2157 path.iter().enumerate().rev().min_by_key(|(_, c)| constraint_interest(c)).unwrap().0
2158 } else {
2159 path.iter().enumerate().min_by_key(|(_, c)| constraint_interest(c)).unwrap().0
2160 };
2161
2162 debug!(?best_choice, ?blame_source);
2163
2164 let best_constraint = if let Some(next) = path.get(best_choice + 1)
2165 && matches!(path[best_choice].category, ConstraintCategory::Return(_))
2166 && next.category == ConstraintCategory::OpaqueType
2167 {
2168 // The return expression is being influenced by the return type being
2169 // impl Trait, point at the return type and not the return expr.
2170 *next
2171 } else if path[best_choice].category == ConstraintCategory::Return(ReturnConstraint::Normal)
2172 && let Some(field) = path.iter().find_map(|p| {
2173 if let ConstraintCategory::ClosureUpvar(f) = p.category { Some(f) } else { None }
2174 })
2175 {
2176 OutlivesConstraint {
2177 category: ConstraintCategory::Return(ReturnConstraint::ClosureUpvar(field)),
2178 ..path[best_choice]
2179 }
2180 } else {
2181 path[best_choice]
2182 };
2183
2184 let blame_constraint = BlameConstraint {
2185 category: best_constraint.category,
2186 from_closure: best_constraint.from_closure,
2187 cause: ObligationCause::new(best_constraint.span, CRATE_DEF_ID, cause_code.clone()),
2188 variance_info: best_constraint.variance_info,
2189 };
2190 (blame_constraint, path)
2191 }
2192
2193 pub(crate) fn universe_info(&self, universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
2194 // Query canonicalization can create local superuniverses (for example in
2195 // `InferCtx::query_response_instantiation_guess`), but they don't have an associated
2196 // `UniverseInfo` explaining why they were created.
2197 // This can cause ICEs if these causes are accessed in diagnostics, for example in issue
2198 // #114907 where this happens via liveness and dropck outlives results.
2199 // Therefore, we return a default value in case that happens, which should at worst emit a
2200 // suboptimal error, instead of the ICE.
2201 self.universe_causes.get(&universe).cloned().unwrap_or_else(UniverseInfo::other)
2202 }
2203
2204 /// Tries to find the terminator of the loop in which the region 'r' resides.
2205 /// Returns the location of the terminator if found.
2206 pub(crate) fn find_loop_terminator_location(
2207 &self,
2208 r: RegionVid,
2209 body: &Body<'_>,
2210 ) -> Option<Location> {
2211 let scc = self.constraint_sccs.scc(r);
2212 let locations = self.scc_values.locations_outlived_by(scc);
2213 for location in locations {
2214 let bb = &body[location.block];
2215 if let Some(terminator) = &bb.terminator {
2216 // terminator of a loop should be TerminatorKind::FalseUnwind
2217 if let TerminatorKind::FalseUnwind { .. } = terminator.kind {
2218 return Some(location);
2219 }
2220 }
2221 }
2222 None
2223 }
2224
2225 /// Access to the SCC constraint graph.
2226 /// This can be used to quickly under-approximate the regions which are equal to each other
2227 /// and their relative orderings.
2228 // This is `pub` because it's used by unstable external borrowck data users, see `consumers.rs`.
2229 pub fn constraint_sccs(&self) -> &ConstraintSccs {
2230 &self.constraint_sccs
2231 }
2232
2233 /// Access to the region graph, built from the outlives constraints.
2234 pub(crate) fn region_graph(&self) -> RegionGraph<'_, 'tcx, graph::Normal> {
2235 self.constraint_graph.region_graph(&self.constraints, self.universal_regions().fr_static)
2236 }
2237
2238 /// Returns the representative `RegionVid` for a given SCC.
2239 /// See `RegionTracker` for how a region variable ID is chosen.
2240 ///
2241 /// It is a hacky way to manage checking regions for equality,
2242 /// since we can 'canonicalize' each region to the representative
2243 /// of its SCC and be sure that -- if they have the same repr --
2244 /// they *must* be equal (though not having the same repr does not
2245 /// mean they are unequal).
2246 fn scc_representative(&self, scc: ConstraintSccIndex) -> RegionVid {
2247 self.scc_annotations[scc].representative
2248 }
2249
2250 pub(crate) fn liveness_constraints(&self) -> &LivenessValues {
2251 &self.liveness_constraints
2252 }
2253
2254 /// When using `-Zpolonius=next`, records the given live loans for the loan scopes and active
2255 /// loans dataflow computations.
2256 pub(crate) fn record_live_loans(&mut self, live_loans: LiveLoans) {
2257 self.liveness_constraints.record_live_loans(live_loans);
2258 }
2259
2260 /// Returns whether the `loan_idx` is live at the given `location`: whether its issuing
2261 /// region is contained within the type of a variable that is live at this point.
2262 /// Note: for now, the sets of live loans is only available when using `-Zpolonius=next`.
2263 pub(crate) fn is_loan_live_at(&self, loan_idx: BorrowIndex, location: Location) -> bool {
2264 let point = self.liveness_constraints.point_from_location(location);
2265 self.liveness_constraints.is_loan_live_at(loan_idx, point)
2266 }
2267}
2268
2269impl<'tcx> RegionDefinition<'tcx> {
2270 fn new(universe: ty::UniverseIndex, rv_origin: RegionVariableOrigin) -> Self {
2271 // Create a new region definition. Note that, for free
2272 // regions, the `external_name` field gets updated later in
2273 // `init_free_and_bound_regions`.
2274
2275 let origin = match rv_origin {
2276 RegionVariableOrigin::Nll(origin) => origin,
2277 _ => NllRegionVariableOrigin::Existential { from_forall: false },
2278 };
2279
2280 Self { origin, universe, external_name: None }
2281 }
2282}
2283
2284#[derive(Clone, Debug)]
2285pub(crate) struct BlameConstraint<'tcx> {
2286 pub category: ConstraintCategory<'tcx>,
2287 pub from_closure: bool,
2288 pub cause: ObligationCause<'tcx>,
2289 pub variance_info: ty::VarianceDiagInfo<TyCtxt<'tcx>>,
2290}