1 use std::collections::VecDeque;
4 use rustc_data_structures::binary_search_util;
5 use rustc_data_structures::frozen::Frozen;
6 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
7 use rustc_data_structures::graph::scc::Sccs;
8 use rustc_errors::Diagnostic;
9 use rustc_hir::def_id::{DefId, CRATE_DEF_ID};
10 use rustc_hir::CRATE_HIR_ID;
11 use rustc_index::vec::IndexVec;
12 use rustc_infer::infer::canonical::QueryOutlivesConstraint;
13 use rustc_infer::infer::outlives::test_type_match;
14 use rustc_infer::infer::region_constraints::{GenericKind, VarInfos, VerifyBound, VerifyIfEq};
15 use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin};
16 use rustc_middle::mir::{
17 Body, ClosureOutlivesRequirement, ClosureOutlivesSubject, ClosureRegionRequirements,
18 ConstraintCategory, Local, Location, ReturnConstraint, TerminatorKind,
20 use rustc_middle::traits::ObligationCause;
21 use rustc_middle::traits::ObligationCauseCode;
22 use rustc_middle::ty::{
23 self, subst::SubstsRef, RegionVid, Ty, TyCtxt, TypeFoldable, TypeVisitable,
29 graph::NormalConstraintGraph, ConstraintSccIndex, OutlivesConstraint, OutlivesConstraintSet,
31 diagnostics::{RegionErrorKind, RegionErrors, UniverseInfo},
32 member_constraints::{MemberConstraintSet, NllMemberConstraintIndex},
33 nll::{PoloniusOutput, ToRegionVid},
34 region_infer::reverse_sccs::ReverseSccGraph,
35 region_infer::values::{
36 LivenessValues, PlaceholderIndices, RegionElement, RegionValueElements, RegionValues,
39 type_check::{free_region_relations::UniversalRegionRelations, Locations},
40 universal_regions::UniversalRegions,
50 pub struct RegionInferenceContext<'tcx> {
51 pub var_infos: VarInfos,
53 /// Contains the definition for every region variable. Region
54 /// variables are identified by their index (`RegionVid`). The
55 /// definition contains information about where the region came
56 /// from as well as its final inferred value.
57 definitions: IndexVec<RegionVid, RegionDefinition<'tcx>>,
59 /// The liveness constraints added to each region. For most
60 /// regions, these start out empty and steadily grow, though for
61 /// each universally quantified region R they start out containing
62 /// the entire CFG and `end(R)`.
63 liveness_constraints: LivenessValues<RegionVid>,
65 /// The outlives constraints computed by the type-check.
66 constraints: Frozen<OutlivesConstraintSet<'tcx>>,
68 /// The constraint-set, but in graph form, making it easy to traverse
69 /// the constraints adjacent to a particular region. Used to construct
70 /// the SCC (see `constraint_sccs`) and for error reporting.
71 constraint_graph: Frozen<NormalConstraintGraph>,
73 /// The SCC computed from `constraints` and the constraint
74 /// graph. We have an edge from SCC A to SCC B if `A: B`. Used to
75 /// compute the values of each region.
76 constraint_sccs: Rc<Sccs<RegionVid, ConstraintSccIndex>>,
78 /// Reverse of the SCC constraint graph -- i.e., an edge `A -> B` exists if
79 /// `B: A`. This is used to compute the universal regions that are required
80 /// to outlive a given SCC. Computed lazily.
81 rev_scc_graph: Option<Rc<ReverseSccGraph>>,
83 /// The "R0 member of [R1..Rn]" constraints, indexed by SCC.
84 member_constraints: Rc<MemberConstraintSet<'tcx, ConstraintSccIndex>>,
86 /// Records the member constraints that we applied to each scc.
87 /// This is useful for error reporting. Once constraint
88 /// propagation is done, this vector is sorted according to
89 /// `member_region_scc`.
90 member_constraints_applied: Vec<AppliedMemberConstraint>,
92 /// Map closure bounds to a `Span` that should be used for error reporting.
93 closure_bounds_mapping:
94 FxHashMap<Location, FxHashMap<(RegionVid, RegionVid), (ConstraintCategory<'tcx>, Span)>>,
96 /// Map universe indexes to information on why we created it.
97 universe_causes: FxHashMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
99 /// Contains the minimum universe of any variable within the same
100 /// SCC. We will ensure that no SCC contains values that are not
101 /// visible from this index.
102 scc_universes: IndexVec<ConstraintSccIndex, ty::UniverseIndex>,
104 /// Contains a "representative" from each SCC. This will be the
105 /// minimal RegionVid belonging to that universe. It is used as a
106 /// kind of hacky way to manage checking outlives relationships,
107 /// since we can 'canonicalize' each region to the representative
108 /// of its SCC and be sure that -- if they have the same repr --
109 /// they *must* be equal (though not having the same repr does not
110 /// mean they are unequal).
111 scc_representatives: IndexVec<ConstraintSccIndex, ty::RegionVid>,
113 /// The final inferred values of the region variables; we compute
114 /// one value per SCC. To get the value for any given *region*,
115 /// you first find which scc it is a part of.
116 scc_values: RegionValues<ConstraintSccIndex>,
118 /// Type constraints that we check after solving.
119 type_tests: Vec<TypeTest<'tcx>>,
121 /// Information about the universally quantified regions in scope
122 /// on this function.
123 universal_regions: Rc<UniversalRegions<'tcx>>,
125 /// Information about how the universally quantified regions in
126 /// scope on this function relate to one another.
127 universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
130 /// Each time that `apply_member_constraint` is successful, it appends
131 /// one of these structs to the `member_constraints_applied` field.
132 /// This is used in error reporting to trace out what happened.
134 /// The way that `apply_member_constraint` works is that it effectively
135 /// adds a new lower bound to the SCC it is analyzing: so you wind up
136 /// with `'R: 'O` where `'R` is the pick-region and `'O` is the
137 /// minimal viable option.
138 pub(crate) struct AppliedMemberConstraint {
139 /// The SCC that was affected. (The "member region".)
141 /// The vector if `AppliedMemberConstraint` elements is kept sorted
143 pub(crate) member_region_scc: ConstraintSccIndex,
145 /// The "best option" that `apply_member_constraint` found -- this was
146 /// added as an "ad-hoc" lower-bound to `member_region_scc`.
147 pub(crate) min_choice: ty::RegionVid,
149 /// The "member constraint index" -- we can find out details about
150 /// the constraint from
151 /// `set.member_constraints[member_constraint_index]`.
152 pub(crate) member_constraint_index: NllMemberConstraintIndex,
155 pub(crate) struct RegionDefinition<'tcx> {
156 /// What kind of variable is this -- a free region? existential
157 /// variable? etc. (See the `NllRegionVariableOrigin` for more
159 pub(crate) origin: NllRegionVariableOrigin,
161 /// Which universe is this region variable defined in? This is
162 /// most often `ty::UniverseIndex::ROOT`, but when we encounter
163 /// forall-quantifiers like `for<'a> { 'a = 'b }`, we would create
164 /// the variable for `'a` in a fresh universe that extends ROOT.
165 pub(crate) universe: ty::UniverseIndex,
167 /// If this is 'static or an early-bound region, then this is
168 /// `Some(X)` where `X` is the name of the region.
169 pub(crate) external_name: Option<ty::Region<'tcx>>,
172 /// N.B., the variants in `Cause` are intentionally ordered. Lower
173 /// values are preferred when it comes to error messages. Do not
174 /// reorder willy nilly.
175 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
176 pub(crate) enum Cause {
177 /// point inserted because Local was live at the given Location
178 LiveVar(Local, Location),
180 /// point inserted because Local was dropped at the given Location
181 DropVar(Local, Location),
184 /// A "type test" corresponds to an outlives constraint between a type
185 /// and a lifetime, like `T: 'x` or `<T as Foo>::Bar: 'x`. They are
186 /// translated from the `Verify` region constraints in the ordinary
187 /// inference context.
189 /// These sorts of constraints are handled differently than ordinary
190 /// constraints, at least at present. During type checking, the
191 /// `InferCtxt::process_registered_region_obligations` method will
192 /// attempt to convert a type test like `T: 'x` into an ordinary
193 /// outlives constraint when possible (for example, `&'a T: 'b` will
194 /// be converted into `'a: 'b` and registered as a `Constraint`).
196 /// In some cases, however, there are outlives relationships that are
197 /// not converted into a region constraint, but rather into one of
198 /// these "type tests". The distinction is that a type test does not
199 /// influence the inference result, but instead just examines the
200 /// values that we ultimately inferred for each region variable and
201 /// checks that they meet certain extra criteria. If not, an error
204 /// One reason for this is that these type tests typically boil down
205 /// to a check like `'a: 'x` where `'a` is a universally quantified
206 /// region -- and therefore not one whose value is really meant to be
207 /// *inferred*, precisely (this is not always the case: one can have a
208 /// type test like `<Foo as Trait<'?0>>::Bar: 'x`, where `'?0` is an
209 /// inference variable). Another reason is that these type tests can
210 /// involve *disjunction* -- that is, they can be satisfied in more
213 /// For more information about this translation, see
214 /// `InferCtxt::process_registered_region_obligations` and
215 /// `InferCtxt::type_must_outlive` in `rustc_infer::infer::InferCtxt`.
216 #[derive(Clone, Debug)]
217 pub struct TypeTest<'tcx> {
218 /// The type `T` that must outlive the region.
219 pub generic_kind: GenericKind<'tcx>,
221 /// The region `'x` that the type must outlive.
222 pub lower_bound: RegionVid,
224 /// Where did this constraint arise and why?
225 pub locations: Locations,
227 /// A test which, if met by the region `'x`, proves that this type
228 /// constraint is satisfied.
229 pub verify_bound: VerifyBound<'tcx>,
232 /// When we have an unmet lifetime constraint, we try to propagate it outward (e.g. to a closure
233 /// environment). If we can't, it is an error.
234 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
235 enum RegionRelationCheckResult {
241 #[derive(Clone, PartialEq, Eq, Debug)]
244 FromOutlivesConstraint(OutlivesConstraint<'tcx>),
248 #[derive(Clone, PartialEq, Eq, Debug)]
249 pub enum ExtraConstraintInfo {
250 PlaceholderFromPredicate(Span),
253 impl<'tcx> RegionInferenceContext<'tcx> {
254 /// Creates a new region inference context with a total of
255 /// `num_region_variables` valid inference variables; the first N
256 /// of those will be constant regions representing the free
257 /// regions defined in `universal_regions`.
259 /// The `outlives_constraints` and `type_tests` are an initial set
260 /// of constraints produced by the MIR type check.
263 universal_regions: Rc<UniversalRegions<'tcx>>,
264 placeholder_indices: Rc<PlaceholderIndices>,
265 universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
266 outlives_constraints: OutlivesConstraintSet<'tcx>,
267 member_constraints_in: MemberConstraintSet<'tcx, RegionVid>,
268 closure_bounds_mapping: FxHashMap<
270 FxHashMap<(RegionVid, RegionVid), (ConstraintCategory<'tcx>, Span)>,
272 universe_causes: FxHashMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
273 type_tests: Vec<TypeTest<'tcx>>,
274 liveness_constraints: LivenessValues<RegionVid>,
275 elements: &Rc<RegionValueElements>,
277 // Create a RegionDefinition for each inference variable.
278 let definitions: IndexVec<_, _> = var_infos
280 .map(|info| RegionDefinition::new(info.universe, info.origin))
283 let constraints = Frozen::freeze(outlives_constraints);
284 let constraint_graph = Frozen::freeze(constraints.graph(definitions.len()));
285 let fr_static = universal_regions.fr_static;
286 let constraint_sccs = Rc::new(constraints.compute_sccs(&constraint_graph, fr_static));
289 RegionValues::new(elements, universal_regions.len(), &placeholder_indices);
291 for region in liveness_constraints.rows() {
292 let scc = constraint_sccs.scc(region);
293 scc_values.merge_liveness(scc, region, &liveness_constraints);
296 let scc_universes = Self::compute_scc_universes(&constraint_sccs, &definitions);
298 let scc_representatives = Self::compute_scc_representatives(&constraint_sccs, &definitions);
300 let member_constraints =
301 Rc::new(member_constraints_in.into_mapped(|r| constraint_sccs.scc(r)));
303 let mut result = Self {
306 liveness_constraints,
312 member_constraints_applied: Vec::new(),
313 closure_bounds_mapping,
320 universal_region_relations,
323 result.init_free_and_bound_regions();
328 /// Each SCC is the combination of many region variables which
329 /// have been equated. Therefore, we can associate a universe with
330 /// each SCC which is minimum of all the universes of its
331 /// constituent regions -- this is because whatever value the SCC
332 /// takes on must be a value that each of the regions within the
333 /// SCC could have as well. This implies that the SCC must have
334 /// the minimum, or narrowest, universe.
335 fn compute_scc_universes(
336 constraint_sccs: &Sccs<RegionVid, ConstraintSccIndex>,
337 definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
338 ) -> IndexVec<ConstraintSccIndex, ty::UniverseIndex> {
339 let num_sccs = constraint_sccs.num_sccs();
340 let mut scc_universes = IndexVec::from_elem_n(ty::UniverseIndex::MAX, num_sccs);
342 debug!("compute_scc_universes()");
344 // For each region R in universe U, ensure that the universe for the SCC
345 // that contains R is "no bigger" than U. This effectively sets the universe
346 // for each SCC to be the minimum of the regions within.
347 for (region_vid, region_definition) in definitions.iter_enumerated() {
348 let scc = constraint_sccs.scc(region_vid);
349 let scc_universe = &mut scc_universes[scc];
350 let scc_min = std::cmp::min(region_definition.universe, *scc_universe);
351 if scc_min != *scc_universe {
352 *scc_universe = scc_min;
354 "compute_scc_universes: lowered universe of {scc:?} to {scc_min:?} \
355 because it contains {region_vid:?} in {region_universe:?}",
358 region_vid = region_vid,
359 region_universe = region_definition.universe,
364 // Walk each SCC `A` and `B` such that `A: B`
365 // and ensure that universe(A) can see universe(B).
367 // This serves to enforce the 'empty/placeholder' hierarchy
368 // (described in more detail on `RegionKind`):
373 // empty(U0) placeholder(U1)
378 // In particular, imagine we have variables R0 in U0 and R1
379 // created in U1, and constraints like this;
382 // R1: !1 // R1 outlives the placeholder in U1
383 // R1: R0 // R1 outlives R0
386 // Here, we wish for R1 to be `'static`, because it
387 // cannot outlive `placeholder(U1)` and `empty(U0)` any other way.
389 // Thanks to this loop, what happens is that the `R1: R0`
390 // constraint lowers the universe of `R1` to `U0`, which in turn
391 // means that the `R1: !1` constraint will (later) cause
392 // `R1` to become `'static`.
393 for scc_a in constraint_sccs.all_sccs() {
394 for &scc_b in constraint_sccs.successors(scc_a) {
395 let scc_universe_a = scc_universes[scc_a];
396 let scc_universe_b = scc_universes[scc_b];
397 let scc_universe_min = std::cmp::min(scc_universe_a, scc_universe_b);
398 if scc_universe_a != scc_universe_min {
399 scc_universes[scc_a] = scc_universe_min;
402 "compute_scc_universes: lowered universe of {scc_a:?} to {scc_universe_min:?} \
403 because {scc_a:?}: {scc_b:?} and {scc_b:?} is in universe {scc_universe_b:?}",
406 scc_universe_min = scc_universe_min,
407 scc_universe_b = scc_universe_b
413 debug!("compute_scc_universes: scc_universe = {:#?}", scc_universes);
418 /// For each SCC, we compute a unique `RegionVid` (in fact, the
419 /// minimal one that belongs to the SCC). See
420 /// `scc_representatives` field of `RegionInferenceContext` for
422 fn compute_scc_representatives(
423 constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
424 definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
425 ) -> IndexVec<ConstraintSccIndex, ty::RegionVid> {
426 let num_sccs = constraints_scc.num_sccs();
427 let next_region_vid = definitions.next_index();
428 let mut scc_representatives = IndexVec::from_elem_n(next_region_vid, num_sccs);
430 for region_vid in definitions.indices() {
431 let scc = constraints_scc.scc(region_vid);
432 let prev_min = scc_representatives[scc];
433 scc_representatives[scc] = region_vid.min(prev_min);
439 /// Initializes the region variables for each universally
440 /// quantified region (lifetime parameter). The first N variables
441 /// always correspond to the regions appearing in the function
442 /// signature (both named and anonymous) and where-clauses. This
443 /// function iterates over those regions and initializes them with
448 /// fn foo<'a, 'b>( /* ... */ ) where 'a: 'b { /* ... */ }
450 /// would initialize two variables like so:
451 /// ```ignore (illustrative)
452 /// R0 = { CFG, R0 } // 'a
453 /// R1 = { CFG, R0, R1 } // 'b
455 /// Here, R0 represents `'a`, and it contains (a) the entire CFG
456 /// and (b) any universally quantified regions that it outlives,
457 /// which in this case is just itself. R1 (`'b`) in contrast also
458 /// outlives `'a` and hence contains R0 and R1.
459 fn init_free_and_bound_regions(&mut self) {
460 // Update the names (if any)
461 for (external_name, variable) in self.universal_regions.named_universal_regions() {
463 "init_universal_regions: region {:?} has external name {:?}",
464 variable, external_name
466 self.definitions[variable].external_name = Some(external_name);
469 for variable in self.definitions.indices() {
470 let scc = self.constraint_sccs.scc(variable);
472 match self.definitions[variable].origin {
473 NllRegionVariableOrigin::FreeRegion => {
474 // For each free, universally quantified region X:
476 // Add all nodes in the CFG to liveness constraints
477 self.liveness_constraints.add_all_points(variable);
478 self.scc_values.add_all_points(scc);
480 // Add `end(X)` into the set for X.
481 self.scc_values.add_element(scc, variable);
484 NllRegionVariableOrigin::Placeholder(placeholder) => {
485 // Each placeholder region is only visible from
486 // its universe `ui` and its extensions. So we
487 // can't just add it into `scc` unless the
488 // universe of the scc can name this region.
489 let scc_universe = self.scc_universes[scc];
490 if scc_universe.can_name(placeholder.universe) {
491 self.scc_values.add_element(scc, placeholder);
494 "init_free_and_bound_regions: placeholder {:?} is \
495 not compatible with universe {:?} of its SCC {:?}",
496 placeholder, scc_universe, scc,
498 self.add_incompatible_universe(scc);
502 NllRegionVariableOrigin::Existential { .. } => {
503 // For existential, regions, nothing to do.
509 /// Returns an iterator over all the region indices.
510 pub fn regions(&self) -> impl Iterator<Item = RegionVid> + 'tcx {
511 self.definitions.indices()
514 /// Given a universal region in scope on the MIR, returns the
515 /// corresponding index.
517 /// (Panics if `r` is not a registered universal region.)
518 pub fn to_region_vid(&self, r: ty::Region<'tcx>) -> RegionVid {
519 self.universal_regions.to_region_vid(r)
522 /// Adds annotations for `#[rustc_regions]`; see `UniversalRegions::annotate`.
523 pub(crate) fn annotate(&self, tcx: TyCtxt<'tcx>, err: &mut Diagnostic) {
524 self.universal_regions.annotate(tcx, err)
527 /// Returns `true` if the region `r` contains the point `p`.
529 /// Panics if called before `solve()` executes,
530 pub(crate) fn region_contains(&self, r: impl ToRegionVid, p: impl ToElementIndex) -> bool {
531 let scc = self.constraint_sccs.scc(r.to_region_vid());
532 self.scc_values.contains(scc, p)
535 /// Returns access to the value of `r` for debugging purposes.
536 pub(crate) fn region_value_str(&self, r: RegionVid) -> String {
537 let scc = self.constraint_sccs.scc(r.to_region_vid());
538 self.scc_values.region_value_str(scc)
541 /// Returns access to the value of `r` for debugging purposes.
542 pub(crate) fn region_universe(&self, r: RegionVid) -> ty::UniverseIndex {
543 let scc = self.constraint_sccs.scc(r.to_region_vid());
544 self.scc_universes[scc]
547 /// Once region solving has completed, this function will return
548 /// the member constraints that were applied to the value of a given
549 /// region `r`. See `AppliedMemberConstraint`.
550 pub(crate) fn applied_member_constraints(
553 ) -> &[AppliedMemberConstraint] {
554 let scc = self.constraint_sccs.scc(r.to_region_vid());
555 binary_search_util::binary_search_slice(
556 &self.member_constraints_applied,
557 |applied| applied.member_region_scc,
562 /// Performs region inference and report errors if we see any
563 /// unsatisfiable constraints. If this is a closure, returns the
564 /// region requirements to propagate to our creator, if any.
565 #[instrument(skip(self, infcx, body, polonius_output), level = "debug")]
568 infcx: &InferCtxt<'tcx>,
569 param_env: ty::ParamEnv<'tcx>,
571 polonius_output: Option<Rc<PoloniusOutput>>,
572 ) -> (Option<ClosureRegionRequirements<'tcx>>, RegionErrors<'tcx>) {
573 let mir_def_id = body.source.def_id();
574 self.propagate_constraints(body);
576 let mut errors_buffer = RegionErrors::new();
578 // If this is a closure, we can propagate unsatisfied
579 // `outlives_requirements` to our creator, so create a vector
580 // to store those. Otherwise, we'll pass in `None` to the
581 // functions below, which will trigger them to report errors
583 let mut outlives_requirements = infcx.tcx.is_typeck_child(mir_def_id).then(Vec::new);
585 self.check_type_tests(
589 outlives_requirements.as_mut(),
593 // In Polonius mode, the errors about missing universal region relations are in the output
594 // and need to be emitted or propagated. Otherwise, we need to check whether the
595 // constraints were too strong, and if so, emit or propagate those errors.
596 if infcx.tcx.sess.opts.unstable_opts.polonius {
597 self.check_polonius_subset_errors(
598 outlives_requirements.as_mut(),
600 polonius_output.expect("Polonius output is unavailable despite `-Z polonius`"),
603 self.check_universal_regions(outlives_requirements.as_mut(), &mut errors_buffer);
606 if errors_buffer.is_empty() {
607 self.check_member_constraints(infcx, &mut errors_buffer);
610 let outlives_requirements = outlives_requirements.unwrap_or_default();
612 if outlives_requirements.is_empty() {
613 (None, errors_buffer)
615 let num_external_vids = self.universal_regions.num_global_and_external_regions();
617 Some(ClosureRegionRequirements { num_external_vids, outlives_requirements }),
623 /// Propagate the region constraints: this will grow the values
624 /// for each region variable until all the constraints are
625 /// satisfied. Note that some values may grow **too** large to be
626 /// feasible, but we check this later.
627 #[instrument(skip(self, _body), level = "debug")]
628 fn propagate_constraints(&mut self, _body: &Body<'tcx>) {
629 debug!("constraints={:#?}", {
630 let mut constraints: Vec<_> = self.constraints.outlives().iter().collect();
631 constraints.sort_by_key(|c| (c.sup, c.sub));
634 .map(|c| (c, self.constraint_sccs.scc(c.sup), self.constraint_sccs.scc(c.sub)))
638 // To propagate constraints, we walk the DAG induced by the
639 // SCC. For each SCC, we visit its successors and compute
640 // their values, then we union all those values to get our
642 let constraint_sccs = self.constraint_sccs.clone();
643 for scc in constraint_sccs.all_sccs() {
644 self.compute_value_for_scc(scc);
647 // Sort the applied member constraints so we can binary search
648 // through them later.
649 self.member_constraints_applied.sort_by_key(|applied| applied.member_region_scc);
652 /// Computes the value of the SCC `scc_a`, which has not yet been
653 /// computed, by unioning the values of its successors.
654 /// Assumes that all successors have been computed already
655 /// (which is assured by iterating over SCCs in dependency order).
656 #[instrument(skip(self), level = "debug")]
657 fn compute_value_for_scc(&mut self, scc_a: ConstraintSccIndex) {
658 let constraint_sccs = self.constraint_sccs.clone();
660 // Walk each SCC `B` such that `A: B`...
661 for &scc_b in constraint_sccs.successors(scc_a) {
664 // ...and add elements from `B` into `A`. One complication
665 // arises because of universes: If `B` contains something
666 // that `A` cannot name, then `A` can only contain `B` if
667 // it outlives static.
668 if self.universe_compatible(scc_b, scc_a) {
669 // `A` can name everything that is in `B`, so just
671 self.scc_values.add_region(scc_a, scc_b);
673 self.add_incompatible_universe(scc_a);
677 // Now take member constraints into account.
678 let member_constraints = self.member_constraints.clone();
679 for m_c_i in member_constraints.indices(scc_a) {
680 self.apply_member_constraint(scc_a, m_c_i, member_constraints.choice_regions(m_c_i));
683 debug!(value = ?self.scc_values.region_value_str(scc_a));
686 /// Invoked for each `R0 member of [R1..Rn]` constraint.
688 /// `scc` is the SCC containing R0, and `choice_regions` are the
689 /// `R1..Rn` regions -- they are always known to be universal
690 /// regions (and if that's not true, we just don't attempt to
691 /// enforce the constraint).
693 /// The current value of `scc` at the time the method is invoked
694 /// is considered a *lower bound*. If possible, we will modify
695 /// the constraint to set it equal to one of the option regions.
696 /// If we make any changes, returns true, else false.
697 #[instrument(skip(self, member_constraint_index), level = "debug")]
698 fn apply_member_constraint(
700 scc: ConstraintSccIndex,
701 member_constraint_index: NllMemberConstraintIndex,
702 choice_regions: &[ty::RegionVid],
704 // Create a mutable vector of the options. We'll try to winnow
706 let mut choice_regions: Vec<ty::RegionVid> = choice_regions.to_vec();
708 // Convert to the SCC representative: sometimes we have inference
709 // variables in the member constraint that wind up equated with
710 // universal regions. The scc representative is the minimal numbered
711 // one from the corresponding scc so it will be the universal region
713 for c_r in &mut choice_regions {
714 let scc = self.constraint_sccs.scc(*c_r);
715 *c_r = self.scc_representatives[scc];
718 // The 'member region' in a member constraint is part of the
719 // hidden type, which must be in the root universe. Therefore,
720 // it cannot have any placeholders in its value.
721 assert!(self.scc_universes[scc] == ty::UniverseIndex::ROOT);
723 self.scc_values.placeholders_contained_in(scc).next().is_none(),
724 "scc {:?} in a member constraint has placeholder value: {:?}",
726 self.scc_values.region_value_str(scc),
729 // The existing value for `scc` is a lower-bound. This will
730 // consist of some set `{P} + {LB}` of points `{P}` and
731 // lower-bound free regions `{LB}`. As each choice region `O`
732 // is a free region, it will outlive the points. But we can
733 // only consider the option `O` if `O: LB`.
734 choice_regions.retain(|&o_r| {
736 .universal_regions_outlived_by(scc)
737 .all(|lb| self.universal_region_relations.outlives(o_r, lb))
739 debug!(?choice_regions, "after lb");
741 // Now find all the *upper bounds* -- that is, each UB is a
742 // free region that must outlive the member region `R0` (`UB:
743 // R0`). Therefore, we need only keep an option `O` if `UB: O`
745 let rev_scc_graph = self.reverse_scc_graph();
746 let universal_region_relations = &self.universal_region_relations;
747 for ub in rev_scc_graph.upper_bounds(scc) {
749 choice_regions.retain(|&o_r| universal_region_relations.outlives(ub, o_r));
751 debug!(?choice_regions, "after ub");
753 // If we ruled everything out, we're done.
754 if choice_regions.is_empty() {
758 // Otherwise, we need to find the minimum remaining choice, if
759 // any, and take that.
760 debug!("choice_regions remaining are {:#?}", choice_regions);
761 let Some(&min_choice) = choice_regions.iter().find(|&r1| {
762 choice_regions.iter().all(|&r2| {
763 self.universal_region_relations.outlives(r2, *r1)
766 debug!("no choice region outlived by all others");
770 let min_choice_scc = self.constraint_sccs.scc(min_choice);
771 debug!(?min_choice, ?min_choice_scc);
772 if self.scc_values.add_region(scc, min_choice_scc) {
773 self.member_constraints_applied.push(AppliedMemberConstraint {
774 member_region_scc: scc,
776 member_constraint_index,
785 /// Returns `true` if all the elements in the value of `scc_b` are nameable
786 /// in `scc_a`. Used during constraint propagation, and only once
787 /// the value of `scc_b` has been computed.
788 fn universe_compatible(&self, scc_b: ConstraintSccIndex, scc_a: ConstraintSccIndex) -> bool {
789 let universe_a = self.scc_universes[scc_a];
791 // Quick check: if scc_b's declared universe is a subset of
792 // scc_a's declared universe (typically, both are ROOT), then
793 // it cannot contain any problematic universe elements.
794 if universe_a.can_name(self.scc_universes[scc_b]) {
798 // Otherwise, we have to iterate over the universe elements in
799 // B's value, and check whether all of them are nameable
801 self.scc_values.placeholders_contained_in(scc_b).all(|p| universe_a.can_name(p.universe))
804 /// Extend `scc` so that it can outlive some placeholder region
805 /// from a universe it can't name; at present, the only way for
806 /// this to be true is if `scc` outlives `'static`. This is
807 /// actually stricter than necessary: ideally, we'd support bounds
808 /// like `for<'a: 'b`>` that might then allow us to approximate
809 /// `'a` with `'b` and not `'static`. But it will have to do for
811 fn add_incompatible_universe(&mut self, scc: ConstraintSccIndex) {
812 debug!("add_incompatible_universe(scc={:?})", scc);
814 let fr_static = self.universal_regions.fr_static;
815 self.scc_values.add_all_points(scc);
816 self.scc_values.add_element(scc, fr_static);
819 /// Once regions have been propagated, this method is used to see
820 /// whether the "type tests" produced by typeck were satisfied;
821 /// type tests encode type-outlives relationships like `T:
822 /// 'a`. See `TypeTest` for more details.
825 infcx: &InferCtxt<'tcx>,
826 param_env: ty::ParamEnv<'tcx>,
828 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
829 errors_buffer: &mut RegionErrors<'tcx>,
833 // Sometimes we register equivalent type-tests that would
834 // result in basically the exact same error being reported to
835 // the user. Avoid that.
836 let mut deduplicate_errors = FxHashSet::default();
838 for type_test in &self.type_tests {
839 debug!("check_type_test: {:?}", type_test);
841 let generic_ty = type_test.generic_kind.to_ty(tcx);
842 if self.eval_verify_bound(
847 type_test.lower_bound,
848 &type_test.verify_bound,
853 if let Some(propagated_outlives_requirements) = &mut propagated_outlives_requirements {
854 if self.try_promote_type_test(
859 propagated_outlives_requirements,
865 // Type-test failed. Report the error.
866 let erased_generic_kind = infcx.tcx.erase_regions(type_test.generic_kind);
868 // Skip duplicate-ish errors.
869 if deduplicate_errors.insert((
871 type_test.lower_bound,
875 "check_type_test: reporting error for erased_generic_kind={:?}, \
876 lower_bound_region={:?}, \
877 type_test.locations={:?}",
878 erased_generic_kind, type_test.lower_bound, type_test.locations,
881 errors_buffer.push(RegionErrorKind::TypeTestError { type_test: type_test.clone() });
886 /// Invoked when we have some type-test (e.g., `T: 'X`) that we cannot
887 /// prove to be satisfied. If this is a closure, we will attempt to
888 /// "promote" this type-test into our `ClosureRegionRequirements` and
889 /// hence pass it up the creator. To do this, we have to phrase the
890 /// type-test in terms of external free regions, as local free
891 /// regions are not nameable by the closure's creator.
893 /// Promotion works as follows: we first check that the type `T`
894 /// contains only regions that the creator knows about. If this is
895 /// true, then -- as a consequence -- we know that all regions in
896 /// the type `T` are free regions that outlive the closure body. If
897 /// false, then promotion fails.
899 /// Once we've promoted T, we have to "promote" `'X` to some region
900 /// that is "external" to the closure. Generally speaking, a region
901 /// may be the union of some points in the closure body as well as
902 /// various free lifetimes. We can ignore the points in the closure
903 /// body: if the type T can be expressed in terms of external regions,
904 /// we know it outlives the points in the closure body. That
905 /// just leaves the free regions.
907 /// The idea then is to lower the `T: 'X` constraint into multiple
908 /// bounds -- e.g., if `'X` is the union of two free lifetimes,
909 /// `'1` and `'2`, then we would create `T: '1` and `T: '2`.
910 #[instrument(level = "debug", skip(self, infcx, propagated_outlives_requirements))]
911 fn try_promote_type_test(
913 infcx: &InferCtxt<'tcx>,
914 param_env: ty::ParamEnv<'tcx>,
916 type_test: &TypeTest<'tcx>,
917 propagated_outlives_requirements: &mut Vec<ClosureOutlivesRequirement<'tcx>>,
921 let TypeTest { generic_kind, lower_bound, locations, verify_bound: _ } = type_test;
923 let generic_ty = generic_kind.to_ty(tcx);
924 let Some(subject) = self.try_promote_type_test_subject(infcx, generic_ty) else {
928 debug!("subject = {:?}", subject);
930 let r_scc = self.constraint_sccs.scc(*lower_bound);
933 "lower_bound = {:?} r_scc={:?} universe={:?}",
934 lower_bound, r_scc, self.scc_universes[r_scc]
937 // If the type test requires that `T: 'a` where `'a` is a
938 // placeholder from another universe, that effectively requires
939 // `T: 'static`, so we have to propagate that requirement.
941 // It doesn't matter *what* universe because the promoted `T` will
942 // always be in the root universe.
943 if let Some(p) = self.scc_values.placeholders_contained_in(r_scc).next() {
944 debug!("encountered placeholder in higher universe: {:?}, requiring 'static", p);
945 let static_r = self.universal_regions.fr_static;
946 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
948 outlived_free_region: static_r,
949 blame_span: locations.span(body),
950 category: ConstraintCategory::Boring,
953 // we can return here -- the code below might push add'l constraints
954 // but they would all be weaker than this one.
958 // For each region outlived by lower_bound find a non-local,
959 // universal region (it may be the same region) and add it to
960 // `ClosureOutlivesRequirement`.
961 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
962 debug!("universal_region_outlived_by ur={:?}", ur);
963 // Check whether we can already prove that the "subject" outlives `ur`.
964 // If so, we don't have to propagate this requirement to our caller.
966 // To continue the example from the function, if we are trying to promote
967 // a requirement that `T: 'X`, and we know that `'X = '1 + '2` (i.e., the union
968 // `'1` and `'2`), then in this loop `ur` will be `'1` (and `'2`). So here
969 // we check whether `T: '1` is something we *can* prove. If so, no need
970 // to propagate that requirement.
972 // This is needed because -- particularly in the case
973 // where `ur` is a local bound -- we are sometimes in a
974 // position to prove things that our caller cannot. See
975 // #53570 for an example.
976 if self.eval_verify_bound(
982 &type_test.verify_bound,
987 let non_local_ub = self.universal_region_relations.non_local_upper_bounds(ur);
988 debug!("try_promote_type_test: non_local_ub={:?}", non_local_ub);
990 // This is slightly too conservative. To show T: '1, given `'2: '1`
991 // and `'3: '1` we only need to prove that T: '2 *or* T: '3, but to
992 // avoid potential non-determinism we approximate this by requiring
994 for upper_bound in non_local_ub {
995 debug_assert!(self.universal_regions.is_universal_region(upper_bound));
996 debug_assert!(!self.universal_regions.is_local_free_region(upper_bound));
998 let requirement = ClosureOutlivesRequirement {
1000 outlived_free_region: upper_bound,
1001 blame_span: locations.span(body),
1002 category: ConstraintCategory::Boring,
1004 debug!("try_promote_type_test: pushing {:#?}", requirement);
1005 propagated_outlives_requirements.push(requirement);
1011 /// When we promote a type test `T: 'r`, we have to convert the
1012 /// type `T` into something we can store in a query result (so
1013 /// something allocated for `'tcx`). This is problematic if `ty`
1014 /// contains regions. During the course of NLL region checking, we
1015 /// will have replaced all of those regions with fresh inference
1016 /// variables. To create a test subject, we want to replace those
1017 /// inference variables with some region from the closure
1018 /// signature -- this is not always possible, so this is a
1019 /// fallible process. Presuming we do find a suitable region, we
1020 /// will use it's *external name*, which will be a `RegionKind`
1021 /// variant that can be used in query responses such as
1023 #[instrument(level = "debug", skip(self, infcx))]
1024 fn try_promote_type_test_subject(
1026 infcx: &InferCtxt<'tcx>,
1028 ) -> Option<ClosureOutlivesSubject<'tcx>> {
1029 let tcx = infcx.tcx;
1031 let ty = tcx.fold_regions(ty, |r, _depth| {
1032 let region_vid = self.to_region_vid(r);
1034 // The challenge if this. We have some region variable `r`
1035 // whose value is a set of CFG points and universal
1036 // regions. We want to find if that set is *equivalent* to
1037 // any of the named regions found in the closure.
1039 // To do so, we compute the
1040 // `non_local_universal_upper_bound`. This will be a
1041 // non-local, universal region that is greater than `r`.
1042 // However, it might not be *contained* within `r`, so
1043 // then we further check whether this bound is contained
1044 // in `r`. If so, we can say that `r` is equivalent to the
1047 // Let's work through a few examples. For these, imagine
1048 // that we have 3 non-local regions (I'll denote them as
1049 // `'static`, `'a`, and `'b`, though of course in the code
1050 // they would be represented with indices) where:
1055 // First, let's assume that `r` is some existential
1056 // variable with an inferred value `{'a, 'static}` (plus
1057 // some CFG nodes). In this case, the non-local upper
1058 // bound is `'static`, since that outlives `'a`. `'static`
1059 // is also a member of `r` and hence we consider `r`
1060 // equivalent to `'static` (and replace it with
1063 // Now let's consider the inferred value `{'a, 'b}`. This
1064 // means `r` is effectively `'a | 'b`. I'm not sure if
1065 // this can come about, actually, but assuming it did, we
1066 // would get a non-local upper bound of `'static`. Since
1067 // `'static` is not contained in `r`, we would fail to
1068 // find an equivalent.
1069 let upper_bound = self.non_local_universal_upper_bound(region_vid);
1070 if self.region_contains(region_vid, upper_bound) {
1071 self.definitions[upper_bound].external_name.unwrap_or(r)
1073 // In the case of a failure, use a `ReVar` result. This will
1074 // cause the `needs_infer` later on to return `None`.
1079 debug!("try_promote_type_test_subject: folded ty = {:?}", ty);
1081 // `needs_infer` will only be true if we failed to promote some region.
1082 if ty.needs_infer() {
1086 Some(ClosureOutlivesSubject::Ty(ty))
1089 /// Given some universal or existential region `r`, finds a
1090 /// non-local, universal region `r+` that outlives `r` at entry to (and
1091 /// exit from) the closure. In the worst case, this will be
1094 /// This is used for two purposes. First, if we are propagated
1095 /// some requirement `T: r`, we can use this method to enlarge `r`
1096 /// to something we can encode for our creator (which only knows
1097 /// about non-local, universal regions). It is also used when
1098 /// encoding `T` as part of `try_promote_type_test_subject` (see
1099 /// that fn for details).
1101 /// This is based on the result `'y` of `universal_upper_bound`,
1102 /// except that it converts further takes the non-local upper
1103 /// bound of `'y`, so that the final result is non-local.
1104 fn non_local_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1105 debug!("non_local_universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1107 let lub = self.universal_upper_bound(r);
1109 // Grow further to get smallest universal region known to
1111 let non_local_lub = self.universal_region_relations.non_local_upper_bound(lub);
1113 debug!("non_local_universal_upper_bound: non_local_lub={:?}", non_local_lub);
1118 /// Returns a universally quantified region that outlives the
1119 /// value of `r` (`r` may be existentially or universally
1122 /// Since `r` is (potentially) an existential region, it has some
1123 /// value which may include (a) any number of points in the CFG
1124 /// and (b) any number of `end('x)` elements of universally
1125 /// quantified regions. To convert this into a single universal
1126 /// region we do as follows:
1128 /// - Ignore the CFG points in `'r`. All universally quantified regions
1129 /// include the CFG anyhow.
1130 /// - For each `end('x)` element in `'r`, compute the mutual LUB, yielding
1132 #[instrument(skip(self), level = "debug", ret)]
1133 pub(crate) fn universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1134 debug!(r = %self.region_value_str(r));
1136 // Find the smallest universal region that contains all other
1137 // universal regions within `region`.
1138 let mut lub = self.universal_regions.fr_fn_body;
1139 let r_scc = self.constraint_sccs.scc(r);
1140 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1141 lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1147 /// Like `universal_upper_bound`, but returns an approximation more suitable
1148 /// for diagnostics. If `r` contains multiple disjoint universal regions
1149 /// (e.g. 'a and 'b in `fn foo<'a, 'b> { ... }`, we pick the lower-numbered region.
1150 /// This corresponds to picking named regions over unnamed regions
1151 /// (e.g. picking early-bound regions over a closure late-bound region).
1153 /// This means that the returned value may not be a true upper bound, since
1154 /// only 'static is known to outlive disjoint universal regions.
1155 /// Therefore, this method should only be used in diagnostic code,
1156 /// where displaying *some* named universal region is better than
1157 /// falling back to 'static.
1158 #[instrument(level = "debug", skip(self))]
1159 pub(crate) fn approx_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1160 debug!("{}", self.region_value_str(r));
1162 // Find the smallest universal region that contains all other
1163 // universal regions within `region`.
1164 let mut lub = self.universal_regions.fr_fn_body;
1165 let r_scc = self.constraint_sccs.scc(r);
1166 let static_r = self.universal_regions.fr_static;
1167 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1168 let new_lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1169 debug!(?ur, ?lub, ?new_lub);
1170 // The upper bound of two non-static regions is static: this
1171 // means we know nothing about the relationship between these
1172 // two regions. Pick a 'better' one to use when constructing
1174 if ur != static_r && lub != static_r && new_lub == static_r {
1175 // Prefer the region with an `external_name` - this
1176 // indicates that the region is early-bound, so working with
1177 // it can produce a nicer error.
1178 if self.region_definition(ur).external_name.is_some() {
1180 } else if self.region_definition(lub).external_name.is_some() {
1181 // Leave lub unchanged
1183 // If we get here, we don't have any reason to prefer
1184 // one region over the other. Just pick the
1185 // one with the lower index for now.
1186 lub = std::cmp::min(ur, lub);
1198 /// Tests if `test` is true when applied to `lower_bound` at
1200 fn eval_verify_bound(
1202 infcx: &InferCtxt<'tcx>,
1203 param_env: ty::ParamEnv<'tcx>,
1205 generic_ty: Ty<'tcx>,
1206 lower_bound: RegionVid,
1207 verify_bound: &VerifyBound<'tcx>,
1209 debug!("eval_verify_bound(lower_bound={:?}, verify_bound={:?})", lower_bound, verify_bound);
1211 match verify_bound {
1212 VerifyBound::IfEq(verify_if_eq_b) => {
1213 self.eval_if_eq(infcx, param_env, generic_ty, lower_bound, *verify_if_eq_b)
1216 VerifyBound::IsEmpty => {
1217 let lower_bound_scc = self.constraint_sccs.scc(lower_bound);
1218 self.scc_values.elements_contained_in(lower_bound_scc).next().is_none()
1221 VerifyBound::OutlivedBy(r) => {
1222 let r_vid = self.to_region_vid(*r);
1223 self.eval_outlives(r_vid, lower_bound)
1226 VerifyBound::AnyBound(verify_bounds) => verify_bounds.iter().any(|verify_bound| {
1227 self.eval_verify_bound(
1237 VerifyBound::AllBounds(verify_bounds) => verify_bounds.iter().all(|verify_bound| {
1238 self.eval_verify_bound(
1252 infcx: &InferCtxt<'tcx>,
1253 param_env: ty::ParamEnv<'tcx>,
1254 generic_ty: Ty<'tcx>,
1255 lower_bound: RegionVid,
1256 verify_if_eq_b: ty::Binder<'tcx, VerifyIfEq<'tcx>>,
1258 let generic_ty = self.normalize_to_scc_representatives(infcx.tcx, generic_ty);
1259 let verify_if_eq_b = self.normalize_to_scc_representatives(infcx.tcx, verify_if_eq_b);
1260 match test_type_match::extract_verify_if_eq(
1267 let r_vid = self.to_region_vid(r);
1268 self.eval_outlives(r_vid, lower_bound)
1274 /// This is a conservative normalization procedure. It takes every
1275 /// free region in `value` and replaces it with the
1276 /// "representative" of its SCC (see `scc_representatives` field).
1277 /// We are guaranteed that if two values normalize to the same
1278 /// thing, then they are equal; this is a conservative check in
1279 /// that they could still be equal even if they normalize to
1280 /// different results. (For example, there might be two regions
1281 /// with the same value that are not in the same SCC).
1283 /// N.B., this is not an ideal approach and I would like to revisit
1284 /// it. However, it works pretty well in practice. In particular,
1285 /// this is needed to deal with projection outlives bounds like
1288 /// <T as Foo<'0>>::Item: '1
1291 /// In particular, this routine winds up being important when
1292 /// there are bounds like `where <T as Foo<'a>>::Item: 'b` in the
1293 /// environment. In this case, if we can show that `'0 == 'a`,
1294 /// and that `'b: '1`, then we know that the clause is
1295 /// satisfied. In such cases, particularly due to limitations of
1296 /// the trait solver =), we usually wind up with a where-clause like
1297 /// `T: Foo<'a>` in scope, which thus forces `'0 == 'a` to be added as
1298 /// a constraint, and thus ensures that they are in the same SCC.
1300 /// So why can't we do a more correct routine? Well, we could
1301 /// *almost* use the `relate_tys` code, but the way it is
1302 /// currently setup it creates inference variables to deal with
1303 /// higher-ranked things and so forth, and right now the inference
1304 /// context is not permitted to make more inference variables. So
1305 /// we use this kind of hacky solution.
1306 fn normalize_to_scc_representatives<T>(&self, tcx: TyCtxt<'tcx>, value: T) -> T
1308 T: TypeFoldable<'tcx>,
1310 tcx.fold_regions(value, |r, _db| {
1311 let vid = self.to_region_vid(r);
1312 let scc = self.constraint_sccs.scc(vid);
1313 let repr = self.scc_representatives[scc];
1314 tcx.mk_region(ty::ReVar(repr))
1318 // Evaluate whether `sup_region == sub_region`.
1319 fn eval_equal(&self, r1: RegionVid, r2: RegionVid) -> bool {
1320 self.eval_outlives(r1, r2) && self.eval_outlives(r2, r1)
1323 // Evaluate whether `sup_region: sub_region`.
1324 #[instrument(skip(self), level = "debug", ret)]
1325 fn eval_outlives(&self, sup_region: RegionVid, sub_region: RegionVid) -> bool {
1327 "sup_region's value = {:?} universal={:?}",
1328 self.region_value_str(sup_region),
1329 self.universal_regions.is_universal_region(sup_region),
1332 "sub_region's value = {:?} universal={:?}",
1333 self.region_value_str(sub_region),
1334 self.universal_regions.is_universal_region(sub_region),
1337 let sub_region_scc = self.constraint_sccs.scc(sub_region);
1338 let sup_region_scc = self.constraint_sccs.scc(sup_region);
1340 // If we are checking that `'sup: 'sub`, and `'sub` contains
1341 // some placeholder that `'sup` cannot name, then this is only
1342 // true if `'sup` outlives static.
1343 if !self.universe_compatible(sub_region_scc, sup_region_scc) {
1345 "sub universe `{sub_region_scc:?}` is not nameable \
1346 by super `{sup_region_scc:?}`, promoting to static",
1349 return self.eval_outlives(sup_region, self.universal_regions.fr_static);
1352 // Both the `sub_region` and `sup_region` consist of the union
1353 // of some number of universal regions (along with the union
1354 // of various points in the CFG; ignore those points for
1355 // now). Therefore, the sup-region outlives the sub-region if,
1356 // for each universal region R1 in the sub-region, there
1357 // exists some region R2 in the sup-region that outlives R1.
1358 let universal_outlives =
1359 self.scc_values.universal_regions_outlived_by(sub_region_scc).all(|r1| {
1361 .universal_regions_outlived_by(sup_region_scc)
1362 .any(|r2| self.universal_region_relations.outlives(r2, r1))
1365 if !universal_outlives {
1366 debug!("sub region contains a universal region not present in super");
1370 // Now we have to compare all the points in the sub region and make
1371 // sure they exist in the sup region.
1373 if self.universal_regions.is_universal_region(sup_region) {
1374 // Micro-opt: universal regions contain all points.
1375 debug!("super is universal and hence contains all points");
1379 debug!("comparison between points in sup/sub");
1381 self.scc_values.contains_points(sup_region_scc, sub_region_scc)
1384 /// Once regions have been propagated, this method is used to see
1385 /// whether any of the constraints were too strong. In particular,
1386 /// we want to check for a case where a universally quantified
1387 /// region exceeded its bounds. Consider:
1389 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1391 /// In this case, returning `x` requires `&'a u32 <: &'b u32`
1392 /// and hence we establish (transitively) a constraint that
1393 /// `'a: 'b`. The `propagate_constraints` code above will
1394 /// therefore add `end('a)` into the region for `'b` -- but we
1395 /// have no evidence that `'b` outlives `'a`, so we want to report
1398 /// If `propagated_outlives_requirements` is `Some`, then we will
1399 /// push unsatisfied obligations into there. Otherwise, we'll
1400 /// report them as errors.
1401 fn check_universal_regions(
1403 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1404 errors_buffer: &mut RegionErrors<'tcx>,
1406 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1407 match fr_definition.origin {
1408 NllRegionVariableOrigin::FreeRegion => {
1409 // Go through each of the universal regions `fr` and check that
1410 // they did not grow too large, accumulating any requirements
1411 // for our caller into the `outlives_requirements` vector.
1412 self.check_universal_region(
1414 &mut propagated_outlives_requirements,
1419 NllRegionVariableOrigin::Placeholder(placeholder) => {
1420 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1423 NllRegionVariableOrigin::Existential { .. } => {
1424 // nothing to check here
1430 /// Checks if Polonius has found any unexpected free region relations.
1432 /// In Polonius terms, a "subset error" (or "illegal subset relation error") is the equivalent
1433 /// of NLL's "checking if any region constraints were too strong": a placeholder origin `'a`
1434 /// was unexpectedly found to be a subset of another placeholder origin `'b`, and means in NLL
1435 /// terms that the "longer free region" `'a` outlived the "shorter free region" `'b`.
1437 /// More details can be found in this blog post by Niko:
1438 /// <https://smallcultfollowing.com/babysteps/blog/2019/01/17/polonius-and-region-errors/>
1440 /// In the canonical example
1442 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1444 /// returning `x` requires `&'a u32 <: &'b u32` and hence we establish (transitively) a
1445 /// constraint that `'a: 'b`. It is an error that we have no evidence that this
1446 /// constraint holds.
1448 /// If `propagated_outlives_requirements` is `Some`, then we will
1449 /// push unsatisfied obligations into there. Otherwise, we'll
1450 /// report them as errors.
1451 fn check_polonius_subset_errors(
1453 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1454 errors_buffer: &mut RegionErrors<'tcx>,
1455 polonius_output: Rc<PoloniusOutput>,
1458 "check_polonius_subset_errors: {} subset_errors",
1459 polonius_output.subset_errors.len()
1462 // Similarly to `check_universal_regions`: a free region relation, which was not explicitly
1463 // declared ("known") was found by Polonius, so emit an error, or propagate the
1464 // requirements for our caller into the `propagated_outlives_requirements` vector.
1466 // Polonius doesn't model regions ("origins") as CFG-subsets or durations, but the
1467 // `longer_fr` and `shorter_fr` terminology will still be used here, for consistency with
1468 // the rest of the NLL infrastructure. The "subset origin" is the "longer free region",
1469 // and the "superset origin" is the outlived "shorter free region".
1471 // Note: Polonius will produce a subset error at every point where the unexpected
1472 // `longer_fr`'s "placeholder loan" is contained in the `shorter_fr`. This can be helpful
1473 // for diagnostics in the future, e.g. to point more precisely at the key locations
1474 // requiring this constraint to hold. However, the error and diagnostics code downstream
1475 // expects that these errors are not duplicated (and that they are in a certain order).
1476 // Otherwise, diagnostics messages such as the ones giving names like `'1` to elided or
1477 // anonymous lifetimes for example, could give these names differently, while others like
1478 // the outlives suggestions or the debug output from `#[rustc_regions]` would be
1479 // duplicated. The polonius subset errors are deduplicated here, while keeping the
1480 // CFG-location ordering.
1481 let mut subset_errors: Vec<_> = polonius_output
1484 .flat_map(|(_location, subset_errors)| subset_errors.iter())
1486 subset_errors.sort();
1487 subset_errors.dedup();
1489 for (longer_fr, shorter_fr) in subset_errors.into_iter() {
1491 "check_polonius_subset_errors: subset_error longer_fr={:?},\
1493 longer_fr, shorter_fr
1496 let propagated = self.try_propagate_universal_region_error(
1499 &mut propagated_outlives_requirements,
1501 if propagated == RegionRelationCheckResult::Error {
1502 errors_buffer.push(RegionErrorKind::RegionError {
1503 longer_fr: *longer_fr,
1504 shorter_fr: *shorter_fr,
1505 fr_origin: NllRegionVariableOrigin::FreeRegion,
1511 // Handle the placeholder errors as usual, until the chalk-rustc-polonius triumvirate has
1512 // a more complete picture on how to separate this responsibility.
1513 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1514 match fr_definition.origin {
1515 NllRegionVariableOrigin::FreeRegion => {
1516 // handled by polonius above
1519 NllRegionVariableOrigin::Placeholder(placeholder) => {
1520 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1523 NllRegionVariableOrigin::Existential { .. } => {
1524 // nothing to check here
1530 /// Checks the final value for the free region `fr` to see if it
1531 /// grew too large. In particular, examine what `end(X)` points
1532 /// wound up in `fr`'s final value; for each `end(X)` where `X !=
1533 /// fr`, we want to check that `fr: X`. If not, that's either an
1534 /// error, or something we have to propagate to our creator.
1536 /// Things that are to be propagated are accumulated into the
1537 /// `outlives_requirements` vector.
1538 #[instrument(skip(self, propagated_outlives_requirements, errors_buffer), level = "debug")]
1539 fn check_universal_region(
1541 longer_fr: RegionVid,
1542 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1543 errors_buffer: &mut RegionErrors<'tcx>,
1545 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1547 // Because this free region must be in the ROOT universe, we
1548 // know it cannot contain any bound universes.
1549 assert!(self.scc_universes[longer_fr_scc] == ty::UniverseIndex::ROOT);
1550 debug_assert!(self.scc_values.placeholders_contained_in(longer_fr_scc).next().is_none());
1552 // Only check all of the relations for the main representative of each
1553 // SCC, otherwise just check that we outlive said representative. This
1554 // reduces the number of redundant relations propagated out of
1556 // Note that the representative will be a universal region if there is
1557 // one in this SCC, so we will always check the representative here.
1558 let representative = self.scc_representatives[longer_fr_scc];
1559 if representative != longer_fr {
1560 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1563 propagated_outlives_requirements,
1565 errors_buffer.push(RegionErrorKind::RegionError {
1567 shorter_fr: representative,
1568 fr_origin: NllRegionVariableOrigin::FreeRegion,
1575 // Find every region `o` such that `fr: o`
1576 // (because `fr` includes `end(o)`).
1577 let mut error_reported = false;
1578 for shorter_fr in self.scc_values.universal_regions_outlived_by(longer_fr_scc) {
1579 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1582 propagated_outlives_requirements,
1584 // We only report the first region error. Subsequent errors are hidden so as
1585 // not to overwhelm the user, but we do record them so as to potentially print
1586 // better diagnostics elsewhere...
1587 errors_buffer.push(RegionErrorKind::RegionError {
1590 fr_origin: NllRegionVariableOrigin::FreeRegion,
1591 is_reported: !error_reported,
1594 error_reported = true;
1599 /// Checks that we can prove that `longer_fr: shorter_fr`. If we can't we attempt to propagate
1600 /// the constraint outward (e.g. to a closure environment), but if that fails, there is an
1602 fn check_universal_region_relation(
1604 longer_fr: RegionVid,
1605 shorter_fr: RegionVid,
1606 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1607 ) -> RegionRelationCheckResult {
1608 // If it is known that `fr: o`, carry on.
1609 if self.universal_region_relations.outlives(longer_fr, shorter_fr) {
1610 RegionRelationCheckResult::Ok
1612 // If we are not in a context where we can't propagate errors, or we
1613 // could not shrink `fr` to something smaller, then just report an
1616 // Note: in this case, we use the unapproximated regions to report the
1617 // error. This gives better error messages in some cases.
1618 self.try_propagate_universal_region_error(
1621 propagated_outlives_requirements,
1626 /// Attempt to propagate a region error (e.g. `'a: 'b`) that is not met to a closure's
1627 /// creator. If we cannot, then the caller should report an error to the user.
1628 fn try_propagate_universal_region_error(
1630 longer_fr: RegionVid,
1631 shorter_fr: RegionVid,
1632 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1633 ) -> RegionRelationCheckResult {
1634 if let Some(propagated_outlives_requirements) = propagated_outlives_requirements {
1635 // Shrink `longer_fr` until we find a non-local region (if we do).
1636 // We'll call it `fr-` -- it's ever so slightly smaller than
1638 if let Some(fr_minus) = self.universal_region_relations.non_local_lower_bound(longer_fr)
1640 debug!("try_propagate_universal_region_error: fr_minus={:?}", fr_minus);
1642 let blame_span_category = self.find_outlives_blame_span(
1644 NllRegionVariableOrigin::FreeRegion,
1648 // Grow `shorter_fr` until we find some non-local regions. (We
1649 // always will.) We'll call them `shorter_fr+` -- they're ever
1650 // so slightly larger than `shorter_fr`.
1651 let shorter_fr_plus =
1652 self.universal_region_relations.non_local_upper_bounds(shorter_fr);
1654 "try_propagate_universal_region_error: shorter_fr_plus={:?}",
1657 for fr in shorter_fr_plus {
1658 // Push the constraint `fr-: shorter_fr+`
1659 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
1660 subject: ClosureOutlivesSubject::Region(fr_minus),
1661 outlived_free_region: fr,
1662 blame_span: blame_span_category.1.span,
1663 category: blame_span_category.0,
1666 return RegionRelationCheckResult::Propagated;
1670 RegionRelationCheckResult::Error
1673 fn check_bound_universal_region(
1675 longer_fr: RegionVid,
1676 placeholder: ty::PlaceholderRegion,
1677 errors_buffer: &mut RegionErrors<'tcx>,
1679 debug!("check_bound_universal_region(fr={:?}, placeholder={:?})", longer_fr, placeholder,);
1681 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1682 debug!("check_bound_universal_region: longer_fr_scc={:?}", longer_fr_scc,);
1684 // If we have some bound universal region `'a`, then the only
1685 // elements it can contain is itself -- we don't know anything
1687 let Some(error_element) = ({
1688 self.scc_values.elements_contained_in(longer_fr_scc).find(|element| match element {
1689 RegionElement::Location(_) => true,
1690 RegionElement::RootUniversalRegion(_) => true,
1691 RegionElement::PlaceholderRegion(placeholder1) => placeholder != *placeholder1,
1696 debug!("check_bound_universal_region: error_element = {:?}", error_element);
1698 // Find the region that introduced this `error_element`.
1699 errors_buffer.push(RegionErrorKind::BoundUniversalRegionError {
1706 fn check_member_constraints(
1708 infcx: &InferCtxt<'tcx>,
1709 errors_buffer: &mut RegionErrors<'tcx>,
1711 let member_constraints = self.member_constraints.clone();
1712 for m_c_i in member_constraints.all_indices() {
1713 debug!("check_member_constraint(m_c_i={:?})", m_c_i);
1714 let m_c = &member_constraints[m_c_i];
1715 let member_region_vid = m_c.member_region_vid;
1717 "check_member_constraint: member_region_vid={:?} with value {}",
1719 self.region_value_str(member_region_vid),
1721 let choice_regions = member_constraints.choice_regions(m_c_i);
1722 debug!("check_member_constraint: choice_regions={:?}", choice_regions);
1724 // Did the member region wind up equal to any of the option regions?
1726 choice_regions.iter().find(|&&o_r| self.eval_equal(o_r, m_c.member_region_vid))
1728 debug!("check_member_constraint: evaluated as equal to {:?}", o);
1732 // If not, report an error.
1733 let member_region = infcx.tcx.mk_region(ty::ReVar(member_region_vid));
1734 errors_buffer.push(RegionErrorKind::UnexpectedHiddenRegion {
1735 span: m_c.definition_span,
1736 hidden_ty: m_c.hidden_ty,
1743 /// We have a constraint `fr1: fr2` that is not satisfied, where
1744 /// `fr2` represents some universal region. Here, `r` is some
1745 /// region where we know that `fr1: r` and this function has the
1746 /// job of determining whether `r` is "to blame" for the fact that
1747 /// `fr1: fr2` is required.
1749 /// This is true under two conditions:
1752 /// - `fr2` is `'static` and `r` is some placeholder in a universe
1753 /// that cannot be named by `fr1`; in that case, we will require
1754 /// that `fr1: 'static` because it is the only way to `fr1: r` to
1755 /// be satisfied. (See `add_incompatible_universe`.)
1756 pub(crate) fn provides_universal_region(
1762 debug!("provides_universal_region(r={:?}, fr1={:?}, fr2={:?})", r, fr1, fr2);
1765 fr2 == self.universal_regions.fr_static && self.cannot_name_placeholder(fr1, r)
1768 debug!("provides_universal_region: result = {:?}", result);
1772 /// If `r2` represents a placeholder region, then this returns
1773 /// `true` if `r1` cannot name that placeholder in its
1774 /// value; otherwise, returns `false`.
1775 pub(crate) fn cannot_name_placeholder(&self, r1: RegionVid, r2: RegionVid) -> bool {
1776 debug!("cannot_name_value_of(r1={:?}, r2={:?})", r1, r2);
1778 match self.definitions[r2].origin {
1779 NllRegionVariableOrigin::Placeholder(placeholder) => {
1780 let universe1 = self.definitions[r1].universe;
1782 "cannot_name_value_of: universe1={:?} placeholder={:?}",
1783 universe1, placeholder
1785 universe1.cannot_name(placeholder.universe)
1788 NllRegionVariableOrigin::FreeRegion | NllRegionVariableOrigin::Existential { .. } => {
1794 pub(crate) fn retrieve_closure_constraint_info(
1796 constraint: OutlivesConstraint<'tcx>,
1797 ) -> Option<(ConstraintCategory<'tcx>, Span)> {
1798 match constraint.locations {
1799 Locations::All(_) => None,
1800 Locations::Single(loc) => {
1801 self.closure_bounds_mapping[&loc].get(&(constraint.sup, constraint.sub)).copied()
1806 /// Finds a good `ObligationCause` to blame for the fact that `fr1` outlives `fr2`.
1807 pub(crate) fn find_outlives_blame_span(
1810 fr1_origin: NllRegionVariableOrigin,
1812 ) -> (ConstraintCategory<'tcx>, ObligationCause<'tcx>) {
1813 let BlameConstraint { category, cause, .. } = self
1814 .best_blame_constraint(fr1, fr1_origin, |r| self.provides_universal_region(r, fr1, fr2))
1819 /// Walks the graph of constraints (where `'a: 'b` is considered
1820 /// an edge `'a -> 'b`) to find all paths from `from_region` to
1821 /// `to_region`. The paths are accumulated into the vector
1822 /// `results`. The paths are stored as a series of
1823 /// `ConstraintIndex` values -- in other words, a list of *edges*.
1825 /// Returns: a series of constraints as well as the region `R`
1826 /// that passed the target test.
1827 pub(crate) fn find_constraint_paths_between_regions(
1829 from_region: RegionVid,
1830 target_test: impl Fn(RegionVid) -> bool,
1831 ) -> Option<(Vec<OutlivesConstraint<'tcx>>, RegionVid)> {
1832 let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions);
1833 context[from_region] = Trace::StartRegion;
1835 // Use a deque so that we do a breadth-first search. We will
1836 // stop at the first match, which ought to be the shortest
1837 // path (fewest constraints).
1838 let mut deque = VecDeque::new();
1839 deque.push_back(from_region);
1841 while let Some(r) = deque.pop_front() {
1843 "find_constraint_paths_between_regions: from_region={:?} r={:?} value={}",
1846 self.region_value_str(r),
1849 // Check if we reached the region we were looking for. If so,
1850 // we can reconstruct the path that led to it and return it.
1852 let mut result = vec![];
1855 match context[p].clone() {
1856 Trace::NotVisited => {
1857 bug!("found unvisited region {:?} on path to {:?}", p, r)
1860 Trace::FromOutlivesConstraint(c) => {
1865 Trace::StartRegion => {
1867 return Some((result, r));
1873 // Otherwise, walk over the outgoing constraints and
1874 // enqueue any regions we find, keeping track of how we
1877 // A constraint like `'r: 'x` can come from our constraint
1879 let fr_static = self.universal_regions.fr_static;
1880 let outgoing_edges_from_graph =
1881 self.constraint_graph.outgoing_edges(r, &self.constraints, fr_static);
1883 // Always inline this closure because it can be hot.
1884 let mut handle_constraint = #[inline(always)]
1885 |constraint: OutlivesConstraint<'tcx>| {
1886 debug_assert_eq!(constraint.sup, r);
1887 let sub_region = constraint.sub;
1888 if let Trace::NotVisited = context[sub_region] {
1889 context[sub_region] = Trace::FromOutlivesConstraint(constraint);
1890 deque.push_back(sub_region);
1894 // This loop can be hot.
1895 for constraint in outgoing_edges_from_graph {
1896 handle_constraint(constraint);
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(r) {
1903 let p_c = &self.member_constraints[constraint.member_constraint_index];
1904 let constraint = OutlivesConstraint {
1906 sub: constraint.min_choice,
1907 locations: Locations::All(p_c.definition_span),
1908 span: p_c.definition_span,
1909 category: ConstraintCategory::OpaqueType,
1910 variance_info: ty::VarianceDiagInfo::default(),
1912 handle_constraint(constraint);
1919 /// Finds some region R such that `fr1: R` and `R` is live at `elem`.
1920 #[instrument(skip(self), level = "trace", ret)]
1921 pub(crate) fn find_sub_region_live_at(&self, fr1: RegionVid, elem: Location) -> RegionVid {
1922 trace!(scc = ?self.constraint_sccs.scc(fr1));
1923 trace!(universe = ?self.scc_universes[self.constraint_sccs.scc(fr1)]);
1924 self.find_constraint_paths_between_regions(fr1, |r| {
1925 // First look for some `r` such that `fr1: r` and `r` is live at `elem`
1926 trace!(?r, liveness_constraints=?self.liveness_constraints.region_value_str(r));
1927 self.liveness_constraints.contains(r, elem)
1930 // If we fail to find that, we may find some `r` such that
1931 // `fr1: r` and `r` is a placeholder from some universe
1932 // `fr1` cannot name. This would force `fr1` to be
1934 self.find_constraint_paths_between_regions(fr1, |r| {
1935 self.cannot_name_placeholder(fr1, r)
1939 // If we fail to find THAT, it may be that `fr1` is a
1940 // placeholder that cannot "fit" into its SCC. In that
1941 // case, there should be some `r` where `fr1: r` and `fr1` is a
1942 // placeholder that `r` cannot name. We can blame that
1945 // Remember that if `R1: R2`, then the universe of R1
1946 // must be able to name the universe of R2, because R2 will
1947 // be at least `'empty(Universe(R2))`, and `R1` must be at
1948 // larger than that.
1949 self.find_constraint_paths_between_regions(fr1, |r| {
1950 self.cannot_name_placeholder(r, fr1)
1953 .map(|(_path, r)| r)
1957 /// Get the region outlived by `longer_fr` and live at `element`.
1958 pub(crate) fn region_from_element(
1960 longer_fr: RegionVid,
1961 element: &RegionElement,
1964 RegionElement::Location(l) => self.find_sub_region_live_at(longer_fr, l),
1965 RegionElement::RootUniversalRegion(r) => r,
1966 RegionElement::PlaceholderRegion(error_placeholder) => self
1969 .find_map(|(r, definition)| match definition.origin {
1970 NllRegionVariableOrigin::Placeholder(p) if p == error_placeholder => Some(r),
1977 /// Get the region definition of `r`.
1978 pub(crate) fn region_definition(&self, r: RegionVid) -> &RegionDefinition<'tcx> {
1979 &self.definitions[r]
1982 /// Check if the SCC of `r` contains `upper`.
1983 pub(crate) fn upper_bound_in_region_scc(&self, r: RegionVid, upper: RegionVid) -> bool {
1984 let r_scc = self.constraint_sccs.scc(r);
1985 self.scc_values.contains(r_scc, upper)
1988 pub(crate) fn universal_regions(&self) -> &UniversalRegions<'tcx> {
1989 self.universal_regions.as_ref()
1992 /// Tries to find the best constraint to blame for the fact that
1993 /// `R: from_region`, where `R` is some region that meets
1994 /// `target_test`. This works by following the constraint graph,
1995 /// creating a constraint path that forces `R` to outlive
1996 /// `from_region`, and then finding the best choices within that
1998 #[instrument(level = "debug", skip(self, target_test))]
1999 pub(crate) fn best_blame_constraint(
2001 from_region: RegionVid,
2002 from_region_origin: NllRegionVariableOrigin,
2003 target_test: impl Fn(RegionVid) -> bool,
2004 ) -> (BlameConstraint<'tcx>, Vec<ExtraConstraintInfo>) {
2006 let (path, target_region) =
2007 self.find_constraint_paths_between_regions(from_region, target_test).unwrap();
2012 "{:?} ({:?}: {:?})",
2014 self.constraint_sccs.scc(c.sup),
2015 self.constraint_sccs.scc(c.sub),
2017 .collect::<Vec<_>>()
2020 let mut extra_info = vec![];
2021 for constraint in path.iter() {
2022 let outlived = constraint.sub;
2023 let Some(origin) = self.var_infos.get(outlived) else { continue; };
2024 let RegionVariableOrigin::Nll(NllRegionVariableOrigin::Placeholder(p)) = origin.origin else { continue; };
2025 debug!(?constraint, ?p);
2026 let ConstraintCategory::Predicate(span) = constraint.category else { continue; };
2027 extra_info.push(ExtraConstraintInfo::PlaceholderFromPredicate(span));
2028 // We only want to point to one
2032 // We try to avoid reporting a `ConstraintCategory::Predicate` as our best constraint.
2033 // Instead, we use it to produce an improved `ObligationCauseCode`.
2034 // FIXME - determine what we should do if we encounter multiple `ConstraintCategory::Predicate`
2035 // constraints. Currently, we just pick the first one.
2036 let cause_code = path
2038 .find_map(|constraint| {
2039 if let ConstraintCategory::Predicate(predicate_span) = constraint.category {
2040 // We currently do not store the `DefId` in the `ConstraintCategory`
2041 // for performances reasons. The error reporting code used by NLL only
2042 // uses the span, so this doesn't cause any problems at the moment.
2043 Some(ObligationCauseCode::BindingObligation(
2044 CRATE_DEF_ID.to_def_id(),
2051 .unwrap_or_else(|| ObligationCauseCode::MiscObligation);
2053 // Classify each of the constraints along the path.
2054 let mut categorized_path: Vec<BlameConstraint<'tcx>> = path
2057 let (category, span, from_closure, cause_code) =
2058 if constraint.category == ConstraintCategory::ClosureBounds {
2059 if let Some((category, span)) =
2060 self.retrieve_closure_constraint_info(*constraint)
2062 (category, span, true, ObligationCauseCode::MiscObligation)
2065 constraint.category,
2068 ObligationCauseCode::MiscObligation,
2072 (constraint.category, constraint.span, false, cause_code.clone())
2077 cause: ObligationCause::new(span, CRATE_HIR_ID, cause_code),
2078 variance_info: constraint.variance_info,
2079 outlives_constraint: *constraint,
2083 debug!("categorized_path={:#?}", categorized_path);
2085 // To find the best span to cite, we first try to look for the
2086 // final constraint that is interesting and where the `sup` is
2087 // not unified with the ultimate target region. The reason
2088 // for this is that we have a chain of constraints that lead
2089 // from the source to the target region, something like:
2091 // '0: '1 ('0 is the source)
2096 // '5: '6 ('6 is the target)
2098 // Some of those regions are unified with `'6` (in the same
2099 // SCC). We want to screen those out. After that point, the
2100 // "closest" constraint we have to the end is going to be the
2101 // most likely to be the point where the value escapes -- but
2102 // we still want to screen for an "interesting" point to
2103 // highlight (e.g., a call site or something).
2104 let target_scc = self.constraint_sccs.scc(target_region);
2105 let mut range = 0..path.len();
2107 // As noted above, when reporting an error, there is typically a chain of constraints
2108 // leading from some "source" region which must outlive some "target" region.
2109 // In most cases, we prefer to "blame" the constraints closer to the target --
2110 // but there is one exception. When constraints arise from higher-ranked subtyping,
2111 // we generally prefer to blame the source value,
2112 // as the "target" in this case tends to be some type annotation that the user gave.
2113 // Therefore, if we find that the region origin is some instantiation
2114 // of a higher-ranked region, we start our search from the "source" point
2115 // rather than the "target", and we also tweak a few other things.
2117 // An example might be this bit of Rust code:
2120 // let x: fn(&'static ()) = |_| {};
2121 // let y: for<'a> fn(&'a ()) = x;
2124 // In MIR, this will be converted into a combination of assignments and type ascriptions.
2125 // In particular, the 'static is imposed through a type ascription:
2129 // AscribeUserType(x, fn(&'static ())
2133 // We wind up ultimately with constraints like
2136 // !a: 'temp1 // from the `y = x` statement
2138 // 'temp2: 'static // from the AscribeUserType
2141 // and here we prefer to blame the source (the y = x statement).
2142 let blame_source = match from_region_origin {
2143 NllRegionVariableOrigin::FreeRegion
2144 | NllRegionVariableOrigin::Existential { from_forall: false } => true,
2145 NllRegionVariableOrigin::Placeholder(_)
2146 | NllRegionVariableOrigin::Existential { from_forall: true } => false,
2149 let find_region = |i: &usize| {
2150 let constraint = &path[*i];
2152 let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
2155 match categorized_path[*i].category {
2156 ConstraintCategory::OpaqueType
2157 | ConstraintCategory::Boring
2158 | ConstraintCategory::BoringNoLocation
2159 | ConstraintCategory::Internal
2160 | ConstraintCategory::Predicate(_) => false,
2161 ConstraintCategory::TypeAnnotation
2162 | ConstraintCategory::Return(_)
2163 | ConstraintCategory::Yield => true,
2164 _ => constraint_sup_scc != target_scc,
2168 categorized_path[*i].category,
2169 ConstraintCategory::OpaqueType
2170 | ConstraintCategory::Boring
2171 | ConstraintCategory::BoringNoLocation
2172 | ConstraintCategory::Internal
2173 | ConstraintCategory::Predicate(_)
2179 if blame_source { range.rev().find(find_region) } else { range.find(find_region) };
2181 debug!(?best_choice, ?blame_source, ?extra_info);
2183 if let Some(i) = best_choice {
2184 if let Some(next) = categorized_path.get(i + 1) {
2185 if matches!(categorized_path[i].category, ConstraintCategory::Return(_))
2186 && next.category == ConstraintCategory::OpaqueType
2188 // The return expression is being influenced by the return type being
2189 // impl Trait, point at the return type and not the return expr.
2190 return (next.clone(), extra_info);
2194 if categorized_path[i].category == ConstraintCategory::Return(ReturnConstraint::Normal)
2196 let field = categorized_path.iter().find_map(|p| {
2197 if let ConstraintCategory::ClosureUpvar(f) = p.category {
2204 if let Some(field) = field {
2205 categorized_path[i].category =
2206 ConstraintCategory::Return(ReturnConstraint::ClosureUpvar(field));
2210 return (categorized_path[i].clone(), extra_info);
2213 // If that search fails, that is.. unusual. Maybe everything
2214 // is in the same SCC or something. In that case, find what
2215 // appears to be the most interesting point to report to the
2216 // user via an even more ad-hoc guess.
2217 categorized_path.sort_by(|p0, p1| p0.category.cmp(&p1.category));
2218 debug!("sorted_path={:#?}", categorized_path);
2220 (categorized_path.remove(0), extra_info)
2223 pub(crate) fn universe_info(&self, universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
2224 self.universe_causes[&universe].clone()
2227 /// Tries to find the terminator of the loop in which the region 'r' resides.
2228 /// Returns the location of the terminator if found.
2229 pub(crate) fn find_loop_terminator_location(
2233 ) -> Option<Location> {
2234 let scc = self.constraint_sccs.scc(r.to_region_vid());
2235 let locations = self.scc_values.locations_outlived_by(scc);
2236 for location in locations {
2237 let bb = &body[location.block];
2238 if let Some(terminator) = &bb.terminator {
2239 // terminator of a loop should be TerminatorKind::FalseUnwind
2240 if let TerminatorKind::FalseUnwind { .. } = terminator.kind {
2241 return Some(location);
2249 impl<'tcx> RegionDefinition<'tcx> {
2250 fn new(universe: ty::UniverseIndex, rv_origin: RegionVariableOrigin) -> Self {
2251 // Create a new region definition. Note that, for free
2252 // regions, the `external_name` field gets updated later in
2253 // `init_universal_regions`.
2255 let origin = match rv_origin {
2256 RegionVariableOrigin::Nll(origin) => origin,
2257 _ => NllRegionVariableOrigin::Existential { from_forall: false },
2260 Self { origin, universe, external_name: None }
2264 pub trait ClosureRegionRequirementsExt<'tcx> {
2265 fn apply_requirements(
2268 closure_def_id: DefId,
2269 closure_substs: SubstsRef<'tcx>,
2270 ) -> Vec<QueryOutlivesConstraint<'tcx>>;
2273 impl<'tcx> ClosureRegionRequirementsExt<'tcx> for ClosureRegionRequirements<'tcx> {
2274 /// Given an instance T of the closure type, this method
2275 /// instantiates the "extra" requirements that we computed for the
2276 /// closure into the inference context. This has the effect of
2277 /// adding new outlives obligations to existing variables.
2279 /// As described on `ClosureRegionRequirements`, the extra
2280 /// requirements are expressed in terms of regionvids that index
2281 /// into the free regions that appear on the closure type. So, to
2282 /// do this, we first copy those regions out from the type T into
2283 /// a vector. Then we can just index into that vector to extract
2284 /// out the corresponding region from T and apply the
2286 fn apply_requirements(
2289 closure_def_id: DefId,
2290 closure_substs: SubstsRef<'tcx>,
2291 ) -> Vec<QueryOutlivesConstraint<'tcx>> {
2293 "apply_requirements(closure_def_id={:?}, closure_substs={:?})",
2294 closure_def_id, closure_substs
2297 // Extract the values of the free regions in `closure_substs`
2298 // into a vector. These are the regions that we will be
2299 // relating to one another.
2300 let closure_mapping = &UniversalRegions::closure_mapping(
2303 self.num_external_vids,
2304 tcx.typeck_root_def_id(closure_def_id),
2306 debug!("apply_requirements: closure_mapping={:?}", closure_mapping);
2308 // Create the predicates.
2309 self.outlives_requirements
2311 .map(|outlives_requirement| {
2312 let outlived_region = closure_mapping[outlives_requirement.outlived_free_region];
2314 match outlives_requirement.subject {
2315 ClosureOutlivesSubject::Region(region) => {
2316 let region = closure_mapping[region];
2318 "apply_requirements: region={:?} \
2319 outlived_region={:?} \
2320 outlives_requirement={:?}",
2321 region, outlived_region, outlives_requirement,
2324 ty::Binder::dummy(ty::OutlivesPredicate(
2328 ConstraintCategory::BoringNoLocation,
2332 ClosureOutlivesSubject::Ty(ty) => {
2334 "apply_requirements: ty={:?} \
2335 outlived_region={:?} \
2336 outlives_requirement={:?}",
2337 ty, outlived_region, outlives_requirement,
2340 ty::Binder::dummy(ty::OutlivesPredicate(ty.into(), outlived_region)),
2341 ConstraintCategory::BoringNoLocation,
2350 #[derive(Clone, Debug)]
2351 pub struct BlameConstraint<'tcx> {
2352 pub category: ConstraintCategory<'tcx>,
2353 pub from_closure: bool,
2354 pub cause: ObligationCause<'tcx>,
2355 pub variance_info: ty::VarianceDiagInfo<'tcx>,
2356 pub outlives_constraint: OutlivesConstraint<'tcx>,