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::CRATE_DEF_ID;
10 use rustc_index::vec::IndexVec;
11 use rustc_infer::infer::outlives::test_type_match;
12 use rustc_infer::infer::region_constraints::{GenericKind, VarInfos, VerifyBound, VerifyIfEq};
13 use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin};
14 use rustc_middle::mir::{
15 Body, ClosureOutlivesRequirement, ClosureOutlivesSubject, ClosureRegionRequirements,
16 ConstraintCategory, Local, Location, ReturnConstraint, TerminatorKind,
18 use rustc_middle::traits::ObligationCause;
19 use rustc_middle::traits::ObligationCauseCode;
20 use rustc_middle::ty::{self, RegionVid, Ty, TyCtxt, TypeFoldable, TypeVisitable};
25 graph::NormalConstraintGraph, ConstraintSccIndex, OutlivesConstraint, OutlivesConstraintSet,
27 diagnostics::{RegionErrorKind, RegionErrors, UniverseInfo},
28 member_constraints::{MemberConstraintSet, NllMemberConstraintIndex},
29 nll::{PoloniusOutput, ToRegionVid},
30 region_infer::reverse_sccs::ReverseSccGraph,
31 region_infer::values::{
32 LivenessValues, PlaceholderIndices, RegionElement, RegionValueElements, RegionValues,
35 type_check::{free_region_relations::UniversalRegionRelations, Locations},
36 universal_regions::UniversalRegions,
46 pub struct RegionInferenceContext<'tcx> {
47 pub var_infos: VarInfos,
49 /// Contains the definition for every region variable. Region
50 /// variables are identified by their index (`RegionVid`). The
51 /// definition contains information about where the region came
52 /// from as well as its final inferred value.
53 definitions: IndexVec<RegionVid, RegionDefinition<'tcx>>,
55 /// The liveness constraints added to each region. For most
56 /// regions, these start out empty and steadily grow, though for
57 /// each universally quantified region R they start out containing
58 /// the entire CFG and `end(R)`.
59 liveness_constraints: LivenessValues<RegionVid>,
61 /// The outlives constraints computed by the type-check.
62 constraints: Frozen<OutlivesConstraintSet<'tcx>>,
64 /// The constraint-set, but in graph form, making it easy to traverse
65 /// the constraints adjacent to a particular region. Used to construct
66 /// the SCC (see `constraint_sccs`) and for error reporting.
67 constraint_graph: Frozen<NormalConstraintGraph>,
69 /// The SCC computed from `constraints` and the constraint
70 /// graph. We have an edge from SCC A to SCC B if `A: B`. Used to
71 /// compute the values of each region.
72 constraint_sccs: Rc<Sccs<RegionVid, ConstraintSccIndex>>,
74 /// Reverse of the SCC constraint graph -- i.e., an edge `A -> B` exists if
75 /// `B: A`. This is used to compute the universal regions that are required
76 /// to outlive a given SCC. Computed lazily.
77 rev_scc_graph: Option<Rc<ReverseSccGraph>>,
79 /// The "R0 member of [R1..Rn]" constraints, indexed by SCC.
80 member_constraints: Rc<MemberConstraintSet<'tcx, ConstraintSccIndex>>,
82 /// Records the member constraints that we applied to each scc.
83 /// This is useful for error reporting. Once constraint
84 /// propagation is done, this vector is sorted according to
85 /// `member_region_scc`.
86 member_constraints_applied: Vec<AppliedMemberConstraint>,
88 /// Map universe indexes to information on why we created it.
89 universe_causes: FxHashMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
91 /// Contains the minimum universe of any variable within the same
92 /// SCC. We will ensure that no SCC contains values that are not
93 /// visible from this index.
94 scc_universes: IndexVec<ConstraintSccIndex, ty::UniverseIndex>,
96 /// Contains a "representative" from each SCC. This will be the
97 /// minimal RegionVid belonging to that universe. It is used as a
98 /// kind of hacky way to manage checking outlives relationships,
99 /// since we can 'canonicalize' each region to the representative
100 /// of its SCC and be sure that -- if they have the same repr --
101 /// they *must* be equal (though not having the same repr does not
102 /// mean they are unequal).
103 scc_representatives: IndexVec<ConstraintSccIndex, ty::RegionVid>,
105 /// The final inferred values of the region variables; we compute
106 /// one value per SCC. To get the value for any given *region*,
107 /// you first find which scc it is a part of.
108 scc_values: RegionValues<ConstraintSccIndex>,
110 /// Type constraints that we check after solving.
111 type_tests: Vec<TypeTest<'tcx>>,
113 /// Information about the universally quantified regions in scope
114 /// on this function.
115 universal_regions: Rc<UniversalRegions<'tcx>>,
117 /// Information about how the universally quantified regions in
118 /// scope on this function relate to one another.
119 universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
122 /// Each time that `apply_member_constraint` is successful, it appends
123 /// one of these structs to the `member_constraints_applied` field.
124 /// This is used in error reporting to trace out what happened.
126 /// The way that `apply_member_constraint` works is that it effectively
127 /// adds a new lower bound to the SCC it is analyzing: so you wind up
128 /// with `'R: 'O` where `'R` is the pick-region and `'O` is the
129 /// minimal viable option.
131 pub(crate) struct AppliedMemberConstraint {
132 /// The SCC that was affected. (The "member region".)
134 /// The vector if `AppliedMemberConstraint` elements is kept sorted
136 pub(crate) member_region_scc: ConstraintSccIndex,
138 /// The "best option" that `apply_member_constraint` found -- this was
139 /// added as an "ad-hoc" lower-bound to `member_region_scc`.
140 pub(crate) min_choice: ty::RegionVid,
142 /// The "member constraint index" -- we can find out details about
143 /// the constraint from
144 /// `set.member_constraints[member_constraint_index]`.
145 pub(crate) member_constraint_index: NllMemberConstraintIndex,
148 pub(crate) struct RegionDefinition<'tcx> {
149 /// What kind of variable is this -- a free region? existential
150 /// variable? etc. (See the `NllRegionVariableOrigin` for more
152 pub(crate) origin: NllRegionVariableOrigin,
154 /// Which universe is this region variable defined in? This is
155 /// most often `ty::UniverseIndex::ROOT`, but when we encounter
156 /// forall-quantifiers like `for<'a> { 'a = 'b }`, we would create
157 /// the variable for `'a` in a fresh universe that extends ROOT.
158 pub(crate) universe: ty::UniverseIndex,
160 /// If this is 'static or an early-bound region, then this is
161 /// `Some(X)` where `X` is the name of the region.
162 pub(crate) external_name: Option<ty::Region<'tcx>>,
165 /// N.B., the variants in `Cause` are intentionally ordered. Lower
166 /// values are preferred when it comes to error messages. Do not
167 /// reorder willy nilly.
168 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
169 pub(crate) enum Cause {
170 /// point inserted because Local was live at the given Location
171 LiveVar(Local, Location),
173 /// point inserted because Local was dropped at the given Location
174 DropVar(Local, Location),
177 /// A "type test" corresponds to an outlives constraint between a type
178 /// and a lifetime, like `T: 'x` or `<T as Foo>::Bar: 'x`. They are
179 /// translated from the `Verify` region constraints in the ordinary
180 /// inference context.
182 /// These sorts of constraints are handled differently than ordinary
183 /// constraints, at least at present. During type checking, the
184 /// `InferCtxt::process_registered_region_obligations` method will
185 /// attempt to convert a type test like `T: 'x` into an ordinary
186 /// outlives constraint when possible (for example, `&'a T: 'b` will
187 /// be converted into `'a: 'b` and registered as a `Constraint`).
189 /// In some cases, however, there are outlives relationships that are
190 /// not converted into a region constraint, but rather into one of
191 /// these "type tests". The distinction is that a type test does not
192 /// influence the inference result, but instead just examines the
193 /// values that we ultimately inferred for each region variable and
194 /// checks that they meet certain extra criteria. If not, an error
197 /// One reason for this is that these type tests typically boil down
198 /// to a check like `'a: 'x` where `'a` is a universally quantified
199 /// region -- and therefore not one whose value is really meant to be
200 /// *inferred*, precisely (this is not always the case: one can have a
201 /// type test like `<Foo as Trait<'?0>>::Bar: 'x`, where `'?0` is an
202 /// inference variable). Another reason is that these type tests can
203 /// involve *disjunction* -- that is, they can be satisfied in more
206 /// For more information about this translation, see
207 /// `InferCtxt::process_registered_region_obligations` and
208 /// `InferCtxt::type_must_outlive` in `rustc_infer::infer::InferCtxt`.
209 #[derive(Clone, Debug)]
210 pub struct TypeTest<'tcx> {
211 /// The type `T` that must outlive the region.
212 pub generic_kind: GenericKind<'tcx>,
214 /// The region `'x` that the type must outlive.
215 pub lower_bound: RegionVid,
217 /// The span to blame.
220 /// A test which, if met by the region `'x`, proves that this type
221 /// constraint is satisfied.
222 pub verify_bound: VerifyBound<'tcx>,
225 /// When we have an unmet lifetime constraint, we try to propagate it outward (e.g. to a closure
226 /// environment). If we can't, it is an error.
227 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
228 enum RegionRelationCheckResult {
234 #[derive(Clone, PartialEq, Eq, Debug)]
237 FromOutlivesConstraint(OutlivesConstraint<'tcx>),
241 #[derive(Clone, PartialEq, Eq, Debug)]
242 pub enum ExtraConstraintInfo {
243 PlaceholderFromPredicate(Span),
246 impl<'tcx> RegionInferenceContext<'tcx> {
247 /// Creates a new region inference context with a total of
248 /// `num_region_variables` valid inference variables; the first N
249 /// of those will be constant regions representing the free
250 /// regions defined in `universal_regions`.
252 /// The `outlives_constraints` and `type_tests` are an initial set
253 /// of constraints produced by the MIR type check.
256 universal_regions: Rc<UniversalRegions<'tcx>>,
257 placeholder_indices: Rc<PlaceholderIndices>,
258 universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
259 outlives_constraints: OutlivesConstraintSet<'tcx>,
260 member_constraints_in: MemberConstraintSet<'tcx, RegionVid>,
261 universe_causes: FxHashMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
262 type_tests: Vec<TypeTest<'tcx>>,
263 liveness_constraints: LivenessValues<RegionVid>,
264 elements: &Rc<RegionValueElements>,
266 // Create a RegionDefinition for each inference variable.
267 let definitions: IndexVec<_, _> = var_infos
269 .map(|info| RegionDefinition::new(info.universe, info.origin))
272 let constraints = Frozen::freeze(outlives_constraints);
273 let constraint_graph = Frozen::freeze(constraints.graph(definitions.len()));
274 let fr_static = universal_regions.fr_static;
275 let constraint_sccs = Rc::new(constraints.compute_sccs(&constraint_graph, fr_static));
278 RegionValues::new(elements, universal_regions.len(), &placeholder_indices);
280 for region in liveness_constraints.rows() {
281 let scc = constraint_sccs.scc(region);
282 scc_values.merge_liveness(scc, region, &liveness_constraints);
285 let scc_universes = Self::compute_scc_universes(&constraint_sccs, &definitions);
287 let scc_representatives = Self::compute_scc_representatives(&constraint_sccs, &definitions);
289 let member_constraints =
290 Rc::new(member_constraints_in.into_mapped(|r| constraint_sccs.scc(r)));
292 let mut result = Self {
295 liveness_constraints,
301 member_constraints_applied: Vec::new(),
308 universal_region_relations,
311 result.init_free_and_bound_regions();
316 /// Each SCC is the combination of many region variables which
317 /// have been equated. Therefore, we can associate a universe with
318 /// each SCC which is minimum of all the universes of its
319 /// constituent regions -- this is because whatever value the SCC
320 /// takes on must be a value that each of the regions within the
321 /// SCC could have as well. This implies that the SCC must have
322 /// the minimum, or narrowest, universe.
323 fn compute_scc_universes(
324 constraint_sccs: &Sccs<RegionVid, ConstraintSccIndex>,
325 definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
326 ) -> IndexVec<ConstraintSccIndex, ty::UniverseIndex> {
327 let num_sccs = constraint_sccs.num_sccs();
328 let mut scc_universes = IndexVec::from_elem_n(ty::UniverseIndex::MAX, num_sccs);
330 debug!("compute_scc_universes()");
332 // For each region R in universe U, ensure that the universe for the SCC
333 // that contains R is "no bigger" than U. This effectively sets the universe
334 // for each SCC to be the minimum of the regions within.
335 for (region_vid, region_definition) in definitions.iter_enumerated() {
336 let scc = constraint_sccs.scc(region_vid);
337 let scc_universe = &mut scc_universes[scc];
338 let scc_min = std::cmp::min(region_definition.universe, *scc_universe);
339 if scc_min != *scc_universe {
340 *scc_universe = scc_min;
342 "compute_scc_universes: lowered universe of {scc:?} to {scc_min:?} \
343 because it contains {region_vid:?} in {region_universe:?}",
346 region_vid = region_vid,
347 region_universe = region_definition.universe,
352 // Walk each SCC `A` and `B` such that `A: B`
353 // and ensure that universe(A) can see universe(B).
355 // This serves to enforce the 'empty/placeholder' hierarchy
356 // (described in more detail on `RegionKind`):
361 // empty(U0) placeholder(U1)
366 // In particular, imagine we have variables R0 in U0 and R1
367 // created in U1, and constraints like this;
370 // R1: !1 // R1 outlives the placeholder in U1
371 // R1: R0 // R1 outlives R0
374 // Here, we wish for R1 to be `'static`, because it
375 // cannot outlive `placeholder(U1)` and `empty(U0)` any other way.
377 // Thanks to this loop, what happens is that the `R1: R0`
378 // constraint lowers the universe of `R1` to `U0`, which in turn
379 // means that the `R1: !1` constraint will (later) cause
380 // `R1` to become `'static`.
381 for scc_a in constraint_sccs.all_sccs() {
382 for &scc_b in constraint_sccs.successors(scc_a) {
383 let scc_universe_a = scc_universes[scc_a];
384 let scc_universe_b = scc_universes[scc_b];
385 let scc_universe_min = std::cmp::min(scc_universe_a, scc_universe_b);
386 if scc_universe_a != scc_universe_min {
387 scc_universes[scc_a] = scc_universe_min;
390 "compute_scc_universes: lowered universe of {scc_a:?} to {scc_universe_min:?} \
391 because {scc_a:?}: {scc_b:?} and {scc_b:?} is in universe {scc_universe_b:?}",
394 scc_universe_min = scc_universe_min,
395 scc_universe_b = scc_universe_b
401 debug!("compute_scc_universes: scc_universe = {:#?}", scc_universes);
406 /// For each SCC, we compute a unique `RegionVid` (in fact, the
407 /// minimal one that belongs to the SCC). See
408 /// `scc_representatives` field of `RegionInferenceContext` for
410 fn compute_scc_representatives(
411 constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
412 definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
413 ) -> IndexVec<ConstraintSccIndex, ty::RegionVid> {
414 let num_sccs = constraints_scc.num_sccs();
415 let next_region_vid = definitions.next_index();
416 let mut scc_representatives = IndexVec::from_elem_n(next_region_vid, num_sccs);
418 for region_vid in definitions.indices() {
419 let scc = constraints_scc.scc(region_vid);
420 let prev_min = scc_representatives[scc];
421 scc_representatives[scc] = region_vid.min(prev_min);
427 /// Initializes the region variables for each universally
428 /// quantified region (lifetime parameter). The first N variables
429 /// always correspond to the regions appearing in the function
430 /// signature (both named and anonymous) and where-clauses. This
431 /// function iterates over those regions and initializes them with
436 /// fn foo<'a, 'b>( /* ... */ ) where 'a: 'b { /* ... */ }
438 /// would initialize two variables like so:
439 /// ```ignore (illustrative)
440 /// R0 = { CFG, R0 } // 'a
441 /// R1 = { CFG, R0, R1 } // 'b
443 /// Here, R0 represents `'a`, and it contains (a) the entire CFG
444 /// and (b) any universally quantified regions that it outlives,
445 /// which in this case is just itself. R1 (`'b`) in contrast also
446 /// outlives `'a` and hence contains R0 and R1.
447 fn init_free_and_bound_regions(&mut self) {
448 // Update the names (if any)
449 for (external_name, variable) in self.universal_regions.named_universal_regions() {
451 "init_universal_regions: region {:?} has external name {:?}",
452 variable, external_name
454 self.definitions[variable].external_name = Some(external_name);
457 for variable in self.definitions.indices() {
458 let scc = self.constraint_sccs.scc(variable);
460 match self.definitions[variable].origin {
461 NllRegionVariableOrigin::FreeRegion => {
462 // For each free, universally quantified region X:
464 // Add all nodes in the CFG to liveness constraints
465 self.liveness_constraints.add_all_points(variable);
466 self.scc_values.add_all_points(scc);
468 // Add `end(X)` into the set for X.
469 self.scc_values.add_element(scc, variable);
472 NllRegionVariableOrigin::Placeholder(placeholder) => {
473 // Each placeholder region is only visible from
474 // its universe `ui` and its extensions. So we
475 // can't just add it into `scc` unless the
476 // universe of the scc can name this region.
477 let scc_universe = self.scc_universes[scc];
478 if scc_universe.can_name(placeholder.universe) {
479 self.scc_values.add_element(scc, placeholder);
482 "init_free_and_bound_regions: placeholder {:?} is \
483 not compatible with universe {:?} of its SCC {:?}",
484 placeholder, scc_universe, scc,
486 self.add_incompatible_universe(scc);
490 NllRegionVariableOrigin::Existential { .. } => {
491 // For existential, regions, nothing to do.
497 /// Returns an iterator over all the region indices.
498 pub fn regions(&self) -> impl Iterator<Item = RegionVid> + 'tcx {
499 self.definitions.indices()
502 /// Given a universal region in scope on the MIR, returns the
503 /// corresponding index.
505 /// (Panics if `r` is not a registered universal region.)
506 pub fn to_region_vid(&self, r: ty::Region<'tcx>) -> RegionVid {
507 self.universal_regions.to_region_vid(r)
510 /// Adds annotations for `#[rustc_regions]`; see `UniversalRegions::annotate`.
511 pub(crate) fn annotate(&self, tcx: TyCtxt<'tcx>, err: &mut Diagnostic) {
512 self.universal_regions.annotate(tcx, err)
515 /// Returns `true` if the region `r` contains the point `p`.
517 /// Panics if called before `solve()` executes,
518 pub(crate) fn region_contains(&self, r: impl ToRegionVid, p: impl ToElementIndex) -> bool {
519 let scc = self.constraint_sccs.scc(r.to_region_vid());
520 self.scc_values.contains(scc, p)
523 /// Returns access to the value of `r` for debugging purposes.
524 pub(crate) fn region_value_str(&self, r: RegionVid) -> String {
525 let scc = self.constraint_sccs.scc(r.to_region_vid());
526 self.scc_values.region_value_str(scc)
529 pub(crate) fn placeholders_contained_in<'a>(
532 ) -> impl Iterator<Item = ty::PlaceholderRegion> + 'a {
533 let scc = self.constraint_sccs.scc(r.to_region_vid());
534 self.scc_values.placeholders_contained_in(scc)
537 /// Returns access to the value of `r` for debugging purposes.
538 pub(crate) fn region_universe(&self, r: RegionVid) -> ty::UniverseIndex {
539 let scc = self.constraint_sccs.scc(r.to_region_vid());
540 self.scc_universes[scc]
543 /// Once region solving has completed, this function will return
544 /// the member constraints that were applied to the value of a given
545 /// region `r`. See `AppliedMemberConstraint`.
546 pub(crate) fn applied_member_constraints(
549 ) -> &[AppliedMemberConstraint] {
550 let scc = self.constraint_sccs.scc(r.to_region_vid());
551 binary_search_util::binary_search_slice(
552 &self.member_constraints_applied,
553 |applied| applied.member_region_scc,
558 /// Performs region inference and report errors if we see any
559 /// unsatisfiable constraints. If this is a closure, returns the
560 /// region requirements to propagate to our creator, if any.
561 #[instrument(skip(self, infcx, body, polonius_output), level = "debug")]
564 infcx: &InferCtxt<'tcx>,
565 param_env: ty::ParamEnv<'tcx>,
567 polonius_output: Option<Rc<PoloniusOutput>>,
568 ) -> (Option<ClosureRegionRequirements<'tcx>>, RegionErrors<'tcx>) {
569 let mir_def_id = body.source.def_id();
570 self.propagate_constraints(body);
572 let mut errors_buffer = RegionErrors::new(infcx.tcx);
574 // If this is a closure, we can propagate unsatisfied
575 // `outlives_requirements` to our creator, so create a vector
576 // to store those. Otherwise, we'll pass in `None` to the
577 // functions below, which will trigger them to report errors
579 let mut outlives_requirements = infcx.tcx.is_typeck_child(mir_def_id).then(Vec::new);
581 self.check_type_tests(
585 outlives_requirements.as_mut(),
589 // In Polonius mode, the errors about missing universal region relations are in the output
590 // and need to be emitted or propagated. Otherwise, we need to check whether the
591 // constraints were too strong, and if so, emit or propagate those errors.
592 if infcx.tcx.sess.opts.unstable_opts.polonius {
593 self.check_polonius_subset_errors(
594 outlives_requirements.as_mut(),
596 polonius_output.expect("Polonius output is unavailable despite `-Z polonius`"),
599 self.check_universal_regions(outlives_requirements.as_mut(), &mut errors_buffer);
602 if errors_buffer.is_empty() {
603 self.check_member_constraints(infcx, &mut errors_buffer);
606 let outlives_requirements = outlives_requirements.unwrap_or_default();
608 if outlives_requirements.is_empty() {
609 (None, errors_buffer)
611 let num_external_vids = self.universal_regions.num_global_and_external_regions();
613 Some(ClosureRegionRequirements { num_external_vids, outlives_requirements }),
619 /// Propagate the region constraints: this will grow the values
620 /// for each region variable until all the constraints are
621 /// satisfied. Note that some values may grow **too** large to be
622 /// feasible, but we check this later.
623 #[instrument(skip(self, _body), level = "debug")]
624 fn propagate_constraints(&mut self, _body: &Body<'tcx>) {
625 debug!("constraints={:#?}", {
626 let mut constraints: Vec<_> = self.constraints.outlives().iter().collect();
627 constraints.sort_by_key(|c| (c.sup, c.sub));
630 .map(|c| (c, self.constraint_sccs.scc(c.sup), self.constraint_sccs.scc(c.sub)))
634 // To propagate constraints, we walk the DAG induced by the
635 // SCC. For each SCC, we visit its successors and compute
636 // their values, then we union all those values to get our
638 let constraint_sccs = self.constraint_sccs.clone();
639 for scc in constraint_sccs.all_sccs() {
640 self.compute_value_for_scc(scc);
643 // Sort the applied member constraints so we can binary search
644 // through them later.
645 self.member_constraints_applied.sort_by_key(|applied| applied.member_region_scc);
648 /// Computes the value of the SCC `scc_a`, which has not yet been
649 /// computed, by unioning the values of its successors.
650 /// Assumes that all successors have been computed already
651 /// (which is assured by iterating over SCCs in dependency order).
652 #[instrument(skip(self), level = "debug")]
653 fn compute_value_for_scc(&mut self, scc_a: ConstraintSccIndex) {
654 let constraint_sccs = self.constraint_sccs.clone();
656 // Walk each SCC `B` such that `A: B`...
657 for &scc_b in constraint_sccs.successors(scc_a) {
660 // ...and add elements from `B` into `A`. One complication
661 // arises because of universes: If `B` contains something
662 // that `A` cannot name, then `A` can only contain `B` if
663 // it outlives static.
664 if self.universe_compatible(scc_b, scc_a) {
665 // `A` can name everything that is in `B`, so just
667 self.scc_values.add_region(scc_a, scc_b);
669 self.add_incompatible_universe(scc_a);
673 // Now take member constraints into account.
674 let member_constraints = self.member_constraints.clone();
675 for m_c_i in member_constraints.indices(scc_a) {
676 self.apply_member_constraint(scc_a, m_c_i, member_constraints.choice_regions(m_c_i));
679 debug!(value = ?self.scc_values.region_value_str(scc_a));
682 /// Invoked for each `R0 member of [R1..Rn]` constraint.
684 /// `scc` is the SCC containing R0, and `choice_regions` are the
685 /// `R1..Rn` regions -- they are always known to be universal
686 /// regions (and if that's not true, we just don't attempt to
687 /// enforce the constraint).
689 /// The current value of `scc` at the time the method is invoked
690 /// is considered a *lower bound*. If possible, we will modify
691 /// the constraint to set it equal to one of the option regions.
692 /// If we make any changes, returns true, else false.
693 #[instrument(skip(self, member_constraint_index), level = "debug")]
694 fn apply_member_constraint(
696 scc: ConstraintSccIndex,
697 member_constraint_index: NllMemberConstraintIndex,
698 choice_regions: &[ty::RegionVid],
700 // Create a mutable vector of the options. We'll try to winnow
702 let mut choice_regions: Vec<ty::RegionVid> = choice_regions.to_vec();
704 // Convert to the SCC representative: sometimes we have inference
705 // variables in the member constraint that wind up equated with
706 // universal regions. The scc representative is the minimal numbered
707 // one from the corresponding scc so it will be the universal region
709 for c_r in &mut choice_regions {
710 let scc = self.constraint_sccs.scc(*c_r);
711 *c_r = self.scc_representatives[scc];
714 // The 'member region' in a member constraint is part of the
715 // hidden type, which must be in the root universe. Therefore,
716 // it cannot have any placeholders in its value.
717 assert!(self.scc_universes[scc] == ty::UniverseIndex::ROOT);
719 self.scc_values.placeholders_contained_in(scc).next().is_none(),
720 "scc {:?} in a member constraint has placeholder value: {:?}",
722 self.scc_values.region_value_str(scc),
725 // The existing value for `scc` is a lower-bound. This will
726 // consist of some set `{P} + {LB}` of points `{P}` and
727 // lower-bound free regions `{LB}`. As each choice region `O`
728 // is a free region, it will outlive the points. But we can
729 // only consider the option `O` if `O: LB`.
730 choice_regions.retain(|&o_r| {
732 .universal_regions_outlived_by(scc)
733 .all(|lb| self.universal_region_relations.outlives(o_r, lb))
735 debug!(?choice_regions, "after lb");
737 // Now find all the *upper bounds* -- that is, each UB is a
738 // free region that must outlive the member region `R0` (`UB:
739 // R0`). Therefore, we need only keep an option `O` if `UB: O`
741 let rev_scc_graph = self.reverse_scc_graph();
742 let universal_region_relations = &self.universal_region_relations;
743 for ub in rev_scc_graph.upper_bounds(scc) {
745 choice_regions.retain(|&o_r| universal_region_relations.outlives(ub, o_r));
747 debug!(?choice_regions, "after ub");
749 // If we ruled everything out, we're done.
750 if choice_regions.is_empty() {
754 // Otherwise, we need to find the minimum remaining choice, if
755 // any, and take that.
756 debug!("choice_regions remaining are {:#?}", choice_regions);
757 let Some(&min_choice) = choice_regions.iter().find(|&r1| {
758 choice_regions.iter().all(|&r2| {
759 self.universal_region_relations.outlives(r2, *r1)
762 debug!("no choice region outlived by all others");
766 let min_choice_scc = self.constraint_sccs.scc(min_choice);
767 debug!(?min_choice, ?min_choice_scc);
768 if self.scc_values.add_region(scc, min_choice_scc) {
769 self.member_constraints_applied.push(AppliedMemberConstraint {
770 member_region_scc: scc,
772 member_constraint_index,
781 /// Returns `true` if all the elements in the value of `scc_b` are nameable
782 /// in `scc_a`. Used during constraint propagation, and only once
783 /// the value of `scc_b` has been computed.
784 fn universe_compatible(&self, scc_b: ConstraintSccIndex, scc_a: ConstraintSccIndex) -> bool {
785 let universe_a = self.scc_universes[scc_a];
787 // Quick check: if scc_b's declared universe is a subset of
788 // scc_a's declared universe (typically, both are ROOT), then
789 // it cannot contain any problematic universe elements.
790 if universe_a.can_name(self.scc_universes[scc_b]) {
794 // Otherwise, we have to iterate over the universe elements in
795 // B's value, and check whether all of them are nameable
797 self.scc_values.placeholders_contained_in(scc_b).all(|p| universe_a.can_name(p.universe))
800 /// Extend `scc` so that it can outlive some placeholder region
801 /// from a universe it can't name; at present, the only way for
802 /// this to be true is if `scc` outlives `'static`. This is
803 /// actually stricter than necessary: ideally, we'd support bounds
804 /// like `for<'a: 'b`>` that might then allow us to approximate
805 /// `'a` with `'b` and not `'static`. But it will have to do for
807 fn add_incompatible_universe(&mut self, scc: ConstraintSccIndex) {
808 debug!("add_incompatible_universe(scc={:?})", scc);
810 let fr_static = self.universal_regions.fr_static;
811 self.scc_values.add_all_points(scc);
812 self.scc_values.add_element(scc, fr_static);
815 /// Once regions have been propagated, this method is used to see
816 /// whether the "type tests" produced by typeck were satisfied;
817 /// type tests encode type-outlives relationships like `T:
818 /// 'a`. See `TypeTest` for more details.
821 infcx: &InferCtxt<'tcx>,
822 param_env: ty::ParamEnv<'tcx>,
824 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
825 errors_buffer: &mut RegionErrors<'tcx>,
829 // Sometimes we register equivalent type-tests that would
830 // result in basically the exact same error being reported to
831 // the user. Avoid that.
832 let mut deduplicate_errors = FxHashSet::default();
834 for type_test in &self.type_tests {
835 debug!("check_type_test: {:?}", type_test);
837 let generic_ty = type_test.generic_kind.to_ty(tcx);
838 if self.eval_verify_bound(
842 type_test.lower_bound,
843 &type_test.verify_bound,
848 if let Some(propagated_outlives_requirements) = &mut propagated_outlives_requirements {
849 if self.try_promote_type_test(
854 propagated_outlives_requirements,
860 // Type-test failed. Report the error.
861 let erased_generic_kind = infcx.tcx.erase_regions(type_test.generic_kind);
863 // Skip duplicate-ish errors.
864 if deduplicate_errors.insert((
866 type_test.lower_bound,
870 "check_type_test: reporting error for erased_generic_kind={:?}, \
871 lower_bound_region={:?}, \
872 type_test.span={:?}",
873 erased_generic_kind, type_test.lower_bound, type_test.span,
876 errors_buffer.push(RegionErrorKind::TypeTestError { type_test: type_test.clone() });
881 /// Invoked when we have some type-test (e.g., `T: 'X`) that we cannot
882 /// prove to be satisfied. If this is a closure, we will attempt to
883 /// "promote" this type-test into our `ClosureRegionRequirements` and
884 /// hence pass it up the creator. To do this, we have to phrase the
885 /// type-test in terms of external free regions, as local free
886 /// regions are not nameable by the closure's creator.
888 /// Promotion works as follows: we first check that the type `T`
889 /// contains only regions that the creator knows about. If this is
890 /// true, then -- as a consequence -- we know that all regions in
891 /// the type `T` are free regions that outlive the closure body. If
892 /// false, then promotion fails.
894 /// Once we've promoted T, we have to "promote" `'X` to some region
895 /// that is "external" to the closure. Generally speaking, a region
896 /// may be the union of some points in the closure body as well as
897 /// various free lifetimes. We can ignore the points in the closure
898 /// body: if the type T can be expressed in terms of external regions,
899 /// we know it outlives the points in the closure body. That
900 /// just leaves the free regions.
902 /// The idea then is to lower the `T: 'X` constraint into multiple
903 /// bounds -- e.g., if `'X` is the union of two free lifetimes,
904 /// `'1` and `'2`, then we would create `T: '1` and `T: '2`.
905 #[instrument(level = "debug", skip(self, infcx, propagated_outlives_requirements))]
906 fn try_promote_type_test(
908 infcx: &InferCtxt<'tcx>,
909 param_env: ty::ParamEnv<'tcx>,
911 type_test: &TypeTest<'tcx>,
912 propagated_outlives_requirements: &mut Vec<ClosureOutlivesRequirement<'tcx>>,
916 let TypeTest { generic_kind, lower_bound, span: _, verify_bound: _ } = type_test;
918 let generic_ty = generic_kind.to_ty(tcx);
919 let Some(subject) = self.try_promote_type_test_subject(infcx, generic_ty) else {
923 debug!("subject = {:?}", subject);
925 let r_scc = self.constraint_sccs.scc(*lower_bound);
928 "lower_bound = {:?} r_scc={:?} universe={:?}",
929 lower_bound, r_scc, self.scc_universes[r_scc]
932 // If the type test requires that `T: 'a` where `'a` is a
933 // placeholder from another universe, that effectively requires
934 // `T: 'static`, so we have to propagate that requirement.
936 // It doesn't matter *what* universe because the promoted `T` will
937 // always be in the root universe.
938 if let Some(p) = self.scc_values.placeholders_contained_in(r_scc).next() {
939 debug!("encountered placeholder in higher universe: {:?}, requiring 'static", p);
940 let static_r = self.universal_regions.fr_static;
941 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
943 outlived_free_region: static_r,
944 blame_span: type_test.span,
945 category: ConstraintCategory::Boring,
948 // we can return here -- the code below might push add'l constraints
949 // but they would all be weaker than this one.
953 // For each region outlived by lower_bound find a non-local,
954 // universal region (it may be the same region) and add it to
955 // `ClosureOutlivesRequirement`.
956 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
957 debug!("universal_region_outlived_by ur={:?}", ur);
958 // Check whether we can already prove that the "subject" outlives `ur`.
959 // If so, we don't have to propagate this requirement to our caller.
961 // To continue the example from the function, if we are trying to promote
962 // a requirement that `T: 'X`, and we know that `'X = '1 + '2` (i.e., the union
963 // `'1` and `'2`), then in this loop `ur` will be `'1` (and `'2`). So here
964 // we check whether `T: '1` is something we *can* prove. If so, no need
965 // to propagate that requirement.
967 // This is needed because -- particularly in the case
968 // where `ur` is a local bound -- we are sometimes in a
969 // position to prove things that our caller cannot. See
970 // #53570 for an example.
971 if self.eval_verify_bound(infcx, param_env, generic_ty, ur, &type_test.verify_bound) {
975 let non_local_ub = self.universal_region_relations.non_local_upper_bounds(ur);
976 debug!("try_promote_type_test: non_local_ub={:?}", non_local_ub);
978 // This is slightly too conservative. To show T: '1, given `'2: '1`
979 // and `'3: '1` we only need to prove that T: '2 *or* T: '3, but to
980 // avoid potential non-determinism we approximate this by requiring
982 for upper_bound in non_local_ub {
983 debug_assert!(self.universal_regions.is_universal_region(upper_bound));
984 debug_assert!(!self.universal_regions.is_local_free_region(upper_bound));
986 let requirement = ClosureOutlivesRequirement {
988 outlived_free_region: upper_bound,
989 blame_span: type_test.span,
990 category: ConstraintCategory::Boring,
992 debug!("try_promote_type_test: pushing {:#?}", requirement);
993 propagated_outlives_requirements.push(requirement);
999 /// When we promote a type test `T: 'r`, we have to convert the
1000 /// type `T` into something we can store in a query result (so
1001 /// something allocated for `'tcx`). This is problematic if `ty`
1002 /// contains regions. During the course of NLL region checking, we
1003 /// will have replaced all of those regions with fresh inference
1004 /// variables. To create a test subject, we want to replace those
1005 /// inference variables with some region from the closure
1006 /// signature -- this is not always possible, so this is a
1007 /// fallible process. Presuming we do find a suitable region, we
1008 /// will use it's *external name*, which will be a `RegionKind`
1009 /// variant that can be used in query responses such as
1011 #[instrument(level = "debug", skip(self, infcx))]
1012 fn try_promote_type_test_subject(
1014 infcx: &InferCtxt<'tcx>,
1016 ) -> Option<ClosureOutlivesSubject<'tcx>> {
1017 let tcx = infcx.tcx;
1019 let ty = tcx.fold_regions(ty, |r, _depth| {
1020 let region_vid = self.to_region_vid(r);
1022 // The challenge if this. We have some region variable `r`
1023 // whose value is a set of CFG points and universal
1024 // regions. We want to find if that set is *equivalent* to
1025 // any of the named regions found in the closure.
1027 // To do so, we compute the
1028 // `non_local_universal_upper_bound`. This will be a
1029 // non-local, universal region that is greater than `r`.
1030 // However, it might not be *contained* within `r`, so
1031 // then we further check whether this bound is contained
1032 // in `r`. If so, we can say that `r` is equivalent to the
1035 // Let's work through a few examples. For these, imagine
1036 // that we have 3 non-local regions (I'll denote them as
1037 // `'static`, `'a`, and `'b`, though of course in the code
1038 // they would be represented with indices) where:
1043 // First, let's assume that `r` is some existential
1044 // variable with an inferred value `{'a, 'static}` (plus
1045 // some CFG nodes). In this case, the non-local upper
1046 // bound is `'static`, since that outlives `'a`. `'static`
1047 // is also a member of `r` and hence we consider `r`
1048 // equivalent to `'static` (and replace it with
1051 // Now let's consider the inferred value `{'a, 'b}`. This
1052 // means `r` is effectively `'a | 'b`. I'm not sure if
1053 // this can come about, actually, but assuming it did, we
1054 // would get a non-local upper bound of `'static`. Since
1055 // `'static` is not contained in `r`, we would fail to
1056 // find an equivalent.
1057 let upper_bound = self.non_local_universal_upper_bound(region_vid);
1058 if self.region_contains(region_vid, upper_bound) {
1059 self.definitions[upper_bound].external_name.unwrap_or(r)
1061 // In the case of a failure, use a `ReVar` result. This will
1062 // cause the `needs_infer` later on to return `None`.
1067 debug!("try_promote_type_test_subject: folded ty = {:?}", ty);
1069 // `needs_infer` will only be true if we failed to promote some region.
1070 if ty.needs_infer() {
1074 Some(ClosureOutlivesSubject::Ty(ty))
1077 /// Given some universal or existential region `r`, finds a
1078 /// non-local, universal region `r+` that outlives `r` at entry to (and
1079 /// exit from) the closure. In the worst case, this will be
1082 /// This is used for two purposes. First, if we are propagated
1083 /// some requirement `T: r`, we can use this method to enlarge `r`
1084 /// to something we can encode for our creator (which only knows
1085 /// about non-local, universal regions). It is also used when
1086 /// encoding `T` as part of `try_promote_type_test_subject` (see
1087 /// that fn for details).
1089 /// This is based on the result `'y` of `universal_upper_bound`,
1090 /// except that it converts further takes the non-local upper
1091 /// bound of `'y`, so that the final result is non-local.
1092 fn non_local_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1093 debug!("non_local_universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1095 let lub = self.universal_upper_bound(r);
1097 // Grow further to get smallest universal region known to
1099 let non_local_lub = self.universal_region_relations.non_local_upper_bound(lub);
1101 debug!("non_local_universal_upper_bound: non_local_lub={:?}", non_local_lub);
1106 /// Returns a universally quantified region that outlives the
1107 /// value of `r` (`r` may be existentially or universally
1110 /// Since `r` is (potentially) an existential region, it has some
1111 /// value which may include (a) any number of points in the CFG
1112 /// and (b) any number of `end('x)` elements of universally
1113 /// quantified regions. To convert this into a single universal
1114 /// region we do as follows:
1116 /// - Ignore the CFG points in `'r`. All universally quantified regions
1117 /// include the CFG anyhow.
1118 /// - For each `end('x)` element in `'r`, compute the mutual LUB, yielding
1120 #[instrument(skip(self), level = "debug", ret)]
1121 pub(crate) fn universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1122 debug!(r = %self.region_value_str(r));
1124 // Find the smallest universal region that contains all other
1125 // universal regions within `region`.
1126 let mut lub = self.universal_regions.fr_fn_body;
1127 let r_scc = self.constraint_sccs.scc(r);
1128 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1129 lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1135 /// Like `universal_upper_bound`, but returns an approximation more suitable
1136 /// for diagnostics. If `r` contains multiple disjoint universal regions
1137 /// (e.g. 'a and 'b in `fn foo<'a, 'b> { ... }`, we pick the lower-numbered region.
1138 /// This corresponds to picking named regions over unnamed regions
1139 /// (e.g. picking early-bound regions over a closure late-bound region).
1141 /// This means that the returned value may not be a true upper bound, since
1142 /// only 'static is known to outlive disjoint universal regions.
1143 /// Therefore, this method should only be used in diagnostic code,
1144 /// where displaying *some* named universal region is better than
1145 /// falling back to 'static.
1146 #[instrument(level = "debug", skip(self))]
1147 pub(crate) fn approx_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1148 debug!("{}", self.region_value_str(r));
1150 // Find the smallest universal region that contains all other
1151 // universal regions within `region`.
1152 let mut lub = self.universal_regions.fr_fn_body;
1153 let r_scc = self.constraint_sccs.scc(r);
1154 let static_r = self.universal_regions.fr_static;
1155 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1156 let new_lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1157 debug!(?ur, ?lub, ?new_lub);
1158 // The upper bound of two non-static regions is static: this
1159 // means we know nothing about the relationship between these
1160 // two regions. Pick a 'better' one to use when constructing
1162 if ur != static_r && lub != static_r && new_lub == static_r {
1163 // Prefer the region with an `external_name` - this
1164 // indicates that the region is early-bound, so working with
1165 // it can produce a nicer error.
1166 if self.region_definition(ur).external_name.is_some() {
1168 } else if self.region_definition(lub).external_name.is_some() {
1169 // Leave lub unchanged
1171 // If we get here, we don't have any reason to prefer
1172 // one region over the other. Just pick the
1173 // one with the lower index for now.
1174 lub = std::cmp::min(ur, lub);
1186 /// Tests if `test` is true when applied to `lower_bound` at
1188 fn eval_verify_bound(
1190 infcx: &InferCtxt<'tcx>,
1191 param_env: ty::ParamEnv<'tcx>,
1192 generic_ty: Ty<'tcx>,
1193 lower_bound: RegionVid,
1194 verify_bound: &VerifyBound<'tcx>,
1196 debug!("eval_verify_bound(lower_bound={:?}, verify_bound={:?})", lower_bound, verify_bound);
1198 match verify_bound {
1199 VerifyBound::IfEq(verify_if_eq_b) => {
1200 self.eval_if_eq(infcx, param_env, generic_ty, lower_bound, *verify_if_eq_b)
1203 VerifyBound::IsEmpty => {
1204 let lower_bound_scc = self.constraint_sccs.scc(lower_bound);
1205 self.scc_values.elements_contained_in(lower_bound_scc).next().is_none()
1208 VerifyBound::OutlivedBy(r) => {
1209 let r_vid = self.to_region_vid(*r);
1210 self.eval_outlives(r_vid, lower_bound)
1213 VerifyBound::AnyBound(verify_bounds) => verify_bounds.iter().any(|verify_bound| {
1214 self.eval_verify_bound(infcx, param_env, generic_ty, lower_bound, verify_bound)
1217 VerifyBound::AllBounds(verify_bounds) => verify_bounds.iter().all(|verify_bound| {
1218 self.eval_verify_bound(infcx, param_env, generic_ty, lower_bound, verify_bound)
1225 infcx: &InferCtxt<'tcx>,
1226 param_env: ty::ParamEnv<'tcx>,
1227 generic_ty: Ty<'tcx>,
1228 lower_bound: RegionVid,
1229 verify_if_eq_b: ty::Binder<'tcx, VerifyIfEq<'tcx>>,
1231 let generic_ty = self.normalize_to_scc_representatives(infcx.tcx, generic_ty);
1232 let verify_if_eq_b = self.normalize_to_scc_representatives(infcx.tcx, verify_if_eq_b);
1233 match test_type_match::extract_verify_if_eq(
1240 let r_vid = self.to_region_vid(r);
1241 self.eval_outlives(r_vid, lower_bound)
1247 /// This is a conservative normalization procedure. It takes every
1248 /// free region in `value` and replaces it with the
1249 /// "representative" of its SCC (see `scc_representatives` field).
1250 /// We are guaranteed that if two values normalize to the same
1251 /// thing, then they are equal; this is a conservative check in
1252 /// that they could still be equal even if they normalize to
1253 /// different results. (For example, there might be two regions
1254 /// with the same value that are not in the same SCC).
1256 /// N.B., this is not an ideal approach and I would like to revisit
1257 /// it. However, it works pretty well in practice. In particular,
1258 /// this is needed to deal with projection outlives bounds like
1261 /// <T as Foo<'0>>::Item: '1
1264 /// In particular, this routine winds up being important when
1265 /// there are bounds like `where <T as Foo<'a>>::Item: 'b` in the
1266 /// environment. In this case, if we can show that `'0 == 'a`,
1267 /// and that `'b: '1`, then we know that the clause is
1268 /// satisfied. In such cases, particularly due to limitations of
1269 /// the trait solver =), we usually wind up with a where-clause like
1270 /// `T: Foo<'a>` in scope, which thus forces `'0 == 'a` to be added as
1271 /// a constraint, and thus ensures that they are in the same SCC.
1273 /// So why can't we do a more correct routine? Well, we could
1274 /// *almost* use the `relate_tys` code, but the way it is
1275 /// currently setup it creates inference variables to deal with
1276 /// higher-ranked things and so forth, and right now the inference
1277 /// context is not permitted to make more inference variables. So
1278 /// we use this kind of hacky solution.
1279 fn normalize_to_scc_representatives<T>(&self, tcx: TyCtxt<'tcx>, value: T) -> T
1281 T: TypeFoldable<'tcx>,
1283 tcx.fold_regions(value, |r, _db| {
1284 let vid = self.to_region_vid(r);
1285 let scc = self.constraint_sccs.scc(vid);
1286 let repr = self.scc_representatives[scc];
1287 tcx.mk_region(ty::ReVar(repr))
1291 // Evaluate whether `sup_region == sub_region`.
1292 fn eval_equal(&self, r1: RegionVid, r2: RegionVid) -> bool {
1293 self.eval_outlives(r1, r2) && self.eval_outlives(r2, r1)
1296 // Evaluate whether `sup_region: sub_region`.
1297 #[instrument(skip(self), level = "debug", ret)]
1298 fn eval_outlives(&self, sup_region: RegionVid, sub_region: RegionVid) -> bool {
1300 "sup_region's value = {:?} universal={:?}",
1301 self.region_value_str(sup_region),
1302 self.universal_regions.is_universal_region(sup_region),
1305 "sub_region's value = {:?} universal={:?}",
1306 self.region_value_str(sub_region),
1307 self.universal_regions.is_universal_region(sub_region),
1310 let sub_region_scc = self.constraint_sccs.scc(sub_region);
1311 let sup_region_scc = self.constraint_sccs.scc(sup_region);
1313 // If we are checking that `'sup: 'sub`, and `'sub` contains
1314 // some placeholder that `'sup` cannot name, then this is only
1315 // true if `'sup` outlives static.
1316 if !self.universe_compatible(sub_region_scc, sup_region_scc) {
1318 "sub universe `{sub_region_scc:?}` is not nameable \
1319 by super `{sup_region_scc:?}`, promoting to static",
1322 return self.eval_outlives(sup_region, self.universal_regions.fr_static);
1325 // Both the `sub_region` and `sup_region` consist of the union
1326 // of some number of universal regions (along with the union
1327 // of various points in the CFG; ignore those points for
1328 // now). Therefore, the sup-region outlives the sub-region if,
1329 // for each universal region R1 in the sub-region, there
1330 // exists some region R2 in the sup-region that outlives R1.
1331 let universal_outlives =
1332 self.scc_values.universal_regions_outlived_by(sub_region_scc).all(|r1| {
1334 .universal_regions_outlived_by(sup_region_scc)
1335 .any(|r2| self.universal_region_relations.outlives(r2, r1))
1338 if !universal_outlives {
1339 debug!("sub region contains a universal region not present in super");
1343 // Now we have to compare all the points in the sub region and make
1344 // sure they exist in the sup region.
1346 if self.universal_regions.is_universal_region(sup_region) {
1347 // Micro-opt: universal regions contain all points.
1348 debug!("super is universal and hence contains all points");
1352 debug!("comparison between points in sup/sub");
1354 self.scc_values.contains_points(sup_region_scc, sub_region_scc)
1357 /// Once regions have been propagated, this method is used to see
1358 /// whether any of the constraints were too strong. In particular,
1359 /// we want to check for a case where a universally quantified
1360 /// region exceeded its bounds. Consider:
1362 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1364 /// In this case, returning `x` requires `&'a u32 <: &'b u32`
1365 /// and hence we establish (transitively) a constraint that
1366 /// `'a: 'b`. The `propagate_constraints` code above will
1367 /// therefore add `end('a)` into the region for `'b` -- but we
1368 /// have no evidence that `'b` outlives `'a`, so we want to report
1371 /// If `propagated_outlives_requirements` is `Some`, then we will
1372 /// push unsatisfied obligations into there. Otherwise, we'll
1373 /// report them as errors.
1374 fn check_universal_regions(
1376 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1377 errors_buffer: &mut RegionErrors<'tcx>,
1379 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1380 match fr_definition.origin {
1381 NllRegionVariableOrigin::FreeRegion => {
1382 // Go through each of the universal regions `fr` and check that
1383 // they did not grow too large, accumulating any requirements
1384 // for our caller into the `outlives_requirements` vector.
1385 self.check_universal_region(
1387 &mut propagated_outlives_requirements,
1392 NllRegionVariableOrigin::Placeholder(placeholder) => {
1393 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1396 NllRegionVariableOrigin::Existential { .. } => {
1397 // nothing to check here
1403 /// Checks if Polonius has found any unexpected free region relations.
1405 /// In Polonius terms, a "subset error" (or "illegal subset relation error") is the equivalent
1406 /// of NLL's "checking if any region constraints were too strong": a placeholder origin `'a`
1407 /// was unexpectedly found to be a subset of another placeholder origin `'b`, and means in NLL
1408 /// terms that the "longer free region" `'a` outlived the "shorter free region" `'b`.
1410 /// More details can be found in this blog post by Niko:
1411 /// <https://smallcultfollowing.com/babysteps/blog/2019/01/17/polonius-and-region-errors/>
1413 /// In the canonical example
1415 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1417 /// returning `x` requires `&'a u32 <: &'b u32` and hence we establish (transitively) a
1418 /// constraint that `'a: 'b`. It is an error that we have no evidence that this
1419 /// constraint holds.
1421 /// If `propagated_outlives_requirements` is `Some`, then we will
1422 /// push unsatisfied obligations into there. Otherwise, we'll
1423 /// report them as errors.
1424 fn check_polonius_subset_errors(
1426 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1427 errors_buffer: &mut RegionErrors<'tcx>,
1428 polonius_output: Rc<PoloniusOutput>,
1431 "check_polonius_subset_errors: {} subset_errors",
1432 polonius_output.subset_errors.len()
1435 // Similarly to `check_universal_regions`: a free region relation, which was not explicitly
1436 // declared ("known") was found by Polonius, so emit an error, or propagate the
1437 // requirements for our caller into the `propagated_outlives_requirements` vector.
1439 // Polonius doesn't model regions ("origins") as CFG-subsets or durations, but the
1440 // `longer_fr` and `shorter_fr` terminology will still be used here, for consistency with
1441 // the rest of the NLL infrastructure. The "subset origin" is the "longer free region",
1442 // and the "superset origin" is the outlived "shorter free region".
1444 // Note: Polonius will produce a subset error at every point where the unexpected
1445 // `longer_fr`'s "placeholder loan" is contained in the `shorter_fr`. This can be helpful
1446 // for diagnostics in the future, e.g. to point more precisely at the key locations
1447 // requiring this constraint to hold. However, the error and diagnostics code downstream
1448 // expects that these errors are not duplicated (and that they are in a certain order).
1449 // Otherwise, diagnostics messages such as the ones giving names like `'1` to elided or
1450 // anonymous lifetimes for example, could give these names differently, while others like
1451 // the outlives suggestions or the debug output from `#[rustc_regions]` would be
1452 // duplicated. The polonius subset errors are deduplicated here, while keeping the
1453 // CFG-location ordering.
1454 let mut subset_errors: Vec<_> = polonius_output
1457 .flat_map(|(_location, subset_errors)| subset_errors.iter())
1459 subset_errors.sort();
1460 subset_errors.dedup();
1462 for (longer_fr, shorter_fr) in subset_errors.into_iter() {
1464 "check_polonius_subset_errors: subset_error longer_fr={:?},\
1466 longer_fr, shorter_fr
1469 let propagated = self.try_propagate_universal_region_error(
1472 &mut propagated_outlives_requirements,
1474 if propagated == RegionRelationCheckResult::Error {
1475 errors_buffer.push(RegionErrorKind::RegionError {
1476 longer_fr: *longer_fr,
1477 shorter_fr: *shorter_fr,
1478 fr_origin: NllRegionVariableOrigin::FreeRegion,
1484 // Handle the placeholder errors as usual, until the chalk-rustc-polonius triumvirate has
1485 // a more complete picture on how to separate this responsibility.
1486 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1487 match fr_definition.origin {
1488 NllRegionVariableOrigin::FreeRegion => {
1489 // handled by polonius above
1492 NllRegionVariableOrigin::Placeholder(placeholder) => {
1493 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1496 NllRegionVariableOrigin::Existential { .. } => {
1497 // nothing to check here
1503 /// Checks the final value for the free region `fr` to see if it
1504 /// grew too large. In particular, examine what `end(X)` points
1505 /// wound up in `fr`'s final value; for each `end(X)` where `X !=
1506 /// fr`, we want to check that `fr: X`. If not, that's either an
1507 /// error, or something we have to propagate to our creator.
1509 /// Things that are to be propagated are accumulated into the
1510 /// `outlives_requirements` vector.
1511 #[instrument(skip(self, propagated_outlives_requirements, errors_buffer), level = "debug")]
1512 fn check_universal_region(
1514 longer_fr: RegionVid,
1515 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1516 errors_buffer: &mut RegionErrors<'tcx>,
1518 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1520 // Because this free region must be in the ROOT universe, we
1521 // know it cannot contain any bound universes.
1522 assert!(self.scc_universes[longer_fr_scc] == ty::UniverseIndex::ROOT);
1523 debug_assert!(self.scc_values.placeholders_contained_in(longer_fr_scc).next().is_none());
1525 // Only check all of the relations for the main representative of each
1526 // SCC, otherwise just check that we outlive said representative. This
1527 // reduces the number of redundant relations propagated out of
1529 // Note that the representative will be a universal region if there is
1530 // one in this SCC, so we will always check the representative here.
1531 let representative = self.scc_representatives[longer_fr_scc];
1532 if representative != longer_fr {
1533 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1536 propagated_outlives_requirements,
1538 errors_buffer.push(RegionErrorKind::RegionError {
1540 shorter_fr: representative,
1541 fr_origin: NllRegionVariableOrigin::FreeRegion,
1548 // Find every region `o` such that `fr: o`
1549 // (because `fr` includes `end(o)`).
1550 let mut error_reported = false;
1551 for shorter_fr in self.scc_values.universal_regions_outlived_by(longer_fr_scc) {
1552 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1555 propagated_outlives_requirements,
1557 // We only report the first region error. Subsequent errors are hidden so as
1558 // not to overwhelm the user, but we do record them so as to potentially print
1559 // better diagnostics elsewhere...
1560 errors_buffer.push(RegionErrorKind::RegionError {
1563 fr_origin: NllRegionVariableOrigin::FreeRegion,
1564 is_reported: !error_reported,
1567 error_reported = true;
1572 /// Checks that we can prove that `longer_fr: shorter_fr`. If we can't we attempt to propagate
1573 /// the constraint outward (e.g. to a closure environment), but if that fails, there is an
1575 fn check_universal_region_relation(
1577 longer_fr: RegionVid,
1578 shorter_fr: RegionVid,
1579 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1580 ) -> RegionRelationCheckResult {
1581 // If it is known that `fr: o`, carry on.
1582 if self.universal_region_relations.outlives(longer_fr, shorter_fr) {
1583 RegionRelationCheckResult::Ok
1585 // If we are not in a context where we can't propagate errors, or we
1586 // could not shrink `fr` to something smaller, then just report an
1589 // Note: in this case, we use the unapproximated regions to report the
1590 // error. This gives better error messages in some cases.
1591 self.try_propagate_universal_region_error(
1594 propagated_outlives_requirements,
1599 /// Attempt to propagate a region error (e.g. `'a: 'b`) that is not met to a closure's
1600 /// creator. If we cannot, then the caller should report an error to the user.
1601 fn try_propagate_universal_region_error(
1603 longer_fr: RegionVid,
1604 shorter_fr: RegionVid,
1605 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1606 ) -> RegionRelationCheckResult {
1607 if let Some(propagated_outlives_requirements) = propagated_outlives_requirements {
1608 // Shrink `longer_fr` until we find a non-local region (if we do).
1609 // We'll call it `fr-` -- it's ever so slightly smaller than
1611 if let Some(fr_minus) = self.universal_region_relations.non_local_lower_bound(longer_fr)
1613 debug!("try_propagate_universal_region_error: fr_minus={:?}", fr_minus);
1615 let blame_span_category = self.find_outlives_blame_span(
1617 NllRegionVariableOrigin::FreeRegion,
1621 // Grow `shorter_fr` until we find some non-local regions. (We
1622 // always will.) We'll call them `shorter_fr+` -- they're ever
1623 // so slightly larger than `shorter_fr`.
1624 let shorter_fr_plus =
1625 self.universal_region_relations.non_local_upper_bounds(shorter_fr);
1627 "try_propagate_universal_region_error: shorter_fr_plus={:?}",
1630 for fr in shorter_fr_plus {
1631 // Push the constraint `fr-: shorter_fr+`
1632 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
1633 subject: ClosureOutlivesSubject::Region(fr_minus),
1634 outlived_free_region: fr,
1635 blame_span: blame_span_category.1.span,
1636 category: blame_span_category.0,
1639 return RegionRelationCheckResult::Propagated;
1643 RegionRelationCheckResult::Error
1646 fn check_bound_universal_region(
1648 longer_fr: RegionVid,
1649 placeholder: ty::PlaceholderRegion,
1650 errors_buffer: &mut RegionErrors<'tcx>,
1652 debug!("check_bound_universal_region(fr={:?}, placeholder={:?})", longer_fr, placeholder,);
1654 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1655 debug!("check_bound_universal_region: longer_fr_scc={:?}", longer_fr_scc,);
1657 for error_element in self.scc_values.elements_contained_in(longer_fr_scc) {
1658 match error_element {
1659 RegionElement::Location(_) | RegionElement::RootUniversalRegion(_) => {}
1660 // If we have some bound universal region `'a`, then the only
1661 // elements it can contain is itself -- we don't know anything
1663 RegionElement::PlaceholderRegion(placeholder1) => {
1664 if placeholder == placeholder1 {
1670 errors_buffer.push(RegionErrorKind::BoundUniversalRegionError {
1676 // Stop after the first error, it gets too noisy otherwise, and does not provide more information.
1679 debug!("check_bound_universal_region: all bounds satisfied");
1682 #[instrument(level = "debug", skip(self, infcx, errors_buffer))]
1683 fn check_member_constraints(
1685 infcx: &InferCtxt<'tcx>,
1686 errors_buffer: &mut RegionErrors<'tcx>,
1688 let member_constraints = self.member_constraints.clone();
1689 for m_c_i in member_constraints.all_indices() {
1691 let m_c = &member_constraints[m_c_i];
1692 let member_region_vid = m_c.member_region_vid;
1695 value = ?self.region_value_str(member_region_vid),
1697 let choice_regions = member_constraints.choice_regions(m_c_i);
1698 debug!(?choice_regions);
1700 // Did the member region wind up equal to any of the option regions?
1702 choice_regions.iter().find(|&&o_r| self.eval_equal(o_r, m_c.member_region_vid))
1704 debug!("evaluated as equal to {:?}", o);
1708 // If not, report an error.
1709 let member_region = infcx.tcx.mk_region(ty::ReVar(member_region_vid));
1710 errors_buffer.push(RegionErrorKind::UnexpectedHiddenRegion {
1711 span: m_c.definition_span,
1712 hidden_ty: m_c.hidden_ty,
1719 /// We have a constraint `fr1: fr2` that is not satisfied, where
1720 /// `fr2` represents some universal region. Here, `r` is some
1721 /// region where we know that `fr1: r` and this function has the
1722 /// job of determining whether `r` is "to blame" for the fact that
1723 /// `fr1: fr2` is required.
1725 /// This is true under two conditions:
1728 /// - `fr2` is `'static` and `r` is some placeholder in a universe
1729 /// that cannot be named by `fr1`; in that case, we will require
1730 /// that `fr1: 'static` because it is the only way to `fr1: r` to
1731 /// be satisfied. (See `add_incompatible_universe`.)
1732 pub(crate) fn provides_universal_region(
1738 debug!("provides_universal_region(r={:?}, fr1={:?}, fr2={:?})", r, fr1, fr2);
1741 fr2 == self.universal_regions.fr_static && self.cannot_name_placeholder(fr1, r)
1744 debug!("provides_universal_region: result = {:?}", result);
1748 /// If `r2` represents a placeholder region, then this returns
1749 /// `true` if `r1` cannot name that placeholder in its
1750 /// value; otherwise, returns `false`.
1751 pub(crate) fn cannot_name_placeholder(&self, r1: RegionVid, r2: RegionVid) -> bool {
1752 debug!("cannot_name_value_of(r1={:?}, r2={:?})", r1, r2);
1754 match self.definitions[r2].origin {
1755 NllRegionVariableOrigin::Placeholder(placeholder) => {
1756 let universe1 = self.definitions[r1].universe;
1758 "cannot_name_value_of: universe1={:?} placeholder={:?}",
1759 universe1, placeholder
1761 universe1.cannot_name(placeholder.universe)
1764 NllRegionVariableOrigin::FreeRegion | NllRegionVariableOrigin::Existential { .. } => {
1770 /// Finds a good `ObligationCause` to blame for the fact that `fr1` outlives `fr2`.
1771 pub(crate) fn find_outlives_blame_span(
1774 fr1_origin: NllRegionVariableOrigin,
1776 ) -> (ConstraintCategory<'tcx>, ObligationCause<'tcx>) {
1777 let BlameConstraint { category, cause, .. } = self
1778 .best_blame_constraint(fr1, fr1_origin, |r| self.provides_universal_region(r, fr1, fr2))
1783 /// Walks the graph of constraints (where `'a: 'b` is considered
1784 /// an edge `'a -> 'b`) to find all paths from `from_region` to
1785 /// `to_region`. The paths are accumulated into the vector
1786 /// `results`. The paths are stored as a series of
1787 /// `ConstraintIndex` values -- in other words, a list of *edges*.
1789 /// Returns: a series of constraints as well as the region `R`
1790 /// that passed the target test.
1791 pub(crate) fn find_constraint_paths_between_regions(
1793 from_region: RegionVid,
1794 target_test: impl Fn(RegionVid) -> bool,
1795 ) -> Option<(Vec<OutlivesConstraint<'tcx>>, RegionVid)> {
1796 let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions);
1797 context[from_region] = Trace::StartRegion;
1799 // Use a deque so that we do a breadth-first search. We will
1800 // stop at the first match, which ought to be the shortest
1801 // path (fewest constraints).
1802 let mut deque = VecDeque::new();
1803 deque.push_back(from_region);
1805 while let Some(r) = deque.pop_front() {
1807 "find_constraint_paths_between_regions: from_region={:?} r={:?} value={}",
1810 self.region_value_str(r),
1813 // Check if we reached the region we were looking for. If so,
1814 // we can reconstruct the path that led to it and return it.
1816 let mut result = vec![];
1819 match context[p].clone() {
1820 Trace::NotVisited => {
1821 bug!("found unvisited region {:?} on path to {:?}", p, r)
1824 Trace::FromOutlivesConstraint(c) => {
1829 Trace::StartRegion => {
1831 return Some((result, r));
1837 // Otherwise, walk over the outgoing constraints and
1838 // enqueue any regions we find, keeping track of how we
1841 // A constraint like `'r: 'x` can come from our constraint
1843 let fr_static = self.universal_regions.fr_static;
1844 let outgoing_edges_from_graph =
1845 self.constraint_graph.outgoing_edges(r, &self.constraints, fr_static);
1847 // Always inline this closure because it can be hot.
1848 let mut handle_constraint = #[inline(always)]
1849 |constraint: OutlivesConstraint<'tcx>| {
1850 debug_assert_eq!(constraint.sup, r);
1851 let sub_region = constraint.sub;
1852 if let Trace::NotVisited = context[sub_region] {
1853 context[sub_region] = Trace::FromOutlivesConstraint(constraint);
1854 deque.push_back(sub_region);
1858 // This loop can be hot.
1859 for constraint in outgoing_edges_from_graph {
1860 handle_constraint(constraint);
1863 // Member constraints can also give rise to `'r: 'x` edges that
1864 // were not part of the graph initially, so watch out for those.
1865 // (But they are extremely rare; this loop is very cold.)
1866 for constraint in self.applied_member_constraints(r) {
1867 let p_c = &self.member_constraints[constraint.member_constraint_index];
1868 let constraint = OutlivesConstraint {
1870 sub: constraint.min_choice,
1871 locations: Locations::All(p_c.definition_span),
1872 span: p_c.definition_span,
1873 category: ConstraintCategory::OpaqueType,
1874 variance_info: ty::VarianceDiagInfo::default(),
1875 from_closure: false,
1877 handle_constraint(constraint);
1884 /// Finds some region R such that `fr1: R` and `R` is live at `elem`.
1885 #[instrument(skip(self), level = "trace", ret)]
1886 pub(crate) fn find_sub_region_live_at(&self, fr1: RegionVid, elem: Location) -> RegionVid {
1887 trace!(scc = ?self.constraint_sccs.scc(fr1));
1888 trace!(universe = ?self.scc_universes[self.constraint_sccs.scc(fr1)]);
1889 self.find_constraint_paths_between_regions(fr1, |r| {
1890 // First look for some `r` such that `fr1: r` and `r` is live at `elem`
1891 trace!(?r, liveness_constraints=?self.liveness_constraints.region_value_str(r));
1892 self.liveness_constraints.contains(r, elem)
1895 // If we fail to find that, we may find some `r` such that
1896 // `fr1: r` and `r` is a placeholder from some universe
1897 // `fr1` cannot name. This would force `fr1` to be
1899 self.find_constraint_paths_between_regions(fr1, |r| {
1900 self.cannot_name_placeholder(fr1, r)
1904 // If we fail to find THAT, it may be that `fr1` is a
1905 // placeholder that cannot "fit" into its SCC. In that
1906 // case, there should be some `r` where `fr1: r` and `fr1` is a
1907 // placeholder that `r` cannot name. We can blame that
1910 // Remember that if `R1: R2`, then the universe of R1
1911 // must be able to name the universe of R2, because R2 will
1912 // be at least `'empty(Universe(R2))`, and `R1` must be at
1913 // larger than that.
1914 self.find_constraint_paths_between_regions(fr1, |r| {
1915 self.cannot_name_placeholder(r, fr1)
1918 .map(|(_path, r)| r)
1922 /// Get the region outlived by `longer_fr` and live at `element`.
1923 pub(crate) fn region_from_element(
1925 longer_fr: RegionVid,
1926 element: &RegionElement,
1929 RegionElement::Location(l) => self.find_sub_region_live_at(longer_fr, l),
1930 RegionElement::RootUniversalRegion(r) => r,
1931 RegionElement::PlaceholderRegion(error_placeholder) => self
1934 .find_map(|(r, definition)| match definition.origin {
1935 NllRegionVariableOrigin::Placeholder(p) if p == error_placeholder => Some(r),
1942 /// Get the region definition of `r`.
1943 pub(crate) fn region_definition(&self, r: RegionVid) -> &RegionDefinition<'tcx> {
1944 &self.definitions[r]
1947 /// Check if the SCC of `r` contains `upper`.
1948 pub(crate) fn upper_bound_in_region_scc(&self, r: RegionVid, upper: RegionVid) -> bool {
1949 let r_scc = self.constraint_sccs.scc(r);
1950 self.scc_values.contains(r_scc, upper)
1953 pub(crate) fn universal_regions(&self) -> &UniversalRegions<'tcx> {
1954 self.universal_regions.as_ref()
1957 /// Tries to find the best constraint to blame for the fact that
1958 /// `R: from_region`, where `R` is some region that meets
1959 /// `target_test`. This works by following the constraint graph,
1960 /// creating a constraint path that forces `R` to outlive
1961 /// `from_region`, and then finding the best choices within that
1963 #[instrument(level = "debug", skip(self, target_test))]
1964 pub(crate) fn best_blame_constraint(
1966 from_region: RegionVid,
1967 from_region_origin: NllRegionVariableOrigin,
1968 target_test: impl Fn(RegionVid) -> bool,
1969 ) -> (BlameConstraint<'tcx>, Vec<ExtraConstraintInfo>) {
1971 let (path, target_region) =
1972 self.find_constraint_paths_between_regions(from_region, target_test).unwrap();
1977 "{:?} ({:?}: {:?})",
1979 self.constraint_sccs.scc(c.sup),
1980 self.constraint_sccs.scc(c.sub),
1982 .collect::<Vec<_>>()
1985 let mut extra_info = vec![];
1986 for constraint in path.iter() {
1987 let outlived = constraint.sub;
1988 let Some(origin) = self.var_infos.get(outlived) else { continue; };
1989 let RegionVariableOrigin::Nll(NllRegionVariableOrigin::Placeholder(p)) = origin.origin else { continue; };
1990 debug!(?constraint, ?p);
1991 let ConstraintCategory::Predicate(span) = constraint.category else { continue; };
1992 extra_info.push(ExtraConstraintInfo::PlaceholderFromPredicate(span));
1993 // We only want to point to one
1997 // We try to avoid reporting a `ConstraintCategory::Predicate` as our best constraint.
1998 // Instead, we use it to produce an improved `ObligationCauseCode`.
1999 // FIXME - determine what we should do if we encounter multiple `ConstraintCategory::Predicate`
2000 // constraints. Currently, we just pick the first one.
2001 let cause_code = path
2003 .find_map(|constraint| {
2004 if let ConstraintCategory::Predicate(predicate_span) = constraint.category {
2005 // We currently do not store the `DefId` in the `ConstraintCategory`
2006 // for performances reasons. The error reporting code used by NLL only
2007 // uses the span, so this doesn't cause any problems at the moment.
2008 Some(ObligationCauseCode::BindingObligation(
2009 CRATE_DEF_ID.to_def_id(),
2016 .unwrap_or_else(|| ObligationCauseCode::MiscObligation);
2018 // Classify each of the constraints along the path.
2019 let mut categorized_path: Vec<BlameConstraint<'tcx>> = path
2021 .map(|constraint| BlameConstraint {
2022 category: constraint.category,
2023 from_closure: constraint.from_closure,
2024 cause: ObligationCause::new(constraint.span, CRATE_DEF_ID, cause_code.clone()),
2025 variance_info: constraint.variance_info,
2026 outlives_constraint: *constraint,
2029 debug!("categorized_path={:#?}", categorized_path);
2031 // To find the best span to cite, we first try to look for the
2032 // final constraint that is interesting and where the `sup` is
2033 // not unified with the ultimate target region. The reason
2034 // for this is that we have a chain of constraints that lead
2035 // from the source to the target region, something like:
2037 // '0: '1 ('0 is the source)
2042 // '5: '6 ('6 is the target)
2044 // Some of those regions are unified with `'6` (in the same
2045 // SCC). We want to screen those out. After that point, the
2046 // "closest" constraint we have to the end is going to be the
2047 // most likely to be the point where the value escapes -- but
2048 // we still want to screen for an "interesting" point to
2049 // highlight (e.g., a call site or something).
2050 let target_scc = self.constraint_sccs.scc(target_region);
2051 let mut range = 0..path.len();
2053 // As noted above, when reporting an error, there is typically a chain of constraints
2054 // leading from some "source" region which must outlive some "target" region.
2055 // In most cases, we prefer to "blame" the constraints closer to the target --
2056 // but there is one exception. When constraints arise from higher-ranked subtyping,
2057 // we generally prefer to blame the source value,
2058 // as the "target" in this case tends to be some type annotation that the user gave.
2059 // Therefore, if we find that the region origin is some instantiation
2060 // of a higher-ranked region, we start our search from the "source" point
2061 // rather than the "target", and we also tweak a few other things.
2063 // An example might be this bit of Rust code:
2066 // let x: fn(&'static ()) = |_| {};
2067 // let y: for<'a> fn(&'a ()) = x;
2070 // In MIR, this will be converted into a combination of assignments and type ascriptions.
2071 // In particular, the 'static is imposed through a type ascription:
2075 // AscribeUserType(x, fn(&'static ())
2079 // We wind up ultimately with constraints like
2082 // !a: 'temp1 // from the `y = x` statement
2084 // 'temp2: 'static // from the AscribeUserType
2087 // and here we prefer to blame the source (the y = x statement).
2088 let blame_source = match from_region_origin {
2089 NllRegionVariableOrigin::FreeRegion
2090 | NllRegionVariableOrigin::Existential { from_forall: false } => true,
2091 NllRegionVariableOrigin::Placeholder(_)
2092 | NllRegionVariableOrigin::Existential { from_forall: true } => false,
2095 let find_region = |i: &usize| {
2096 let constraint = &path[*i];
2098 let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
2101 match categorized_path[*i].category {
2102 ConstraintCategory::OpaqueType
2103 | ConstraintCategory::Boring
2104 | ConstraintCategory::BoringNoLocation
2105 | ConstraintCategory::Internal
2106 | ConstraintCategory::Predicate(_) => false,
2107 ConstraintCategory::TypeAnnotation
2108 | ConstraintCategory::Return(_)
2109 | ConstraintCategory::Yield => true,
2110 _ => constraint_sup_scc != target_scc,
2114 categorized_path[*i].category,
2115 ConstraintCategory::OpaqueType
2116 | ConstraintCategory::Boring
2117 | ConstraintCategory::BoringNoLocation
2118 | ConstraintCategory::Internal
2119 | ConstraintCategory::Predicate(_)
2125 if blame_source { range.rev().find(find_region) } else { range.find(find_region) };
2127 debug!(?best_choice, ?blame_source, ?extra_info);
2129 if let Some(i) = best_choice {
2130 if let Some(next) = categorized_path.get(i + 1) {
2131 if matches!(categorized_path[i].category, ConstraintCategory::Return(_))
2132 && next.category == ConstraintCategory::OpaqueType
2134 // The return expression is being influenced by the return type being
2135 // impl Trait, point at the return type and not the return expr.
2136 return (next.clone(), extra_info);
2140 if categorized_path[i].category == ConstraintCategory::Return(ReturnConstraint::Normal)
2142 let field = categorized_path.iter().find_map(|p| {
2143 if let ConstraintCategory::ClosureUpvar(f) = p.category {
2150 if let Some(field) = field {
2151 categorized_path[i].category =
2152 ConstraintCategory::Return(ReturnConstraint::ClosureUpvar(field));
2156 return (categorized_path[i].clone(), extra_info);
2159 // If that search fails, that is.. unusual. Maybe everything
2160 // is in the same SCC or something. In that case, find what
2161 // appears to be the most interesting point to report to the
2162 // user via an even more ad-hoc guess.
2163 categorized_path.sort_by(|p0, p1| p0.category.cmp(&p1.category));
2164 debug!("sorted_path={:#?}", categorized_path);
2166 (categorized_path.remove(0), extra_info)
2169 pub(crate) fn universe_info(&self, universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
2170 self.universe_causes[&universe].clone()
2173 /// Tries to find the terminator of the loop in which the region 'r' resides.
2174 /// Returns the location of the terminator if found.
2175 pub(crate) fn find_loop_terminator_location(
2179 ) -> Option<Location> {
2180 let scc = self.constraint_sccs.scc(r.to_region_vid());
2181 let locations = self.scc_values.locations_outlived_by(scc);
2182 for location in locations {
2183 let bb = &body[location.block];
2184 if let Some(terminator) = &bb.terminator {
2185 // terminator of a loop should be TerminatorKind::FalseUnwind
2186 if let TerminatorKind::FalseUnwind { .. } = terminator.kind {
2187 return Some(location);
2195 impl<'tcx> RegionDefinition<'tcx> {
2196 fn new(universe: ty::UniverseIndex, rv_origin: RegionVariableOrigin) -> Self {
2197 // Create a new region definition. Note that, for free
2198 // regions, the `external_name` field gets updated later in
2199 // `init_universal_regions`.
2201 let origin = match rv_origin {
2202 RegionVariableOrigin::Nll(origin) => origin,
2203 _ => NllRegionVariableOrigin::Existential { from_forall: false },
2206 Self { origin, universe, external_name: None }
2210 #[derive(Clone, Debug)]
2211 pub struct BlameConstraint<'tcx> {
2212 pub category: ConstraintCategory<'tcx>,
2213 pub from_closure: bool,
2214 pub cause: ObligationCause<'tcx>,
2215 pub variance_info: ty::VarianceDiagInfo<'tcx>,
2216 pub outlives_constraint: OutlivesConstraint<'tcx>,