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_hir::CRATE_HIR_ID;
11 use rustc_index::vec::IndexVec;
12 use rustc_infer::infer::outlives::test_type_match;
13 use rustc_infer::infer::region_constraints::{GenericKind, VarInfos, VerifyBound, VerifyIfEq};
14 use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin};
15 use rustc_middle::mir::{
16 Body, ClosureOutlivesRequirement, ClosureOutlivesSubject, ClosureRegionRequirements,
17 ConstraintCategory, Local, Location, ReturnConstraint, TerminatorKind,
19 use rustc_middle::traits::ObligationCause;
20 use rustc_middle::traits::ObligationCauseCode;
21 use rustc_middle::ty::{self, RegionVid, Ty, TyCtxt, TypeFoldable, TypeVisitable};
26 graph::NormalConstraintGraph, ConstraintSccIndex, OutlivesConstraint, OutlivesConstraintSet,
28 diagnostics::{RegionErrorKind, RegionErrors, UniverseInfo},
29 member_constraints::{MemberConstraintSet, NllMemberConstraintIndex},
30 nll::{PoloniusOutput, ToRegionVid},
31 region_infer::reverse_sccs::ReverseSccGraph,
32 region_infer::values::{
33 LivenessValues, PlaceholderIndices, RegionElement, RegionValueElements, RegionValues,
36 type_check::{free_region_relations::UniversalRegionRelations, Locations},
37 universal_regions::UniversalRegions,
47 pub struct RegionInferenceContext<'tcx> {
48 pub var_infos: VarInfos,
50 /// Contains the definition for every region variable. Region
51 /// variables are identified by their index (`RegionVid`). The
52 /// definition contains information about where the region came
53 /// from as well as its final inferred value.
54 definitions: IndexVec<RegionVid, RegionDefinition<'tcx>>,
56 /// The liveness constraints added to each region. For most
57 /// regions, these start out empty and steadily grow, though for
58 /// each universally quantified region R they start out containing
59 /// the entire CFG and `end(R)`.
60 liveness_constraints: LivenessValues<RegionVid>,
62 /// The outlives constraints computed by the type-check.
63 constraints: Frozen<OutlivesConstraintSet<'tcx>>,
65 /// The constraint-set, but in graph form, making it easy to traverse
66 /// the constraints adjacent to a particular region. Used to construct
67 /// the SCC (see `constraint_sccs`) and for error reporting.
68 constraint_graph: Frozen<NormalConstraintGraph>,
70 /// The SCC computed from `constraints` and the constraint
71 /// graph. We have an edge from SCC A to SCC B if `A: B`. Used to
72 /// compute the values of each region.
73 constraint_sccs: Rc<Sccs<RegionVid, ConstraintSccIndex>>,
75 /// Reverse of the SCC constraint graph -- i.e., an edge `A -> B` exists if
76 /// `B: A`. This is used to compute the universal regions that are required
77 /// to outlive a given SCC. Computed lazily.
78 rev_scc_graph: Option<Rc<ReverseSccGraph>>,
80 /// The "R0 member of [R1..Rn]" constraints, indexed by SCC.
81 member_constraints: Rc<MemberConstraintSet<'tcx, ConstraintSccIndex>>,
83 /// Records the member constraints that we applied to each scc.
84 /// This is useful for error reporting. Once constraint
85 /// propagation is done, this vector is sorted according to
86 /// `member_region_scc`.
87 member_constraints_applied: Vec<AppliedMemberConstraint>,
89 /// Map universe indexes to information on why we created it.
90 universe_causes: FxHashMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
92 /// Contains the minimum universe of any variable within the same
93 /// SCC. We will ensure that no SCC contains values that are not
94 /// visible from this index.
95 scc_universes: IndexVec<ConstraintSccIndex, ty::UniverseIndex>,
97 /// Contains a "representative" from each SCC. This will be the
98 /// minimal RegionVid belonging to that universe. It is used as a
99 /// kind of hacky way to manage checking outlives relationships,
100 /// since we can 'canonicalize' each region to the representative
101 /// of its SCC and be sure that -- if they have the same repr --
102 /// they *must* be equal (though not having the same repr does not
103 /// mean they are unequal).
104 scc_representatives: IndexVec<ConstraintSccIndex, ty::RegionVid>,
106 /// The final inferred values of the region variables; we compute
107 /// one value per SCC. To get the value for any given *region*,
108 /// you first find which scc it is a part of.
109 scc_values: RegionValues<ConstraintSccIndex>,
111 /// Type constraints that we check after solving.
112 type_tests: Vec<TypeTest<'tcx>>,
114 /// Information about the universally quantified regions in scope
115 /// on this function.
116 universal_regions: Rc<UniversalRegions<'tcx>>,
118 /// Information about how the universally quantified regions in
119 /// scope on this function relate to one another.
120 universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
123 /// Each time that `apply_member_constraint` is successful, it appends
124 /// one of these structs to the `member_constraints_applied` field.
125 /// This is used in error reporting to trace out what happened.
127 /// The way that `apply_member_constraint` works is that it effectively
128 /// adds a new lower bound to the SCC it is analyzing: so you wind up
129 /// with `'R: 'O` where `'R` is the pick-region and `'O` is the
130 /// 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 /// Returns access to the value of `r` for debugging purposes.
530 pub(crate) fn region_universe(&self, r: RegionVid) -> ty::UniverseIndex {
531 let scc = self.constraint_sccs.scc(r.to_region_vid());
532 self.scc_universes[scc]
535 /// Once region solving has completed, this function will return
536 /// the member constraints that were applied to the value of a given
537 /// region `r`. See `AppliedMemberConstraint`.
538 pub(crate) fn applied_member_constraints(
541 ) -> &[AppliedMemberConstraint] {
542 let scc = self.constraint_sccs.scc(r.to_region_vid());
543 binary_search_util::binary_search_slice(
544 &self.member_constraints_applied,
545 |applied| applied.member_region_scc,
550 /// Performs region inference and report errors if we see any
551 /// unsatisfiable constraints. If this is a closure, returns the
552 /// region requirements to propagate to our creator, if any.
553 #[instrument(skip(self, infcx, body, polonius_output), level = "debug")]
556 infcx: &InferCtxt<'tcx>,
557 param_env: ty::ParamEnv<'tcx>,
559 polonius_output: Option<Rc<PoloniusOutput>>,
560 ) -> (Option<ClosureRegionRequirements<'tcx>>, RegionErrors<'tcx>) {
561 let mir_def_id = body.source.def_id();
562 self.propagate_constraints(body);
564 let mut errors_buffer = RegionErrors::new();
566 // If this is a closure, we can propagate unsatisfied
567 // `outlives_requirements` to our creator, so create a vector
568 // to store those. Otherwise, we'll pass in `None` to the
569 // functions below, which will trigger them to report errors
571 let mut outlives_requirements = infcx.tcx.is_typeck_child(mir_def_id).then(Vec::new);
573 self.check_type_tests(
577 outlives_requirements.as_mut(),
581 // In Polonius mode, the errors about missing universal region relations are in the output
582 // and need to be emitted or propagated. Otherwise, we need to check whether the
583 // constraints were too strong, and if so, emit or propagate those errors.
584 if infcx.tcx.sess.opts.unstable_opts.polonius {
585 self.check_polonius_subset_errors(
586 outlives_requirements.as_mut(),
588 polonius_output.expect("Polonius output is unavailable despite `-Z polonius`"),
591 self.check_universal_regions(outlives_requirements.as_mut(), &mut errors_buffer);
594 if errors_buffer.is_empty() {
595 self.check_member_constraints(infcx, &mut errors_buffer);
598 let outlives_requirements = outlives_requirements.unwrap_or_default();
600 if outlives_requirements.is_empty() {
601 (None, errors_buffer)
603 let num_external_vids = self.universal_regions.num_global_and_external_regions();
605 Some(ClosureRegionRequirements { num_external_vids, outlives_requirements }),
611 /// Propagate the region constraints: this will grow the values
612 /// for each region variable until all the constraints are
613 /// satisfied. Note that some values may grow **too** large to be
614 /// feasible, but we check this later.
615 #[instrument(skip(self, _body), level = "debug")]
616 fn propagate_constraints(&mut self, _body: &Body<'tcx>) {
617 debug!("constraints={:#?}", {
618 let mut constraints: Vec<_> = self.constraints.outlives().iter().collect();
619 constraints.sort_by_key(|c| (c.sup, c.sub));
622 .map(|c| (c, self.constraint_sccs.scc(c.sup), self.constraint_sccs.scc(c.sub)))
626 // To propagate constraints, we walk the DAG induced by the
627 // SCC. For each SCC, we visit its successors and compute
628 // their values, then we union all those values to get our
630 let constraint_sccs = self.constraint_sccs.clone();
631 for scc in constraint_sccs.all_sccs() {
632 self.compute_value_for_scc(scc);
635 // Sort the applied member constraints so we can binary search
636 // through them later.
637 self.member_constraints_applied.sort_by_key(|applied| applied.member_region_scc);
640 /// Computes the value of the SCC `scc_a`, which has not yet been
641 /// computed, by unioning the values of its successors.
642 /// Assumes that all successors have been computed already
643 /// (which is assured by iterating over SCCs in dependency order).
644 #[instrument(skip(self), level = "debug")]
645 fn compute_value_for_scc(&mut self, scc_a: ConstraintSccIndex) {
646 let constraint_sccs = self.constraint_sccs.clone();
648 // Walk each SCC `B` such that `A: B`...
649 for &scc_b in constraint_sccs.successors(scc_a) {
652 // ...and add elements from `B` into `A`. One complication
653 // arises because of universes: If `B` contains something
654 // that `A` cannot name, then `A` can only contain `B` if
655 // it outlives static.
656 if self.universe_compatible(scc_b, scc_a) {
657 // `A` can name everything that is in `B`, so just
659 self.scc_values.add_region(scc_a, scc_b);
661 self.add_incompatible_universe(scc_a);
665 // Now take member constraints into account.
666 let member_constraints = self.member_constraints.clone();
667 for m_c_i in member_constraints.indices(scc_a) {
668 self.apply_member_constraint(scc_a, m_c_i, member_constraints.choice_regions(m_c_i));
671 debug!(value = ?self.scc_values.region_value_str(scc_a));
674 /// Invoked for each `R0 member of [R1..Rn]` constraint.
676 /// `scc` is the SCC containing R0, and `choice_regions` are the
677 /// `R1..Rn` regions -- they are always known to be universal
678 /// regions (and if that's not true, we just don't attempt to
679 /// enforce the constraint).
681 /// The current value of `scc` at the time the method is invoked
682 /// is considered a *lower bound*. If possible, we will modify
683 /// the constraint to set it equal to one of the option regions.
684 /// If we make any changes, returns true, else false.
685 #[instrument(skip(self, member_constraint_index), level = "debug")]
686 fn apply_member_constraint(
688 scc: ConstraintSccIndex,
689 member_constraint_index: NllMemberConstraintIndex,
690 choice_regions: &[ty::RegionVid],
692 // Create a mutable vector of the options. We'll try to winnow
694 let mut choice_regions: Vec<ty::RegionVid> = choice_regions.to_vec();
696 // Convert to the SCC representative: sometimes we have inference
697 // variables in the member constraint that wind up equated with
698 // universal regions. The scc representative is the minimal numbered
699 // one from the corresponding scc so it will be the universal region
701 for c_r in &mut choice_regions {
702 let scc = self.constraint_sccs.scc(*c_r);
703 *c_r = self.scc_representatives[scc];
706 // The 'member region' in a member constraint is part of the
707 // hidden type, which must be in the root universe. Therefore,
708 // it cannot have any placeholders in its value.
709 assert!(self.scc_universes[scc] == ty::UniverseIndex::ROOT);
711 self.scc_values.placeholders_contained_in(scc).next().is_none(),
712 "scc {:?} in a member constraint has placeholder value: {:?}",
714 self.scc_values.region_value_str(scc),
717 // The existing value for `scc` is a lower-bound. This will
718 // consist of some set `{P} + {LB}` of points `{P}` and
719 // lower-bound free regions `{LB}`. As each choice region `O`
720 // is a free region, it will outlive the points. But we can
721 // only consider the option `O` if `O: LB`.
722 choice_regions.retain(|&o_r| {
724 .universal_regions_outlived_by(scc)
725 .all(|lb| self.universal_region_relations.outlives(o_r, lb))
727 debug!(?choice_regions, "after lb");
729 // Now find all the *upper bounds* -- that is, each UB is a
730 // free region that must outlive the member region `R0` (`UB:
731 // R0`). Therefore, we need only keep an option `O` if `UB: O`
733 let rev_scc_graph = self.reverse_scc_graph();
734 let universal_region_relations = &self.universal_region_relations;
735 for ub in rev_scc_graph.upper_bounds(scc) {
737 choice_regions.retain(|&o_r| universal_region_relations.outlives(ub, o_r));
739 debug!(?choice_regions, "after ub");
741 // If we ruled everything out, we're done.
742 if choice_regions.is_empty() {
746 // Otherwise, we need to find the minimum remaining choice, if
747 // any, and take that.
748 debug!("choice_regions remaining are {:#?}", choice_regions);
749 let min = |r1: ty::RegionVid, r2: ty::RegionVid| -> Option<ty::RegionVid> {
750 let r1_outlives_r2 = self.universal_region_relations.outlives(r1, r2);
751 let r2_outlives_r1 = self.universal_region_relations.outlives(r2, r1);
752 match (r1_outlives_r2, r2_outlives_r1) {
753 (true, true) => Some(r1.min(r2)),
754 (true, false) => Some(r2),
755 (false, true) => Some(r1),
756 (false, false) => None,
759 let mut min_choice = choice_regions[0];
760 for &other_option in &choice_regions[1..] {
761 debug!(?min_choice, ?other_option,);
762 match min(min_choice, other_option) {
763 Some(m) => min_choice = m,
765 debug!(?min_choice, ?other_option, "incomparable; no min choice",);
771 let min_choice_scc = self.constraint_sccs.scc(min_choice);
772 debug!(?min_choice, ?min_choice_scc);
773 if self.scc_values.add_region(scc, min_choice_scc) {
774 self.member_constraints_applied.push(AppliedMemberConstraint {
775 member_region_scc: scc,
777 member_constraint_index,
786 /// Returns `true` if all the elements in the value of `scc_b` are nameable
787 /// in `scc_a`. Used during constraint propagation, and only once
788 /// the value of `scc_b` has been computed.
789 fn universe_compatible(&self, scc_b: ConstraintSccIndex, scc_a: ConstraintSccIndex) -> bool {
790 let universe_a = self.scc_universes[scc_a];
792 // Quick check: if scc_b's declared universe is a subset of
793 // scc_a's declared universe (typically, both are ROOT), then
794 // it cannot contain any problematic universe elements.
795 if universe_a.can_name(self.scc_universes[scc_b]) {
799 // Otherwise, we have to iterate over the universe elements in
800 // B's value, and check whether all of them are nameable
802 self.scc_values.placeholders_contained_in(scc_b).all(|p| universe_a.can_name(p.universe))
805 /// Extend `scc` so that it can outlive some placeholder region
806 /// from a universe it can't name; at present, the only way for
807 /// this to be true is if `scc` outlives `'static`. This is
808 /// actually stricter than necessary: ideally, we'd support bounds
809 /// like `for<'a: 'b`>` that might then allow us to approximate
810 /// `'a` with `'b` and not `'static`. But it will have to do for
812 fn add_incompatible_universe(&mut self, scc: ConstraintSccIndex) {
813 debug!("add_incompatible_universe(scc={:?})", scc);
815 let fr_static = self.universal_regions.fr_static;
816 self.scc_values.add_all_points(scc);
817 self.scc_values.add_element(scc, fr_static);
820 /// Once regions have been propagated, this method is used to see
821 /// whether the "type tests" produced by typeck were satisfied;
822 /// type tests encode type-outlives relationships like `T:
823 /// 'a`. See `TypeTest` for more details.
826 infcx: &InferCtxt<'tcx>,
827 param_env: ty::ParamEnv<'tcx>,
829 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
830 errors_buffer: &mut RegionErrors<'tcx>,
834 // Sometimes we register equivalent type-tests that would
835 // result in basically the exact same error being reported to
836 // the user. Avoid that.
837 let mut deduplicate_errors = FxHashSet::default();
839 for type_test in &self.type_tests {
840 debug!("check_type_test: {:?}", type_test);
842 let generic_ty = type_test.generic_kind.to_ty(tcx);
843 if self.eval_verify_bound(
848 type_test.lower_bound,
849 &type_test.verify_bound,
854 if let Some(propagated_outlives_requirements) = &mut propagated_outlives_requirements {
855 if self.try_promote_type_test(
860 propagated_outlives_requirements,
866 // Type-test failed. Report the error.
867 let erased_generic_kind = infcx.tcx.erase_regions(type_test.generic_kind);
869 // Skip duplicate-ish errors.
870 if deduplicate_errors.insert((
872 type_test.lower_bound,
876 "check_type_test: reporting error for erased_generic_kind={:?}, \
877 lower_bound_region={:?}, \
878 type_test.span={:?}",
879 erased_generic_kind, type_test.lower_bound, type_test.span,
882 errors_buffer.push(RegionErrorKind::TypeTestError { type_test: type_test.clone() });
887 /// Invoked when we have some type-test (e.g., `T: 'X`) that we cannot
888 /// prove to be satisfied. If this is a closure, we will attempt to
889 /// "promote" this type-test into our `ClosureRegionRequirements` and
890 /// hence pass it up the creator. To do this, we have to phrase the
891 /// type-test in terms of external free regions, as local free
892 /// regions are not nameable by the closure's creator.
894 /// Promotion works as follows: we first check that the type `T`
895 /// contains only regions that the creator knows about. If this is
896 /// true, then -- as a consequence -- we know that all regions in
897 /// the type `T` are free regions that outlive the closure body. If
898 /// false, then promotion fails.
900 /// Once we've promoted T, we have to "promote" `'X` to some region
901 /// that is "external" to the closure. Generally speaking, a region
902 /// may be the union of some points in the closure body as well as
903 /// various free lifetimes. We can ignore the points in the closure
904 /// body: if the type T can be expressed in terms of external regions,
905 /// we know it outlives the points in the closure body. That
906 /// just leaves the free regions.
908 /// The idea then is to lower the `T: 'X` constraint into multiple
909 /// bounds -- e.g., if `'X` is the union of two free lifetimes,
910 /// `'1` and `'2`, then we would create `T: '1` and `T: '2`.
911 #[instrument(level = "debug", skip(self, infcx, propagated_outlives_requirements))]
912 fn try_promote_type_test(
914 infcx: &InferCtxt<'tcx>,
915 param_env: ty::ParamEnv<'tcx>,
917 type_test: &TypeTest<'tcx>,
918 propagated_outlives_requirements: &mut Vec<ClosureOutlivesRequirement<'tcx>>,
922 let TypeTest { generic_kind, lower_bound, span: _, verify_bound: _ } = type_test;
924 let generic_ty = generic_kind.to_ty(tcx);
925 let Some(subject) = self.try_promote_type_test_subject(infcx, generic_ty) else {
929 debug!("subject = {:?}", subject);
931 let r_scc = self.constraint_sccs.scc(*lower_bound);
934 "lower_bound = {:?} r_scc={:?} universe={:?}",
935 lower_bound, r_scc, self.scc_universes[r_scc]
938 // If the type test requires that `T: 'a` where `'a` is a
939 // placeholder from another universe, that effectively requires
940 // `T: 'static`, so we have to propagate that requirement.
942 // It doesn't matter *what* universe because the promoted `T` will
943 // always be in the root universe.
944 if let Some(p) = self.scc_values.placeholders_contained_in(r_scc).next() {
945 debug!("encountered placeholder in higher universe: {:?}, requiring 'static", p);
946 let static_r = self.universal_regions.fr_static;
947 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
949 outlived_free_region: static_r,
950 blame_span: type_test.span,
951 category: ConstraintCategory::Boring,
954 // we can return here -- the code below might push add'l constraints
955 // but they would all be weaker than this one.
959 // For each region outlived by lower_bound find a non-local,
960 // universal region (it may be the same region) and add it to
961 // `ClosureOutlivesRequirement`.
962 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
963 debug!("universal_region_outlived_by ur={:?}", ur);
964 // Check whether we can already prove that the "subject" outlives `ur`.
965 // If so, we don't have to propagate this requirement to our caller.
967 // To continue the example from the function, if we are trying to promote
968 // a requirement that `T: 'X`, and we know that `'X = '1 + '2` (i.e., the union
969 // `'1` and `'2`), then in this loop `ur` will be `'1` (and `'2`). So here
970 // we check whether `T: '1` is something we *can* prove. If so, no need
971 // to propagate that requirement.
973 // This is needed because -- particularly in the case
974 // where `ur` is a local bound -- we are sometimes in a
975 // position to prove things that our caller cannot. See
976 // #53570 for an example.
977 if self.eval_verify_bound(
983 &type_test.verify_bound,
988 let non_local_ub = self.universal_region_relations.non_local_upper_bounds(ur);
989 debug!("try_promote_type_test: non_local_ub={:?}", non_local_ub);
991 // This is slightly too conservative. To show T: '1, given `'2: '1`
992 // and `'3: '1` we only need to prove that T: '2 *or* T: '3, but to
993 // avoid potential non-determinism we approximate this by requiring
995 for upper_bound in non_local_ub {
996 debug_assert!(self.universal_regions.is_universal_region(upper_bound));
997 debug_assert!(!self.universal_regions.is_local_free_region(upper_bound));
999 let requirement = ClosureOutlivesRequirement {
1001 outlived_free_region: upper_bound,
1002 blame_span: type_test.span,
1003 category: ConstraintCategory::Boring,
1005 debug!("try_promote_type_test: pushing {:#?}", requirement);
1006 propagated_outlives_requirements.push(requirement);
1012 /// When we promote a type test `T: 'r`, we have to convert the
1013 /// type `T` into something we can store in a query result (so
1014 /// something allocated for `'tcx`). This is problematic if `ty`
1015 /// contains regions. During the course of NLL region checking, we
1016 /// will have replaced all of those regions with fresh inference
1017 /// variables. To create a test subject, we want to replace those
1018 /// inference variables with some region from the closure
1019 /// signature -- this is not always possible, so this is a
1020 /// fallible process. Presuming we do find a suitable region, we
1021 /// will use it's *external name*, which will be a `RegionKind`
1022 /// variant that can be used in query responses such as
1024 #[instrument(level = "debug", skip(self, infcx))]
1025 fn try_promote_type_test_subject(
1027 infcx: &InferCtxt<'tcx>,
1029 ) -> Option<ClosureOutlivesSubject<'tcx>> {
1030 let tcx = infcx.tcx;
1032 let ty = tcx.fold_regions(ty, |r, _depth| {
1033 let region_vid = self.to_region_vid(r);
1035 // The challenge if this. We have some region variable `r`
1036 // whose value is a set of CFG points and universal
1037 // regions. We want to find if that set is *equivalent* to
1038 // any of the named regions found in the closure.
1040 // To do so, we compute the
1041 // `non_local_universal_upper_bound`. This will be a
1042 // non-local, universal region that is greater than `r`.
1043 // However, it might not be *contained* within `r`, so
1044 // then we further check whether this bound is contained
1045 // in `r`. If so, we can say that `r` is equivalent to the
1048 // Let's work through a few examples. For these, imagine
1049 // that we have 3 non-local regions (I'll denote them as
1050 // `'static`, `'a`, and `'b`, though of course in the code
1051 // they would be represented with indices) where:
1056 // First, let's assume that `r` is some existential
1057 // variable with an inferred value `{'a, 'static}` (plus
1058 // some CFG nodes). In this case, the non-local upper
1059 // bound is `'static`, since that outlives `'a`. `'static`
1060 // is also a member of `r` and hence we consider `r`
1061 // equivalent to `'static` (and replace it with
1064 // Now let's consider the inferred value `{'a, 'b}`. This
1065 // means `r` is effectively `'a | 'b`. I'm not sure if
1066 // this can come about, actually, but assuming it did, we
1067 // would get a non-local upper bound of `'static`. Since
1068 // `'static` is not contained in `r`, we would fail to
1069 // find an equivalent.
1070 let upper_bound = self.non_local_universal_upper_bound(region_vid);
1071 if self.region_contains(region_vid, upper_bound) {
1072 self.definitions[upper_bound].external_name.unwrap_or(r)
1074 // In the case of a failure, use a `ReVar` result. This will
1075 // cause the `needs_infer` later on to return `None`.
1080 debug!("try_promote_type_test_subject: folded ty = {:?}", ty);
1082 // `needs_infer` will only be true if we failed to promote some region.
1083 if ty.needs_infer() {
1087 Some(ClosureOutlivesSubject::Ty(ty))
1090 /// Given some universal or existential region `r`, finds a
1091 /// non-local, universal region `r+` that outlives `r` at entry to (and
1092 /// exit from) the closure. In the worst case, this will be
1095 /// This is used for two purposes. First, if we are propagated
1096 /// some requirement `T: r`, we can use this method to enlarge `r`
1097 /// to something we can encode for our creator (which only knows
1098 /// about non-local, universal regions). It is also used when
1099 /// encoding `T` as part of `try_promote_type_test_subject` (see
1100 /// that fn for details).
1102 /// This is based on the result `'y` of `universal_upper_bound`,
1103 /// except that it converts further takes the non-local upper
1104 /// bound of `'y`, so that the final result is non-local.
1105 fn non_local_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1106 debug!("non_local_universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1108 let lub = self.universal_upper_bound(r);
1110 // Grow further to get smallest universal region known to
1112 let non_local_lub = self.universal_region_relations.non_local_upper_bound(lub);
1114 debug!("non_local_universal_upper_bound: non_local_lub={:?}", non_local_lub);
1119 /// Returns a universally quantified region that outlives the
1120 /// value of `r` (`r` may be existentially or universally
1123 /// Since `r` is (potentially) an existential region, it has some
1124 /// value which may include (a) any number of points in the CFG
1125 /// and (b) any number of `end('x)` elements of universally
1126 /// quantified regions. To convert this into a single universal
1127 /// region we do as follows:
1129 /// - Ignore the CFG points in `'r`. All universally quantified regions
1130 /// include the CFG anyhow.
1131 /// - For each `end('x)` element in `'r`, compute the mutual LUB, yielding
1133 #[instrument(skip(self), level = "debug", ret)]
1134 pub(crate) fn universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1135 debug!(r = %self.region_value_str(r));
1137 // Find the smallest universal region that contains all other
1138 // universal regions within `region`.
1139 let mut lub = self.universal_regions.fr_fn_body;
1140 let r_scc = self.constraint_sccs.scc(r);
1141 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1142 lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1148 /// Like `universal_upper_bound`, but returns an approximation more suitable
1149 /// for diagnostics. If `r` contains multiple disjoint universal regions
1150 /// (e.g. 'a and 'b in `fn foo<'a, 'b> { ... }`, we pick the lower-numbered region.
1151 /// This corresponds to picking named regions over unnamed regions
1152 /// (e.g. picking early-bound regions over a closure late-bound region).
1154 /// This means that the returned value may not be a true upper bound, since
1155 /// only 'static is known to outlive disjoint universal regions.
1156 /// Therefore, this method should only be used in diagnostic code,
1157 /// where displaying *some* named universal region is better than
1158 /// falling back to 'static.
1159 #[instrument(level = "debug", skip(self))]
1160 pub(crate) fn approx_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1161 debug!("{}", self.region_value_str(r));
1163 // Find the smallest universal region that contains all other
1164 // universal regions within `region`.
1165 let mut lub = self.universal_regions.fr_fn_body;
1166 let r_scc = self.constraint_sccs.scc(r);
1167 let static_r = self.universal_regions.fr_static;
1168 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1169 let new_lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1170 debug!(?ur, ?lub, ?new_lub);
1171 // The upper bound of two non-static regions is static: this
1172 // means we know nothing about the relationship between these
1173 // two regions. Pick a 'better' one to use when constructing
1175 if ur != static_r && lub != static_r && new_lub == static_r {
1176 // Prefer the region with an `external_name` - this
1177 // indicates that the region is early-bound, so working with
1178 // it can produce a nicer error.
1179 if self.region_definition(ur).external_name.is_some() {
1181 } else if self.region_definition(lub).external_name.is_some() {
1182 // Leave lub unchanged
1184 // If we get here, we don't have any reason to prefer
1185 // one region over the other. Just pick the
1186 // one with the lower index for now.
1187 lub = std::cmp::min(ur, lub);
1199 /// Tests if `test` is true when applied to `lower_bound` at
1201 fn eval_verify_bound(
1203 infcx: &InferCtxt<'tcx>,
1204 param_env: ty::ParamEnv<'tcx>,
1206 generic_ty: Ty<'tcx>,
1207 lower_bound: RegionVid,
1208 verify_bound: &VerifyBound<'tcx>,
1210 debug!("eval_verify_bound(lower_bound={:?}, verify_bound={:?})", lower_bound, verify_bound);
1212 match verify_bound {
1213 VerifyBound::IfEq(verify_if_eq_b) => {
1214 self.eval_if_eq(infcx, param_env, generic_ty, lower_bound, *verify_if_eq_b)
1217 VerifyBound::IsEmpty => {
1218 let lower_bound_scc = self.constraint_sccs.scc(lower_bound);
1219 self.scc_values.elements_contained_in(lower_bound_scc).next().is_none()
1222 VerifyBound::OutlivedBy(r) => {
1223 let r_vid = self.to_region_vid(*r);
1224 self.eval_outlives(r_vid, lower_bound)
1227 VerifyBound::AnyBound(verify_bounds) => verify_bounds.iter().any(|verify_bound| {
1228 self.eval_verify_bound(
1238 VerifyBound::AllBounds(verify_bounds) => verify_bounds.iter().all(|verify_bound| {
1239 self.eval_verify_bound(
1253 infcx: &InferCtxt<'tcx>,
1254 param_env: ty::ParamEnv<'tcx>,
1255 generic_ty: Ty<'tcx>,
1256 lower_bound: RegionVid,
1257 verify_if_eq_b: ty::Binder<'tcx, VerifyIfEq<'tcx>>,
1259 let generic_ty = self.normalize_to_scc_representatives(infcx.tcx, generic_ty);
1260 let verify_if_eq_b = self.normalize_to_scc_representatives(infcx.tcx, verify_if_eq_b);
1261 match test_type_match::extract_verify_if_eq(
1268 let r_vid = self.to_region_vid(r);
1269 self.eval_outlives(r_vid, lower_bound)
1275 /// This is a conservative normalization procedure. It takes every
1276 /// free region in `value` and replaces it with the
1277 /// "representative" of its SCC (see `scc_representatives` field).
1278 /// We are guaranteed that if two values normalize to the same
1279 /// thing, then they are equal; this is a conservative check in
1280 /// that they could still be equal even if they normalize to
1281 /// different results. (For example, there might be two regions
1282 /// with the same value that are not in the same SCC).
1284 /// N.B., this is not an ideal approach and I would like to revisit
1285 /// it. However, it works pretty well in practice. In particular,
1286 /// this is needed to deal with projection outlives bounds like
1289 /// <T as Foo<'0>>::Item: '1
1292 /// In particular, this routine winds up being important when
1293 /// there are bounds like `where <T as Foo<'a>>::Item: 'b` in the
1294 /// environment. In this case, if we can show that `'0 == 'a`,
1295 /// and that `'b: '1`, then we know that the clause is
1296 /// satisfied. In such cases, particularly due to limitations of
1297 /// the trait solver =), we usually wind up with a where-clause like
1298 /// `T: Foo<'a>` in scope, which thus forces `'0 == 'a` to be added as
1299 /// a constraint, and thus ensures that they are in the same SCC.
1301 /// So why can't we do a more correct routine? Well, we could
1302 /// *almost* use the `relate_tys` code, but the way it is
1303 /// currently setup it creates inference variables to deal with
1304 /// higher-ranked things and so forth, and right now the inference
1305 /// context is not permitted to make more inference variables. So
1306 /// we use this kind of hacky solution.
1307 fn normalize_to_scc_representatives<T>(&self, tcx: TyCtxt<'tcx>, value: T) -> T
1309 T: TypeFoldable<'tcx>,
1311 tcx.fold_regions(value, |r, _db| {
1312 let vid = self.to_region_vid(r);
1313 let scc = self.constraint_sccs.scc(vid);
1314 let repr = self.scc_representatives[scc];
1315 tcx.mk_region(ty::ReVar(repr))
1319 // Evaluate whether `sup_region == sub_region`.
1320 fn eval_equal(&self, r1: RegionVid, r2: RegionVid) -> bool {
1321 self.eval_outlives(r1, r2) && self.eval_outlives(r2, r1)
1324 // Evaluate whether `sup_region: sub_region`.
1325 #[instrument(skip(self), level = "debug", ret)]
1326 fn eval_outlives(&self, sup_region: RegionVid, sub_region: RegionVid) -> bool {
1328 "sup_region's value = {:?} universal={:?}",
1329 self.region_value_str(sup_region),
1330 self.universal_regions.is_universal_region(sup_region),
1333 "sub_region's value = {:?} universal={:?}",
1334 self.region_value_str(sub_region),
1335 self.universal_regions.is_universal_region(sub_region),
1338 let sub_region_scc = self.constraint_sccs.scc(sub_region);
1339 let sup_region_scc = self.constraint_sccs.scc(sup_region);
1341 // If we are checking that `'sup: 'sub`, and `'sub` contains
1342 // some placeholder that `'sup` cannot name, then this is only
1343 // true if `'sup` outlives static.
1344 if !self.universe_compatible(sub_region_scc, sup_region_scc) {
1346 "sub universe `{sub_region_scc:?}` is not nameable \
1347 by super `{sup_region_scc:?}`, promoting to static",
1350 return self.eval_outlives(sup_region, self.universal_regions.fr_static);
1353 // Both the `sub_region` and `sup_region` consist of the union
1354 // of some number of universal regions (along with the union
1355 // of various points in the CFG; ignore those points for
1356 // now). Therefore, the sup-region outlives the sub-region if,
1357 // for each universal region R1 in the sub-region, there
1358 // exists some region R2 in the sup-region that outlives R1.
1359 let universal_outlives =
1360 self.scc_values.universal_regions_outlived_by(sub_region_scc).all(|r1| {
1362 .universal_regions_outlived_by(sup_region_scc)
1363 .any(|r2| self.universal_region_relations.outlives(r2, r1))
1366 if !universal_outlives {
1367 debug!("sub region contains a universal region not present in super");
1371 // Now we have to compare all the points in the sub region and make
1372 // sure they exist in the sup region.
1374 if self.universal_regions.is_universal_region(sup_region) {
1375 // Micro-opt: universal regions contain all points.
1376 debug!("super is universal and hence contains all points");
1380 debug!("comparison between points in sup/sub");
1382 self.scc_values.contains_points(sup_region_scc, sub_region_scc)
1385 /// Once regions have been propagated, this method is used to see
1386 /// whether any of the constraints were too strong. In particular,
1387 /// we want to check for a case where a universally quantified
1388 /// region exceeded its bounds. Consider:
1390 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1392 /// In this case, returning `x` requires `&'a u32 <: &'b u32`
1393 /// and hence we establish (transitively) a constraint that
1394 /// `'a: 'b`. The `propagate_constraints` code above will
1395 /// therefore add `end('a)` into the region for `'b` -- but we
1396 /// have no evidence that `'b` outlives `'a`, so we want to report
1399 /// If `propagated_outlives_requirements` is `Some`, then we will
1400 /// push unsatisfied obligations into there. Otherwise, we'll
1401 /// report them as errors.
1402 fn check_universal_regions(
1404 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1405 errors_buffer: &mut RegionErrors<'tcx>,
1407 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1408 match fr_definition.origin {
1409 NllRegionVariableOrigin::FreeRegion => {
1410 // Go through each of the universal regions `fr` and check that
1411 // they did not grow too large, accumulating any requirements
1412 // for our caller into the `outlives_requirements` vector.
1413 self.check_universal_region(
1415 &mut propagated_outlives_requirements,
1420 NllRegionVariableOrigin::Placeholder(placeholder) => {
1421 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1424 NllRegionVariableOrigin::Existential { .. } => {
1425 // nothing to check here
1431 /// Checks if Polonius has found any unexpected free region relations.
1433 /// In Polonius terms, a "subset error" (or "illegal subset relation error") is the equivalent
1434 /// of NLL's "checking if any region constraints were too strong": a placeholder origin `'a`
1435 /// was unexpectedly found to be a subset of another placeholder origin `'b`, and means in NLL
1436 /// terms that the "longer free region" `'a` outlived the "shorter free region" `'b`.
1438 /// More details can be found in this blog post by Niko:
1439 /// <https://smallcultfollowing.com/babysteps/blog/2019/01/17/polonius-and-region-errors/>
1441 /// In the canonical example
1443 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1445 /// returning `x` requires `&'a u32 <: &'b u32` and hence we establish (transitively) a
1446 /// constraint that `'a: 'b`. It is an error that we have no evidence that this
1447 /// constraint holds.
1449 /// If `propagated_outlives_requirements` is `Some`, then we will
1450 /// push unsatisfied obligations into there. Otherwise, we'll
1451 /// report them as errors.
1452 fn check_polonius_subset_errors(
1454 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1455 errors_buffer: &mut RegionErrors<'tcx>,
1456 polonius_output: Rc<PoloniusOutput>,
1459 "check_polonius_subset_errors: {} subset_errors",
1460 polonius_output.subset_errors.len()
1463 // Similarly to `check_universal_regions`: a free region relation, which was not explicitly
1464 // declared ("known") was found by Polonius, so emit an error, or propagate the
1465 // requirements for our caller into the `propagated_outlives_requirements` vector.
1467 // Polonius doesn't model regions ("origins") as CFG-subsets or durations, but the
1468 // `longer_fr` and `shorter_fr` terminology will still be used here, for consistency with
1469 // the rest of the NLL infrastructure. The "subset origin" is the "longer free region",
1470 // and the "superset origin" is the outlived "shorter free region".
1472 // Note: Polonius will produce a subset error at every point where the unexpected
1473 // `longer_fr`'s "placeholder loan" is contained in the `shorter_fr`. This can be helpful
1474 // for diagnostics in the future, e.g. to point more precisely at the key locations
1475 // requiring this constraint to hold. However, the error and diagnostics code downstream
1476 // expects that these errors are not duplicated (and that they are in a certain order).
1477 // Otherwise, diagnostics messages such as the ones giving names like `'1` to elided or
1478 // anonymous lifetimes for example, could give these names differently, while others like
1479 // the outlives suggestions or the debug output from `#[rustc_regions]` would be
1480 // duplicated. The polonius subset errors are deduplicated here, while keeping the
1481 // CFG-location ordering.
1482 let mut subset_errors: Vec<_> = polonius_output
1485 .flat_map(|(_location, subset_errors)| subset_errors.iter())
1487 subset_errors.sort();
1488 subset_errors.dedup();
1490 for (longer_fr, shorter_fr) in subset_errors.into_iter() {
1492 "check_polonius_subset_errors: subset_error longer_fr={:?},\
1494 longer_fr, shorter_fr
1497 let propagated = self.try_propagate_universal_region_error(
1500 &mut propagated_outlives_requirements,
1502 if propagated == RegionRelationCheckResult::Error {
1503 errors_buffer.push(RegionErrorKind::RegionError {
1504 longer_fr: *longer_fr,
1505 shorter_fr: *shorter_fr,
1506 fr_origin: NllRegionVariableOrigin::FreeRegion,
1512 // Handle the placeholder errors as usual, until the chalk-rustc-polonius triumvirate has
1513 // a more complete picture on how to separate this responsibility.
1514 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1515 match fr_definition.origin {
1516 NllRegionVariableOrigin::FreeRegion => {
1517 // handled by polonius above
1520 NllRegionVariableOrigin::Placeholder(placeholder) => {
1521 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1524 NllRegionVariableOrigin::Existential { .. } => {
1525 // nothing to check here
1531 /// Checks the final value for the free region `fr` to see if it
1532 /// grew too large. In particular, examine what `end(X)` points
1533 /// wound up in `fr`'s final value; for each `end(X)` where `X !=
1534 /// fr`, we want to check that `fr: X`. If not, that's either an
1535 /// error, or something we have to propagate to our creator.
1537 /// Things that are to be propagated are accumulated into the
1538 /// `outlives_requirements` vector.
1539 #[instrument(skip(self, propagated_outlives_requirements, errors_buffer), level = "debug")]
1540 fn check_universal_region(
1542 longer_fr: RegionVid,
1543 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1544 errors_buffer: &mut RegionErrors<'tcx>,
1546 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1548 // Because this free region must be in the ROOT universe, we
1549 // know it cannot contain any bound universes.
1550 assert!(self.scc_universes[longer_fr_scc] == ty::UniverseIndex::ROOT);
1551 debug_assert!(self.scc_values.placeholders_contained_in(longer_fr_scc).next().is_none());
1553 // Only check all of the relations for the main representative of each
1554 // SCC, otherwise just check that we outlive said representative. This
1555 // reduces the number of redundant relations propagated out of
1557 // Note that the representative will be a universal region if there is
1558 // one in this SCC, so we will always check the representative here.
1559 let representative = self.scc_representatives[longer_fr_scc];
1560 if representative != longer_fr {
1561 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1564 propagated_outlives_requirements,
1566 errors_buffer.push(RegionErrorKind::RegionError {
1568 shorter_fr: representative,
1569 fr_origin: NllRegionVariableOrigin::FreeRegion,
1576 // Find every region `o` such that `fr: o`
1577 // (because `fr` includes `end(o)`).
1578 let mut error_reported = false;
1579 for shorter_fr in self.scc_values.universal_regions_outlived_by(longer_fr_scc) {
1580 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1583 propagated_outlives_requirements,
1585 // We only report the first region error. Subsequent errors are hidden so as
1586 // not to overwhelm the user, but we do record them so as to potentially print
1587 // better diagnostics elsewhere...
1588 errors_buffer.push(RegionErrorKind::RegionError {
1591 fr_origin: NllRegionVariableOrigin::FreeRegion,
1592 is_reported: !error_reported,
1595 error_reported = true;
1600 /// Checks that we can prove that `longer_fr: shorter_fr`. If we can't we attempt to propagate
1601 /// the constraint outward (e.g. to a closure environment), but if that fails, there is an
1603 fn check_universal_region_relation(
1605 longer_fr: RegionVid,
1606 shorter_fr: RegionVid,
1607 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1608 ) -> RegionRelationCheckResult {
1609 // If it is known that `fr: o`, carry on.
1610 if self.universal_region_relations.outlives(longer_fr, shorter_fr) {
1611 RegionRelationCheckResult::Ok
1613 // If we are not in a context where we can't propagate errors, or we
1614 // could not shrink `fr` to something smaller, then just report an
1617 // Note: in this case, we use the unapproximated regions to report the
1618 // error. This gives better error messages in some cases.
1619 self.try_propagate_universal_region_error(
1622 propagated_outlives_requirements,
1627 /// Attempt to propagate a region error (e.g. `'a: 'b`) that is not met to a closure's
1628 /// creator. If we cannot, then the caller should report an error to the user.
1629 fn try_propagate_universal_region_error(
1631 longer_fr: RegionVid,
1632 shorter_fr: RegionVid,
1633 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1634 ) -> RegionRelationCheckResult {
1635 if let Some(propagated_outlives_requirements) = propagated_outlives_requirements {
1636 // Shrink `longer_fr` until we find a non-local region (if we do).
1637 // We'll call it `fr-` -- it's ever so slightly smaller than
1639 if let Some(fr_minus) = self.universal_region_relations.non_local_lower_bound(longer_fr)
1641 debug!("try_propagate_universal_region_error: fr_minus={:?}", fr_minus);
1643 let blame_span_category = self.find_outlives_blame_span(
1645 NllRegionVariableOrigin::FreeRegion,
1649 // Grow `shorter_fr` until we find some non-local regions. (We
1650 // always will.) We'll call them `shorter_fr+` -- they're ever
1651 // so slightly larger than `shorter_fr`.
1652 let shorter_fr_plus =
1653 self.universal_region_relations.non_local_upper_bounds(shorter_fr);
1655 "try_propagate_universal_region_error: shorter_fr_plus={:?}",
1658 for fr in shorter_fr_plus {
1659 // Push the constraint `fr-: shorter_fr+`
1660 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
1661 subject: ClosureOutlivesSubject::Region(fr_minus),
1662 outlived_free_region: fr,
1663 blame_span: blame_span_category.1.span,
1664 category: blame_span_category.0,
1667 return RegionRelationCheckResult::Propagated;
1671 RegionRelationCheckResult::Error
1674 fn check_bound_universal_region(
1676 longer_fr: RegionVid,
1677 placeholder: ty::PlaceholderRegion,
1678 errors_buffer: &mut RegionErrors<'tcx>,
1680 debug!("check_bound_universal_region(fr={:?}, placeholder={:?})", longer_fr, placeholder,);
1682 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1683 debug!("check_bound_universal_region: longer_fr_scc={:?}", longer_fr_scc,);
1685 // If we have some bound universal region `'a`, then the only
1686 // elements it can contain is itself -- we don't know anything
1688 let Some(error_element) = ({
1689 self.scc_values.elements_contained_in(longer_fr_scc).find(|element| match element {
1690 RegionElement::Location(_) => true,
1691 RegionElement::RootUniversalRegion(_) => true,
1692 RegionElement::PlaceholderRegion(placeholder1) => placeholder != *placeholder1,
1697 debug!("check_bound_universal_region: error_element = {:?}", error_element);
1699 // Find the region that introduced this `error_element`.
1700 errors_buffer.push(RegionErrorKind::BoundUniversalRegionError {
1707 fn check_member_constraints(
1709 infcx: &InferCtxt<'tcx>,
1710 errors_buffer: &mut RegionErrors<'tcx>,
1712 let member_constraints = self.member_constraints.clone();
1713 for m_c_i in member_constraints.all_indices() {
1714 debug!("check_member_constraint(m_c_i={:?})", m_c_i);
1715 let m_c = &member_constraints[m_c_i];
1716 let member_region_vid = m_c.member_region_vid;
1718 "check_member_constraint: member_region_vid={:?} with value {}",
1720 self.region_value_str(member_region_vid),
1722 let choice_regions = member_constraints.choice_regions(m_c_i);
1723 debug!("check_member_constraint: choice_regions={:?}", choice_regions);
1725 // Did the member region wind up equal to any of the option regions?
1727 choice_regions.iter().find(|&&o_r| self.eval_equal(o_r, m_c.member_region_vid))
1729 debug!("check_member_constraint: evaluated as equal to {:?}", o);
1733 // If not, report an error.
1734 let member_region = infcx.tcx.mk_region(ty::ReVar(member_region_vid));
1735 errors_buffer.push(RegionErrorKind::UnexpectedHiddenRegion {
1736 span: m_c.definition_span,
1737 hidden_ty: m_c.hidden_ty,
1744 /// We have a constraint `fr1: fr2` that is not satisfied, where
1745 /// `fr2` represents some universal region. Here, `r` is some
1746 /// region where we know that `fr1: r` and this function has the
1747 /// job of determining whether `r` is "to blame" for the fact that
1748 /// `fr1: fr2` is required.
1750 /// This is true under two conditions:
1753 /// - `fr2` is `'static` and `r` is some placeholder in a universe
1754 /// that cannot be named by `fr1`; in that case, we will require
1755 /// that `fr1: 'static` because it is the only way to `fr1: r` to
1756 /// be satisfied. (See `add_incompatible_universe`.)
1757 pub(crate) fn provides_universal_region(
1763 debug!("provides_universal_region(r={:?}, fr1={:?}, fr2={:?})", r, fr1, fr2);
1766 fr2 == self.universal_regions.fr_static && self.cannot_name_placeholder(fr1, r)
1769 debug!("provides_universal_region: result = {:?}", result);
1773 /// If `r2` represents a placeholder region, then this returns
1774 /// `true` if `r1` cannot name that placeholder in its
1775 /// value; otherwise, returns `false`.
1776 pub(crate) fn cannot_name_placeholder(&self, r1: RegionVid, r2: RegionVid) -> bool {
1777 debug!("cannot_name_value_of(r1={:?}, r2={:?})", r1, r2);
1779 match self.definitions[r2].origin {
1780 NllRegionVariableOrigin::Placeholder(placeholder) => {
1781 let universe1 = self.definitions[r1].universe;
1783 "cannot_name_value_of: universe1={:?} placeholder={:?}",
1784 universe1, placeholder
1786 universe1.cannot_name(placeholder.universe)
1789 NllRegionVariableOrigin::FreeRegion | NllRegionVariableOrigin::Existential { .. } => {
1795 /// Finds a good `ObligationCause` to blame for the fact that `fr1` outlives `fr2`.
1796 pub(crate) fn find_outlives_blame_span(
1799 fr1_origin: NllRegionVariableOrigin,
1801 ) -> (ConstraintCategory<'tcx>, ObligationCause<'tcx>) {
1802 let BlameConstraint { category, cause, .. } = self
1803 .best_blame_constraint(fr1, fr1_origin, |r| self.provides_universal_region(r, fr1, fr2))
1808 /// Walks the graph of constraints (where `'a: 'b` is considered
1809 /// an edge `'a -> 'b`) to find all paths from `from_region` to
1810 /// `to_region`. The paths are accumulated into the vector
1811 /// `results`. The paths are stored as a series of
1812 /// `ConstraintIndex` values -- in other words, a list of *edges*.
1814 /// Returns: a series of constraints as well as the region `R`
1815 /// that passed the target test.
1816 pub(crate) fn find_constraint_paths_between_regions(
1818 from_region: RegionVid,
1819 target_test: impl Fn(RegionVid) -> bool,
1820 ) -> Option<(Vec<OutlivesConstraint<'tcx>>, RegionVid)> {
1821 let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions);
1822 context[from_region] = Trace::StartRegion;
1824 // Use a deque so that we do a breadth-first search. We will
1825 // stop at the first match, which ought to be the shortest
1826 // path (fewest constraints).
1827 let mut deque = VecDeque::new();
1828 deque.push_back(from_region);
1830 while let Some(r) = deque.pop_front() {
1832 "find_constraint_paths_between_regions: from_region={:?} r={:?} value={}",
1835 self.region_value_str(r),
1838 // Check if we reached the region we were looking for. If so,
1839 // we can reconstruct the path that led to it and return it.
1841 let mut result = vec![];
1844 match context[p].clone() {
1845 Trace::NotVisited => {
1846 bug!("found unvisited region {:?} on path to {:?}", p, r)
1849 Trace::FromOutlivesConstraint(c) => {
1854 Trace::StartRegion => {
1856 return Some((result, r));
1862 // Otherwise, walk over the outgoing constraints and
1863 // enqueue any regions we find, keeping track of how we
1866 // A constraint like `'r: 'x` can come from our constraint
1868 let fr_static = self.universal_regions.fr_static;
1869 let outgoing_edges_from_graph =
1870 self.constraint_graph.outgoing_edges(r, &self.constraints, fr_static);
1872 // Always inline this closure because it can be hot.
1873 let mut handle_constraint = #[inline(always)]
1874 |constraint: OutlivesConstraint<'tcx>| {
1875 debug_assert_eq!(constraint.sup, r);
1876 let sub_region = constraint.sub;
1877 if let Trace::NotVisited = context[sub_region] {
1878 context[sub_region] = Trace::FromOutlivesConstraint(constraint);
1879 deque.push_back(sub_region);
1883 // This loop can be hot.
1884 for constraint in outgoing_edges_from_graph {
1885 handle_constraint(constraint);
1888 // Member constraints can also give rise to `'r: 'x` edges that
1889 // were not part of the graph initially, so watch out for those.
1890 // (But they are extremely rare; this loop is very cold.)
1891 for constraint in self.applied_member_constraints(r) {
1892 let p_c = &self.member_constraints[constraint.member_constraint_index];
1893 let constraint = OutlivesConstraint {
1895 sub: constraint.min_choice,
1896 locations: Locations::All(p_c.definition_span),
1897 span: p_c.definition_span,
1898 category: ConstraintCategory::OpaqueType,
1899 variance_info: ty::VarianceDiagInfo::default(),
1900 from_closure: false,
1902 handle_constraint(constraint);
1909 /// Finds some region R such that `fr1: R` and `R` is live at `elem`.
1910 #[instrument(skip(self), level = "trace", ret)]
1911 pub(crate) fn find_sub_region_live_at(&self, fr1: RegionVid, elem: Location) -> RegionVid {
1912 trace!(scc = ?self.constraint_sccs.scc(fr1));
1913 trace!(universe = ?self.scc_universes[self.constraint_sccs.scc(fr1)]);
1914 self.find_constraint_paths_between_regions(fr1, |r| {
1915 // First look for some `r` such that `fr1: r` and `r` is live at `elem`
1916 trace!(?r, liveness_constraints=?self.liveness_constraints.region_value_str(r));
1917 self.liveness_constraints.contains(r, elem)
1920 // If we fail to find that, we may find some `r` such that
1921 // `fr1: r` and `r` is a placeholder from some universe
1922 // `fr1` cannot name. This would force `fr1` to be
1924 self.find_constraint_paths_between_regions(fr1, |r| {
1925 self.cannot_name_placeholder(fr1, r)
1929 // If we fail to find THAT, it may be that `fr1` is a
1930 // placeholder that cannot "fit" into its SCC. In that
1931 // case, there should be some `r` where `fr1: r` and `fr1` is a
1932 // placeholder that `r` cannot name. We can blame that
1935 // Remember that if `R1: R2`, then the universe of R1
1936 // must be able to name the universe of R2, because R2 will
1937 // be at least `'empty(Universe(R2))`, and `R1` must be at
1938 // larger than that.
1939 self.find_constraint_paths_between_regions(fr1, |r| {
1940 self.cannot_name_placeholder(r, fr1)
1943 .map(|(_path, r)| r)
1947 /// Get the region outlived by `longer_fr` and live at `element`.
1948 pub(crate) fn region_from_element(
1950 longer_fr: RegionVid,
1951 element: &RegionElement,
1954 RegionElement::Location(l) => self.find_sub_region_live_at(longer_fr, l),
1955 RegionElement::RootUniversalRegion(r) => r,
1956 RegionElement::PlaceholderRegion(error_placeholder) => self
1959 .find_map(|(r, definition)| match definition.origin {
1960 NllRegionVariableOrigin::Placeholder(p) if p == error_placeholder => Some(r),
1967 /// Get the region definition of `r`.
1968 pub(crate) fn region_definition(&self, r: RegionVid) -> &RegionDefinition<'tcx> {
1969 &self.definitions[r]
1972 /// Check if the SCC of `r` contains `upper`.
1973 pub(crate) fn upper_bound_in_region_scc(&self, r: RegionVid, upper: RegionVid) -> bool {
1974 let r_scc = self.constraint_sccs.scc(r);
1975 self.scc_values.contains(r_scc, upper)
1978 pub(crate) fn universal_regions(&self) -> &UniversalRegions<'tcx> {
1979 self.universal_regions.as_ref()
1982 /// Tries to find the best constraint to blame for the fact that
1983 /// `R: from_region`, where `R` is some region that meets
1984 /// `target_test`. This works by following the constraint graph,
1985 /// creating a constraint path that forces `R` to outlive
1986 /// `from_region`, and then finding the best choices within that
1988 #[instrument(level = "debug", skip(self, target_test))]
1989 pub(crate) fn best_blame_constraint(
1991 from_region: RegionVid,
1992 from_region_origin: NllRegionVariableOrigin,
1993 target_test: impl Fn(RegionVid) -> bool,
1994 ) -> (BlameConstraint<'tcx>, Vec<ExtraConstraintInfo>) {
1996 let (path, target_region) =
1997 self.find_constraint_paths_between_regions(from_region, target_test).unwrap();
2002 "{:?} ({:?}: {:?})",
2004 self.constraint_sccs.scc(c.sup),
2005 self.constraint_sccs.scc(c.sub),
2007 .collect::<Vec<_>>()
2010 let mut extra_info = vec![];
2011 for constraint in path.iter() {
2012 let outlived = constraint.sub;
2013 let Some(origin) = self.var_infos.get(outlived) else { continue; };
2014 let RegionVariableOrigin::Nll(NllRegionVariableOrigin::Placeholder(p)) = origin.origin else { continue; };
2015 debug!(?constraint, ?p);
2016 let ConstraintCategory::Predicate(span) = constraint.category else { continue; };
2017 extra_info.push(ExtraConstraintInfo::PlaceholderFromPredicate(span));
2018 // We only want to point to one
2022 // We try to avoid reporting a `ConstraintCategory::Predicate` as our best constraint.
2023 // Instead, we use it to produce an improved `ObligationCauseCode`.
2024 // FIXME - determine what we should do if we encounter multiple `ConstraintCategory::Predicate`
2025 // constraints. Currently, we just pick the first one.
2026 let cause_code = path
2028 .find_map(|constraint| {
2029 if let ConstraintCategory::Predicate(predicate_span) = constraint.category {
2030 // We currently do not store the `DefId` in the `ConstraintCategory`
2031 // for performances reasons. The error reporting code used by NLL only
2032 // uses the span, so this doesn't cause any problems at the moment.
2033 Some(ObligationCauseCode::BindingObligation(
2034 CRATE_DEF_ID.to_def_id(),
2041 .unwrap_or_else(|| ObligationCauseCode::MiscObligation);
2043 // Classify each of the constraints along the path.
2044 let mut categorized_path: Vec<BlameConstraint<'tcx>> = path
2046 .map(|constraint| BlameConstraint {
2047 category: constraint.category,
2048 from_closure: constraint.from_closure,
2049 cause: ObligationCause::new(constraint.span, CRATE_HIR_ID, cause_code.clone()),
2050 variance_info: constraint.variance_info,
2051 outlives_constraint: *constraint,
2054 debug!("categorized_path={:#?}", categorized_path);
2056 // To find the best span to cite, we first try to look for the
2057 // final constraint that is interesting and where the `sup` is
2058 // not unified with the ultimate target region. The reason
2059 // for this is that we have a chain of constraints that lead
2060 // from the source to the target region, something like:
2062 // '0: '1 ('0 is the source)
2067 // '5: '6 ('6 is the target)
2069 // Some of those regions are unified with `'6` (in the same
2070 // SCC). We want to screen those out. After that point, the
2071 // "closest" constraint we have to the end is going to be the
2072 // most likely to be the point where the value escapes -- but
2073 // we still want to screen for an "interesting" point to
2074 // highlight (e.g., a call site or something).
2075 let target_scc = self.constraint_sccs.scc(target_region);
2076 let mut range = 0..path.len();
2078 // As noted above, when reporting an error, there is typically a chain of constraints
2079 // leading from some "source" region which must outlive some "target" region.
2080 // In most cases, we prefer to "blame" the constraints closer to the target --
2081 // but there is one exception. When constraints arise from higher-ranked subtyping,
2082 // we generally prefer to blame the source value,
2083 // as the "target" in this case tends to be some type annotation that the user gave.
2084 // Therefore, if we find that the region origin is some instantiation
2085 // of a higher-ranked region, we start our search from the "source" point
2086 // rather than the "target", and we also tweak a few other things.
2088 // An example might be this bit of Rust code:
2091 // let x: fn(&'static ()) = |_| {};
2092 // let y: for<'a> fn(&'a ()) = x;
2095 // In MIR, this will be converted into a combination of assignments and type ascriptions.
2096 // In particular, the 'static is imposed through a type ascription:
2100 // AscribeUserType(x, fn(&'static ())
2104 // We wind up ultimately with constraints like
2107 // !a: 'temp1 // from the `y = x` statement
2109 // 'temp2: 'static // from the AscribeUserType
2112 // and here we prefer to blame the source (the y = x statement).
2113 let blame_source = match from_region_origin {
2114 NllRegionVariableOrigin::FreeRegion
2115 | NllRegionVariableOrigin::Existential { from_forall: false } => true,
2116 NllRegionVariableOrigin::Placeholder(_)
2117 | NllRegionVariableOrigin::Existential { from_forall: true } => false,
2120 let find_region = |i: &usize| {
2121 let constraint = &path[*i];
2123 let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
2126 match categorized_path[*i].category {
2127 ConstraintCategory::OpaqueType
2128 | ConstraintCategory::Boring
2129 | ConstraintCategory::BoringNoLocation
2130 | ConstraintCategory::Internal
2131 | ConstraintCategory::Predicate(_) => false,
2132 ConstraintCategory::TypeAnnotation
2133 | ConstraintCategory::Return(_)
2134 | ConstraintCategory::Yield => true,
2135 _ => constraint_sup_scc != target_scc,
2139 categorized_path[*i].category,
2140 ConstraintCategory::OpaqueType
2141 | ConstraintCategory::Boring
2142 | ConstraintCategory::BoringNoLocation
2143 | ConstraintCategory::Internal
2144 | ConstraintCategory::Predicate(_)
2150 if blame_source { range.rev().find(find_region) } else { range.find(find_region) };
2152 debug!(?best_choice, ?blame_source, ?extra_info);
2154 if let Some(i) = best_choice {
2155 if let Some(next) = categorized_path.get(i + 1) {
2156 if matches!(categorized_path[i].category, ConstraintCategory::Return(_))
2157 && next.category == ConstraintCategory::OpaqueType
2159 // The return expression is being influenced by the return type being
2160 // impl Trait, point at the return type and not the return expr.
2161 return (next.clone(), extra_info);
2165 if categorized_path[i].category == ConstraintCategory::Return(ReturnConstraint::Normal)
2167 let field = categorized_path.iter().find_map(|p| {
2168 if let ConstraintCategory::ClosureUpvar(f) = p.category {
2175 if let Some(field) = field {
2176 categorized_path[i].category =
2177 ConstraintCategory::Return(ReturnConstraint::ClosureUpvar(field));
2181 return (categorized_path[i].clone(), extra_info);
2184 // If that search fails, that is.. unusual. Maybe everything
2185 // is in the same SCC or something. In that case, find what
2186 // appears to be the most interesting point to report to the
2187 // user via an even more ad-hoc guess.
2188 categorized_path.sort_by(|p0, p1| p0.category.cmp(&p1.category));
2189 debug!("sorted_path={:#?}", categorized_path);
2191 (categorized_path.remove(0), extra_info)
2194 pub(crate) fn universe_info(&self, universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
2195 self.universe_causes[&universe].clone()
2198 /// Tries to find the terminator of the loop in which the region 'r' resides.
2199 /// Returns the location of the terminator if found.
2200 pub(crate) fn find_loop_terminator_location(
2204 ) -> Option<Location> {
2205 let scc = self.constraint_sccs.scc(r.to_region_vid());
2206 let locations = self.scc_values.locations_outlived_by(scc);
2207 for location in locations {
2208 let bb = &body[location.block];
2209 if let Some(terminator) = &bb.terminator {
2210 // terminator of a loop should be TerminatorKind::FalseUnwind
2211 if let TerminatorKind::FalseUnwind { .. } = terminator.kind {
2212 return Some(location);
2220 impl<'tcx> RegionDefinition<'tcx> {
2221 fn new(universe: ty::UniverseIndex, rv_origin: RegionVariableOrigin) -> Self {
2222 // Create a new region definition. Note that, for free
2223 // regions, the `external_name` field gets updated later in
2224 // `init_universal_regions`.
2226 let origin = match rv_origin {
2227 RegionVariableOrigin::Nll(origin) => origin,
2228 _ => NllRegionVariableOrigin::Existential { from_forall: false },
2231 Self { origin, universe, external_name: None }
2235 #[derive(Clone, Debug)]
2236 pub struct BlameConstraint<'tcx> {
2237 pub category: ConstraintCategory<'tcx>,
2238 pub from_closure: bool,
2239 pub cause: ObligationCause<'tcx>,
2240 pub variance_info: ty::VarianceDiagInfo<'tcx>,
2241 pub outlives_constraint: OutlivesConstraint<'tcx>,