1 use std::collections::VecDeque;
4 use rustc_data_structures::binary_search_util;
5 use rustc_data_structures::frozen::Frozen;
6 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
7 use rustc_data_structures::graph::scc::Sccs;
8 use rustc_errors::Diagnostic;
9 use rustc_hir::def_id::{DefId, CRATE_DEF_ID};
10 use rustc_hir::CRATE_HIR_ID;
11 use rustc_index::vec::IndexVec;
12 use rustc_infer::infer::canonical::QueryOutlivesConstraint;
13 use rustc_infer::infer::region_constraints::{GenericKind, VarInfos, VerifyBound};
14 use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin};
15 use rustc_middle::mir::{
16 Body, ClosureOutlivesRequirement, ClosureOutlivesSubject, ClosureRegionRequirements,
17 ConstraintCategory, Local, Location, ReturnConstraint,
19 use rustc_middle::traits::ObligationCause;
20 use rustc_middle::traits::ObligationCauseCode;
21 use rustc_middle::ty::{self, subst::SubstsRef, RegionVid, Ty, TyCtxt, TypeFoldable};
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,
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 closure bounds to a `Span` that should be used for error reporting.
89 closure_bounds_mapping:
90 FxHashMap<Location, FxHashMap<(RegionVid, RegionVid), (ConstraintCategory, Span)>>,
92 /// Map universe indexes to information on why we created it.
93 universe_causes: FxHashMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
95 /// Contains the minimum universe of any variable within the same
96 /// SCC. We will ensure that no SCC contains values that are not
97 /// visible from this index.
98 scc_universes: IndexVec<ConstraintSccIndex, ty::UniverseIndex>,
100 /// Contains a "representative" from each SCC. This will be the
101 /// minimal RegionVid belonging to that universe. It is used as a
102 /// kind of hacky way to manage checking outlives relationships,
103 /// since we can 'canonicalize' each region to the representative
104 /// of its SCC and be sure that -- if they have the same repr --
105 /// they *must* be equal (though not having the same repr does not
106 /// mean they are unequal).
107 scc_representatives: IndexVec<ConstraintSccIndex, ty::RegionVid>,
109 /// The final inferred values of the region variables; we compute
110 /// one value per SCC. To get the value for any given *region*,
111 /// you first find which scc it is a part of.
112 scc_values: RegionValues<ConstraintSccIndex>,
114 /// Type constraints that we check after solving.
115 type_tests: Vec<TypeTest<'tcx>>,
117 /// Information about the universally quantified regions in scope
118 /// on this function.
119 universal_regions: Rc<UniversalRegions<'tcx>>,
121 /// Information about how the universally quantified regions in
122 /// scope on this function relate to one another.
123 universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
126 /// Each time that `apply_member_constraint` is successful, it appends
127 /// one of these structs to the `member_constraints_applied` field.
128 /// This is used in error reporting to trace out what happened.
130 /// The way that `apply_member_constraint` works is that it effectively
131 /// adds a new lower bound to the SCC it is analyzing: so you wind up
132 /// with `'R: 'O` where `'R` is the pick-region and `'O` is the
133 /// minimal viable option.
134 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd)]
135 pub(crate) struct AppliedMemberConstraint {
136 /// The SCC that was affected. (The "member region".)
138 /// The vector if `AppliedMemberConstraint` elements is kept sorted
140 pub(crate) member_region_scc: ConstraintSccIndex,
142 /// The "best option" that `apply_member_constraint` found -- this was
143 /// added as an "ad-hoc" lower-bound to `member_region_scc`.
144 pub(crate) min_choice: ty::RegionVid,
146 /// The "member constraint index" -- we can find out details about
147 /// the constraint from
148 /// `set.member_constraints[member_constraint_index]`.
149 pub(crate) member_constraint_index: NllMemberConstraintIndex,
152 pub(crate) struct RegionDefinition<'tcx> {
153 /// What kind of variable is this -- a free region? existential
154 /// variable? etc. (See the `NllRegionVariableOrigin` for more
156 pub(crate) origin: NllRegionVariableOrigin,
158 /// Which universe is this region variable defined in? This is
159 /// most often `ty::UniverseIndex::ROOT`, but when we encounter
160 /// forall-quantifiers like `for<'a> { 'a = 'b }`, we would create
161 /// the variable for `'a` in a fresh universe that extends ROOT.
162 pub(crate) universe: ty::UniverseIndex,
164 /// If this is 'static or an early-bound region, then this is
165 /// `Some(X)` where `X` is the name of the region.
166 pub(crate) external_name: Option<ty::Region<'tcx>>,
169 /// N.B., the variants in `Cause` are intentionally ordered. Lower
170 /// values are preferred when it comes to error messages. Do not
171 /// reorder willy nilly.
172 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
173 pub(crate) enum Cause {
174 /// point inserted because Local was live at the given Location
175 LiveVar(Local, Location),
177 /// point inserted because Local was dropped at the given Location
178 DropVar(Local, Location),
181 /// A "type test" corresponds to an outlives constraint between a type
182 /// and a lifetime, like `T: 'x` or `<T as Foo>::Bar: 'x`. They are
183 /// translated from the `Verify` region constraints in the ordinary
184 /// inference context.
186 /// These sorts of constraints are handled differently than ordinary
187 /// constraints, at least at present. During type checking, the
188 /// `InferCtxt::process_registered_region_obligations` method will
189 /// attempt to convert a type test like `T: 'x` into an ordinary
190 /// outlives constraint when possible (for example, `&'a T: 'b` will
191 /// be converted into `'a: 'b` and registered as a `Constraint`).
193 /// In some cases, however, there are outlives relationships that are
194 /// not converted into a region constraint, but rather into one of
195 /// these "type tests". The distinction is that a type test does not
196 /// influence the inference result, but instead just examines the
197 /// values that we ultimately inferred for each region variable and
198 /// checks that they meet certain extra criteria. If not, an error
201 /// One reason for this is that these type tests typically boil down
202 /// to a check like `'a: 'x` where `'a` is a universally quantified
203 /// region -- and therefore not one whose value is really meant to be
204 /// *inferred*, precisely (this is not always the case: one can have a
205 /// type test like `<Foo as Trait<'?0>>::Bar: 'x`, where `'?0` is an
206 /// inference variable). Another reason is that these type tests can
207 /// involve *disjunction* -- that is, they can be satisfied in more
210 /// For more information about this translation, see
211 /// `InferCtxt::process_registered_region_obligations` and
212 /// `InferCtxt::type_must_outlive` in `rustc_infer::infer::InferCtxt`.
213 #[derive(Clone, Debug)]
214 pub struct TypeTest<'tcx> {
215 /// The type `T` that must outlive the region.
216 pub generic_kind: GenericKind<'tcx>,
218 /// The region `'x` that the type must outlive.
219 pub lower_bound: RegionVid,
221 /// Where did this constraint arise and why?
222 pub locations: Locations,
224 /// A test which, if met by the region `'x`, proves that this type
225 /// constraint is satisfied.
226 pub verify_bound: VerifyBound<'tcx>,
229 /// When we have an unmet lifetime constraint, we try to propagate it outward (e.g. to a closure
230 /// environment). If we can't, it is an error.
231 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
232 enum RegionRelationCheckResult {
238 #[derive(Clone, PartialEq, Eq, Debug)]
241 FromOutlivesConstraint(OutlivesConstraint<'tcx>),
245 impl<'tcx> RegionInferenceContext<'tcx> {
246 /// Creates a new region inference context with a total of
247 /// `num_region_variables` valid inference variables; the first N
248 /// of those will be constant regions representing the free
249 /// regions defined in `universal_regions`.
251 /// The `outlives_constraints` and `type_tests` are an initial set
252 /// of constraints produced by the MIR type check.
255 universal_regions: Rc<UniversalRegions<'tcx>>,
256 placeholder_indices: Rc<PlaceholderIndices>,
257 universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
258 outlives_constraints: OutlivesConstraintSet<'tcx>,
259 member_constraints_in: MemberConstraintSet<'tcx, RegionVid>,
260 closure_bounds_mapping: FxHashMap<
262 FxHashMap<(RegionVid, RegionVid), (ConstraintCategory, Span)>,
264 universe_causes: FxHashMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
265 type_tests: Vec<TypeTest<'tcx>>,
266 liveness_constraints: LivenessValues<RegionVid>,
267 elements: &Rc<RegionValueElements>,
269 // Create a RegionDefinition for each inference variable.
270 let definitions: IndexVec<_, _> = var_infos
272 .map(|info| RegionDefinition::new(info.universe, info.origin))
275 let constraints = Frozen::freeze(outlives_constraints);
276 let constraint_graph = Frozen::freeze(constraints.graph(definitions.len()));
277 let fr_static = universal_regions.fr_static;
278 let constraint_sccs = Rc::new(constraints.compute_sccs(&constraint_graph, fr_static));
281 RegionValues::new(elements, universal_regions.len(), &placeholder_indices);
283 for region in liveness_constraints.rows() {
284 let scc = constraint_sccs.scc(region);
285 scc_values.merge_liveness(scc, region, &liveness_constraints);
288 let scc_universes = Self::compute_scc_universes(&constraint_sccs, &definitions);
290 let scc_representatives = Self::compute_scc_representatives(&constraint_sccs, &definitions);
292 let member_constraints =
293 Rc::new(member_constraints_in.into_mapped(|r| constraint_sccs.scc(r)));
295 let mut result = Self {
298 liveness_constraints,
304 member_constraints_applied: Vec::new(),
305 closure_bounds_mapping,
312 universal_region_relations,
315 result.init_free_and_bound_regions();
320 /// Each SCC is the combination of many region variables which
321 /// have been equated. Therefore, we can associate a universe with
322 /// each SCC which is minimum of all the universes of its
323 /// constituent regions -- this is because whatever value the SCC
324 /// takes on must be a value that each of the regions within the
325 /// SCC could have as well. This implies that the SCC must have
326 /// the minimum, or narrowest, universe.
327 fn compute_scc_universes(
328 constraint_sccs: &Sccs<RegionVid, ConstraintSccIndex>,
329 definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
330 ) -> IndexVec<ConstraintSccIndex, ty::UniverseIndex> {
331 let num_sccs = constraint_sccs.num_sccs();
332 let mut scc_universes = IndexVec::from_elem_n(ty::UniverseIndex::MAX, num_sccs);
334 debug!("compute_scc_universes()");
336 // For each region R in universe U, ensure that the universe for the SCC
337 // that contains R is "no bigger" than U. This effectively sets the universe
338 // for each SCC to be the minimum of the regions within.
339 for (region_vid, region_definition) in definitions.iter_enumerated() {
340 let scc = constraint_sccs.scc(region_vid);
341 let scc_universe = &mut scc_universes[scc];
342 let scc_min = std::cmp::min(region_definition.universe, *scc_universe);
343 if scc_min != *scc_universe {
344 *scc_universe = scc_min;
346 "compute_scc_universes: lowered universe of {scc:?} to {scc_min:?} \
347 because it contains {region_vid:?} in {region_universe:?}",
350 region_vid = region_vid,
351 region_universe = region_definition.universe,
356 // Walk each SCC `A` and `B` such that `A: B`
357 // and ensure that universe(A) can see universe(B).
359 // This serves to enforce the 'empty/placeholder' hierarchy
360 // (described in more detail on `RegionKind`):
365 // empty(U0) placeholder(U1)
370 // In particular, imagine we have variables R0 in U0 and R1
371 // created in U1, and constraints like this;
374 // R1: !1 // R1 outlives the placeholder in U1
375 // R1: R0 // R1 outlives R0
378 // Here, we wish for R1 to be `'static`, because it
379 // cannot outlive `placeholder(U1)` and `empty(U0)` any other way.
381 // Thanks to this loop, what happens is that the `R1: R0`
382 // constraint lowers the universe of `R1` to `U0`, which in turn
383 // means that the `R1: !1` constraint will (later) cause
384 // `R1` to become `'static`.
385 for scc_a in constraint_sccs.all_sccs() {
386 for &scc_b in constraint_sccs.successors(scc_a) {
387 let scc_universe_a = scc_universes[scc_a];
388 let scc_universe_b = scc_universes[scc_b];
389 let scc_universe_min = std::cmp::min(scc_universe_a, scc_universe_b);
390 if scc_universe_a != scc_universe_min {
391 scc_universes[scc_a] = scc_universe_min;
394 "compute_scc_universes: lowered universe of {scc_a:?} to {scc_universe_min:?} \
395 because {scc_a:?}: {scc_b:?} and {scc_b:?} is in universe {scc_universe_b:?}",
398 scc_universe_min = scc_universe_min,
399 scc_universe_b = scc_universe_b
405 debug!("compute_scc_universes: scc_universe = {:#?}", scc_universes);
410 /// For each SCC, we compute a unique `RegionVid` (in fact, the
411 /// minimal one that belongs to the SCC). See
412 /// `scc_representatives` field of `RegionInferenceContext` for
414 fn compute_scc_representatives(
415 constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
416 definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
417 ) -> IndexVec<ConstraintSccIndex, ty::RegionVid> {
418 let num_sccs = constraints_scc.num_sccs();
419 let next_region_vid = definitions.next_index();
420 let mut scc_representatives = IndexVec::from_elem_n(next_region_vid, num_sccs);
422 for region_vid in definitions.indices() {
423 let scc = constraints_scc.scc(region_vid);
424 let prev_min = scc_representatives[scc];
425 scc_representatives[scc] = region_vid.min(prev_min);
431 /// Initializes the region variables for each universally
432 /// quantified region (lifetime parameter). The first N variables
433 /// always correspond to the regions appearing in the function
434 /// signature (both named and anonymous) and where-clauses. This
435 /// function iterates over those regions and initializes them with
440 /// fn foo<'a, 'b>( /* ... */ ) where 'a: 'b { /* ... */ }
442 /// would initialize two variables like so:
443 /// ```ignore (illustrative)
444 /// R0 = { CFG, R0 } // 'a
445 /// R1 = { CFG, R0, R1 } // 'b
447 /// Here, R0 represents `'a`, and it contains (a) the entire CFG
448 /// and (b) any universally quantified regions that it outlives,
449 /// which in this case is just itself. R1 (`'b`) in contrast also
450 /// outlives `'a` and hence contains R0 and R1.
451 fn init_free_and_bound_regions(&mut self) {
452 // Update the names (if any)
453 for (external_name, variable) in self.universal_regions.named_universal_regions() {
455 "init_universal_regions: region {:?} has external name {:?}",
456 variable, external_name
458 self.definitions[variable].external_name = Some(external_name);
461 for variable in self.definitions.indices() {
462 let scc = self.constraint_sccs.scc(variable);
464 match self.definitions[variable].origin {
465 NllRegionVariableOrigin::FreeRegion => {
466 // For each free, universally quantified region X:
468 // Add all nodes in the CFG to liveness constraints
469 self.liveness_constraints.add_all_points(variable);
470 self.scc_values.add_all_points(scc);
472 // Add `end(X)` into the set for X.
473 self.scc_values.add_element(scc, variable);
476 NllRegionVariableOrigin::Placeholder(placeholder) => {
477 // Each placeholder region is only visible from
478 // its universe `ui` and its extensions. So we
479 // can't just add it into `scc` unless the
480 // universe of the scc can name this region.
481 let scc_universe = self.scc_universes[scc];
482 if scc_universe.can_name(placeholder.universe) {
483 self.scc_values.add_element(scc, placeholder);
486 "init_free_and_bound_regions: placeholder {:?} is \
487 not compatible with universe {:?} of its SCC {:?}",
488 placeholder, scc_universe, scc,
490 self.add_incompatible_universe(scc);
494 NllRegionVariableOrigin::RootEmptyRegion
495 | NllRegionVariableOrigin::Existential { .. } => {
496 // For existential, regions, nothing to do.
502 /// Returns an iterator over all the region indices.
503 pub fn regions(&self) -> impl Iterator<Item = RegionVid> + '_ {
504 self.definitions.indices()
507 /// Given a universal region in scope on the MIR, returns the
508 /// corresponding index.
510 /// (Panics if `r` is not a registered universal region.)
511 pub fn to_region_vid(&self, r: ty::Region<'tcx>) -> RegionVid {
512 self.universal_regions.to_region_vid(r)
515 /// Adds annotations for `#[rustc_regions]`; see `UniversalRegions::annotate`.
516 crate fn annotate(&self, tcx: TyCtxt<'tcx>, err: &mut Diagnostic) {
517 self.universal_regions.annotate(tcx, err)
520 /// Returns `true` if the region `r` contains the point `p`.
522 /// Panics if called before `solve()` executes,
523 crate fn region_contains(&self, r: impl ToRegionVid, p: impl ToElementIndex) -> bool {
524 let scc = self.constraint_sccs.scc(r.to_region_vid());
525 self.scc_values.contains(scc, p)
528 /// Returns access to the value of `r` for debugging purposes.
529 crate fn region_value_str(&self, r: RegionVid) -> String {
530 let scc = self.constraint_sccs.scc(r.to_region_vid());
531 self.scc_values.region_value_str(scc)
534 /// Returns access to the value of `r` for debugging purposes.
535 crate fn region_universe(&self, r: RegionVid) -> ty::UniverseIndex {
536 let scc = self.constraint_sccs.scc(r.to_region_vid());
537 self.scc_universes[scc]
540 /// Once region solving has completed, this function will return
541 /// the member constraints that were applied to the value of a given
542 /// region `r`. See `AppliedMemberConstraint`.
543 pub(crate) fn applied_member_constraints(
546 ) -> &[AppliedMemberConstraint] {
547 let scc = self.constraint_sccs.scc(r.to_region_vid());
548 binary_search_util::binary_search_slice(
549 &self.member_constraints_applied,
550 |applied| applied.member_region_scc,
555 /// Performs region inference and report errors if we see any
556 /// unsatisfiable constraints. If this is a closure, returns the
557 /// region requirements to propagate to our creator, if any.
558 #[instrument(skip(self, infcx, body, polonius_output), level = "debug")]
561 infcx: &InferCtxt<'_, 'tcx>,
563 polonius_output: Option<Rc<PoloniusOutput>>,
564 ) -> (Option<ClosureRegionRequirements<'tcx>>, RegionErrors<'tcx>) {
565 let mir_def_id = body.source.def_id();
566 self.propagate_constraints(body);
568 let mut errors_buffer = RegionErrors::new();
570 // If this is a closure, we can propagate unsatisfied
571 // `outlives_requirements` to our creator, so create a vector
572 // to store those. Otherwise, we'll pass in `None` to the
573 // functions below, which will trigger them to report errors
575 let mut outlives_requirements = infcx.tcx.is_typeck_child(mir_def_id).then(Vec::new);
577 self.check_type_tests(infcx, body, outlives_requirements.as_mut(), &mut errors_buffer);
579 // In Polonius mode, the errors about missing universal region relations are in the output
580 // and need to be emitted or propagated. Otherwise, we need to check whether the
581 // constraints were too strong, and if so, emit or propagate those errors.
582 if infcx.tcx.sess.opts.debugging_opts.polonius {
583 self.check_polonius_subset_errors(
585 outlives_requirements.as_mut(),
587 polonius_output.expect("Polonius output is unavailable despite `-Z polonius`"),
590 self.check_universal_regions(body, outlives_requirements.as_mut(), &mut errors_buffer);
593 if errors_buffer.is_empty() {
594 self.check_member_constraints(infcx, &mut errors_buffer);
597 let outlives_requirements = outlives_requirements.unwrap_or_default();
599 if outlives_requirements.is_empty() {
600 (None, errors_buffer)
602 let num_external_vids = self.universal_regions.num_global_and_external_regions();
604 Some(ClosureRegionRequirements { num_external_vids, outlives_requirements }),
610 /// Propagate the region constraints: this will grow the values
611 /// for each region variable until all the constraints are
612 /// satisfied. Note that some values may grow **too** large to be
613 /// feasible, but we check this later.
614 #[instrument(skip(self, _body), level = "debug")]
615 fn propagate_constraints(&mut self, _body: &Body<'tcx>) {
616 debug!("constraints={:#?}", {
617 let mut constraints: Vec<_> = self.constraints.outlives().iter().collect();
618 constraints.sort_by_key(|c| (c.sup, c.sub));
621 .map(|c| (c, self.constraint_sccs.scc(c.sup), self.constraint_sccs.scc(c.sub)))
625 // To propagate constraints, we walk the DAG induced by the
626 // SCC. For each SCC, we visit its successors and compute
627 // their values, then we union all those values to get our
629 let constraint_sccs = self.constraint_sccs.clone();
630 for scc in constraint_sccs.all_sccs() {
631 self.compute_value_for_scc(scc);
634 // Sort the applied member constraints so we can binary search
635 // through them later.
636 self.member_constraints_applied.sort_by_key(|applied| applied.member_region_scc);
639 /// Computes the value of the SCC `scc_a`, which has not yet been
640 /// computed, by unioning the values of its successors.
641 /// Assumes that all successors have been computed already
642 /// (which is assured by iterating over SCCs in dependency order).
643 #[instrument(skip(self), level = "debug")]
644 fn compute_value_for_scc(&mut self, scc_a: ConstraintSccIndex) {
645 let constraint_sccs = self.constraint_sccs.clone();
647 // Walk each SCC `B` such that `A: B`...
648 for &scc_b in constraint_sccs.successors(scc_a) {
651 // ...and add elements from `B` into `A`. One complication
652 // arises because of universes: If `B` contains something
653 // that `A` cannot name, then `A` can only contain `B` if
654 // it outlives static.
655 if self.universe_compatible(scc_b, scc_a) {
656 // `A` can name everything that is in `B`, so just
658 self.scc_values.add_region(scc_a, scc_b);
660 self.add_incompatible_universe(scc_a);
664 // Now take member constraints into account.
665 let member_constraints = self.member_constraints.clone();
666 for m_c_i in member_constraints.indices(scc_a) {
667 self.apply_member_constraint(scc_a, m_c_i, member_constraints.choice_regions(m_c_i));
670 debug!(value = ?self.scc_values.region_value_str(scc_a));
673 /// Invoked for each `R0 member of [R1..Rn]` constraint.
675 /// `scc` is the SCC containing R0, and `choice_regions` are the
676 /// `R1..Rn` regions -- they are always known to be universal
677 /// regions (and if that's not true, we just don't attempt to
678 /// enforce the constraint).
680 /// The current value of `scc` at the time the method is invoked
681 /// is considered a *lower bound*. If possible, we will modify
682 /// the constraint to set it equal to one of the option regions.
683 /// If we make any changes, returns true, else false.
684 #[instrument(skip(self, member_constraint_index), level = "debug")]
685 fn apply_member_constraint(
687 scc: ConstraintSccIndex,
688 member_constraint_index: NllMemberConstraintIndex,
689 choice_regions: &[ty::RegionVid],
691 // Create a mutable vector of the options. We'll try to winnow
693 let mut choice_regions: Vec<ty::RegionVid> = choice_regions.to_vec();
695 // Convert to the SCC representative: sometimes we have inference
696 // variables in the member constraint that wind up equated with
697 // universal regions. The scc representative is the minimal numbered
698 // one from the corresponding scc so it will be the universal region
700 for c_r in &mut choice_regions {
701 let scc = self.constraint_sccs.scc(*c_r);
702 *c_r = self.scc_representatives[scc];
705 // The 'member region' in a member constraint is part of the
706 // hidden type, which must be in the root universe. Therefore,
707 // it cannot have any placeholders in its value.
708 assert!(self.scc_universes[scc] == ty::UniverseIndex::ROOT);
710 self.scc_values.placeholders_contained_in(scc).next().is_none(),
711 "scc {:?} in a member constraint has placeholder value: {:?}",
713 self.scc_values.region_value_str(scc),
716 // The existing value for `scc` is a lower-bound. This will
717 // consist of some set `{P} + {LB}` of points `{P}` and
718 // lower-bound free regions `{LB}`. As each choice region `O`
719 // is a free region, it will outlive the points. But we can
720 // only consider the option `O` if `O: LB`.
721 choice_regions.retain(|&o_r| {
723 .universal_regions_outlived_by(scc)
724 .all(|lb| self.universal_region_relations.outlives(o_r, lb))
726 debug!(?choice_regions, "after lb");
728 // Now find all the *upper bounds* -- that is, each UB is a
729 // free region that must outlive the member region `R0` (`UB:
730 // R0`). Therefore, we need only keep an option `O` if `UB: O`
732 let rev_scc_graph = self.reverse_scc_graph();
733 let universal_region_relations = &self.universal_region_relations;
734 for ub in rev_scc_graph.upper_bounds(scc) {
736 choice_regions.retain(|&o_r| universal_region_relations.outlives(ub, o_r));
738 debug!(?choice_regions, "after ub");
740 // If we ruled everything out, we're done.
741 if choice_regions.is_empty() {
745 // Otherwise, we need to find the minimum remaining choice, if
746 // any, and take that.
747 debug!("choice_regions remaining are {:#?}", choice_regions);
748 let min = |r1: ty::RegionVid, r2: ty::RegionVid| -> Option<ty::RegionVid> {
749 let r1_outlives_r2 = self.universal_region_relations.outlives(r1, r2);
750 let r2_outlives_r1 = self.universal_region_relations.outlives(r2, r1);
751 match (r1_outlives_r2, r2_outlives_r1) {
752 (true, true) => Some(r1.min(r2)),
753 (true, false) => Some(r2),
754 (false, true) => Some(r1),
755 (false, false) => None,
758 let mut min_choice = choice_regions[0];
759 for &other_option in &choice_regions[1..] {
760 debug!(?min_choice, ?other_option,);
761 match min(min_choice, other_option) {
762 Some(m) => min_choice = m,
764 debug!(?min_choice, ?other_option, "incomparable; no min choice",);
770 let min_choice_scc = self.constraint_sccs.scc(min_choice);
771 debug!(?min_choice, ?min_choice_scc);
772 if self.scc_values.add_region(scc, min_choice_scc) {
773 self.member_constraints_applied.push(AppliedMemberConstraint {
774 member_region_scc: scc,
776 member_constraint_index,
785 /// Returns `true` if all the elements in the value of `scc_b` are nameable
786 /// in `scc_a`. Used during constraint propagation, and only once
787 /// the value of `scc_b` has been computed.
788 fn universe_compatible(&self, scc_b: ConstraintSccIndex, scc_a: ConstraintSccIndex) -> bool {
789 let universe_a = self.scc_universes[scc_a];
791 // Quick check: if scc_b's declared universe is a subset of
792 // scc_a's declared universe (typically, both are ROOT), then
793 // it cannot contain any problematic universe elements.
794 if universe_a.can_name(self.scc_universes[scc_b]) {
798 // Otherwise, we have to iterate over the universe elements in
799 // B's value, and check whether all of them are nameable
801 self.scc_values.placeholders_contained_in(scc_b).all(|p| universe_a.can_name(p.universe))
804 /// Extend `scc` so that it can outlive some placeholder region
805 /// from a universe it can't name; at present, the only way for
806 /// this to be true is if `scc` outlives `'static`. This is
807 /// actually stricter than necessary: ideally, we'd support bounds
808 /// like `for<'a: 'b`>` that might then allow us to approximate
809 /// `'a` with `'b` and not `'static`. But it will have to do for
811 fn add_incompatible_universe(&mut self, scc: ConstraintSccIndex) {
812 debug!("add_incompatible_universe(scc={:?})", scc);
814 let fr_static = self.universal_regions.fr_static;
815 self.scc_values.add_all_points(scc);
816 self.scc_values.add_element(scc, fr_static);
819 /// Once regions have been propagated, this method is used to see
820 /// whether the "type tests" produced by typeck were satisfied;
821 /// type tests encode type-outlives relationships like `T:
822 /// 'a`. See `TypeTest` for more details.
825 infcx: &InferCtxt<'_, 'tcx>,
827 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
828 errors_buffer: &mut RegionErrors<'tcx>,
832 // Sometimes we register equivalent type-tests that would
833 // result in basically the exact same error being reported to
834 // the user. Avoid that.
835 let mut deduplicate_errors = FxHashSet::default();
837 for type_test in &self.type_tests {
838 debug!("check_type_test: {:?}", type_test);
840 let generic_ty = type_test.generic_kind.to_ty(tcx);
841 if self.eval_verify_bound(
845 type_test.lower_bound,
846 &type_test.verify_bound,
851 if let Some(propagated_outlives_requirements) = &mut propagated_outlives_requirements {
852 if self.try_promote_type_test(
856 propagated_outlives_requirements,
862 // Type-test failed. Report the error.
863 let erased_generic_kind = infcx.tcx.erase_regions(type_test.generic_kind);
865 // Skip duplicate-ish errors.
866 if deduplicate_errors.insert((
868 type_test.lower_bound,
872 "check_type_test: reporting error for erased_generic_kind={:?}, \
873 lower_bound_region={:?}, \
874 type_test.locations={:?}",
875 erased_generic_kind, type_test.lower_bound, type_test.locations,
878 errors_buffer.push(RegionErrorKind::TypeTestError { type_test: type_test.clone() });
883 /// Invoked when we have some type-test (e.g., `T: 'X`) that we cannot
884 /// prove to be satisfied. If this is a closure, we will attempt to
885 /// "promote" this type-test into our `ClosureRegionRequirements` and
886 /// hence pass it up the creator. To do this, we have to phrase the
887 /// type-test in terms of external free regions, as local free
888 /// regions are not nameable by the closure's creator.
890 /// Promotion works as follows: we first check that the type `T`
891 /// contains only regions that the creator knows about. If this is
892 /// true, then -- as a consequence -- we know that all regions in
893 /// the type `T` are free regions that outlive the closure body. If
894 /// false, then promotion fails.
896 /// Once we've promoted T, we have to "promote" `'X` to some region
897 /// that is "external" to the closure. Generally speaking, a region
898 /// may be the union of some points in the closure body as well as
899 /// various free lifetimes. We can ignore the points in the closure
900 /// body: if the type T can be expressed in terms of external regions,
901 /// we know it outlives the points in the closure body. That
902 /// just leaves the free regions.
904 /// The idea then is to lower the `T: 'X` constraint into multiple
905 /// bounds -- e.g., if `'X` is the union of two free lifetimes,
906 /// `'1` and `'2`, then we would create `T: '1` and `T: '2`.
907 fn try_promote_type_test(
909 infcx: &InferCtxt<'_, 'tcx>,
911 type_test: &TypeTest<'tcx>,
912 propagated_outlives_requirements: &mut Vec<ClosureOutlivesRequirement<'tcx>>,
916 let TypeTest { generic_kind, lower_bound, locations, 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 // For each region outlived by lower_bound find a non-local,
924 // universal region (it may be the same region) and add it to
925 // `ClosureOutlivesRequirement`.
926 let r_scc = self.constraint_sccs.scc(*lower_bound);
927 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
928 // Check whether we can already prove that the "subject" outlives `ur`.
929 // If so, we don't have to propagate this requirement to our caller.
931 // To continue the example from the function, if we are trying to promote
932 // a requirement that `T: 'X`, and we know that `'X = '1 + '2` (i.e., the union
933 // `'1` and `'2`), then in this loop `ur` will be `'1` (and `'2`). So here
934 // we check whether `T: '1` is something we *can* prove. If so, no need
935 // to propagate that requirement.
937 // This is needed because -- particularly in the case
938 // where `ur` is a local bound -- we are sometimes in a
939 // position to prove things that our caller cannot. See
940 // #53570 for an example.
941 if self.eval_verify_bound(tcx, body, generic_ty, ur, &type_test.verify_bound) {
945 debug!("try_promote_type_test: ur={:?}", ur);
947 let non_local_ub = self.universal_region_relations.non_local_upper_bounds(ur);
948 debug!("try_promote_type_test: non_local_ub={:?}", non_local_ub);
950 // This is slightly too conservative. To show T: '1, given `'2: '1`
951 // and `'3: '1` we only need to prove that T: '2 *or* T: '3, but to
952 // avoid potential non-determinism we approximate this by requiring
954 for upper_bound in non_local_ub {
955 debug_assert!(self.universal_regions.is_universal_region(upper_bound));
956 debug_assert!(!self.universal_regions.is_local_free_region(upper_bound));
958 let requirement = ClosureOutlivesRequirement {
960 outlived_free_region: upper_bound,
961 blame_span: locations.span(body),
962 category: ConstraintCategory::Boring,
964 debug!("try_promote_type_test: pushing {:#?}", requirement);
965 propagated_outlives_requirements.push(requirement);
971 /// When we promote a type test `T: 'r`, we have to convert the
972 /// type `T` into something we can store in a query result (so
973 /// something allocated for `'tcx`). This is problematic if `ty`
974 /// contains regions. During the course of NLL region checking, we
975 /// will have replaced all of those regions with fresh inference
976 /// variables. To create a test subject, we want to replace those
977 /// inference variables with some region from the closure
978 /// signature -- this is not always possible, so this is a
979 /// fallible process. Presuming we do find a suitable region, we
980 /// will use it's *external name*, which will be a `RegionKind`
981 /// variant that can be used in query responses such as
983 fn try_promote_type_test_subject(
985 infcx: &InferCtxt<'_, 'tcx>,
987 ) -> Option<ClosureOutlivesSubject<'tcx>> {
990 debug!("try_promote_type_test_subject(ty = {:?})", ty);
992 let ty = tcx.fold_regions(ty, &mut false, |r, _depth| {
993 let region_vid = self.to_region_vid(r);
995 // The challenge if this. We have some region variable `r`
996 // whose value is a set of CFG points and universal
997 // regions. We want to find if that set is *equivalent* to
998 // any of the named regions found in the closure.
1000 // To do so, we compute the
1001 // `non_local_universal_upper_bound`. This will be a
1002 // non-local, universal region that is greater than `r`.
1003 // However, it might not be *contained* within `r`, so
1004 // then we further check whether this bound is contained
1005 // in `r`. If so, we can say that `r` is equivalent to the
1008 // Let's work through a few examples. For these, imagine
1009 // that we have 3 non-local regions (I'll denote them as
1010 // `'static`, `'a`, and `'b`, though of course in the code
1011 // they would be represented with indices) where:
1016 // First, let's assume that `r` is some existential
1017 // variable with an inferred value `{'a, 'static}` (plus
1018 // some CFG nodes). In this case, the non-local upper
1019 // bound is `'static`, since that outlives `'a`. `'static`
1020 // is also a member of `r` and hence we consider `r`
1021 // equivalent to `'static` (and replace it with
1024 // Now let's consider the inferred value `{'a, 'b}`. This
1025 // means `r` is effectively `'a | 'b`. I'm not sure if
1026 // this can come about, actually, but assuming it did, we
1027 // would get a non-local upper bound of `'static`. Since
1028 // `'static` is not contained in `r`, we would fail to
1029 // find an equivalent.
1030 let upper_bound = self.non_local_universal_upper_bound(region_vid);
1031 if self.region_contains(region_vid, upper_bound) {
1032 self.definitions[upper_bound].external_name.unwrap_or(r)
1034 // In the case of a failure, use a `ReVar` result. This will
1035 // cause the `needs_infer` later on to return `None`.
1040 debug!("try_promote_type_test_subject: folded ty = {:?}", ty);
1042 // `needs_infer` will only be true if we failed to promote some region.
1043 if ty.needs_infer() {
1047 Some(ClosureOutlivesSubject::Ty(ty))
1050 /// Given some universal or existential region `r`, finds a
1051 /// non-local, universal region `r+` that outlives `r` at entry to (and
1052 /// exit from) the closure. In the worst case, this will be
1055 /// This is used for two purposes. First, if we are propagated
1056 /// some requirement `T: r`, we can use this method to enlarge `r`
1057 /// to something we can encode for our creator (which only knows
1058 /// about non-local, universal regions). It is also used when
1059 /// encoding `T` as part of `try_promote_type_test_subject` (see
1060 /// that fn for details).
1062 /// This is based on the result `'y` of `universal_upper_bound`,
1063 /// except that it converts further takes the non-local upper
1064 /// bound of `'y`, so that the final result is non-local.
1065 fn non_local_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1066 debug!("non_local_universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1068 let lub = self.universal_upper_bound(r);
1070 // Grow further to get smallest universal region known to
1072 let non_local_lub = self.universal_region_relations.non_local_upper_bound(lub);
1074 debug!("non_local_universal_upper_bound: non_local_lub={:?}", non_local_lub);
1079 /// Returns a universally quantified region that outlives the
1080 /// value of `r` (`r` may be existentially or universally
1083 /// Since `r` is (potentially) an existential region, it has some
1084 /// value which may include (a) any number of points in the CFG
1085 /// and (b) any number of `end('x)` elements of universally
1086 /// quantified regions. To convert this into a single universal
1087 /// region we do as follows:
1089 /// - Ignore the CFG points in `'r`. All universally quantified regions
1090 /// include the CFG anyhow.
1091 /// - For each `end('x)` element in `'r`, compute the mutual LUB, yielding
1093 #[instrument(skip(self), level = "debug")]
1094 pub(crate) fn universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1095 debug!(r = %self.region_value_str(r));
1097 // Find the smallest universal region that contains all other
1098 // universal regions within `region`.
1099 let mut lub = self.universal_regions.fr_fn_body;
1100 let r_scc = self.constraint_sccs.scc(r);
1101 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1102 lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1110 /// Like `universal_upper_bound`, but returns an approximation more suitable
1111 /// for diagnostics. If `r` contains multiple disjoint universal regions
1112 /// (e.g. 'a and 'b in `fn foo<'a, 'b> { ... }`, we pick the lower-numbered region.
1113 /// This corresponds to picking named regions over unnamed regions
1114 /// (e.g. picking early-bound regions over a closure late-bound region).
1116 /// This means that the returned value may not be a true upper bound, since
1117 /// only 'static is known to outlive disjoint universal regions.
1118 /// Therefore, this method should only be used in diagnostic code,
1119 /// where displaying *some* named universal region is better than
1120 /// falling back to 'static.
1121 pub(crate) fn approx_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1122 debug!("approx_universal_upper_bound(r={:?}={})", 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 let static_r = self.universal_regions.fr_static;
1129 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1130 let new_lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1131 debug!("approx_universal_upper_bound: ur={:?} lub={:?} new_lub={:?}", ur, lub, new_lub);
1132 // The upper bound of two non-static regions is static: this
1133 // means we know nothing about the relationship between these
1134 // two regions. Pick a 'better' one to use when constructing
1136 if ur != static_r && lub != static_r && new_lub == static_r {
1137 // Prefer the region with an `external_name` - this
1138 // indicates that the region is early-bound, so working with
1139 // it can produce a nicer error.
1140 if self.region_definition(ur).external_name.is_some() {
1142 } else if self.region_definition(lub).external_name.is_some() {
1143 // Leave lub unchanged
1145 // If we get here, we don't have any reason to prefer
1146 // one region over the other. Just pick the
1147 // one with the lower index for now.
1148 lub = std::cmp::min(ur, lub);
1155 debug!("approx_universal_upper_bound: r={:?} lub={:?}", r, lub);
1160 /// Tests if `test` is true when applied to `lower_bound` at
1162 fn eval_verify_bound(
1166 generic_ty: Ty<'tcx>,
1167 lower_bound: RegionVid,
1168 verify_bound: &VerifyBound<'tcx>,
1170 debug!("eval_verify_bound(lower_bound={:?}, verify_bound={:?})", lower_bound, verify_bound);
1172 match verify_bound {
1173 VerifyBound::IfEq(test_ty, verify_bound1) => {
1174 self.eval_if_eq(tcx, body, generic_ty, lower_bound, *test_ty, verify_bound1)
1177 VerifyBound::IsEmpty => {
1178 let lower_bound_scc = self.constraint_sccs.scc(lower_bound);
1179 self.scc_values.elements_contained_in(lower_bound_scc).next().is_none()
1182 VerifyBound::OutlivedBy(r) => {
1183 let r_vid = self.to_region_vid(*r);
1184 self.eval_outlives(r_vid, lower_bound)
1187 VerifyBound::AnyBound(verify_bounds) => verify_bounds.iter().any(|verify_bound| {
1188 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1191 VerifyBound::AllBounds(verify_bounds) => verify_bounds.iter().all(|verify_bound| {
1192 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1201 generic_ty: Ty<'tcx>,
1202 lower_bound: RegionVid,
1204 verify_bound: &VerifyBound<'tcx>,
1206 let generic_ty_normalized = self.normalize_to_scc_representatives(tcx, generic_ty);
1207 let test_ty_normalized = self.normalize_to_scc_representatives(tcx, test_ty);
1208 if generic_ty_normalized == test_ty_normalized {
1209 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1215 /// This is a conservative normalization procedure. It takes every
1216 /// free region in `value` and replaces it with the
1217 /// "representative" of its SCC (see `scc_representatives` field).
1218 /// We are guaranteed that if two values normalize to the same
1219 /// thing, then they are equal; this is a conservative check in
1220 /// that they could still be equal even if they normalize to
1221 /// different results. (For example, there might be two regions
1222 /// with the same value that are not in the same SCC).
1224 /// N.B., this is not an ideal approach and I would like to revisit
1225 /// it. However, it works pretty well in practice. In particular,
1226 /// this is needed to deal with projection outlives bounds like
1229 /// <T as Foo<'0>>::Item: '1
1232 /// In particular, this routine winds up being important when
1233 /// there are bounds like `where <T as Foo<'a>>::Item: 'b` in the
1234 /// environment. In this case, if we can show that `'0 == 'a`,
1235 /// and that `'b: '1`, then we know that the clause is
1236 /// satisfied. In such cases, particularly due to limitations of
1237 /// the trait solver =), we usually wind up with a where-clause like
1238 /// `T: Foo<'a>` in scope, which thus forces `'0 == 'a` to be added as
1239 /// a constraint, and thus ensures that they are in the same SCC.
1241 /// So why can't we do a more correct routine? Well, we could
1242 /// *almost* use the `relate_tys` code, but the way it is
1243 /// currently setup it creates inference variables to deal with
1244 /// higher-ranked things and so forth, and right now the inference
1245 /// context is not permitted to make more inference variables. So
1246 /// we use this kind of hacky solution.
1247 fn normalize_to_scc_representatives<T>(&self, tcx: TyCtxt<'tcx>, value: T) -> T
1249 T: TypeFoldable<'tcx>,
1251 tcx.fold_regions(value, &mut false, |r, _db| {
1252 let vid = self.to_region_vid(r);
1253 let scc = self.constraint_sccs.scc(vid);
1254 let repr = self.scc_representatives[scc];
1255 tcx.mk_region(ty::ReVar(repr))
1259 // Evaluate whether `sup_region == sub_region`.
1260 fn eval_equal(&self, r1: RegionVid, r2: RegionVid) -> bool {
1261 self.eval_outlives(r1, r2) && self.eval_outlives(r2, r1)
1264 // Evaluate whether `sup_region: sub_region`.
1265 #[instrument(skip(self), level = "debug")]
1266 fn eval_outlives(&self, sup_region: RegionVid, sub_region: RegionVid) -> bool {
1268 "eval_outlives: sup_region's value = {:?} universal={:?}",
1269 self.region_value_str(sup_region),
1270 self.universal_regions.is_universal_region(sup_region),
1273 "eval_outlives: sub_region's value = {:?} universal={:?}",
1274 self.region_value_str(sub_region),
1275 self.universal_regions.is_universal_region(sub_region),
1278 let sub_region_scc = self.constraint_sccs.scc(sub_region);
1279 let sup_region_scc = self.constraint_sccs.scc(sup_region);
1281 // Both the `sub_region` and `sup_region` consist of the union
1282 // of some number of universal regions (along with the union
1283 // of various points in the CFG; ignore those points for
1284 // now). Therefore, the sup-region outlives the sub-region if,
1285 // for each universal region R1 in the sub-region, there
1286 // exists some region R2 in the sup-region that outlives R1.
1287 let universal_outlives =
1288 self.scc_values.universal_regions_outlived_by(sub_region_scc).all(|r1| {
1290 .universal_regions_outlived_by(sup_region_scc)
1291 .any(|r2| self.universal_region_relations.outlives(r2, r1))
1294 if !universal_outlives {
1298 // Now we have to compare all the points in the sub region and make
1299 // sure they exist in the sup region.
1301 if self.universal_regions.is_universal_region(sup_region) {
1302 // Micro-opt: universal regions contain all points.
1306 self.scc_values.contains_points(sup_region_scc, sub_region_scc)
1309 /// Once regions have been propagated, this method is used to see
1310 /// whether any of the constraints were too strong. In particular,
1311 /// we want to check for a case where a universally quantified
1312 /// region exceeded its bounds. Consider:
1313 /// ```compile_fail,E0312
1314 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1316 /// In this case, returning `x` requires `&'a u32 <: &'b u32`
1317 /// and hence we establish (transitively) a constraint that
1318 /// `'a: 'b`. The `propagate_constraints` code above will
1319 /// therefore add `end('a)` into the region for `'b` -- but we
1320 /// have no evidence that `'b` outlives `'a`, so we want to report
1323 /// If `propagated_outlives_requirements` is `Some`, then we will
1324 /// push unsatisfied obligations into there. Otherwise, we'll
1325 /// report them as errors.
1326 fn check_universal_regions(
1329 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1330 errors_buffer: &mut RegionErrors<'tcx>,
1332 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1333 match fr_definition.origin {
1334 NllRegionVariableOrigin::FreeRegion => {
1335 // Go through each of the universal regions `fr` and check that
1336 // they did not grow too large, accumulating any requirements
1337 // for our caller into the `outlives_requirements` vector.
1338 self.check_universal_region(
1341 &mut propagated_outlives_requirements,
1346 NllRegionVariableOrigin::Placeholder(placeholder) => {
1347 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1350 NllRegionVariableOrigin::RootEmptyRegion
1351 | NllRegionVariableOrigin::Existential { .. } => {
1352 // nothing to check here
1358 /// Checks if Polonius has found any unexpected free region relations.
1360 /// In Polonius terms, a "subset error" (or "illegal subset relation error") is the equivalent
1361 /// of NLL's "checking if any region constraints were too strong": a placeholder origin `'a`
1362 /// was unexpectedly found to be a subset of another placeholder origin `'b`, and means in NLL
1363 /// terms that the "longer free region" `'a` outlived the "shorter free region" `'b`.
1365 /// More details can be found in this blog post by Niko:
1366 /// <https://smallcultfollowing.com/babysteps/blog/2019/01/17/polonius-and-region-errors/>
1368 /// In the canonical example
1369 /// ```compile_fail,E0312
1370 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1372 /// returning `x` requires `&'a u32 <: &'b u32` and hence we establish (transitively) a
1373 /// constraint that `'a: 'b`. It is an error that we have no evidence that this
1374 /// constraint holds.
1376 /// If `propagated_outlives_requirements` is `Some`, then we will
1377 /// push unsatisfied obligations into there. Otherwise, we'll
1378 /// report them as errors.
1379 fn check_polonius_subset_errors(
1382 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1383 errors_buffer: &mut RegionErrors<'tcx>,
1384 polonius_output: Rc<PoloniusOutput>,
1387 "check_polonius_subset_errors: {} subset_errors",
1388 polonius_output.subset_errors.len()
1391 // Similarly to `check_universal_regions`: a free region relation, which was not explicitly
1392 // declared ("known") was found by Polonius, so emit an error, or propagate the
1393 // requirements for our caller into the `propagated_outlives_requirements` vector.
1395 // Polonius doesn't model regions ("origins") as CFG-subsets or durations, but the
1396 // `longer_fr` and `shorter_fr` terminology will still be used here, for consistency with
1397 // the rest of the NLL infrastructure. The "subset origin" is the "longer free region",
1398 // and the "superset origin" is the outlived "shorter free region".
1400 // Note: Polonius will produce a subset error at every point where the unexpected
1401 // `longer_fr`'s "placeholder loan" is contained in the `shorter_fr`. This can be helpful
1402 // for diagnostics in the future, e.g. to point more precisely at the key locations
1403 // requiring this constraint to hold. However, the error and diagnostics code downstream
1404 // expects that these errors are not duplicated (and that they are in a certain order).
1405 // Otherwise, diagnostics messages such as the ones giving names like `'1` to elided or
1406 // anonymous lifetimes for example, could give these names differently, while others like
1407 // the outlives suggestions or the debug output from `#[rustc_regions]` would be
1408 // duplicated. The polonius subset errors are deduplicated here, while keeping the
1409 // CFG-location ordering.
1410 let mut subset_errors: Vec<_> = polonius_output
1413 .flat_map(|(_location, subset_errors)| subset_errors.iter())
1415 subset_errors.sort();
1416 subset_errors.dedup();
1418 for (longer_fr, shorter_fr) in subset_errors.into_iter() {
1420 "check_polonius_subset_errors: subset_error longer_fr={:?},\
1422 longer_fr, shorter_fr
1425 let propagated = self.try_propagate_universal_region_error(
1429 &mut propagated_outlives_requirements,
1431 if propagated == RegionRelationCheckResult::Error {
1432 errors_buffer.push(RegionErrorKind::RegionError {
1433 longer_fr: *longer_fr,
1434 shorter_fr: *shorter_fr,
1435 fr_origin: NllRegionVariableOrigin::FreeRegion,
1441 // Handle the placeholder errors as usual, until the chalk-rustc-polonius triumvirate has
1442 // a more complete picture on how to separate this responsibility.
1443 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1444 match fr_definition.origin {
1445 NllRegionVariableOrigin::FreeRegion => {
1446 // handled by polonius above
1449 NllRegionVariableOrigin::Placeholder(placeholder) => {
1450 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1453 NllRegionVariableOrigin::RootEmptyRegion
1454 | NllRegionVariableOrigin::Existential { .. } => {
1455 // nothing to check here
1461 /// Checks the final value for the free region `fr` to see if it
1462 /// grew too large. In particular, examine what `end(X)` points
1463 /// wound up in `fr`'s final value; for each `end(X)` where `X !=
1464 /// fr`, we want to check that `fr: X`. If not, that's either an
1465 /// error, or something we have to propagate to our creator.
1467 /// Things that are to be propagated are accumulated into the
1468 /// `outlives_requirements` vector.
1470 skip(self, body, propagated_outlives_requirements, errors_buffer),
1473 fn check_universal_region(
1476 longer_fr: RegionVid,
1477 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1478 errors_buffer: &mut RegionErrors<'tcx>,
1480 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1482 // Because this free region must be in the ROOT universe, we
1483 // know it cannot contain any bound universes.
1484 assert!(self.scc_universes[longer_fr_scc] == ty::UniverseIndex::ROOT);
1485 debug_assert!(self.scc_values.placeholders_contained_in(longer_fr_scc).next().is_none());
1487 // Only check all of the relations for the main representative of each
1488 // SCC, otherwise just check that we outlive said representative. This
1489 // reduces the number of redundant relations propagated out of
1491 // Note that the representative will be a universal region if there is
1492 // one in this SCC, so we will always check the representative here.
1493 let representative = self.scc_representatives[longer_fr_scc];
1494 if representative != longer_fr {
1495 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1499 propagated_outlives_requirements,
1501 errors_buffer.push(RegionErrorKind::RegionError {
1503 shorter_fr: representative,
1504 fr_origin: NllRegionVariableOrigin::FreeRegion,
1511 // Find every region `o` such that `fr: o`
1512 // (because `fr` includes `end(o)`).
1513 let mut error_reported = false;
1514 for shorter_fr in self.scc_values.universal_regions_outlived_by(longer_fr_scc) {
1515 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1519 propagated_outlives_requirements,
1521 // We only report the first region error. Subsequent errors are hidden so as
1522 // not to overwhelm the user, but we do record them so as to potentially print
1523 // better diagnostics elsewhere...
1524 errors_buffer.push(RegionErrorKind::RegionError {
1527 fr_origin: NllRegionVariableOrigin::FreeRegion,
1528 is_reported: !error_reported,
1531 error_reported = true;
1536 /// Checks that we can prove that `longer_fr: shorter_fr`. If we can't we attempt to propagate
1537 /// the constraint outward (e.g. to a closure environment), but if that fails, there is an
1539 fn check_universal_region_relation(
1541 longer_fr: RegionVid,
1542 shorter_fr: RegionVid,
1544 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1545 ) -> RegionRelationCheckResult {
1546 // If it is known that `fr: o`, carry on.
1547 if self.universal_region_relations.outlives(longer_fr, shorter_fr) {
1548 RegionRelationCheckResult::Ok
1550 // If we are not in a context where we can't propagate errors, or we
1551 // could not shrink `fr` to something smaller, then just report an
1554 // Note: in this case, we use the unapproximated regions to report the
1555 // error. This gives better error messages in some cases.
1556 self.try_propagate_universal_region_error(
1560 propagated_outlives_requirements,
1565 /// Attempt to propagate a region error (e.g. `'a: 'b`) that is not met to a closure's
1566 /// creator. If we cannot, then the caller should report an error to the user.
1567 fn try_propagate_universal_region_error(
1569 longer_fr: RegionVid,
1570 shorter_fr: RegionVid,
1572 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1573 ) -> RegionRelationCheckResult {
1574 if let Some(propagated_outlives_requirements) = propagated_outlives_requirements {
1575 // Shrink `longer_fr` until we find a non-local region (if we do).
1576 // We'll call it `fr-` -- it's ever so slightly smaller than
1578 if let Some(fr_minus) = self.universal_region_relations.non_local_lower_bound(longer_fr)
1580 debug!("try_propagate_universal_region_error: fr_minus={:?}", fr_minus);
1582 let blame_span_category = self.find_outlives_blame_span(
1585 NllRegionVariableOrigin::FreeRegion,
1589 // Grow `shorter_fr` until we find some non-local regions. (We
1590 // always will.) We'll call them `shorter_fr+` -- they're ever
1591 // so slightly larger than `shorter_fr`.
1592 let shorter_fr_plus =
1593 self.universal_region_relations.non_local_upper_bounds(shorter_fr);
1595 "try_propagate_universal_region_error: shorter_fr_plus={:?}",
1598 for fr in shorter_fr_plus {
1599 // Push the constraint `fr-: shorter_fr+`
1600 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
1601 subject: ClosureOutlivesSubject::Region(fr_minus),
1602 outlived_free_region: fr,
1603 blame_span: blame_span_category.1.span,
1604 category: blame_span_category.0,
1607 return RegionRelationCheckResult::Propagated;
1611 RegionRelationCheckResult::Error
1614 fn check_bound_universal_region(
1616 longer_fr: RegionVid,
1617 placeholder: ty::PlaceholderRegion,
1618 errors_buffer: &mut RegionErrors<'tcx>,
1620 debug!("check_bound_universal_region(fr={:?}, placeholder={:?})", longer_fr, placeholder,);
1622 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1623 debug!("check_bound_universal_region: longer_fr_scc={:?}", longer_fr_scc,);
1625 // If we have some bound universal region `'a`, then the only
1626 // elements it can contain is itself -- we don't know anything
1628 let Some(error_element) = ({
1629 self.scc_values.elements_contained_in(longer_fr_scc).find(|element| match element {
1630 RegionElement::Location(_) => true,
1631 RegionElement::RootUniversalRegion(_) => true,
1632 RegionElement::PlaceholderRegion(placeholder1) => placeholder != *placeholder1,
1637 debug!("check_bound_universal_region: error_element = {:?}", error_element);
1639 // Find the region that introduced this `error_element`.
1640 errors_buffer.push(RegionErrorKind::BoundUniversalRegionError {
1647 fn check_member_constraints(
1649 infcx: &InferCtxt<'_, 'tcx>,
1650 errors_buffer: &mut RegionErrors<'tcx>,
1652 let member_constraints = self.member_constraints.clone();
1653 for m_c_i in member_constraints.all_indices() {
1654 debug!("check_member_constraint(m_c_i={:?})", m_c_i);
1655 let m_c = &member_constraints[m_c_i];
1656 let member_region_vid = m_c.member_region_vid;
1658 "check_member_constraint: member_region_vid={:?} with value {}",
1660 self.region_value_str(member_region_vid),
1662 let choice_regions = member_constraints.choice_regions(m_c_i);
1663 debug!("check_member_constraint: choice_regions={:?}", choice_regions);
1665 // Did the member region wind up equal to any of the option regions?
1667 choice_regions.iter().find(|&&o_r| self.eval_equal(o_r, m_c.member_region_vid))
1669 debug!("check_member_constraint: evaluated as equal to {:?}", o);
1673 // If not, report an error.
1674 let member_region = infcx.tcx.mk_region(ty::ReVar(member_region_vid));
1675 errors_buffer.push(RegionErrorKind::UnexpectedHiddenRegion {
1676 span: m_c.definition_span,
1677 hidden_ty: m_c.hidden_ty,
1683 /// We have a constraint `fr1: fr2` that is not satisfied, where
1684 /// `fr2` represents some universal region. Here, `r` is some
1685 /// region where we know that `fr1: r` and this function has the
1686 /// job of determining whether `r` is "to blame" for the fact that
1687 /// `fr1: fr2` is required.
1689 /// This is true under two conditions:
1692 /// - `fr2` is `'static` and `r` is some placeholder in a universe
1693 /// that cannot be named by `fr1`; in that case, we will require
1694 /// that `fr1: 'static` because it is the only way to `fr1: r` to
1695 /// be satisfied. (See `add_incompatible_universe`.)
1696 crate fn provides_universal_region(
1702 debug!("provides_universal_region(r={:?}, fr1={:?}, fr2={:?})", r, fr1, fr2);
1705 fr2 == self.universal_regions.fr_static && self.cannot_name_placeholder(fr1, r)
1708 debug!("provides_universal_region: result = {:?}", result);
1712 /// If `r2` represents a placeholder region, then this returns
1713 /// `true` if `r1` cannot name that placeholder in its
1714 /// value; otherwise, returns `false`.
1715 crate fn cannot_name_placeholder(&self, r1: RegionVid, r2: RegionVid) -> bool {
1716 debug!("cannot_name_value_of(r1={:?}, r2={:?})", r1, r2);
1718 match self.definitions[r2].origin {
1719 NllRegionVariableOrigin::Placeholder(placeholder) => {
1720 let universe1 = self.definitions[r1].universe;
1722 "cannot_name_value_of: universe1={:?} placeholder={:?}",
1723 universe1, placeholder
1725 universe1.cannot_name(placeholder.universe)
1728 NllRegionVariableOrigin::RootEmptyRegion
1729 | NllRegionVariableOrigin::FreeRegion
1730 | NllRegionVariableOrigin::Existential { .. } => false,
1734 crate fn retrieve_closure_constraint_info(
1737 constraint: &OutlivesConstraint<'tcx>,
1738 ) -> BlameConstraint<'tcx> {
1739 let loc = match constraint.locations {
1740 Locations::All(span) => {
1741 return BlameConstraint {
1742 category: constraint.category,
1743 from_closure: false,
1744 cause: ObligationCause::dummy_with_span(span),
1745 variance_info: constraint.variance_info,
1748 Locations::Single(loc) => loc,
1751 let opt_span_category =
1752 self.closure_bounds_mapping[&loc].get(&(constraint.sup, constraint.sub));
1754 .map(|&(category, span)| BlameConstraint {
1757 cause: ObligationCause::dummy_with_span(span),
1758 variance_info: constraint.variance_info,
1760 .unwrap_or(BlameConstraint {
1761 category: constraint.category,
1762 from_closure: false,
1763 cause: ObligationCause::dummy_with_span(constraint.span),
1764 variance_info: constraint.variance_info,
1768 /// Finds a good `ObligationCause` to blame for the fact that `fr1` outlives `fr2`.
1769 crate fn find_outlives_blame_span(
1773 fr1_origin: NllRegionVariableOrigin,
1775 ) -> (ConstraintCategory, ObligationCause<'tcx>) {
1776 let BlameConstraint { category, cause, .. } =
1777 self.best_blame_constraint(body, fr1, fr1_origin, |r| {
1778 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 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(),
1876 handle_constraint(constraint);
1883 /// Finds some region R such that `fr1: R` and `R` is live at `elem`.
1884 #[instrument(skip(self), level = "trace")]
1885 crate fn find_sub_region_live_at(&self, fr1: RegionVid, elem: Location) -> RegionVid {
1886 trace!(scc = ?self.constraint_sccs.scc(fr1));
1887 trace!(universe = ?self.scc_universes[self.constraint_sccs.scc(fr1)]);
1888 self.find_constraint_paths_between_regions(fr1, |r| {
1889 // First look for some `r` such that `fr1: r` and `r` is live at `elem`
1890 trace!(?r, liveness_constraints=?self.liveness_constraints.region_value_str(r));
1891 self.liveness_constraints.contains(r, elem)
1894 // If we fail to find that, we may find some `r` such that
1895 // `fr1: r` and `r` is a placeholder from some universe
1896 // `fr1` cannot name. This would force `fr1` to be
1898 self.find_constraint_paths_between_regions(fr1, |r| {
1899 self.cannot_name_placeholder(fr1, r)
1903 // If we fail to find THAT, it may be that `fr1` is a
1904 // placeholder that cannot "fit" into its SCC. In that
1905 // case, there should be some `r` where `fr1: r` and `fr1` is a
1906 // placeholder that `r` cannot name. We can blame that
1909 // Remember that if `R1: R2`, then the universe of R1
1910 // must be able to name the universe of R2, because R2 will
1911 // be at least `'empty(Universe(R2))`, and `R1` must be at
1912 // larger than that.
1913 self.find_constraint_paths_between_regions(fr1, |r| {
1914 self.cannot_name_placeholder(r, fr1)
1917 .map(|(_path, r)| r)
1921 /// Get the region outlived by `longer_fr` and live at `element`.
1922 crate fn region_from_element(
1924 longer_fr: RegionVid,
1925 element: &RegionElement,
1928 RegionElement::Location(l) => self.find_sub_region_live_at(longer_fr, l),
1929 RegionElement::RootUniversalRegion(r) => r,
1930 RegionElement::PlaceholderRegion(error_placeholder) => self
1933 .find_map(|(r, definition)| match definition.origin {
1934 NllRegionVariableOrigin::Placeholder(p) if p == error_placeholder => Some(r),
1941 /// Get the region definition of `r`.
1942 crate fn region_definition(&self, r: RegionVid) -> &RegionDefinition<'tcx> {
1943 &self.definitions[r]
1946 /// Check if the SCC of `r` contains `upper`.
1947 crate fn upper_bound_in_region_scc(&self, r: RegionVid, upper: RegionVid) -> bool {
1948 let r_scc = self.constraint_sccs.scc(r);
1949 self.scc_values.contains(r_scc, upper)
1952 crate fn universal_regions(&self) -> &UniversalRegions<'tcx> {
1953 self.universal_regions.as_ref()
1956 /// Tries to find the best constraint to blame for the fact that
1957 /// `R: from_region`, where `R` is some region that meets
1958 /// `target_test`. This works by following the constraint graph,
1959 /// creating a constraint path that forces `R` to outlive
1960 /// `from_region`, and then finding the best choices within that
1962 crate fn best_blame_constraint(
1965 from_region: RegionVid,
1966 from_region_origin: NllRegionVariableOrigin,
1967 target_test: impl Fn(RegionVid) -> bool,
1968 ) -> BlameConstraint<'tcx> {
1970 "best_blame_constraint(from_region={:?}, from_region_origin={:?})",
1971 from_region, from_region_origin
1975 let (path, target_region) =
1976 self.find_constraint_paths_between_regions(from_region, target_test).unwrap();
1978 "best_blame_constraint: path={:#?}",
1981 "{:?} ({:?}: {:?})",
1983 self.constraint_sccs.scc(c.sup),
1984 self.constraint_sccs.scc(c.sub),
1986 .collect::<Vec<_>>()
1989 // We try to avoid reporting a `ConstraintCategory::Predicate` as our best constraint.
1990 // Instead, we use it to produce an improved `ObligationCauseCode`.
1991 // FIXME - determine what we should do if we encounter multiple `ConstraintCategory::Predicate`
1992 // constraints. Currently, we just pick the first one.
1993 let cause_code = path
1995 .find_map(|constraint| {
1996 if let ConstraintCategory::Predicate(predicate_span) = constraint.category {
1997 // We currently do not store the `DefId` in the `ConstraintCategory`
1998 // for performances reasons. The error reporting code used by NLL only
1999 // uses the span, so this doesn't cause any problems at the moment.
2000 Some(ObligationCauseCode::BindingObligation(
2001 CRATE_DEF_ID.to_def_id(),
2008 .unwrap_or_else(|| ObligationCauseCode::MiscObligation);
2010 // Classify each of the constraints along the path.
2011 let mut categorized_path: Vec<BlameConstraint<'tcx>> = path
2014 if constraint.category == ConstraintCategory::ClosureBounds {
2015 self.retrieve_closure_constraint_info(body, &constraint)
2018 category: constraint.category,
2019 from_closure: false,
2020 cause: ObligationCause::new(
2025 variance_info: constraint.variance_info,
2030 debug!("best_blame_constraint: categorized_path={:#?}", categorized_path);
2032 // To find the best span to cite, we first try to look for the
2033 // final constraint that is interesting and where the `sup` is
2034 // not unified with the ultimate target region. The reason
2035 // for this is that we have a chain of constraints that lead
2036 // from the source to the target region, something like:
2038 // '0: '1 ('0 is the source)
2043 // '5: '6 ('6 is the target)
2045 // Some of those regions are unified with `'6` (in the same
2046 // SCC). We want to screen those out. After that point, the
2047 // "closest" constraint we have to the end is going to be the
2048 // most likely to be the point where the value escapes -- but
2049 // we still want to screen for an "interesting" point to
2050 // highlight (e.g., a call site or something).
2051 let target_scc = self.constraint_sccs.scc(target_region);
2052 let mut range = 0..path.len();
2054 // As noted above, when reporting an error, there is typically a chain of constraints
2055 // leading from some "source" region which must outlive some "target" region.
2056 // In most cases, we prefer to "blame" the constraints closer to the target --
2057 // but there is one exception. When constraints arise from higher-ranked subtyping,
2058 // we generally prefer to blame the source value,
2059 // as the "target" in this case tends to be some type annotation that the user gave.
2060 // Therefore, if we find that the region origin is some instantiation
2061 // of a higher-ranked region, we start our search from the "source" point
2062 // rather than the "target", and we also tweak a few other things.
2064 // An example might be this bit of Rust code:
2067 // let x: fn(&'static ()) = |_| {};
2068 // let y: for<'a> fn(&'a ()) = x;
2071 // In MIR, this will be converted into a combination of assignments and type ascriptions.
2072 // In particular, the 'static is imposed through a type ascription:
2076 // AscribeUserType(x, fn(&'static ())
2080 // We wind up ultimately with constraints like
2083 // !a: 'temp1 // from the `y = x` statement
2085 // 'temp2: 'static // from the AscribeUserType
2088 // and here we prefer to blame the source (the y = x statement).
2089 let blame_source = match from_region_origin {
2090 NllRegionVariableOrigin::FreeRegion
2091 | NllRegionVariableOrigin::Existential { from_forall: false } => true,
2092 NllRegionVariableOrigin::RootEmptyRegion
2093 | NllRegionVariableOrigin::Placeholder(_)
2094 | NllRegionVariableOrigin::Existential { from_forall: true } => false,
2097 let find_region = |i: &usize| {
2098 let constraint = &path[*i];
2100 let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
2103 match categorized_path[*i].category {
2104 ConstraintCategory::OpaqueType
2105 | ConstraintCategory::Boring
2106 | ConstraintCategory::BoringNoLocation
2107 | ConstraintCategory::Internal
2108 | ConstraintCategory::Predicate(_) => false,
2109 ConstraintCategory::TypeAnnotation
2110 | ConstraintCategory::Return(_)
2111 | ConstraintCategory::Yield => true,
2112 _ => constraint_sup_scc != target_scc,
2116 categorized_path[*i].category,
2117 ConstraintCategory::OpaqueType
2118 | ConstraintCategory::Boring
2119 | ConstraintCategory::BoringNoLocation
2120 | ConstraintCategory::Internal
2121 | ConstraintCategory::Predicate(_)
2127 if blame_source { range.rev().find(find_region) } else { range.find(find_region) };
2130 "best_blame_constraint: best_choice={:?} blame_source={}",
2131 best_choice, blame_source
2134 if let Some(i) = best_choice {
2135 if let Some(next) = categorized_path.get(i + 1) {
2136 if matches!(categorized_path[i].category, ConstraintCategory::Return(_))
2137 && next.category == ConstraintCategory::OpaqueType
2139 // The return expression is being influenced by the return type being
2140 // impl Trait, point at the return type and not the return expr.
2141 return next.clone();
2145 if categorized_path[i].category == ConstraintCategory::Return(ReturnConstraint::Normal)
2147 let field = categorized_path.iter().find_map(|p| {
2148 if let ConstraintCategory::ClosureUpvar(f) = p.category {
2155 if let Some(field) = field {
2156 categorized_path[i].category =
2157 ConstraintCategory::Return(ReturnConstraint::ClosureUpvar(field));
2161 return categorized_path[i].clone();
2164 // If that search fails, that is.. unusual. Maybe everything
2165 // is in the same SCC or something. In that case, find what
2166 // appears to be the most interesting point to report to the
2167 // user via an even more ad-hoc guess.
2168 categorized_path.sort_by(|p0, p1| p0.category.cmp(&p1.category));
2169 debug!("best_blame_constraint: sorted_path={:#?}", categorized_path);
2171 categorized_path.remove(0)
2174 crate fn universe_info(&self, universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
2175 self.universe_causes[&universe].clone()
2179 impl<'tcx> RegionDefinition<'tcx> {
2180 fn new(universe: ty::UniverseIndex, rv_origin: RegionVariableOrigin) -> Self {
2181 // Create a new region definition. Note that, for free
2182 // regions, the `external_name` field gets updated later in
2183 // `init_universal_regions`.
2185 let origin = match rv_origin {
2186 RegionVariableOrigin::Nll(origin) => origin,
2187 _ => NllRegionVariableOrigin::Existential { from_forall: false },
2190 Self { origin, universe, external_name: None }
2194 pub trait ClosureRegionRequirementsExt<'tcx> {
2195 fn apply_requirements(
2198 closure_def_id: DefId,
2199 closure_substs: SubstsRef<'tcx>,
2200 ) -> Vec<QueryOutlivesConstraint<'tcx>>;
2203 impl<'tcx> ClosureRegionRequirementsExt<'tcx> for ClosureRegionRequirements<'tcx> {
2204 /// Given an instance T of the closure type, this method
2205 /// instantiates the "extra" requirements that we computed for the
2206 /// closure into the inference context. This has the effect of
2207 /// adding new outlives obligations to existing variables.
2209 /// As described on `ClosureRegionRequirements`, the extra
2210 /// requirements are expressed in terms of regionvids that index
2211 /// into the free regions that appear on the closure type. So, to
2212 /// do this, we first copy those regions out from the type T into
2213 /// a vector. Then we can just index into that vector to extract
2214 /// out the corresponding region from T and apply the
2216 fn apply_requirements(
2219 closure_def_id: DefId,
2220 closure_substs: SubstsRef<'tcx>,
2221 ) -> Vec<QueryOutlivesConstraint<'tcx>> {
2223 "apply_requirements(closure_def_id={:?}, closure_substs={:?})",
2224 closure_def_id, closure_substs
2227 // Extract the values of the free regions in `closure_substs`
2228 // into a vector. These are the regions that we will be
2229 // relating to one another.
2230 let closure_mapping = &UniversalRegions::closure_mapping(
2233 self.num_external_vids,
2234 tcx.typeck_root_def_id(closure_def_id),
2236 debug!("apply_requirements: closure_mapping={:?}", closure_mapping);
2238 // Create the predicates.
2239 self.outlives_requirements
2241 .map(|outlives_requirement| {
2242 let outlived_region = closure_mapping[outlives_requirement.outlived_free_region];
2244 match outlives_requirement.subject {
2245 ClosureOutlivesSubject::Region(region) => {
2246 let region = closure_mapping[region];
2248 "apply_requirements: region={:?} \
2249 outlived_region={:?} \
2250 outlives_requirement={:?}",
2251 region, outlived_region, outlives_requirement,
2253 ty::Binder::dummy(ty::OutlivesPredicate(region.into(), outlived_region))
2256 ClosureOutlivesSubject::Ty(ty) => {
2258 "apply_requirements: ty={:?} \
2259 outlived_region={:?} \
2260 outlives_requirement={:?}",
2261 ty, outlived_region, outlives_requirement,
2263 ty::Binder::dummy(ty::OutlivesPredicate(ty.into(), outlived_region))
2271 #[derive(Clone, Debug)]
2272 pub struct BlameConstraint<'tcx> {
2273 pub category: ConstraintCategory,
2274 pub from_closure: bool,
2275 pub cause: ObligationCause<'tcx>,
2276 pub variance_info: ty::VarianceDiagInfo<'tcx>,