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.
132 pub(crate) struct AppliedMemberConstraint {
133 /// The SCC that was affected. (The "member region".)
135 /// The vector if `AppliedMemberConstraint` elements is kept sorted
137 pub(crate) member_region_scc: ConstraintSccIndex,
139 /// The "best option" that `apply_member_constraint` found -- this was
140 /// added as an "ad-hoc" lower-bound to `member_region_scc`.
141 pub(crate) min_choice: ty::RegionVid,
143 /// The "member constraint index" -- we can find out details about
144 /// the constraint from
145 /// `set.member_constraints[member_constraint_index]`.
146 pub(crate) member_constraint_index: NllMemberConstraintIndex,
149 pub(crate) struct RegionDefinition<'tcx> {
150 /// What kind of variable is this -- a free region? existential
151 /// variable? etc. (See the `NllRegionVariableOrigin` for more
153 pub(crate) origin: NllRegionVariableOrigin,
155 /// Which universe is this region variable defined in? This is
156 /// most often `ty::UniverseIndex::ROOT`, but when we encounter
157 /// forall-quantifiers like `for<'a> { 'a = 'b }`, we would create
158 /// the variable for `'a` in a fresh universe that extends ROOT.
159 pub(crate) universe: ty::UniverseIndex,
161 /// If this is 'static or an early-bound region, then this is
162 /// `Some(X)` where `X` is the name of the region.
163 pub(crate) external_name: Option<ty::Region<'tcx>>,
166 /// N.B., the variants in `Cause` are intentionally ordered. Lower
167 /// values are preferred when it comes to error messages. Do not
168 /// reorder willy nilly.
169 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
170 pub(crate) enum Cause {
171 /// point inserted because Local was live at the given Location
172 LiveVar(Local, Location),
174 /// point inserted because Local was dropped at the given Location
175 DropVar(Local, Location),
178 /// A "type test" corresponds to an outlives constraint between a type
179 /// and a lifetime, like `T: 'x` or `<T as Foo>::Bar: 'x`. They are
180 /// translated from the `Verify` region constraints in the ordinary
181 /// inference context.
183 /// These sorts of constraints are handled differently than ordinary
184 /// constraints, at least at present. During type checking, the
185 /// `InferCtxt::process_registered_region_obligations` method will
186 /// attempt to convert a type test like `T: 'x` into an ordinary
187 /// outlives constraint when possible (for example, `&'a T: 'b` will
188 /// be converted into `'a: 'b` and registered as a `Constraint`).
190 /// In some cases, however, there are outlives relationships that are
191 /// not converted into a region constraint, but rather into one of
192 /// these "type tests". The distinction is that a type test does not
193 /// influence the inference result, but instead just examines the
194 /// values that we ultimately inferred for each region variable and
195 /// checks that they meet certain extra criteria. If not, an error
198 /// One reason for this is that these type tests typically boil down
199 /// to a check like `'a: 'x` where `'a` is a universally quantified
200 /// region -- and therefore not one whose value is really meant to be
201 /// *inferred*, precisely (this is not always the case: one can have a
202 /// type test like `<Foo as Trait<'?0>>::Bar: 'x`, where `'?0` is an
203 /// inference variable). Another reason is that these type tests can
204 /// involve *disjunction* -- that is, they can be satisfied in more
207 /// For more information about this translation, see
208 /// `InferCtxt::process_registered_region_obligations` and
209 /// `InferCtxt::type_must_outlive` in `rustc_infer::infer::InferCtxt`.
210 #[derive(Clone, Debug)]
211 pub struct TypeTest<'tcx> {
212 /// The type `T` that must outlive the region.
213 pub generic_kind: GenericKind<'tcx>,
215 /// The region `'x` that the type must outlive.
216 pub lower_bound: RegionVid,
218 /// The span to blame.
221 /// A test which, if met by the region `'x`, proves that this type
222 /// constraint is satisfied.
223 pub verify_bound: VerifyBound<'tcx>,
226 /// When we have an unmet lifetime constraint, we try to propagate it outward (e.g. to a closure
227 /// environment). If we can't, it is an error.
228 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
229 enum RegionRelationCheckResult {
235 #[derive(Clone, PartialEq, Eq, Debug)]
238 FromOutlivesConstraint(OutlivesConstraint<'tcx>),
242 #[derive(Clone, PartialEq, Eq, Debug)]
243 pub enum ExtraConstraintInfo {
244 PlaceholderFromPredicate(Span),
247 impl<'tcx> RegionInferenceContext<'tcx> {
248 /// Creates a new region inference context with a total of
249 /// `num_region_variables` valid inference variables; the first N
250 /// of those will be constant regions representing the free
251 /// regions defined in `universal_regions`.
253 /// The `outlives_constraints` and `type_tests` are an initial set
254 /// of constraints produced by the MIR type check.
257 universal_regions: Rc<UniversalRegions<'tcx>>,
258 placeholder_indices: Rc<PlaceholderIndices>,
259 universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
260 outlives_constraints: OutlivesConstraintSet<'tcx>,
261 member_constraints_in: MemberConstraintSet<'tcx, RegionVid>,
262 universe_causes: FxHashMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
263 type_tests: Vec<TypeTest<'tcx>>,
264 liveness_constraints: LivenessValues<RegionVid>,
265 elements: &Rc<RegionValueElements>,
267 // Create a RegionDefinition for each inference variable.
268 let definitions: IndexVec<_, _> = var_infos
270 .map(|info| RegionDefinition::new(info.universe, info.origin))
273 let constraints = Frozen::freeze(outlives_constraints);
274 let constraint_graph = Frozen::freeze(constraints.graph(definitions.len()));
275 let fr_static = universal_regions.fr_static;
276 let constraint_sccs = Rc::new(constraints.compute_sccs(&constraint_graph, fr_static));
279 RegionValues::new(elements, universal_regions.len(), &placeholder_indices);
281 for region in liveness_constraints.rows() {
282 let scc = constraint_sccs.scc(region);
283 scc_values.merge_liveness(scc, region, &liveness_constraints);
286 let scc_universes = Self::compute_scc_universes(&constraint_sccs, &definitions);
288 let scc_representatives = Self::compute_scc_representatives(&constraint_sccs, &definitions);
290 let member_constraints =
291 Rc::new(member_constraints_in.into_mapped(|r| constraint_sccs.scc(r)));
293 let mut result = Self {
296 liveness_constraints,
302 member_constraints_applied: Vec::new(),
309 universal_region_relations,
312 result.init_free_and_bound_regions();
317 /// Each SCC is the combination of many region variables which
318 /// have been equated. Therefore, we can associate a universe with
319 /// each SCC which is minimum of all the universes of its
320 /// constituent regions -- this is because whatever value the SCC
321 /// takes on must be a value that each of the regions within the
322 /// SCC could have as well. This implies that the SCC must have
323 /// the minimum, or narrowest, universe.
324 fn compute_scc_universes(
325 constraint_sccs: &Sccs<RegionVid, ConstraintSccIndex>,
326 definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
327 ) -> IndexVec<ConstraintSccIndex, ty::UniverseIndex> {
328 let num_sccs = constraint_sccs.num_sccs();
329 let mut scc_universes = IndexVec::from_elem_n(ty::UniverseIndex::MAX, num_sccs);
331 debug!("compute_scc_universes()");
333 // For each region R in universe U, ensure that the universe for the SCC
334 // that contains R is "no bigger" than U. This effectively sets the universe
335 // for each SCC to be the minimum of the regions within.
336 for (region_vid, region_definition) in definitions.iter_enumerated() {
337 let scc = constraint_sccs.scc(region_vid);
338 let scc_universe = &mut scc_universes[scc];
339 let scc_min = std::cmp::min(region_definition.universe, *scc_universe);
340 if scc_min != *scc_universe {
341 *scc_universe = scc_min;
343 "compute_scc_universes: lowered universe of {scc:?} to {scc_min:?} \
344 because it contains {region_vid:?} in {region_universe:?}",
347 region_vid = region_vid,
348 region_universe = region_definition.universe,
353 // Walk each SCC `A` and `B` such that `A: B`
354 // and ensure that universe(A) can see universe(B).
356 // This serves to enforce the 'empty/placeholder' hierarchy
357 // (described in more detail on `RegionKind`):
362 // empty(U0) placeholder(U1)
367 // In particular, imagine we have variables R0 in U0 and R1
368 // created in U1, and constraints like this;
371 // R1: !1 // R1 outlives the placeholder in U1
372 // R1: R0 // R1 outlives R0
375 // Here, we wish for R1 to be `'static`, because it
376 // cannot outlive `placeholder(U1)` and `empty(U0)` any other way.
378 // Thanks to this loop, what happens is that the `R1: R0`
379 // constraint lowers the universe of `R1` to `U0`, which in turn
380 // means that the `R1: !1` constraint will (later) cause
381 // `R1` to become `'static`.
382 for scc_a in constraint_sccs.all_sccs() {
383 for &scc_b in constraint_sccs.successors(scc_a) {
384 let scc_universe_a = scc_universes[scc_a];
385 let scc_universe_b = scc_universes[scc_b];
386 let scc_universe_min = std::cmp::min(scc_universe_a, scc_universe_b);
387 if scc_universe_a != scc_universe_min {
388 scc_universes[scc_a] = scc_universe_min;
391 "compute_scc_universes: lowered universe of {scc_a:?} to {scc_universe_min:?} \
392 because {scc_a:?}: {scc_b:?} and {scc_b:?} is in universe {scc_universe_b:?}",
395 scc_universe_min = scc_universe_min,
396 scc_universe_b = scc_universe_b
402 debug!("compute_scc_universes: scc_universe = {:#?}", scc_universes);
407 /// For each SCC, we compute a unique `RegionVid` (in fact, the
408 /// minimal one that belongs to the SCC). See
409 /// `scc_representatives` field of `RegionInferenceContext` for
411 fn compute_scc_representatives(
412 constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
413 definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
414 ) -> IndexVec<ConstraintSccIndex, ty::RegionVid> {
415 let num_sccs = constraints_scc.num_sccs();
416 let next_region_vid = definitions.next_index();
417 let mut scc_representatives = IndexVec::from_elem_n(next_region_vid, num_sccs);
419 for region_vid in definitions.indices() {
420 let scc = constraints_scc.scc(region_vid);
421 let prev_min = scc_representatives[scc];
422 scc_representatives[scc] = region_vid.min(prev_min);
428 /// Initializes the region variables for each universally
429 /// quantified region (lifetime parameter). The first N variables
430 /// always correspond to the regions appearing in the function
431 /// signature (both named and anonymous) and where-clauses. This
432 /// function iterates over those regions and initializes them with
437 /// fn foo<'a, 'b>( /* ... */ ) where 'a: 'b { /* ... */ }
439 /// would initialize two variables like so:
440 /// ```ignore (illustrative)
441 /// R0 = { CFG, R0 } // 'a
442 /// R1 = { CFG, R0, R1 } // 'b
444 /// Here, R0 represents `'a`, and it contains (a) the entire CFG
445 /// and (b) any universally quantified regions that it outlives,
446 /// which in this case is just itself. R1 (`'b`) in contrast also
447 /// outlives `'a` and hence contains R0 and R1.
448 fn init_free_and_bound_regions(&mut self) {
449 // Update the names (if any)
450 for (external_name, variable) in self.universal_regions.named_universal_regions() {
452 "init_universal_regions: region {:?} has external name {:?}",
453 variable, external_name
455 self.definitions[variable].external_name = Some(external_name);
458 for variable in self.definitions.indices() {
459 let scc = self.constraint_sccs.scc(variable);
461 match self.definitions[variable].origin {
462 NllRegionVariableOrigin::FreeRegion => {
463 // For each free, universally quantified region X:
465 // Add all nodes in the CFG to liveness constraints
466 self.liveness_constraints.add_all_points(variable);
467 self.scc_values.add_all_points(scc);
469 // Add `end(X)` into the set for X.
470 self.scc_values.add_element(scc, variable);
473 NllRegionVariableOrigin::Placeholder(placeholder) => {
474 // Each placeholder region is only visible from
475 // its universe `ui` and its extensions. So we
476 // can't just add it into `scc` unless the
477 // universe of the scc can name this region.
478 let scc_universe = self.scc_universes[scc];
479 if scc_universe.can_name(placeholder.universe) {
480 self.scc_values.add_element(scc, placeholder);
483 "init_free_and_bound_regions: placeholder {:?} is \
484 not compatible with universe {:?} of its SCC {:?}",
485 placeholder, scc_universe, scc,
487 self.add_incompatible_universe(scc);
491 NllRegionVariableOrigin::Existential { .. } => {
492 // For existential, regions, nothing to do.
498 /// Returns an iterator over all the region indices.
499 pub fn regions(&self) -> impl Iterator<Item = RegionVid> + 'tcx {
500 self.definitions.indices()
503 /// Given a universal region in scope on the MIR, returns the
504 /// corresponding index.
506 /// (Panics if `r` is not a registered universal region.)
507 pub fn to_region_vid(&self, r: ty::Region<'tcx>) -> RegionVid {
508 self.universal_regions.to_region_vid(r)
511 /// Adds annotations for `#[rustc_regions]`; see `UniversalRegions::annotate`.
512 pub(crate) fn annotate(&self, tcx: TyCtxt<'tcx>, err: &mut Diagnostic) {
513 self.universal_regions.annotate(tcx, err)
516 /// Returns `true` if the region `r` contains the point `p`.
518 /// Panics if called before `solve()` executes,
519 pub(crate) fn region_contains(&self, r: impl ToRegionVid, p: impl ToElementIndex) -> bool {
520 let scc = self.constraint_sccs.scc(r.to_region_vid());
521 self.scc_values.contains(scc, p)
524 /// Returns access to the value of `r` for debugging purposes.
525 pub(crate) fn region_value_str(&self, r: RegionVid) -> String {
526 let scc = self.constraint_sccs.scc(r.to_region_vid());
527 self.scc_values.region_value_str(scc)
530 /// Returns access to the value of `r` for debugging purposes.
531 pub(crate) fn region_universe(&self, r: RegionVid) -> ty::UniverseIndex {
532 let scc = self.constraint_sccs.scc(r.to_region_vid());
533 self.scc_universes[scc]
536 /// Once region solving has completed, this function will return
537 /// the member constraints that were applied to the value of a given
538 /// region `r`. See `AppliedMemberConstraint`.
539 pub(crate) fn applied_member_constraints(
542 ) -> &[AppliedMemberConstraint] {
543 let scc = self.constraint_sccs.scc(r.to_region_vid());
544 binary_search_util::binary_search_slice(
545 &self.member_constraints_applied,
546 |applied| applied.member_region_scc,
551 /// Performs region inference and report errors if we see any
552 /// unsatisfiable constraints. If this is a closure, returns the
553 /// region requirements to propagate to our creator, if any.
554 #[instrument(skip(self, infcx, body, polonius_output), level = "debug")]
557 infcx: &InferCtxt<'tcx>,
558 param_env: ty::ParamEnv<'tcx>,
560 polonius_output: Option<Rc<PoloniusOutput>>,
561 ) -> (Option<ClosureRegionRequirements<'tcx>>, RegionErrors<'tcx>) {
562 let mir_def_id = body.source.def_id();
563 self.propagate_constraints(body);
565 let mut errors_buffer = RegionErrors::new(infcx.tcx);
567 // If this is a closure, we can propagate unsatisfied
568 // `outlives_requirements` to our creator, so create a vector
569 // to store those. Otherwise, we'll pass in `None` to the
570 // functions below, which will trigger them to report errors
572 let mut outlives_requirements = infcx.tcx.is_typeck_child(mir_def_id).then(Vec::new);
574 self.check_type_tests(
578 outlives_requirements.as_mut(),
582 // In Polonius mode, the errors about missing universal region relations are in the output
583 // and need to be emitted or propagated. Otherwise, we need to check whether the
584 // constraints were too strong, and if so, emit or propagate those errors.
585 if infcx.tcx.sess.opts.unstable_opts.polonius {
586 self.check_polonius_subset_errors(
587 outlives_requirements.as_mut(),
589 polonius_output.expect("Polonius output is unavailable despite `-Z polonius`"),
592 self.check_universal_regions(outlives_requirements.as_mut(), &mut errors_buffer);
595 if errors_buffer.is_empty() {
596 self.check_member_constraints(infcx, &mut errors_buffer);
599 let outlives_requirements = outlives_requirements.unwrap_or_default();
601 if outlives_requirements.is_empty() {
602 (None, errors_buffer)
604 let num_external_vids = self.universal_regions.num_global_and_external_regions();
606 Some(ClosureRegionRequirements { num_external_vids, outlives_requirements }),
612 /// Propagate the region constraints: this will grow the values
613 /// for each region variable until all the constraints are
614 /// satisfied. Note that some values may grow **too** large to be
615 /// feasible, but we check this later.
616 #[instrument(skip(self, _body), level = "debug")]
617 fn propagate_constraints(&mut self, _body: &Body<'tcx>) {
618 debug!("constraints={:#?}", {
619 let mut constraints: Vec<_> = self.constraints.outlives().iter().collect();
620 constraints.sort_by_key(|c| (c.sup, c.sub));
623 .map(|c| (c, self.constraint_sccs.scc(c.sup), self.constraint_sccs.scc(c.sub)))
627 // To propagate constraints, we walk the DAG induced by the
628 // SCC. For each SCC, we visit its successors and compute
629 // their values, then we union all those values to get our
631 let constraint_sccs = self.constraint_sccs.clone();
632 for scc in constraint_sccs.all_sccs() {
633 self.compute_value_for_scc(scc);
636 // Sort the applied member constraints so we can binary search
637 // through them later.
638 self.member_constraints_applied.sort_by_key(|applied| applied.member_region_scc);
641 /// Computes the value of the SCC `scc_a`, which has not yet been
642 /// computed, by unioning the values of its successors.
643 /// Assumes that all successors have been computed already
644 /// (which is assured by iterating over SCCs in dependency order).
645 #[instrument(skip(self), level = "debug")]
646 fn compute_value_for_scc(&mut self, scc_a: ConstraintSccIndex) {
647 let constraint_sccs = self.constraint_sccs.clone();
649 // Walk each SCC `B` such that `A: B`...
650 for &scc_b in constraint_sccs.successors(scc_a) {
653 // ...and add elements from `B` into `A`. One complication
654 // arises because of universes: If `B` contains something
655 // that `A` cannot name, then `A` can only contain `B` if
656 // it outlives static.
657 if self.universe_compatible(scc_b, scc_a) {
658 // `A` can name everything that is in `B`, so just
660 self.scc_values.add_region(scc_a, scc_b);
662 self.add_incompatible_universe(scc_a);
666 // Now take member constraints into account.
667 let member_constraints = self.member_constraints.clone();
668 for m_c_i in member_constraints.indices(scc_a) {
669 self.apply_member_constraint(scc_a, m_c_i, member_constraints.choice_regions(m_c_i));
672 debug!(value = ?self.scc_values.region_value_str(scc_a));
675 /// Invoked for each `R0 member of [R1..Rn]` constraint.
677 /// `scc` is the SCC containing R0, and `choice_regions` are the
678 /// `R1..Rn` regions -- they are always known to be universal
679 /// regions (and if that's not true, we just don't attempt to
680 /// enforce the constraint).
682 /// The current value of `scc` at the time the method is invoked
683 /// is considered a *lower bound*. If possible, we will modify
684 /// the constraint to set it equal to one of the option regions.
685 /// If we make any changes, returns true, else false.
686 #[instrument(skip(self, member_constraint_index), level = "debug")]
687 fn apply_member_constraint(
689 scc: ConstraintSccIndex,
690 member_constraint_index: NllMemberConstraintIndex,
691 choice_regions: &[ty::RegionVid],
693 // Create a mutable vector of the options. We'll try to winnow
695 let mut choice_regions: Vec<ty::RegionVid> = choice_regions.to_vec();
697 // Convert to the SCC representative: sometimes we have inference
698 // variables in the member constraint that wind up equated with
699 // universal regions. The scc representative is the minimal numbered
700 // one from the corresponding scc so it will be the universal region
702 for c_r in &mut choice_regions {
703 let scc = self.constraint_sccs.scc(*c_r);
704 *c_r = self.scc_representatives[scc];
707 // The 'member region' in a member constraint is part of the
708 // hidden type, which must be in the root universe. Therefore,
709 // it cannot have any placeholders in its value.
710 assert!(self.scc_universes[scc] == ty::UniverseIndex::ROOT);
712 self.scc_values.placeholders_contained_in(scc).next().is_none(),
713 "scc {:?} in a member constraint has placeholder value: {:?}",
715 self.scc_values.region_value_str(scc),
718 // The existing value for `scc` is a lower-bound. This will
719 // consist of some set `{P} + {LB}` of points `{P}` and
720 // lower-bound free regions `{LB}`. As each choice region `O`
721 // is a free region, it will outlive the points. But we can
722 // only consider the option `O` if `O: LB`.
723 choice_regions.retain(|&o_r| {
725 .universal_regions_outlived_by(scc)
726 .all(|lb| self.universal_region_relations.outlives(o_r, lb))
728 debug!(?choice_regions, "after lb");
730 // Now find all the *upper bounds* -- that is, each UB is a
731 // free region that must outlive the member region `R0` (`UB:
732 // R0`). Therefore, we need only keep an option `O` if `UB: O`
734 let rev_scc_graph = self.reverse_scc_graph();
735 let universal_region_relations = &self.universal_region_relations;
736 for ub in rev_scc_graph.upper_bounds(scc) {
738 choice_regions.retain(|&o_r| universal_region_relations.outlives(ub, o_r));
740 debug!(?choice_regions, "after ub");
742 // If we ruled everything out, we're done.
743 if choice_regions.is_empty() {
747 // Otherwise, we need to find the minimum remaining choice, if
748 // any, and take that.
749 debug!("choice_regions remaining are {:#?}", choice_regions);
750 let Some(&min_choice) = choice_regions.iter().find(|&r1| {
751 choice_regions.iter().all(|&r2| {
752 self.universal_region_relations.outlives(r2, *r1)
755 debug!("no choice region outlived by all others");
759 let min_choice_scc = self.constraint_sccs.scc(min_choice);
760 debug!(?min_choice, ?min_choice_scc);
761 if self.scc_values.add_region(scc, min_choice_scc) {
762 self.member_constraints_applied.push(AppliedMemberConstraint {
763 member_region_scc: scc,
765 member_constraint_index,
774 /// Returns `true` if all the elements in the value of `scc_b` are nameable
775 /// in `scc_a`. Used during constraint propagation, and only once
776 /// the value of `scc_b` has been computed.
777 fn universe_compatible(&self, scc_b: ConstraintSccIndex, scc_a: ConstraintSccIndex) -> bool {
778 let universe_a = self.scc_universes[scc_a];
780 // Quick check: if scc_b's declared universe is a subset of
781 // scc_a's declared universe (typically, both are ROOT), then
782 // it cannot contain any problematic universe elements.
783 if universe_a.can_name(self.scc_universes[scc_b]) {
787 // Otherwise, we have to iterate over the universe elements in
788 // B's value, and check whether all of them are nameable
790 self.scc_values.placeholders_contained_in(scc_b).all(|p| universe_a.can_name(p.universe))
793 /// Extend `scc` so that it can outlive some placeholder region
794 /// from a universe it can't name; at present, the only way for
795 /// this to be true is if `scc` outlives `'static`. This is
796 /// actually stricter than necessary: ideally, we'd support bounds
797 /// like `for<'a: 'b`>` that might then allow us to approximate
798 /// `'a` with `'b` and not `'static`. But it will have to do for
800 fn add_incompatible_universe(&mut self, scc: ConstraintSccIndex) {
801 debug!("add_incompatible_universe(scc={:?})", scc);
803 let fr_static = self.universal_regions.fr_static;
804 self.scc_values.add_all_points(scc);
805 self.scc_values.add_element(scc, fr_static);
808 /// Once regions have been propagated, this method is used to see
809 /// whether the "type tests" produced by typeck were satisfied;
810 /// type tests encode type-outlives relationships like `T:
811 /// 'a`. See `TypeTest` for more details.
814 infcx: &InferCtxt<'tcx>,
815 param_env: ty::ParamEnv<'tcx>,
817 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
818 errors_buffer: &mut RegionErrors<'tcx>,
822 // Sometimes we register equivalent type-tests that would
823 // result in basically the exact same error being reported to
824 // the user. Avoid that.
825 let mut deduplicate_errors = FxHashSet::default();
827 for type_test in &self.type_tests {
828 debug!("check_type_test: {:?}", type_test);
830 let generic_ty = type_test.generic_kind.to_ty(tcx);
831 if self.eval_verify_bound(
835 type_test.lower_bound,
836 &type_test.verify_bound,
841 if let Some(propagated_outlives_requirements) = &mut propagated_outlives_requirements {
842 if self.try_promote_type_test(
847 propagated_outlives_requirements,
853 // Type-test failed. Report the error.
854 let erased_generic_kind = infcx.tcx.erase_regions(type_test.generic_kind);
856 // Skip duplicate-ish errors.
857 if deduplicate_errors.insert((
859 type_test.lower_bound,
863 "check_type_test: reporting error for erased_generic_kind={:?}, \
864 lower_bound_region={:?}, \
865 type_test.span={:?}",
866 erased_generic_kind, type_test.lower_bound, type_test.span,
869 errors_buffer.push(RegionErrorKind::TypeTestError { type_test: type_test.clone() });
874 /// Invoked when we have some type-test (e.g., `T: 'X`) that we cannot
875 /// prove to be satisfied. If this is a closure, we will attempt to
876 /// "promote" this type-test into our `ClosureRegionRequirements` and
877 /// hence pass it up the creator. To do this, we have to phrase the
878 /// type-test in terms of external free regions, as local free
879 /// regions are not nameable by the closure's creator.
881 /// Promotion works as follows: we first check that the type `T`
882 /// contains only regions that the creator knows about. If this is
883 /// true, then -- as a consequence -- we know that all regions in
884 /// the type `T` are free regions that outlive the closure body. If
885 /// false, then promotion fails.
887 /// Once we've promoted T, we have to "promote" `'X` to some region
888 /// that is "external" to the closure. Generally speaking, a region
889 /// may be the union of some points in the closure body as well as
890 /// various free lifetimes. We can ignore the points in the closure
891 /// body: if the type T can be expressed in terms of external regions,
892 /// we know it outlives the points in the closure body. That
893 /// just leaves the free regions.
895 /// The idea then is to lower the `T: 'X` constraint into multiple
896 /// bounds -- e.g., if `'X` is the union of two free lifetimes,
897 /// `'1` and `'2`, then we would create `T: '1` and `T: '2`.
898 #[instrument(level = "debug", skip(self, infcx, propagated_outlives_requirements))]
899 fn try_promote_type_test(
901 infcx: &InferCtxt<'tcx>,
902 param_env: ty::ParamEnv<'tcx>,
904 type_test: &TypeTest<'tcx>,
905 propagated_outlives_requirements: &mut Vec<ClosureOutlivesRequirement<'tcx>>,
909 let TypeTest { generic_kind, lower_bound, span: _, verify_bound: _ } = type_test;
911 let generic_ty = generic_kind.to_ty(tcx);
912 let Some(subject) = self.try_promote_type_test_subject(infcx, generic_ty) else {
916 debug!("subject = {:?}", subject);
918 let r_scc = self.constraint_sccs.scc(*lower_bound);
921 "lower_bound = {:?} r_scc={:?} universe={:?}",
922 lower_bound, r_scc, self.scc_universes[r_scc]
925 // If the type test requires that `T: 'a` where `'a` is a
926 // placeholder from another universe, that effectively requires
927 // `T: 'static`, so we have to propagate that requirement.
929 // It doesn't matter *what* universe because the promoted `T` will
930 // always be in the root universe.
931 if let Some(p) = self.scc_values.placeholders_contained_in(r_scc).next() {
932 debug!("encountered placeholder in higher universe: {:?}, requiring 'static", p);
933 let static_r = self.universal_regions.fr_static;
934 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
936 outlived_free_region: static_r,
937 blame_span: type_test.span,
938 category: ConstraintCategory::Boring,
941 // we can return here -- the code below might push add'l constraints
942 // but they would all be weaker than this one.
946 // For each region outlived by lower_bound find a non-local,
947 // universal region (it may be the same region) and add it to
948 // `ClosureOutlivesRequirement`.
949 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
950 debug!("universal_region_outlived_by ur={:?}", ur);
951 // Check whether we can already prove that the "subject" outlives `ur`.
952 // If so, we don't have to propagate this requirement to our caller.
954 // To continue the example from the function, if we are trying to promote
955 // a requirement that `T: 'X`, and we know that `'X = '1 + '2` (i.e., the union
956 // `'1` and `'2`), then in this loop `ur` will be `'1` (and `'2`). So here
957 // we check whether `T: '1` is something we *can* prove. If so, no need
958 // to propagate that requirement.
960 // This is needed because -- particularly in the case
961 // where `ur` is a local bound -- we are sometimes in a
962 // position to prove things that our caller cannot. See
963 // #53570 for an example.
964 if self.eval_verify_bound(infcx, param_env, generic_ty, ur, &type_test.verify_bound) {
968 let non_local_ub = self.universal_region_relations.non_local_upper_bounds(ur);
969 debug!("try_promote_type_test: non_local_ub={:?}", non_local_ub);
971 // This is slightly too conservative. To show T: '1, given `'2: '1`
972 // and `'3: '1` we only need to prove that T: '2 *or* T: '3, but to
973 // avoid potential non-determinism we approximate this by requiring
975 for upper_bound in non_local_ub {
976 debug_assert!(self.universal_regions.is_universal_region(upper_bound));
977 debug_assert!(!self.universal_regions.is_local_free_region(upper_bound));
979 let requirement = ClosureOutlivesRequirement {
981 outlived_free_region: upper_bound,
982 blame_span: type_test.span,
983 category: ConstraintCategory::Boring,
985 debug!("try_promote_type_test: pushing {:#?}", requirement);
986 propagated_outlives_requirements.push(requirement);
992 /// When we promote a type test `T: 'r`, we have to convert the
993 /// type `T` into something we can store in a query result (so
994 /// something allocated for `'tcx`). This is problematic if `ty`
995 /// contains regions. During the course of NLL region checking, we
996 /// will have replaced all of those regions with fresh inference
997 /// variables. To create a test subject, we want to replace those
998 /// inference variables with some region from the closure
999 /// signature -- this is not always possible, so this is a
1000 /// fallible process. Presuming we do find a suitable region, we
1001 /// will use it's *external name*, which will be a `RegionKind`
1002 /// variant that can be used in query responses such as
1004 #[instrument(level = "debug", skip(self, infcx))]
1005 fn try_promote_type_test_subject(
1007 infcx: &InferCtxt<'tcx>,
1009 ) -> Option<ClosureOutlivesSubject<'tcx>> {
1010 let tcx = infcx.tcx;
1012 let ty = tcx.fold_regions(ty, |r, _depth| {
1013 let region_vid = self.to_region_vid(r);
1015 // The challenge if this. We have some region variable `r`
1016 // whose value is a set of CFG points and universal
1017 // regions. We want to find if that set is *equivalent* to
1018 // any of the named regions found in the closure.
1020 // To do so, we compute the
1021 // `non_local_universal_upper_bound`. This will be a
1022 // non-local, universal region that is greater than `r`.
1023 // However, it might not be *contained* within `r`, so
1024 // then we further check whether this bound is contained
1025 // in `r`. If so, we can say that `r` is equivalent to the
1028 // Let's work through a few examples. For these, imagine
1029 // that we have 3 non-local regions (I'll denote them as
1030 // `'static`, `'a`, and `'b`, though of course in the code
1031 // they would be represented with indices) where:
1036 // First, let's assume that `r` is some existential
1037 // variable with an inferred value `{'a, 'static}` (plus
1038 // some CFG nodes). In this case, the non-local upper
1039 // bound is `'static`, since that outlives `'a`. `'static`
1040 // is also a member of `r` and hence we consider `r`
1041 // equivalent to `'static` (and replace it with
1044 // Now let's consider the inferred value `{'a, 'b}`. This
1045 // means `r` is effectively `'a | 'b`. I'm not sure if
1046 // this can come about, actually, but assuming it did, we
1047 // would get a non-local upper bound of `'static`. Since
1048 // `'static` is not contained in `r`, we would fail to
1049 // find an equivalent.
1050 let upper_bound = self.non_local_universal_upper_bound(region_vid);
1051 if self.region_contains(region_vid, upper_bound) {
1052 self.definitions[upper_bound].external_name.unwrap_or(r)
1054 // In the case of a failure, use a `ReVar` result. This will
1055 // cause the `needs_infer` later on to return `None`.
1060 debug!("try_promote_type_test_subject: folded ty = {:?}", ty);
1062 // `needs_infer` will only be true if we failed to promote some region.
1063 if ty.needs_infer() {
1067 Some(ClosureOutlivesSubject::Ty(ty))
1070 /// Given some universal or existential region `r`, finds a
1071 /// non-local, universal region `r+` that outlives `r` at entry to (and
1072 /// exit from) the closure. In the worst case, this will be
1075 /// This is used for two purposes. First, if we are propagated
1076 /// some requirement `T: r`, we can use this method to enlarge `r`
1077 /// to something we can encode for our creator (which only knows
1078 /// about non-local, universal regions). It is also used when
1079 /// encoding `T` as part of `try_promote_type_test_subject` (see
1080 /// that fn for details).
1082 /// This is based on the result `'y` of `universal_upper_bound`,
1083 /// except that it converts further takes the non-local upper
1084 /// bound of `'y`, so that the final result is non-local.
1085 fn non_local_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1086 debug!("non_local_universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1088 let lub = self.universal_upper_bound(r);
1090 // Grow further to get smallest universal region known to
1092 let non_local_lub = self.universal_region_relations.non_local_upper_bound(lub);
1094 debug!("non_local_universal_upper_bound: non_local_lub={:?}", non_local_lub);
1099 /// Returns a universally quantified region that outlives the
1100 /// value of `r` (`r` may be existentially or universally
1103 /// Since `r` is (potentially) an existential region, it has some
1104 /// value which may include (a) any number of points in the CFG
1105 /// and (b) any number of `end('x)` elements of universally
1106 /// quantified regions. To convert this into a single universal
1107 /// region we do as follows:
1109 /// - Ignore the CFG points in `'r`. All universally quantified regions
1110 /// include the CFG anyhow.
1111 /// - For each `end('x)` element in `'r`, compute the mutual LUB, yielding
1113 #[instrument(skip(self), level = "debug", ret)]
1114 pub(crate) fn universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1115 debug!(r = %self.region_value_str(r));
1117 // Find the smallest universal region that contains all other
1118 // universal regions within `region`.
1119 let mut lub = self.universal_regions.fr_fn_body;
1120 let r_scc = self.constraint_sccs.scc(r);
1121 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1122 lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1128 /// Like `universal_upper_bound`, but returns an approximation more suitable
1129 /// for diagnostics. If `r` contains multiple disjoint universal regions
1130 /// (e.g. 'a and 'b in `fn foo<'a, 'b> { ... }`, we pick the lower-numbered region.
1131 /// This corresponds to picking named regions over unnamed regions
1132 /// (e.g. picking early-bound regions over a closure late-bound region).
1134 /// This means that the returned value may not be a true upper bound, since
1135 /// only 'static is known to outlive disjoint universal regions.
1136 /// Therefore, this method should only be used in diagnostic code,
1137 /// where displaying *some* named universal region is better than
1138 /// falling back to 'static.
1139 #[instrument(level = "debug", skip(self))]
1140 pub(crate) fn approx_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1141 debug!("{}", self.region_value_str(r));
1143 // Find the smallest universal region that contains all other
1144 // universal regions within `region`.
1145 let mut lub = self.universal_regions.fr_fn_body;
1146 let r_scc = self.constraint_sccs.scc(r);
1147 let static_r = self.universal_regions.fr_static;
1148 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1149 let new_lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1150 debug!(?ur, ?lub, ?new_lub);
1151 // The upper bound of two non-static regions is static: this
1152 // means we know nothing about the relationship between these
1153 // two regions. Pick a 'better' one to use when constructing
1155 if ur != static_r && lub != static_r && new_lub == static_r {
1156 // Prefer the region with an `external_name` - this
1157 // indicates that the region is early-bound, so working with
1158 // it can produce a nicer error.
1159 if self.region_definition(ur).external_name.is_some() {
1161 } else if self.region_definition(lub).external_name.is_some() {
1162 // Leave lub unchanged
1164 // If we get here, we don't have any reason to prefer
1165 // one region over the other. Just pick the
1166 // one with the lower index for now.
1167 lub = std::cmp::min(ur, lub);
1179 /// Tests if `test` is true when applied to `lower_bound` at
1181 fn eval_verify_bound(
1183 infcx: &InferCtxt<'tcx>,
1184 param_env: ty::ParamEnv<'tcx>,
1185 generic_ty: Ty<'tcx>,
1186 lower_bound: RegionVid,
1187 verify_bound: &VerifyBound<'tcx>,
1189 debug!("eval_verify_bound(lower_bound={:?}, verify_bound={:?})", lower_bound, verify_bound);
1191 match verify_bound {
1192 VerifyBound::IfEq(verify_if_eq_b) => {
1193 self.eval_if_eq(infcx, param_env, generic_ty, lower_bound, *verify_if_eq_b)
1196 VerifyBound::IsEmpty => {
1197 let lower_bound_scc = self.constraint_sccs.scc(lower_bound);
1198 self.scc_values.elements_contained_in(lower_bound_scc).next().is_none()
1201 VerifyBound::OutlivedBy(r) => {
1202 let r_vid = self.to_region_vid(*r);
1203 self.eval_outlives(r_vid, lower_bound)
1206 VerifyBound::AnyBound(verify_bounds) => verify_bounds.iter().any(|verify_bound| {
1207 self.eval_verify_bound(infcx, param_env, generic_ty, lower_bound, verify_bound)
1210 VerifyBound::AllBounds(verify_bounds) => verify_bounds.iter().all(|verify_bound| {
1211 self.eval_verify_bound(infcx, param_env, generic_ty, lower_bound, verify_bound)
1218 infcx: &InferCtxt<'tcx>,
1219 param_env: ty::ParamEnv<'tcx>,
1220 generic_ty: Ty<'tcx>,
1221 lower_bound: RegionVid,
1222 verify_if_eq_b: ty::Binder<'tcx, VerifyIfEq<'tcx>>,
1224 let generic_ty = self.normalize_to_scc_representatives(infcx.tcx, generic_ty);
1225 let verify_if_eq_b = self.normalize_to_scc_representatives(infcx.tcx, verify_if_eq_b);
1226 match test_type_match::extract_verify_if_eq(
1233 let r_vid = self.to_region_vid(r);
1234 self.eval_outlives(r_vid, lower_bound)
1240 /// This is a conservative normalization procedure. It takes every
1241 /// free region in `value` and replaces it with the
1242 /// "representative" of its SCC (see `scc_representatives` field).
1243 /// We are guaranteed that if two values normalize to the same
1244 /// thing, then they are equal; this is a conservative check in
1245 /// that they could still be equal even if they normalize to
1246 /// different results. (For example, there might be two regions
1247 /// with the same value that are not in the same SCC).
1249 /// N.B., this is not an ideal approach and I would like to revisit
1250 /// it. However, it works pretty well in practice. In particular,
1251 /// this is needed to deal with projection outlives bounds like
1254 /// <T as Foo<'0>>::Item: '1
1257 /// In particular, this routine winds up being important when
1258 /// there are bounds like `where <T as Foo<'a>>::Item: 'b` in the
1259 /// environment. In this case, if we can show that `'0 == 'a`,
1260 /// and that `'b: '1`, then we know that the clause is
1261 /// satisfied. In such cases, particularly due to limitations of
1262 /// the trait solver =), we usually wind up with a where-clause like
1263 /// `T: Foo<'a>` in scope, which thus forces `'0 == 'a` to be added as
1264 /// a constraint, and thus ensures that they are in the same SCC.
1266 /// So why can't we do a more correct routine? Well, we could
1267 /// *almost* use the `relate_tys` code, but the way it is
1268 /// currently setup it creates inference variables to deal with
1269 /// higher-ranked things and so forth, and right now the inference
1270 /// context is not permitted to make more inference variables. So
1271 /// we use this kind of hacky solution.
1272 fn normalize_to_scc_representatives<T>(&self, tcx: TyCtxt<'tcx>, value: T) -> T
1274 T: TypeFoldable<'tcx>,
1276 tcx.fold_regions(value, |r, _db| {
1277 let vid = self.to_region_vid(r);
1278 let scc = self.constraint_sccs.scc(vid);
1279 let repr = self.scc_representatives[scc];
1280 tcx.mk_region(ty::ReVar(repr))
1284 // Evaluate whether `sup_region == sub_region`.
1285 fn eval_equal(&self, r1: RegionVid, r2: RegionVid) -> bool {
1286 self.eval_outlives(r1, r2) && self.eval_outlives(r2, r1)
1289 // Evaluate whether `sup_region: sub_region`.
1290 #[instrument(skip(self), level = "debug", ret)]
1291 fn eval_outlives(&self, sup_region: RegionVid, sub_region: RegionVid) -> bool {
1293 "sup_region's value = {:?} universal={:?}",
1294 self.region_value_str(sup_region),
1295 self.universal_regions.is_universal_region(sup_region),
1298 "sub_region's value = {:?} universal={:?}",
1299 self.region_value_str(sub_region),
1300 self.universal_regions.is_universal_region(sub_region),
1303 let sub_region_scc = self.constraint_sccs.scc(sub_region);
1304 let sup_region_scc = self.constraint_sccs.scc(sup_region);
1306 // If we are checking that `'sup: 'sub`, and `'sub` contains
1307 // some placeholder that `'sup` cannot name, then this is only
1308 // true if `'sup` outlives static.
1309 if !self.universe_compatible(sub_region_scc, sup_region_scc) {
1311 "sub universe `{sub_region_scc:?}` is not nameable \
1312 by super `{sup_region_scc:?}`, promoting to static",
1315 return self.eval_outlives(sup_region, self.universal_regions.fr_static);
1318 // Both the `sub_region` and `sup_region` consist of the union
1319 // of some number of universal regions (along with the union
1320 // of various points in the CFG; ignore those points for
1321 // now). Therefore, the sup-region outlives the sub-region if,
1322 // for each universal region R1 in the sub-region, there
1323 // exists some region R2 in the sup-region that outlives R1.
1324 let universal_outlives =
1325 self.scc_values.universal_regions_outlived_by(sub_region_scc).all(|r1| {
1327 .universal_regions_outlived_by(sup_region_scc)
1328 .any(|r2| self.universal_region_relations.outlives(r2, r1))
1331 if !universal_outlives {
1332 debug!("sub region contains a universal region not present in super");
1336 // Now we have to compare all the points in the sub region and make
1337 // sure they exist in the sup region.
1339 if self.universal_regions.is_universal_region(sup_region) {
1340 // Micro-opt: universal regions contain all points.
1341 debug!("super is universal and hence contains all points");
1345 debug!("comparison between points in sup/sub");
1347 self.scc_values.contains_points(sup_region_scc, sub_region_scc)
1350 /// Once regions have been propagated, this method is used to see
1351 /// whether any of the constraints were too strong. In particular,
1352 /// we want to check for a case where a universally quantified
1353 /// region exceeded its bounds. Consider:
1355 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1357 /// In this case, returning `x` requires `&'a u32 <: &'b u32`
1358 /// and hence we establish (transitively) a constraint that
1359 /// `'a: 'b`. The `propagate_constraints` code above will
1360 /// therefore add `end('a)` into the region for `'b` -- but we
1361 /// have no evidence that `'b` outlives `'a`, so we want to report
1364 /// If `propagated_outlives_requirements` is `Some`, then we will
1365 /// push unsatisfied obligations into there. Otherwise, we'll
1366 /// report them as errors.
1367 fn check_universal_regions(
1369 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1370 errors_buffer: &mut RegionErrors<'tcx>,
1372 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1373 match fr_definition.origin {
1374 NllRegionVariableOrigin::FreeRegion => {
1375 // Go through each of the universal regions `fr` and check that
1376 // they did not grow too large, accumulating any requirements
1377 // for our caller into the `outlives_requirements` vector.
1378 self.check_universal_region(
1380 &mut propagated_outlives_requirements,
1385 NllRegionVariableOrigin::Placeholder(placeholder) => {
1386 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1389 NllRegionVariableOrigin::Existential { .. } => {
1390 // nothing to check here
1396 /// Checks if Polonius has found any unexpected free region relations.
1398 /// In Polonius terms, a "subset error" (or "illegal subset relation error") is the equivalent
1399 /// of NLL's "checking if any region constraints were too strong": a placeholder origin `'a`
1400 /// was unexpectedly found to be a subset of another placeholder origin `'b`, and means in NLL
1401 /// terms that the "longer free region" `'a` outlived the "shorter free region" `'b`.
1403 /// More details can be found in this blog post by Niko:
1404 /// <https://smallcultfollowing.com/babysteps/blog/2019/01/17/polonius-and-region-errors/>
1406 /// In the canonical example
1408 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1410 /// returning `x` requires `&'a u32 <: &'b u32` and hence we establish (transitively) a
1411 /// constraint that `'a: 'b`. It is an error that we have no evidence that this
1412 /// constraint holds.
1414 /// If `propagated_outlives_requirements` is `Some`, then we will
1415 /// push unsatisfied obligations into there. Otherwise, we'll
1416 /// report them as errors.
1417 fn check_polonius_subset_errors(
1419 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1420 errors_buffer: &mut RegionErrors<'tcx>,
1421 polonius_output: Rc<PoloniusOutput>,
1424 "check_polonius_subset_errors: {} subset_errors",
1425 polonius_output.subset_errors.len()
1428 // Similarly to `check_universal_regions`: a free region relation, which was not explicitly
1429 // declared ("known") was found by Polonius, so emit an error, or propagate the
1430 // requirements for our caller into the `propagated_outlives_requirements` vector.
1432 // Polonius doesn't model regions ("origins") as CFG-subsets or durations, but the
1433 // `longer_fr` and `shorter_fr` terminology will still be used here, for consistency with
1434 // the rest of the NLL infrastructure. The "subset origin" is the "longer free region",
1435 // and the "superset origin" is the outlived "shorter free region".
1437 // Note: Polonius will produce a subset error at every point where the unexpected
1438 // `longer_fr`'s "placeholder loan" is contained in the `shorter_fr`. This can be helpful
1439 // for diagnostics in the future, e.g. to point more precisely at the key locations
1440 // requiring this constraint to hold. However, the error and diagnostics code downstream
1441 // expects that these errors are not duplicated (and that they are in a certain order).
1442 // Otherwise, diagnostics messages such as the ones giving names like `'1` to elided or
1443 // anonymous lifetimes for example, could give these names differently, while others like
1444 // the outlives suggestions or the debug output from `#[rustc_regions]` would be
1445 // duplicated. The polonius subset errors are deduplicated here, while keeping the
1446 // CFG-location ordering.
1447 let mut subset_errors: Vec<_> = polonius_output
1450 .flat_map(|(_location, subset_errors)| subset_errors.iter())
1452 subset_errors.sort();
1453 subset_errors.dedup();
1455 for (longer_fr, shorter_fr) in subset_errors.into_iter() {
1457 "check_polonius_subset_errors: subset_error longer_fr={:?},\
1459 longer_fr, shorter_fr
1462 let propagated = self.try_propagate_universal_region_error(
1465 &mut propagated_outlives_requirements,
1467 if propagated == RegionRelationCheckResult::Error {
1468 errors_buffer.push(RegionErrorKind::RegionError {
1469 longer_fr: *longer_fr,
1470 shorter_fr: *shorter_fr,
1471 fr_origin: NllRegionVariableOrigin::FreeRegion,
1477 // Handle the placeholder errors as usual, until the chalk-rustc-polonius triumvirate has
1478 // a more complete picture on how to separate this responsibility.
1479 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1480 match fr_definition.origin {
1481 NllRegionVariableOrigin::FreeRegion => {
1482 // handled by polonius above
1485 NllRegionVariableOrigin::Placeholder(placeholder) => {
1486 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1489 NllRegionVariableOrigin::Existential { .. } => {
1490 // nothing to check here
1496 /// Checks the final value for the free region `fr` to see if it
1497 /// grew too large. In particular, examine what `end(X)` points
1498 /// wound up in `fr`'s final value; for each `end(X)` where `X !=
1499 /// fr`, we want to check that `fr: X`. If not, that's either an
1500 /// error, or something we have to propagate to our creator.
1502 /// Things that are to be propagated are accumulated into the
1503 /// `outlives_requirements` vector.
1504 #[instrument(skip(self, propagated_outlives_requirements, errors_buffer), level = "debug")]
1505 fn check_universal_region(
1507 longer_fr: RegionVid,
1508 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1509 errors_buffer: &mut RegionErrors<'tcx>,
1511 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1513 // Because this free region must be in the ROOT universe, we
1514 // know it cannot contain any bound universes.
1515 assert!(self.scc_universes[longer_fr_scc] == ty::UniverseIndex::ROOT);
1516 debug_assert!(self.scc_values.placeholders_contained_in(longer_fr_scc).next().is_none());
1518 // Only check all of the relations for the main representative of each
1519 // SCC, otherwise just check that we outlive said representative. This
1520 // reduces the number of redundant relations propagated out of
1522 // Note that the representative will be a universal region if there is
1523 // one in this SCC, so we will always check the representative here.
1524 let representative = self.scc_representatives[longer_fr_scc];
1525 if representative != longer_fr {
1526 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1529 propagated_outlives_requirements,
1531 errors_buffer.push(RegionErrorKind::RegionError {
1533 shorter_fr: representative,
1534 fr_origin: NllRegionVariableOrigin::FreeRegion,
1541 // Find every region `o` such that `fr: o`
1542 // (because `fr` includes `end(o)`).
1543 let mut error_reported = false;
1544 for shorter_fr in self.scc_values.universal_regions_outlived_by(longer_fr_scc) {
1545 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1548 propagated_outlives_requirements,
1550 // We only report the first region error. Subsequent errors are hidden so as
1551 // not to overwhelm the user, but we do record them so as to potentially print
1552 // better diagnostics elsewhere...
1553 errors_buffer.push(RegionErrorKind::RegionError {
1556 fr_origin: NllRegionVariableOrigin::FreeRegion,
1557 is_reported: !error_reported,
1560 error_reported = true;
1565 /// Checks that we can prove that `longer_fr: shorter_fr`. If we can't we attempt to propagate
1566 /// the constraint outward (e.g. to a closure environment), but if that fails, there is an
1568 fn check_universal_region_relation(
1570 longer_fr: RegionVid,
1571 shorter_fr: RegionVid,
1572 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1573 ) -> RegionRelationCheckResult {
1574 // If it is known that `fr: o`, carry on.
1575 if self.universal_region_relations.outlives(longer_fr, shorter_fr) {
1576 RegionRelationCheckResult::Ok
1578 // If we are not in a context where we can't propagate errors, or we
1579 // could not shrink `fr` to something smaller, then just report an
1582 // Note: in this case, we use the unapproximated regions to report the
1583 // error. This gives better error messages in some cases.
1584 self.try_propagate_universal_region_error(
1587 propagated_outlives_requirements,
1592 /// Attempt to propagate a region error (e.g. `'a: 'b`) that is not met to a closure's
1593 /// creator. If we cannot, then the caller should report an error to the user.
1594 fn try_propagate_universal_region_error(
1596 longer_fr: RegionVid,
1597 shorter_fr: RegionVid,
1598 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1599 ) -> RegionRelationCheckResult {
1600 if let Some(propagated_outlives_requirements) = propagated_outlives_requirements {
1601 // Shrink `longer_fr` until we find a non-local region (if we do).
1602 // We'll call it `fr-` -- it's ever so slightly smaller than
1604 if let Some(fr_minus) = self.universal_region_relations.non_local_lower_bound(longer_fr)
1606 debug!("try_propagate_universal_region_error: fr_minus={:?}", fr_minus);
1608 let blame_span_category = self.find_outlives_blame_span(
1610 NllRegionVariableOrigin::FreeRegion,
1614 // Grow `shorter_fr` until we find some non-local regions. (We
1615 // always will.) We'll call them `shorter_fr+` -- they're ever
1616 // so slightly larger than `shorter_fr`.
1617 let shorter_fr_plus =
1618 self.universal_region_relations.non_local_upper_bounds(shorter_fr);
1620 "try_propagate_universal_region_error: shorter_fr_plus={:?}",
1623 for fr in shorter_fr_plus {
1624 // Push the constraint `fr-: shorter_fr+`
1625 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
1626 subject: ClosureOutlivesSubject::Region(fr_minus),
1627 outlived_free_region: fr,
1628 blame_span: blame_span_category.1.span,
1629 category: blame_span_category.0,
1632 return RegionRelationCheckResult::Propagated;
1636 RegionRelationCheckResult::Error
1639 fn check_bound_universal_region(
1641 longer_fr: RegionVid,
1642 placeholder: ty::PlaceholderRegion,
1643 errors_buffer: &mut RegionErrors<'tcx>,
1645 debug!("check_bound_universal_region(fr={:?}, placeholder={:?})", longer_fr, placeholder,);
1647 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1648 debug!("check_bound_universal_region: longer_fr_scc={:?}", longer_fr_scc,);
1650 for error_element in self.scc_values.elements_contained_in(longer_fr_scc) {
1651 match error_element {
1652 RegionElement::Location(_) | RegionElement::RootUniversalRegion(_) => {}
1653 // If we have some bound universal region `'a`, then the only
1654 // elements it can contain is itself -- we don't know anything
1656 RegionElement::PlaceholderRegion(placeholder1) => {
1657 if placeholder == placeholder1 {
1663 errors_buffer.push(RegionErrorKind::BoundUniversalRegionError {
1669 // Stop after the first error, it gets too noisy otherwise, and does not provide more information.
1672 debug!("check_bound_universal_region: all bounds satisfied");
1675 #[instrument(level = "debug", skip(self, infcx, errors_buffer))]
1676 fn check_member_constraints(
1678 infcx: &InferCtxt<'tcx>,
1679 errors_buffer: &mut RegionErrors<'tcx>,
1681 let member_constraints = self.member_constraints.clone();
1682 for m_c_i in member_constraints.all_indices() {
1684 let m_c = &member_constraints[m_c_i];
1685 let member_region_vid = m_c.member_region_vid;
1688 value = ?self.region_value_str(member_region_vid),
1690 let choice_regions = member_constraints.choice_regions(m_c_i);
1691 debug!(?choice_regions);
1693 // Did the member region wind up equal to any of the option regions?
1695 choice_regions.iter().find(|&&o_r| self.eval_equal(o_r, m_c.member_region_vid))
1697 debug!("evaluated as equal to {:?}", o);
1701 // If not, report an error.
1702 let member_region = infcx.tcx.mk_region(ty::ReVar(member_region_vid));
1703 errors_buffer.push(RegionErrorKind::UnexpectedHiddenRegion {
1704 span: m_c.definition_span,
1705 hidden_ty: m_c.hidden_ty,
1712 /// We have a constraint `fr1: fr2` that is not satisfied, where
1713 /// `fr2` represents some universal region. Here, `r` is some
1714 /// region where we know that `fr1: r` and this function has the
1715 /// job of determining whether `r` is "to blame" for the fact that
1716 /// `fr1: fr2` is required.
1718 /// This is true under two conditions:
1721 /// - `fr2` is `'static` and `r` is some placeholder in a universe
1722 /// that cannot be named by `fr1`; in that case, we will require
1723 /// that `fr1: 'static` because it is the only way to `fr1: r` to
1724 /// be satisfied. (See `add_incompatible_universe`.)
1725 pub(crate) fn provides_universal_region(
1731 debug!("provides_universal_region(r={:?}, fr1={:?}, fr2={:?})", r, fr1, fr2);
1734 fr2 == self.universal_regions.fr_static && self.cannot_name_placeholder(fr1, r)
1737 debug!("provides_universal_region: result = {:?}", result);
1741 /// If `r2` represents a placeholder region, then this returns
1742 /// `true` if `r1` cannot name that placeholder in its
1743 /// value; otherwise, returns `false`.
1744 pub(crate) fn cannot_name_placeholder(&self, r1: RegionVid, r2: RegionVid) -> bool {
1745 debug!("cannot_name_value_of(r1={:?}, r2={:?})", r1, r2);
1747 match self.definitions[r2].origin {
1748 NllRegionVariableOrigin::Placeholder(placeholder) => {
1749 let universe1 = self.definitions[r1].universe;
1751 "cannot_name_value_of: universe1={:?} placeholder={:?}",
1752 universe1, placeholder
1754 universe1.cannot_name(placeholder.universe)
1757 NllRegionVariableOrigin::FreeRegion | NllRegionVariableOrigin::Existential { .. } => {
1763 /// Finds a good `ObligationCause` to blame for the fact that `fr1` outlives `fr2`.
1764 pub(crate) fn find_outlives_blame_span(
1767 fr1_origin: NllRegionVariableOrigin,
1769 ) -> (ConstraintCategory<'tcx>, ObligationCause<'tcx>) {
1770 let BlameConstraint { category, cause, .. } = self
1771 .best_blame_constraint(fr1, fr1_origin, |r| self.provides_universal_region(r, fr1, fr2))
1776 /// Walks the graph of constraints (where `'a: 'b` is considered
1777 /// an edge `'a -> 'b`) to find all paths from `from_region` to
1778 /// `to_region`. The paths are accumulated into the vector
1779 /// `results`. The paths are stored as a series of
1780 /// `ConstraintIndex` values -- in other words, a list of *edges*.
1782 /// Returns: a series of constraints as well as the region `R`
1783 /// that passed the target test.
1784 pub(crate) fn find_constraint_paths_between_regions(
1786 from_region: RegionVid,
1787 target_test: impl Fn(RegionVid) -> bool,
1788 ) -> Option<(Vec<OutlivesConstraint<'tcx>>, RegionVid)> {
1789 let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions);
1790 context[from_region] = Trace::StartRegion;
1792 // Use a deque so that we do a breadth-first search. We will
1793 // stop at the first match, which ought to be the shortest
1794 // path (fewest constraints).
1795 let mut deque = VecDeque::new();
1796 deque.push_back(from_region);
1798 while let Some(r) = deque.pop_front() {
1800 "find_constraint_paths_between_regions: from_region={:?} r={:?} value={}",
1803 self.region_value_str(r),
1806 // Check if we reached the region we were looking for. If so,
1807 // we can reconstruct the path that led to it and return it.
1809 let mut result = vec![];
1812 match context[p].clone() {
1813 Trace::NotVisited => {
1814 bug!("found unvisited region {:?} on path to {:?}", p, r)
1817 Trace::FromOutlivesConstraint(c) => {
1822 Trace::StartRegion => {
1824 return Some((result, r));
1830 // Otherwise, walk over the outgoing constraints and
1831 // enqueue any regions we find, keeping track of how we
1834 // A constraint like `'r: 'x` can come from our constraint
1836 let fr_static = self.universal_regions.fr_static;
1837 let outgoing_edges_from_graph =
1838 self.constraint_graph.outgoing_edges(r, &self.constraints, fr_static);
1840 // Always inline this closure because it can be hot.
1841 let mut handle_constraint = #[inline(always)]
1842 |constraint: OutlivesConstraint<'tcx>| {
1843 debug_assert_eq!(constraint.sup, r);
1844 let sub_region = constraint.sub;
1845 if let Trace::NotVisited = context[sub_region] {
1846 context[sub_region] = Trace::FromOutlivesConstraint(constraint);
1847 deque.push_back(sub_region);
1851 // This loop can be hot.
1852 for constraint in outgoing_edges_from_graph {
1853 handle_constraint(constraint);
1856 // Member constraints can also give rise to `'r: 'x` edges that
1857 // were not part of the graph initially, so watch out for those.
1858 // (But they are extremely rare; this loop is very cold.)
1859 for constraint in self.applied_member_constraints(r) {
1860 let p_c = &self.member_constraints[constraint.member_constraint_index];
1861 let constraint = OutlivesConstraint {
1863 sub: constraint.min_choice,
1864 locations: Locations::All(p_c.definition_span),
1865 span: p_c.definition_span,
1866 category: ConstraintCategory::OpaqueType,
1867 variance_info: ty::VarianceDiagInfo::default(),
1868 from_closure: false,
1870 handle_constraint(constraint);
1877 /// Finds some region R such that `fr1: R` and `R` is live at `elem`.
1878 #[instrument(skip(self), level = "trace", ret)]
1879 pub(crate) fn find_sub_region_live_at(&self, fr1: RegionVid, elem: Location) -> RegionVid {
1880 trace!(scc = ?self.constraint_sccs.scc(fr1));
1881 trace!(universe = ?self.scc_universes[self.constraint_sccs.scc(fr1)]);
1882 self.find_constraint_paths_between_regions(fr1, |r| {
1883 // First look for some `r` such that `fr1: r` and `r` is live at `elem`
1884 trace!(?r, liveness_constraints=?self.liveness_constraints.region_value_str(r));
1885 self.liveness_constraints.contains(r, elem)
1888 // If we fail to find that, we may find some `r` such that
1889 // `fr1: r` and `r` is a placeholder from some universe
1890 // `fr1` cannot name. This would force `fr1` to be
1892 self.find_constraint_paths_between_regions(fr1, |r| {
1893 self.cannot_name_placeholder(fr1, r)
1897 // If we fail to find THAT, it may be that `fr1` is a
1898 // placeholder that cannot "fit" into its SCC. In that
1899 // case, there should be some `r` where `fr1: r` and `fr1` is a
1900 // placeholder that `r` cannot name. We can blame that
1903 // Remember that if `R1: R2`, then the universe of R1
1904 // must be able to name the universe of R2, because R2 will
1905 // be at least `'empty(Universe(R2))`, and `R1` must be at
1906 // larger than that.
1907 self.find_constraint_paths_between_regions(fr1, |r| {
1908 self.cannot_name_placeholder(r, fr1)
1911 .map(|(_path, r)| r)
1915 /// Get the region outlived by `longer_fr` and live at `element`.
1916 pub(crate) fn region_from_element(
1918 longer_fr: RegionVid,
1919 element: &RegionElement,
1922 RegionElement::Location(l) => self.find_sub_region_live_at(longer_fr, l),
1923 RegionElement::RootUniversalRegion(r) => r,
1924 RegionElement::PlaceholderRegion(error_placeholder) => self
1927 .find_map(|(r, definition)| match definition.origin {
1928 NllRegionVariableOrigin::Placeholder(p) if p == error_placeholder => Some(r),
1935 /// Get the region definition of `r`.
1936 pub(crate) fn region_definition(&self, r: RegionVid) -> &RegionDefinition<'tcx> {
1937 &self.definitions[r]
1940 /// Check if the SCC of `r` contains `upper`.
1941 pub(crate) fn upper_bound_in_region_scc(&self, r: RegionVid, upper: RegionVid) -> bool {
1942 let r_scc = self.constraint_sccs.scc(r);
1943 self.scc_values.contains(r_scc, upper)
1946 pub(crate) fn universal_regions(&self) -> &UniversalRegions<'tcx> {
1947 self.universal_regions.as_ref()
1950 /// Tries to find the best constraint to blame for the fact that
1951 /// `R: from_region`, where `R` is some region that meets
1952 /// `target_test`. This works by following the constraint graph,
1953 /// creating a constraint path that forces `R` to outlive
1954 /// `from_region`, and then finding the best choices within that
1956 #[instrument(level = "debug", skip(self, target_test))]
1957 pub(crate) fn best_blame_constraint(
1959 from_region: RegionVid,
1960 from_region_origin: NllRegionVariableOrigin,
1961 target_test: impl Fn(RegionVid) -> bool,
1962 ) -> (BlameConstraint<'tcx>, Vec<ExtraConstraintInfo>) {
1964 let (path, target_region) =
1965 self.find_constraint_paths_between_regions(from_region, target_test).unwrap();
1970 "{:?} ({:?}: {:?})",
1972 self.constraint_sccs.scc(c.sup),
1973 self.constraint_sccs.scc(c.sub),
1975 .collect::<Vec<_>>()
1978 let mut extra_info = vec![];
1979 for constraint in path.iter() {
1980 let outlived = constraint.sub;
1981 let Some(origin) = self.var_infos.get(outlived) else { continue; };
1982 let RegionVariableOrigin::Nll(NllRegionVariableOrigin::Placeholder(p)) = origin.origin else { continue; };
1983 debug!(?constraint, ?p);
1984 let ConstraintCategory::Predicate(span) = constraint.category else { continue; };
1985 extra_info.push(ExtraConstraintInfo::PlaceholderFromPredicate(span));
1986 // We only want to point to one
1990 // We try to avoid reporting a `ConstraintCategory::Predicate` as our best constraint.
1991 // Instead, we use it to produce an improved `ObligationCauseCode`.
1992 // FIXME - determine what we should do if we encounter multiple `ConstraintCategory::Predicate`
1993 // constraints. Currently, we just pick the first one.
1994 let cause_code = path
1996 .find_map(|constraint| {
1997 if let ConstraintCategory::Predicate(predicate_span) = constraint.category {
1998 // We currently do not store the `DefId` in the `ConstraintCategory`
1999 // for performances reasons. The error reporting code used by NLL only
2000 // uses the span, so this doesn't cause any problems at the moment.
2001 Some(ObligationCauseCode::BindingObligation(
2002 CRATE_DEF_ID.to_def_id(),
2009 .unwrap_or_else(|| ObligationCauseCode::MiscObligation);
2011 // Classify each of the constraints along the path.
2012 let mut categorized_path: Vec<BlameConstraint<'tcx>> = path
2014 .map(|constraint| BlameConstraint {
2015 category: constraint.category,
2016 from_closure: constraint.from_closure,
2017 cause: ObligationCause::new(constraint.span, CRATE_HIR_ID, cause_code.clone()),
2018 variance_info: constraint.variance_info,
2019 outlives_constraint: *constraint,
2022 debug!("categorized_path={:#?}", categorized_path);
2024 // To find the best span to cite, we first try to look for the
2025 // final constraint that is interesting and where the `sup` is
2026 // not unified with the ultimate target region. The reason
2027 // for this is that we have a chain of constraints that lead
2028 // from the source to the target region, something like:
2030 // '0: '1 ('0 is the source)
2035 // '5: '6 ('6 is the target)
2037 // Some of those regions are unified with `'6` (in the same
2038 // SCC). We want to screen those out. After that point, the
2039 // "closest" constraint we have to the end is going to be the
2040 // most likely to be the point where the value escapes -- but
2041 // we still want to screen for an "interesting" point to
2042 // highlight (e.g., a call site or something).
2043 let target_scc = self.constraint_sccs.scc(target_region);
2044 let mut range = 0..path.len();
2046 // As noted above, when reporting an error, there is typically a chain of constraints
2047 // leading from some "source" region which must outlive some "target" region.
2048 // In most cases, we prefer to "blame" the constraints closer to the target --
2049 // but there is one exception. When constraints arise from higher-ranked subtyping,
2050 // we generally prefer to blame the source value,
2051 // as the "target" in this case tends to be some type annotation that the user gave.
2052 // Therefore, if we find that the region origin is some instantiation
2053 // of a higher-ranked region, we start our search from the "source" point
2054 // rather than the "target", and we also tweak a few other things.
2056 // An example might be this bit of Rust code:
2059 // let x: fn(&'static ()) = |_| {};
2060 // let y: for<'a> fn(&'a ()) = x;
2063 // In MIR, this will be converted into a combination of assignments and type ascriptions.
2064 // In particular, the 'static is imposed through a type ascription:
2068 // AscribeUserType(x, fn(&'static ())
2072 // We wind up ultimately with constraints like
2075 // !a: 'temp1 // from the `y = x` statement
2077 // 'temp2: 'static // from the AscribeUserType
2080 // and here we prefer to blame the source (the y = x statement).
2081 let blame_source = match from_region_origin {
2082 NllRegionVariableOrigin::FreeRegion
2083 | NllRegionVariableOrigin::Existential { from_forall: false } => true,
2084 NllRegionVariableOrigin::Placeholder(_)
2085 | NllRegionVariableOrigin::Existential { from_forall: true } => false,
2088 let find_region = |i: &usize| {
2089 let constraint = &path[*i];
2091 let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
2094 match categorized_path[*i].category {
2095 ConstraintCategory::OpaqueType
2096 | ConstraintCategory::Boring
2097 | ConstraintCategory::BoringNoLocation
2098 | ConstraintCategory::Internal
2099 | ConstraintCategory::Predicate(_) => false,
2100 ConstraintCategory::TypeAnnotation
2101 | ConstraintCategory::Return(_)
2102 | ConstraintCategory::Yield => true,
2103 _ => constraint_sup_scc != target_scc,
2107 categorized_path[*i].category,
2108 ConstraintCategory::OpaqueType
2109 | ConstraintCategory::Boring
2110 | ConstraintCategory::BoringNoLocation
2111 | ConstraintCategory::Internal
2112 | ConstraintCategory::Predicate(_)
2118 if blame_source { range.rev().find(find_region) } else { range.find(find_region) };
2120 debug!(?best_choice, ?blame_source, ?extra_info);
2122 if let Some(i) = best_choice {
2123 if let Some(next) = categorized_path.get(i + 1) {
2124 if matches!(categorized_path[i].category, ConstraintCategory::Return(_))
2125 && next.category == ConstraintCategory::OpaqueType
2127 // The return expression is being influenced by the return type being
2128 // impl Trait, point at the return type and not the return expr.
2129 return (next.clone(), extra_info);
2133 if categorized_path[i].category == ConstraintCategory::Return(ReturnConstraint::Normal)
2135 let field = categorized_path.iter().find_map(|p| {
2136 if let ConstraintCategory::ClosureUpvar(f) = p.category {
2143 if let Some(field) = field {
2144 categorized_path[i].category =
2145 ConstraintCategory::Return(ReturnConstraint::ClosureUpvar(field));
2149 return (categorized_path[i].clone(), extra_info);
2152 // If that search fails, that is.. unusual. Maybe everything
2153 // is in the same SCC or something. In that case, find what
2154 // appears to be the most interesting point to report to the
2155 // user via an even more ad-hoc guess.
2156 categorized_path.sort_by(|p0, p1| p0.category.cmp(&p1.category));
2157 debug!("sorted_path={:#?}", categorized_path);
2159 (categorized_path.remove(0), extra_info)
2162 pub(crate) fn universe_info(&self, universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
2163 self.universe_causes[&universe].clone()
2166 /// Tries to find the terminator of the loop in which the region 'r' resides.
2167 /// Returns the location of the terminator if found.
2168 pub(crate) fn find_loop_terminator_location(
2172 ) -> Option<Location> {
2173 let scc = self.constraint_sccs.scc(r.to_region_vid());
2174 let locations = self.scc_values.locations_outlived_by(scc);
2175 for location in locations {
2176 let bb = &body[location.block];
2177 if let Some(terminator) = &bb.terminator {
2178 // terminator of a loop should be TerminatorKind::FalseUnwind
2179 if let TerminatorKind::FalseUnwind { .. } = terminator.kind {
2180 return Some(location);
2188 impl<'tcx> RegionDefinition<'tcx> {
2189 fn new(universe: ty::UniverseIndex, rv_origin: RegionVariableOrigin) -> Self {
2190 // Create a new region definition. Note that, for free
2191 // regions, the `external_name` field gets updated later in
2192 // `init_universal_regions`.
2194 let origin = match rv_origin {
2195 RegionVariableOrigin::Nll(origin) => origin,
2196 _ => NllRegionVariableOrigin::Existential { from_forall: false },
2199 Self { origin, universe, external_name: None }
2203 #[derive(Clone, Debug)]
2204 pub struct BlameConstraint<'tcx> {
2205 pub category: ConstraintCategory<'tcx>,
2206 pub from_closure: bool,
2207 pub cause: ObligationCause<'tcx>,
2208 pub variance_info: ty::VarianceDiagInfo<'tcx>,
2209 pub outlives_constraint: OutlivesConstraint<'tcx>,