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_hir::def_id::DefId;
9 use rustc_index::vec::IndexVec;
10 use rustc_infer::infer::canonical::QueryOutlivesConstraint;
11 use rustc_infer::infer::region_constraints::{GenericKind, VarInfos, VerifyBound};
12 use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin};
13 use rustc_middle::mir::{
14 Body, ClosureOutlivesRequirement, ClosureOutlivesSubject, ClosureRegionRequirements,
15 ConstraintCategory, Local, Location, ReturnConstraint,
17 use rustc_middle::ty::{self, subst::SubstsRef, RegionVid, Ty, TyCtxt, TypeFoldable};
20 use crate::borrow_check::{
22 graph::NormalConstraintGraph, ConstraintSccIndex, OutlivesConstraint, OutlivesConstraintSet,
24 diagnostics::{RegionErrorKind, RegionErrors},
25 member_constraints::{MemberConstraintSet, NllMemberConstraintIndex},
26 nll::{PoloniusOutput, ToRegionVid},
27 region_infer::reverse_sccs::ReverseSccGraph,
28 region_infer::values::{
29 LivenessValues, PlaceholderIndices, RegionElement, RegionValueElements, RegionValues,
32 type_check::{free_region_relations::UniversalRegionRelations, Locations},
33 universal_regions::UniversalRegions,
43 pub struct RegionInferenceContext<'tcx> {
44 /// Contains the definition for every region variable. Region
45 /// variables are identified by their index (`RegionVid`). The
46 /// definition contains information about where the region came
47 /// from as well as its final inferred value.
48 definitions: IndexVec<RegionVid, RegionDefinition<'tcx>>,
50 /// The liveness constraints added to each region. For most
51 /// regions, these start out empty and steadily grow, though for
52 /// each universally quantified region R they start out containing
53 /// the entire CFG and `end(R)`.
54 liveness_constraints: LivenessValues<RegionVid>,
56 /// The outlives constraints computed by the type-check.
57 constraints: Frozen<OutlivesConstraintSet<'tcx>>,
59 /// The constraint-set, but in graph form, making it easy to traverse
60 /// the constraints adjacent to a particular region. Used to construct
61 /// the SCC (see `constraint_sccs`) and for error reporting.
62 constraint_graph: Frozen<NormalConstraintGraph>,
64 /// The SCC computed from `constraints` and the constraint
65 /// graph. We have an edge from SCC A to SCC B if `A: B`. Used to
66 /// compute the values of each region.
67 constraint_sccs: Rc<Sccs<RegionVid, ConstraintSccIndex>>,
69 /// Reverse of the SCC constraint graph -- i.e., an edge `A -> B` exists if
70 /// `B: A`. This is used to compute the universal regions that are required
71 /// to outlive a given SCC. Computed lazily.
72 rev_scc_graph: Option<Rc<ReverseSccGraph>>,
74 /// The "R0 member of [R1..Rn]" constraints, indexed by SCC.
75 member_constraints: Rc<MemberConstraintSet<'tcx, ConstraintSccIndex>>,
77 /// Records the member constraints that we applied to each scc.
78 /// This is useful for error reporting. Once constraint
79 /// propagation is done, this vector is sorted according to
80 /// `member_region_scc`.
81 member_constraints_applied: Vec<AppliedMemberConstraint>,
83 /// Map closure bounds to a `Span` that should be used for error reporting.
84 closure_bounds_mapping:
85 FxHashMap<Location, FxHashMap<(RegionVid, RegionVid), (ConstraintCategory, Span)>>,
87 /// Contains the minimum universe of any variable within the same
88 /// SCC. We will ensure that no SCC contains values that are not
89 /// visible from this index.
90 scc_universes: IndexVec<ConstraintSccIndex, ty::UniverseIndex>,
92 /// Contains a "representative" from each SCC. This will be the
93 /// minimal RegionVid belonging to that universe. It is used as a
94 /// kind of hacky way to manage checking outlives relationships,
95 /// since we can 'canonicalize' each region to the representative
96 /// of its SCC and be sure that -- if they have the same repr --
97 /// they *must* be equal (though not having the same repr does not
98 /// mean they are unequal).
99 scc_representatives: IndexVec<ConstraintSccIndex, ty::RegionVid>,
101 /// The final inferred values of the region variables; we compute
102 /// one value per SCC. To get the value for any given *region*,
103 /// you first find which scc it is a part of.
104 scc_values: RegionValues<ConstraintSccIndex>,
106 /// Type constraints that we check after solving.
107 type_tests: Vec<TypeTest<'tcx>>,
109 /// Information about the universally quantified regions in scope
110 /// on this function.
111 universal_regions: Rc<UniversalRegions<'tcx>>,
113 /// Information about how the universally quantified regions in
114 /// scope on this function relate to one another.
115 universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
118 /// Each time that `apply_member_constraint` is successful, it appends
119 /// one of these structs to the `member_constraints_applied` field.
120 /// This is used in error reporting to trace out what happened.
122 /// The way that `apply_member_constraint` works is that it effectively
123 /// adds a new lower bound to the SCC it is analyzing: so you wind up
124 /// with `'R: 'O` where `'R` is the pick-region and `'O` is the
125 /// minimal viable option.
126 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd)]
127 pub(crate) struct AppliedMemberConstraint {
128 /// The SCC that was affected. (The "member region".)
130 /// The vector if `AppliedMemberConstraint` elements is kept sorted
132 pub(in crate::borrow_check) member_region_scc: ConstraintSccIndex,
134 /// The "best option" that `apply_member_constraint` found -- this was
135 /// added as an "ad-hoc" lower-bound to `member_region_scc`.
136 pub(in crate::borrow_check) min_choice: ty::RegionVid,
138 /// The "member constraint index" -- we can find out details about
139 /// the constraint from
140 /// `set.member_constraints[member_constraint_index]`.
141 pub(in crate::borrow_check) member_constraint_index: NllMemberConstraintIndex,
144 pub(crate) struct RegionDefinition<'tcx> {
145 /// What kind of variable is this -- a free region? existential
146 /// variable? etc. (See the `NllRegionVariableOrigin` for more
148 pub(in crate::borrow_check) origin: NllRegionVariableOrigin,
150 /// Which universe is this region variable defined in? This is
151 /// most often `ty::UniverseIndex::ROOT`, but when we encounter
152 /// forall-quantifiers like `for<'a> { 'a = 'b }`, we would create
153 /// the variable for `'a` in a fresh universe that extends ROOT.
154 pub(in crate::borrow_check) universe: ty::UniverseIndex,
156 /// If this is 'static or an early-bound region, then this is
157 /// `Some(X)` where `X` is the name of the region.
158 pub(in crate::borrow_check) external_name: Option<ty::Region<'tcx>>,
161 /// N.B., the variants in `Cause` are intentionally ordered. Lower
162 /// values are preferred when it comes to error messages. Do not
163 /// reorder willy nilly.
164 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
165 pub(crate) enum Cause {
166 /// point inserted because Local was live at the given Location
167 LiveVar(Local, Location),
169 /// point inserted because Local was dropped at the given Location
170 DropVar(Local, Location),
173 /// A "type test" corresponds to an outlives constraint between a type
174 /// and a lifetime, like `T: 'x` or `<T as Foo>::Bar: 'x`. They are
175 /// translated from the `Verify` region constraints in the ordinary
176 /// inference context.
178 /// These sorts of constraints are handled differently than ordinary
179 /// constraints, at least at present. During type checking, the
180 /// `InferCtxt::process_registered_region_obligations` method will
181 /// attempt to convert a type test like `T: 'x` into an ordinary
182 /// outlives constraint when possible (for example, `&'a T: 'b` will
183 /// be converted into `'a: 'b` and registered as a `Constraint`).
185 /// In some cases, however, there are outlives relationships that are
186 /// not converted into a region constraint, but rather into one of
187 /// these "type tests". The distinction is that a type test does not
188 /// influence the inference result, but instead just examines the
189 /// values that we ultimately inferred for each region variable and
190 /// checks that they meet certain extra criteria. If not, an error
193 /// One reason for this is that these type tests typically boil down
194 /// to a check like `'a: 'x` where `'a` is a universally quantified
195 /// region -- and therefore not one whose value is really meant to be
196 /// *inferred*, precisely (this is not always the case: one can have a
197 /// type test like `<Foo as Trait<'?0>>::Bar: 'x`, where `'?0` is an
198 /// inference variable). Another reason is that these type tests can
199 /// involve *disjunction* -- that is, they can be satisfied in more
202 /// For more information about this translation, see
203 /// `InferCtxt::process_registered_region_obligations` and
204 /// `InferCtxt::type_must_outlive` in `rustc_infer::infer::InferCtxt`.
205 #[derive(Clone, Debug)]
206 pub struct TypeTest<'tcx> {
207 /// The type `T` that must outlive the region.
208 pub generic_kind: GenericKind<'tcx>,
210 /// The region `'x` that the type must outlive.
211 pub lower_bound: RegionVid,
213 /// Where did this constraint arise and why?
214 pub locations: Locations,
216 /// A test which, if met by the region `'x`, proves that this type
217 /// constraint is satisfied.
218 pub verify_bound: VerifyBound<'tcx>,
221 /// When we have an unmet lifetime constraint, we try to propagate it outward (e.g. to a closure
222 /// environment). If we can't, it is an error.
223 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
224 enum RegionRelationCheckResult {
230 #[derive(Clone, PartialEq, Eq, Debug)]
233 FromOutlivesConstraint(OutlivesConstraint<'tcx>),
237 impl<'tcx> RegionInferenceContext<'tcx> {
238 /// Creates a new region inference context with a total of
239 /// `num_region_variables` valid inference variables; the first N
240 /// of those will be constant regions representing the free
241 /// regions defined in `universal_regions`.
243 /// The `outlives_constraints` and `type_tests` are an initial set
244 /// of constraints produced by the MIR type check.
245 pub(in crate::borrow_check) fn new(
247 universal_regions: Rc<UniversalRegions<'tcx>>,
248 placeholder_indices: Rc<PlaceholderIndices>,
249 universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
250 outlives_constraints: OutlivesConstraintSet<'tcx>,
251 member_constraints_in: MemberConstraintSet<'tcx, RegionVid>,
252 closure_bounds_mapping: FxHashMap<
254 FxHashMap<(RegionVid, RegionVid), (ConstraintCategory, Span)>,
256 type_tests: Vec<TypeTest<'tcx>>,
257 liveness_constraints: LivenessValues<RegionVid>,
258 elements: &Rc<RegionValueElements>,
260 // Create a RegionDefinition for each inference variable.
261 let definitions: IndexVec<_, _> = var_infos
263 .map(|info| RegionDefinition::new(info.universe, info.origin))
266 let constraints = Frozen::freeze(outlives_constraints);
267 let constraint_graph = Frozen::freeze(constraints.graph(definitions.len()));
268 let fr_static = universal_regions.fr_static;
269 let constraint_sccs = Rc::new(constraints.compute_sccs(&constraint_graph, fr_static));
272 RegionValues::new(elements, universal_regions.len(), &placeholder_indices);
274 for region in liveness_constraints.rows() {
275 let scc = constraint_sccs.scc(region);
276 scc_values.merge_liveness(scc, region, &liveness_constraints);
279 let scc_universes = Self::compute_scc_universes(&constraint_sccs, &definitions);
281 let scc_representatives = Self::compute_scc_representatives(&constraint_sccs, &definitions);
283 let member_constraints =
284 Rc::new(member_constraints_in.into_mapped(|r| constraint_sccs.scc(r)));
286 let mut result = Self {
288 liveness_constraints,
294 member_constraints_applied: Vec::new(),
295 closure_bounds_mapping,
301 universal_region_relations,
304 result.init_free_and_bound_regions();
309 /// Each SCC is the combination of many region variables which
310 /// have been equated. Therefore, we can associate a universe with
311 /// each SCC which is minimum of all the universes of its
312 /// constituent regions -- this is because whatever value the SCC
313 /// takes on must be a value that each of the regions within the
314 /// SCC could have as well. This implies that the SCC must have
315 /// the minimum, or narrowest, universe.
316 fn compute_scc_universes(
317 constraint_sccs: &Sccs<RegionVid, ConstraintSccIndex>,
318 definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
319 ) -> IndexVec<ConstraintSccIndex, ty::UniverseIndex> {
320 let num_sccs = constraint_sccs.num_sccs();
321 let mut scc_universes = IndexVec::from_elem_n(ty::UniverseIndex::MAX, num_sccs);
323 debug!("compute_scc_universes()");
325 // For each region R in universe U, ensure that the universe for the SCC
326 // that contains R is "no bigger" than U. This effectively sets the universe
327 // for each SCC to be the minimum of the regions within.
328 for (region_vid, region_definition) in definitions.iter_enumerated() {
329 let scc = constraint_sccs.scc(region_vid);
330 let scc_universe = &mut scc_universes[scc];
331 let scc_min = std::cmp::min(region_definition.universe, *scc_universe);
332 if scc_min != *scc_universe {
333 *scc_universe = scc_min;
335 "compute_scc_universes: lowered universe of {scc:?} to {scc_min:?} \
336 because it contains {region_vid:?} in {region_universe:?}",
339 region_vid = region_vid,
340 region_universe = region_definition.universe,
345 // Walk each SCC `A` and `B` such that `A: B`
346 // and ensure that universe(A) can see universe(B).
348 // This serves to enforce the 'empty/placeholder' hierarchy
349 // (described in more detail on `RegionKind`):
354 // empty(U0) placeholder(U1)
359 // In particular, imagine we have variables R0 in U0 and R1
360 // created in U1, and constraints like this;
363 // R1: !1 // R1 outlives the placeholder in U1
364 // R1: R0 // R1 outlives R0
367 // Here, we wish for R1 to be `'static`, because it
368 // cannot outlive `placeholder(U1)` and `empty(U0)` any other way.
370 // Thanks to this loop, what happens is that the `R1: R0`
371 // constraint lowers the universe of `R1` to `U0`, which in turn
372 // means that the `R1: !1` constraint will (later) cause
373 // `R1` to become `'static`.
374 for scc_a in constraint_sccs.all_sccs() {
375 for &scc_b in constraint_sccs.successors(scc_a) {
376 let scc_universe_a = scc_universes[scc_a];
377 let scc_universe_b = scc_universes[scc_b];
378 let scc_universe_min = std::cmp::min(scc_universe_a, scc_universe_b);
379 if scc_universe_a != scc_universe_min {
380 scc_universes[scc_a] = scc_universe_min;
383 "compute_scc_universes: lowered universe of {scc_a:?} to {scc_universe_min:?} \
384 because {scc_a:?}: {scc_b:?} and {scc_b:?} is in universe {scc_universe_b:?}",
387 scc_universe_min = scc_universe_min,
388 scc_universe_b = scc_universe_b
394 debug!("compute_scc_universes: scc_universe = {:#?}", scc_universes);
399 /// For each SCC, we compute a unique `RegionVid` (in fact, the
400 /// minimal one that belongs to the SCC). See
401 /// `scc_representatives` field of `RegionInferenceContext` for
403 fn compute_scc_representatives(
404 constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
405 definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
406 ) -> IndexVec<ConstraintSccIndex, ty::RegionVid> {
407 let num_sccs = constraints_scc.num_sccs();
408 let next_region_vid = definitions.next_index();
409 let mut scc_representatives = IndexVec::from_elem_n(next_region_vid, num_sccs);
411 for region_vid in definitions.indices() {
412 let scc = constraints_scc.scc(region_vid);
413 let prev_min = scc_representatives[scc];
414 scc_representatives[scc] = region_vid.min(prev_min);
420 /// Initializes the region variables for each universally
421 /// quantified region (lifetime parameter). The first N variables
422 /// always correspond to the regions appearing in the function
423 /// signature (both named and anonymous) and where-clauses. This
424 /// function iterates over those regions and initializes them with
429 /// fn foo<'a, 'b>(..) where 'a: 'b
431 /// would initialize two variables like so:
433 /// R0 = { CFG, R0 } // 'a
434 /// R1 = { CFG, R0, R1 } // 'b
436 /// Here, R0 represents `'a`, and it contains (a) the entire CFG
437 /// and (b) any universally quantified regions that it outlives,
438 /// which in this case is just itself. R1 (`'b`) in contrast also
439 /// outlives `'a` and hence contains R0 and R1.
440 fn init_free_and_bound_regions(&mut self) {
441 // Update the names (if any)
442 for (external_name, variable) in self.universal_regions.named_universal_regions() {
444 "init_universal_regions: region {:?} has external name {:?}",
445 variable, external_name
447 self.definitions[variable].external_name = Some(external_name);
450 for variable in self.definitions.indices() {
451 let scc = self.constraint_sccs.scc(variable);
453 match self.definitions[variable].origin {
454 NllRegionVariableOrigin::FreeRegion => {
455 // For each free, universally quantified region X:
457 // Add all nodes in the CFG to liveness constraints
458 self.liveness_constraints.add_all_points(variable);
459 self.scc_values.add_all_points(scc);
461 // Add `end(X)` into the set for X.
462 self.scc_values.add_element(scc, variable);
465 NllRegionVariableOrigin::Placeholder(placeholder) => {
466 // Each placeholder region is only visible from
467 // its universe `ui` and its extensions. So we
468 // can't just add it into `scc` unless the
469 // universe of the scc can name this region.
470 let scc_universe = self.scc_universes[scc];
471 if scc_universe.can_name(placeholder.universe) {
472 self.scc_values.add_element(scc, placeholder);
475 "init_free_and_bound_regions: placeholder {:?} is \
476 not compatible with universe {:?} of its SCC {:?}",
477 placeholder, scc_universe, scc,
479 self.add_incompatible_universe(scc);
483 NllRegionVariableOrigin::RootEmptyRegion
484 | NllRegionVariableOrigin::Existential { .. } => {
485 // For existential, regions, nothing to do.
491 /// Returns an iterator over all the region indices.
492 pub fn regions(&self) -> impl Iterator<Item = RegionVid> {
493 self.definitions.indices()
496 /// Given a universal region in scope on the MIR, returns the
497 /// corresponding index.
499 /// (Panics if `r` is not a registered universal region.)
500 pub fn to_region_vid(&self, r: ty::Region<'tcx>) -> RegionVid {
501 self.universal_regions.to_region_vid(r)
504 /// Adds annotations for `#[rustc_regions]`; see `UniversalRegions::annotate`.
505 crate fn annotate(&self, tcx: TyCtxt<'tcx>, err: &mut rustc_errors::DiagnosticBuilder<'_>) {
506 self.universal_regions.annotate(tcx, err)
509 /// Returns `true` if the region `r` contains the point `p`.
511 /// Panics if called before `solve()` executes,
512 crate fn region_contains(&self, r: impl ToRegionVid, p: impl ToElementIndex) -> bool {
513 let scc = self.constraint_sccs.scc(r.to_region_vid());
514 self.scc_values.contains(scc, p)
517 /// Returns access to the value of `r` for debugging purposes.
518 crate fn region_value_str(&self, r: RegionVid) -> String {
519 let scc = self.constraint_sccs.scc(r.to_region_vid());
520 self.scc_values.region_value_str(scc)
523 /// Returns access to the value of `r` for debugging purposes.
524 crate fn region_universe(&self, r: RegionVid) -> ty::UniverseIndex {
525 let scc = self.constraint_sccs.scc(r.to_region_vid());
526 self.scc_universes[scc]
529 /// Once region solving has completed, this function will return
530 /// the member constraints that were applied to the value of a given
531 /// region `r`. See `AppliedMemberConstraint`.
532 pub(in crate::borrow_check) fn applied_member_constraints(
535 ) -> &[AppliedMemberConstraint] {
536 let scc = self.constraint_sccs.scc(r.to_region_vid());
537 binary_search_util::binary_search_slice(
538 &self.member_constraints_applied,
539 |applied| applied.member_region_scc,
544 /// Performs region inference and report errors if we see any
545 /// unsatisfiable constraints. If this is a closure, returns the
546 /// region requirements to propagate to our creator, if any.
549 infcx: &InferCtxt<'_, 'tcx>,
551 polonius_output: Option<Rc<PoloniusOutput>>,
552 ) -> (Option<ClosureRegionRequirements<'tcx>>, RegionErrors<'tcx>) {
553 let mir_def_id = body.source.def_id();
554 self.propagate_constraints(body, infcx.tcx);
556 let mut errors_buffer = RegionErrors::new();
558 // If this is a closure, we can propagate unsatisfied
559 // `outlives_requirements` to our creator, so create a vector
560 // to store those. Otherwise, we'll pass in `None` to the
561 // functions below, which will trigger them to report errors
563 let mut outlives_requirements = infcx.tcx.is_closure(mir_def_id).then(Vec::new);
565 self.check_type_tests(infcx, body, outlives_requirements.as_mut(), &mut errors_buffer);
567 // In Polonius mode, the errors about missing universal region relations are in the output
568 // and need to be emitted or propagated. Otherwise, we need to check whether the
569 // constraints were too strong, and if so, emit or propagate those errors.
570 if infcx.tcx.sess.opts.debugging_opts.polonius {
571 self.check_polonius_subset_errors(
573 outlives_requirements.as_mut(),
575 polonius_output.expect("Polonius output is unavailable despite `-Z polonius`"),
578 self.check_universal_regions(body, outlives_requirements.as_mut(), &mut errors_buffer);
581 if errors_buffer.is_empty() {
582 self.check_member_constraints(infcx, &mut errors_buffer);
585 let outlives_requirements = outlives_requirements.unwrap_or_default();
587 if outlives_requirements.is_empty() {
588 (None, errors_buffer)
590 let num_external_vids = self.universal_regions.num_global_and_external_regions();
592 Some(ClosureRegionRequirements { num_external_vids, outlives_requirements }),
598 /// Propagate the region constraints: this will grow the values
599 /// for each region variable until all the constraints are
600 /// satisfied. Note that some values may grow **too** large to be
601 /// feasible, but we check this later.
602 fn propagate_constraints(&mut self, _body: &Body<'tcx>, tcx: TyCtxt<'tcx>) {
603 debug!("propagate_constraints()");
605 debug!("propagate_constraints: constraints={:#?}", {
606 let mut constraints: Vec<_> = self.constraints.outlives().iter().collect();
610 .map(|c| (c, self.constraint_sccs.scc(c.sup), self.constraint_sccs.scc(c.sub)))
614 // To propagate constraints, we walk the DAG induced by the
615 // SCC. For each SCC, we visit its successors and compute
616 // their values, then we union all those values to get our
618 let constraint_sccs = self.constraint_sccs.clone();
619 for scc in constraint_sccs.all_sccs() {
620 self.compute_value_for_scc(scc, tcx);
623 // Sort the applied member constraints so we can binary search
624 // through them later.
625 self.member_constraints_applied.sort_by_key(|applied| applied.member_region_scc);
628 /// Computes the value of the SCC `scc_a`, which has not yet been
629 /// computed, by unioning the values of its successors.
630 /// Assumes that all successors have been computed already
631 /// (which is assured by iterating over SCCs in dependency order).
632 fn compute_value_for_scc(&mut self, scc_a: ConstraintSccIndex, tcx: TyCtxt<'tcx>) {
633 let constraint_sccs = self.constraint_sccs.clone();
635 // Walk each SCC `B` such that `A: B`...
636 for &scc_b in constraint_sccs.successors(scc_a) {
637 debug!("propagate_constraint_sccs: scc_a = {:?} scc_b = {:?}", scc_a, scc_b);
639 // ...and add elements from `B` into `A`. One complication
640 // arises because of universes: If `B` contains something
641 // that `A` cannot name, then `A` can only contain `B` if
642 // it outlives static.
643 if self.universe_compatible(scc_b, scc_a) {
644 // `A` can name everything that is in `B`, so just
646 self.scc_values.add_region(scc_a, scc_b);
648 self.add_incompatible_universe(scc_a);
652 // Now take member constraints into account.
653 let member_constraints = self.member_constraints.clone();
654 for m_c_i in member_constraints.indices(scc_a) {
655 self.apply_member_constraint(
659 member_constraints.choice_regions(m_c_i),
664 "propagate_constraint_sccs: scc_a = {:?} has value {:?}",
666 self.scc_values.region_value_str(scc_a),
670 /// Invoked for each `R0 member of [R1..Rn]` constraint.
672 /// `scc` is the SCC containing R0, and `choice_regions` are the
673 /// `R1..Rn` regions -- they are always known to be universal
674 /// regions (and if that's not true, we just don't attempt to
675 /// enforce the constraint).
677 /// The current value of `scc` at the time the method is invoked
678 /// is considered a *lower bound*. If possible, we will modify
679 /// the constraint to set it equal to one of the option regions.
680 /// If we make any changes, returns true, else false.
681 fn apply_member_constraint(
684 scc: ConstraintSccIndex,
685 member_constraint_index: NllMemberConstraintIndex,
686 choice_regions: &[ty::RegionVid],
688 debug!("apply_member_constraint(scc={:?}, choice_regions={:#?})", scc, choice_regions,);
691 choice_regions.iter().find(|&&r| !self.universal_regions.is_universal_region(r))
693 // FIXME(#61773): This case can only occur with
694 // `impl_trait_in_bindings`, I believe, and we are just
695 // opting not to handle it for now. See #61773 for
697 tcx.sess.delay_span_bug(
698 self.member_constraints[member_constraint_index].definition_span,
700 "member constraint for `{:?}` has an option region `{:?}` \
701 that is not a universal region",
702 self.member_constraints[member_constraint_index].opaque_type_def_id, uh_oh,
708 // Create a mutable vector of the options. We'll try to winnow
710 let mut choice_regions: Vec<ty::RegionVid> = choice_regions.to_vec();
712 // The 'member region' in a member constraint is part of the
713 // hidden type, which must be in the root universe. Therefore,
714 // it cannot have any placeholders in its value.
715 assert!(self.scc_universes[scc] == ty::UniverseIndex::ROOT);
717 self.scc_values.placeholders_contained_in(scc).next().is_none(),
718 "scc {:?} in a member constraint has placeholder value: {:?}",
720 self.scc_values.region_value_str(scc),
723 // The existing value for `scc` is a lower-bound. This will
724 // consist of some set `{P} + {LB}` of points `{P}` and
725 // lower-bound free regions `{LB}`. As each choice region `O`
726 // is a free region, it will outlive the points. But we can
727 // only consider the option `O` if `O: LB`.
728 choice_regions.retain(|&o_r| {
730 .universal_regions_outlived_by(scc)
731 .all(|lb| self.universal_region_relations.outlives(o_r, lb))
733 debug!("apply_member_constraint: after lb, choice_regions={:?}", choice_regions);
735 // Now find all the *upper bounds* -- that is, each UB is a
736 // free region that must outlive the member region `R0` (`UB:
737 // R0`). Therefore, we need only keep an option `O` if `UB: O`
739 let rev_scc_graph = self.reverse_scc_graph();
740 let universal_region_relations = &self.universal_region_relations;
741 for ub in rev_scc_graph.upper_bounds(scc) {
742 debug!("apply_member_constraint: ub={:?}", ub);
743 choice_regions.retain(|&o_r| universal_region_relations.outlives(ub, o_r));
745 debug!("apply_member_constraint: after ub, choice_regions={:?}", choice_regions);
747 // If we ruled everything out, we're done.
748 if choice_regions.is_empty() {
752 // Otherwise, we need to find the minimum remaining choice, if
753 // any, and take that.
754 debug!("apply_member_constraint: choice_regions remaining are {:#?}", choice_regions);
755 let min = |r1: ty::RegionVid, r2: ty::RegionVid| -> Option<ty::RegionVid> {
756 let r1_outlives_r2 = self.universal_region_relations.outlives(r1, r2);
757 let r2_outlives_r1 = self.universal_region_relations.outlives(r2, r1);
758 match (r1_outlives_r2, r2_outlives_r1) {
759 (true, true) => Some(r1.min(r2)),
760 (true, false) => Some(r2),
761 (false, true) => Some(r1),
762 (false, false) => None,
765 let mut min_choice = choice_regions[0];
766 for &other_option in &choice_regions[1..] {
768 "apply_member_constraint: min_choice={:?} other_option={:?}",
769 min_choice, other_option,
771 match min(min_choice, other_option) {
772 Some(m) => min_choice = m,
775 "apply_member_constraint: {:?} and {:?} are incomparable; no min choice",
776 min_choice, other_option,
783 let min_choice_scc = self.constraint_sccs.scc(min_choice);
785 "apply_member_constraint: min_choice={:?} best_choice_scc={:?}",
786 min_choice, min_choice_scc,
788 if self.scc_values.add_region(scc, min_choice_scc) {
789 self.member_constraints_applied.push(AppliedMemberConstraint {
790 member_region_scc: scc,
792 member_constraint_index,
801 /// Returns `true` if all the elements in the value of `scc_b` are nameable
802 /// in `scc_a`. Used during constraint propagation, and only once
803 /// the value of `scc_b` has been computed.
804 fn universe_compatible(&self, scc_b: ConstraintSccIndex, scc_a: ConstraintSccIndex) -> bool {
805 let universe_a = self.scc_universes[scc_a];
807 // Quick check: if scc_b's declared universe is a subset of
808 // scc_a's declared univese (typically, both are ROOT), then
809 // it cannot contain any problematic universe elements.
810 if universe_a.can_name(self.scc_universes[scc_b]) {
814 // Otherwise, we have to iterate over the universe elements in
815 // B's value, and check whether all of them are nameable
817 self.scc_values.placeholders_contained_in(scc_b).all(|p| universe_a.can_name(p.universe))
820 /// Extend `scc` so that it can outlive some placeholder region
821 /// from a universe it can't name; at present, the only way for
822 /// this to be true is if `scc` outlives `'static`. This is
823 /// actually stricter than necessary: ideally, we'd support bounds
824 /// like `for<'a: 'b`>` that might then allow us to approximate
825 /// `'a` with `'b` and not `'static`. But it will have to do for
827 fn add_incompatible_universe(&mut self, scc: ConstraintSccIndex) {
828 debug!("add_incompatible_universe(scc={:?})", scc);
830 let fr_static = self.universal_regions.fr_static;
831 self.scc_values.add_all_points(scc);
832 self.scc_values.add_element(scc, fr_static);
835 /// Once regions have been propagated, this method is used to see
836 /// whether the "type tests" produced by typeck were satisfied;
837 /// type tests encode type-outlives relationships like `T:
838 /// 'a`. See `TypeTest` for more details.
841 infcx: &InferCtxt<'_, 'tcx>,
843 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
844 errors_buffer: &mut RegionErrors<'tcx>,
848 // Sometimes we register equivalent type-tests that would
849 // result in basically the exact same error being reported to
850 // the user. Avoid that.
851 let mut deduplicate_errors = FxHashSet::default();
853 for type_test in &self.type_tests {
854 debug!("check_type_test: {:?}", type_test);
856 let generic_ty = type_test.generic_kind.to_ty(tcx);
857 if self.eval_verify_bound(
861 type_test.lower_bound,
862 &type_test.verify_bound,
867 if let Some(propagated_outlives_requirements) = &mut propagated_outlives_requirements {
868 if self.try_promote_type_test(
872 propagated_outlives_requirements,
878 // Type-test failed. Report the error.
879 let erased_generic_kind = infcx.tcx.erase_regions(type_test.generic_kind);
881 // Skip duplicate-ish errors.
882 if deduplicate_errors.insert((
884 type_test.lower_bound,
888 "check_type_test: reporting error for erased_generic_kind={:?}, \
889 lower_bound_region={:?}, \
890 type_test.locations={:?}",
891 erased_generic_kind, type_test.lower_bound, type_test.locations,
894 errors_buffer.push(RegionErrorKind::TypeTestError { type_test: type_test.clone() });
899 /// Invoked when we have some type-test (e.g., `T: 'X`) that we cannot
900 /// prove to be satisfied. If this is a closure, we will attempt to
901 /// "promote" this type-test into our `ClosureRegionRequirements` and
902 /// hence pass it up the creator. To do this, we have to phrase the
903 /// type-test in terms of external free regions, as local free
904 /// regions are not nameable by the closure's creator.
906 /// Promotion works as follows: we first check that the type `T`
907 /// contains only regions that the creator knows about. If this is
908 /// true, then -- as a consequence -- we know that all regions in
909 /// the type `T` are free regions that outlive the closure body. If
910 /// false, then promotion fails.
912 /// Once we've promoted T, we have to "promote" `'X` to some region
913 /// that is "external" to the closure. Generally speaking, a region
914 /// may be the union of some points in the closure body as well as
915 /// various free lifetimes. We can ignore the points in the closure
916 /// body: if the type T can be expressed in terms of external regions,
917 /// we know it outlives the points in the closure body. That
918 /// just leaves the free regions.
920 /// The idea then is to lower the `T: 'X` constraint into multiple
921 /// bounds -- e.g., if `'X` is the union of two free lifetimes,
922 /// `'1` and `'2`, then we would create `T: '1` and `T: '2`.
923 fn try_promote_type_test(
925 infcx: &InferCtxt<'_, 'tcx>,
927 type_test: &TypeTest<'tcx>,
928 propagated_outlives_requirements: &mut Vec<ClosureOutlivesRequirement<'tcx>>,
932 let TypeTest { generic_kind, lower_bound, locations, verify_bound: _ } = type_test;
934 let generic_ty = generic_kind.to_ty(tcx);
935 let subject = match self.try_promote_type_test_subject(infcx, generic_ty) {
937 None => return false,
940 // For each region outlived by lower_bound find a non-local,
941 // universal region (it may be the same region) and add it to
942 // `ClosureOutlivesRequirement`.
943 let r_scc = self.constraint_sccs.scc(*lower_bound);
944 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
945 // Check whether we can already prove that the "subject" outlives `ur`.
946 // If so, we don't have to propagate this requirement to our caller.
948 // To continue the example from the function, if we are trying to promote
949 // a requirement that `T: 'X`, and we know that `'X = '1 + '2` (i.e., the union
950 // `'1` and `'2`), then in this loop `ur` will be `'1` (and `'2`). So here
951 // we check whether `T: '1` is something we *can* prove. If so, no need
952 // to propagate that requirement.
954 // This is needed because -- particularly in the case
955 // where `ur` is a local bound -- we are sometimes in a
956 // position to prove things that our caller cannot. See
957 // #53570 for an example.
958 if self.eval_verify_bound(tcx, body, generic_ty, ur, &type_test.verify_bound) {
962 debug!("try_promote_type_test: ur={:?}", ur);
964 let non_local_ub = self.universal_region_relations.non_local_upper_bounds(&ur);
965 debug!("try_promote_type_test: non_local_ub={:?}", non_local_ub);
967 // This is slightly too conservative. To show T: '1, given `'2: '1`
968 // and `'3: '1` we only need to prove that T: '2 *or* T: '3, but to
969 // avoid potential non-determinism we approximate this by requiring
971 for &upper_bound in non_local_ub {
972 debug_assert!(self.universal_regions.is_universal_region(upper_bound));
973 debug_assert!(!self.universal_regions.is_local_free_region(upper_bound));
975 let requirement = ClosureOutlivesRequirement {
977 outlived_free_region: upper_bound,
978 blame_span: locations.span(body),
979 category: ConstraintCategory::Boring,
981 debug!("try_promote_type_test: pushing {:#?}", requirement);
982 propagated_outlives_requirements.push(requirement);
988 /// When we promote a type test `T: 'r`, we have to convert the
989 /// type `T` into something we can store in a query result (so
990 /// something allocated for `'tcx`). This is problematic if `ty`
991 /// contains regions. During the course of NLL region checking, we
992 /// will have replaced all of those regions with fresh inference
993 /// variables. To create a test subject, we want to replace those
994 /// inference variables with some region from the closure
995 /// signature -- this is not always possible, so this is a
996 /// fallible process. Presuming we do find a suitable region, we
997 /// will use it's *external name*, which will be a `RegionKind`
998 /// variant that can be used in query responses such as
1000 fn try_promote_type_test_subject(
1002 infcx: &InferCtxt<'_, 'tcx>,
1004 ) -> Option<ClosureOutlivesSubject<'tcx>> {
1005 let tcx = infcx.tcx;
1007 debug!("try_promote_type_test_subject(ty = {:?})", ty);
1009 let ty = tcx.fold_regions(ty, &mut false, |r, _depth| {
1010 let region_vid = self.to_region_vid(r);
1012 // The challenge if this. We have some region variable `r`
1013 // whose value is a set of CFG points and universal
1014 // regions. We want to find if that set is *equivalent* to
1015 // any of the named regions found in the closure.
1017 // To do so, we compute the
1018 // `non_local_universal_upper_bound`. This will be a
1019 // non-local, universal region that is greater than `r`.
1020 // However, it might not be *contained* within `r`, so
1021 // then we further check whether this bound is contained
1022 // in `r`. If so, we can say that `r` is equivalent to the
1025 // Let's work through a few examples. For these, imagine
1026 // that we have 3 non-local regions (I'll denote them as
1027 // `'static`, `'a`, and `'b`, though of course in the code
1028 // they would be represented with indices) where:
1033 // First, let's assume that `r` is some existential
1034 // variable with an inferred value `{'a, 'static}` (plus
1035 // some CFG nodes). In this case, the non-local upper
1036 // bound is `'static`, since that outlives `'a`. `'static`
1037 // is also a member of `r` and hence we consider `r`
1038 // equivalent to `'static` (and replace it with
1041 // Now let's consider the inferred value `{'a, 'b}`. This
1042 // means `r` is effectively `'a | 'b`. I'm not sure if
1043 // this can come about, actually, but assuming it did, we
1044 // would get a non-local upper bound of `'static`. Since
1045 // `'static` is not contained in `r`, we would fail to
1046 // find an equivalent.
1047 let upper_bound = self.non_local_universal_upper_bound(region_vid);
1048 if self.region_contains(region_vid, upper_bound) {
1049 self.definitions[upper_bound].external_name.unwrap_or(r)
1051 // In the case of a failure, use a `ReVar` result. This will
1052 // cause the `needs_infer` later on to return `None`.
1057 debug!("try_promote_type_test_subject: folded ty = {:?}", ty);
1059 // `needs_infer` will only be true if we failed to promote some region.
1060 if ty.needs_infer() {
1064 Some(ClosureOutlivesSubject::Ty(ty))
1067 /// Given some universal or existential region `r`, finds a
1068 /// non-local, universal region `r+` that outlives `r` at entry to (and
1069 /// exit from) the closure. In the worst case, this will be
1072 /// This is used for two purposes. First, if we are propagated
1073 /// some requirement `T: r`, we can use this method to enlarge `r`
1074 /// to something we can encode for our creator (which only knows
1075 /// about non-local, universal regions). It is also used when
1076 /// encoding `T` as part of `try_promote_type_test_subject` (see
1077 /// that fn for details).
1079 /// This is based on the result `'y` of `universal_upper_bound`,
1080 /// except that it converts further takes the non-local upper
1081 /// bound of `'y`, so that the final result is non-local.
1082 fn non_local_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1083 debug!("non_local_universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1085 let lub = self.universal_upper_bound(r);
1087 // Grow further to get smallest universal region known to
1089 let non_local_lub = self.universal_region_relations.non_local_upper_bound(lub);
1091 debug!("non_local_universal_upper_bound: non_local_lub={:?}", non_local_lub);
1096 /// Returns a universally quantified region that outlives the
1097 /// value of `r` (`r` may be existentially or universally
1100 /// Since `r` is (potentially) an existential region, it has some
1101 /// value which may include (a) any number of points in the CFG
1102 /// and (b) any number of `end('x)` elements of universally
1103 /// quantified regions. To convert this into a single universal
1104 /// region we do as follows:
1106 /// - Ignore the CFG points in `'r`. All universally quantified regions
1107 /// include the CFG anyhow.
1108 /// - For each `end('x)` element in `'r`, compute the mutual LUB, yielding
1110 pub(in crate::borrow_check) fn universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1111 debug!("universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1113 // Find the smallest universal region that contains all other
1114 // universal regions within `region`.
1115 let mut lub = self.universal_regions.fr_fn_body;
1116 let r_scc = self.constraint_sccs.scc(r);
1117 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1118 lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1121 debug!("universal_upper_bound: r={:?} lub={:?}", r, lub);
1126 /// Like `universal_upper_bound`, but returns an approximation more suitable
1127 /// for diagnostics. If `r` contains multiple disjoint universal regions
1128 /// (e.g. 'a and 'b in `fn foo<'a, 'b> { ... }`, we pick the lower-numbered region.
1129 /// This corresponds to picking named regions over unnamed regions
1130 /// (e.g. picking early-bound regions over a closure late-bound region).
1132 /// This means that the returned value may not be a true upper bound, since
1133 /// only 'static is known to outlive disjoint universal regions.
1134 /// Therefore, this method should only be used in diagnostic code,
1135 /// where displaying *some* named universal region is better than
1136 /// falling back to 'static.
1137 pub(in crate::borrow_check) fn approx_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1138 debug!("approx_universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1140 // Find the smallest universal region that contains all other
1141 // universal regions within `region`.
1142 let mut lub = self.universal_regions.fr_fn_body;
1143 let r_scc = self.constraint_sccs.scc(r);
1144 let static_r = self.universal_regions.fr_static;
1145 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1146 let new_lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1147 debug!("approx_universal_upper_bound: ur={:?} lub={:?} new_lub={:?}", ur, lub, new_lub);
1148 // The upper bound of two non-static regions is static: this
1149 // means we know nothing about the relationship between these
1150 // two regions. Pick a 'better' one to use when constructing
1152 if ur != static_r && lub != static_r && new_lub == static_r {
1153 // Prefer the region with an `external_name` - this
1154 // indicates that the region is early-bound, so working with
1155 // it can produce a nicer error.
1156 if self.region_definition(ur).external_name.is_some() {
1158 } else if self.region_definition(lub).external_name.is_some() {
1159 // Leave lub unchanged
1161 // If we get here, we don't have any reason to prefer
1162 // one region over the other. Just pick the
1163 // one with the lower index for now.
1164 lub = std::cmp::min(ur, lub);
1171 debug!("approx_universal_upper_bound: r={:?} lub={:?}", r, lub);
1176 /// Tests if `test` is true when applied to `lower_bound` at
1178 fn eval_verify_bound(
1182 generic_ty: Ty<'tcx>,
1183 lower_bound: RegionVid,
1184 verify_bound: &VerifyBound<'tcx>,
1186 debug!("eval_verify_bound(lower_bound={:?}, verify_bound={:?})", lower_bound, verify_bound);
1188 match verify_bound {
1189 VerifyBound::IfEq(test_ty, verify_bound1) => {
1190 self.eval_if_eq(tcx, body, generic_ty, lower_bound, test_ty, verify_bound1)
1193 VerifyBound::IsEmpty => {
1194 let lower_bound_scc = self.constraint_sccs.scc(lower_bound);
1195 self.scc_values.elements_contained_in(lower_bound_scc).next().is_none()
1198 VerifyBound::OutlivedBy(r) => {
1199 let r_vid = self.to_region_vid(r);
1200 self.eval_outlives(r_vid, lower_bound)
1203 VerifyBound::AnyBound(verify_bounds) => verify_bounds.iter().any(|verify_bound| {
1204 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1207 VerifyBound::AllBounds(verify_bounds) => verify_bounds.iter().all(|verify_bound| {
1208 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1217 generic_ty: Ty<'tcx>,
1218 lower_bound: RegionVid,
1220 verify_bound: &VerifyBound<'tcx>,
1222 let generic_ty_normalized = self.normalize_to_scc_representatives(tcx, generic_ty);
1223 let test_ty_normalized = self.normalize_to_scc_representatives(tcx, test_ty);
1224 if generic_ty_normalized == test_ty_normalized {
1225 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1231 /// This is a conservative normalization procedure. It takes every
1232 /// free region in `value` and replaces it with the
1233 /// "representative" of its SCC (see `scc_representatives` field).
1234 /// We are guaranteed that if two values normalize to the same
1235 /// thing, then they are equal; this is a conservative check in
1236 /// that they could still be equal even if they normalize to
1237 /// different results. (For example, there might be two regions
1238 /// with the same value that are not in the same SCC).
1240 /// N.B., this is not an ideal approach and I would like to revisit
1241 /// it. However, it works pretty well in practice. In particular,
1242 /// this is needed to deal with projection outlives bounds like
1245 /// <T as Foo<'0>>::Item: '1
1248 /// In particular, this routine winds up being important when
1249 /// there are bounds like `where <T as Foo<'a>>::Item: 'b` in the
1250 /// environment. In this case, if we can show that `'0 == 'a`,
1251 /// and that `'b: '1`, then we know that the clause is
1252 /// satisfied. In such cases, particularly due to limitations of
1253 /// the trait solver =), we usually wind up with a where-clause like
1254 /// `T: Foo<'a>` in scope, which thus forces `'0 == 'a` to be added as
1255 /// a constraint, and thus ensures that they are in the same SCC.
1257 /// So why can't we do a more correct routine? Well, we could
1258 /// *almost* use the `relate_tys` code, but the way it is
1259 /// currently setup it creates inference variables to deal with
1260 /// higher-ranked things and so forth, and right now the inference
1261 /// context is not permitted to make more inference variables. So
1262 /// we use this kind of hacky solution.
1263 fn normalize_to_scc_representatives<T>(&self, tcx: TyCtxt<'tcx>, value: T) -> T
1265 T: TypeFoldable<'tcx>,
1267 tcx.fold_regions(value, &mut false, |r, _db| {
1268 let vid = self.to_region_vid(r);
1269 let scc = self.constraint_sccs.scc(vid);
1270 let repr = self.scc_representatives[scc];
1271 tcx.mk_region(ty::ReVar(repr))
1275 // Evaluate whether `sup_region == sub_region`.
1276 fn eval_equal(&self, r1: RegionVid, r2: RegionVid) -> bool {
1277 self.eval_outlives(r1, r2) && self.eval_outlives(r2, r1)
1280 // Evaluate whether `sup_region: sub_region`.
1281 fn eval_outlives(&self, sup_region: RegionVid, sub_region: RegionVid) -> bool {
1282 debug!("eval_outlives({:?}: {:?})", sup_region, sub_region);
1285 "eval_outlives: sup_region's value = {:?} universal={:?}",
1286 self.region_value_str(sup_region),
1287 self.universal_regions.is_universal_region(sup_region),
1290 "eval_outlives: sub_region's value = {:?} universal={:?}",
1291 self.region_value_str(sub_region),
1292 self.universal_regions.is_universal_region(sub_region),
1295 let sub_region_scc = self.constraint_sccs.scc(sub_region);
1296 let sup_region_scc = self.constraint_sccs.scc(sup_region);
1298 // Both the `sub_region` and `sup_region` consist of the union
1299 // of some number of universal regions (along with the union
1300 // of various points in the CFG; ignore those points for
1301 // now). Therefore, the sup-region outlives the sub-region if,
1302 // for each universal region R1 in the sub-region, there
1303 // exists some region R2 in the sup-region that outlives R1.
1304 let universal_outlives =
1305 self.scc_values.universal_regions_outlived_by(sub_region_scc).all(|r1| {
1307 .universal_regions_outlived_by(sup_region_scc)
1308 .any(|r2| self.universal_region_relations.outlives(r2, r1))
1311 if !universal_outlives {
1315 // Now we have to compare all the points in the sub region and make
1316 // sure they exist in the sup region.
1318 if self.universal_regions.is_universal_region(sup_region) {
1319 // Micro-opt: universal regions contain all points.
1323 self.scc_values.contains_points(sup_region_scc, sub_region_scc)
1326 /// Once regions have been propagated, this method is used to see
1327 /// whether any of the constraints were too strong. In particular,
1328 /// we want to check for a case where a universally quantified
1329 /// region exceeded its bounds. Consider:
1331 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1333 /// In this case, returning `x` requires `&'a u32 <: &'b u32`
1334 /// and hence we establish (transitively) a constraint that
1335 /// `'a: 'b`. The `propagate_constraints` code above will
1336 /// therefore add `end('a)` into the region for `'b` -- but we
1337 /// have no evidence that `'b` outlives `'a`, so we want to report
1340 /// If `propagated_outlives_requirements` is `Some`, then we will
1341 /// push unsatisfied obligations into there. Otherwise, we'll
1342 /// report them as errors.
1343 fn check_universal_regions(
1346 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1347 errors_buffer: &mut RegionErrors<'tcx>,
1349 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1350 match fr_definition.origin {
1351 NllRegionVariableOrigin::FreeRegion => {
1352 // Go through each of the universal regions `fr` and check that
1353 // they did not grow too large, accumulating any requirements
1354 // for our caller into the `outlives_requirements` vector.
1355 self.check_universal_region(
1358 &mut propagated_outlives_requirements,
1363 NllRegionVariableOrigin::Placeholder(placeholder) => {
1364 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1367 NllRegionVariableOrigin::RootEmptyRegion
1368 | NllRegionVariableOrigin::Existential { .. } => {
1369 // nothing to check here
1375 /// Checks if Polonius has found any unexpected free region relations.
1377 /// In Polonius terms, a "subset error" (or "illegal subset relation error") is the equivalent
1378 /// of NLL's "checking if any region constraints were too strong": a placeholder origin `'a`
1379 /// was unexpectedly found to be a subset of another placeholder origin `'b`, and means in NLL
1380 /// terms that the "longer free region" `'a` outlived the "shorter free region" `'b`.
1382 /// More details can be found in this blog post by Niko:
1383 /// <https://smallcultfollowing.com/babysteps/blog/2019/01/17/polonius-and-region-errors/>
1385 /// In the canonical example
1387 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1389 /// returning `x` requires `&'a u32 <: &'b u32` and hence we establish (transitively) a
1390 /// constraint that `'a: 'b`. It is an error that we have no evidence that this
1391 /// constraint holds.
1393 /// If `propagated_outlives_requirements` is `Some`, then we will
1394 /// push unsatisfied obligations into there. Otherwise, we'll
1395 /// report them as errors.
1396 fn check_polonius_subset_errors(
1399 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1400 errors_buffer: &mut RegionErrors<'tcx>,
1401 polonius_output: Rc<PoloniusOutput>,
1404 "check_polonius_subset_errors: {} subset_errors",
1405 polonius_output.subset_errors.len()
1408 // Similarly to `check_universal_regions`: a free region relation, which was not explicitly
1409 // declared ("known") was found by Polonius, so emit an error, or propagate the
1410 // requirements for our caller into the `propagated_outlives_requirements` vector.
1412 // Polonius doesn't model regions ("origins") as CFG-subsets or durations, but the
1413 // `longer_fr` and `shorter_fr` terminology will still be used here, for consistency with
1414 // the rest of the NLL infrastructure. The "subset origin" is the "longer free region",
1415 // and the "superset origin" is the outlived "shorter free region".
1417 // Note: Polonius will produce a subset error at every point where the unexpected
1418 // `longer_fr`'s "placeholder loan" is contained in the `shorter_fr`. This can be helpful
1419 // for diagnostics in the future, e.g. to point more precisely at the key locations
1420 // requiring this constraint to hold. However, the error and diagnostics code downstream
1421 // expects that these errors are not duplicated (and that they are in a certain order).
1422 // Otherwise, diagnostics messages such as the ones giving names like `'1` to elided or
1423 // anonymous lifetimes for example, could give these names differently, while others like
1424 // the outlives suggestions or the debug output from `#[rustc_regions]` would be
1425 // duplicated. The polonius subset errors are deduplicated here, while keeping the
1426 // CFG-location ordering.
1427 let mut subset_errors: Vec<_> = polonius_output
1430 .flat_map(|(_location, subset_errors)| subset_errors.iter())
1432 subset_errors.sort();
1433 subset_errors.dedup();
1435 for (longer_fr, shorter_fr) in subset_errors.into_iter() {
1437 "check_polonius_subset_errors: subset_error longer_fr={:?},\
1439 longer_fr, shorter_fr
1442 let propagated = self.try_propagate_universal_region_error(
1446 &mut propagated_outlives_requirements,
1448 if propagated == RegionRelationCheckResult::Error {
1449 errors_buffer.push(RegionErrorKind::RegionError {
1450 longer_fr: *longer_fr,
1451 shorter_fr: *shorter_fr,
1452 fr_origin: NllRegionVariableOrigin::FreeRegion,
1458 // Handle the placeholder errors as usual, until the chalk-rustc-polonius triumvirate has
1459 // a more complete picture on how to separate this responsibility.
1460 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1461 match fr_definition.origin {
1462 NllRegionVariableOrigin::FreeRegion => {
1463 // handled by polonius above
1466 NllRegionVariableOrigin::Placeholder(placeholder) => {
1467 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1470 NllRegionVariableOrigin::RootEmptyRegion
1471 | NllRegionVariableOrigin::Existential { .. } => {
1472 // nothing to check here
1478 /// Checks the final value for the free region `fr` to see if it
1479 /// grew too large. In particular, examine what `end(X)` points
1480 /// wound up in `fr`'s final value; for each `end(X)` where `X !=
1481 /// fr`, we want to check that `fr: X`. If not, that's either an
1482 /// error, or something we have to propagate to our creator.
1484 /// Things that are to be propagated are accumulated into the
1485 /// `outlives_requirements` vector.
1486 fn check_universal_region(
1489 longer_fr: RegionVid,
1490 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1491 errors_buffer: &mut RegionErrors<'tcx>,
1493 debug!("check_universal_region(fr={:?})", longer_fr);
1495 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1497 // Because this free region must be in the ROOT universe, we
1498 // know it cannot contain any bound universes.
1499 assert!(self.scc_universes[longer_fr_scc] == ty::UniverseIndex::ROOT);
1500 debug_assert!(self.scc_values.placeholders_contained_in(longer_fr_scc).next().is_none());
1502 // Only check all of the relations for the main representative of each
1503 // SCC, otherwise just check that we outlive said representative. This
1504 // reduces the number of redundant relations propagated out of
1506 // Note that the representative will be a universal region if there is
1507 // one in this SCC, so we will always check the representative here.
1508 let representative = self.scc_representatives[longer_fr_scc];
1509 if representative != longer_fr {
1510 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1514 propagated_outlives_requirements,
1516 errors_buffer.push(RegionErrorKind::RegionError {
1518 shorter_fr: representative,
1519 fr_origin: NllRegionVariableOrigin::FreeRegion,
1526 // Find every region `o` such that `fr: o`
1527 // (because `fr` includes `end(o)`).
1528 let mut error_reported = false;
1529 for shorter_fr in self.scc_values.universal_regions_outlived_by(longer_fr_scc) {
1530 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1534 propagated_outlives_requirements,
1536 // We only report the first region error. Subsequent errors are hidden so as
1537 // not to overwhelm the user, but we do record them so as to potentially print
1538 // better diagnostics elsewhere...
1539 errors_buffer.push(RegionErrorKind::RegionError {
1542 fr_origin: NllRegionVariableOrigin::FreeRegion,
1543 is_reported: !error_reported,
1546 error_reported = true;
1551 /// Checks that we can prove that `longer_fr: shorter_fr`. If we can't we attempt to propagate
1552 /// the constraint outward (e.g. to a closure environment), but if that fails, there is an
1554 fn check_universal_region_relation(
1556 longer_fr: RegionVid,
1557 shorter_fr: RegionVid,
1559 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1560 ) -> RegionRelationCheckResult {
1561 // If it is known that `fr: o`, carry on.
1562 if self.universal_region_relations.outlives(longer_fr, shorter_fr) {
1563 RegionRelationCheckResult::Ok
1565 // If we are not in a context where we can't propagate errors, or we
1566 // could not shrink `fr` to something smaller, then just report an
1569 // Note: in this case, we use the unapproximated regions to report the
1570 // error. This gives better error messages in some cases.
1571 self.try_propagate_universal_region_error(
1575 propagated_outlives_requirements,
1580 /// Attempt to propagate a region error (e.g. `'a: 'b`) that is not met to a closure's
1581 /// creator. If we cannot, then the caller should report an error to the user.
1582 fn try_propagate_universal_region_error(
1584 longer_fr: RegionVid,
1585 shorter_fr: RegionVid,
1587 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1588 ) -> RegionRelationCheckResult {
1589 if let Some(propagated_outlives_requirements) = propagated_outlives_requirements {
1590 // Shrink `longer_fr` until we find a non-local region (if we do).
1591 // We'll call it `fr-` -- it's ever so slightly smaller than
1593 if let Some(fr_minus) = self.universal_region_relations.non_local_lower_bound(longer_fr)
1595 debug!("try_propagate_universal_region_error: fr_minus={:?}", fr_minus);
1597 let blame_span_category = self.find_outlives_blame_span(
1600 NllRegionVariableOrigin::FreeRegion,
1604 // Grow `shorter_fr` until we find some non-local regions. (We
1605 // always will.) We'll call them `shorter_fr+` -- they're ever
1606 // so slightly larger than `shorter_fr`.
1607 let shorter_fr_plus =
1608 self.universal_region_relations.non_local_upper_bounds(&shorter_fr);
1610 "try_propagate_universal_region_error: shorter_fr_plus={:?}",
1613 for &&fr in &shorter_fr_plus {
1614 // Push the constraint `fr-: shorter_fr+`
1615 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
1616 subject: ClosureOutlivesSubject::Region(fr_minus),
1617 outlived_free_region: fr,
1618 blame_span: blame_span_category.1,
1619 category: blame_span_category.0,
1622 return RegionRelationCheckResult::Propagated;
1626 RegionRelationCheckResult::Error
1629 fn check_bound_universal_region(
1631 longer_fr: RegionVid,
1632 placeholder: ty::PlaceholderRegion,
1633 errors_buffer: &mut RegionErrors<'tcx>,
1635 debug!("check_bound_universal_region(fr={:?}, placeholder={:?})", longer_fr, placeholder,);
1637 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1638 debug!("check_bound_universal_region: longer_fr_scc={:?}", longer_fr_scc,);
1640 // If we have some bound universal region `'a`, then the only
1641 // elements it can contain is itself -- we don't know anything
1643 let error_element = match {
1644 self.scc_values.elements_contained_in(longer_fr_scc).find(|element| match element {
1645 RegionElement::Location(_) => true,
1646 RegionElement::RootUniversalRegion(_) => true,
1647 RegionElement::PlaceholderRegion(placeholder1) => placeholder != *placeholder1,
1653 debug!("check_bound_universal_region: error_element = {:?}", error_element);
1655 // Find the region that introduced this `error_element`.
1656 errors_buffer.push(RegionErrorKind::BoundUniversalRegionError {
1659 fr_origin: NllRegionVariableOrigin::Placeholder(placeholder),
1663 fn check_member_constraints(
1665 infcx: &InferCtxt<'_, 'tcx>,
1666 errors_buffer: &mut RegionErrors<'tcx>,
1668 let member_constraints = self.member_constraints.clone();
1669 for m_c_i in member_constraints.all_indices() {
1670 debug!("check_member_constraint(m_c_i={:?})", m_c_i);
1671 let m_c = &member_constraints[m_c_i];
1672 let member_region_vid = m_c.member_region_vid;
1674 "check_member_constraint: member_region_vid={:?} with value {}",
1676 self.region_value_str(member_region_vid),
1678 let choice_regions = member_constraints.choice_regions(m_c_i);
1679 debug!("check_member_constraint: choice_regions={:?}", choice_regions);
1681 // Did the member region wind up equal to any of the option regions?
1683 choice_regions.iter().find(|&&o_r| self.eval_equal(o_r, m_c.member_region_vid))
1685 debug!("check_member_constraint: evaluated as equal to {:?}", o);
1689 // If not, report an error.
1690 let member_region = infcx.tcx.mk_region(ty::ReVar(member_region_vid));
1691 errors_buffer.push(RegionErrorKind::UnexpectedHiddenRegion {
1692 span: m_c.definition_span,
1693 hidden_ty: m_c.hidden_ty,
1699 /// We have a constraint `fr1: fr2` that is not satisfied, where
1700 /// `fr2` represents some universal region. Here, `r` is some
1701 /// region where we know that `fr1: r` and this function has the
1702 /// job of determining whether `r` is "to blame" for the fact that
1703 /// `fr1: fr2` is required.
1705 /// This is true under two conditions:
1708 /// - `fr2` is `'static` and `r` is some placeholder in a universe
1709 /// that cannot be named by `fr1`; in that case, we will require
1710 /// that `fr1: 'static` because it is the only way to `fr1: r` to
1711 /// be satisfied. (See `add_incompatible_universe`.)
1712 crate fn provides_universal_region(
1718 debug!("provides_universal_region(r={:?}, fr1={:?}, fr2={:?})", r, fr1, fr2);
1721 fr2 == self.universal_regions.fr_static && self.cannot_name_placeholder(fr1, r)
1724 debug!("provides_universal_region: result = {:?}", result);
1728 /// If `r2` represents a placeholder region, then this returns
1729 /// `true` if `r1` cannot name that placeholder in its
1730 /// value; otherwise, returns `false`.
1731 crate fn cannot_name_placeholder(&self, r1: RegionVid, r2: RegionVid) -> bool {
1732 debug!("cannot_name_value_of(r1={:?}, r2={:?})", r1, r2);
1734 match self.definitions[r2].origin {
1735 NllRegionVariableOrigin::Placeholder(placeholder) => {
1736 let universe1 = self.definitions[r1].universe;
1738 "cannot_name_value_of: universe1={:?} placeholder={:?}",
1739 universe1, placeholder
1741 universe1.cannot_name(placeholder.universe)
1744 NllRegionVariableOrigin::RootEmptyRegion
1745 | NllRegionVariableOrigin::FreeRegion
1746 | NllRegionVariableOrigin::Existential { .. } => false,
1750 crate fn retrieve_closure_constraint_info(
1753 constraint: &OutlivesConstraint<'tcx>,
1754 ) -> BlameConstraint<'tcx> {
1755 let loc = match constraint.locations {
1756 Locations::All(span) => {
1757 return BlameConstraint {
1758 category: constraint.category,
1759 from_closure: false,
1761 variance_info: constraint.variance_info.clone(),
1764 Locations::Single(loc) => loc,
1767 let opt_span_category =
1768 self.closure_bounds_mapping[&loc].get(&(constraint.sup, constraint.sub));
1770 .map(|&(category, span)| BlameConstraint {
1774 variance_info: constraint.variance_info.clone(),
1776 .unwrap_or(BlameConstraint {
1777 category: constraint.category,
1778 from_closure: false,
1779 span: body.source_info(loc).span,
1780 variance_info: constraint.variance_info.clone(),
1784 /// Finds a good span to blame for the fact that `fr1` outlives `fr2`.
1785 crate fn find_outlives_blame_span(
1789 fr1_origin: NllRegionVariableOrigin,
1791 ) -> (ConstraintCategory, Span) {
1792 let BlameConstraint { category, span, .. } =
1793 self.best_blame_constraint(body, fr1, fr1_origin, |r| {
1794 self.provides_universal_region(r, fr1, fr2)
1799 /// Walks the graph of constraints (where `'a: 'b` is considered
1800 /// an edge `'a -> 'b`) to find all paths from `from_region` to
1801 /// `to_region`. The paths are accumulated into the vector
1802 /// `results`. The paths are stored as a series of
1803 /// `ConstraintIndex` values -- in other words, a list of *edges*.
1805 /// Returns: a series of constraints as well as the region `R`
1806 /// that passed the target test.
1807 crate fn find_constraint_paths_between_regions(
1809 from_region: RegionVid,
1810 target_test: impl Fn(RegionVid) -> bool,
1811 ) -> Option<(Vec<OutlivesConstraint<'tcx>>, RegionVid)> {
1812 let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions);
1813 context[from_region] = Trace::StartRegion;
1815 // Use a deque so that we do a breadth-first search. We will
1816 // stop at the first match, which ought to be the shortest
1817 // path (fewest constraints).
1818 let mut deque = VecDeque::new();
1819 deque.push_back(from_region);
1821 while let Some(r) = deque.pop_front() {
1823 "find_constraint_paths_between_regions: from_region={:?} r={:?} value={}",
1826 self.region_value_str(r),
1829 // Check if we reached the region we were looking for. If so,
1830 // we can reconstruct the path that led to it and return it.
1832 let mut result = vec![];
1835 match context[p].clone() {
1836 Trace::NotVisited => {
1837 bug!("found unvisited region {:?} on path to {:?}", p, r)
1840 Trace::FromOutlivesConstraint(c) => {
1845 Trace::StartRegion => {
1847 return Some((result, r));
1853 // Otherwise, walk over the outgoing constraints and
1854 // enqueue any regions we find, keeping track of how we
1857 // A constraint like `'r: 'x` can come from our constraint
1859 let fr_static = self.universal_regions.fr_static;
1860 let outgoing_edges_from_graph =
1861 self.constraint_graph.outgoing_edges(r, &self.constraints, fr_static);
1863 // Always inline this closure because it can be hot.
1864 let mut handle_constraint = #[inline(always)]
1865 |constraint: OutlivesConstraint<'tcx>| {
1866 debug_assert_eq!(constraint.sup, r);
1867 let sub_region = constraint.sub;
1868 if let Trace::NotVisited = context[sub_region] {
1869 context[sub_region] = Trace::FromOutlivesConstraint(constraint);
1870 deque.push_back(sub_region);
1874 // This loop can be hot.
1875 for constraint in outgoing_edges_from_graph {
1876 handle_constraint(constraint);
1879 // Member constraints can also give rise to `'r: 'x` edges that
1880 // were not part of the graph initially, so watch out for those.
1881 // (But they are extremely rare; this loop is very cold.)
1882 for constraint in self.applied_member_constraints(r) {
1883 let p_c = &self.member_constraints[constraint.member_constraint_index];
1884 let constraint = OutlivesConstraint {
1886 sub: constraint.min_choice,
1887 locations: Locations::All(p_c.definition_span),
1888 category: ConstraintCategory::OpaqueType,
1889 variance_info: ty::VarianceDiagInfo::default(),
1891 handle_constraint(constraint);
1898 /// Finds some region R such that `fr1: R` and `R` is live at `elem`.
1899 crate fn find_sub_region_live_at(&self, fr1: RegionVid, elem: Location) -> RegionVid {
1900 debug!("find_sub_region_live_at(fr1={:?}, elem={:?})", fr1, elem);
1901 debug!("find_sub_region_live_at: {:?} is in scc {:?}", fr1, self.constraint_sccs.scc(fr1));
1903 "find_sub_region_live_at: {:?} is in universe {:?}",
1905 self.scc_universes[self.constraint_sccs.scc(fr1)]
1907 self.find_constraint_paths_between_regions(fr1, |r| {
1908 // First look for some `r` such that `fr1: r` and `r` is live at `elem`
1910 "find_sub_region_live_at: liveness_constraints for {:?} are {:?}",
1912 self.liveness_constraints.region_value_str(r),
1914 self.liveness_constraints.contains(r, elem)
1917 // If we fail to find that, we may find some `r` such that
1918 // `fr1: r` and `r` is a placeholder from some universe
1919 // `fr1` cannot name. This would force `fr1` to be
1921 self.find_constraint_paths_between_regions(fr1, |r| {
1922 self.cannot_name_placeholder(fr1, r)
1926 // If we fail to find THAT, it may be that `fr1` is a
1927 // placeholder that cannot "fit" into its SCC. In that
1928 // case, there should be some `r` where `fr1: r` and `fr1` is a
1929 // placeholder that `r` cannot name. We can blame that
1932 // Remember that if `R1: R2`, then the universe of R1
1933 // must be able to name the universe of R2, because R2 will
1934 // be at least `'empty(Universe(R2))`, and `R1` must be at
1935 // larger than that.
1936 self.find_constraint_paths_between_regions(fr1, |r| {
1937 self.cannot_name_placeholder(r, fr1)
1940 .map(|(_path, r)| r)
1944 /// Get the region outlived by `longer_fr` and live at `element`.
1945 crate fn region_from_element(&self, longer_fr: RegionVid, element: RegionElement) -> RegionVid {
1947 RegionElement::Location(l) => self.find_sub_region_live_at(longer_fr, l),
1948 RegionElement::RootUniversalRegion(r) => r,
1949 RegionElement::PlaceholderRegion(error_placeholder) => self
1952 .find_map(|(r, definition)| match definition.origin {
1953 NllRegionVariableOrigin::Placeholder(p) if p == error_placeholder => Some(r),
1960 /// Get the region definition of `r`.
1961 crate fn region_definition(&self, r: RegionVid) -> &RegionDefinition<'tcx> {
1962 &self.definitions[r]
1965 /// Check if the SCC of `r` contains `upper`.
1966 crate fn upper_bound_in_region_scc(&self, r: RegionVid, upper: RegionVid) -> bool {
1967 let r_scc = self.constraint_sccs.scc(r);
1968 self.scc_values.contains(r_scc, upper)
1971 crate fn universal_regions(&self) -> &UniversalRegions<'tcx> {
1972 self.universal_regions.as_ref()
1975 /// Tries to find the best constraint to blame for the fact that
1976 /// `R: from_region`, where `R` is some region that meets
1977 /// `target_test`. This works by following the constraint graph,
1978 /// creating a constraint path that forces `R` to outlive
1979 /// `from_region`, and then finding the best choices within that
1981 crate fn best_blame_constraint(
1984 from_region: RegionVid,
1985 from_region_origin: NllRegionVariableOrigin,
1986 target_test: impl Fn(RegionVid) -> bool,
1987 ) -> BlameConstraint<'tcx> {
1989 "best_blame_constraint(from_region={:?}, from_region_origin={:?})",
1990 from_region, from_region_origin
1994 let (path, target_region) =
1995 self.find_constraint_paths_between_regions(from_region, target_test).unwrap();
1997 "best_blame_constraint: path={:#?}",
2000 "{:?} ({:?}: {:?})",
2002 self.constraint_sccs.scc(c.sup),
2003 self.constraint_sccs.scc(c.sub),
2005 .collect::<Vec<_>>()
2008 // Classify each of the constraints along the path.
2009 let mut categorized_path: Vec<BlameConstraint<'tcx>> = path
2012 if constraint.category == ConstraintCategory::ClosureBounds {
2013 self.retrieve_closure_constraint_info(body, &constraint)
2016 category: constraint.category,
2017 from_closure: false,
2018 span: constraint.locations.span(body),
2019 variance_info: constraint.variance_info.clone(),
2024 debug!("best_blame_constraint: categorized_path={:#?}", categorized_path);
2026 // To find the best span to cite, we first try to look for the
2027 // final constraint that is interesting and where the `sup` is
2028 // not unified with the ultimate target region. The reason
2029 // for this is that we have a chain of constraints that lead
2030 // from the source to the target region, something like:
2032 // '0: '1 ('0 is the source)
2037 // '5: '6 ('6 is the target)
2039 // Some of those regions are unified with `'6` (in the same
2040 // SCC). We want to screen those out. After that point, the
2041 // "closest" constraint we have to the end is going to be the
2042 // most likely to be the point where the value escapes -- but
2043 // we still want to screen for an "interesting" point to
2044 // highlight (e.g., a call site or something).
2045 let target_scc = self.constraint_sccs.scc(target_region);
2046 let mut range = 0..path.len();
2048 // As noted above, when reporting an error, there is typically a chain of constraints
2049 // leading from some "source" region which must outlive some "target" region.
2050 // In most cases, we prefer to "blame" the constraints closer to the target --
2051 // but there is one exception. When constraints arise from higher-ranked subtyping,
2052 // we generally prefer to blame the source value,
2053 // as the "target" in this case tends to be some type annotation that the user gave.
2054 // Therefore, if we find that the region origin is some instantiation
2055 // of a higher-ranked region, we start our search from the "source" point
2056 // rather than the "target", and we also tweak a few other things.
2058 // An example might be this bit of Rust code:
2061 // let x: fn(&'static ()) = |_| {};
2062 // let y: for<'a> fn(&'a ()) = x;
2065 // In MIR, this will be converted into a combination of assignments and type ascriptions.
2066 // In particular, the 'static is imposed through a type ascription:
2070 // AscribeUserType(x, fn(&'static ())
2074 // We wind up ultimately with constraints like
2077 // !a: 'temp1 // from the `y = x` statement
2079 // 'temp2: 'static // from the AscribeUserType
2082 // and here we prefer to blame the source (the y = x statement).
2083 let blame_source = match from_region_origin {
2084 NllRegionVariableOrigin::FreeRegion
2085 | NllRegionVariableOrigin::Existential { from_forall: false } => true,
2086 NllRegionVariableOrigin::RootEmptyRegion
2087 | NllRegionVariableOrigin::Placeholder(_)
2088 | NllRegionVariableOrigin::Existential { from_forall: true } => false,
2091 let find_region = |i: &usize| {
2092 let constraint = &path[*i];
2094 let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
2097 match categorized_path[*i].category {
2098 ConstraintCategory::OpaqueType
2099 | ConstraintCategory::Boring
2100 | ConstraintCategory::BoringNoLocation
2101 | ConstraintCategory::Internal => false,
2102 ConstraintCategory::TypeAnnotation
2103 | ConstraintCategory::Return(_)
2104 | ConstraintCategory::Yield => true,
2105 _ => constraint_sup_scc != target_scc,
2108 match categorized_path[*i].category {
2109 ConstraintCategory::OpaqueType
2110 | ConstraintCategory::Boring
2111 | ConstraintCategory::BoringNoLocation
2112 | ConstraintCategory::Internal => false,
2119 if blame_source { range.rev().find(find_region) } else { range.find(find_region) };
2122 "best_blame_constraint: best_choice={:?} blame_source={}",
2123 best_choice, blame_source
2126 if let Some(i) = best_choice {
2127 if let Some(next) = categorized_path.get(i + 1) {
2128 if matches!(categorized_path[i].category, ConstraintCategory::Return(_))
2129 && next.category == ConstraintCategory::OpaqueType
2131 // The return expression is being influenced by the return type being
2132 // impl Trait, point at the return type and not the return expr.
2133 return next.clone();
2137 if categorized_path[i].category == ConstraintCategory::Return(ReturnConstraint::Normal)
2139 let field = categorized_path.iter().find_map(|p| {
2140 if let ConstraintCategory::ClosureUpvar(f) = p.category {
2147 if let Some(field) = field {
2148 categorized_path[i].category =
2149 ConstraintCategory::Return(ReturnConstraint::ClosureUpvar(field));
2153 return categorized_path[i].clone();
2156 // If that search fails, that is.. unusual. Maybe everything
2157 // is in the same SCC or something. In that case, find what
2158 // appears to be the most interesting point to report to the
2159 // user via an even more ad-hoc guess.
2160 categorized_path.sort_by(|p0, p1| p0.category.cmp(&p1.category));
2161 debug!("`: sorted_path={:#?}", categorized_path);
2163 categorized_path.remove(0)
2167 impl<'tcx> RegionDefinition<'tcx> {
2168 fn new(universe: ty::UniverseIndex, rv_origin: RegionVariableOrigin) -> Self {
2169 // Create a new region definition. Note that, for free
2170 // regions, the `external_name` field gets updated later in
2171 // `init_universal_regions`.
2173 let origin = match rv_origin {
2174 RegionVariableOrigin::Nll(origin) => origin,
2175 _ => NllRegionVariableOrigin::Existential { from_forall: false },
2178 Self { origin, universe, external_name: None }
2182 pub trait ClosureRegionRequirementsExt<'tcx> {
2183 fn apply_requirements(
2186 closure_def_id: DefId,
2187 closure_substs: SubstsRef<'tcx>,
2188 ) -> Vec<QueryOutlivesConstraint<'tcx>>;
2191 impl<'tcx> ClosureRegionRequirementsExt<'tcx> for ClosureRegionRequirements<'tcx> {
2192 /// Given an instance T of the closure type, this method
2193 /// instantiates the "extra" requirements that we computed for the
2194 /// closure into the inference context. This has the effect of
2195 /// adding new outlives obligations to existing variables.
2197 /// As described on `ClosureRegionRequirements`, the extra
2198 /// requirements are expressed in terms of regionvids that index
2199 /// into the free regions that appear on the closure type. So, to
2200 /// do this, we first copy those regions out from the type T into
2201 /// a vector. Then we can just index into that vector to extract
2202 /// out the corresponding region from T and apply the
2204 fn apply_requirements(
2207 closure_def_id: DefId,
2208 closure_substs: SubstsRef<'tcx>,
2209 ) -> Vec<QueryOutlivesConstraint<'tcx>> {
2211 "apply_requirements(closure_def_id={:?}, closure_substs={:?})",
2212 closure_def_id, closure_substs
2215 // Extract the values of the free regions in `closure_substs`
2216 // into a vector. These are the regions that we will be
2217 // relating to one another.
2218 let closure_mapping = &UniversalRegions::closure_mapping(
2221 self.num_external_vids,
2222 tcx.closure_base_def_id(closure_def_id),
2224 debug!("apply_requirements: closure_mapping={:?}", closure_mapping);
2226 // Create the predicates.
2227 self.outlives_requirements
2229 .map(|outlives_requirement| {
2230 let outlived_region = closure_mapping[outlives_requirement.outlived_free_region];
2232 match outlives_requirement.subject {
2233 ClosureOutlivesSubject::Region(region) => {
2234 let region = closure_mapping[region];
2236 "apply_requirements: region={:?} \
2237 outlived_region={:?} \
2238 outlives_requirement={:?}",
2239 region, outlived_region, outlives_requirement,
2241 ty::Binder::dummy(ty::OutlivesPredicate(region.into(), outlived_region))
2244 ClosureOutlivesSubject::Ty(ty) => {
2246 "apply_requirements: ty={:?} \
2247 outlived_region={:?} \
2248 outlives_requirement={:?}",
2249 ty, outlived_region, outlives_requirement,
2251 ty::Binder::dummy(ty::OutlivesPredicate(ty.into(), outlived_region))
2259 #[derive(Clone, Debug)]
2260 pub struct BlameConstraint<'tcx> {
2261 pub category: ConstraintCategory,
2262 pub from_closure: bool,
2264 pub variance_info: ty::VarianceDiagInfo<'tcx>,