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);
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>) {
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);
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) {
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(scc_a, m_c_i, member_constraints.choice_regions(m_c_i));
659 "propagate_constraint_sccs: scc_a = {:?} has value {:?}",
661 self.scc_values.region_value_str(scc_a),
665 /// Invoked for each `R0 member of [R1..Rn]` constraint.
667 /// `scc` is the SCC containing R0, and `choice_regions` are the
668 /// `R1..Rn` regions -- they are always known to be universal
669 /// regions (and if that's not true, we just don't attempt to
670 /// enforce the constraint).
672 /// The current value of `scc` at the time the method is invoked
673 /// is considered a *lower bound*. If possible, we will modify
674 /// the constraint to set it equal to one of the option regions.
675 /// If we make any changes, returns true, else false.
676 fn apply_member_constraint(
678 scc: ConstraintSccIndex,
679 member_constraint_index: NllMemberConstraintIndex,
680 choice_regions: &[ty::RegionVid],
682 debug!("apply_member_constraint(scc={:?}, choice_regions={:#?})", scc, choice_regions,);
684 // Create a mutable vector of the options. We'll try to winnow
686 let mut choice_regions: Vec<ty::RegionVid> = choice_regions.to_vec();
688 // The 'member region' in a member constraint is part of the
689 // hidden type, which must be in the root universe. Therefore,
690 // it cannot have any placeholders in its value.
691 assert!(self.scc_universes[scc] == ty::UniverseIndex::ROOT);
693 self.scc_values.placeholders_contained_in(scc).next().is_none(),
694 "scc {:?} in a member constraint has placeholder value: {:?}",
696 self.scc_values.region_value_str(scc),
699 // The existing value for `scc` is a lower-bound. This will
700 // consist of some set `{P} + {LB}` of points `{P}` and
701 // lower-bound free regions `{LB}`. As each choice region `O`
702 // is a free region, it will outlive the points. But we can
703 // only consider the option `O` if `O: LB`.
704 choice_regions.retain(|&o_r| {
706 .universal_regions_outlived_by(scc)
707 .all(|lb| self.universal_region_relations.outlives(o_r, lb))
709 debug!("apply_member_constraint: after lb, choice_regions={:?}", choice_regions);
711 // Now find all the *upper bounds* -- that is, each UB is a
712 // free region that must outlive the member region `R0` (`UB:
713 // R0`). Therefore, we need only keep an option `O` if `UB: O`
715 let rev_scc_graph = self.reverse_scc_graph();
716 let universal_region_relations = &self.universal_region_relations;
717 for ub in rev_scc_graph.upper_bounds(scc) {
718 debug!("apply_member_constraint: ub={:?}", ub);
719 choice_regions.retain(|&o_r| universal_region_relations.outlives(ub, o_r));
721 debug!("apply_member_constraint: after ub, choice_regions={:?}", choice_regions);
723 // If we ruled everything out, we're done.
724 if choice_regions.is_empty() {
728 // Otherwise, we need to find the minimum remaining choice, if
729 // any, and take that.
730 debug!("apply_member_constraint: choice_regions remaining are {:#?}", choice_regions);
731 let min = |r1: ty::RegionVid, r2: ty::RegionVid| -> Option<ty::RegionVid> {
732 let r1_outlives_r2 = self.universal_region_relations.outlives(r1, r2);
733 let r2_outlives_r1 = self.universal_region_relations.outlives(r2, r1);
734 match (r1_outlives_r2, r2_outlives_r1) {
735 (true, true) => Some(r1.min(r2)),
736 (true, false) => Some(r2),
737 (false, true) => Some(r1),
738 (false, false) => None,
741 let mut min_choice = choice_regions[0];
742 for &other_option in &choice_regions[1..] {
744 "apply_member_constraint: min_choice={:?} other_option={:?}",
745 min_choice, other_option,
747 match min(min_choice, other_option) {
748 Some(m) => min_choice = m,
751 "apply_member_constraint: {:?} and {:?} are incomparable; no min choice",
752 min_choice, other_option,
759 let min_choice_scc = self.constraint_sccs.scc(min_choice);
761 "apply_member_constraint: min_choice={:?} best_choice_scc={:?}",
762 min_choice, min_choice_scc,
764 if self.scc_values.add_region(scc, min_choice_scc) {
765 self.member_constraints_applied.push(AppliedMemberConstraint {
766 member_region_scc: scc,
768 member_constraint_index,
777 /// Returns `true` if all the elements in the value of `scc_b` are nameable
778 /// in `scc_a`. Used during constraint propagation, and only once
779 /// the value of `scc_b` has been computed.
780 fn universe_compatible(&self, scc_b: ConstraintSccIndex, scc_a: ConstraintSccIndex) -> bool {
781 let universe_a = self.scc_universes[scc_a];
783 // Quick check: if scc_b's declared universe is a subset of
784 // scc_a's declared univese (typically, both are ROOT), then
785 // it cannot contain any problematic universe elements.
786 if universe_a.can_name(self.scc_universes[scc_b]) {
790 // Otherwise, we have to iterate over the universe elements in
791 // B's value, and check whether all of them are nameable
793 self.scc_values.placeholders_contained_in(scc_b).all(|p| universe_a.can_name(p.universe))
796 /// Extend `scc` so that it can outlive some placeholder region
797 /// from a universe it can't name; at present, the only way for
798 /// this to be true is if `scc` outlives `'static`. This is
799 /// actually stricter than necessary: ideally, we'd support bounds
800 /// like `for<'a: 'b`>` that might then allow us to approximate
801 /// `'a` with `'b` and not `'static`. But it will have to do for
803 fn add_incompatible_universe(&mut self, scc: ConstraintSccIndex) {
804 debug!("add_incompatible_universe(scc={:?})", scc);
806 let fr_static = self.universal_regions.fr_static;
807 self.scc_values.add_all_points(scc);
808 self.scc_values.add_element(scc, fr_static);
811 /// Once regions have been propagated, this method is used to see
812 /// whether the "type tests" produced by typeck were satisfied;
813 /// type tests encode type-outlives relationships like `T:
814 /// 'a`. See `TypeTest` for more details.
817 infcx: &InferCtxt<'_, 'tcx>,
819 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
820 errors_buffer: &mut RegionErrors<'tcx>,
824 // Sometimes we register equivalent type-tests that would
825 // result in basically the exact same error being reported to
826 // the user. Avoid that.
827 let mut deduplicate_errors = FxHashSet::default();
829 for type_test in &self.type_tests {
830 debug!("check_type_test: {:?}", type_test);
832 let generic_ty = type_test.generic_kind.to_ty(tcx);
833 if self.eval_verify_bound(
837 type_test.lower_bound,
838 &type_test.verify_bound,
843 if let Some(propagated_outlives_requirements) = &mut propagated_outlives_requirements {
844 if self.try_promote_type_test(
848 propagated_outlives_requirements,
854 // Type-test failed. Report the error.
855 let erased_generic_kind = infcx.tcx.erase_regions(type_test.generic_kind);
857 // Skip duplicate-ish errors.
858 if deduplicate_errors.insert((
860 type_test.lower_bound,
864 "check_type_test: reporting error for erased_generic_kind={:?}, \
865 lower_bound_region={:?}, \
866 type_test.locations={:?}",
867 erased_generic_kind, type_test.lower_bound, type_test.locations,
870 errors_buffer.push(RegionErrorKind::TypeTestError { type_test: type_test.clone() });
875 /// Invoked when we have some type-test (e.g., `T: 'X`) that we cannot
876 /// prove to be satisfied. If this is a closure, we will attempt to
877 /// "promote" this type-test into our `ClosureRegionRequirements` and
878 /// hence pass it up the creator. To do this, we have to phrase the
879 /// type-test in terms of external free regions, as local free
880 /// regions are not nameable by the closure's creator.
882 /// Promotion works as follows: we first check that the type `T`
883 /// contains only regions that the creator knows about. If this is
884 /// true, then -- as a consequence -- we know that all regions in
885 /// the type `T` are free regions that outlive the closure body. If
886 /// false, then promotion fails.
888 /// Once we've promoted T, we have to "promote" `'X` to some region
889 /// that is "external" to the closure. Generally speaking, a region
890 /// may be the union of some points in the closure body as well as
891 /// various free lifetimes. We can ignore the points in the closure
892 /// body: if the type T can be expressed in terms of external regions,
893 /// we know it outlives the points in the closure body. That
894 /// just leaves the free regions.
896 /// The idea then is to lower the `T: 'X` constraint into multiple
897 /// bounds -- e.g., if `'X` is the union of two free lifetimes,
898 /// `'1` and `'2`, then we would create `T: '1` and `T: '2`.
899 fn try_promote_type_test(
901 infcx: &InferCtxt<'_, 'tcx>,
903 type_test: &TypeTest<'tcx>,
904 propagated_outlives_requirements: &mut Vec<ClosureOutlivesRequirement<'tcx>>,
908 let TypeTest { generic_kind, lower_bound, locations, verify_bound: _ } = type_test;
910 let generic_ty = generic_kind.to_ty(tcx);
911 let subject = match self.try_promote_type_test_subject(infcx, generic_ty) {
913 None => return false,
916 // For each region outlived by lower_bound find a non-local,
917 // universal region (it may be the same region) and add it to
918 // `ClosureOutlivesRequirement`.
919 let r_scc = self.constraint_sccs.scc(*lower_bound);
920 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
921 // Check whether we can already prove that the "subject" outlives `ur`.
922 // If so, we don't have to propagate this requirement to our caller.
924 // To continue the example from the function, if we are trying to promote
925 // a requirement that `T: 'X`, and we know that `'X = '1 + '2` (i.e., the union
926 // `'1` and `'2`), then in this loop `ur` will be `'1` (and `'2`). So here
927 // we check whether `T: '1` is something we *can* prove. If so, no need
928 // to propagate that requirement.
930 // This is needed because -- particularly in the case
931 // where `ur` is a local bound -- we are sometimes in a
932 // position to prove things that our caller cannot. See
933 // #53570 for an example.
934 if self.eval_verify_bound(tcx, body, generic_ty, ur, &type_test.verify_bound) {
938 debug!("try_promote_type_test: ur={:?}", ur);
940 let non_local_ub = self.universal_region_relations.non_local_upper_bounds(&ur);
941 debug!("try_promote_type_test: non_local_ub={:?}", non_local_ub);
943 // This is slightly too conservative. To show T: '1, given `'2: '1`
944 // and `'3: '1` we only need to prove that T: '2 *or* T: '3, but to
945 // avoid potential non-determinism we approximate this by requiring
947 for &upper_bound in non_local_ub {
948 debug_assert!(self.universal_regions.is_universal_region(upper_bound));
949 debug_assert!(!self.universal_regions.is_local_free_region(upper_bound));
951 let requirement = ClosureOutlivesRequirement {
953 outlived_free_region: upper_bound,
954 blame_span: locations.span(body),
955 category: ConstraintCategory::Boring,
957 debug!("try_promote_type_test: pushing {:#?}", requirement);
958 propagated_outlives_requirements.push(requirement);
964 /// When we promote a type test `T: 'r`, we have to convert the
965 /// type `T` into something we can store in a query result (so
966 /// something allocated for `'tcx`). This is problematic if `ty`
967 /// contains regions. During the course of NLL region checking, we
968 /// will have replaced all of those regions with fresh inference
969 /// variables. To create a test subject, we want to replace those
970 /// inference variables with some region from the closure
971 /// signature -- this is not always possible, so this is a
972 /// fallible process. Presuming we do find a suitable region, we
973 /// will use it's *external name*, which will be a `RegionKind`
974 /// variant that can be used in query responses such as
976 fn try_promote_type_test_subject(
978 infcx: &InferCtxt<'_, 'tcx>,
980 ) -> Option<ClosureOutlivesSubject<'tcx>> {
983 debug!("try_promote_type_test_subject(ty = {:?})", ty);
985 let ty = tcx.fold_regions(ty, &mut false, |r, _depth| {
986 let region_vid = self.to_region_vid(r);
988 // The challenge if this. We have some region variable `r`
989 // whose value is a set of CFG points and universal
990 // regions. We want to find if that set is *equivalent* to
991 // any of the named regions found in the closure.
993 // To do so, we compute the
994 // `non_local_universal_upper_bound`. This will be a
995 // non-local, universal region that is greater than `r`.
996 // However, it might not be *contained* within `r`, so
997 // then we further check whether this bound is contained
998 // in `r`. If so, we can say that `r` is equivalent to the
1001 // Let's work through a few examples. For these, imagine
1002 // that we have 3 non-local regions (I'll denote them as
1003 // `'static`, `'a`, and `'b`, though of course in the code
1004 // they would be represented with indices) where:
1009 // First, let's assume that `r` is some existential
1010 // variable with an inferred value `{'a, 'static}` (plus
1011 // some CFG nodes). In this case, the non-local upper
1012 // bound is `'static`, since that outlives `'a`. `'static`
1013 // is also a member of `r` and hence we consider `r`
1014 // equivalent to `'static` (and replace it with
1017 // Now let's consider the inferred value `{'a, 'b}`. This
1018 // means `r` is effectively `'a | 'b`. I'm not sure if
1019 // this can come about, actually, but assuming it did, we
1020 // would get a non-local upper bound of `'static`. Since
1021 // `'static` is not contained in `r`, we would fail to
1022 // find an equivalent.
1023 let upper_bound = self.non_local_universal_upper_bound(region_vid);
1024 if self.region_contains(region_vid, upper_bound) {
1025 self.definitions[upper_bound].external_name.unwrap_or(r)
1027 // In the case of a failure, use a `ReVar` result. This will
1028 // cause the `needs_infer` later on to return `None`.
1033 debug!("try_promote_type_test_subject: folded ty = {:?}", ty);
1035 // `needs_infer` will only be true if we failed to promote some region.
1036 if ty.needs_infer() {
1040 Some(ClosureOutlivesSubject::Ty(ty))
1043 /// Given some universal or existential region `r`, finds a
1044 /// non-local, universal region `r+` that outlives `r` at entry to (and
1045 /// exit from) the closure. In the worst case, this will be
1048 /// This is used for two purposes. First, if we are propagated
1049 /// some requirement `T: r`, we can use this method to enlarge `r`
1050 /// to something we can encode for our creator (which only knows
1051 /// about non-local, universal regions). It is also used when
1052 /// encoding `T` as part of `try_promote_type_test_subject` (see
1053 /// that fn for details).
1055 /// This is based on the result `'y` of `universal_upper_bound`,
1056 /// except that it converts further takes the non-local upper
1057 /// bound of `'y`, so that the final result is non-local.
1058 fn non_local_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1059 debug!("non_local_universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1061 let lub = self.universal_upper_bound(r);
1063 // Grow further to get smallest universal region known to
1065 let non_local_lub = self.universal_region_relations.non_local_upper_bound(lub);
1067 debug!("non_local_universal_upper_bound: non_local_lub={:?}", non_local_lub);
1072 /// Returns a universally quantified region that outlives the
1073 /// value of `r` (`r` may be existentially or universally
1076 /// Since `r` is (potentially) an existential region, it has some
1077 /// value which may include (a) any number of points in the CFG
1078 /// and (b) any number of `end('x)` elements of universally
1079 /// quantified regions. To convert this into a single universal
1080 /// region we do as follows:
1082 /// - Ignore the CFG points in `'r`. All universally quantified regions
1083 /// include the CFG anyhow.
1084 /// - For each `end('x)` element in `'r`, compute the mutual LUB, yielding
1086 pub(in crate::borrow_check) fn universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1087 debug!("universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1089 // Find the smallest universal region that contains all other
1090 // universal regions within `region`.
1091 let mut lub = self.universal_regions.fr_fn_body;
1092 let r_scc = self.constraint_sccs.scc(r);
1093 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1094 lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1097 debug!("universal_upper_bound: r={:?} lub={:?}", r, lub);
1102 /// Like `universal_upper_bound`, but returns an approximation more suitable
1103 /// for diagnostics. If `r` contains multiple disjoint universal regions
1104 /// (e.g. 'a and 'b in `fn foo<'a, 'b> { ... }`, we pick the lower-numbered region.
1105 /// This corresponds to picking named regions over unnamed regions
1106 /// (e.g. picking early-bound regions over a closure late-bound region).
1108 /// This means that the returned value may not be a true upper bound, since
1109 /// only 'static is known to outlive disjoint universal regions.
1110 /// Therefore, this method should only be used in diagnostic code,
1111 /// where displaying *some* named universal region is better than
1112 /// falling back to 'static.
1113 pub(in crate::borrow_check) fn approx_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1114 debug!("approx_universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1116 // Find the smallest universal region that contains all other
1117 // universal regions within `region`.
1118 let mut lub = self.universal_regions.fr_fn_body;
1119 let r_scc = self.constraint_sccs.scc(r);
1120 let static_r = self.universal_regions.fr_static;
1121 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1122 let new_lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1123 debug!("approx_universal_upper_bound: ur={:?} lub={:?} new_lub={:?}", ur, lub, new_lub);
1124 // The upper bound of two non-static regions is static: this
1125 // means we know nothing about the relationship between these
1126 // two regions. Pick a 'better' one to use when constructing
1128 if ur != static_r && lub != static_r && new_lub == static_r {
1129 // Prefer the region with an `external_name` - this
1130 // indicates that the region is early-bound, so working with
1131 // it can produce a nicer error.
1132 if self.region_definition(ur).external_name.is_some() {
1134 } else if self.region_definition(lub).external_name.is_some() {
1135 // Leave lub unchanged
1137 // If we get here, we don't have any reason to prefer
1138 // one region over the other. Just pick the
1139 // one with the lower index for now.
1140 lub = std::cmp::min(ur, lub);
1147 debug!("approx_universal_upper_bound: r={:?} lub={:?}", r, lub);
1152 /// Tests if `test` is true when applied to `lower_bound` at
1154 fn eval_verify_bound(
1158 generic_ty: Ty<'tcx>,
1159 lower_bound: RegionVid,
1160 verify_bound: &VerifyBound<'tcx>,
1162 debug!("eval_verify_bound(lower_bound={:?}, verify_bound={:?})", lower_bound, verify_bound);
1164 match verify_bound {
1165 VerifyBound::IfEq(test_ty, verify_bound1) => {
1166 self.eval_if_eq(tcx, body, generic_ty, lower_bound, test_ty, verify_bound1)
1169 VerifyBound::IsEmpty => {
1170 let lower_bound_scc = self.constraint_sccs.scc(lower_bound);
1171 self.scc_values.elements_contained_in(lower_bound_scc).next().is_none()
1174 VerifyBound::OutlivedBy(r) => {
1175 let r_vid = self.to_region_vid(r);
1176 self.eval_outlives(r_vid, lower_bound)
1179 VerifyBound::AnyBound(verify_bounds) => verify_bounds.iter().any(|verify_bound| {
1180 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1183 VerifyBound::AllBounds(verify_bounds) => verify_bounds.iter().all(|verify_bound| {
1184 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1193 generic_ty: Ty<'tcx>,
1194 lower_bound: RegionVid,
1196 verify_bound: &VerifyBound<'tcx>,
1198 let generic_ty_normalized = self.normalize_to_scc_representatives(tcx, generic_ty);
1199 let test_ty_normalized = self.normalize_to_scc_representatives(tcx, test_ty);
1200 if generic_ty_normalized == test_ty_normalized {
1201 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1207 /// This is a conservative normalization procedure. It takes every
1208 /// free region in `value` and replaces it with the
1209 /// "representative" of its SCC (see `scc_representatives` field).
1210 /// We are guaranteed that if two values normalize to the same
1211 /// thing, then they are equal; this is a conservative check in
1212 /// that they could still be equal even if they normalize to
1213 /// different results. (For example, there might be two regions
1214 /// with the same value that are not in the same SCC).
1216 /// N.B., this is not an ideal approach and I would like to revisit
1217 /// it. However, it works pretty well in practice. In particular,
1218 /// this is needed to deal with projection outlives bounds like
1221 /// <T as Foo<'0>>::Item: '1
1224 /// In particular, this routine winds up being important when
1225 /// there are bounds like `where <T as Foo<'a>>::Item: 'b` in the
1226 /// environment. In this case, if we can show that `'0 == 'a`,
1227 /// and that `'b: '1`, then we know that the clause is
1228 /// satisfied. In such cases, particularly due to limitations of
1229 /// the trait solver =), we usually wind up with a where-clause like
1230 /// `T: Foo<'a>` in scope, which thus forces `'0 == 'a` to be added as
1231 /// a constraint, and thus ensures that they are in the same SCC.
1233 /// So why can't we do a more correct routine? Well, we could
1234 /// *almost* use the `relate_tys` code, but the way it is
1235 /// currently setup it creates inference variables to deal with
1236 /// higher-ranked things and so forth, and right now the inference
1237 /// context is not permitted to make more inference variables. So
1238 /// we use this kind of hacky solution.
1239 fn normalize_to_scc_representatives<T>(&self, tcx: TyCtxt<'tcx>, value: T) -> T
1241 T: TypeFoldable<'tcx>,
1243 tcx.fold_regions(value, &mut false, |r, _db| {
1244 let vid = self.to_region_vid(r);
1245 let scc = self.constraint_sccs.scc(vid);
1246 let repr = self.scc_representatives[scc];
1247 tcx.mk_region(ty::ReVar(repr))
1251 // Evaluate whether `sup_region == sub_region`.
1252 fn eval_equal(&self, r1: RegionVid, r2: RegionVid) -> bool {
1253 self.eval_outlives(r1, r2) && self.eval_outlives(r2, r1)
1256 // Evaluate whether `sup_region: sub_region`.
1257 fn eval_outlives(&self, sup_region: RegionVid, sub_region: RegionVid) -> bool {
1258 debug!("eval_outlives({:?}: {:?})", sup_region, sub_region);
1261 "eval_outlives: sup_region's value = {:?} universal={:?}",
1262 self.region_value_str(sup_region),
1263 self.universal_regions.is_universal_region(sup_region),
1266 "eval_outlives: sub_region's value = {:?} universal={:?}",
1267 self.region_value_str(sub_region),
1268 self.universal_regions.is_universal_region(sub_region),
1271 let sub_region_scc = self.constraint_sccs.scc(sub_region);
1272 let sup_region_scc = self.constraint_sccs.scc(sup_region);
1274 // Both the `sub_region` and `sup_region` consist of the union
1275 // of some number of universal regions (along with the union
1276 // of various points in the CFG; ignore those points for
1277 // now). Therefore, the sup-region outlives the sub-region if,
1278 // for each universal region R1 in the sub-region, there
1279 // exists some region R2 in the sup-region that outlives R1.
1280 let universal_outlives =
1281 self.scc_values.universal_regions_outlived_by(sub_region_scc).all(|r1| {
1283 .universal_regions_outlived_by(sup_region_scc)
1284 .any(|r2| self.universal_region_relations.outlives(r2, r1))
1287 if !universal_outlives {
1291 // Now we have to compare all the points in the sub region and make
1292 // sure they exist in the sup region.
1294 if self.universal_regions.is_universal_region(sup_region) {
1295 // Micro-opt: universal regions contain all points.
1299 self.scc_values.contains_points(sup_region_scc, sub_region_scc)
1302 /// Once regions have been propagated, this method is used to see
1303 /// whether any of the constraints were too strong. In particular,
1304 /// we want to check for a case where a universally quantified
1305 /// region exceeded its bounds. Consider:
1307 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1309 /// In this case, returning `x` requires `&'a u32 <: &'b u32`
1310 /// and hence we establish (transitively) a constraint that
1311 /// `'a: 'b`. The `propagate_constraints` code above will
1312 /// therefore add `end('a)` into the region for `'b` -- but we
1313 /// have no evidence that `'b` outlives `'a`, so we want to report
1316 /// If `propagated_outlives_requirements` is `Some`, then we will
1317 /// push unsatisfied obligations into there. Otherwise, we'll
1318 /// report them as errors.
1319 fn check_universal_regions(
1322 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1323 errors_buffer: &mut RegionErrors<'tcx>,
1325 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1326 match fr_definition.origin {
1327 NllRegionVariableOrigin::FreeRegion => {
1328 // Go through each of the universal regions `fr` and check that
1329 // they did not grow too large, accumulating any requirements
1330 // for our caller into the `outlives_requirements` vector.
1331 self.check_universal_region(
1334 &mut propagated_outlives_requirements,
1339 NllRegionVariableOrigin::Placeholder(placeholder) => {
1340 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1343 NllRegionVariableOrigin::RootEmptyRegion
1344 | NllRegionVariableOrigin::Existential { .. } => {
1345 // nothing to check here
1351 /// Checks if Polonius has found any unexpected free region relations.
1353 /// In Polonius terms, a "subset error" (or "illegal subset relation error") is the equivalent
1354 /// of NLL's "checking if any region constraints were too strong": a placeholder origin `'a`
1355 /// was unexpectedly found to be a subset of another placeholder origin `'b`, and means in NLL
1356 /// terms that the "longer free region" `'a` outlived the "shorter free region" `'b`.
1358 /// More details can be found in this blog post by Niko:
1359 /// <https://smallcultfollowing.com/babysteps/blog/2019/01/17/polonius-and-region-errors/>
1361 /// In the canonical example
1363 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1365 /// returning `x` requires `&'a u32 <: &'b u32` and hence we establish (transitively) a
1366 /// constraint that `'a: 'b`. It is an error that we have no evidence that this
1367 /// constraint holds.
1369 /// If `propagated_outlives_requirements` is `Some`, then we will
1370 /// push unsatisfied obligations into there. Otherwise, we'll
1371 /// report them as errors.
1372 fn check_polonius_subset_errors(
1375 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1376 errors_buffer: &mut RegionErrors<'tcx>,
1377 polonius_output: Rc<PoloniusOutput>,
1380 "check_polonius_subset_errors: {} subset_errors",
1381 polonius_output.subset_errors.len()
1384 // Similarly to `check_universal_regions`: a free region relation, which was not explicitly
1385 // declared ("known") was found by Polonius, so emit an error, or propagate the
1386 // requirements for our caller into the `propagated_outlives_requirements` vector.
1388 // Polonius doesn't model regions ("origins") as CFG-subsets or durations, but the
1389 // `longer_fr` and `shorter_fr` terminology will still be used here, for consistency with
1390 // the rest of the NLL infrastructure. The "subset origin" is the "longer free region",
1391 // and the "superset origin" is the outlived "shorter free region".
1393 // Note: Polonius will produce a subset error at every point where the unexpected
1394 // `longer_fr`'s "placeholder loan" is contained in the `shorter_fr`. This can be helpful
1395 // for diagnostics in the future, e.g. to point more precisely at the key locations
1396 // requiring this constraint to hold. However, the error and diagnostics code downstream
1397 // expects that these errors are not duplicated (and that they are in a certain order).
1398 // Otherwise, diagnostics messages such as the ones giving names like `'1` to elided or
1399 // anonymous lifetimes for example, could give these names differently, while others like
1400 // the outlives suggestions or the debug output from `#[rustc_regions]` would be
1401 // duplicated. The polonius subset errors are deduplicated here, while keeping the
1402 // CFG-location ordering.
1403 let mut subset_errors: Vec<_> = polonius_output
1406 .flat_map(|(_location, subset_errors)| subset_errors.iter())
1408 subset_errors.sort();
1409 subset_errors.dedup();
1411 for (longer_fr, shorter_fr) in subset_errors.into_iter() {
1413 "check_polonius_subset_errors: subset_error longer_fr={:?},\
1415 longer_fr, shorter_fr
1418 let propagated = self.try_propagate_universal_region_error(
1422 &mut propagated_outlives_requirements,
1424 if propagated == RegionRelationCheckResult::Error {
1425 errors_buffer.push(RegionErrorKind::RegionError {
1426 longer_fr: *longer_fr,
1427 shorter_fr: *shorter_fr,
1428 fr_origin: NllRegionVariableOrigin::FreeRegion,
1434 // Handle the placeholder errors as usual, until the chalk-rustc-polonius triumvirate has
1435 // a more complete picture on how to separate this responsibility.
1436 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1437 match fr_definition.origin {
1438 NllRegionVariableOrigin::FreeRegion => {
1439 // handled by polonius above
1442 NllRegionVariableOrigin::Placeholder(placeholder) => {
1443 self.check_bound_universal_region(fr, placeholder, errors_buffer);
1446 NllRegionVariableOrigin::RootEmptyRegion
1447 | NllRegionVariableOrigin::Existential { .. } => {
1448 // nothing to check here
1454 /// Checks the final value for the free region `fr` to see if it
1455 /// grew too large. In particular, examine what `end(X)` points
1456 /// wound up in `fr`'s final value; for each `end(X)` where `X !=
1457 /// fr`, we want to check that `fr: X`. If not, that's either an
1458 /// error, or something we have to propagate to our creator.
1460 /// Things that are to be propagated are accumulated into the
1461 /// `outlives_requirements` vector.
1462 fn check_universal_region(
1465 longer_fr: RegionVid,
1466 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1467 errors_buffer: &mut RegionErrors<'tcx>,
1469 debug!("check_universal_region(fr={:?})", longer_fr);
1471 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1473 // Because this free region must be in the ROOT universe, we
1474 // know it cannot contain any bound universes.
1475 assert!(self.scc_universes[longer_fr_scc] == ty::UniverseIndex::ROOT);
1476 debug_assert!(self.scc_values.placeholders_contained_in(longer_fr_scc).next().is_none());
1478 // Only check all of the relations for the main representative of each
1479 // SCC, otherwise just check that we outlive said representative. This
1480 // reduces the number of redundant relations propagated out of
1482 // Note that the representative will be a universal region if there is
1483 // one in this SCC, so we will always check the representative here.
1484 let representative = self.scc_representatives[longer_fr_scc];
1485 if representative != longer_fr {
1486 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1490 propagated_outlives_requirements,
1492 errors_buffer.push(RegionErrorKind::RegionError {
1494 shorter_fr: representative,
1495 fr_origin: NllRegionVariableOrigin::FreeRegion,
1502 // Find every region `o` such that `fr: o`
1503 // (because `fr` includes `end(o)`).
1504 let mut error_reported = false;
1505 for shorter_fr in self.scc_values.universal_regions_outlived_by(longer_fr_scc) {
1506 if let RegionRelationCheckResult::Error = self.check_universal_region_relation(
1510 propagated_outlives_requirements,
1512 // We only report the first region error. Subsequent errors are hidden so as
1513 // not to overwhelm the user, but we do record them so as to potentially print
1514 // better diagnostics elsewhere...
1515 errors_buffer.push(RegionErrorKind::RegionError {
1518 fr_origin: NllRegionVariableOrigin::FreeRegion,
1519 is_reported: !error_reported,
1522 error_reported = true;
1527 /// Checks that we can prove that `longer_fr: shorter_fr`. If we can't we attempt to propagate
1528 /// the constraint outward (e.g. to a closure environment), but if that fails, there is an
1530 fn check_universal_region_relation(
1532 longer_fr: RegionVid,
1533 shorter_fr: RegionVid,
1535 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1536 ) -> RegionRelationCheckResult {
1537 // If it is known that `fr: o`, carry on.
1538 if self.universal_region_relations.outlives(longer_fr, shorter_fr) {
1539 RegionRelationCheckResult::Ok
1541 // If we are not in a context where we can't propagate errors, or we
1542 // could not shrink `fr` to something smaller, then just report an
1545 // Note: in this case, we use the unapproximated regions to report the
1546 // error. This gives better error messages in some cases.
1547 self.try_propagate_universal_region_error(
1551 propagated_outlives_requirements,
1556 /// Attempt to propagate a region error (e.g. `'a: 'b`) that is not met to a closure's
1557 /// creator. If we cannot, then the caller should report an error to the user.
1558 fn try_propagate_universal_region_error(
1560 longer_fr: RegionVid,
1561 shorter_fr: RegionVid,
1563 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1564 ) -> RegionRelationCheckResult {
1565 if let Some(propagated_outlives_requirements) = propagated_outlives_requirements {
1566 // Shrink `longer_fr` until we find a non-local region (if we do).
1567 // We'll call it `fr-` -- it's ever so slightly smaller than
1569 if let Some(fr_minus) = self.universal_region_relations.non_local_lower_bound(longer_fr)
1571 debug!("try_propagate_universal_region_error: fr_minus={:?}", fr_minus);
1573 let blame_span_category = self.find_outlives_blame_span(
1576 NllRegionVariableOrigin::FreeRegion,
1580 // Grow `shorter_fr` until we find some non-local regions. (We
1581 // always will.) We'll call them `shorter_fr+` -- they're ever
1582 // so slightly larger than `shorter_fr`.
1583 let shorter_fr_plus =
1584 self.universal_region_relations.non_local_upper_bounds(&shorter_fr);
1586 "try_propagate_universal_region_error: shorter_fr_plus={:?}",
1589 for &&fr in &shorter_fr_plus {
1590 // Push the constraint `fr-: shorter_fr+`
1591 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
1592 subject: ClosureOutlivesSubject::Region(fr_minus),
1593 outlived_free_region: fr,
1594 blame_span: blame_span_category.1,
1595 category: blame_span_category.0,
1598 return RegionRelationCheckResult::Propagated;
1602 RegionRelationCheckResult::Error
1605 fn check_bound_universal_region(
1607 longer_fr: RegionVid,
1608 placeholder: ty::PlaceholderRegion,
1609 errors_buffer: &mut RegionErrors<'tcx>,
1611 debug!("check_bound_universal_region(fr={:?}, placeholder={:?})", longer_fr, placeholder,);
1613 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1614 debug!("check_bound_universal_region: longer_fr_scc={:?}", longer_fr_scc,);
1616 // If we have some bound universal region `'a`, then the only
1617 // elements it can contain is itself -- we don't know anything
1619 let error_element = match {
1620 self.scc_values.elements_contained_in(longer_fr_scc).find(|element| match element {
1621 RegionElement::Location(_) => true,
1622 RegionElement::RootUniversalRegion(_) => true,
1623 RegionElement::PlaceholderRegion(placeholder1) => placeholder != *placeholder1,
1629 debug!("check_bound_universal_region: error_element = {:?}", error_element);
1631 // Find the region that introduced this `error_element`.
1632 errors_buffer.push(RegionErrorKind::BoundUniversalRegionError {
1635 fr_origin: NllRegionVariableOrigin::Placeholder(placeholder),
1639 fn check_member_constraints(
1641 infcx: &InferCtxt<'_, 'tcx>,
1642 errors_buffer: &mut RegionErrors<'tcx>,
1644 let member_constraints = self.member_constraints.clone();
1645 for m_c_i in member_constraints.all_indices() {
1646 debug!("check_member_constraint(m_c_i={:?})", m_c_i);
1647 let m_c = &member_constraints[m_c_i];
1648 let member_region_vid = m_c.member_region_vid;
1650 "check_member_constraint: member_region_vid={:?} with value {}",
1652 self.region_value_str(member_region_vid),
1654 let choice_regions = member_constraints.choice_regions(m_c_i);
1655 debug!("check_member_constraint: choice_regions={:?}", choice_regions);
1657 // Did the member region wind up equal to any of the option regions?
1659 choice_regions.iter().find(|&&o_r| self.eval_equal(o_r, m_c.member_region_vid))
1661 debug!("check_member_constraint: evaluated as equal to {:?}", o);
1665 // If not, report an error.
1666 let member_region = infcx.tcx.mk_region(ty::ReVar(member_region_vid));
1667 errors_buffer.push(RegionErrorKind::UnexpectedHiddenRegion {
1668 span: m_c.definition_span,
1669 hidden_ty: m_c.hidden_ty,
1675 /// We have a constraint `fr1: fr2` that is not satisfied, where
1676 /// `fr2` represents some universal region. Here, `r` is some
1677 /// region where we know that `fr1: r` and this function has the
1678 /// job of determining whether `r` is "to blame" for the fact that
1679 /// `fr1: fr2` is required.
1681 /// This is true under two conditions:
1684 /// - `fr2` is `'static` and `r` is some placeholder in a universe
1685 /// that cannot be named by `fr1`; in that case, we will require
1686 /// that `fr1: 'static` because it is the only way to `fr1: r` to
1687 /// be satisfied. (See `add_incompatible_universe`.)
1688 crate fn provides_universal_region(
1694 debug!("provides_universal_region(r={:?}, fr1={:?}, fr2={:?})", r, fr1, fr2);
1697 fr2 == self.universal_regions.fr_static && self.cannot_name_placeholder(fr1, r)
1700 debug!("provides_universal_region: result = {:?}", result);
1704 /// If `r2` represents a placeholder region, then this returns
1705 /// `true` if `r1` cannot name that placeholder in its
1706 /// value; otherwise, returns `false`.
1707 crate fn cannot_name_placeholder(&self, r1: RegionVid, r2: RegionVid) -> bool {
1708 debug!("cannot_name_value_of(r1={:?}, r2={:?})", r1, r2);
1710 match self.definitions[r2].origin {
1711 NllRegionVariableOrigin::Placeholder(placeholder) => {
1712 let universe1 = self.definitions[r1].universe;
1714 "cannot_name_value_of: universe1={:?} placeholder={:?}",
1715 universe1, placeholder
1717 universe1.cannot_name(placeholder.universe)
1720 NllRegionVariableOrigin::RootEmptyRegion
1721 | NllRegionVariableOrigin::FreeRegion
1722 | NllRegionVariableOrigin::Existential { .. } => false,
1726 crate fn retrieve_closure_constraint_info(
1729 constraint: &OutlivesConstraint<'tcx>,
1730 ) -> BlameConstraint<'tcx> {
1731 let loc = match constraint.locations {
1732 Locations::All(span) => {
1733 return BlameConstraint {
1734 category: constraint.category,
1735 from_closure: false,
1737 variance_info: constraint.variance_info.clone(),
1740 Locations::Single(loc) => loc,
1743 let opt_span_category =
1744 self.closure_bounds_mapping[&loc].get(&(constraint.sup, constraint.sub));
1746 .map(|&(category, span)| BlameConstraint {
1750 variance_info: constraint.variance_info.clone(),
1752 .unwrap_or(BlameConstraint {
1753 category: constraint.category,
1754 from_closure: false,
1755 span: body.source_info(loc).span,
1756 variance_info: constraint.variance_info.clone(),
1760 /// Finds a good span to blame for the fact that `fr1` outlives `fr2`.
1761 crate fn find_outlives_blame_span(
1765 fr1_origin: NllRegionVariableOrigin,
1767 ) -> (ConstraintCategory, Span) {
1768 let BlameConstraint { category, span, .. } =
1769 self.best_blame_constraint(body, fr1, fr1_origin, |r| {
1770 self.provides_universal_region(r, fr1, fr2)
1775 /// Walks the graph of constraints (where `'a: 'b` is considered
1776 /// an edge `'a -> 'b`) to find all paths from `from_region` to
1777 /// `to_region`. The paths are accumulated into the vector
1778 /// `results`. The paths are stored as a series of
1779 /// `ConstraintIndex` values -- in other words, a list of *edges*.
1781 /// Returns: a series of constraints as well as the region `R`
1782 /// that passed the target test.
1783 crate fn find_constraint_paths_between_regions(
1785 from_region: RegionVid,
1786 target_test: impl Fn(RegionVid) -> bool,
1787 ) -> Option<(Vec<OutlivesConstraint<'tcx>>, RegionVid)> {
1788 let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions);
1789 context[from_region] = Trace::StartRegion;
1791 // Use a deque so that we do a breadth-first search. We will
1792 // stop at the first match, which ought to be the shortest
1793 // path (fewest constraints).
1794 let mut deque = VecDeque::new();
1795 deque.push_back(from_region);
1797 while let Some(r) = deque.pop_front() {
1799 "find_constraint_paths_between_regions: from_region={:?} r={:?} value={}",
1802 self.region_value_str(r),
1805 // Check if we reached the region we were looking for. If so,
1806 // we can reconstruct the path that led to it and return it.
1808 let mut result = vec![];
1811 match context[p].clone() {
1812 Trace::NotVisited => {
1813 bug!("found unvisited region {:?} on path to {:?}", p, r)
1816 Trace::FromOutlivesConstraint(c) => {
1821 Trace::StartRegion => {
1823 return Some((result, r));
1829 // Otherwise, walk over the outgoing constraints and
1830 // enqueue any regions we find, keeping track of how we
1833 // A constraint like `'r: 'x` can come from our constraint
1835 let fr_static = self.universal_regions.fr_static;
1836 let outgoing_edges_from_graph =
1837 self.constraint_graph.outgoing_edges(r, &self.constraints, fr_static);
1839 // Always inline this closure because it can be hot.
1840 let mut handle_constraint = #[inline(always)]
1841 |constraint: OutlivesConstraint<'tcx>| {
1842 debug_assert_eq!(constraint.sup, r);
1843 let sub_region = constraint.sub;
1844 if let Trace::NotVisited = context[sub_region] {
1845 context[sub_region] = Trace::FromOutlivesConstraint(constraint);
1846 deque.push_back(sub_region);
1850 // This loop can be hot.
1851 for constraint in outgoing_edges_from_graph {
1852 handle_constraint(constraint);
1855 // Member constraints can also give rise to `'r: 'x` edges that
1856 // were not part of the graph initially, so watch out for those.
1857 // (But they are extremely rare; this loop is very cold.)
1858 for constraint in self.applied_member_constraints(r) {
1859 let p_c = &self.member_constraints[constraint.member_constraint_index];
1860 let constraint = OutlivesConstraint {
1862 sub: constraint.min_choice,
1863 locations: Locations::All(p_c.definition_span),
1864 category: ConstraintCategory::OpaqueType,
1865 variance_info: ty::VarianceDiagInfo::default(),
1867 handle_constraint(constraint);
1874 /// Finds some region R such that `fr1: R` and `R` is live at `elem`.
1875 crate fn find_sub_region_live_at(&self, fr1: RegionVid, elem: Location) -> RegionVid {
1876 debug!("find_sub_region_live_at(fr1={:?}, elem={:?})", fr1, elem);
1877 debug!("find_sub_region_live_at: {:?} is in scc {:?}", fr1, self.constraint_sccs.scc(fr1));
1879 "find_sub_region_live_at: {:?} is in universe {:?}",
1881 self.scc_universes[self.constraint_sccs.scc(fr1)]
1883 self.find_constraint_paths_between_regions(fr1, |r| {
1884 // First look for some `r` such that `fr1: r` and `r` is live at `elem`
1886 "find_sub_region_live_at: liveness_constraints for {:?} are {:?}",
1888 self.liveness_constraints.region_value_str(r),
1890 self.liveness_constraints.contains(r, elem)
1893 // If we fail to find that, we may find some `r` such that
1894 // `fr1: r` and `r` is a placeholder from some universe
1895 // `fr1` cannot name. This would force `fr1` to be
1897 self.find_constraint_paths_between_regions(fr1, |r| {
1898 self.cannot_name_placeholder(fr1, r)
1902 // If we fail to find THAT, it may be that `fr1` is a
1903 // placeholder that cannot "fit" into its SCC. In that
1904 // case, there should be some `r` where `fr1: r` and `fr1` is a
1905 // placeholder that `r` cannot name. We can blame that
1908 // Remember that if `R1: R2`, then the universe of R1
1909 // must be able to name the universe of R2, because R2 will
1910 // be at least `'empty(Universe(R2))`, and `R1` must be at
1911 // larger than that.
1912 self.find_constraint_paths_between_regions(fr1, |r| {
1913 self.cannot_name_placeholder(r, fr1)
1916 .map(|(_path, r)| r)
1920 /// Get the region outlived by `longer_fr` and live at `element`.
1921 crate fn region_from_element(&self, longer_fr: RegionVid, element: RegionElement) -> RegionVid {
1923 RegionElement::Location(l) => self.find_sub_region_live_at(longer_fr, l),
1924 RegionElement::RootUniversalRegion(r) => r,
1925 RegionElement::PlaceholderRegion(error_placeholder) => self
1928 .find_map(|(r, definition)| match definition.origin {
1929 NllRegionVariableOrigin::Placeholder(p) if p == error_placeholder => Some(r),
1936 /// Get the region definition of `r`.
1937 crate fn region_definition(&self, r: RegionVid) -> &RegionDefinition<'tcx> {
1938 &self.definitions[r]
1941 /// Check if the SCC of `r` contains `upper`.
1942 crate fn upper_bound_in_region_scc(&self, r: RegionVid, upper: RegionVid) -> bool {
1943 let r_scc = self.constraint_sccs.scc(r);
1944 self.scc_values.contains(r_scc, upper)
1947 crate fn universal_regions(&self) -> &UniversalRegions<'tcx> {
1948 self.universal_regions.as_ref()
1951 /// Tries to find the best constraint to blame for the fact that
1952 /// `R: from_region`, where `R` is some region that meets
1953 /// `target_test`. This works by following the constraint graph,
1954 /// creating a constraint path that forces `R` to outlive
1955 /// `from_region`, and then finding the best choices within that
1957 crate fn best_blame_constraint(
1960 from_region: RegionVid,
1961 from_region_origin: NllRegionVariableOrigin,
1962 target_test: impl Fn(RegionVid) -> bool,
1963 ) -> BlameConstraint<'tcx> {
1965 "best_blame_constraint(from_region={:?}, from_region_origin={:?})",
1966 from_region, from_region_origin
1970 let (path, target_region) =
1971 self.find_constraint_paths_between_regions(from_region, target_test).unwrap();
1973 "best_blame_constraint: path={:#?}",
1976 "{:?} ({:?}: {:?})",
1978 self.constraint_sccs.scc(c.sup),
1979 self.constraint_sccs.scc(c.sub),
1981 .collect::<Vec<_>>()
1984 // Classify each of the constraints along the path.
1985 let mut categorized_path: Vec<BlameConstraint<'tcx>> = path
1988 if constraint.category == ConstraintCategory::ClosureBounds {
1989 self.retrieve_closure_constraint_info(body, &constraint)
1992 category: constraint.category,
1993 from_closure: false,
1994 span: constraint.locations.span(body),
1995 variance_info: constraint.variance_info.clone(),
2000 debug!("best_blame_constraint: categorized_path={:#?}", categorized_path);
2002 // To find the best span to cite, we first try to look for the
2003 // final constraint that is interesting and where the `sup` is
2004 // not unified with the ultimate target region. The reason
2005 // for this is that we have a chain of constraints that lead
2006 // from the source to the target region, something like:
2008 // '0: '1 ('0 is the source)
2013 // '5: '6 ('6 is the target)
2015 // Some of those regions are unified with `'6` (in the same
2016 // SCC). We want to screen those out. After that point, the
2017 // "closest" constraint we have to the end is going to be the
2018 // most likely to be the point where the value escapes -- but
2019 // we still want to screen for an "interesting" point to
2020 // highlight (e.g., a call site or something).
2021 let target_scc = self.constraint_sccs.scc(target_region);
2022 let mut range = 0..path.len();
2024 // As noted above, when reporting an error, there is typically a chain of constraints
2025 // leading from some "source" region which must outlive some "target" region.
2026 // In most cases, we prefer to "blame" the constraints closer to the target --
2027 // but there is one exception. When constraints arise from higher-ranked subtyping,
2028 // we generally prefer to blame the source value,
2029 // as the "target" in this case tends to be some type annotation that the user gave.
2030 // Therefore, if we find that the region origin is some instantiation
2031 // of a higher-ranked region, we start our search from the "source" point
2032 // rather than the "target", and we also tweak a few other things.
2034 // An example might be this bit of Rust code:
2037 // let x: fn(&'static ()) = |_| {};
2038 // let y: for<'a> fn(&'a ()) = x;
2041 // In MIR, this will be converted into a combination of assignments and type ascriptions.
2042 // In particular, the 'static is imposed through a type ascription:
2046 // AscribeUserType(x, fn(&'static ())
2050 // We wind up ultimately with constraints like
2053 // !a: 'temp1 // from the `y = x` statement
2055 // 'temp2: 'static // from the AscribeUserType
2058 // and here we prefer to blame the source (the y = x statement).
2059 let blame_source = match from_region_origin {
2060 NllRegionVariableOrigin::FreeRegion
2061 | NllRegionVariableOrigin::Existential { from_forall: false } => true,
2062 NllRegionVariableOrigin::RootEmptyRegion
2063 | NllRegionVariableOrigin::Placeholder(_)
2064 | NllRegionVariableOrigin::Existential { from_forall: true } => false,
2067 let find_region = |i: &usize| {
2068 let constraint = &path[*i];
2070 let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
2073 match categorized_path[*i].category {
2074 ConstraintCategory::OpaqueType
2075 | ConstraintCategory::Boring
2076 | ConstraintCategory::BoringNoLocation
2077 | ConstraintCategory::Internal => false,
2078 ConstraintCategory::TypeAnnotation
2079 | ConstraintCategory::Return(_)
2080 | ConstraintCategory::Yield => true,
2081 _ => constraint_sup_scc != target_scc,
2084 match categorized_path[*i].category {
2085 ConstraintCategory::OpaqueType
2086 | ConstraintCategory::Boring
2087 | ConstraintCategory::BoringNoLocation
2088 | ConstraintCategory::Internal => false,
2095 if blame_source { range.rev().find(find_region) } else { range.find(find_region) };
2098 "best_blame_constraint: best_choice={:?} blame_source={}",
2099 best_choice, blame_source
2102 if let Some(i) = best_choice {
2103 if let Some(next) = categorized_path.get(i + 1) {
2104 if matches!(categorized_path[i].category, ConstraintCategory::Return(_))
2105 && next.category == ConstraintCategory::OpaqueType
2107 // The return expression is being influenced by the return type being
2108 // impl Trait, point at the return type and not the return expr.
2109 return next.clone();
2113 if categorized_path[i].category == ConstraintCategory::Return(ReturnConstraint::Normal)
2115 let field = categorized_path.iter().find_map(|p| {
2116 if let ConstraintCategory::ClosureUpvar(f) = p.category {
2123 if let Some(field) = field {
2124 categorized_path[i].category =
2125 ConstraintCategory::Return(ReturnConstraint::ClosureUpvar(field));
2129 return categorized_path[i].clone();
2132 // If that search fails, that is.. unusual. Maybe everything
2133 // is in the same SCC or something. In that case, find what
2134 // appears to be the most interesting point to report to the
2135 // user via an even more ad-hoc guess.
2136 categorized_path.sort_by(|p0, p1| p0.category.cmp(&p1.category));
2137 debug!("`: sorted_path={:#?}", categorized_path);
2139 categorized_path.remove(0)
2143 impl<'tcx> RegionDefinition<'tcx> {
2144 fn new(universe: ty::UniverseIndex, rv_origin: RegionVariableOrigin) -> Self {
2145 // Create a new region definition. Note that, for free
2146 // regions, the `external_name` field gets updated later in
2147 // `init_universal_regions`.
2149 let origin = match rv_origin {
2150 RegionVariableOrigin::Nll(origin) => origin,
2151 _ => NllRegionVariableOrigin::Existential { from_forall: false },
2154 Self { origin, universe, external_name: None }
2158 pub trait ClosureRegionRequirementsExt<'tcx> {
2159 fn apply_requirements(
2162 closure_def_id: DefId,
2163 closure_substs: SubstsRef<'tcx>,
2164 ) -> Vec<QueryOutlivesConstraint<'tcx>>;
2167 impl<'tcx> ClosureRegionRequirementsExt<'tcx> for ClosureRegionRequirements<'tcx> {
2168 /// Given an instance T of the closure type, this method
2169 /// instantiates the "extra" requirements that we computed for the
2170 /// closure into the inference context. This has the effect of
2171 /// adding new outlives obligations to existing variables.
2173 /// As described on `ClosureRegionRequirements`, the extra
2174 /// requirements are expressed in terms of regionvids that index
2175 /// into the free regions that appear on the closure type. So, to
2176 /// do this, we first copy those regions out from the type T into
2177 /// a vector. Then we can just index into that vector to extract
2178 /// out the corresponding region from T and apply the
2180 fn apply_requirements(
2183 closure_def_id: DefId,
2184 closure_substs: SubstsRef<'tcx>,
2185 ) -> Vec<QueryOutlivesConstraint<'tcx>> {
2187 "apply_requirements(closure_def_id={:?}, closure_substs={:?})",
2188 closure_def_id, closure_substs
2191 // Extract the values of the free regions in `closure_substs`
2192 // into a vector. These are the regions that we will be
2193 // relating to one another.
2194 let closure_mapping = &UniversalRegions::closure_mapping(
2197 self.num_external_vids,
2198 tcx.closure_base_def_id(closure_def_id),
2200 debug!("apply_requirements: closure_mapping={:?}", closure_mapping);
2202 // Create the predicates.
2203 self.outlives_requirements
2205 .map(|outlives_requirement| {
2206 let outlived_region = closure_mapping[outlives_requirement.outlived_free_region];
2208 match outlives_requirement.subject {
2209 ClosureOutlivesSubject::Region(region) => {
2210 let region = closure_mapping[region];
2212 "apply_requirements: region={:?} \
2213 outlived_region={:?} \
2214 outlives_requirement={:?}",
2215 region, outlived_region, outlives_requirement,
2217 ty::Binder::dummy(ty::OutlivesPredicate(region.into(), outlived_region))
2220 ClosureOutlivesSubject::Ty(ty) => {
2222 "apply_requirements: ty={:?} \
2223 outlived_region={:?} \
2224 outlives_requirement={:?}",
2225 ty, outlived_region, outlives_requirement,
2227 ty::Binder::dummy(ty::OutlivesPredicate(ty.into(), outlived_region))
2235 #[derive(Clone, Debug)]
2236 pub struct BlameConstraint<'tcx> {
2237 pub category: ConstraintCategory,
2238 pub from_closure: bool,
2240 pub variance_info: ty::VarianceDiagInfo<'tcx>,