1 use super::universal_regions::UniversalRegions;
2 use crate::borrow_check::nll::constraints::graph::NormalConstraintGraph;
3 use crate::borrow_check::nll::constraints::{
4 ConstraintSccIndex, OutlivesConstraint, OutlivesConstraintSet,
6 use crate::borrow_check::nll::member_constraints::{MemberConstraintSet, NllMemberConstraintIndex};
7 use crate::borrow_check::nll::region_infer::values::{
8 PlaceholderIndices, RegionElement, ToElementIndex,
10 use crate::borrow_check::nll::type_check::free_region_relations::UniversalRegionRelations;
11 use crate::borrow_check::nll::type_check::Locations;
12 use crate::borrow_check::Upvar;
13 use rustc::hir::def_id::DefId;
14 use rustc::infer::canonical::QueryOutlivesConstraint;
15 use rustc::infer::opaque_types;
16 use rustc::infer::region_constraints::{GenericKind, VarInfos, VerifyBound};
17 use rustc::infer::{InferCtxt, NLLRegionVariableOrigin, RegionVariableOrigin};
19 Body, ClosureOutlivesRequirement, ClosureOutlivesSubject, ClosureRegionRequirements,
20 ConstraintCategory, Local, Location,
22 use rustc::ty::{self, subst::SubstsRef, RegionVid, Ty, TyCtxt, TypeFoldable};
23 use rustc::util::common::{self, ErrorReported};
24 use rustc_data_structures::binary_search_util;
25 use rustc_data_structures::bit_set::BitSet;
26 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
27 use rustc_data_structures::graph::WithSuccessors;
28 use rustc_data_structures::graph::scc::Sccs;
29 use rustc_data_structures::graph::vec_graph::VecGraph;
30 use rustc_data_structures::indexed_vec::IndexVec;
31 use rustc_errors::{Diagnostic, DiagnosticBuilder};
38 crate use self::error_reporting::{RegionName, RegionNameSource};
41 use self::values::{LivenessValues, RegionValueElements, RegionValues};
43 use super::ToRegionVid;
45 pub struct RegionInferenceContext<'tcx> {
46 /// Contains the definition for every region variable. Region
47 /// variables are identified by their index (`RegionVid`). The
48 /// definition contains information about where the region came
49 /// from as well as its final inferred value.
50 definitions: IndexVec<RegionVid, RegionDefinition<'tcx>>,
52 /// The liveness constraints added to each region. For most
53 /// regions, these start out empty and steadily grow, though for
54 /// each universally quantified region R they start out containing
55 /// the entire CFG and `end(R)`.
56 liveness_constraints: LivenessValues<RegionVid>,
58 /// The outlives constraints computed by the type-check.
59 constraints: Rc<OutlivesConstraintSet>,
61 /// The constraint-set, but in graph form, making it easy to traverse
62 /// the constraints adjacent to a particular region. Used to construct
63 /// the SCC (see `constraint_sccs`) and for error reporting.
64 constraint_graph: Rc<NormalConstraintGraph>,
66 /// The SCC computed from `constraints` and the constraint
67 /// graph. We have an edge from SCC A to SCC B if `A: B`. Used to
68 /// compute the values of each region.
69 constraint_sccs: Rc<Sccs<RegionVid, ConstraintSccIndex>>,
71 /// Reverse of the SCC constraint graph -- i.e., an edge `A -> B`
72 /// exists if `B: A`. Computed lazilly.
73 rev_constraint_graph: Option<Rc<VecGraph<ConstraintSccIndex>>>,
75 /// The "R0 member of [R1..Rn]" constraints, indexed by SCC.
76 member_constraints: Rc<MemberConstraintSet<'tcx, ConstraintSccIndex>>,
78 /// Records the pick-constraints that we applied to each scc.
79 /// This is useful for error reporting. Once constraint
80 /// propagation is done, this vector is sorted according to
81 /// `member_region_scc`.
82 member_constraints_applied: Vec<AppliedMemberConstraint>,
84 /// Map closure bounds to a `Span` that should be used for error reporting.
85 closure_bounds_mapping:
86 FxHashMap<Location, FxHashMap<(RegionVid, RegionVid), (ConstraintCategory, Span)>>,
88 /// Contains the minimum universe of any variable within the same
89 /// SCC. We will ensure that no SCC contains values that are not
90 /// visible from this index.
91 scc_universes: IndexVec<ConstraintSccIndex, ty::UniverseIndex>,
93 /// Contains a "representative" from each SCC. This will be the
94 /// minimal RegionVid belonging to that universe. It is used as a
95 /// kind of hacky way to manage checking outlives relationships,
96 /// since we can 'canonicalize' each region to the representative
97 /// of its SCC and be sure that -- if they have the same repr --
98 /// they *must* be equal (though not having the same repr does not
99 /// mean they are unequal).
100 scc_representatives: IndexVec<ConstraintSccIndex, ty::RegionVid>,
102 /// The final inferred values of the region variables; we compute
103 /// one value per SCC. To get the value for any given *region*,
104 /// you first find which scc it is a part of.
105 scc_values: RegionValues<ConstraintSccIndex>,
107 /// Type constraints that we check after solving.
108 type_tests: Vec<TypeTest<'tcx>>,
110 /// Information about the universally quantified regions in scope
111 /// on this function.
112 universal_regions: Rc<UniversalRegions<'tcx>>,
114 /// Information about how the universally quantified regions in
115 /// scope on this function relate to one another.
116 universal_region_relations: Rc<UniversalRegionRelations<'tcx>>,
119 /// Each time that `apply_member_constraint` is successful, it appends
120 /// one of these structs to the `member_constraints_applied` field.
121 /// This is used in error reporting to trace out what happened.
123 /// The way that `apply_member_constraint` works is that it effectively
124 /// adds a new lower bound to the SCC it is analyzing: so you wind up
125 /// with `'R: 'O` where `'R` is the pick-region and `'O` is the
126 /// minimal viable option.
127 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd)]
128 struct AppliedMemberConstraint {
129 /// The SCC that was affected. (The "member region".)
131 /// The vector if `AppliedMemberConstraint` elements is kept sorted
133 member_region_scc: ConstraintSccIndex,
135 /// The "best option" that `apply_member_constraint` found -- this was
136 /// added as an "ad-hoc" lower-bound to `member_region_scc`.
137 min_choice: ty::RegionVid,
139 /// The "member constraint index" -- we can find out details about
140 /// the constraint from
141 /// `set.member_constraints[member_constraint_index]`.
142 member_constraint_index: NllMemberConstraintIndex,
145 struct RegionDefinition<'tcx> {
146 /// What kind of variable is this -- a free region? existential
147 /// variable? etc. (See the `NLLRegionVariableOrigin` for more
149 origin: NLLRegionVariableOrigin,
151 /// Which universe is this region variable defined in? This is
152 /// most often `ty::UniverseIndex::ROOT`, but when we encounter
153 /// forall-quantifiers like `for<'a> { 'a = 'b }`, we would create
154 /// the variable for `'a` in a fresh universe that extends ROOT.
155 universe: ty::UniverseIndex,
157 /// If this is 'static or an early-bound region, then this is
158 /// `Some(X)` where `X` is the name of the region.
159 external_name: Option<ty::Region<'tcx>>,
162 /// N.B., the variants in `Cause` are intentionally ordered. Lower
163 /// values are preferred when it comes to error messages. Do not
164 /// reorder willy nilly.
165 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
166 pub(crate) enum Cause {
167 /// point inserted because Local was live at the given Location
168 LiveVar(Local, Location),
170 /// point inserted because Local was dropped at the given Location
171 DropVar(Local, Location),
174 /// A "type test" corresponds to an outlives constraint between a type
175 /// and a lifetime, like `T: 'x` or `<T as Foo>::Bar: 'x`. They are
176 /// translated from the `Verify` region constraints in the ordinary
177 /// inference context.
179 /// These sorts of constraints are handled differently than ordinary
180 /// constraints, at least at present. During type checking, the
181 /// `InferCtxt::process_registered_region_obligations` method will
182 /// attempt to convert a type test like `T: 'x` into an ordinary
183 /// outlives constraint when possible (for example, `&'a T: 'b` will
184 /// be converted into `'a: 'b` and registered as a `Constraint`).
186 /// In some cases, however, there are outlives relationships that are
187 /// not converted into a region constraint, but rather into one of
188 /// these "type tests". The distinction is that a type test does not
189 /// influence the inference result, but instead just examines the
190 /// values that we ultimately inferred for each region variable and
191 /// checks that they meet certain extra criteria. If not, an error
194 /// One reason for this is that these type tests typically boil down
195 /// to a check like `'a: 'x` where `'a` is a universally quantified
196 /// region -- and therefore not one whose value is really meant to be
197 /// *inferred*, precisely (this is not always the case: one can have a
198 /// type test like `<Foo as Trait<'?0>>::Bar: 'x`, where `'?0` is an
199 /// inference variable). Another reason is that these type tests can
200 /// involve *disjunction* -- that is, they can be satisfied in more
203 /// For more information about this translation, see
204 /// `InferCtxt::process_registered_region_obligations` and
205 /// `InferCtxt::type_must_outlive` in `rustc::infer::outlives`.
206 #[derive(Clone, Debug)]
207 pub struct TypeTest<'tcx> {
208 /// The type `T` that must outlive the region.
209 pub generic_kind: GenericKind<'tcx>,
211 /// The region `'x` that the type must outlive.
212 pub lower_bound: RegionVid,
214 /// Where did this constraint arise and why?
215 pub locations: Locations,
217 /// A test which, if met by the region `'x`, proves that this type
218 /// constraint is satisfied.
219 pub verify_bound: VerifyBound<'tcx>,
222 impl<'tcx> RegionInferenceContext<'tcx> {
223 /// Creates a new region inference context with a total of
224 /// `num_region_variables` valid inference variables; the first N
225 /// of those will be constant regions representing the free
226 /// regions defined in `universal_regions`.
228 /// The `outlives_constraints` and `type_tests` are an initial set
229 /// of constraints produced by the MIR type check.
232 universal_regions: Rc<UniversalRegions<'tcx>>,
233 placeholder_indices: Rc<PlaceholderIndices>,
234 universal_region_relations: Rc<UniversalRegionRelations<'tcx>>,
236 outlives_constraints: OutlivesConstraintSet,
237 member_constraints_in: MemberConstraintSet<'tcx, RegionVid>,
238 closure_bounds_mapping: FxHashMap<
240 FxHashMap<(RegionVid, RegionVid), (ConstraintCategory, Span)>,
242 type_tests: Vec<TypeTest<'tcx>>,
243 liveness_constraints: LivenessValues<RegionVid>,
244 elements: &Rc<RegionValueElements>,
246 // Create a RegionDefinition for each inference variable.
247 let definitions: IndexVec<_, _> = var_infos
249 .map(|info| RegionDefinition::new(info.universe, info.origin))
252 let constraints = Rc::new(outlives_constraints); // freeze constraints
253 let constraint_graph = Rc::new(constraints.graph(definitions.len()));
254 let fr_static = universal_regions.fr_static;
255 let constraint_sccs = Rc::new(constraints.compute_sccs(&constraint_graph, fr_static));
258 RegionValues::new(elements, universal_regions.len(), &placeholder_indices);
260 for region in liveness_constraints.rows() {
261 let scc = constraint_sccs.scc(region);
262 scc_values.merge_liveness(scc, region, &liveness_constraints);
265 let scc_universes = Self::compute_scc_universes(&constraint_sccs, &definitions);
267 let scc_representatives = Self::compute_scc_representatives(&constraint_sccs, &definitions);
269 let member_constraints =
270 Rc::new(member_constraints_in.into_mapped(|r| constraint_sccs.scc(r)));
272 let mut result = Self {
274 liveness_constraints,
278 rev_constraint_graph: None,
280 member_constraints_applied: Vec::new(),
281 closure_bounds_mapping,
287 universal_region_relations,
290 result.init_free_and_bound_regions();
295 /// Each SCC is the combination of many region variables which
296 /// have been equated. Therefore, we can associate a universe with
297 /// each SCC which is minimum of all the universes of its
298 /// constituent regions -- this is because whatever value the SCC
299 /// takes on must be a value that each of the regions within the
300 /// SCC could have as well. This implies that the SCC must have
301 /// the minimum, or narrowest, universe.
302 fn compute_scc_universes(
303 constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
304 definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
305 ) -> IndexVec<ConstraintSccIndex, ty::UniverseIndex> {
306 let num_sccs = constraints_scc.num_sccs();
307 let mut scc_universes = IndexVec::from_elem_n(ty::UniverseIndex::MAX, num_sccs);
309 for (region_vid, region_definition) in definitions.iter_enumerated() {
310 let scc = constraints_scc.scc(region_vid);
311 let scc_universe = &mut scc_universes[scc];
312 *scc_universe = ::std::cmp::min(*scc_universe, region_definition.universe);
315 debug!("compute_scc_universes: scc_universe = {:#?}", scc_universes);
320 /// For each SCC, we compute a unique `RegionVid` (in fact, the
321 /// minimal one that belongs to the SCC). See
322 /// `scc_representatives` field of `RegionInferenceContext` for
324 fn compute_scc_representatives(
325 constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
326 definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
327 ) -> IndexVec<ConstraintSccIndex, ty::RegionVid> {
328 let num_sccs = constraints_scc.num_sccs();
329 let next_region_vid = definitions.next_index();
330 let mut scc_representatives = IndexVec::from_elem_n(next_region_vid, num_sccs);
332 for region_vid in definitions.indices() {
333 let scc = constraints_scc.scc(region_vid);
334 let prev_min = scc_representatives[scc];
335 scc_representatives[scc] = region_vid.min(prev_min);
341 /// Initializes the region variables for each universally
342 /// quantified region (lifetime parameter). The first N variables
343 /// always correspond to the regions appearing in the function
344 /// signature (both named and anonymous) and where-clauses. This
345 /// function iterates over those regions and initializes them with
350 /// fn foo<'a, 'b>(..) where 'a: 'b
352 /// would initialize two variables like so:
354 /// R0 = { CFG, R0 } // 'a
355 /// R1 = { CFG, R0, R1 } // 'b
357 /// Here, R0 represents `'a`, and it contains (a) the entire CFG
358 /// and (b) any universally quantified regions that it outlives,
359 /// which in this case is just itself. R1 (`'b`) in contrast also
360 /// outlives `'a` and hence contains R0 and R1.
361 fn init_free_and_bound_regions(&mut self) {
362 // Update the names (if any)
363 for (external_name, variable) in self.universal_regions.named_universal_regions() {
365 "init_universal_regions: region {:?} has external name {:?}",
366 variable, external_name
368 self.definitions[variable].external_name = Some(external_name);
371 for variable in self.definitions.indices() {
372 let scc = self.constraint_sccs.scc(variable);
374 match self.definitions[variable].origin {
375 NLLRegionVariableOrigin::FreeRegion => {
376 // For each free, universally quantified region X:
378 // Add all nodes in the CFG to liveness constraints
379 self.liveness_constraints.add_all_points(variable);
380 self.scc_values.add_all_points(scc);
382 // Add `end(X)` into the set for X.
383 self.scc_values.add_element(scc, variable);
386 NLLRegionVariableOrigin::Placeholder(placeholder) => {
387 // Each placeholder region is only visible from
388 // its universe `ui` and its extensions. So we
389 // can't just add it into `scc` unless the
390 // universe of the scc can name this region.
391 let scc_universe = self.scc_universes[scc];
392 if scc_universe.can_name(placeholder.universe) {
393 self.scc_values.add_element(scc, placeholder);
396 "init_free_and_bound_regions: placeholder {:?} is \
397 not compatible with universe {:?} of its SCC {:?}",
398 placeholder, scc_universe, scc,
400 self.add_incompatible_universe(scc);
404 NLLRegionVariableOrigin::Existential => {
405 // For existential, regions, nothing to do.
411 /// Returns an iterator over all the region indices.
412 pub fn regions(&self) -> impl Iterator<Item = RegionVid> {
413 self.definitions.indices()
416 /// Given a universal region in scope on the MIR, returns the
417 /// corresponding index.
419 /// (Panics if `r` is not a registered universal region.)
420 pub fn to_region_vid(&self, r: ty::Region<'tcx>) -> RegionVid {
421 self.universal_regions.to_region_vid(r)
424 /// Adds annotations for `#[rustc_regions]`; see `UniversalRegions::annotate`.
425 crate fn annotate(&self, tcx: TyCtxt<'tcx>, err: &mut DiagnosticBuilder<'_>) {
426 self.universal_regions.annotate(tcx, err)
429 /// Returns `true` if the region `r` contains the point `p`.
431 /// Panics if called before `solve()` executes,
432 crate fn region_contains(&self, r: impl ToRegionVid, p: impl ToElementIndex) -> bool {
433 let scc = self.constraint_sccs.scc(r.to_region_vid());
434 self.scc_values.contains(scc, p)
437 /// Returns access to the value of `r` for debugging purposes.
438 crate fn region_value_str(&self, r: RegionVid) -> String {
439 let scc = self.constraint_sccs.scc(r.to_region_vid());
440 self.scc_values.region_value_str(scc)
443 /// Returns access to the value of `r` for debugging purposes.
444 crate fn region_universe(&self, r: RegionVid) -> ty::UniverseIndex {
445 let scc = self.constraint_sccs.scc(r.to_region_vid());
446 self.scc_universes[scc]
449 /// Once region solving has completed, this function will return
450 /// the pick-constraints that were applied to the value of a given
451 /// region `r`. See `AppliedMemberConstraint`.
452 fn applied_member_constraints(&self, r: impl ToRegionVid) -> &[AppliedMemberConstraint] {
453 let scc = self.constraint_sccs.scc(r.to_region_vid());
454 binary_search_util::binary_search_slice(
455 &self.member_constraints_applied,
456 |applied| applied.member_region_scc,
461 /// Performs region inference and report errors if we see any
462 /// unsatisfiable constraints. If this is a closure, returns the
463 /// region requirements to propagate to our creator, if any.
466 infcx: &InferCtxt<'_, 'tcx>,
470 errors_buffer: &mut Vec<Diagnostic>,
471 ) -> Option<ClosureRegionRequirements<'tcx>> {
473 infcx.tcx.sess.time_extended(),
474 Some(infcx.tcx.sess),
475 &format!("solve_nll_region_constraints({:?})", mir_def_id),
476 || self.solve_inner(infcx, body, upvars, mir_def_id, errors_buffer),
482 infcx: &InferCtxt<'_, 'tcx>,
486 errors_buffer: &mut Vec<Diagnostic>,
487 ) -> Option<ClosureRegionRequirements<'tcx>> {
488 self.propagate_constraints(body);
490 // If this is a closure, we can propagate unsatisfied
491 // `outlives_requirements` to our creator, so create a vector
492 // to store those. Otherwise, we'll pass in `None` to the
493 // functions below, which will trigger them to report errors
495 let mut outlives_requirements =
496 if infcx.tcx.is_closure(mir_def_id) { Some(vec![]) } else { None };
498 self.check_type_tests(
502 outlives_requirements.as_mut(),
506 self.check_universal_regions(
511 outlives_requirements.as_mut(),
515 self.check_member_constraints(infcx, mir_def_id, errors_buffer);
517 let outlives_requirements = outlives_requirements.unwrap_or(vec![]);
519 if outlives_requirements.is_empty() {
522 let num_external_vids = self.universal_regions.num_global_and_external_regions();
523 Some(ClosureRegionRequirements { num_external_vids, outlives_requirements })
527 /// Propagate the region constraints: this will grow the values
528 /// for each region variable until all the constraints are
529 /// satisfied. Note that some values may grow **too** large to be
530 /// feasible, but we check this later.
531 fn propagate_constraints(&mut self, _body: &Body<'tcx>) {
532 debug!("propagate_constraints()");
534 debug!("propagate_constraints: constraints={:#?}", {
535 let mut constraints: Vec<_> = self.constraints.outlives().iter().collect();
539 .map(|c| (c, self.constraint_sccs.scc(c.sup), self.constraint_sccs.scc(c.sub)))
543 // To propagate constraints, we walk the DAG induced by the
544 // SCC. For each SCC, we visit its successors and compute
545 // their values, then we union all those values to get our
547 let visited = &mut BitSet::new_empty(self.constraint_sccs.num_sccs());
548 for scc_index in self.constraint_sccs.all_sccs() {
549 self.propagate_constraint_sccs_if_new(scc_index, visited);
552 // Sort the applied member constraints so we can binary search
553 // through them later.
554 self.member_constraints_applied.sort_by_key(|applied| applied.member_region_scc);
557 /// Computes the value of the SCC `scc_a` if it has not already
558 /// been computed. The `visited` parameter is a bitset
560 fn propagate_constraint_sccs_if_new(
562 scc_a: ConstraintSccIndex,
563 visited: &mut BitSet<ConstraintSccIndex>,
565 if visited.insert(scc_a) {
566 self.propagate_constraint_sccs_new(scc_a, visited);
570 /// Computes the value of the SCC `scc_a`, which has not yet been
571 /// computed. This works by first computing all successors of the
572 /// SCC (if they haven't been computed already) and then unioning
573 /// together their elements.
574 fn propagate_constraint_sccs_new(
576 scc_a: ConstraintSccIndex,
577 visited: &mut BitSet<ConstraintSccIndex>,
579 let constraint_sccs = self.constraint_sccs.clone();
581 // Walk each SCC `B` such that `A: B`...
582 for &scc_b in constraint_sccs.successors(scc_a) {
583 debug!("propagate_constraint_sccs: scc_a = {:?} scc_b = {:?}", scc_a, scc_b);
585 // ...compute the value of `B`...
586 self.propagate_constraint_sccs_if_new(scc_b, visited);
588 // ...and add elements from `B` into `A`. One complication
589 // arises because of universes: If `B` contains something
590 // that `A` cannot name, then `A` can only contain `B` if
591 // it outlives static.
592 if self.universe_compatible(scc_b, scc_a) {
593 // `A` can name everything that is in `B`, so just
595 self.scc_values.add_region(scc_a, scc_b);
597 self.add_incompatible_universe(scc_a);
601 // Now take member constraints into account
602 let member_constraints = self.member_constraints.clone();
603 for m_c_i in member_constraints.indices(scc_a) {
604 self.apply_member_constraint(
607 member_constraints.choice_regions(m_c_i),
612 "propagate_constraint_sccs: scc_a = {:?} has value {:?}",
614 self.scc_values.region_value_str(scc_a),
618 /// Invoked for each `member R0 of [R1..Rn]` constraint.
620 /// `scc` is the SCC containing R0, and `choice_regions` are the
621 /// `R1..Rn` regions -- they are always known to be universal
622 /// regions (and if that's not true, we just don't attempt to
623 /// enforce the constraint).
625 /// The current value of `scc` at the time the method is invoked
626 /// is considered a *lower bound*. If possible, we will modify
627 /// the constraint to set it equal to one of the option regions.
628 /// If we make any changes, returns true, else false.
629 fn apply_member_constraint(
631 scc: ConstraintSccIndex,
632 member_constraint_index: NllMemberConstraintIndex,
633 choice_regions: &[ty::RegionVid],
635 debug!("apply_member_constraint(scc={:?}, choice_regions={:#?})", scc, choice_regions,);
638 choice_regions.iter().find(|&&r| !self.universal_regions.is_universal_region(r))
640 // FIXME(#61773): This case can only occur with
641 // `impl_trait_in_bindings`, I believe, and we are just
642 // opting not to handle it for now. See #61773 for
645 "member constraint for `{:?}` has an option region `{:?}` \
646 that is not a universal region",
647 self.member_constraints[member_constraint_index].opaque_type_def_id,
652 // Create a mutable vector of the options. We'll try to winnow
654 let mut choice_regions: Vec<ty::RegionVid> = choice_regions.to_vec();
656 // The 'member region' in a member constraint is part of the
657 // hidden type, which must be in the root universe. Therefore,
658 // it cannot have any placeholders in its value.
659 assert!(self.scc_universes[scc] == ty::UniverseIndex::ROOT);
661 self.scc_values.placeholders_contained_in(scc).next().is_none(),
662 "scc {:?} in a pick-constraint has placeholder value: {:?}",
664 self.scc_values.region_value_str(scc),
667 // The existing value for `scc` is a lower-bound. This will
668 // consist of some set {P} + {LB} of points {P} and
669 // lower-bound free regions {LB}. As each choice region O is a
670 // free region, it will outlive the points. But we can only
671 // consider the option O if O: LB.
672 choice_regions.retain(|&o_r| {
674 .universal_regions_outlived_by(scc)
675 .all(|lb| self.universal_region_relations.outlives(o_r, lb))
677 debug!("apply_member_constraint: after lb, choice_regions={:?}", choice_regions);
679 // Now find all the *upper bounds* -- that is, each UB is a
680 // free region that must outlive the member region R0 (`UB:
681 // R0`). Therefore, we need only keep an option O if `UB: O`
683 if choice_regions.len() > 1 {
684 let universal_region_relations = self.universal_region_relations.clone();
685 for ub in self.upper_bounds(scc) {
686 debug!("apply_member_constraint: ub={:?}", ub);
687 choice_regions.retain(|&o_r| universal_region_relations.outlives(ub, o_r));
689 debug!("apply_member_constraint: after ub, choice_regions={:?}", choice_regions);
692 // If we ruled everything out, we're done.
693 if choice_regions.is_empty() {
697 // Otherwise, we need to find the minimum remaining choice, if
698 // any, and take that.
699 debug!("apply_member_constraint: choice_regions remaining are {:#?}", choice_regions);
700 let min = |r1: ty::RegionVid, r2: ty::RegionVid| -> Option<ty::RegionVid> {
701 let r1_outlives_r2 = self.universal_region_relations.outlives(r1, r2);
702 let r2_outlives_r1 = self.universal_region_relations.outlives(r2, r1);
703 if r1_outlives_r2 && r2_outlives_r1 {
705 } else if r1_outlives_r2 {
707 } else if r2_outlives_r1 {
713 let mut min_choice = choice_regions[0];
714 for &other_option in &choice_regions[1..] {
716 "apply_member_constraint: min_choice={:?} other_option={:?}",
717 min_choice, other_option,
719 match min(min_choice, other_option) {
720 Some(m) => min_choice = m,
723 "apply_member_constraint: {:?} and {:?} are incomparable; no min choice",
724 min_choice, other_option,
731 let min_choice_scc = self.constraint_sccs.scc(min_choice);
733 "apply_member_constraint: min_choice={:?} best_choice_scc={:?}",
737 if self.scc_values.add_region(scc, min_choice_scc) {
738 self.member_constraints_applied.push(AppliedMemberConstraint {
739 member_region_scc: scc,
741 member_constraint_index,
750 /// Compute and return the reverse SCC-based constraint graph (lazilly).
753 scc0: ConstraintSccIndex,
754 ) -> Vec<RegionVid> {
755 // I wanted to return an `impl Iterator` here, but it's
756 // annoying because the `rev_constraint_graph` is in a local
757 // variable. We'd need a "once-cell" or some such thing to let
758 // us borrow it for the right amount of time.
759 let rev_constraint_graph = self.rev_constraint_graph();
760 let scc_values = &self.scc_values;
761 let mut duplicates = FxHashSet::default();
763 .depth_first_search(scc0)
765 .flat_map(|scc1| scc_values.universal_regions_outlived_by(scc1))
766 .filter(|&r| duplicates.insert(r))
770 /// Compute and return the reverse SCC-based constraint graph (lazilly).
771 fn rev_constraint_graph(
773 ) -> Rc<VecGraph<ConstraintSccIndex>> {
774 if let Some(g) = &self.rev_constraint_graph {
778 let rev_graph = Rc::new(self.constraint_sccs.reverse());
779 self.rev_constraint_graph = Some(rev_graph.clone());
783 /// Returns `true` if all the elements in the value of `scc_b` are nameable
784 /// in `scc_a`. Used during constraint propagation, and only once
785 /// the value of `scc_b` has been computed.
786 fn universe_compatible(&self, scc_b: ConstraintSccIndex, scc_a: ConstraintSccIndex) -> bool {
787 let universe_a = self.scc_universes[scc_a];
789 // Quick check: if scc_b's declared universe is a subset of
790 // scc_a's declared univese (typically, both are ROOT), then
791 // it cannot contain any problematic universe elements.
792 if universe_a.can_name(self.scc_universes[scc_b]) {
796 // Otherwise, we have to iterate over the universe elements in
797 // B's value, and check whether all of them are nameable
799 self.scc_values.placeholders_contained_in(scc_b).all(|p| universe_a.can_name(p.universe))
802 /// Extend `scc` so that it can outlive some placeholder region
803 /// from a universe it can't name; at present, the only way for
804 /// this to be true is if `scc` outlives `'static`. This is
805 /// actually stricter than necessary: ideally, we'd support bounds
806 /// like `for<'a: 'b`>` that might then allow us to approximate
807 /// `'a` with `'b` and not `'static`. But it will have to do for
809 fn add_incompatible_universe(&mut self, scc: ConstraintSccIndex) {
810 debug!("add_incompatible_universe(scc={:?})", scc);
812 let fr_static = self.universal_regions.fr_static;
813 self.scc_values.add_all_points(scc);
814 self.scc_values.add_element(scc, fr_static);
817 /// Once regions have been propagated, this method is used to see
818 /// whether the "type tests" produced by typeck were satisfied;
819 /// type tests encode type-outlives relationships like `T:
820 /// 'a`. See `TypeTest` for more details.
823 infcx: &InferCtxt<'_, 'tcx>,
826 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
827 errors_buffer: &mut Vec<Diagnostic>,
831 // Sometimes we register equivalent type-tests that would
832 // result in basically the exact same error being reported to
833 // the user. Avoid that.
834 let mut deduplicate_errors = FxHashSet::default();
836 for type_test in &self.type_tests {
837 debug!("check_type_test: {:?}", type_test);
839 let generic_ty = type_test.generic_kind.to_ty(tcx);
840 if self.eval_verify_bound(
844 type_test.lower_bound,
845 &type_test.verify_bound,
850 if let Some(propagated_outlives_requirements) = &mut propagated_outlives_requirements {
851 if self.try_promote_type_test(
855 propagated_outlives_requirements,
861 // Type-test failed. Report the error.
863 // Try to convert the lower-bound region into something named we can print for the user.
864 let lower_bound_region = self.to_error_region(type_test.lower_bound);
866 // Skip duplicate-ish errors.
867 let type_test_span = type_test.locations.span(body);
868 let erased_generic_kind = tcx.erase_regions(&type_test.generic_kind);
869 if !deduplicate_errors.insert((
877 "check_type_test: reporting error for erased_generic_kind={:?}, \
878 lower_bound_region={:?}, \
879 type_test.locations={:?}",
880 erased_generic_kind, lower_bound_region, type_test.locations,
884 if let Some(lower_bound_region) = lower_bound_region {
885 let region_scope_tree = &tcx.region_scope_tree(mir_def_id);
887 .construct_generic_bound_failure(
891 type_test.generic_kind,
894 .buffer(errors_buffer);
896 // FIXME. We should handle this case better. It
897 // indicates that we have e.g., some region variable
898 // whose value is like `'a+'b` where `'a` and `'b` are
899 // distinct unrelated univesal regions that are not
900 // known to outlive one another. It'd be nice to have
901 // some examples where this arises to decide how best
902 // to report it; we could probably handle it by
903 // iterating over the universal regions and reporting
904 // an error that multiple bounds are required.
908 &format!("`{}` does not live long enough", type_test.generic_kind,),
910 .buffer(errors_buffer);
915 /// Converts a region inference variable into a `ty::Region` that
916 /// we can use for error reporting. If `r` is universally bound,
917 /// then we use the name that we have on record for it. If `r` is
918 /// existentially bound, then we check its inferred value and try
919 /// to find a good name from that. Returns `None` if we can't find
920 /// one (e.g., this is just some random part of the CFG).
921 pub fn to_error_region(&self, r: RegionVid) -> Option<ty::Region<'tcx>> {
922 self.to_error_region_vid(r).and_then(|r| self.definitions[r].external_name)
925 /// Returns the [RegionVid] corresponding to the region returned by
926 /// `to_error_region`.
927 pub fn to_error_region_vid(&self, r: RegionVid) -> Option<RegionVid> {
928 if self.universal_regions.is_universal_region(r) {
931 let r_scc = self.constraint_sccs.scc(r);
932 let upper_bound = self.universal_upper_bound(r);
933 if self.scc_values.contains(r_scc, upper_bound) {
934 self.to_error_region_vid(upper_bound)
941 /// Invoked when we have some type-test (e.g., `T: 'X`) that we cannot
942 /// prove to be satisfied. If this is a closure, we will attempt to
943 /// "promote" this type-test into our `ClosureRegionRequirements` and
944 /// hence pass it up the creator. To do this, we have to phrase the
945 /// type-test in terms of external free regions, as local free
946 /// regions are not nameable by the closure's creator.
948 /// Promotion works as follows: we first check that the type `T`
949 /// contains only regions that the creator knows about. If this is
950 /// true, then -- as a consequence -- we know that all regions in
951 /// the type `T` are free regions that outlive the closure body. If
952 /// false, then promotion fails.
954 /// Once we've promoted T, we have to "promote" `'X` to some region
955 /// that is "external" to the closure. Generally speaking, a region
956 /// may be the union of some points in the closure body as well as
957 /// various free lifetimes. We can ignore the points in the closure
958 /// body: if the type T can be expressed in terms of external regions,
959 /// we know it outlives the points in the closure body. That
960 /// just leaves the free regions.
962 /// The idea then is to lower the `T: 'X` constraint into multiple
963 /// bounds -- e.g., if `'X` is the union of two free lifetimes,
964 /// `'1` and `'2`, then we would create `T: '1` and `T: '2`.
965 fn try_promote_type_test(
967 infcx: &InferCtxt<'_, 'tcx>,
969 type_test: &TypeTest<'tcx>,
970 propagated_outlives_requirements: &mut Vec<ClosureOutlivesRequirement<'tcx>>,
974 let TypeTest { generic_kind, lower_bound, locations, verify_bound: _ } = type_test;
976 let generic_ty = generic_kind.to_ty(tcx);
977 let subject = match self.try_promote_type_test_subject(infcx, generic_ty) {
979 None => return false,
982 // For each region outlived by lower_bound find a non-local,
983 // universal region (it may be the same region) and add it to
984 // `ClosureOutlivesRequirement`.
985 let r_scc = self.constraint_sccs.scc(*lower_bound);
986 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
987 // Check whether we can already prove that the "subject" outlives `ur`.
988 // If so, we don't have to propagate this requirement to our caller.
990 // To continue the example from the function, if we are trying to promote
991 // a requirement that `T: 'X`, and we know that `'X = '1 + '2` (i.e., the union
992 // `'1` and `'2`), then in this loop `ur` will be `'1` (and `'2`). So here
993 // we check whether `T: '1` is something we *can* prove. If so, no need
994 // to propagate that requirement.
996 // This is needed because -- particularly in the case
997 // where `ur` is a local bound -- we are sometimes in a
998 // position to prove things that our caller cannot. See
999 // #53570 for an example.
1000 if self.eval_verify_bound(tcx, body, generic_ty, ur, &type_test.verify_bound) {
1004 debug!("try_promote_type_test: ur={:?}", ur);
1006 let non_local_ub = self.universal_region_relations.non_local_upper_bounds(&ur);
1007 debug!("try_promote_type_test: non_local_ub={:?}", non_local_ub);
1009 // This is slightly too conservative. To show T: '1, given `'2: '1`
1010 // and `'3: '1` we only need to prove that T: '2 *or* T: '3, but to
1011 // avoid potential non-determinism we approximate this by requiring
1013 for &upper_bound in non_local_ub {
1014 debug_assert!(self.universal_regions.is_universal_region(upper_bound));
1015 debug_assert!(!self.universal_regions.is_local_free_region(upper_bound));
1017 let requirement = ClosureOutlivesRequirement {
1019 outlived_free_region: upper_bound,
1020 blame_span: locations.span(body),
1021 category: ConstraintCategory::Boring,
1023 debug!("try_promote_type_test: pushing {:#?}", requirement);
1024 propagated_outlives_requirements.push(requirement);
1030 /// When we promote a type test `T: 'r`, we have to convert the
1031 /// type `T` into something we can store in a query result (so
1032 /// something allocated for `'tcx`). This is problematic if `ty`
1033 /// contains regions. During the course of NLL region checking, we
1034 /// will have replaced all of those regions with fresh inference
1035 /// variables. To create a test subject, we want to replace those
1036 /// inference variables with some region from the closure
1037 /// signature -- this is not always possible, so this is a
1038 /// fallible process. Presuming we do find a suitable region, we
1039 /// will represent it with a `ReClosureBound`, which is a
1040 /// `RegionKind` variant that can be allocated in the gcx.
1041 fn try_promote_type_test_subject(
1043 infcx: &InferCtxt<'_, 'tcx>,
1045 ) -> Option<ClosureOutlivesSubject<'tcx>> {
1046 let tcx = infcx.tcx;
1048 debug!("try_promote_type_test_subject(ty = {:?})", ty);
1050 let ty = tcx.fold_regions(&ty, &mut false, |r, _depth| {
1051 let region_vid = self.to_region_vid(r);
1053 // The challenge if this. We have some region variable `r`
1054 // whose value is a set of CFG points and universal
1055 // regions. We want to find if that set is *equivalent* to
1056 // any of the named regions found in the closure.
1058 // To do so, we compute the
1059 // `non_local_universal_upper_bound`. This will be a
1060 // non-local, universal region that is greater than `r`.
1061 // However, it might not be *contained* within `r`, so
1062 // then we further check whether this bound is contained
1063 // in `r`. If so, we can say that `r` is equivalent to the
1066 // Let's work through a few examples. For these, imagine
1067 // that we have 3 non-local regions (I'll denote them as
1068 // `'static`, `'a`, and `'b`, though of course in the code
1069 // they would be represented with indices) where:
1074 // First, let's assume that `r` is some existential
1075 // variable with an inferred value `{'a, 'static}` (plus
1076 // some CFG nodes). In this case, the non-local upper
1077 // bound is `'static`, since that outlives `'a`. `'static`
1078 // is also a member of `r` and hence we consider `r`
1079 // equivalent to `'static` (and replace it with
1082 // Now let's consider the inferred value `{'a, 'b}`. This
1083 // means `r` is effectively `'a | 'b`. I'm not sure if
1084 // this can come about, actually, but assuming it did, we
1085 // would get a non-local upper bound of `'static`. Since
1086 // `'static` is not contained in `r`, we would fail to
1087 // find an equivalent.
1088 let upper_bound = self.non_local_universal_upper_bound(region_vid);
1089 if self.region_contains(region_vid, upper_bound) {
1090 tcx.mk_region(ty::ReClosureBound(upper_bound))
1092 // In the case of a failure, use a `ReVar`
1093 // result. This will cause the `lift` later on to
1098 debug!("try_promote_type_test_subject: folded ty = {:?}", ty);
1100 // `has_local_value` will only be true if we failed to promote some region.
1101 if ty.has_local_value() {
1105 Some(ClosureOutlivesSubject::Ty(ty))
1108 /// Given some universal or existential region `r`, finds a
1109 /// non-local, universal region `r+` that outlives `r` at entry to (and
1110 /// exit from) the closure. In the worst case, this will be
1113 /// This is used for two purposes. First, if we are propagated
1114 /// some requirement `T: r`, we can use this method to enlarge `r`
1115 /// to something we can encode for our creator (which only knows
1116 /// about non-local, universal regions). It is also used when
1117 /// encoding `T` as part of `try_promote_type_test_subject` (see
1118 /// that fn for details).
1120 /// This is based on the result `'y` of `universal_upper_bound`,
1121 /// except that it converts further takes the non-local upper
1122 /// bound of `'y`, so that the final result is non-local.
1123 fn non_local_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1124 debug!("non_local_universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1126 let lub = self.universal_upper_bound(r);
1128 // Grow further to get smallest universal region known to
1130 let non_local_lub = self.universal_region_relations.non_local_upper_bound(lub);
1132 debug!("non_local_universal_upper_bound: non_local_lub={:?}", non_local_lub);
1137 /// Returns a universally quantified region that outlives the
1138 /// value of `r` (`r` may be existentially or universally
1141 /// Since `r` is (potentially) an existential region, it has some
1142 /// value which may include (a) any number of points in the CFG
1143 /// and (b) any number of `end('x)` elements of universally
1144 /// quantified regions. To convert this into a single universal
1145 /// region we do as follows:
1147 /// - Ignore the CFG points in `'r`. All universally quantified regions
1148 /// include the CFG anyhow.
1149 /// - For each `end('x)` element in `'r`, compute the mutual LUB, yielding
1151 fn universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1152 debug!("universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1154 // Find the smallest universal region that contains all other
1155 // universal regions within `region`.
1156 let mut lub = self.universal_regions.fr_fn_body;
1157 let r_scc = self.constraint_sccs.scc(r);
1158 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1159 lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1162 debug!("universal_upper_bound: r={:?} lub={:?}", r, lub);
1167 /// Tests if `test` is true when applied to `lower_bound` at
1169 fn eval_verify_bound(
1173 generic_ty: Ty<'tcx>,
1174 lower_bound: RegionVid,
1175 verify_bound: &VerifyBound<'tcx>,
1177 debug!("eval_verify_bound(lower_bound={:?}, verify_bound={:?})", lower_bound, verify_bound);
1179 match verify_bound {
1180 VerifyBound::IfEq(test_ty, verify_bound1) => {
1181 self.eval_if_eq(tcx, body, generic_ty, lower_bound, test_ty, verify_bound1)
1184 VerifyBound::OutlivedBy(r) => {
1185 let r_vid = self.to_region_vid(r);
1186 self.eval_outlives(r_vid, lower_bound)
1189 VerifyBound::AnyBound(verify_bounds) => verify_bounds.iter().any(|verify_bound| {
1190 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1193 VerifyBound::AllBounds(verify_bounds) => verify_bounds.iter().all(|verify_bound| {
1194 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1203 generic_ty: Ty<'tcx>,
1204 lower_bound: RegionVid,
1206 verify_bound: &VerifyBound<'tcx>,
1208 let generic_ty_normalized = self.normalize_to_scc_representatives(tcx, generic_ty);
1209 let test_ty_normalized = self.normalize_to_scc_representatives(tcx, test_ty);
1210 if generic_ty_normalized == test_ty_normalized {
1211 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1217 /// This is a conservative normalization procedure. It takes every
1218 /// free region in `value` and replaces it with the
1219 /// "representative" of its SCC (see `scc_representatives` field).
1220 /// We are guaranteed that if two values normalize to the same
1221 /// thing, then they are equal; this is a conservative check in
1222 /// that they could still be equal even if they normalize to
1223 /// different results. (For example, there might be two regions
1224 /// with the same value that are not in the same SCC).
1226 /// N.B., this is not an ideal approach and I would like to revisit
1227 /// it. However, it works pretty well in practice. In particular,
1228 /// this is needed to deal with projection outlives bounds like
1230 /// <T as Foo<'0>>::Item: '1
1232 /// In particular, this routine winds up being important when
1233 /// there are bounds like `where <T as Foo<'a>>::Item: 'b` in the
1234 /// environment. In this case, if we can show that `'0 == 'a`,
1235 /// and that `'b: '1`, then we know that the clause is
1236 /// satisfied. In such cases, particularly due to limitations of
1237 /// the trait solver =), we usually wind up with a where-clause like
1238 /// `T: Foo<'a>` in scope, which thus forces `'0 == 'a` to be added as
1239 /// a constraint, and thus ensures that they are in the same SCC.
1241 /// So why can't we do a more correct routine? Well, we could
1242 /// *almost* use the `relate_tys` code, but the way it is
1243 /// currently setup it creates inference variables to deal with
1244 /// higher-ranked things and so forth, and right now the inference
1245 /// context is not permitted to make more inference variables. So
1246 /// we use this kind of hacky solution.
1247 fn normalize_to_scc_representatives<T>(&self, tcx: TyCtxt<'tcx>, value: T) -> T
1249 T: TypeFoldable<'tcx>,
1251 tcx.fold_regions(&value, &mut false, |r, _db| {
1252 let vid = self.to_region_vid(r);
1253 let scc = self.constraint_sccs.scc(vid);
1254 let repr = self.scc_representatives[scc];
1255 tcx.mk_region(ty::ReVar(repr))
1259 // Evaluate whether `sup_region == sub_region`.
1260 fn eval_equal(&self, r1: RegionVid, r2: RegionVid) -> bool {
1261 self.eval_outlives(r1, r2) && self.eval_outlives(r2, r1)
1264 // Evaluate whether `sup_region: sub_region`.
1265 fn eval_outlives(&self, sup_region: RegionVid, sub_region: RegionVid) -> bool {
1266 debug!("eval_outlives({:?}: {:?})", sup_region, sub_region);
1269 "eval_outlives: sup_region's value = {:?} universal={:?}",
1270 self.region_value_str(sup_region),
1271 self.universal_regions.is_universal_region(sup_region),
1274 "eval_outlives: sub_region's value = {:?} universal={:?}",
1275 self.region_value_str(sub_region),
1276 self.universal_regions.is_universal_region(sub_region),
1279 let sub_region_scc = self.constraint_sccs.scc(sub_region);
1280 let sup_region_scc = self.constraint_sccs.scc(sup_region);
1282 // Both the `sub_region` and `sup_region` consist of the union
1283 // of some number of universal regions (along with the union
1284 // of various points in the CFG; ignore those points for
1285 // now). Therefore, the sup-region outlives the sub-region if,
1286 // for each universal region R1 in the sub-region, there
1287 // exists some region R2 in the sup-region that outlives R1.
1288 let universal_outlives =
1289 self.scc_values.universal_regions_outlived_by(sub_region_scc).all(|r1| {
1291 .universal_regions_outlived_by(sup_region_scc)
1292 .any(|r2| self.universal_region_relations.outlives(r2, r1))
1295 if !universal_outlives {
1299 // Now we have to compare all the points in the sub region and make
1300 // sure they exist in the sup region.
1302 if self.universal_regions.is_universal_region(sup_region) {
1303 // Micro-opt: universal regions contain all points.
1307 self.scc_values.contains_points(sup_region_scc, sub_region_scc)
1310 /// Once regions have been propagated, this method is used to see
1311 /// whether any of the constraints were too strong. In particular,
1312 /// we want to check for a case where a universally quantified
1313 /// region exceeded its bounds. Consider:
1315 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1317 /// In this case, returning `x` requires `&'a u32 <: &'b u32`
1318 /// and hence we establish (transitively) a constraint that
1319 /// `'a: 'b`. The `propagate_constraints` code above will
1320 /// therefore add `end('a)` into the region for `'b` -- but we
1321 /// have no evidence that `'b` outlives `'a`, so we want to report
1324 /// If `propagated_outlives_requirements` is `Some`, then we will
1325 /// push unsatisfied obligations into there. Otherwise, we'll
1326 /// report them as errors.
1327 fn check_universal_regions(
1329 infcx: &InferCtxt<'_, 'tcx>,
1333 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1334 errors_buffer: &mut Vec<Diagnostic>,
1336 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1337 match fr_definition.origin {
1338 NLLRegionVariableOrigin::FreeRegion => {
1339 // Go through each of the universal regions `fr` and check that
1340 // they did not grow too large, accumulating any requirements
1341 // for our caller into the `outlives_requirements` vector.
1342 self.check_universal_region(
1348 &mut propagated_outlives_requirements,
1353 NLLRegionVariableOrigin::Placeholder(placeholder) => {
1354 self.check_bound_universal_region(infcx, body, mir_def_id, fr, placeholder);
1357 NLLRegionVariableOrigin::Existential => {
1358 // nothing to check here
1364 /// Checks the final value for the free region `fr` to see if it
1365 /// grew too large. In particular, examine what `end(X)` points
1366 /// wound up in `fr`'s final value; for each `end(X)` where `X !=
1367 /// fr`, we want to check that `fr: X`. If not, that's either an
1368 /// error, or something we have to propagate to our creator.
1370 /// Things that are to be propagated are accumulated into the
1371 /// `outlives_requirements` vector.
1372 fn check_universal_region(
1374 infcx: &InferCtxt<'_, 'tcx>,
1378 longer_fr: RegionVid,
1379 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1380 errors_buffer: &mut Vec<Diagnostic>,
1382 debug!("check_universal_region(fr={:?})", longer_fr);
1384 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1386 // Because this free region must be in the ROOT universe, we
1387 // know it cannot contain any bound universes.
1388 assert!(self.scc_universes[longer_fr_scc] == ty::UniverseIndex::ROOT);
1389 debug_assert!(self.scc_values.placeholders_contained_in(longer_fr_scc).next().is_none());
1391 // Only check all of the relations for the main representative of each
1392 // SCC, otherwise just check that we outlive said representative. This
1393 // reduces the number of redundant relations propagated out of
1395 // Note that the representative will be a universal region if there is
1396 // one in this SCC, so we will always check the representative here.
1397 let representative = self.scc_representatives[longer_fr_scc];
1398 if representative != longer_fr {
1399 self.check_universal_region_relation(
1406 propagated_outlives_requirements,
1412 // Find every region `o` such that `fr: o`
1413 // (because `fr` includes `end(o)`).
1414 for shorter_fr in self.scc_values.universal_regions_outlived_by(longer_fr_scc) {
1415 if let Some(ErrorReported) = self.check_universal_region_relation(
1422 propagated_outlives_requirements,
1425 // continuing to iterate just reports more errors than necessary
1431 fn check_universal_region_relation(
1433 longer_fr: RegionVid,
1434 shorter_fr: RegionVid,
1435 infcx: &InferCtxt<'_, 'tcx>,
1439 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1440 errors_buffer: &mut Vec<Diagnostic>,
1441 ) -> Option<ErrorReported> {
1442 // If it is known that `fr: o`, carry on.
1443 if self.universal_region_relations.outlives(longer_fr, shorter_fr) {
1448 "check_universal_region_relation: fr={:?} does not outlive shorter_fr={:?}",
1449 longer_fr, shorter_fr,
1452 if let Some(propagated_outlives_requirements) = propagated_outlives_requirements {
1453 // Shrink `longer_fr` until we find a non-local region (if we do).
1454 // We'll call it `fr-` -- it's ever so slightly smaller than
1457 if let Some(fr_minus) = self.universal_region_relations.non_local_lower_bound(longer_fr)
1459 debug!("check_universal_region: fr_minus={:?}", fr_minus);
1461 let blame_span_category =
1462 self.find_outlives_blame_span(body, longer_fr, shorter_fr);
1464 // Grow `shorter_fr` until we find some non-local regions. (We
1465 // always will.) We'll call them `shorter_fr+` -- they're ever
1466 // so slightly larger than `shorter_fr`.
1467 let shorter_fr_plus =
1468 self.universal_region_relations.non_local_upper_bounds(&shorter_fr);
1469 debug!("check_universal_region: shorter_fr_plus={:?}", shorter_fr_plus);
1470 for &&fr in &shorter_fr_plus {
1471 // Push the constraint `fr-: shorter_fr+`
1472 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
1473 subject: ClosureOutlivesSubject::Region(fr_minus),
1474 outlived_free_region: fr,
1475 blame_span: blame_span_category.1,
1476 category: blame_span_category.0,
1483 // If we are not in a context where we can't propagate errors, or we
1484 // could not shrink `fr` to something smaller, then just report an
1487 // Note: in this case, we use the unapproximated regions to report the
1488 // error. This gives better error messages in some cases.
1489 self.report_error(body, upvars, infcx, mir_def_id, longer_fr, shorter_fr, errors_buffer);
1493 fn check_bound_universal_region(
1495 infcx: &InferCtxt<'_, 'tcx>,
1498 longer_fr: RegionVid,
1499 placeholder: ty::PlaceholderRegion,
1501 debug!("check_bound_universal_region(fr={:?}, placeholder={:?})", longer_fr, placeholder,);
1503 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1504 debug!("check_bound_universal_region: longer_fr_scc={:?}", longer_fr_scc,);
1506 // If we have some bound universal region `'a`, then the only
1507 // elements it can contain is itself -- we don't know anything
1509 let error_element = match {
1510 self.scc_values.elements_contained_in(longer_fr_scc).find(|element| match element {
1511 RegionElement::Location(_) => true,
1512 RegionElement::RootUniversalRegion(_) => true,
1513 RegionElement::PlaceholderRegion(placeholder1) => placeholder != *placeholder1,
1519 debug!("check_bound_universal_region: error_element = {:?}", error_element);
1521 // Find the region that introduced this `error_element`.
1522 let error_region = match error_element {
1523 RegionElement::Location(l) => self.find_sub_region_live_at(longer_fr, l),
1524 RegionElement::RootUniversalRegion(r) => r,
1525 RegionElement::PlaceholderRegion(error_placeholder) => self
1528 .filter_map(|(r, definition)| match definition.origin {
1529 NLLRegionVariableOrigin::Placeholder(p) if p == error_placeholder => Some(r),
1536 // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
1537 let (_, span) = self.find_outlives_blame_span(body, longer_fr, error_region);
1539 // Obviously, this error message is far from satisfactory.
1540 // At present, though, it only appears in unit tests --
1541 // the AST-based checker uses a more conservative check,
1542 // so to even see this error, one must pass in a special
1544 let mut diag = infcx.tcx.sess.struct_span_err(span, "higher-ranked subtype error");
1548 fn check_member_constraints(
1550 infcx: &InferCtxt<'_, 'tcx>,
1552 errors_buffer: &mut Vec<Diagnostic>,
1554 let member_constraints = self.member_constraints.clone();
1555 for m_c_i in member_constraints.all_indices() {
1556 debug!("check_member_constraint(m_c_i={:?})", m_c_i);
1557 let m_c = &member_constraints[m_c_i];
1558 let member_region_vid = m_c.member_region_vid;
1560 "check_member_constraint: member_region_vid={:?} with value {}",
1562 self.region_value_str(member_region_vid),
1564 let choice_regions = member_constraints.choice_regions(m_c_i);
1565 debug!("check_member_constraint: choice_regions={:?}", choice_regions);
1567 // did the pick-region wind up equal to any of the option regions?
1568 if let Some(o) = choice_regions.iter().find(|&&o_r| {
1569 self.eval_equal(o_r, m_c.member_region_vid)
1571 debug!("check_member_constraint: evaluated as equal to {:?}", o);
1575 // if not, report an error
1576 let region_scope_tree = &infcx.tcx.region_scope_tree(mir_def_id);
1577 let member_region = infcx.tcx.mk_region(ty::ReVar(member_region_vid));
1578 opaque_types::unexpected_hidden_region_diagnostic(
1580 Some(region_scope_tree),
1581 m_c.opaque_type_def_id,
1585 .buffer(errors_buffer);
1590 impl<'tcx> RegionDefinition<'tcx> {
1591 fn new(universe: ty::UniverseIndex, rv_origin: RegionVariableOrigin) -> Self {
1592 // Create a new region definition. Note that, for free
1593 // regions, the `external_name` field gets updated later in
1594 // `init_universal_regions`.
1596 let origin = match rv_origin {
1597 RegionVariableOrigin::NLL(origin) => origin,
1598 _ => NLLRegionVariableOrigin::Existential,
1601 Self { origin, universe, external_name: None }
1605 pub trait ClosureRegionRequirementsExt<'tcx> {
1606 fn apply_requirements(
1609 closure_def_id: DefId,
1610 closure_substs: SubstsRef<'tcx>,
1611 ) -> Vec<QueryOutlivesConstraint<'tcx>>;
1613 fn subst_closure_mapping<T>(
1616 closure_mapping: &IndexVec<RegionVid, ty::Region<'tcx>>,
1620 T: TypeFoldable<'tcx>;
1623 impl<'tcx> ClosureRegionRequirementsExt<'tcx> for ClosureRegionRequirements<'tcx> {
1624 /// Given an instance T of the closure type, this method
1625 /// instantiates the "extra" requirements that we computed for the
1626 /// closure into the inference context. This has the effect of
1627 /// adding new outlives obligations to existing variables.
1629 /// As described on `ClosureRegionRequirements`, the extra
1630 /// requirements are expressed in terms of regionvids that index
1631 /// into the free regions that appear on the closure type. So, to
1632 /// do this, we first copy those regions out from the type T into
1633 /// a vector. Then we can just index into that vector to extract
1634 /// out the corresponding region from T and apply the
1636 fn apply_requirements(
1639 closure_def_id: DefId,
1640 closure_substs: SubstsRef<'tcx>,
1641 ) -> Vec<QueryOutlivesConstraint<'tcx>> {
1643 "apply_requirements(closure_def_id={:?}, closure_substs={:?})",
1644 closure_def_id, closure_substs
1647 // Extract the values of the free regions in `closure_substs`
1648 // into a vector. These are the regions that we will be
1649 // relating to one another.
1650 let closure_mapping = &UniversalRegions::closure_mapping(
1653 self.num_external_vids,
1654 tcx.closure_base_def_id(closure_def_id),
1656 debug!("apply_requirements: closure_mapping={:?}", closure_mapping);
1658 // Create the predicates.
1659 self.outlives_requirements
1661 .map(|outlives_requirement| {
1662 let outlived_region = closure_mapping[outlives_requirement.outlived_free_region];
1664 match outlives_requirement.subject {
1665 ClosureOutlivesSubject::Region(region) => {
1666 let region = closure_mapping[region];
1668 "apply_requirements: region={:?} \
1669 outlived_region={:?} \
1670 outlives_requirement={:?}",
1671 region, outlived_region, outlives_requirement,
1673 ty::Binder::dummy(ty::OutlivesPredicate(region.into(), outlived_region))
1676 ClosureOutlivesSubject::Ty(ty) => {
1677 let ty = self.subst_closure_mapping(tcx, closure_mapping, &ty);
1679 "apply_requirements: ty={:?} \
1680 outlived_region={:?} \
1681 outlives_requirement={:?}",
1682 ty, outlived_region, outlives_requirement,
1684 ty::Binder::dummy(ty::OutlivesPredicate(ty.into(), outlived_region))
1691 fn subst_closure_mapping<T>(
1694 closure_mapping: &IndexVec<RegionVid, ty::Region<'tcx>>,
1698 T: TypeFoldable<'tcx>,
1700 tcx.fold_regions(value, &mut false, |r, _depth| {
1701 if let ty::ReClosureBound(vid) = r {
1702 closure_mapping[*vid]
1704 bug!("subst_closure_mapping: encountered non-closure bound free region {:?}", r)