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::pick_constraints::PickConstraintSet;
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::bit_set::BitSet;
25 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
26 use crate::rustc_data_structures::graph::WithSuccessors;
27 use rustc_data_structures::graph::scc::Sccs;
28 use rustc_data_structures::graph::vec_graph::VecGraph;
29 use rustc_data_structures::indexed_vec::IndexVec;
30 use rustc_errors::{Diagnostic, DiagnosticBuilder};
37 crate use self::error_reporting::{RegionName, RegionNameSource};
40 use self::values::{LivenessValues, RegionValueElements, RegionValues};
42 use super::ToRegionVid;
44 pub struct RegionInferenceContext<'tcx> {
45 /// Contains the definition for every region variable. Region
46 /// variables are identified by their index (`RegionVid`). The
47 /// definition contains information about where the region came
48 /// from as well as its final inferred value.
49 definitions: IndexVec<RegionVid, RegionDefinition<'tcx>>,
51 /// The liveness constraints added to each region. For most
52 /// regions, these start out empty and steadily grow, though for
53 /// each universally quantified region R they start out containing
54 /// the entire CFG and `end(R)`.
55 liveness_constraints: LivenessValues<RegionVid>,
57 /// The outlives constraints computed by the type-check.
58 constraints: Rc<OutlivesConstraintSet>,
60 /// The constraint-set, but in graph form, making it easy to traverse
61 /// the constraints adjacent to a particular region. Used to construct
62 /// the SCC (see `constraint_sccs`) and for error reporting.
63 constraint_graph: Rc<NormalConstraintGraph>,
65 /// The SCC computed from `constraints` and the constraint
66 /// graph. We have an edge from SCC A to SCC B if `A: B`. Used to
67 /// compute the values of each region.
68 constraint_sccs: Rc<Sccs<RegionVid, ConstraintSccIndex>>,
70 /// Reverse of the SCC constraint graph -- i.e., an edge `A -> B`
71 /// exists if `B: A`. Computed lazilly.
72 rev_constraint_graph: Option<Rc<VecGraph<ConstraintSccIndex>>>,
74 /// The "pick R0 from [R1..Rn]" constraints, indexed by SCC.
75 pick_constraints: Rc<PickConstraintSet<'tcx, ConstraintSccIndex>>,
77 /// Map closure bounds to a `Span` that should be used for error reporting.
78 closure_bounds_mapping:
79 FxHashMap<Location, FxHashMap<(RegionVid, RegionVid), (ConstraintCategory, Span)>>,
81 /// Contains the minimum universe of any variable within the same
82 /// SCC. We will ensure that no SCC contains values that are not
83 /// visible from this index.
84 scc_universes: IndexVec<ConstraintSccIndex, ty::UniverseIndex>,
86 /// Contains a "representative" from each SCC. This will be the
87 /// minimal RegionVid belonging to that universe. It is used as a
88 /// kind of hacky way to manage checking outlives relationships,
89 /// since we can 'canonicalize' each region to the representative
90 /// of its SCC and be sure that -- if they have the same repr --
91 /// they *must* be equal (though not having the same repr does not
92 /// mean they are unequal).
93 scc_representatives: IndexVec<ConstraintSccIndex, ty::RegionVid>,
95 /// The final inferred values of the region variables; we compute
96 /// one value per SCC. To get the value for any given *region*,
97 /// you first find which scc it is a part of.
98 scc_values: RegionValues<ConstraintSccIndex>,
100 /// Type constraints that we check after solving.
101 type_tests: Vec<TypeTest<'tcx>>,
103 /// Information about the universally quantified regions in scope
104 /// on this function.
105 universal_regions: Rc<UniversalRegions<'tcx>>,
107 /// Information about how the universally quantified regions in
108 /// scope on this function relate to one another.
109 universal_region_relations: Rc<UniversalRegionRelations<'tcx>>,
112 struct RegionDefinition<'tcx> {
113 /// What kind of variable is this -- a free region? existential
114 /// variable? etc. (See the `NLLRegionVariableOrigin` for more
116 origin: NLLRegionVariableOrigin,
118 /// Which universe is this region variable defined in? This is
119 /// most often `ty::UniverseIndex::ROOT`, but when we encounter
120 /// forall-quantifiers like `for<'a> { 'a = 'b }`, we would create
121 /// the variable for `'a` in a fresh universe that extends ROOT.
122 universe: ty::UniverseIndex,
124 /// If this is 'static or an early-bound region, then this is
125 /// `Some(X)` where `X` is the name of the region.
126 external_name: Option<ty::Region<'tcx>>,
129 /// N.B., the variants in `Cause` are intentionally ordered. Lower
130 /// values are preferred when it comes to error messages. Do not
131 /// reorder willy nilly.
132 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
133 pub(crate) enum Cause {
134 /// point inserted because Local was live at the given Location
135 LiveVar(Local, Location),
137 /// point inserted because Local was dropped at the given Location
138 DropVar(Local, Location),
141 /// A "type test" corresponds to an outlives constraint between a type
142 /// and a lifetime, like `T: 'x` or `<T as Foo>::Bar: 'x`. They are
143 /// translated from the `Verify` region constraints in the ordinary
144 /// inference context.
146 /// These sorts of constraints are handled differently than ordinary
147 /// constraints, at least at present. During type checking, the
148 /// `InferCtxt::process_registered_region_obligations` method will
149 /// attempt to convert a type test like `T: 'x` into an ordinary
150 /// outlives constraint when possible (for example, `&'a T: 'b` will
151 /// be converted into `'a: 'b` and registered as a `Constraint`).
153 /// In some cases, however, there are outlives relationships that are
154 /// not converted into a region constraint, but rather into one of
155 /// these "type tests". The distinction is that a type test does not
156 /// influence the inference result, but instead just examines the
157 /// values that we ultimately inferred for each region variable and
158 /// checks that they meet certain extra criteria. If not, an error
161 /// One reason for this is that these type tests typically boil down
162 /// to a check like `'a: 'x` where `'a` is a universally quantified
163 /// region -- and therefore not one whose value is really meant to be
164 /// *inferred*, precisely (this is not always the case: one can have a
165 /// type test like `<Foo as Trait<'?0>>::Bar: 'x`, where `'?0` is an
166 /// inference variable). Another reason is that these type tests can
167 /// involve *disjunction* -- that is, they can be satisfied in more
170 /// For more information about this translation, see
171 /// `InferCtxt::process_registered_region_obligations` and
172 /// `InferCtxt::type_must_outlive` in `rustc::infer::outlives`.
173 #[derive(Clone, Debug)]
174 pub struct TypeTest<'tcx> {
175 /// The type `T` that must outlive the region.
176 pub generic_kind: GenericKind<'tcx>,
178 /// The region `'x` that the type must outlive.
179 pub lower_bound: RegionVid,
181 /// Where did this constraint arise and why?
182 pub locations: Locations,
184 /// A test which, if met by the region `'x`, proves that this type
185 /// constraint is satisfied.
186 pub verify_bound: VerifyBound<'tcx>,
189 impl<'tcx> RegionInferenceContext<'tcx> {
190 /// Creates a new region inference context with a total of
191 /// `num_region_variables` valid inference variables; the first N
192 /// of those will be constant regions representing the free
193 /// regions defined in `universal_regions`.
195 /// The `outlives_constraints` and `type_tests` are an initial set
196 /// of constraints produced by the MIR type check.
199 universal_regions: Rc<UniversalRegions<'tcx>>,
200 placeholder_indices: Rc<PlaceholderIndices>,
201 universal_region_relations: Rc<UniversalRegionRelations<'tcx>>,
203 outlives_constraints: OutlivesConstraintSet,
204 pick_constraints_in: PickConstraintSet<'tcx, RegionVid>,
205 closure_bounds_mapping: FxHashMap<
207 FxHashMap<(RegionVid, RegionVid), (ConstraintCategory, Span)>,
209 type_tests: Vec<TypeTest<'tcx>>,
210 liveness_constraints: LivenessValues<RegionVid>,
211 elements: &Rc<RegionValueElements>,
213 // Create a RegionDefinition for each inference variable.
214 let definitions: IndexVec<_, _> = var_infos
216 .map(|info| RegionDefinition::new(info.universe, info.origin))
219 let constraints = Rc::new(outlives_constraints); // freeze constraints
220 let constraint_graph = Rc::new(constraints.graph(definitions.len()));
221 let fr_static = universal_regions.fr_static;
222 let constraint_sccs = Rc::new(constraints.compute_sccs(&constraint_graph, fr_static));
225 RegionValues::new(elements, universal_regions.len(), &placeholder_indices);
227 for region in liveness_constraints.rows() {
228 let scc = constraint_sccs.scc(region);
229 scc_values.merge_liveness(scc, region, &liveness_constraints);
232 let scc_universes = Self::compute_scc_universes(&constraint_sccs, &definitions);
234 let scc_representatives = Self::compute_scc_representatives(&constraint_sccs, &definitions);
236 let pick_constraints = Rc::new(pick_constraints_in.into_mapped(|r| constraint_sccs.scc(r)));
238 let mut result = Self {
240 liveness_constraints,
244 rev_constraint_graph: None,
246 closure_bounds_mapping,
252 universal_region_relations,
255 result.init_free_and_bound_regions();
260 /// Each SCC is the combination of many region variables which
261 /// have been equated. Therefore, we can associate a universe with
262 /// each SCC which is minimum of all the universes of its
263 /// constituent regions -- this is because whatever value the SCC
264 /// takes on must be a value that each of the regions within the
265 /// SCC could have as well. This implies that the SCC must have
266 /// the minimum, or narrowest, universe.
267 fn compute_scc_universes(
268 constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
269 definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
270 ) -> IndexVec<ConstraintSccIndex, ty::UniverseIndex> {
271 let num_sccs = constraints_scc.num_sccs();
272 let mut scc_universes = IndexVec::from_elem_n(ty::UniverseIndex::MAX, num_sccs);
274 for (region_vid, region_definition) in definitions.iter_enumerated() {
275 let scc = constraints_scc.scc(region_vid);
276 let scc_universe = &mut scc_universes[scc];
277 *scc_universe = ::std::cmp::min(*scc_universe, region_definition.universe);
280 debug!("compute_scc_universes: scc_universe = {:#?}", scc_universes);
285 /// For each SCC, we compute a unique `RegionVid` (in fact, the
286 /// minimal one that belongs to the SCC). See
287 /// `scc_representatives` field of `RegionInferenceContext` for
289 fn compute_scc_representatives(
290 constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
291 definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
292 ) -> IndexVec<ConstraintSccIndex, ty::RegionVid> {
293 let num_sccs = constraints_scc.num_sccs();
294 let next_region_vid = definitions.next_index();
295 let mut scc_representatives = IndexVec::from_elem_n(next_region_vid, num_sccs);
297 for region_vid in definitions.indices() {
298 let scc = constraints_scc.scc(region_vid);
299 let prev_min = scc_representatives[scc];
300 scc_representatives[scc] = region_vid.min(prev_min);
306 /// Initializes the region variables for each universally
307 /// quantified region (lifetime parameter). The first N variables
308 /// always correspond to the regions appearing in the function
309 /// signature (both named and anonymous) and where-clauses. This
310 /// function iterates over those regions and initializes them with
315 /// fn foo<'a, 'b>(..) where 'a: 'b
317 /// would initialize two variables like so:
319 /// R0 = { CFG, R0 } // 'a
320 /// R1 = { CFG, R0, R1 } // 'b
322 /// Here, R0 represents `'a`, and it contains (a) the entire CFG
323 /// and (b) any universally quantified regions that it outlives,
324 /// which in this case is just itself. R1 (`'b`) in contrast also
325 /// outlives `'a` and hence contains R0 and R1.
326 fn init_free_and_bound_regions(&mut self) {
327 // Update the names (if any)
328 for (external_name, variable) in self.universal_regions.named_universal_regions() {
330 "init_universal_regions: region {:?} has external name {:?}",
331 variable, external_name
333 self.definitions[variable].external_name = Some(external_name);
336 for variable in self.definitions.indices() {
337 let scc = self.constraint_sccs.scc(variable);
339 match self.definitions[variable].origin {
340 NLLRegionVariableOrigin::FreeRegion => {
341 // For each free, universally quantified region X:
343 // Add all nodes in the CFG to liveness constraints
344 self.liveness_constraints.add_all_points(variable);
345 self.scc_values.add_all_points(scc);
347 // Add `end(X)` into the set for X.
348 self.scc_values.add_element(scc, variable);
351 NLLRegionVariableOrigin::Placeholder(placeholder) => {
352 // Each placeholder region is only visible from
353 // its universe `ui` and its extensions. So we
354 // can't just add it into `scc` unless the
355 // universe of the scc can name this region.
356 let scc_universe = self.scc_universes[scc];
357 if scc_universe.can_name(placeholder.universe) {
358 self.scc_values.add_element(scc, placeholder);
361 "init_free_and_bound_regions: placeholder {:?} is \
362 not compatible with universe {:?} of its SCC {:?}",
363 placeholder, scc_universe, scc,
365 self.add_incompatible_universe(scc);
369 NLLRegionVariableOrigin::Existential => {
370 // For existential, regions, nothing to do.
376 /// Returns an iterator over all the region indices.
377 pub fn regions(&self) -> impl Iterator<Item = RegionVid> {
378 self.definitions.indices()
381 /// Given a universal region in scope on the MIR, returns the
382 /// corresponding index.
384 /// (Panics if `r` is not a registered universal region.)
385 pub fn to_region_vid(&self, r: ty::Region<'tcx>) -> RegionVid {
386 self.universal_regions.to_region_vid(r)
389 /// Adds annotations for `#[rustc_regions]`; see `UniversalRegions::annotate`.
390 crate fn annotate(&self, tcx: TyCtxt<'tcx>, err: &mut DiagnosticBuilder<'_>) {
391 self.universal_regions.annotate(tcx, err)
394 /// Returns `true` if the region `r` contains the point `p`.
396 /// Panics if called before `solve()` executes,
397 crate fn region_contains(&self, r: impl ToRegionVid, p: impl ToElementIndex) -> bool {
398 let scc = self.constraint_sccs.scc(r.to_region_vid());
399 self.scc_values.contains(scc, p)
402 /// Returns access to the value of `r` for debugging purposes.
403 crate fn region_value_str(&self, r: RegionVid) -> String {
404 let scc = self.constraint_sccs.scc(r.to_region_vid());
405 self.scc_values.region_value_str(scc)
408 /// Returns access to the value of `r` for debugging purposes.
409 crate fn region_universe(&self, r: RegionVid) -> ty::UniverseIndex {
410 let scc = self.constraint_sccs.scc(r.to_region_vid());
411 self.scc_universes[scc]
414 /// Performs region inference and report errors if we see any
415 /// unsatisfiable constraints. If this is a closure, returns the
416 /// region requirements to propagate to our creator, if any.
419 infcx: &InferCtxt<'_, 'tcx>,
423 errors_buffer: &mut Vec<Diagnostic>,
424 ) -> Option<ClosureRegionRequirements<'tcx>> {
426 infcx.tcx.sess.time_extended(),
427 Some(infcx.tcx.sess),
428 &format!("solve_nll_region_constraints({:?})", mir_def_id),
429 || self.solve_inner(infcx, body, upvars, mir_def_id, errors_buffer),
435 infcx: &InferCtxt<'_, 'tcx>,
439 errors_buffer: &mut Vec<Diagnostic>,
440 ) -> Option<ClosureRegionRequirements<'tcx>> {
441 self.propagate_constraints(body);
443 // If this is a closure, we can propagate unsatisfied
444 // `outlives_requirements` to our creator, so create a vector
445 // to store those. Otherwise, we'll pass in `None` to the
446 // functions below, which will trigger them to report errors
448 let mut outlives_requirements =
449 if infcx.tcx.is_closure(mir_def_id) { Some(vec![]) } else { None };
451 self.check_type_tests(
455 outlives_requirements.as_mut(),
459 self.check_universal_regions(
464 outlives_requirements.as_mut(),
468 self.check_pick_constraints(infcx, mir_def_id, errors_buffer);
470 let outlives_requirements = outlives_requirements.unwrap_or(vec![]);
472 if outlives_requirements.is_empty() {
475 let num_external_vids = self.universal_regions.num_global_and_external_regions();
476 Some(ClosureRegionRequirements { num_external_vids, outlives_requirements })
480 /// Propagate the region constraints: this will grow the values
481 /// for each region variable until all the constraints are
482 /// satisfied. Note that some values may grow **too** large to be
483 /// feasible, but we check this later.
484 fn propagate_constraints(&mut self, _body: &Body<'tcx>) {
485 debug!("propagate_constraints()");
487 debug!("propagate_constraints: constraints={:#?}", {
488 let mut constraints: Vec<_> = self.constraints.outlives().iter().collect();
492 .map(|c| (c, self.constraint_sccs.scc(c.sup), self.constraint_sccs.scc(c.sub)))
496 // To propagate constraints, we walk the DAG induced by the
497 // SCC. For each SCC, we visit its successors and compute
498 // their values, then we union all those values to get our
500 let visited = &mut BitSet::new_empty(self.constraint_sccs.num_sccs());
501 for scc_index in self.constraint_sccs.all_sccs() {
502 self.propagate_constraint_sccs_if_new(scc_index, visited);
506 /// Computes the value of the SCC `scc_a` if it has not already
507 /// been computed. The `visited` parameter is a bitset
509 fn propagate_constraint_sccs_if_new(
511 scc_a: ConstraintSccIndex,
512 visited: &mut BitSet<ConstraintSccIndex>,
514 if visited.insert(scc_a) {
515 self.propagate_constraint_sccs_new(scc_a, visited);
519 /// Computes the value of the SCC `scc_a`, which has not yet been
520 /// computed. This works by first computing all successors of the
521 /// SCC (if they haven't been computed already) and then unioning
522 /// together their elements.
523 fn propagate_constraint_sccs_new(
525 scc_a: ConstraintSccIndex,
526 visited: &mut BitSet<ConstraintSccIndex>,
528 let constraint_sccs = self.constraint_sccs.clone();
530 // Walk each SCC `B` such that `A: B`...
531 for &scc_b in constraint_sccs.successors(scc_a) {
532 debug!("propagate_constraint_sccs: scc_a = {:?} scc_b = {:?}", scc_a, scc_b);
534 // ...compute the value of `B`...
535 self.propagate_constraint_sccs_if_new(scc_b, visited);
537 // ...and add elements from `B` into `A`. One complication
538 // arises because of universes: If `B` contains something
539 // that `A` cannot name, then `A` can only contain `B` if
540 // it outlives static.
541 if self.universe_compatible(scc_b, scc_a) {
542 // `A` can name everything that is in `B`, so just
544 self.scc_values.add_region(scc_a, scc_b);
546 self.add_incompatible_universe(scc_a);
550 // Now take pick constraints into account
551 let pick_constraints = self.pick_constraints.clone();
552 for p_c_i in pick_constraints.indices(scc_a) {
553 self.apply_pick_constraint(
555 pick_constraints[p_c_i].opaque_type_def_id,
556 pick_constraints.option_regions(p_c_i),
561 "propagate_constraint_sccs: scc_a = {:?} has value {:?}",
563 self.scc_values.region_value_str(scc_a),
567 /// Invoked for each `pick R0 from [R1..Rn]` constraint.
569 /// `scc` is the SCC containing R0, and `option_regions` are the
570 /// `R1..Rn` regions -- they are always known to be universal
571 /// regions (and if that's not true, we just don't attempt to
572 /// enforce the constraint).
574 /// The current value of `scc` at the time the method is invoked
575 /// is considered a *lower bound*. If possible, we will modify
576 /// the constraint to set it equal to one of the option regions.
577 /// If we make any changes, returns true, else false.
578 fn apply_pick_constraint(
580 scc: ConstraintSccIndex,
581 opaque_type_def_id: DefId,
582 option_regions: &[ty::RegionVid],
584 debug!("apply_pick_constraint(scc={:?}, option_regions={:#?})", scc, option_regions,);
587 option_regions.iter().find(|&&r| !self.universal_regions.is_universal_region(r))
589 // FIXME(#61773): This case can only occur with
590 // `impl_trait_in_bindings`, I believe, and we are just
591 // opting not to handle it for now. See #61773 for
594 "pick constraint for `{:?}` has an option region `{:?}` \
595 that is not a universal region",
601 // Create a mutable vector of the options. We'll try to winnow
603 let mut option_regions: Vec<ty::RegionVid> = option_regions.to_vec();
605 // The 'pick-region' in a pick-constraint is part of the
606 // hidden type, which must be in the root universe. Therefore,
607 // it cannot have any placeholders in its value.
608 assert!(self.scc_universes[scc] == ty::UniverseIndex::ROOT);
610 self.scc_values.placeholders_contained_in(scc).next().is_none(),
611 "scc {:?} in a pick-constraint has placeholder value: {:?}",
613 self.scc_values.region_value_str(scc),
616 // The existing value for `scc` is a lower-bound. This will
617 // consist of some set {P} + {LB} of points {P} and
618 // lower-bound free regions {LB}. As each option region O is a
619 // free region, it will outlive the points. But we can only
620 // consider the option O if O: LB.
621 option_regions.retain(|&o_r| {
623 .universal_regions_outlived_by(scc)
624 .all(|lb| self.universal_region_relations.outlives(o_r, lb))
626 debug!("apply_pick_constraint: after lb, option_regions={:?}", option_regions);
628 // Now find all the *upper bounds* -- that is, each UB is a free
629 // region that must outlive pick region R0 (`UB: R0`). Therefore,
630 // we need only keep an option O if `UB: O` for all UB.
631 if option_regions.len() > 1 {
632 let universal_region_relations = self.universal_region_relations.clone();
633 for ub in self.upper_bounds(scc) {
634 debug!("apply_pick_constraint: ub={:?}", ub);
635 option_regions.retain(|&o_r| universal_region_relations.outlives(ub, o_r));
637 debug!("apply_pick_constraint: after ub, option_regions={:?}", option_regions);
640 // If we ruled everything out, we're done.
641 if option_regions.is_empty() {
645 // Otherwise, we need to find the minimum option, if any, and take that.
646 debug!("apply_pick_constraint: option_regions remaining are {:#?}", option_regions);
647 let min = |r1: ty::RegionVid, r2: ty::RegionVid| -> Option<ty::RegionVid> {
648 let r1_outlives_r2 = self.universal_region_relations.outlives(r1, r2);
649 let r2_outlives_r1 = self.universal_region_relations.outlives(r2, r1);
650 if r1_outlives_r2 && r2_outlives_r1 {
652 } else if r1_outlives_r2 {
654 } else if r2_outlives_r1 {
660 let mut best_option = option_regions[0];
661 for &other_option in &option_regions[1..] {
663 "apply_pick_constraint: best_option={:?} other_option={:?}",
664 best_option, other_option,
666 match min(best_option, other_option) {
667 Some(m) => best_option = m,
670 "apply_pick_constraint: {:?} and {:?} are incomparable --> no best choice",
671 best_option, other_option,
678 let best_option_scc = self.constraint_sccs.scc(best_option);
680 "apply_pick_constraint: best_choice={:?} best_option_scc={:?}",
684 self.scc_values.add_region(scc, best_option_scc)
687 /// Compute and return the reverse SCC-based constraint graph (lazilly).
690 scc0: ConstraintSccIndex,
691 ) -> Vec<RegionVid> {
692 // I wanted to return an `impl Iterator` here, but it's
693 // annoying because the `rev_constraint_graph` is in a local
694 // variable. We'd need a "once-cell" or some such thing to let
695 // us borrow it for the right amount of time.
696 let rev_constraint_graph = self.rev_constraint_graph();
697 let scc_values = &self.scc_values;
698 let mut duplicates = FxHashSet::default();
700 .depth_first_search(scc0)
702 .flat_map(|scc1| scc_values.universal_regions_outlived_by(scc1))
703 .filter(|&r| duplicates.insert(r))
707 /// Compute and return the reverse SCC-based constraint graph (lazilly).
708 fn rev_constraint_graph(
710 ) -> Rc<VecGraph<ConstraintSccIndex>> {
711 if let Some(g) = &self.rev_constraint_graph {
715 let rev_graph = Rc::new(self.constraint_sccs.reverse());
716 self.rev_constraint_graph = Some(rev_graph.clone());
720 /// Returns `true` if all the elements in the value of `scc_b` are nameable
721 /// in `scc_a`. Used during constraint propagation, and only once
722 /// the value of `scc_b` has been computed.
723 fn universe_compatible(&self, scc_b: ConstraintSccIndex, scc_a: ConstraintSccIndex) -> bool {
724 let universe_a = self.scc_universes[scc_a];
726 // Quick check: if scc_b's declared universe is a subset of
727 // scc_a's declared univese (typically, both are ROOT), then
728 // it cannot contain any problematic universe elements.
729 if universe_a.can_name(self.scc_universes[scc_b]) {
733 // Otherwise, we have to iterate over the universe elements in
734 // B's value, and check whether all of them are nameable
736 self.scc_values.placeholders_contained_in(scc_b).all(|p| universe_a.can_name(p.universe))
739 /// Extend `scc` so that it can outlive some placeholder region
740 /// from a universe it can't name; at present, the only way for
741 /// this to be true is if `scc` outlives `'static`. This is
742 /// actually stricter than necessary: ideally, we'd support bounds
743 /// like `for<'a: 'b`>` that might then allow us to approximate
744 /// `'a` with `'b` and not `'static`. But it will have to do for
746 fn add_incompatible_universe(&mut self, scc: ConstraintSccIndex) {
747 debug!("add_incompatible_universe(scc={:?})", scc);
749 let fr_static = self.universal_regions.fr_static;
750 self.scc_values.add_all_points(scc);
751 self.scc_values.add_element(scc, fr_static);
754 /// Once regions have been propagated, this method is used to see
755 /// whether the "type tests" produced by typeck were satisfied;
756 /// type tests encode type-outlives relationships like `T:
757 /// 'a`. See `TypeTest` for more details.
760 infcx: &InferCtxt<'_, 'tcx>,
763 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
764 errors_buffer: &mut Vec<Diagnostic>,
768 // Sometimes we register equivalent type-tests that would
769 // result in basically the exact same error being reported to
770 // the user. Avoid that.
771 let mut deduplicate_errors = FxHashSet::default();
773 for type_test in &self.type_tests {
774 debug!("check_type_test: {:?}", type_test);
776 let generic_ty = type_test.generic_kind.to_ty(tcx);
777 if self.eval_verify_bound(
781 type_test.lower_bound,
782 &type_test.verify_bound,
787 if let Some(propagated_outlives_requirements) = &mut propagated_outlives_requirements {
788 if self.try_promote_type_test(
792 propagated_outlives_requirements,
798 // Type-test failed. Report the error.
800 // Try to convert the lower-bound region into something named we can print for the user.
801 let lower_bound_region = self.to_error_region(type_test.lower_bound);
803 // Skip duplicate-ish errors.
804 let type_test_span = type_test.locations.span(body);
805 let erased_generic_kind = tcx.erase_regions(&type_test.generic_kind);
806 if !deduplicate_errors.insert((
814 "check_type_test: reporting error for erased_generic_kind={:?}, \
815 lower_bound_region={:?}, \
816 type_test.locations={:?}",
817 erased_generic_kind, lower_bound_region, type_test.locations,
821 if let Some(lower_bound_region) = lower_bound_region {
822 let region_scope_tree = &tcx.region_scope_tree(mir_def_id);
824 .construct_generic_bound_failure(
828 type_test.generic_kind,
831 .buffer(errors_buffer);
833 // FIXME. We should handle this case better. It
834 // indicates that we have e.g., some region variable
835 // whose value is like `'a+'b` where `'a` and `'b` are
836 // distinct unrelated univesal regions that are not
837 // known to outlive one another. It'd be nice to have
838 // some examples where this arises to decide how best
839 // to report it; we could probably handle it by
840 // iterating over the universal regions and reporting
841 // an error that multiple bounds are required.
845 &format!("`{}` does not live long enough", type_test.generic_kind,),
847 .buffer(errors_buffer);
852 /// Converts a region inference variable into a `ty::Region` that
853 /// we can use for error reporting. If `r` is universally bound,
854 /// then we use the name that we have on record for it. If `r` is
855 /// existentially bound, then we check its inferred value and try
856 /// to find a good name from that. Returns `None` if we can't find
857 /// one (e.g., this is just some random part of the CFG).
858 pub fn to_error_region(&self, r: RegionVid) -> Option<ty::Region<'tcx>> {
859 self.to_error_region_vid(r).and_then(|r| self.definitions[r].external_name)
862 /// Returns the [RegionVid] corresponding to the region returned by
863 /// `to_error_region`.
864 pub fn to_error_region_vid(&self, r: RegionVid) -> Option<RegionVid> {
865 if self.universal_regions.is_universal_region(r) {
868 let r_scc = self.constraint_sccs.scc(r);
869 let upper_bound = self.universal_upper_bound(r);
870 if self.scc_values.contains(r_scc, upper_bound) {
871 self.to_error_region_vid(upper_bound)
878 /// Invoked when we have some type-test (e.g., `T: 'X`) that we cannot
879 /// prove to be satisfied. If this is a closure, we will attempt to
880 /// "promote" this type-test into our `ClosureRegionRequirements` and
881 /// hence pass it up the creator. To do this, we have to phrase the
882 /// type-test in terms of external free regions, as local free
883 /// regions are not nameable by the closure's creator.
885 /// Promotion works as follows: we first check that the type `T`
886 /// contains only regions that the creator knows about. If this is
887 /// true, then -- as a consequence -- we know that all regions in
888 /// the type `T` are free regions that outlive the closure body. If
889 /// false, then promotion fails.
891 /// Once we've promoted T, we have to "promote" `'X` to some region
892 /// that is "external" to the closure. Generally speaking, a region
893 /// may be the union of some points in the closure body as well as
894 /// various free lifetimes. We can ignore the points in the closure
895 /// body: if the type T can be expressed in terms of external regions,
896 /// we know it outlives the points in the closure body. That
897 /// just leaves the free regions.
899 /// The idea then is to lower the `T: 'X` constraint into multiple
900 /// bounds -- e.g., if `'X` is the union of two free lifetimes,
901 /// `'1` and `'2`, then we would create `T: '1` and `T: '2`.
902 fn try_promote_type_test(
904 infcx: &InferCtxt<'_, 'tcx>,
906 type_test: &TypeTest<'tcx>,
907 propagated_outlives_requirements: &mut Vec<ClosureOutlivesRequirement<'tcx>>,
911 let TypeTest { generic_kind, lower_bound, locations, verify_bound: _ } = type_test;
913 let generic_ty = generic_kind.to_ty(tcx);
914 let subject = match self.try_promote_type_test_subject(infcx, generic_ty) {
916 None => return false,
919 // For each region outlived by lower_bound find a non-local,
920 // universal region (it may be the same region) and add it to
921 // `ClosureOutlivesRequirement`.
922 let r_scc = self.constraint_sccs.scc(*lower_bound);
923 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
924 // Check whether we can already prove that the "subject" outlives `ur`.
925 // If so, we don't have to propagate this requirement to our caller.
927 // To continue the example from the function, if we are trying to promote
928 // a requirement that `T: 'X`, and we know that `'X = '1 + '2` (i.e., the union
929 // `'1` and `'2`), then in this loop `ur` will be `'1` (and `'2`). So here
930 // we check whether `T: '1` is something we *can* prove. If so, no need
931 // to propagate that requirement.
933 // This is needed because -- particularly in the case
934 // where `ur` is a local bound -- we are sometimes in a
935 // position to prove things that our caller cannot. See
936 // #53570 for an example.
937 if self.eval_verify_bound(tcx, body, generic_ty, ur, &type_test.verify_bound) {
941 debug!("try_promote_type_test: ur={:?}", ur);
943 let non_local_ub = self.universal_region_relations.non_local_upper_bounds(&ur);
944 debug!("try_promote_type_test: non_local_ub={:?}", non_local_ub);
946 // This is slightly too conservative. To show T: '1, given `'2: '1`
947 // and `'3: '1` we only need to prove that T: '2 *or* T: '3, but to
948 // avoid potential non-determinism we approximate this by requiring
950 for &upper_bound in non_local_ub {
951 debug_assert!(self.universal_regions.is_universal_region(upper_bound));
952 debug_assert!(!self.universal_regions.is_local_free_region(upper_bound));
954 let requirement = ClosureOutlivesRequirement {
956 outlived_free_region: upper_bound,
957 blame_span: locations.span(body),
958 category: ConstraintCategory::Boring,
960 debug!("try_promote_type_test: pushing {:#?}", requirement);
961 propagated_outlives_requirements.push(requirement);
967 /// When we promote a type test `T: 'r`, we have to convert the
968 /// type `T` into something we can store in a query result (so
969 /// something allocated for `'tcx`). This is problematic if `ty`
970 /// contains regions. During the course of NLL region checking, we
971 /// will have replaced all of those regions with fresh inference
972 /// variables. To create a test subject, we want to replace those
973 /// inference variables with some region from the closure
974 /// signature -- this is not always possible, so this is a
975 /// fallible process. Presuming we do find a suitable region, we
976 /// will represent it with a `ReClosureBound`, which is a
977 /// `RegionKind` variant that can be allocated in the gcx.
978 fn try_promote_type_test_subject(
980 infcx: &InferCtxt<'_, 'tcx>,
982 ) -> Option<ClosureOutlivesSubject<'tcx>> {
985 debug!("try_promote_type_test_subject(ty = {:?})", ty);
987 let ty = tcx.fold_regions(&ty, &mut false, |r, _depth| {
988 let region_vid = self.to_region_vid(r);
990 // The challenge if this. We have some region variable `r`
991 // whose value is a set of CFG points and universal
992 // regions. We want to find if that set is *equivalent* to
993 // any of the named regions found in the closure.
995 // To do so, we compute the
996 // `non_local_universal_upper_bound`. This will be a
997 // non-local, universal region that is greater than `r`.
998 // However, it might not be *contained* within `r`, so
999 // then we further check whether this bound is contained
1000 // in `r`. If so, we can say that `r` is equivalent to the
1003 // Let's work through a few examples. For these, imagine
1004 // that we have 3 non-local regions (I'll denote them as
1005 // `'static`, `'a`, and `'b`, though of course in the code
1006 // they would be represented with indices) where:
1011 // First, let's assume that `r` is some existential
1012 // variable with an inferred value `{'a, 'static}` (plus
1013 // some CFG nodes). In this case, the non-local upper
1014 // bound is `'static`, since that outlives `'a`. `'static`
1015 // is also a member of `r` and hence we consider `r`
1016 // equivalent to `'static` (and replace it with
1019 // Now let's consider the inferred value `{'a, 'b}`. This
1020 // means `r` is effectively `'a | 'b`. I'm not sure if
1021 // this can come about, actually, but assuming it did, we
1022 // would get a non-local upper bound of `'static`. Since
1023 // `'static` is not contained in `r`, we would fail to
1024 // find an equivalent.
1025 let upper_bound = self.non_local_universal_upper_bound(region_vid);
1026 if self.region_contains(region_vid, upper_bound) {
1027 tcx.mk_region(ty::ReClosureBound(upper_bound))
1029 // In the case of a failure, use a `ReVar`
1030 // result. This will cause the `lift` later on to
1035 debug!("try_promote_type_test_subject: folded ty = {:?}", ty);
1037 // `has_local_value` will only be true if we failed to promote some region.
1038 if ty.has_local_value() {
1042 Some(ClosureOutlivesSubject::Ty(ty))
1045 /// Given some universal or existential region `r`, finds a
1046 /// non-local, universal region `r+` that outlives `r` at entry to (and
1047 /// exit from) the closure. In the worst case, this will be
1050 /// This is used for two purposes. First, if we are propagated
1051 /// some requirement `T: r`, we can use this method to enlarge `r`
1052 /// to something we can encode for our creator (which only knows
1053 /// about non-local, universal regions). It is also used when
1054 /// encoding `T` as part of `try_promote_type_test_subject` (see
1055 /// that fn for details).
1057 /// This is based on the result `'y` of `universal_upper_bound`,
1058 /// except that it converts further takes the non-local upper
1059 /// bound of `'y`, so that the final result is non-local.
1060 fn non_local_universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1061 debug!("non_local_universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1063 let lub = self.universal_upper_bound(r);
1065 // Grow further to get smallest universal region known to
1067 let non_local_lub = self.universal_region_relations.non_local_upper_bound(lub);
1069 debug!("non_local_universal_upper_bound: non_local_lub={:?}", non_local_lub);
1074 /// Returns a universally quantified region that outlives the
1075 /// value of `r` (`r` may be existentially or universally
1078 /// Since `r` is (potentially) an existential region, it has some
1079 /// value which may include (a) any number of points in the CFG
1080 /// and (b) any number of `end('x)` elements of universally
1081 /// quantified regions. To convert this into a single universal
1082 /// region we do as follows:
1084 /// - Ignore the CFG points in `'r`. All universally quantified regions
1085 /// include the CFG anyhow.
1086 /// - For each `end('x)` element in `'r`, compute the mutual LUB, yielding
1088 fn universal_upper_bound(&self, r: RegionVid) -> RegionVid {
1089 debug!("universal_upper_bound(r={:?}={})", r, self.region_value_str(r));
1091 // Find the smallest universal region that contains all other
1092 // universal regions within `region`.
1093 let mut lub = self.universal_regions.fr_fn_body;
1094 let r_scc = self.constraint_sccs.scc(r);
1095 for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
1096 lub = self.universal_region_relations.postdom_upper_bound(lub, ur);
1099 debug!("universal_upper_bound: r={:?} lub={:?}", r, lub);
1104 /// Tests if `test` is true when applied to `lower_bound` at
1106 fn eval_verify_bound(
1110 generic_ty: Ty<'tcx>,
1111 lower_bound: RegionVid,
1112 verify_bound: &VerifyBound<'tcx>,
1114 debug!("eval_verify_bound(lower_bound={:?}, verify_bound={:?})", lower_bound, verify_bound);
1116 match verify_bound {
1117 VerifyBound::IfEq(test_ty, verify_bound1) => {
1118 self.eval_if_eq(tcx, body, generic_ty, lower_bound, test_ty, verify_bound1)
1121 VerifyBound::OutlivedBy(r) => {
1122 let r_vid = self.to_region_vid(r);
1123 self.eval_outlives(r_vid, lower_bound)
1126 VerifyBound::AnyBound(verify_bounds) => verify_bounds.iter().any(|verify_bound| {
1127 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1130 VerifyBound::AllBounds(verify_bounds) => verify_bounds.iter().all(|verify_bound| {
1131 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1140 generic_ty: Ty<'tcx>,
1141 lower_bound: RegionVid,
1143 verify_bound: &VerifyBound<'tcx>,
1145 let generic_ty_normalized = self.normalize_to_scc_representatives(tcx, generic_ty);
1146 let test_ty_normalized = self.normalize_to_scc_representatives(tcx, test_ty);
1147 if generic_ty_normalized == test_ty_normalized {
1148 self.eval_verify_bound(tcx, body, generic_ty, lower_bound, verify_bound)
1154 /// This is a conservative normalization procedure. It takes every
1155 /// free region in `value` and replaces it with the
1156 /// "representative" of its SCC (see `scc_representatives` field).
1157 /// We are guaranteed that if two values normalize to the same
1158 /// thing, then they are equal; this is a conservative check in
1159 /// that they could still be equal even if they normalize to
1160 /// different results. (For example, there might be two regions
1161 /// with the same value that are not in the same SCC).
1163 /// N.B., this is not an ideal approach and I would like to revisit
1164 /// it. However, it works pretty well in practice. In particular,
1165 /// this is needed to deal with projection outlives bounds like
1167 /// <T as Foo<'0>>::Item: '1
1169 /// In particular, this routine winds up being important when
1170 /// there are bounds like `where <T as Foo<'a>>::Item: 'b` in the
1171 /// environment. In this case, if we can show that `'0 == 'a`,
1172 /// and that `'b: '1`, then we know that the clause is
1173 /// satisfied. In such cases, particularly due to limitations of
1174 /// the trait solver =), we usually wind up with a where-clause like
1175 /// `T: Foo<'a>` in scope, which thus forces `'0 == 'a` to be added as
1176 /// a constraint, and thus ensures that they are in the same SCC.
1178 /// So why can't we do a more correct routine? Well, we could
1179 /// *almost* use the `relate_tys` code, but the way it is
1180 /// currently setup it creates inference variables to deal with
1181 /// higher-ranked things and so forth, and right now the inference
1182 /// context is not permitted to make more inference variables. So
1183 /// we use this kind of hacky solution.
1184 fn normalize_to_scc_representatives<T>(&self, tcx: TyCtxt<'tcx>, value: T) -> T
1186 T: TypeFoldable<'tcx>,
1188 tcx.fold_regions(&value, &mut false, |r, _db| {
1189 let vid = self.to_region_vid(r);
1190 let scc = self.constraint_sccs.scc(vid);
1191 let repr = self.scc_representatives[scc];
1192 tcx.mk_region(ty::ReVar(repr))
1196 // Evaluate whether `sup_region == sub_region`.
1197 fn eval_equal(&self, r1: RegionVid, r2: RegionVid) -> bool {
1198 self.eval_outlives(r1, r2) && self.eval_outlives(r2, r1)
1201 // Evaluate whether `sup_region: sub_region`.
1202 fn eval_outlives(&self, sup_region: RegionVid, sub_region: RegionVid) -> bool {
1203 debug!("eval_outlives({:?}: {:?})", sup_region, sub_region);
1206 "eval_outlives: sup_region's value = {:?} universal={:?}",
1207 self.region_value_str(sup_region),
1208 self.universal_regions.is_universal_region(sup_region),
1211 "eval_outlives: sub_region's value = {:?} universal={:?}",
1212 self.region_value_str(sub_region),
1213 self.universal_regions.is_universal_region(sub_region),
1216 let sub_region_scc = self.constraint_sccs.scc(sub_region);
1217 let sup_region_scc = self.constraint_sccs.scc(sup_region);
1219 // Both the `sub_region` and `sup_region` consist of the union
1220 // of some number of universal regions (along with the union
1221 // of various points in the CFG; ignore those points for
1222 // now). Therefore, the sup-region outlives the sub-region if,
1223 // for each universal region R1 in the sub-region, there
1224 // exists some region R2 in the sup-region that outlives R1.
1225 let universal_outlives =
1226 self.scc_values.universal_regions_outlived_by(sub_region_scc).all(|r1| {
1228 .universal_regions_outlived_by(sup_region_scc)
1229 .any(|r2| self.universal_region_relations.outlives(r2, r1))
1232 if !universal_outlives {
1236 // Now we have to compare all the points in the sub region and make
1237 // sure they exist in the sup region.
1239 if self.universal_regions.is_universal_region(sup_region) {
1240 // Micro-opt: universal regions contain all points.
1244 self.scc_values.contains_points(sup_region_scc, sub_region_scc)
1247 /// Once regions have been propagated, this method is used to see
1248 /// whether any of the constraints were too strong. In particular,
1249 /// we want to check for a case where a universally quantified
1250 /// region exceeded its bounds. Consider:
1252 /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
1254 /// In this case, returning `x` requires `&'a u32 <: &'b u32`
1255 /// and hence we establish (transitively) a constraint that
1256 /// `'a: 'b`. The `propagate_constraints` code above will
1257 /// therefore add `end('a)` into the region for `'b` -- but we
1258 /// have no evidence that `'b` outlives `'a`, so we want to report
1261 /// If `propagated_outlives_requirements` is `Some`, then we will
1262 /// push unsatisfied obligations into there. Otherwise, we'll
1263 /// report them as errors.
1264 fn check_universal_regions(
1266 infcx: &InferCtxt<'_, 'tcx>,
1270 mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1271 errors_buffer: &mut Vec<Diagnostic>,
1273 for (fr, fr_definition) in self.definitions.iter_enumerated() {
1274 match fr_definition.origin {
1275 NLLRegionVariableOrigin::FreeRegion => {
1276 // Go through each of the universal regions `fr` and check that
1277 // they did not grow too large, accumulating any requirements
1278 // for our caller into the `outlives_requirements` vector.
1279 self.check_universal_region(
1285 &mut propagated_outlives_requirements,
1290 NLLRegionVariableOrigin::Placeholder(placeholder) => {
1291 self.check_bound_universal_region(infcx, body, mir_def_id, fr, placeholder);
1294 NLLRegionVariableOrigin::Existential => {
1295 // nothing to check here
1301 /// Checks the final value for the free region `fr` to see if it
1302 /// grew too large. In particular, examine what `end(X)` points
1303 /// wound up in `fr`'s final value; for each `end(X)` where `X !=
1304 /// fr`, we want to check that `fr: X`. If not, that's either an
1305 /// error, or something we have to propagate to our creator.
1307 /// Things that are to be propagated are accumulated into the
1308 /// `outlives_requirements` vector.
1309 fn check_universal_region(
1311 infcx: &InferCtxt<'_, 'tcx>,
1315 longer_fr: RegionVid,
1316 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1317 errors_buffer: &mut Vec<Diagnostic>,
1319 debug!("check_universal_region(fr={:?})", longer_fr);
1321 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1323 // Because this free region must be in the ROOT universe, we
1324 // know it cannot contain any bound universes.
1325 assert!(self.scc_universes[longer_fr_scc] == ty::UniverseIndex::ROOT);
1326 debug_assert!(self.scc_values.placeholders_contained_in(longer_fr_scc).next().is_none());
1328 // Only check all of the relations for the main representative of each
1329 // SCC, otherwise just check that we outlive said representative. This
1330 // reduces the number of redundant relations propagated out of
1332 // Note that the representative will be a universal region if there is
1333 // one in this SCC, so we will always check the representative here.
1334 let representative = self.scc_representatives[longer_fr_scc];
1335 if representative != longer_fr {
1336 self.check_universal_region_relation(
1343 propagated_outlives_requirements,
1349 // Find every region `o` such that `fr: o`
1350 // (because `fr` includes `end(o)`).
1351 for shorter_fr in self.scc_values.universal_regions_outlived_by(longer_fr_scc) {
1352 if let Some(ErrorReported) = self.check_universal_region_relation(
1359 propagated_outlives_requirements,
1362 // continuing to iterate just reports more errors than necessary
1368 fn check_universal_region_relation(
1370 longer_fr: RegionVid,
1371 shorter_fr: RegionVid,
1372 infcx: &InferCtxt<'_, 'tcx>,
1376 propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
1377 errors_buffer: &mut Vec<Diagnostic>,
1378 ) -> Option<ErrorReported> {
1379 // If it is known that `fr: o`, carry on.
1380 if self.universal_region_relations.outlives(longer_fr, shorter_fr) {
1385 "check_universal_region_relation: fr={:?} does not outlive shorter_fr={:?}",
1386 longer_fr, shorter_fr,
1389 if let Some(propagated_outlives_requirements) = propagated_outlives_requirements {
1390 // Shrink `longer_fr` until we find a non-local region (if we do).
1391 // We'll call it `fr-` -- it's ever so slightly smaller than
1394 if let Some(fr_minus) = self.universal_region_relations.non_local_lower_bound(longer_fr)
1396 debug!("check_universal_region: fr_minus={:?}", fr_minus);
1398 let blame_span_category =
1399 self.find_outlives_blame_span(body, longer_fr, shorter_fr);
1401 // Grow `shorter_fr` until we find some non-local regions. (We
1402 // always will.) We'll call them `shorter_fr+` -- they're ever
1403 // so slightly larger than `shorter_fr`.
1404 let shorter_fr_plus =
1405 self.universal_region_relations.non_local_upper_bounds(&shorter_fr);
1406 debug!("check_universal_region: shorter_fr_plus={:?}", shorter_fr_plus);
1407 for &&fr in &shorter_fr_plus {
1408 // Push the constraint `fr-: shorter_fr+`
1409 propagated_outlives_requirements.push(ClosureOutlivesRequirement {
1410 subject: ClosureOutlivesSubject::Region(fr_minus),
1411 outlived_free_region: fr,
1412 blame_span: blame_span_category.1,
1413 category: blame_span_category.0,
1420 // If we are not in a context where we can't propagate errors, or we
1421 // could not shrink `fr` to something smaller, then just report an
1424 // Note: in this case, we use the unapproximated regions to report the
1425 // error. This gives better error messages in some cases.
1426 self.report_error(body, upvars, infcx, mir_def_id, longer_fr, shorter_fr, errors_buffer);
1430 fn check_bound_universal_region(
1432 infcx: &InferCtxt<'_, 'tcx>,
1435 longer_fr: RegionVid,
1436 placeholder: ty::PlaceholderRegion,
1438 debug!("check_bound_universal_region(fr={:?}, placeholder={:?})", longer_fr, placeholder,);
1440 let longer_fr_scc = self.constraint_sccs.scc(longer_fr);
1441 debug!("check_bound_universal_region: longer_fr_scc={:?}", longer_fr_scc,);
1443 // If we have some bound universal region `'a`, then the only
1444 // elements it can contain is itself -- we don't know anything
1446 let error_element = match {
1447 self.scc_values.elements_contained_in(longer_fr_scc).find(|element| match element {
1448 RegionElement::Location(_) => true,
1449 RegionElement::RootUniversalRegion(_) => true,
1450 RegionElement::PlaceholderRegion(placeholder1) => placeholder != *placeholder1,
1456 debug!("check_bound_universal_region: error_element = {:?}", error_element);
1458 // Find the region that introduced this `error_element`.
1459 let error_region = match error_element {
1460 RegionElement::Location(l) => self.find_sub_region_live_at(longer_fr, l),
1461 RegionElement::RootUniversalRegion(r) => r,
1462 RegionElement::PlaceholderRegion(error_placeholder) => self
1465 .filter_map(|(r, definition)| match definition.origin {
1466 NLLRegionVariableOrigin::Placeholder(p) if p == error_placeholder => Some(r),
1473 // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
1474 let (_, span) = self.find_outlives_blame_span(body, longer_fr, error_region);
1476 // Obviously, this error message is far from satisfactory.
1477 // At present, though, it only appears in unit tests --
1478 // the AST-based checker uses a more conservative check,
1479 // so to even see this error, one must pass in a special
1481 let mut diag = infcx.tcx.sess.struct_span_err(span, "higher-ranked subtype error");
1485 fn check_pick_constraints(
1487 infcx: &InferCtxt<'_, 'tcx>,
1489 errors_buffer: &mut Vec<Diagnostic>,
1491 let pick_constraints = self.pick_constraints.clone();
1492 for p_c_i in pick_constraints.all_indices() {
1493 debug!("check_pick_constraint(p_c_i={:?})", p_c_i);
1494 let p_c = &pick_constraints[p_c_i];
1495 let pick_region_vid = p_c.pick_region_vid;
1496 debug!("check_pick_constraint: pick_region_vid={:?} with value {}", pick_region_vid, self.region_value_str(pick_region_vid));
1497 let option_regions = pick_constraints.option_regions(p_c_i);
1498 debug!("check_pick_constraint: option_regions={:?}", option_regions);
1500 // did the pick-region wind up equal to any of the option regions?
1501 if let Some(o) = option_regions.iter().find(|&&o_r| self.eval_equal(o_r, p_c.pick_region_vid)) {
1502 debug!("check_pick_constraint: evaluated as equal to {:?}", o);
1506 // if not, report an error
1507 let region_scope_tree = &infcx.tcx.region_scope_tree(mir_def_id);
1508 let pick_region = infcx.tcx.mk_region(ty::ReVar(pick_region_vid)); // XXX
1509 opaque_types::unexpected_hidden_region_diagnostic(
1511 Some(region_scope_tree),
1512 p_c.opaque_type_def_id,
1516 .buffer(errors_buffer);
1521 impl<'tcx> RegionDefinition<'tcx> {
1522 fn new(universe: ty::UniverseIndex, rv_origin: RegionVariableOrigin) -> Self {
1523 // Create a new region definition. Note that, for free
1524 // regions, the `external_name` field gets updated later in
1525 // `init_universal_regions`.
1527 let origin = match rv_origin {
1528 RegionVariableOrigin::NLL(origin) => origin,
1529 _ => NLLRegionVariableOrigin::Existential,
1532 Self { origin, universe, external_name: None }
1536 pub trait ClosureRegionRequirementsExt<'tcx> {
1537 fn apply_requirements(
1540 closure_def_id: DefId,
1541 closure_substs: SubstsRef<'tcx>,
1542 ) -> Vec<QueryOutlivesConstraint<'tcx>>;
1544 fn subst_closure_mapping<T>(
1547 closure_mapping: &IndexVec<RegionVid, ty::Region<'tcx>>,
1551 T: TypeFoldable<'tcx>;
1554 impl<'tcx> ClosureRegionRequirementsExt<'tcx> for ClosureRegionRequirements<'tcx> {
1555 /// Given an instance T of the closure type, this method
1556 /// instantiates the "extra" requirements that we computed for the
1557 /// closure into the inference context. This has the effect of
1558 /// adding new outlives obligations to existing variables.
1560 /// As described on `ClosureRegionRequirements`, the extra
1561 /// requirements are expressed in terms of regionvids that index
1562 /// into the free regions that appear on the closure type. So, to
1563 /// do this, we first copy those regions out from the type T into
1564 /// a vector. Then we can just index into that vector to extract
1565 /// out the corresponding region from T and apply the
1567 fn apply_requirements(
1570 closure_def_id: DefId,
1571 closure_substs: SubstsRef<'tcx>,
1572 ) -> Vec<QueryOutlivesConstraint<'tcx>> {
1574 "apply_requirements(closure_def_id={:?}, closure_substs={:?})",
1575 closure_def_id, closure_substs
1578 // Extract the values of the free regions in `closure_substs`
1579 // into a vector. These are the regions that we will be
1580 // relating to one another.
1581 let closure_mapping = &UniversalRegions::closure_mapping(
1584 self.num_external_vids,
1585 tcx.closure_base_def_id(closure_def_id),
1587 debug!("apply_requirements: closure_mapping={:?}", closure_mapping);
1589 // Create the predicates.
1590 self.outlives_requirements
1592 .map(|outlives_requirement| {
1593 let outlived_region = closure_mapping[outlives_requirement.outlived_free_region];
1595 match outlives_requirement.subject {
1596 ClosureOutlivesSubject::Region(region) => {
1597 let region = closure_mapping[region];
1599 "apply_requirements: region={:?} \
1600 outlived_region={:?} \
1601 outlives_requirement={:?}",
1602 region, outlived_region, outlives_requirement,
1604 ty::Binder::dummy(ty::OutlivesPredicate(region.into(), outlived_region))
1607 ClosureOutlivesSubject::Ty(ty) => {
1608 let ty = self.subst_closure_mapping(tcx, closure_mapping, &ty);
1610 "apply_requirements: ty={:?} \
1611 outlived_region={:?} \
1612 outlives_requirement={:?}",
1613 ty, outlived_region, outlives_requirement,
1615 ty::Binder::dummy(ty::OutlivesPredicate(ty.into(), outlived_region))
1622 fn subst_closure_mapping<T>(
1625 closure_mapping: &IndexVec<RegionVid, ty::Region<'tcx>>,
1629 T: TypeFoldable<'tcx>,
1631 tcx.fold_regions(value, &mut false, |r, _depth| {
1632 if let ty::ReClosureBound(vid) = r {
1633 closure_mapping[*vid]
1635 bug!("subst_closure_mapping: encountered non-closure bound free region {:?}", r)