1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Helper routines for higher-ranked things. See the `doc` module at
12 //! the end of the file for details.
14 use super::{CombinedSnapshot,
20 use super::combine::CombineFields;
21 use super::region_inference::{TaintDirections};
23 use ty::{self, TyCtxt, Binder, TypeFoldable};
24 use ty::error::TypeError;
25 use ty::relate::{Relate, RelateResult, TypeRelation};
27 use util::nodemap::{FxHashMap, FxHashSet};
29 pub struct HrMatchResult<U> {
32 /// Normally, when we do a higher-ranked match operation, we
33 /// expect all higher-ranked regions to be constrained as part of
34 /// the match operation. However, in the transition period for
35 /// #32330, it can happen that we sometimes have unconstrained
36 /// regions that get instantiated with fresh variables. In that
37 /// case, we collect the set of unconstrained bound regions here
38 /// and replace them with fresh variables.
39 pub unconstrained_regions: Vec<ty::BoundRegion>,
42 impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
43 pub fn higher_ranked_sub<T>(&mut self, a: &Binder<T>, b: &Binder<T>, a_is_expected: bool)
44 -> RelateResult<'tcx, Binder<T>>
47 debug!("higher_ranked_sub(a={:?}, b={:?})",
50 // Rather than checking the subtype relationship between `a` and `b`
51 // as-is, we need to do some extra work here in order to make sure
52 // that function subtyping works correctly with respect to regions
54 // Note: this is a subtle algorithm. For a full explanation,
55 // please see the large comment at the end of the file in the (inlined) module
58 // Start a snapshot so we can examine "all bindings that were
59 // created as part of this type comparison".
60 return self.infcx.commit_if_ok(|snapshot| {
61 let span = self.trace.cause.span;
63 // First, we instantiate each bound region in the subtype with a fresh
66 self.infcx.replace_late_bound_regions_with_fresh_var(
71 // Second, we instantiate each bound region in the supertype with a
72 // fresh concrete region.
73 let (b_prime, skol_map) =
74 self.infcx.skolemize_late_bound_regions(b, snapshot);
76 debug!("a_prime={:?}", a_prime);
77 debug!("b_prime={:?}", b_prime);
79 // Compare types now that bound regions have been replaced.
80 let result = self.sub(a_is_expected).relate(&a_prime, &b_prime)?;
82 // Presuming type comparison succeeds, we need to check
83 // that the skolemized regions do not "leak".
84 self.infcx.leak_check(!a_is_expected, span, &skol_map, snapshot)?;
86 // We are finished with the skolemized regions now so pop
88 self.infcx.pop_skolemized(skol_map, snapshot);
90 debug!("higher_ranked_sub: OK result={:?}", result);
92 Ok(ty::Binder(result))
96 /// The value consists of a pair `(t, u)` where `t` is the
97 /// *matcher* and `u` is a *value*. The idea is to find a
98 /// substitution `S` such that `S(t) == b`, and then return
99 /// `S(u)`. In other words, find values for the late-bound regions
100 /// in `a` that can make `t == b` and then replace the LBR in `u`
101 /// with those values.
103 /// This routine is (as of this writing) used in trait matching,
104 /// particularly projection.
106 /// NB. It should not happen that there are LBR appearing in `U`
107 /// that do not appear in `T`. If that happens, those regions are
108 /// unconstrained, and this routine replaces them with `'static`.
109 pub fn higher_ranked_match<T, U>(&mut self,
111 a_pair: &Binder<(T, U)>,
114 -> RelateResult<'tcx, HrMatchResult<U>>
115 where T: Relate<'tcx>,
116 U: TypeFoldable<'tcx>
118 debug!("higher_ranked_match(a={:?}, b={:?})",
121 // Start a snapshot so we can examine "all bindings that were
122 // created as part of this type comparison".
123 return self.infcx.commit_if_ok(|snapshot| {
124 // First, we instantiate each bound region in the matcher
125 // with a skolemized region.
126 let ((a_match, a_value), skol_map) =
127 self.infcx.skolemize_late_bound_regions(a_pair, snapshot);
129 debug!("higher_ranked_match: a_match={:?}", a_match);
130 debug!("higher_ranked_match: skol_map={:?}", skol_map);
132 // Equate types now that bound regions have been replaced.
133 self.equate(a_is_expected).relate(&a_match, &b_match)?;
135 // Map each skolemized region to a vector of other regions that it
136 // must be equated with. (Note that this vector may include other
137 // skolemized regions from `skol_map`.)
138 let skol_resolution_map: FxHashMap<_, _> =
141 .map(|(&br, &skol)| {
142 let tainted_regions =
143 self.infcx.tainted_regions(snapshot,
145 TaintDirections::incoming()); // [1]
147 // [1] this routine executes after the skolemized
148 // regions have been *equated* with something
149 // else, so examining the incoming edges ought to
150 // be enough to collect all constraints
152 (skol, (br, tainted_regions))
156 // For each skolemized region, pick a representative -- which can
157 // be any region from the sets above, except for other members of
158 // `skol_map`. There should always be a representative if things
159 // are properly well-formed.
160 let mut unconstrained_regions = vec![];
161 let skol_representatives: FxHashMap<_, _> =
164 .map(|(&skol, &(br, ref regions))| {
167 .filter(|&&r| !skol_resolution_map.contains_key(r))
170 .unwrap_or_else(|| { // [1]
171 unconstrained_regions.push(br);
172 self.infcx.next_region_var(
173 LateBoundRegion(span, br, HigherRankedType))
176 // [1] There should always be a representative,
177 // unless the higher-ranked region did not appear
178 // in the values being matched. We should reject
179 // as ill-formed cases that can lead to this, but
180 // right now we sometimes issue warnings (see
183 (skol, representative)
187 // Equate all the members of each skolemization set with the
189 for (skol, &(_br, ref regions)) in &skol_resolution_map {
190 let representative = &skol_representatives[skol];
191 debug!("higher_ranked_match: \
192 skol={:?} representative={:?} regions={:?}",
193 skol, representative, regions);
194 for region in regions.iter()
195 .filter(|&r| !skol_resolution_map.contains_key(r))
196 .filter(|&r| r != representative)
198 let origin = SubregionOrigin::Subtype(self.trace.clone());
199 self.infcx.region_vars.make_eqregion(origin,
205 // Replace the skolemized regions appearing in value with
206 // their representatives
211 |r, _| skol_representatives.get(&r).cloned().unwrap_or(r));
213 debug!("higher_ranked_match: value={:?}", a_value);
215 // We are now done with these skolemized variables.
216 self.infcx.pop_skolemized(skol_map, snapshot);
220 unconstrained_regions: unconstrained_regions,
225 pub fn higher_ranked_lub<T>(&mut self, a: &Binder<T>, b: &Binder<T>, a_is_expected: bool)
226 -> RelateResult<'tcx, Binder<T>>
227 where T: Relate<'tcx>
229 // Start a snapshot so we can examine "all bindings that were
230 // created as part of this type comparison".
231 return self.infcx.commit_if_ok(|snapshot| {
232 // Instantiate each bound region with a fresh region variable.
233 let span = self.trace.cause.span;
234 let (a_with_fresh, a_map) =
235 self.infcx.replace_late_bound_regions_with_fresh_var(
236 span, HigherRankedType, a);
237 let (b_with_fresh, _) =
238 self.infcx.replace_late_bound_regions_with_fresh_var(
239 span, HigherRankedType, b);
241 // Collect constraints.
243 self.lub(a_is_expected).relate(&a_with_fresh, &b_with_fresh)?;
245 self.infcx.resolve_type_vars_if_possible(&result0);
246 debug!("lub result0 = {:?}", result0);
248 // Generalize the regions appearing in result0 if possible
249 let new_vars = self.infcx.region_vars_confined_to_snapshot(snapshot);
250 let span = self.trace.cause.span;
255 |r, debruijn| generalize_region(self.infcx, span, snapshot, debruijn,
256 &new_vars, &a_map, r));
258 debug!("lub({:?},{:?}) = {:?}",
263 Ok(ty::Binder(result1))
266 fn generalize_region<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
268 snapshot: &CombinedSnapshot,
269 debruijn: ty::DebruijnIndex,
270 new_vars: &[ty::RegionVid],
271 a_map: &FxHashMap<ty::BoundRegion, &'tcx ty::Region>,
272 r0: &'tcx ty::Region)
273 -> &'tcx ty::Region {
274 // Regions that pre-dated the LUB computation stay as they are.
275 if !is_var_in_set(new_vars, r0) {
276 assert!(!r0.is_bound());
277 debug!("generalize_region(r0={:?}): not new variable", r0);
281 let tainted = infcx.tainted_regions(snapshot, r0, TaintDirections::both());
283 // Variables created during LUB computation which are
284 // *related* to regions that pre-date the LUB computation
286 if !tainted.iter().all(|r| is_var_in_set(new_vars, *r)) {
287 debug!("generalize_region(r0={:?}): \
288 non-new-variables found in {:?}",
290 assert!(!r0.is_bound());
294 // Otherwise, the variable must be associated with at
295 // least one of the variables representing bound regions
296 // in both A and B. Replace the variable with the "first"
297 // bound region from A that we find it to be associated
299 for (a_br, a_r) in a_map {
300 if tainted.iter().any(|x| x == a_r) {
301 debug!("generalize_region(r0={:?}): \
302 replacing with {:?}, tainted={:?}",
304 return infcx.tcx.mk_region(ty::ReLateBound(debruijn, *a_br));
310 "region {:?} is not associated with any bound region from A!",
315 pub fn higher_ranked_glb<T>(&mut self, a: &Binder<T>, b: &Binder<T>, a_is_expected: bool)
316 -> RelateResult<'tcx, Binder<T>>
317 where T: Relate<'tcx>
319 debug!("higher_ranked_glb({:?}, {:?})",
322 // Make a snapshot so we can examine "all bindings that were
323 // created as part of this type comparison".
324 return self.infcx.commit_if_ok(|snapshot| {
325 // Instantiate each bound region with a fresh region variable.
326 let (a_with_fresh, a_map) =
327 self.infcx.replace_late_bound_regions_with_fresh_var(
328 self.trace.cause.span, HigherRankedType, a);
329 let (b_with_fresh, b_map) =
330 self.infcx.replace_late_bound_regions_with_fresh_var(
331 self.trace.cause.span, HigherRankedType, b);
332 let a_vars = var_ids(self, &a_map);
333 let b_vars = var_ids(self, &b_map);
335 // Collect constraints.
337 self.glb(a_is_expected).relate(&a_with_fresh, &b_with_fresh)?;
339 self.infcx.resolve_type_vars_if_possible(&result0);
340 debug!("glb result0 = {:?}", result0);
342 // Generalize the regions appearing in result0 if possible
343 let new_vars = self.infcx.region_vars_confined_to_snapshot(snapshot);
344 let span = self.trace.cause.span;
349 |r, debruijn| generalize_region(self.infcx, span, snapshot, debruijn,
351 &a_map, &a_vars, &b_vars,
354 debug!("glb({:?},{:?}) = {:?}",
359 Ok(ty::Binder(result1))
362 fn generalize_region<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
364 snapshot: &CombinedSnapshot,
365 debruijn: ty::DebruijnIndex,
366 new_vars: &[ty::RegionVid],
367 a_map: &FxHashMap<ty::BoundRegion, &'tcx ty::Region>,
368 a_vars: &[ty::RegionVid],
369 b_vars: &[ty::RegionVid],
370 r0: &'tcx ty::Region)
371 -> &'tcx ty::Region {
372 if !is_var_in_set(new_vars, r0) {
373 assert!(!r0.is_bound());
377 let tainted = infcx.tainted_regions(snapshot, r0, TaintDirections::both());
381 let mut only_new_vars = true;
383 if is_var_in_set(a_vars, *r) {
385 return fresh_bound_variable(infcx, debruijn);
389 } else if is_var_in_set(b_vars, *r) {
391 return fresh_bound_variable(infcx, debruijn);
395 } else if !is_var_in_set(new_vars, *r) {
396 only_new_vars = false;
400 // NB---I do not believe this algorithm computes
401 // (necessarily) the GLB. As written it can
402 // spuriously fail. In particular, if there is a case
403 // like: |fn(&a)| and fn(fn(&b)), where a and b are
404 // free, it will return fn(&c) where c = GLB(a,b). If
405 // however this GLB is not defined, then the result is
406 // an error, even though something like
407 // "fn<X>(fn(&X))" where X is bound would be a
408 // subtype of both of those.
410 // The problem is that if we were to return a bound
411 // variable, we'd be computing a lower-bound, but not
412 // necessarily the *greatest* lower-bound.
414 // Unfortunately, this problem is non-trivial to solve,
415 // because we do not know at the time of computing the GLB
416 // whether a GLB(a,b) exists or not, because we haven't
417 // run region inference (or indeed, even fully computed
418 // the region hierarchy!). The current algorithm seems to
419 // works ok in practice.
421 if a_r.is_some() && b_r.is_some() && only_new_vars {
422 // Related to exactly one bound variable from each fn:
423 return rev_lookup(infcx, span, a_map, a_r.unwrap());
424 } else if a_r.is_none() && b_r.is_none() {
425 // Not related to bound variables from either fn:
426 assert!(!r0.is_bound());
430 return fresh_bound_variable(infcx, debruijn);
434 fn rev_lookup<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
436 a_map: &FxHashMap<ty::BoundRegion, &'tcx ty::Region>,
437 r: &'tcx ty::Region) -> &'tcx ty::Region
439 for (a_br, a_r) in a_map {
441 return infcx.tcx.mk_region(ty::ReLateBound(ty::DebruijnIndex::new(1), *a_br));
446 "could not find original bound region for {:?}",
450 fn fresh_bound_variable<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
451 debruijn: ty::DebruijnIndex)
452 -> &'tcx ty::Region {
453 infcx.region_vars.new_bound(debruijn)
458 fn var_ids<'a, 'gcx, 'tcx>(fields: &CombineFields<'a, 'gcx, 'tcx>,
459 map: &FxHashMap<ty::BoundRegion, &'tcx ty::Region>)
460 -> Vec<ty::RegionVid> {
462 .map(|(_, &r)| match *r {
463 ty::ReVar(r) => { r }
466 fields.trace.cause.span,
467 "found non-region-vid: {:?}",
474 fn is_var_in_set(new_vars: &[ty::RegionVid], r: &ty::Region) -> bool {
476 ty::ReVar(ref v) => new_vars.iter().any(|x| x == v),
481 fn fold_regions_in<'a, 'gcx, 'tcx, T, F>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
485 where T: TypeFoldable<'tcx>,
486 F: FnMut(&'tcx ty::Region, ty::DebruijnIndex) -> &'tcx ty::Region,
488 tcx.fold_regions(unbound_value, &mut false, |region, current_depth| {
489 // we should only be encountering "escaping" late-bound regions here,
490 // because the ones at the current level should have been replaced
491 // with fresh variables
492 assert!(match *region {
493 ty::ReLateBound(..) => false,
497 fldr(region, ty::DebruijnIndex::new(current_depth))
501 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
502 fn tainted_regions(&self,
503 snapshot: &CombinedSnapshot,
505 directions: TaintDirections)
506 -> FxHashSet<&'tcx ty::Region> {
507 self.region_vars.tainted(&snapshot.region_vars_snapshot, r, directions)
510 fn region_vars_confined_to_snapshot(&self,
511 snapshot: &CombinedSnapshot)
512 -> Vec<ty::RegionVid>
515 * Returns the set of region variables that do not affect any
516 * types/regions which existed before `snapshot` was
517 * started. This is used in the sub/lub/glb computations. The
518 * idea here is that when we are computing lub/glb of two
519 * regions, we sometimes create intermediate region variables.
520 * Those region variables may touch some of the skolemized or
521 * other "forbidden" regions we created to replace bound
522 * regions, but they don't really represent an "external"
525 * However, sometimes fresh variables are created for other
526 * purposes too, and those *may* represent an external
527 * constraint. In particular, when a type variable is
528 * instantiated, we create region variables for all the
529 * regions that appear within, and if that type variable
530 * pre-existed the snapshot, then those region variables
531 * represent external constraints.
533 * An example appears in the unit test
534 * `sub_free_bound_false_infer`. In this test, we want to
538 * fn(_#0t) <: for<'a> fn(&'a int)
541 * Note that the subtype has a type variable. Because the type
542 * variable can't be instantiated with a region that is bound
543 * in the fn signature, this comparison ought to fail. But if
544 * we're not careful, it will succeed.
546 * The reason is that when we walk through the subtyping
547 * algorith, we begin by replacing `'a` with a skolemized
548 * variable `'1`. We then have `fn(_#0t) <: fn(&'1 int)`. This
549 * can be made true by unifying `_#0t` with `&'1 int`. In the
550 * process, we create a fresh variable for the skolemized
551 * region, `'$2`, and hence we have that `_#0t == &'$2
552 * int`. However, because `'$2` was created during the sub
553 * computation, if we're not careful we will erroneously
554 * assume it is one of the transient region variables
555 * representing a lub/glb internally. Not good.
557 * To prevent this, we check for type variables which were
558 * unified during the snapshot, and say that any region
559 * variable created during the snapshot but which finds its
560 * way into a type variable is considered to "escape" the
564 let mut region_vars =
565 self.region_vars.vars_created_since_snapshot(&snapshot.region_vars_snapshot);
568 self.type_variables.borrow_mut().types_escaping_snapshot(&snapshot.type_snapshot);
570 let mut escaping_region_vars = FxHashSet();
571 for ty in &escaping_types {
572 self.tcx.collect_regions(ty, &mut escaping_region_vars);
575 region_vars.retain(|®ion_vid| {
576 let r = ty::ReVar(region_vid);
577 !escaping_region_vars.contains(&r)
580 debug!("region_vars_confined_to_snapshot: region_vars={:?} escaping_types={:?}",
587 /// Replace all regions bound by `binder` with skolemized regions and
588 /// return a map indicating which bound-region was replaced with what
589 /// skolemized region. This is the first step of checking subtyping
590 /// when higher-ranked things are involved.
592 /// **Important:** you must call this function from within a snapshot.
593 /// Moreover, before committing the snapshot, you must eventually call
594 /// either `plug_leaks` or `pop_skolemized` to remove the skolemized
595 /// regions. If you rollback the snapshot (or are using a probe), then
596 /// the pop occurs as part of the rollback, so an explicit call is not
597 /// needed (but is also permitted).
599 /// See `README.md` for more details.
600 pub fn skolemize_late_bound_regions<T>(&self,
601 binder: &ty::Binder<T>,
602 snapshot: &CombinedSnapshot)
603 -> (T, SkolemizationMap<'tcx>)
604 where T : TypeFoldable<'tcx>
606 let (result, map) = self.tcx.replace_late_bound_regions(binder, |br| {
607 self.region_vars.push_skolemized(br, &snapshot.region_vars_snapshot)
610 debug!("skolemize_bound_regions(binder={:?}, result={:?}, map={:?})",
618 /// Searches the region constriants created since `snapshot` was started
619 /// and checks to determine whether any of the skolemized regions created
620 /// in `skol_map` would "escape" -- meaning that they are related to
621 /// other regions in some way. If so, the higher-ranked subtyping doesn't
622 /// hold. See `README.md` for more details.
623 pub fn leak_check(&self,
624 overly_polymorphic: bool,
626 skol_map: &SkolemizationMap<'tcx>,
627 snapshot: &CombinedSnapshot)
628 -> RelateResult<'tcx, ()>
630 debug!("leak_check: skol_map={:?}",
633 // ## Issue #32330 warnings
635 // When Issue #32330 is fixed, a certain number of late-bound
636 // regions (LBR) will become early-bound. We wish to issue
637 // warnings when the result of `leak_check` relies on such LBR, as
638 // that means that compilation will likely start to fail.
640 // Recall that when we do a "HR subtype" check, we replace all
641 // late-bound regions (LBR) in the subtype with fresh variables,
642 // and skolemize the late-bound regions in the supertype. If those
643 // skolemized regions from the supertype wind up being
644 // super-regions (directly or indirectly) of either
646 // - another skolemized region; or,
647 // - some region that pre-exists the HR subtype check
648 // - e.g., a region variable that is not one of those created
649 // to represent bound regions in the subtype
651 // then leak-check (and hence the subtype check) fails.
653 // What will change when we fix #32330 is that some of the LBR in the
654 // subtype may become early-bound. In that case, they would no longer be in
655 // the "permitted set" of variables that can be related to a skolemized
658 // So the foundation for this warning is to collect variables that we found
659 // to be related to a skolemized type. For each of them, we have a
660 // `BoundRegion` which carries a `Issue32330` flag. We check whether any of
661 // those flags indicate that this variable was created from a lifetime
662 // that will change from late- to early-bound. If so, we issue a warning
663 // indicating that the results of compilation may change.
665 // This is imperfect, since there are other kinds of code that will not
666 // compile once #32330 is fixed. However, it fixes the errors observed in
667 // practice on crater runs.
668 let mut warnings = vec![];
670 let new_vars = self.region_vars_confined_to_snapshot(snapshot);
671 for (&skol_br, &skol) in skol_map {
672 // The inputs to a skolemized variable can only
673 // be itself or other new variables.
674 let incoming_taints = self.tainted_regions(snapshot,
676 TaintDirections::both());
677 for &tainted_region in &incoming_taints {
678 // Each skolemized should only be relatable to itself
680 match *tainted_region {
682 if new_vars.contains(&vid) {
684 match self.region_vars.var_origin(vid) {
694 if tainted_region == skol { continue; }
698 debug!("{:?} (which replaced {:?}) is tainted by {:?}",
703 if overly_polymorphic {
704 debug!("Overly polymorphic!");
705 return Err(TypeError::RegionsOverlyPolymorphic(skol_br,
708 debug!("Not as polymorphic!");
709 return Err(TypeError::RegionsInsufficientlyPolymorphic(skol_br,
715 self.issue_32330_warnings(span, &warnings);
720 /// This code converts from skolemized regions back to late-bound
721 /// regions. It works by replacing each region in the taint set of a
722 /// skolemized region with a bound-region. The bound region will be bound
723 /// by the outer-most binder in `value`; the caller must ensure that there is
724 /// such a binder and it is the right place.
726 /// This routine is only intended to be used when the leak-check has
727 /// passed; currently, it's used in the trait matching code to create
728 /// a set of nested obligations frmo an impl that matches against
729 /// something higher-ranked. More details can be found in
730 /// `librustc/middle/traits/README.md`.
732 /// As a brief example, consider the obligation `for<'a> Fn(&'a int)
733 /// -> &'a int`, and the impl:
735 /// impl<A,R> Fn<A,R> for SomethingOrOther
739 /// Here we will have replaced `'a` with a skolemized region
740 /// `'0`. This means that our substitution will be `{A=>&'0
741 /// int, R=>&'0 int}`.
743 /// When we apply the substitution to the bounds, we will wind up with
744 /// `&'0 int : Clone` as a predicate. As a last step, we then go and
745 /// replace `'0` with a late-bound region `'a`. The depth is matched
746 /// to the depth of the predicate, in this case 1, so that the final
747 /// predicate is `for<'a> &'a int : Clone`.
748 pub fn plug_leaks<T>(&self,
749 skol_map: SkolemizationMap<'tcx>,
750 snapshot: &CombinedSnapshot,
752 where T : TypeFoldable<'tcx>
754 debug!("plug_leaks(skol_map={:?}, value={:?})",
758 if skol_map.is_empty() {
762 // Compute a mapping from the "taint set" of each skolemized
763 // region back to the `ty::BoundRegion` that it originally
764 // represented. Because `leak_check` passed, we know that
765 // these taint sets are mutually disjoint.
766 let inv_skol_map: FxHashMap<&'tcx ty::Region, ty::BoundRegion> =
769 .flat_map(|(&skol_br, &skol)| {
770 self.tainted_regions(snapshot, skol, TaintDirections::both())
772 .map(move |tainted_region| (tainted_region, skol_br))
776 debug!("plug_leaks: inv_skol_map={:?}",
779 // Remove any instantiated type variables from `value`; those can hide
780 // references to regions from the `fold_regions` code below.
781 let value = self.resolve_type_vars_if_possible(&value);
783 // Map any skolemization byproducts back to a late-bound
784 // region. Put that late-bound region at whatever the outermost
785 // binder is that we encountered in `value`. The caller is
786 // responsible for ensuring that (a) `value` contains at least one
787 // binder and (b) that binder is the one we want to use.
788 let result = self.tcx.fold_regions(&value, &mut false, |r, current_depth| {
789 match inv_skol_map.get(&r) {
792 // It is the responsibility of the caller to ensure
793 // that each skolemized region appears within a
794 // binder. In practice, this routine is only used by
795 // trait checking, and all of the skolemized regions
796 // appear inside predicates, which always have
797 // binders, so this assert is satisfied.
798 assert!(current_depth > 1);
800 // since leak-check passed, this skolemized region
801 // should only have incoming edges from variables
802 // (which ought not to escape the snapshot, but we
803 // don't check that) or itself
806 ty::ReVar(_) => true,
807 ty::ReSkolemized(_, ref br1) => br == br1,
810 "leak-check would have us replace {:?} with {:?}",
813 self.tcx.mk_region(ty::ReLateBound(
814 ty::DebruijnIndex::new(current_depth - 1), br.clone()))
819 self.pop_skolemized(skol_map, snapshot);
821 debug!("plug_leaks: result={:?}", result);
826 /// Pops the skolemized regions found in `skol_map` from the region
827 /// inference context. Whenever you create skolemized regions via
828 /// `skolemize_late_bound_regions`, they must be popped before you
829 /// commit the enclosing snapshot (if you do not commit, e.g. within a
830 /// probe or as a result of an error, then this is not necessary, as
831 /// popping happens as part of the rollback).
833 /// Note: popping also occurs implicitly as part of `leak_check`.
834 pub fn pop_skolemized(&self,
835 skol_map: SkolemizationMap<'tcx>,
836 snapshot: &CombinedSnapshot)
838 debug!("pop_skolemized({:?})", skol_map);
839 let skol_regions: FxHashSet<_> = skol_map.values().cloned().collect();
840 self.region_vars.pop_skolemized(&skol_regions, &snapshot.region_vars_snapshot);
841 if !skol_map.is_empty() {
842 self.projection_cache.borrow_mut().rollback_skolemized(
843 &snapshot.projection_cache_snapshot);