src/librustc/infer/higher_ranked/mod.rs

   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.
   4 //
   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.
  10
  11 //! Helper routines for higher-ranked things. See the `doc` module at
  12 //! the end of the file for details.
  13
  14 use super::{CombinedSnapshot,
  15             InferCtxt,
  16             LateBoundRegion,
  17             HigherRankedType,
  18             SubregionOrigin,
  19             SkolemizationMap};
  20 use super::combine::CombineFields;
  21 use super::region_inference::{TaintDirections};
  22
  23 use ty::{self, TyCtxt, Binder, TypeFoldable};
  24 use ty::error::TypeError;
  25 use ty::relate::{Relate, RelateResult, TypeRelation};
  26 use syntax_pos::Span;
  27 use util::nodemap::{FnvHashMap, FnvHashSet};
  28
  29 pub struct HrMatchResult<U> {
  30     pub value: U,
  31
  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>,
  40 }
  41
  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>>
  45         where T: Relate<'tcx>
  46     {
  47         debug!("higher_ranked_sub(a={:?}, b={:?})",
  48                a, b);
  49
  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
  53         //
  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
  56         // `doc`.
  57
  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.origin.span();
  62
  63             // First, we instantiate each bound region in the subtype with a fresh
  64             // region variable.
  65             let (a_prime, _) =
  66                 self.infcx.replace_late_bound_regions_with_fresh_var(
  67                     span,
  68                     HigherRankedType,
  69                     a);
  70
  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);
  75
  76             debug!("a_prime={:?}", a_prime);
  77             debug!("b_prime={:?}", b_prime);
  78
  79             // Compare types now that bound regions have been replaced.
  80             let result = self.sub(a_is_expected).relate(&a_prime, &b_prime)?;
  81
  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)?;
  85
  86             // We are finished with the skolemized regions now so pop
  87             // them off.
  88             self.infcx.pop_skolemized(skol_map, snapshot);
  89
  90             debug!("higher_ranked_sub: OK result={:?}", result);
  91
  92             Ok(ty::Binder(result))
  93         });
  94     }
  95
  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.
 102     ///
 103     /// This routine is (as of this writing) used in trait matching,
 104     /// particularly projection.
 105     ///
 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,
 110                                      span: Span,
 111                                      a_pair: &Binder<(T, U)>,
 112                                      b_match: &T,
 113                                      a_is_expected: bool)
 114                                      -> RelateResult<'tcx, HrMatchResult<U>>
 115         where T: Relate<'tcx>,
 116               U: TypeFoldable<'tcx>
 117     {
 118         debug!("higher_ranked_match(a={:?}, b={:?})",
 119                a_pair, b_match);
 120
 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);
 128
 129             debug!("higher_ranked_match: a_match={:?}", a_match);
 130             debug!("higher_ranked_match: skol_map={:?}", skol_map);
 131
 132             // Equate types now that bound regions have been replaced.
 133             self.equate(a_is_expected).relate(&a_match, &b_match)?;
 134
 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: FnvHashMap<_, _> =
 139                 skol_map
 140                 .iter()
 141                 .map(|(&br, &skol)| {
 142                     let tainted_regions =
 143                         self.infcx.tainted_regions(snapshot,
 144                                                    skol,
 145                                                    TaintDirections::incoming()); // [1]
 146
 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
 151
 152                     (skol, (br, tainted_regions))
 153                 })
 154                 .collect();
 155
 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: FnvHashMap<_, _> =
 162                 skol_resolution_map
 163                 .iter()
 164                 .map(|(&skol, &(br, ref regions))| {
 165                     let representative =
 166                         regions.iter()
 167                                .filter(|&&r| !skol_resolution_map.contains_key(r))
 168                                .cloned()
 169                                .next()
 170                                .unwrap_or_else(|| { // [1]
 171                                    unconstrained_regions.push(br);
 172                                    self.infcx.next_region_var(
 173                                        LateBoundRegion(span, br, HigherRankedType))
 174                                });
 175
 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
 181                     // #32330).
 182
 183                     (skol, representative)
 184                 })
 185                 .collect();
 186
 187             // Equate all the members of each skolemization set with the
 188             // representative.
 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)
 197                 {
 198                     let origin = SubregionOrigin::Subtype(self.trace.clone());
 199                     self.infcx.region_vars.make_eqregion(origin,
 200                                                          *representative,
 201                                                          *region);
 202                 }
 203             }
 204
 205             // Replace the skolemized regions appearing in value with
 206             // their representatives
 207             let a_value =
 208                 fold_regions_in(
 209                     self.tcx(),
 210                     &a_value,
 211                     |r, _| skol_representatives.get(&r).cloned().unwrap_or(r));
 212
 213             debug!("higher_ranked_match: value={:?}", a_value);
 214
 215             // We are now done with these skolemized variables.
 216             self.infcx.pop_skolemized(skol_map, snapshot);
 217
 218             Ok(HrMatchResult {
 219                 value: a_value,
 220                 unconstrained_regions: unconstrained_regions,
 221             })
 222         });
 223     }
 224
 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>
 228     {
 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.origin.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);
 240
 241             // Collect constraints.
 242             let result0 =
 243                 self.lub(a_is_expected).relate(&a_with_fresh, &b_with_fresh)?;
 244             let result0 =
 245                 self.infcx.resolve_type_vars_if_possible(&result0);
 246             debug!("lub result0 = {:?}", result0);
 247
 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.origin.span();
 251             let result1 =
 252                 fold_regions_in(
 253                     self.tcx(),
 254                     &result0,
 255                     |r, debruijn| generalize_region(self.infcx, span, snapshot, debruijn,
 256                                                     &new_vars, &a_map, r));
 257
 258             debug!("lub({:?},{:?}) = {:?}",
 259                    a,
 260                    b,
 261                    result1);
 262
 263             Ok(ty::Binder(result1))
 264         });
 265
 266         fn generalize_region<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 267                                              span: Span,
 268                                              snapshot: &CombinedSnapshot,
 269                                              debruijn: ty::DebruijnIndex,
 270                                              new_vars: &[ty::RegionVid],
 271                                              a_map: &FnvHashMap<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);
 278                 return r0;
 279             }
 280
 281             let tainted = infcx.tainted_regions(snapshot, r0, TaintDirections::both());
 282
 283             // Variables created during LUB computation which are
 284             // *related* to regions that pre-date the LUB computation
 285             // stay as they are.
 286             if !tainted.iter().all(|r| is_var_in_set(new_vars, *r)) {
 287                 debug!("generalize_region(r0={:?}): \
 288                         non-new-variables found in {:?}",
 289                        r0, tainted);
 290                 assert!(!r0.is_bound());
 291                 return r0;
 292             }
 293
 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
 298             // with.
 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={:?}",
 303                            r0, *a_br, tainted);
 304                     return infcx.tcx.mk_region(ty::ReLateBound(debruijn, *a_br));
 305                 }
 306             }
 307
 308             span_bug!(
 309                 span,
 310                 "region {:?} is not associated with any bound region from A!",
 311                 r0)
 312         }
 313     }
 314
 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>
 318     {
 319         debug!("higher_ranked_glb({:?}, {:?})",
 320                a, b);
 321
 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.origin.span(), HigherRankedType, a);
 329             let (b_with_fresh, b_map) =
 330                 self.infcx.replace_late_bound_regions_with_fresh_var(
 331                     self.trace.origin.span(), HigherRankedType, b);
 332             let a_vars = var_ids(self, &a_map);
 333             let b_vars = var_ids(self, &b_map);
 334
 335             // Collect constraints.
 336             let result0 =
 337                 self.glb(a_is_expected).relate(&a_with_fresh, &b_with_fresh)?;
 338             let result0 =
 339                 self.infcx.resolve_type_vars_if_possible(&result0);
 340             debug!("glb result0 = {:?}", result0);
 341
 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.origin.span();
 345             let result1 =
 346                 fold_regions_in(
 347                     self.tcx(),
 348                     &result0,
 349                     |r, debruijn| generalize_region(self.infcx, span, snapshot, debruijn,
 350                                                     &new_vars,
 351                                                     &a_map, &a_vars, &b_vars,
 352                                                     r));
 353
 354             debug!("glb({:?},{:?}) = {:?}",
 355                    a,
 356                    b,
 357                    result1);
 358
 359             Ok(ty::Binder(result1))
 360         });
 361
 362         fn generalize_region<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 363                                              span: Span,
 364                                              snapshot: &CombinedSnapshot,
 365                                              debruijn: ty::DebruijnIndex,
 366                                              new_vars: &[ty::RegionVid],
 367                                              a_map: &FnvHashMap<ty::BoundRegion,
 368                                                                 &'tcx ty::Region>,
 369                                              a_vars: &[ty::RegionVid],
 370                                              b_vars: &[ty::RegionVid],
 371                                              r0: &'tcx ty::Region)
 372                                              -> &'tcx ty::Region {
 373             if !is_var_in_set(new_vars, r0) {
 374                 assert!(!r0.is_bound());
 375                 return r0;
 376             }
 377
 378             let tainted = infcx.tainted_regions(snapshot, r0, TaintDirections::both());
 379
 380             let mut a_r = None;
 381             let mut b_r = None;
 382             let mut only_new_vars = true;
 383             for r in &tainted {
 384                 if is_var_in_set(a_vars, *r) {
 385                     if a_r.is_some() {
 386                         return fresh_bound_variable(infcx, debruijn);
 387                     } else {
 388                         a_r = Some(*r);
 389                     }
 390                 } else if is_var_in_set(b_vars, *r) {
 391                     if b_r.is_some() {
 392                         return fresh_bound_variable(infcx, debruijn);
 393                     } else {
 394                         b_r = Some(*r);
 395                     }
 396                 } else if !is_var_in_set(new_vars, *r) {
 397                     only_new_vars = false;
 398                 }
 399             }
 400
 401             // NB---I do not believe this algorithm computes
 402             // (necessarily) the GLB.  As written it can
 403             // spuriously fail. In particular, if there is a case
 404             // like: |fn(&a)| and fn(fn(&b)), where a and b are
 405             // free, it will return fn(&c) where c = GLB(a,b).  If
 406             // however this GLB is not defined, then the result is
 407             // an error, even though something like
 408             // "fn<X>(fn(&X))" where X is bound would be a
 409             // subtype of both of those.
 410             //
 411             // The problem is that if we were to return a bound
 412             // variable, we'd be computing a lower-bound, but not
 413             // necessarily the *greatest* lower-bound.
 414             //
 415             // Unfortunately, this problem is non-trivial to solve,
 416             // because we do not know at the time of computing the GLB
 417             // whether a GLB(a,b) exists or not, because we haven't
 418             // run region inference (or indeed, even fully computed
 419             // the region hierarchy!). The current algorithm seems to
 420             // works ok in practice.
 421
 422             if a_r.is_some() && b_r.is_some() && only_new_vars {
 423                 // Related to exactly one bound variable from each fn:
 424                 return rev_lookup(infcx, span, a_map, a_r.unwrap());
 425             } else if a_r.is_none() && b_r.is_none() {
 426                 // Not related to bound variables from either fn:
 427                 assert!(!r0.is_bound());
 428                 return r0;
 429             } else {
 430                 // Other:
 431                 return fresh_bound_variable(infcx, debruijn);
 432             }
 433         }
 434
 435         fn rev_lookup<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 436                                       span: Span,
 437                                       a_map: &FnvHashMap<ty::BoundRegion, &'tcx ty::Region>,
 438                                       r: &'tcx ty::Region) -> &'tcx ty::Region
 439         {
 440             for (a_br, a_r) in a_map {
 441                 if *a_r == r {
 442                     return infcx.tcx.mk_region(ty::ReLateBound(ty::DebruijnIndex::new(1), *a_br));
 443                 }
 444             }
 445             span_bug!(
 446                 span,
 447                 "could not find original bound region for {:?}",
 448                 r);
 449         }
 450
 451         fn fresh_bound_variable<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 452                                                 debruijn: ty::DebruijnIndex)
 453                                                 -> &'tcx ty::Region {
 454             infcx.region_vars.new_bound(debruijn)
 455         }
 456     }
 457 }
 458
 459 fn var_ids<'a, 'gcx, 'tcx>(fields: &CombineFields<'a, 'gcx, 'tcx>,
 460                            map: &FnvHashMap<ty::BoundRegion, &'tcx ty::Region>)
 461                            -> Vec<ty::RegionVid> {
 462     map.iter()
 463        .map(|(_, &r)| match *r {
 464            ty::ReVar(r) => { r }
 465            _ => {
 466                span_bug!(
 467                    fields.trace.origin.span(),
 468                    "found non-region-vid: {:?}",
 469                    r);
 470            }
 471        })
 472        .collect()
 473 }
 474
 475 fn is_var_in_set(new_vars: &[ty::RegionVid], r: &ty::Region) -> bool {
 476     match *r {
 477         ty::ReVar(ref v) => new_vars.iter().any(|x| x == v),
 478         _ => false
 479     }
 480 }
 481
 482 fn fold_regions_in<'a, 'gcx, 'tcx, T, F>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
 483                                          unbound_value: &T,
 484                                          mut fldr: F)
 485                                          -> T
 486     where T: TypeFoldable<'tcx>,
 487           F: FnMut(&'tcx ty::Region, ty::DebruijnIndex) -> &'tcx ty::Region,
 488 {
 489     tcx.fold_regions(unbound_value, &mut false, |region, current_depth| {
 490         // we should only be encountering "escaping" late-bound regions here,
 491         // because the ones at the current level should have been replaced
 492         // with fresh variables
 493         assert!(match *region {
 494             ty::ReLateBound(..) => false,
 495             _ => true
 496         });
 497
 498         fldr(region, ty::DebruijnIndex::new(current_depth))
 499     })
 500 }
 501
 502 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 503     fn tainted_regions(&self,
 504                        snapshot: &CombinedSnapshot,
 505                        r: &'tcx ty::Region,
 506                        directions: TaintDirections)
 507                        -> FnvHashSet<&'tcx ty::Region> {
 508         self.region_vars.tainted(&snapshot.region_vars_snapshot, r, directions)
 509     }
 510
 511     fn region_vars_confined_to_snapshot(&self,
 512                                         snapshot: &CombinedSnapshot)
 513                                         -> Vec<ty::RegionVid>
 514     {
 515         /*!
 516          * Returns the set of region variables that do not affect any
 517          * types/regions which existed before `snapshot` was
 518          * started. This is used in the sub/lub/glb computations. The
 519          * idea here is that when we are computing lub/glb of two
 520          * regions, we sometimes create intermediate region variables.
 521          * Those region variables may touch some of the skolemized or
 522          * other "forbidden" regions we created to replace bound
 523          * regions, but they don't really represent an "external"
 524          * constraint.
 525          *
 526          * However, sometimes fresh variables are created for other
 527          * purposes too, and those *may* represent an external
 528          * constraint. In particular, when a type variable is
 529          * instantiated, we create region variables for all the
 530          * regions that appear within, and if that type variable
 531          * pre-existed the snapshot, then those region variables
 532          * represent external constraints.
 533          *
 534          * An example appears in the unit test
 535          * `sub_free_bound_false_infer`.  In this test, we want to
 536          * know whether
 537          *
 538          * ```rust
 539          * fn(_#0t) <: for<'a> fn(&'a int)
 540          * ```
 541          *
 542          * Note that the subtype has a type variable. Because the type
 543          * variable can't be instantiated with a region that is bound
 544          * in the fn signature, this comparison ought to fail. But if
 545          * we're not careful, it will succeed.
 546          *
 547          * The reason is that when we walk through the subtyping
 548          * algorith, we begin by replacing `'a` with a skolemized
 549          * variable `'1`. We then have `fn(_#0t) <: fn(&'1 int)`. This
 550          * can be made true by unifying `_#0t` with `&'1 int`. In the
 551          * process, we create a fresh variable for the skolemized
 552          * region, `'$2`, and hence we have that `_#0t == &'$2
 553          * int`. However, because `'$2` was created during the sub
 554          * computation, if we're not careful we will erroneously
 555          * assume it is one of the transient region variables
 556          * representing a lub/glb internally. Not good.
 557          *
 558          * To prevent this, we check for type variables which were
 559          * unified during the snapshot, and say that any region
 560          * variable created during the snapshot but which finds its
 561          * way into a type variable is considered to "escape" the
 562          * snapshot.
 563          */
 564
 565         let mut region_vars =
 566             self.region_vars.vars_created_since_snapshot(&snapshot.region_vars_snapshot);
 567
 568         let escaping_types =
 569             self.type_variables.borrow_mut().types_escaping_snapshot(&snapshot.type_snapshot);
 570
 571         let mut escaping_region_vars = FnvHashSet();
 572         for ty in &escaping_types {
 573             self.tcx.collect_regions(ty, &mut escaping_region_vars);
 574         }
 575
 576         region_vars.retain(|&region_vid| {
 577             let r = ty::ReVar(region_vid);
 578             !escaping_region_vars.contains(&r)
 579         });
 580
 581         debug!("region_vars_confined_to_snapshot: region_vars={:?} escaping_types={:?}",
 582                region_vars,
 583                escaping_types);
 584
 585         region_vars
 586     }
 587
 588     /// Replace all regions bound by `binder` with skolemized regions and
 589     /// return a map indicating which bound-region was replaced with what
 590     /// skolemized region. This is the first step of checking subtyping
 591     /// when higher-ranked things are involved.
 592     ///
 593     /// **Important:** you must call this function from within a snapshot.
 594     /// Moreover, before committing the snapshot, you must eventually call
 595     /// either `plug_leaks` or `pop_skolemized` to remove the skolemized
 596     /// regions. If you rollback the snapshot (or are using a probe), then
 597     /// the pop occurs as part of the rollback, so an explicit call is not
 598     /// needed (but is also permitted).
 599     ///
 600     /// See `README.md` for more details.
 601     pub fn skolemize_late_bound_regions<T>(&self,
 602                                            binder: &ty::Binder<T>,
 603                                            snapshot: &CombinedSnapshot)
 604                                            -> (T, SkolemizationMap<'tcx>)
 605         where T : TypeFoldable<'tcx>
 606     {
 607         let (result, map) = self.tcx.replace_late_bound_regions(binder, |br| {
 608             self.region_vars.push_skolemized(br, &snapshot.region_vars_snapshot)
 609         });
 610
 611         debug!("skolemize_bound_regions(binder={:?}, result={:?}, map={:?})",
 612                binder,
 613                result,
 614                map);
 615
 616         (result, map)
 617     }
 618
 619     /// Searches the region constriants created since `snapshot` was started
 620     /// and checks to determine whether any of the skolemized regions created
 621     /// in `skol_map` would "escape" -- meaning that they are related to
 622     /// other regions in some way. If so, the higher-ranked subtyping doesn't
 623     /// hold. See `README.md` for more details.
 624     pub fn leak_check(&self,
 625                       overly_polymorphic: bool,
 626                       span: Span,
 627                       skol_map: &SkolemizationMap<'tcx>,
 628                       snapshot: &CombinedSnapshot)
 629                       -> RelateResult<'tcx, ()>
 630     {
 631         debug!("leak_check: skol_map={:?}",
 632                skol_map);
 633
 634         // ## Issue #32330 warnings
 635         //
 636         // When Issue #32330 is fixed, a certain number of late-bound
 637         // regions (LBR) will become early-bound. We wish to issue
 638         // warnings when the result of `leak_check` relies on such LBR, as
 639         // that means that compilation will likely start to fail.
 640         //
 641         // Recall that when we do a "HR subtype" check, we replace all
 642         // late-bound regions (LBR) in the subtype with fresh variables,
 643         // and skolemize the late-bound regions in the supertype. If those
 644         // skolemized regions from the supertype wind up being
 645         // super-regions (directly or indirectly) of either
 646         //
 647         // - another skolemized region; or,
 648         // - some region that pre-exists the HR subtype check
 649         //   - e.g., a region variable that is not one of those created
 650         //     to represent bound regions in the subtype
 651         //
 652         // then leak-check (and hence the subtype check) fails.
 653         //
 654         // What will change when we fix #32330 is that some of the LBR in the
 655         // subtype may become early-bound. In that case, they would no longer be in
 656         // the "permitted set" of variables that can be related to a skolemized
 657         // type.
 658         //
 659         // So the foundation for this warning is to collect variables that we found
 660         // to be related to a skolemized type. For each of them, we have a
 661         // `BoundRegion` which carries a `Issue32330` flag. We check whether any of
 662         // those flags indicate that this variable was created from a lifetime
 663         // that will change from late- to early-bound. If so, we issue a warning
 664         // indicating that the results of compilation may change.
 665         //
 666         // This is imperfect, since there are other kinds of code that will not
 667         // compile once #32330 is fixed. However, it fixes the errors observed in
 668         // practice on crater runs.
 669         let mut warnings = vec![];
 670
 671         let new_vars = self.region_vars_confined_to_snapshot(snapshot);
 672         for (&skol_br, &skol) in skol_map {
 673             // The inputs to a skolemized variable can only
 674             // be itself or other new variables.
 675             let incoming_taints = self.tainted_regions(snapshot,
 676                                                        skol,
 677                                                        TaintDirections::both());
 678             for &tainted_region in &incoming_taints {
 679                 // Each skolemized should only be relatable to itself
 680                 // or new variables:
 681                 match *tainted_region {
 682                     ty::ReVar(vid) => {
 683                         if new_vars.contains(&vid) {
 684                             warnings.extend(
 685                                 match self.region_vars.var_origin(vid) {
 686                                     LateBoundRegion(_,
 687                                                     ty::BrNamed(.., wc),
 688                                                     _) => Some(wc),
 689                                     _ => None,
 690                                 });
 691                             continue;
 692                         }
 693                     }
 694                     _ => {
 695                         if tainted_region == skol { continue; }
 696                     }
 697                 };
 698
 699                 debug!("{:?} (which replaced {:?}) is tainted by {:?}",
 700                        skol,
 701                        skol_br,
 702                        tainted_region);
 703
 704                 if overly_polymorphic {
 705                     debug!("Overly polymorphic!");
 706                     return Err(TypeError::RegionsOverlyPolymorphic(skol_br,
 707                                                                    tainted_region));
 708                 } else {
 709                     debug!("Not as polymorphic!");
 710                     return Err(TypeError::RegionsInsufficientlyPolymorphic(skol_br,
 711                                                                            tainted_region));
 712                 }
 713             }
 714         }
 715
 716         self.issue_32330_warnings(span, &warnings);
 717
 718         Ok(())
 719     }
 720
 721     /// This code converts from skolemized regions back to late-bound
 722     /// regions. It works by replacing each region in the taint set of a
 723     /// skolemized region with a bound-region. The bound region will be bound
 724     /// by the outer-most binder in `value`; the caller must ensure that there is
 725     /// such a binder and it is the right place.
 726     ///
 727     /// This routine is only intended to be used when the leak-check has
 728     /// passed; currently, it's used in the trait matching code to create
 729     /// a set of nested obligations frmo an impl that matches against
 730     /// something higher-ranked.  More details can be found in
 731     /// `librustc/middle/traits/README.md`.
 732     ///
 733     /// As a brief example, consider the obligation `for<'a> Fn(&'a int)
 734     /// -> &'a int`, and the impl:
 735     ///
 736     ///     impl<A,R> Fn<A,R> for SomethingOrOther
 737     ///         where A : Clone
 738     ///     { ... }
 739     ///
 740     /// Here we will have replaced `'a` with a skolemized region
 741     /// `'0`. This means that our substitution will be `{A=>&'0
 742     /// int, R=>&'0 int}`.
 743     ///
 744     /// When we apply the substitution to the bounds, we will wind up with
 745     /// `&'0 int : Clone` as a predicate. As a last step, we then go and
 746     /// replace `'0` with a late-bound region `'a`.  The depth is matched
 747     /// to the depth of the predicate, in this case 1, so that the final
 748     /// predicate is `for<'a> &'a int : Clone`.
 749     pub fn plug_leaks<T>(&self,
 750                          skol_map: SkolemizationMap<'tcx>,
 751                          snapshot: &CombinedSnapshot,
 752                          value: T) -> T
 753         where T : TypeFoldable<'tcx>
 754     {
 755         debug!("plug_leaks(skol_map={:?}, value={:?})",
 756                skol_map,
 757                value);
 758
 759         if skol_map.is_empty() {
 760             return value;
 761         }
 762
 763         // Compute a mapping from the "taint set" of each skolemized
 764         // region back to the `ty::BoundRegion` that it originally
 765         // represented. Because `leak_check` passed, we know that
 766         // these taint sets are mutually disjoint.
 767         let inv_skol_map: FnvHashMap<&'tcx ty::Region, ty::BoundRegion> =
 768             skol_map
 769             .iter()
 770             .flat_map(|(&skol_br, &skol)| {
 771                 self.tainted_regions(snapshot, skol, TaintDirections::both())
 772                     .into_iter()
 773                     .map(move |tainted_region| (tainted_region, skol_br))
 774             })
 775             .collect();
 776
 777         debug!("plug_leaks: inv_skol_map={:?}",
 778                inv_skol_map);
 779
 780         // Remove any instantiated type variables from `value`; those can hide
 781         // references to regions from the `fold_regions` code below.
 782         let value = self.resolve_type_vars_if_possible(&value);
 783
 784         // Map any skolemization byproducts back to a late-bound
 785         // region. Put that late-bound region at whatever the outermost
 786         // binder is that we encountered in `value`. The caller is
 787         // responsible for ensuring that (a) `value` contains at least one
 788         // binder and (b) that binder is the one we want to use.
 789         let result = self.tcx.fold_regions(&value, &mut false, |r, current_depth| {
 790             match inv_skol_map.get(&r) {
 791                 None => r,
 792                 Some(br) => {
 793                     // It is the responsibility of the caller to ensure
 794                     // that each skolemized region appears within a
 795                     // binder. In practice, this routine is only used by
 796                     // trait checking, and all of the skolemized regions
 797                     // appear inside predicates, which always have
 798                     // binders, so this assert is satisfied.
 799                     assert!(current_depth > 1);
 800
 801                     // since leak-check passed, this skolemized region
 802                     // should only have incoming edges from variables
 803                     // (which ought not to escape the snapshot, but we
 804                     // don't check that) or itself
 805                     assert!(
 806                         match *r {
 807                             ty::ReVar(_) => true,
 808                             ty::ReSkolemized(_, ref br1) => br == br1,
 809                             _ => false,
 810                         },
 811                         "leak-check would have us replace {:?} with {:?}",
 812                         r, br);
 813
 814                     self.tcx.mk_region(ty::ReLateBound(
 815                         ty::DebruijnIndex::new(current_depth - 1), br.clone()))
 816                 }
 817             }
 818         });
 819
 820         self.pop_skolemized(skol_map, snapshot);
 821
 822         debug!("plug_leaks: result={:?}", result);
 823
 824         result
 825     }
 826
 827     /// Pops the skolemized regions found in `skol_map` from the region
 828     /// inference context. Whenever you create skolemized regions via
 829     /// `skolemize_late_bound_regions`, they must be popped before you
 830     /// commit the enclosing snapshot (if you do not commit, e.g. within a
 831     /// probe or as a result of an error, then this is not necessary, as
 832     /// popping happens as part of the rollback).
 833     ///
 834     /// Note: popping also occurs implicitly as part of `leak_check`.
 835     pub fn pop_skolemized(&self,
 836                           skol_map: SkolemizationMap<'tcx>,
 837                           snapshot: &CombinedSnapshot)
 838     {
 839         debug!("pop_skolemized({:?})", skol_map);
 840         let skol_regions: FnvHashSet<_> = skol_map.values().cloned().collect();
 841         self.region_vars.pop_skolemized(&skol_regions, &snapshot.region_vars_snapshot);
 842         if !skol_map.is_empty() {
 843             self.projection_cache.borrow_mut().rollback_skolemized(
 844                 &snapshot.projection_cache_snapshot);
 845         }
 846     }
 847 }