src/librustc/middle/ty/fold.rs

   1 // Copyright 2012-2013 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 //! Generalized type folding mechanism. The setup is a bit convoluted
  12 //! but allows for convenient usage. Let T be an instance of some
  13 //! "foldable type" (one which implements `TypeFoldable`) and F be an
  14 //! instance of a "folder" (a type which implements `TypeFolder`). Then
  15 //! the setup is intended to be:
  16 //!
  17 //!   T.fold_with(F) --calls--> F.fold_T(T) --calls--> T.super_fold_with(F)
  18 //!
  19 //! This way, when you define a new folder F, you can override
  20 //! `fold_T()` to customize the behavior, and invoke `T.super_fold_with()`
  21 //! to get the original behavior. Meanwhile, to actually fold
  22 //! something, you can just write `T.fold_with(F)`, which is
  23 //! convenient. (Note that `fold_with` will also transparently handle
  24 //! things like a `Vec<T>` where T is foldable and so on.)
  25 //!
  26 //! In this ideal setup, the only function that actually *does*
  27 //! anything is `T.super_fold_with()`, which traverses the type `T`.
  28 //! Moreover, `T.super_fold_with()` should only ever call `T.fold_with()`.
  29 //!
  30 //! In some cases, we follow a degenerate pattern where we do not have
  31 //! a `fold_T` method. Instead, `T.fold_with` traverses the structure directly.
  32 //! This is suboptimal because the behavior cannot be overridden, but it's
  33 //! much less work to implement. If you ever *do* need an override that
  34 //! doesn't exist, it's not hard to convert the degenerate pattern into the
  35 //! proper thing.
  36 //!
  37 //! A `TypeFoldable` T can also be visited by a `TypeVisitor` V using similar setup:
  38 //!   T.visit_with(V) --calls--> V.visit_T(T) --calls--> T.super_visit_with(V).
  39 //! These methods return true to indicate that the visitor has found what it is looking for
  40 //! and does not need to visit anything else.
  41
  42 use middle::region;
  43 use middle::subst;
  44 use middle::ty::adjustment;
  45 use middle::ty::{self, Binder, Ty, TypeFlags};
  46
  47 use std::fmt;
  48 use util::nodemap::{FnvHashMap, FnvHashSet};
  49
  50 /// The TypeFoldable trait is implemented for every type that can be folded.
  51 /// Basically, every type that has a corresponding method in TypeFolder.
  52 pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
  53     fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self;
  54     fn fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
  55         self.super_fold_with(folder)
  56     }
  57
  58     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool;
  59     fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
  60         self.super_visit_with(visitor)
  61     }
  62
  63     fn has_regions_escaping_depth(&self, depth: u32) -> bool {
  64         self.visit_with(&mut HasEscapingRegionsVisitor { depth: depth })
  65     }
  66     fn has_escaping_regions(&self) -> bool {
  67         self.has_regions_escaping_depth(0)
  68     }
  69
  70     fn has_type_flags(&self, flags: TypeFlags) -> bool {
  71         self.visit_with(&mut HasTypeFlagsVisitor { flags: flags })
  72     }
  73     fn has_projection_types(&self) -> bool {
  74         self.has_type_flags(TypeFlags::HAS_PROJECTION)
  75     }
  76     fn references_error(&self) -> bool {
  77         self.has_type_flags(TypeFlags::HAS_TY_ERR)
  78     }
  79     fn has_param_types(&self) -> bool {
  80         self.has_type_flags(TypeFlags::HAS_PARAMS)
  81     }
  82     fn has_self_ty(&self) -> bool {
  83         self.has_type_flags(TypeFlags::HAS_SELF)
  84     }
  85     fn has_infer_types(&self) -> bool {
  86         self.has_type_flags(TypeFlags::HAS_TY_INFER)
  87     }
  88     fn needs_infer(&self) -> bool {
  89         self.has_type_flags(TypeFlags::HAS_TY_INFER | TypeFlags::HAS_RE_INFER)
  90     }
  91     fn needs_subst(&self) -> bool {
  92         self.has_type_flags(TypeFlags::NEEDS_SUBST)
  93     }
  94     fn has_closure_types(&self) -> bool {
  95         self.has_type_flags(TypeFlags::HAS_TY_CLOSURE)
  96     }
  97     fn has_erasable_regions(&self) -> bool {
  98         self.has_type_flags(TypeFlags::HAS_RE_EARLY_BOUND |
  99                             TypeFlags::HAS_RE_INFER |
 100                             TypeFlags::HAS_FREE_REGIONS)
 101     }
 102     /// Indicates whether this value references only 'global'
 103     /// types/lifetimes that are the same regardless of what fn we are
 104     /// in. This is used for caching. Errs on the side of returning
 105     /// false.
 106     fn is_global(&self) -> bool {
 107         !self.has_type_flags(TypeFlags::HAS_LOCAL_NAMES)
 108     }
 109 }
 110
 111 /// The TypeFolder trait defines the actual *folding*. There is a
 112 /// method defined for every foldable type. Each of these has a
 113 /// default implementation that does an "identity" fold. Within each
 114 /// identity fold, it should invoke `foo.fold_with(self)` to fold each
 115 /// sub-item.
 116 pub trait TypeFolder<'tcx> : Sized {
 117     fn tcx<'a>(&'a self) -> &'a ty::ctxt<'tcx>;
 118
 119     /// Invoked by the `super_*` routines when we enter a region
 120     /// binding level (for example, when entering a function
 121     /// signature). This is used by clients that want to track the
 122     /// Debruijn index nesting level.
 123     fn enter_region_binder(&mut self) { }
 124
 125     /// Invoked by the `super_*` routines when we exit a region
 126     /// binding level. This is used by clients that want to
 127     /// track the Debruijn index nesting level.
 128     fn exit_region_binder(&mut self) { }
 129
 130     fn fold_binder<T>(&mut self, t: &Binder<T>) -> Binder<T>
 131         where T : TypeFoldable<'tcx>
 132     {
 133         // FIXME(#20526) this should replace `enter_region_binder`/`exit_region_binder`.
 134         t.super_fold_with(self)
 135     }
 136
 137     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 138         t.super_fold_with(self)
 139     }
 140
 141     fn fold_mt(&mut self, t: &ty::TypeAndMut<'tcx>) -> ty::TypeAndMut<'tcx> {
 142         t.super_fold_with(self)
 143     }
 144
 145     fn fold_trait_ref(&mut self, t: &ty::TraitRef<'tcx>) -> ty::TraitRef<'tcx> {
 146         t.super_fold_with(self)
 147     }
 148
 149     fn fold_substs(&mut self,
 150                    substs: &subst::Substs<'tcx>)
 151                    -> subst::Substs<'tcx> {
 152         substs.super_fold_with(self)
 153     }
 154
 155     fn fold_fn_sig(&mut self,
 156                    sig: &ty::FnSig<'tcx>)
 157                    -> ty::FnSig<'tcx> {
 158         sig.super_fold_with(self)
 159     }
 160
 161     fn fold_output(&mut self,
 162                       output: &ty::FnOutput<'tcx>)
 163                       -> ty::FnOutput<'tcx> {
 164         output.super_fold_with(self)
 165     }
 166
 167     fn fold_bare_fn_ty(&mut self,
 168                        fty: &ty::BareFnTy<'tcx>)
 169                        -> ty::BareFnTy<'tcx>
 170     {
 171         fty.super_fold_with(self)
 172     }
 173
 174     fn fold_closure_ty(&mut self,
 175                        fty: &ty::ClosureTy<'tcx>)
 176                        -> ty::ClosureTy<'tcx> {
 177         fty.super_fold_with(self)
 178     }
 179
 180     fn fold_region(&mut self, r: ty::Region) -> ty::Region {
 181         r.super_fold_with(self)
 182     }
 183
 184     fn fold_existential_bounds(&mut self, s: &ty::ExistentialBounds<'tcx>)
 185                                -> ty::ExistentialBounds<'tcx> {
 186         s.super_fold_with(self)
 187     }
 188
 189     fn fold_autoref(&mut self, ar: &adjustment::AutoRef<'tcx>)
 190                     -> adjustment::AutoRef<'tcx> {
 191         ar.super_fold_with(self)
 192     }
 193 }
 194
 195 pub trait TypeVisitor<'tcx> : Sized {
 196     fn enter_region_binder(&mut self) { }
 197     fn exit_region_binder(&mut self) { }
 198
 199     fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
 200         t.super_visit_with(self)
 201     }
 202
 203     fn visit_region(&mut self, r: ty::Region) -> bool {
 204         r.super_visit_with(self)
 205     }
 206 }
 207
 208 ///////////////////////////////////////////////////////////////////////////
 209 // Some sample folders
 210
 211 pub struct BottomUpFolder<'a, 'tcx: 'a, F> where F: FnMut(Ty<'tcx>) -> Ty<'tcx> {
 212     pub tcx: &'a ty::ctxt<'tcx>,
 213     pub fldop: F,
 214 }
 215
 216 impl<'a, 'tcx, F> TypeFolder<'tcx> for BottomUpFolder<'a, 'tcx, F> where
 217     F: FnMut(Ty<'tcx>) -> Ty<'tcx>,
 218 {
 219     fn tcx(&self) -> &ty::ctxt<'tcx> { self.tcx }
 220
 221     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 222         let t1 = ty.super_fold_with(self);
 223         (self.fldop)(t1)
 224     }
 225 }
 226
 227 ///////////////////////////////////////////////////////////////////////////
 228 // Region folder
 229
 230 impl<'tcx> ty::ctxt<'tcx> {
 231     /// Collects the free and escaping regions in `value` into `region_set`. Returns
 232     /// whether any late-bound regions were skipped
 233     pub fn collect_regions<T>(&self,
 234         value: &T,
 235         region_set: &mut FnvHashSet<ty::Region>)
 236         -> bool
 237         where T : TypeFoldable<'tcx>
 238     {
 239         let mut have_bound_regions = false;
 240         self.fold_regions(value, &mut have_bound_regions,
 241                           |r, d| { region_set.insert(r.from_depth(d)); r });
 242         have_bound_regions
 243     }
 244
 245     /// Folds the escaping and free regions in `value` using `f`, and
 246     /// sets `skipped_regions` to true if any late-bound region was found
 247     /// and skipped.
 248     pub fn fold_regions<T,F>(&self,
 249         value: &T,
 250         skipped_regions: &mut bool,
 251         mut f: F)
 252         -> T
 253         where F : FnMut(ty::Region, u32) -> ty::Region,
 254               T : TypeFoldable<'tcx>,
 255     {
 256         value.fold_with(&mut RegionFolder::new(self, skipped_regions, &mut f))
 257     }
 258 }
 259
 260 /// Folds over the substructure of a type, visiting its component
 261 /// types and all regions that occur *free* within it.
 262 ///
 263 /// That is, `Ty` can contain function or method types that bind
 264 /// regions at the call site (`ReLateBound`), and occurrences of
 265 /// regions (aka "lifetimes") that are bound within a type are not
 266 /// visited by this folder; only regions that occur free will be
 267 /// visited by `fld_r`.
 268
 269 pub struct RegionFolder<'a, 'tcx: 'a> {
 270     tcx: &'a ty::ctxt<'tcx>,
 271     skipped_regions: &'a mut bool,
 272     current_depth: u32,
 273     fld_r: &'a mut (FnMut(ty::Region, u32) -> ty::Region + 'a),
 274 }
 275
 276 impl<'a, 'tcx> RegionFolder<'a, 'tcx> {
 277     pub fn new<F>(tcx: &'a ty::ctxt<'tcx>,
 278                   skipped_regions: &'a mut bool,
 279                   fld_r: &'a mut F) -> RegionFolder<'a, 'tcx>
 280         where F : FnMut(ty::Region, u32) -> ty::Region
 281     {
 282         RegionFolder {
 283             tcx: tcx,
 284             skipped_regions: skipped_regions,
 285             current_depth: 1,
 286             fld_r: fld_r,
 287         }
 288     }
 289 }
 290
 291 impl<'a, 'tcx> TypeFolder<'tcx> for RegionFolder<'a, 'tcx>
 292 {
 293     fn tcx(&self) -> &ty::ctxt<'tcx> { self.tcx }
 294
 295     fn enter_region_binder(&mut self) {
 296         self.current_depth += 1;
 297     }
 298
 299     fn exit_region_binder(&mut self) {
 300         self.current_depth -= 1;
 301     }
 302
 303     fn fold_region(&mut self, r: ty::Region) -> ty::Region {
 304         match r {
 305             ty::ReLateBound(debruijn, _) if debruijn.depth < self.current_depth => {
 306                 debug!("RegionFolder.fold_region({:?}) skipped bound region (current depth={})",
 307                        r, self.current_depth);
 308                 *self.skipped_regions = true;
 309                 r
 310             }
 311             _ => {
 312                 debug!("RegionFolder.fold_region({:?}) folding free region (current_depth={})",
 313                        r, self.current_depth);
 314                 (self.fld_r)(r, self.current_depth)
 315             }
 316         }
 317     }
 318 }
 319
 320 ///////////////////////////////////////////////////////////////////////////
 321 // Late-bound region replacer
 322
 323 // Replaces the escaping regions in a type.
 324
 325 struct RegionReplacer<'a, 'tcx: 'a> {
 326     tcx: &'a ty::ctxt<'tcx>,
 327     current_depth: u32,
 328     fld_r: &'a mut (FnMut(ty::BoundRegion) -> ty::Region + 'a),
 329     map: FnvHashMap<ty::BoundRegion, ty::Region>
 330 }
 331
 332 impl<'tcx> ty::ctxt<'tcx> {
 333     pub fn replace_late_bound_regions<T,F>(&self,
 334         value: &Binder<T>,
 335         mut f: F)
 336         -> (T, FnvHashMap<ty::BoundRegion, ty::Region>)
 337         where F : FnMut(ty::BoundRegion) -> ty::Region,
 338               T : TypeFoldable<'tcx>,
 339     {
 340         debug!("replace_late_bound_regions({:?})", value);
 341         let mut replacer = RegionReplacer::new(self, &mut f);
 342         let result = value.skip_binder().fold_with(&mut replacer);
 343         (result, replacer.map)
 344     }
 345
 346
 347     /// Replace any late-bound regions bound in `value` with free variants attached to scope-id
 348     /// `scope_id`.
 349     pub fn liberate_late_bound_regions<T>(&self,
 350         all_outlive_scope: region::CodeExtent,
 351         value: &Binder<T>)
 352         -> T
 353         where T : TypeFoldable<'tcx>
 354     {
 355         self.replace_late_bound_regions(value, |br| {
 356             ty::ReFree(ty::FreeRegion{scope: all_outlive_scope, bound_region: br})
 357         }).0
 358     }
 359
 360     /// Flattens two binding levels into one. So `for<'a> for<'b> Foo`
 361     /// becomes `for<'a,'b> Foo`.
 362     pub fn flatten_late_bound_regions<T>(&self, bound2_value: &Binder<Binder<T>>)
 363                                          -> Binder<T>
 364         where T: TypeFoldable<'tcx>
 365     {
 366         let bound0_value = bound2_value.skip_binder().skip_binder();
 367         let value = self.fold_regions(bound0_value, &mut false,
 368                                       |region, current_depth| {
 369             match region {
 370                 ty::ReLateBound(debruijn, br) if debruijn.depth >= current_depth => {
 371                     // should be true if no escaping regions from bound2_value
 372                     assert!(debruijn.depth - current_depth <= 1);
 373                     ty::ReLateBound(ty::DebruijnIndex::new(current_depth), br)
 374                 }
 375                 _ => {
 376                     region
 377                 }
 378             }
 379         });
 380         Binder(value)
 381     }
 382
 383     pub fn no_late_bound_regions<T>(&self, value: &Binder<T>) -> Option<T>
 384         where T : TypeFoldable<'tcx>
 385     {
 386         if value.0.has_escaping_regions() {
 387             None
 388         } else {
 389             Some(value.0.clone())
 390         }
 391     }
 392
 393     /// Replace any late-bound regions bound in `value` with `'static`. Useful in trans but also
 394     /// method lookup and a few other places where precise region relationships are not required.
 395     pub fn erase_late_bound_regions<T>(&self, value: &Binder<T>) -> T
 396         where T : TypeFoldable<'tcx>
 397     {
 398         self.replace_late_bound_regions(value, |_| ty::ReStatic).0
 399     }
 400
 401     /// Rewrite any late-bound regions so that they are anonymous.  Region numbers are
 402     /// assigned starting at 1 and increasing monotonically in the order traversed
 403     /// by the fold operation.
 404     ///
 405     /// The chief purpose of this function is to canonicalize regions so that two
 406     /// `FnSig`s or `TraitRef`s which are equivalent up to region naming will become
 407     /// structurally identical.  For example, `for<'a, 'b> fn(&'a isize, &'b isize)` and
 408     /// `for<'a, 'b> fn(&'b isize, &'a isize)` will become identical after anonymization.
 409     pub fn anonymize_late_bound_regions<T>(&self, sig: &Binder<T>) -> Binder<T>
 410         where T : TypeFoldable<'tcx>,
 411     {
 412         let mut counter = 0;
 413         Binder(self.replace_late_bound_regions(sig, |_| {
 414             counter += 1;
 415             ty::ReLateBound(ty::DebruijnIndex::new(1), ty::BrAnon(counter))
 416         }).0)
 417     }
 418 }
 419
 420 impl<'a, 'tcx> RegionReplacer<'a, 'tcx> {
 421     fn new<F>(tcx: &'a ty::ctxt<'tcx>, fld_r: &'a mut F) -> RegionReplacer<'a, 'tcx>
 422         where F : FnMut(ty::BoundRegion) -> ty::Region
 423     {
 424         RegionReplacer {
 425             tcx: tcx,
 426             current_depth: 1,
 427             fld_r: fld_r,
 428             map: FnvHashMap()
 429         }
 430     }
 431 }
 432
 433 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx>
 434 {
 435     fn tcx(&self) -> &ty::ctxt<'tcx> { self.tcx }
 436
 437     fn enter_region_binder(&mut self) {
 438         self.current_depth += 1;
 439     }
 440
 441     fn exit_region_binder(&mut self) {
 442         self.current_depth -= 1;
 443     }
 444
 445     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 446         if !t.has_regions_escaping_depth(self.current_depth-1) {
 447             return t;
 448         }
 449
 450         t.super_fold_with(self)
 451     }
 452
 453     fn fold_region(&mut self, r: ty::Region) -> ty::Region {
 454         match r {
 455             ty::ReLateBound(debruijn, br) if debruijn.depth == self.current_depth => {
 456                 debug!("RegionReplacer.fold_region({:?}) folding region (current_depth={})",
 457                        r, self.current_depth);
 458                 let fld_r = &mut self.fld_r;
 459                 let region = *self.map.entry(br).or_insert_with(|| fld_r(br));
 460                 if let ty::ReLateBound(debruijn1, br) = region {
 461                     // If the callback returns a late-bound region,
 462                     // that region should always use depth 1. Then we
 463                     // adjust it to the correct depth.
 464                     assert_eq!(debruijn1.depth, 1);
 465                     ty::ReLateBound(debruijn, br)
 466                 } else {
 467                     region
 468                 }
 469             }
 470             r => r
 471         }
 472     }
 473 }
 474
 475 ///////////////////////////////////////////////////////////////////////////
 476 // Region eraser
 477
 478 impl<'tcx> ty::ctxt<'tcx> {
 479     /// Returns an equivalent value with all free regions removed (note
 480     /// that late-bound regions remain, because they are important for
 481     /// subtyping, but they are anonymized and normalized as well)..
 482     pub fn erase_regions<T>(&self, value: &T) -> T
 483         where T : TypeFoldable<'tcx>
 484     {
 485         let value1 = value.fold_with(&mut RegionEraser(self));
 486         debug!("erase_regions({:?}) = {:?}",
 487                value, value1);
 488         return value1;
 489
 490         struct RegionEraser<'a, 'tcx: 'a>(&'a ty::ctxt<'tcx>);
 491
 492         impl<'a, 'tcx> TypeFolder<'tcx> for RegionEraser<'a, 'tcx> {
 493             fn tcx(&self) -> &ty::ctxt<'tcx> { self.0 }
 494
 495             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 496                 match self.tcx().normalized_cache.borrow().get(&ty).cloned() {
 497                     None => {}
 498                     Some(u) => return u
 499                 }
 500
 501                 let t_norm = ty.super_fold_with(self);
 502                 self.tcx().normalized_cache.borrow_mut().insert(ty, t_norm);
 503                 return t_norm;
 504             }
 505
 506             fn fold_binder<T>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T>
 507                 where T : TypeFoldable<'tcx>
 508             {
 509                 let u = self.tcx().anonymize_late_bound_regions(t);
 510                 u.super_fold_with(self)
 511             }
 512
 513             fn fold_region(&mut self, r: ty::Region) -> ty::Region {
 514                 // because late-bound regions affect subtyping, we can't
 515                 // erase the bound/free distinction, but we can replace
 516                 // all free regions with 'static.
 517                 //
 518                 // Note that we *CAN* replace early-bound regions -- the
 519                 // type system never "sees" those, they get substituted
 520                 // away. In trans, they will always be erased to 'static
 521                 // whenever a substitution occurs.
 522                 match r {
 523                     ty::ReLateBound(..) => r,
 524                     _ => ty::ReStatic
 525                 }
 526             }
 527
 528             fn fold_substs(&mut self,
 529                            substs: &subst::Substs<'tcx>)
 530                            -> subst::Substs<'tcx> {
 531                 subst::Substs { regions: subst::ErasedRegions,
 532                                 types: substs.types.fold_with(self) }
 533             }
 534         }
 535     }
 536 }
 537
 538 ///////////////////////////////////////////////////////////////////////////
 539 // Region shifter
 540 //
 541 // Shifts the De Bruijn indices on all escaping bound regions by a
 542 // fixed amount. Useful in substitution or when otherwise introducing
 543 // a binding level that is not intended to capture the existing bound
 544 // regions. See comment on `shift_regions_through_binders` method in
 545 // `subst.rs` for more details.
 546
 547 pub fn shift_region(region: ty::Region, amount: u32) -> ty::Region {
 548     match region {
 549         ty::ReLateBound(debruijn, br) => {
 550             ty::ReLateBound(debruijn.shifted(amount), br)
 551         }
 552         _ => {
 553             region
 554         }
 555     }
 556 }
 557
 558 pub fn shift_regions<'tcx, T:TypeFoldable<'tcx>>(tcx: &ty::ctxt<'tcx>,
 559                                                  amount: u32, value: &T) -> T {
 560     debug!("shift_regions(value={:?}, amount={})",
 561            value, amount);
 562
 563     value.fold_with(&mut RegionFolder::new(tcx, &mut false, &mut |region, _current_depth| {
 564         shift_region(region, amount)
 565     }))
 566 }
 567
 568 /// An "escaping region" is a bound region whose binder is not part of `t`.
 569 ///
 570 /// So, for example, consider a type like the following, which has two binders:
 571 ///
 572 ///    for<'a> fn(x: for<'b> fn(&'a isize, &'b isize))
 573 ///    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ outer scope
 574 ///                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~  inner scope
 575 ///
 576 /// This type has *bound regions* (`'a`, `'b`), but it does not have escaping regions, because the
 577 /// binders of both `'a` and `'b` are part of the type itself. However, if we consider the *inner
 578 /// fn type*, that type has an escaping region: `'a`.
 579 ///
 580 /// Note that what I'm calling an "escaping region" is often just called a "free region". However,
 581 /// we already use the term "free region". It refers to the regions that we use to represent bound
 582 /// regions on a fn definition while we are typechecking its body.
 583 ///
 584 /// To clarify, conceptually there is no particular difference between an "escaping" region and a
 585 /// "free" region. However, there is a big difference in practice. Basically, when "entering" a
 586 /// binding level, one is generally required to do some sort of processing to a bound region, such
 587 /// as replacing it with a fresh/skolemized region, or making an entry in the environment to
 588 /// represent the scope to which it is attached, etc. An escaping region represents a bound region
 589 /// for which this processing has not yet been done.
 590 struct HasEscapingRegionsVisitor {
 591     depth: u32,
 592 }
 593
 594 impl<'tcx> TypeVisitor<'tcx> for HasEscapingRegionsVisitor {
 595     fn enter_region_binder(&mut self) {
 596         self.depth += 1;
 597     }
 598
 599     fn exit_region_binder(&mut self) {
 600         self.depth -= 1;
 601     }
 602
 603     fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
 604         t.region_depth > self.depth
 605     }
 606
 607     fn visit_region(&mut self, r: ty::Region) -> bool {
 608         r.escapes_depth(self.depth)
 609     }
 610 }
 611
 612 struct HasTypeFlagsVisitor {
 613     flags: ty::TypeFlags,
 614 }
 615
 616 impl<'tcx> TypeVisitor<'tcx> for HasTypeFlagsVisitor {
 617     fn visit_ty(&mut self, t: Ty) -> bool {
 618         t.flags.get().intersects(self.flags)
 619     }
 620
 621     fn visit_region(&mut self, r: ty::Region) -> bool {
 622         if self.flags.intersects(ty::TypeFlags::HAS_LOCAL_NAMES) {
 623             // does this represent a region that cannot be named
 624             // in a global way? used in fulfillment caching.
 625             match r {
 626                 ty::ReStatic | ty::ReEmpty => {}
 627                 _ => return true,
 628             }
 629         }
 630         if self.flags.intersects(ty::TypeFlags::HAS_RE_INFER) {
 631             match r {
 632                 ty::ReVar(_) | ty::ReSkolemized(..) => { return true }
 633                 _ => {}
 634             }
 635         }
 636         false
 637     }
 638 }