src/librustc/ty/fold.rs

   1 //! Generalized type folding mechanism. The setup is a bit convoluted
   2 //! but allows for convenient usage. Let T be an instance of some
   3 //! "foldable type" (one which implements `TypeFoldable`) and F be an
   4 //! instance of a "folder" (a type which implements `TypeFolder`). Then
   5 //! the setup is intended to be:
   6 //!
   7 //!     T.fold_with(F) --calls--> F.fold_T(T) --calls--> T.super_fold_with(F)
   8 //!
   9 //! This way, when you define a new folder F, you can override
  10 //! `fold_T()` to customize the behavior, and invoke `T.super_fold_with()`
  11 //! to get the original behavior. Meanwhile, to actually fold
  12 //! something, you can just write `T.fold_with(F)`, which is
  13 //! convenient. (Note that `fold_with` will also transparently handle
  14 //! things like a `Vec<T>` where T is foldable and so on.)
  15 //!
  16 //! In this ideal setup, the only function that actually *does*
  17 //! anything is `T.super_fold_with()`, which traverses the type `T`.
  18 //! Moreover, `T.super_fold_with()` should only ever call `T.fold_with()`.
  19 //!
  20 //! In some cases, we follow a degenerate pattern where we do not have
  21 //! a `fold_T` method. Instead, `T.fold_with` traverses the structure directly.
  22 //! This is suboptimal because the behavior cannot be overridden, but it's
  23 //! much less work to implement. If you ever *do* need an override that
  24 //! doesn't exist, it's not hard to convert the degenerate pattern into the
  25 //! proper thing.
  26 //!
  27 //! A `TypeFoldable` T can also be visited by a `TypeVisitor` V using similar setup:
  28 //!
  29 //!     T.visit_with(V) --calls--> V.visit_T(T) --calls--> T.super_visit_with(V).
  30 //!
  31 //! These methods return true to indicate that the visitor has found what it is
  32 //! looking for, and does not need to visit anything else.
  33
  34 use crate::hir::def_id::DefId;
  35 use crate::mir::interpret::ConstValue;
  36 use crate::ty::{self, Binder, Ty, TyCtxt, TypeFlags, flags::FlagComputation};
  37
  38 use std::collections::BTreeMap;
  39 use std::fmt;
  40 use crate::util::nodemap::FxHashSet;
  41
  42 /// This trait is implemented for every type that can be folded.
  43 /// Basically, every type that has a corresponding method in `TypeFolder`.
  44 ///
  45 /// To implement this conveniently, use the
  46 /// `BraceStructTypeFoldableImpl` etc macros found in `macros.rs`.
  47 pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
  48     fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self;
  49     fn fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
  50         self.super_fold_with(folder)
  51     }
  52
  53     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool;
  54     fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
  55         self.super_visit_with(visitor)
  56     }
  57
  58     /// Returns `true` if `self` has any late-bound regions that are either
  59     /// bound by `binder` or bound by some binder outside of `binder`.
  60     /// If `binder` is `ty::INNERMOST`, this indicates whether
  61     /// there are any late-bound regions that appear free.
  62     fn has_vars_bound_at_or_above(&self, binder: ty::DebruijnIndex) -> bool {
  63         self.visit_with(&mut HasEscapingVarsVisitor { outer_index: binder })
  64     }
  65
  66     /// Returns `true` if this `self` has any regions that escape `binder` (and
  67     /// hence are not bound by it).
  68     fn has_vars_bound_above(&self, binder: ty::DebruijnIndex) -> bool {
  69         self.has_vars_bound_at_or_above(binder.shifted_in(1))
  70     }
  71
  72     fn has_escaping_bound_vars(&self) -> bool {
  73         self.has_vars_bound_at_or_above(ty::INNERMOST)
  74     }
  75
  76     fn has_type_flags(&self, flags: TypeFlags) -> bool {
  77         self.visit_with(&mut HasTypeFlagsVisitor { flags })
  78     }
  79     fn has_projections(&self) -> bool {
  80         self.has_type_flags(TypeFlags::HAS_PROJECTION)
  81     }
  82     fn references_error(&self) -> bool {
  83         self.has_type_flags(TypeFlags::HAS_TY_ERR)
  84     }
  85     fn has_param_types(&self) -> bool {
  86         self.has_type_flags(TypeFlags::HAS_PARAMS)
  87     }
  88     fn has_infer_types(&self) -> bool {
  89         self.has_type_flags(TypeFlags::HAS_TY_INFER)
  90     }
  91     fn has_infer_consts(&self) -> bool {
  92         self.has_type_flags(TypeFlags::HAS_CT_INFER)
  93     }
  94     fn has_local_value(&self) -> bool {
  95         self.has_type_flags(TypeFlags::KEEP_IN_LOCAL_TCX)
  96     }
  97     fn needs_infer(&self) -> bool {
  98         self.has_type_flags(
  99             TypeFlags::HAS_TY_INFER | TypeFlags::HAS_RE_INFER | TypeFlags::HAS_CT_INFER
 100         )
 101     }
 102     fn has_placeholders(&self) -> bool {
 103         self.has_type_flags(
 104             TypeFlags::HAS_RE_PLACEHOLDER |
 105             TypeFlags::HAS_TY_PLACEHOLDER |
 106             TypeFlags::HAS_CT_PLACEHOLDER
 107         )
 108     }
 109     fn needs_subst(&self) -> bool {
 110         self.has_type_flags(TypeFlags::NEEDS_SUBST)
 111     }
 112     fn has_re_placeholders(&self) -> bool {
 113         self.has_type_flags(TypeFlags::HAS_RE_PLACEHOLDER)
 114     }
 115     fn has_closure_types(&self) -> bool {
 116         self.has_type_flags(TypeFlags::HAS_TY_CLOSURE)
 117     }
 118     /// "Free" regions in this context means that it has any region
 119     /// that is not (a) erased or (b) late-bound.
 120     fn has_free_regions(&self) -> bool {
 121         self.has_type_flags(TypeFlags::HAS_FREE_REGIONS)
 122     }
 123
 124     /// True if there are any un-erased free regions.
 125     fn has_erasable_regions(&self) -> bool {
 126         self.has_type_flags(TypeFlags::HAS_FREE_REGIONS)
 127     }
 128
 129     /// Indicates whether this value references only 'global'
 130     /// generic parameters that are the same regardless of what fn we are
 131     /// in. This is used for caching.
 132     fn is_global(&self) -> bool {
 133         !self.has_type_flags(TypeFlags::HAS_FREE_LOCAL_NAMES)
 134     }
 135
 136     /// True if there are any late-bound regions
 137     fn has_late_bound_regions(&self) -> bool {
 138         self.has_type_flags(TypeFlags::HAS_RE_LATE_BOUND)
 139     }
 140
 141     /// A visitor that does not recurse into types, works like `fn walk_shallow` in `Ty`.
 142     fn visit_tys_shallow(&self, visit: impl FnMut(Ty<'tcx>) -> bool) -> bool {
 143
 144         pub struct Visitor<F>(F);
 145
 146         impl<'tcx, F: FnMut(Ty<'tcx>) -> bool> TypeVisitor<'tcx> for Visitor<F> {
 147             fn visit_ty(&mut self, ty: Ty<'tcx>) -> bool {
 148                 self.0(ty)
 149             }
 150         }
 151
 152         self.visit_with(&mut Visitor(visit))
 153     }
 154 }
 155
 156 /// The `TypeFolder` trait defines the actual *folding*. There is a
 157 /// method defined for every foldable type. Each of these has a
 158 /// default implementation that does an "identity" fold. Within each
 159 /// identity fold, it should invoke `foo.fold_with(self)` to fold each
 160 /// sub-item.
 161 pub trait TypeFolder<'tcx>: Sized {
 162     fn tcx<'a>(&'a self) -> TyCtxt<'tcx>;
 163
 164     fn fold_binder<T>(&mut self, t: &Binder<T>) -> Binder<T>
 165         where T : TypeFoldable<'tcx>
 166     {
 167         t.super_fold_with(self)
 168     }
 169
 170     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 171         t.super_fold_with(self)
 172     }
 173
 174     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 175         r.super_fold_with(self)
 176     }
 177
 178     fn fold_const(&mut self, c: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 179         c.super_fold_with(self)
 180     }
 181 }
 182
 183 pub trait TypeVisitor<'tcx> : Sized {
 184     fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
 185         t.super_visit_with(self)
 186     }
 187
 188     fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
 189         t.super_visit_with(self)
 190     }
 191
 192     fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
 193         r.super_visit_with(self)
 194     }
 195
 196     fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> bool {
 197         c.super_visit_with(self)
 198     }
 199 }
 200
 201 ///////////////////////////////////////////////////////////////////////////
 202 // Some sample folders
 203
 204 pub struct BottomUpFolder<'tcx, F, G, H>
 205 where
 206     F: FnMut(Ty<'tcx>) -> Ty<'tcx>,
 207     G: FnMut(ty::Region<'tcx>) -> ty::Region<'tcx>,
 208     H: FnMut(&'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx>,
 209 {
 210     pub tcx: TyCtxt<'tcx>,
 211     pub ty_op: F,
 212     pub lt_op: G,
 213     pub ct_op: H,
 214 }
 215
 216 impl<'tcx, F, G, H> TypeFolder<'tcx> for BottomUpFolder<'tcx, F, G, H>
 217 where
 218     F: FnMut(Ty<'tcx>) -> Ty<'tcx>,
 219     G: FnMut(ty::Region<'tcx>) -> ty::Region<'tcx>,
 220     H: FnMut(&'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx>,
 221 {
 222     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 223         self.tcx
 224         }
 225
 226     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 227         let t = ty.super_fold_with(self);
 228         (self.ty_op)(t)
 229     }
 230
 231     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 232         let r = r.super_fold_with(self);
 233         (self.lt_op)(r)
 234     }
 235
 236     fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 237         let ct = ct.super_fold_with(self);
 238         (self.ct_op)(ct)
 239     }
 240 }
 241
 242 ///////////////////////////////////////////////////////////////////////////
 243 // Region folder
 244
 245 impl<'tcx> TyCtxt<'tcx> {
 246     /// Collects the free and escaping regions in `value` into `region_set`. Returns
 247     /// whether any late-bound regions were skipped
 248     pub fn collect_regions<T>(self,
 249         value: &T,
 250         region_set: &mut FxHashSet<ty::Region<'tcx>>)
 251         -> bool
 252         where T : TypeFoldable<'tcx>
 253     {
 254         let mut have_bound_regions = false;
 255         self.fold_regions(value, &mut have_bound_regions, |r, d| {
 256             region_set.insert(self.mk_region(r.shifted_out_to_binder(d)));
 257             r
 258         });
 259         have_bound_regions
 260     }
 261
 262     /// Folds the escaping and free regions in `value` using `f`, and
 263     /// sets `skipped_regions` to true if any late-bound region was found
 264     /// and skipped.
 265     pub fn fold_regions<T>(
 266         self,
 267         value: &T,
 268         skipped_regions: &mut bool,
 269         mut f: impl FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
 270     ) -> T
 271     where
 272         T : TypeFoldable<'tcx>,
 273     {
 274         value.fold_with(&mut RegionFolder::new(self, skipped_regions, &mut f))
 275     }
 276
 277     /// Invoke `callback` on every region appearing free in `value`.
 278     pub fn for_each_free_region(
 279         self,
 280         value: &impl TypeFoldable<'tcx>,
 281         mut callback: impl FnMut(ty::Region<'tcx>),
 282     ) {
 283         self.any_free_region_meets(value, |r| {
 284             callback(r);
 285             false
 286         });
 287     }
 288
 289     /// Returns `true` if `callback` returns true for every region appearing free in `value`.
 290     pub fn all_free_regions_meet(
 291         self,
 292         value: &impl TypeFoldable<'tcx>,
 293         mut callback: impl FnMut(ty::Region<'tcx>) -> bool,
 294     ) -> bool {
 295         !self.any_free_region_meets(value, |r| !callback(r))
 296     }
 297
 298     /// Returns `true` if `callback` returns true for some region appearing free in `value`.
 299     pub fn any_free_region_meets(
 300         self,
 301         value: &impl TypeFoldable<'tcx>,
 302         callback: impl FnMut(ty::Region<'tcx>) -> bool,
 303     ) -> bool {
 304         return value.visit_with(&mut RegionVisitor {
 305             outer_index: ty::INNERMOST,
 306             callback
 307         });
 308
 309         struct RegionVisitor<F> {
 310             /// The index of a binder *just outside* the things we have
 311             /// traversed. If we encounter a bound region bound by this
 312             /// binder or one outer to it, it appears free. Example:
 313             ///
 314             /// ```
 315             ///    for<'a> fn(for<'b> fn(), T)
 316             /// ^          ^          ^     ^
 317             /// |          |          |     | here, would be shifted in 1
 318             /// |          |          | here, would be shifted in 2
 319             /// |          | here, would be `INNERMOST` shifted in by 1
 320             /// | here, initially, binder would be `INNERMOST`
 321             /// ```
 322             ///
 323             /// You see that, initially, *any* bound value is free,
 324             /// because we've not traversed any binders. As we pass
 325             /// through a binder, we shift the `outer_index` by 1 to
 326             /// account for the new binder that encloses us.
 327             outer_index: ty::DebruijnIndex,
 328             callback: F,
 329         }
 330
 331         impl<'tcx, F> TypeVisitor<'tcx> for RegionVisitor<F>
 332             where F: FnMut(ty::Region<'tcx>) -> bool
 333         {
 334             fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
 335                 self.outer_index.shift_in(1);
 336                 let result = t.skip_binder().visit_with(self);
 337                 self.outer_index.shift_out(1);
 338                 result
 339             }
 340
 341             fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
 342                 match *r {
 343                     ty::ReLateBound(debruijn, _) if debruijn < self.outer_index => {
 344                         false // ignore bound regions, keep visiting
 345                     }
 346                     _ => (self.callback)(r),
 347                 }
 348             }
 349
 350             fn visit_ty(&mut self, ty: Ty<'tcx>) -> bool {
 351                 // We're only interested in types involving regions
 352                 if ty.flags.intersects(TypeFlags::HAS_FREE_REGIONS) {
 353                     ty.super_visit_with(self)
 354                 } else {
 355                     false // keep visiting
 356                 }
 357             }
 358         }
 359     }
 360 }
 361
 362 /// Folds over the substructure of a type, visiting its component
 363 /// types and all regions that occur *free* within it.
 364 ///
 365 /// That is, `Ty` can contain function or method types that bind
 366 /// regions at the call site (`ReLateBound`), and occurrences of
 367 /// regions (aka "lifetimes") that are bound within a type are not
 368 /// visited by this folder; only regions that occur free will be
 369 /// visited by `fld_r`.
 370
 371 pub struct RegionFolder<'a, 'tcx> {
 372     tcx: TyCtxt<'tcx>,
 373     skipped_regions: &'a mut bool,
 374
 375     /// Stores the index of a binder *just outside* the stuff we have
 376     /// visited.  So this begins as INNERMOST; when we pass through a
 377     /// binder, it is incremented (via `shift_in`).
 378     current_index: ty::DebruijnIndex,
 379
 380     /// Callback invokes for each free region. The `DebruijnIndex`
 381     /// points to the binder *just outside* the ones we have passed
 382     /// through.
 383     fold_region_fn:
 384         &'a mut (dyn FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx> + 'a),
 385 }
 386
 387 impl<'a, 'tcx> RegionFolder<'a, 'tcx> {
 388     #[inline]
 389     pub fn new(
 390         tcx: TyCtxt<'tcx>,
 391         skipped_regions: &'a mut bool,
 392         fold_region_fn: &'a mut dyn FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
 393     ) -> RegionFolder<'a, 'tcx> {
 394         RegionFolder {
 395             tcx,
 396             skipped_regions,
 397             current_index: ty::INNERMOST,
 398             fold_region_fn,
 399         }
 400     }
 401 }
 402
 403 impl<'a, 'tcx> TypeFolder<'tcx> for RegionFolder<'a, 'tcx> {
 404     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 405         self.tcx
 406         }
 407
 408     fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
 409         self.current_index.shift_in(1);
 410         let t = t.super_fold_with(self);
 411         self.current_index.shift_out(1);
 412         t
 413     }
 414
 415     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 416         match *r {
 417             ty::ReLateBound(debruijn, _) if debruijn < self.current_index => {
 418                 debug!("RegionFolder.fold_region({:?}) skipped bound region (current index={:?})",
 419                        r, self.current_index);
 420                 *self.skipped_regions = true;
 421                 r
 422             }
 423             _ => {
 424                 debug!("RegionFolder.fold_region({:?}) folding free region (current_index={:?})",
 425                        r, self.current_index);
 426                 (self.fold_region_fn)(r, self.current_index)
 427             }
 428         }
 429     }
 430 }
 431
 432 ///////////////////////////////////////////////////////////////////////////
 433 // Bound vars replacer
 434
 435 /// Replaces the escaping bound vars (late bound regions or bound types) in a type.
 436 struct BoundVarReplacer<'a, 'tcx> {
 437     tcx: TyCtxt<'tcx>,
 438
 439     /// As with `RegionFolder`, represents the index of a binder *just outside*
 440     /// the ones we have visited.
 441     current_index: ty::DebruijnIndex,
 442
 443     fld_r: &'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a),
 444     fld_t: &'a mut (dyn FnMut(ty::BoundTy) -> Ty<'tcx> + 'a),
 445     fld_c: &'a mut (dyn FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx> + 'a),
 446 }
 447
 448 impl<'a, 'tcx> BoundVarReplacer<'a, 'tcx> {
 449     fn new<F, G, H>(tcx: TyCtxt<'tcx>, fld_r: &'a mut F, fld_t: &'a mut G, fld_c: &'a mut H) -> Self
 450     where
 451         F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
 452         G: FnMut(ty::BoundTy) -> Ty<'tcx>,
 453         H: FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx>,
 454     {
 455         BoundVarReplacer {
 456             tcx,
 457             current_index: ty::INNERMOST,
 458             fld_r,
 459             fld_t,
 460             fld_c,
 461         }
 462     }
 463 }
 464
 465 impl<'a, 'tcx> TypeFolder<'tcx> for BoundVarReplacer<'a, 'tcx> {
 466     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 467         self.tcx
 468         }
 469
 470     fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
 471         self.current_index.shift_in(1);
 472         let t = t.super_fold_with(self);
 473         self.current_index.shift_out(1);
 474         t
 475     }
 476
 477     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 478         match t.kind {
 479             ty::Bound(debruijn, bound_ty) => {
 480                 if debruijn == self.current_index {
 481                     let fld_t = &mut self.fld_t;
 482                     let ty = fld_t(bound_ty);
 483                     ty::fold::shift_vars(
 484                         self.tcx,
 485                         &ty,
 486                         self.current_index.as_u32()
 487                     )
 488                 } else {
 489                     t
 490                 }
 491             }
 492             _ => {
 493                 if !t.has_vars_bound_at_or_above(self.current_index) {
 494                     // Nothing more to substitute.
 495                     t
 496                 } else {
 497                     t.super_fold_with(self)
 498                 }
 499             }
 500         }
 501     }
 502
 503     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 504         match *r {
 505             ty::ReLateBound(debruijn, br) if debruijn == self.current_index => {
 506                 let fld_r = &mut self.fld_r;
 507                 let region = fld_r(br);
 508                 if let ty::ReLateBound(debruijn1, br) = *region {
 509                     // If the callback returns a late-bound region,
 510                     // that region should always use the INNERMOST
 511                     // debruijn index. Then we adjust it to the
 512                     // correct depth.
 513                     assert_eq!(debruijn1, ty::INNERMOST);
 514                     self.tcx.mk_region(ty::ReLateBound(debruijn, br))
 515                 } else {
 516                     region
 517                 }
 518             }
 519             _ => r
 520         }
 521     }
 522
 523     fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 524         if let ty::Const { val: ConstValue::Bound(debruijn, bound_const), ty } = *ct {
 525             if debruijn == self.current_index {
 526                 let fld_c = &mut self.fld_c;
 527                 let ct = fld_c(bound_const, ty);
 528                 ty::fold::shift_vars(
 529                     self.tcx,
 530                     &ct,
 531                     self.current_index.as_u32()
 532                 )
 533             } else {
 534                 ct
 535             }
 536         } else {
 537             if !ct.has_vars_bound_at_or_above(self.current_index) {
 538                 // Nothing more to substitute.
 539                 ct
 540             } else {
 541                 ct.super_fold_with(self)
 542             }
 543         }
 544     }
 545 }
 546
 547 impl<'tcx> TyCtxt<'tcx> {
 548     /// Replaces all regions bound by the given `Binder` with the
 549     /// results returned by the closure; the closure is expected to
 550     /// return a free region (relative to this binder), and hence the
 551     /// binder is removed in the return type. The closure is invoked
 552     /// once for each unique `BoundRegion`; multiple references to the
 553     /// same `BoundRegion` will reuse the previous result. A map is
 554     /// returned at the end with each bound region and the free region
 555     /// that replaced it.
 556     ///
 557     /// This method only replaces late bound regions and the result may still
 558     /// contain escaping bound types.
 559     pub fn replace_late_bound_regions<T, F>(
 560         self,
 561         value: &Binder<T>,
 562         fld_r: F
 563     ) -> (T, BTreeMap<ty::BoundRegion, ty::Region<'tcx>>)
 564         where F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
 565               T: TypeFoldable<'tcx>
 566     {
 567         // identity for bound types and consts
 568         let fld_t = |bound_ty| self.mk_ty(ty::Bound(ty::INNERMOST, bound_ty));
 569         let fld_c = |bound_ct, ty| {
 570             self.mk_const(ty::Const {
 571                 val: ConstValue::Bound(ty::INNERMOST, bound_ct),
 572                 ty,
 573             })
 574         };
 575         self.replace_escaping_bound_vars(value.skip_binder(), fld_r, fld_t, fld_c)
 576     }
 577
 578     /// Replaces all escaping bound vars. The `fld_r` closure replaces escaping
 579     /// bound regions; the `fld_t` closure replaces escaping bound types and the `fld_c`
 580     /// closure replaces escaping bound consts.
 581     pub fn replace_escaping_bound_vars<T, F, G, H>(
 582         self,
 583         value: &T,
 584         mut fld_r: F,
 585         mut fld_t: G,
 586         mut fld_c: H,
 587     ) -> (T, BTreeMap<ty::BoundRegion, ty::Region<'tcx>>)
 588         where F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
 589               G: FnMut(ty::BoundTy) -> Ty<'tcx>,
 590               H: FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx>,
 591               T: TypeFoldable<'tcx>,
 592     {
 593         use rustc_data_structures::fx::FxHashMap;
 594
 595         let mut region_map = BTreeMap::new();
 596         let mut type_map = FxHashMap::default();
 597         let mut const_map = FxHashMap::default();
 598
 599         if !value.has_escaping_bound_vars() {
 600             (value.clone(), region_map)
 601         } else {
 602             let mut real_fld_r = |br| {
 603                 *region_map.entry(br).or_insert_with(|| fld_r(br))
 604             };
 605
 606             let mut real_fld_t = |bound_ty| {
 607                 *type_map.entry(bound_ty).or_insert_with(|| fld_t(bound_ty))
 608             };
 609
 610             let mut real_fld_c = |bound_ct, ty| {
 611                 *const_map.entry(bound_ct).or_insert_with(|| fld_c(bound_ct, ty))
 612             };
 613
 614             let mut replacer = BoundVarReplacer::new(
 615                 self,
 616                 &mut real_fld_r,
 617                 &mut real_fld_t,
 618                 &mut real_fld_c,
 619             );
 620             let result = value.fold_with(&mut replacer);
 621             (result, region_map)
 622         }
 623     }
 624
 625     /// Replaces all types or regions bound by the given `Binder`. The `fld_r`
 626     /// closure replaces bound regions while the `fld_t` closure replaces bound
 627     /// types.
 628     pub fn replace_bound_vars<T, F, G, H>(
 629         self,
 630         value: &Binder<T>,
 631         fld_r: F,
 632         fld_t: G,
 633         fld_c: H,
 634     ) -> (T, BTreeMap<ty::BoundRegion, ty::Region<'tcx>>)
 635         where F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
 636               G: FnMut(ty::BoundTy) -> Ty<'tcx>,
 637               H: FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx>,
 638               T: TypeFoldable<'tcx>
 639     {
 640         self.replace_escaping_bound_vars(value.skip_binder(), fld_r, fld_t, fld_c)
 641     }
 642
 643     /// Replaces any late-bound regions bound in `value` with
 644     /// free variants attached to `all_outlive_scope`.
 645     pub fn liberate_late_bound_regions<T>(
 646         &self,
 647         all_outlive_scope: DefId,
 648         value: &ty::Binder<T>
 649     ) -> T
 650     where T: TypeFoldable<'tcx> {
 651         self.replace_late_bound_regions(value, |br| {
 652             self.mk_region(ty::ReFree(ty::FreeRegion {
 653                 scope: all_outlive_scope,
 654                 bound_region: br
 655             }))
 656         }).0
 657     }
 658
 659     /// Returns a set of all late-bound regions that are constrained
 660     /// by `value`, meaning that if we instantiate those LBR with
 661     /// variables and equate `value` with something else, those
 662     /// variables will also be equated.
 663     pub fn collect_constrained_late_bound_regions<T>(&self, value: &Binder<T>)
 664                                                      -> FxHashSet<ty::BoundRegion>
 665         where T : TypeFoldable<'tcx>
 666     {
 667         self.collect_late_bound_regions(value, true)
 668     }
 669
 670     /// Returns a set of all late-bound regions that appear in `value` anywhere.
 671     pub fn collect_referenced_late_bound_regions<T>(&self, value: &Binder<T>)
 672                                                     -> FxHashSet<ty::BoundRegion>
 673         where T : TypeFoldable<'tcx>
 674     {
 675         self.collect_late_bound_regions(value, false)
 676     }
 677
 678     fn collect_late_bound_regions<T>(&self, value: &Binder<T>, just_constraint: bool)
 679                                      -> FxHashSet<ty::BoundRegion>
 680         where T : TypeFoldable<'tcx>
 681     {
 682         let mut collector = LateBoundRegionsCollector::new(just_constraint);
 683         let result = value.skip_binder().visit_with(&mut collector);
 684         assert!(!result); // should never have stopped early
 685         collector.regions
 686     }
 687
 688     /// Replaces any late-bound regions bound in `value` with `'erased`. Useful in codegen but also
 689     /// method lookup and a few other places where precise region relationships are not required.
 690     pub fn erase_late_bound_regions<T>(self, value: &Binder<T>) -> T
 691         where T : TypeFoldable<'tcx>
 692     {
 693         self.replace_late_bound_regions(value, |_| self.lifetimes.re_erased).0
 694     }
 695
 696     /// Rewrite any late-bound regions so that they are anonymous. Region numbers are
 697     /// assigned starting at 1 and increasing monotonically in the order traversed
 698     /// by the fold operation.
 699     ///
 700     /// The chief purpose of this function is to canonicalize regions so that two
 701     /// `FnSig`s or `TraitRef`s which are equivalent up to region naming will become
 702     /// structurally identical. For example, `for<'a, 'b> fn(&'a isize, &'b isize)` and
 703     /// `for<'a, 'b> fn(&'b isize, &'a isize)` will become identical after anonymization.
 704     pub fn anonymize_late_bound_regions<T>(self, sig: &Binder<T>) -> Binder<T>
 705         where T : TypeFoldable<'tcx>,
 706     {
 707         let mut counter = 0;
 708         Binder::bind(self.replace_late_bound_regions(sig, |_| {
 709             counter += 1;
 710             self.mk_region(ty::ReLateBound(ty::INNERMOST, ty::BrAnon(counter)))
 711         }).0)
 712     }
 713 }
 714
 715 ///////////////////////////////////////////////////////////////////////////
 716 // Shifter
 717 //
 718 // Shifts the De Bruijn indices on all escaping bound vars by a
 719 // fixed amount. Useful in substitution or when otherwise introducing
 720 // a binding level that is not intended to capture the existing bound
 721 // vars. See comment on `shift_vars_through_binders` method in
 722 // `subst.rs` for more details.
 723
 724 enum Direction {
 725     In,
 726     Out,
 727 }
 728
 729 struct Shifter<'tcx> {
 730     tcx: TyCtxt<'tcx>,
 731     current_index: ty::DebruijnIndex,
 732     amount: u32,
 733     direction: Direction,
 734 }
 735
 736 impl Shifter<'tcx> {
 737     pub fn new(tcx: TyCtxt<'tcx>, amount: u32, direction: Direction) -> Self {
 738         Shifter {
 739             tcx,
 740             current_index: ty::INNERMOST,
 741             amount,
 742             direction,
 743         }
 744     }
 745 }
 746
 747 impl TypeFolder<'tcx> for Shifter<'tcx> {
 748     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 749         self.tcx
 750         }
 751
 752     fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
 753         self.current_index.shift_in(1);
 754         let t = t.super_fold_with(self);
 755         self.current_index.shift_out(1);
 756         t
 757     }
 758
 759     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 760         match *r {
 761             ty::ReLateBound(debruijn, br) => {
 762                 if self.amount == 0 || debruijn < self.current_index {
 763                     r
 764                 } else {
 765                     let debruijn = match self.direction {
 766                         Direction::In => debruijn.shifted_in(self.amount),
 767                         Direction::Out => {
 768                             assert!(debruijn.as_u32() >= self.amount);
 769                             debruijn.shifted_out(self.amount)
 770                         }
 771                     };
 772                     let shifted = ty::ReLateBound(debruijn, br);
 773                     self.tcx.mk_region(shifted)
 774                 }
 775             }
 776             _ => r
 777         }
 778     }
 779
 780     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 781         match ty.kind {
 782             ty::Bound(debruijn, bound_ty) => {
 783                 if self.amount == 0 || debruijn < self.current_index {
 784                     ty
 785                 } else {
 786                     let debruijn = match self.direction {
 787                         Direction::In => debruijn.shifted_in(self.amount),
 788                         Direction::Out => {
 789                             assert!(debruijn.as_u32() >= self.amount);
 790                             debruijn.shifted_out(self.amount)
 791                         }
 792                     };
 793                     self.tcx.mk_ty(
 794                         ty::Bound(debruijn, bound_ty)
 795                     )
 796                 }
 797             }
 798
 799             _ => ty.super_fold_with(self),
 800         }
 801     }
 802
 803     fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 804         if let ty::Const { val: ConstValue::Bound(debruijn, bound_ct), ty } = *ct {
 805             if self.amount == 0 || debruijn < self.current_index {
 806                 ct
 807             } else {
 808                 let debruijn = match self.direction {
 809                     Direction::In => debruijn.shifted_in(self.amount),
 810                     Direction::Out => {
 811                         assert!(debruijn.as_u32() >= self.amount);
 812                         debruijn.shifted_out(self.amount)
 813                     }
 814                 };
 815                 self.tcx.mk_const(ty::Const {
 816                     val: ConstValue::Bound(debruijn, bound_ct),
 817                     ty,
 818                 })
 819             }
 820         } else {
 821             ct.super_fold_with(self)
 822         }
 823     }
 824 }
 825
 826 pub fn shift_region<'tcx>(
 827     tcx: TyCtxt<'tcx>,
 828     region: ty::Region<'tcx>,
 829     amount: u32,
 830 ) -> ty::Region<'tcx> {
 831     match region {
 832         ty::ReLateBound(debruijn, br) if amount > 0 => {
 833             tcx.mk_region(ty::ReLateBound(debruijn.shifted_in(amount), *br))
 834         }
 835         _ => {
 836             region
 837         }
 838     }
 839 }
 840
 841 pub fn shift_vars<'tcx, T>(tcx: TyCtxt<'tcx>, value: &T, amount: u32) -> T
 842 where
 843     T: TypeFoldable<'tcx>,
 844 {
 845     debug!("shift_vars(value={:?}, amount={})",
 846            value, amount);
 847
 848     value.fold_with(&mut Shifter::new(tcx, amount, Direction::In))
 849 }
 850
 851 pub fn shift_out_vars<'tcx, T>(tcx: TyCtxt<'tcx>, value: &T, amount: u32) -> T
 852 where
 853     T: TypeFoldable<'tcx>,
 854 {
 855     debug!("shift_out_vars(value={:?}, amount={})",
 856            value, amount);
 857
 858     value.fold_with(&mut Shifter::new(tcx, amount, Direction::Out))
 859 }
 860
 861 /// An "escaping var" is a bound var whose binder is not part of `t`. A bound var can be a
 862 /// bound region or a bound type.
 863 ///
 864 /// So, for example, consider a type like the following, which has two binders:
 865 ///
 866 ///    for<'a> fn(x: for<'b> fn(&'a isize, &'b isize))
 867 ///    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ outer scope
 868 ///                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~  inner scope
 869 ///
 870 /// This type has *bound regions* (`'a`, `'b`), but it does not have escaping regions, because the
 871 /// binders of both `'a` and `'b` are part of the type itself. However, if we consider the *inner
 872 /// fn type*, that type has an escaping region: `'a`.
 873 ///
 874 /// Note that what I'm calling an "escaping var" is often just called a "free var". However,
 875 /// we already use the term "free var". It refers to the regions or types that we use to represent
 876 /// bound regions or type params on a fn definition while we are type checking its body.
 877 ///
 878 /// To clarify, conceptually there is no particular difference between
 879 /// an "escaping" var and a "free" var. However, there is a big
 880 /// difference in practice. Basically, when "entering" a binding
 881 /// level, one is generally required to do some sort of processing to
 882 /// a bound var, such as replacing it with a fresh/placeholder
 883 /// var, or making an entry in the environment to represent the
 884 /// scope to which it is attached, etc. An escaping var represents
 885 /// a bound var for which this processing has not yet been done.
 886 struct HasEscapingVarsVisitor {
 887     /// Anything bound by `outer_index` or "above" is escaping.
 888     outer_index: ty::DebruijnIndex,
 889 }
 890
 891 impl<'tcx> TypeVisitor<'tcx> for HasEscapingVarsVisitor {
 892     fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
 893         self.outer_index.shift_in(1);
 894         let result = t.super_visit_with(self);
 895         self.outer_index.shift_out(1);
 896         result
 897     }
 898
 899     fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
 900         // If the outer-exclusive-binder is *strictly greater* than
 901         // `outer_index`, that means that `t` contains some content
 902         // bound at `outer_index` or above (because
 903         // `outer_exclusive_binder` is always 1 higher than the
 904         // content in `t`). Therefore, `t` has some escaping vars.
 905         t.outer_exclusive_binder > self.outer_index
 906     }
 907
 908     fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
 909         // If the region is bound by `outer_index` or anything outside
 910         // of outer index, then it escapes the binders we have
 911         // visited.
 912         r.bound_at_or_above_binder(self.outer_index)
 913     }
 914
 915     fn visit_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> bool {
 916         // we don't have a `visit_infer_const` callback, so we have to
 917         // hook in here to catch this case (annoying...), but
 918         // otherwise we do want to remember to visit the rest of the
 919         // const, as it has types/regions embedded in a lot of other
 920         // places.
 921         match ct.val {
 922             ConstValue::Bound(debruijn, _) if debruijn >= self.outer_index => true,
 923             _ => ct.super_visit_with(self),
 924         }
 925     }
 926 }
 927
 928 // FIXME: Optimize for checking for infer flags
 929 struct HasTypeFlagsVisitor {
 930     flags: ty::TypeFlags,
 931 }
 932
 933 impl<'tcx> TypeVisitor<'tcx> for HasTypeFlagsVisitor {
 934     fn visit_ty(&mut self, t: Ty<'_>) -> bool {
 935         debug!("HasTypeFlagsVisitor: t={:?} t.flags={:?} self.flags={:?}", t, t.flags, self.flags);
 936         t.flags.intersects(self.flags)
 937     }
 938
 939     fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
 940         let flags = r.type_flags();
 941         debug!("HasTypeFlagsVisitor: r={:?} r.flags={:?} self.flags={:?}", r, flags, self.flags);
 942         flags.intersects(self.flags)
 943     }
 944
 945     fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> bool {
 946         let flags = FlagComputation::for_const(c);
 947         debug!("HasTypeFlagsVisitor: c={:?} c.flags={:?} self.flags={:?}", c, flags, self.flags);
 948         flags.intersects(self.flags)
 949     }
 950 }
 951
 952 /// Collects all the late-bound regions at the innermost binding level
 953 /// into a hash set.
 954 struct LateBoundRegionsCollector {
 955     current_index: ty::DebruijnIndex,
 956     regions: FxHashSet<ty::BoundRegion>,
 957
 958     /// `true` if we only want regions that are known to be
 959     /// "constrained" when you equate this type with another type. In
 960     /// particular, if you have e.g., `&'a u32` and `&'b u32`, equating
 961     /// them constraints `'a == 'b`. But if you have `<&'a u32 as
 962     /// Trait>::Foo` and `<&'b u32 as Trait>::Foo`, normalizing those
 963     /// types may mean that `'a` and `'b` don't appear in the results,
 964     /// so they are not considered *constrained*.
 965     just_constrained: bool,
 966 }
 967
 968 impl LateBoundRegionsCollector {
 969     fn new(just_constrained: bool) -> Self {
 970         LateBoundRegionsCollector {
 971             current_index: ty::INNERMOST,
 972             regions: Default::default(),
 973             just_constrained,
 974         }
 975     }
 976 }
 977
 978 impl<'tcx> TypeVisitor<'tcx> for LateBoundRegionsCollector {
 979     fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
 980         self.current_index.shift_in(1);
 981         let result = t.super_visit_with(self);
 982         self.current_index.shift_out(1);
 983         result
 984     }
 985
 986     fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
 987         // if we are only looking for "constrained" region, we have to
 988         // ignore the inputs to a projection, as they may not appear
 989         // in the normalized form
 990         if self.just_constrained {
 991             match t.kind {
 992                 ty::Projection(..) | ty::Opaque(..) => { return false; }
 993                 _ => { }
 994             }
 995         }
 996
 997         t.super_visit_with(self)
 998     }
 999
1000     fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
1001         if let ty::ReLateBound(debruijn, br) = *r {
1002              if debruijn == self.current_index {
1003                 self.regions.insert(br);
1004             }
1005         }
1006         false
1007     }
1008 }