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