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