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