src/librustc/ty/fold.rs

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