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::mir;
  34 use crate::ty::{self, flags::FlagComputation, Binder, Ty, TyCtxt, TypeFlags};
  35 use rustc_hir as hir;
  36 use rustc_hir::def_id::DefId;
  37
  38 use rustc_data_structures::fx::FxHashSet;
  39 use rustc_data_structures::sso::SsoHashSet;
  40 use std::collections::BTreeMap;
  41 use std::fmt;
  42 use std::ops::ControlFlow;
  43
  44 /// This trait is implemented for every type that can be folded.
  45 /// Basically, every type that has a corresponding method in `TypeFolder`.
  46 ///
  47 /// To implement this conveniently, use the derive macro located in `rustc_macros`.
  48 pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
  49     fn super_fold_with<F: TypeFolder<'tcx>>(self, folder: &mut F) -> Self;
  50     fn fold_with<F: TypeFolder<'tcx>>(self, folder: &mut F) -> Self {
  51         self.super_fold_with(folder)
  52     }
  53
  54     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy>;
  55     fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
  56         self.super_visit_with(visitor)
  57     }
  58
  59     /// Returns `true` if `self` has any late-bound regions that are either
  60     /// bound by `binder` or bound by some binder outside of `binder`.
  61     /// If `binder` is `ty::INNERMOST`, this indicates whether
  62     /// there are any late-bound regions that appear free.
  63     fn has_vars_bound_at_or_above(&self, binder: ty::DebruijnIndex) -> bool {
  64         self.visit_with(&mut HasEscapingVarsVisitor { outer_index: binder }).is_break()
  65     }
  66
  67     /// Returns `true` if this `self` has any regions that escape `binder` (and
  68     /// hence are not bound by it).
  69     fn has_vars_bound_above(&self, binder: ty::DebruijnIndex) -> bool {
  70         self.has_vars_bound_at_or_above(binder.shifted_in(1))
  71     }
  72
  73     fn has_escaping_bound_vars(&self) -> bool {
  74         self.has_vars_bound_at_or_above(ty::INNERMOST)
  75     }
  76
  77     fn definitely_has_type_flags(&self, tcx: TyCtxt<'tcx>, flags: TypeFlags) -> bool {
  78         self.visit_with(&mut HasTypeFlagsVisitor { tcx: Some(tcx), flags }).break_value()
  79             == Some(FoundFlags)
  80     }
  81
  82     fn has_type_flags(&self, flags: TypeFlags) -> bool {
  83         self.visit_with(&mut HasTypeFlagsVisitor { tcx: None, flags }).break_value()
  84             == Some(FoundFlags)
  85     }
  86     fn has_projections(&self) -> bool {
  87         self.has_type_flags(TypeFlags::HAS_PROJECTION)
  88     }
  89     fn has_opaque_types(&self) -> bool {
  90         self.has_type_flags(TypeFlags::HAS_TY_OPAQUE)
  91     }
  92     fn references_error(&self) -> bool {
  93         self.has_type_flags(TypeFlags::HAS_ERROR)
  94     }
  95     fn potentially_has_param_types_or_consts(&self) -> bool {
  96         self.has_type_flags(
  97             TypeFlags::HAS_KNOWN_TY_PARAM
  98                 | TypeFlags::HAS_KNOWN_CT_PARAM
  99                 | TypeFlags::HAS_UNKNOWN_DEFAULT_CONST_SUBSTS,
 100         )
 101     }
 102     fn definitely_has_param_types_or_consts(&self, tcx: TyCtxt<'tcx>) -> bool {
 103         self.definitely_has_type_flags(
 104             tcx,
 105             TypeFlags::HAS_KNOWN_TY_PARAM | TypeFlags::HAS_KNOWN_CT_PARAM,
 106         )
 107     }
 108     fn has_infer_regions(&self) -> bool {
 109         self.has_type_flags(TypeFlags::HAS_RE_INFER)
 110     }
 111     fn has_infer_types(&self) -> bool {
 112         self.has_type_flags(TypeFlags::HAS_TY_INFER)
 113     }
 114     fn has_infer_types_or_consts(&self) -> bool {
 115         self.has_type_flags(TypeFlags::HAS_TY_INFER | TypeFlags::HAS_CT_INFER)
 116     }
 117     fn needs_infer(&self) -> bool {
 118         self.has_type_flags(TypeFlags::NEEDS_INFER)
 119     }
 120     fn has_placeholders(&self) -> bool {
 121         self.has_type_flags(
 122             TypeFlags::HAS_RE_PLACEHOLDER
 123                 | TypeFlags::HAS_TY_PLACEHOLDER
 124                 | TypeFlags::HAS_CT_PLACEHOLDER,
 125         )
 126     }
 127     fn potentially_needs_subst(&self) -> bool {
 128         self.has_type_flags(
 129             TypeFlags::KNOWN_NEEDS_SUBST | TypeFlags::HAS_UNKNOWN_DEFAULT_CONST_SUBSTS,
 130         )
 131     }
 132     fn definitely_needs_subst(&self, tcx: TyCtxt<'tcx>) -> bool {
 133         self.definitely_has_type_flags(tcx, TypeFlags::KNOWN_NEEDS_SUBST)
 134     }
 135     /// "Free" regions in this context means that it has any region
 136     /// that is not (a) erased or (b) late-bound.
 137     fn has_free_regions(&self, tcx: TyCtxt<'tcx>) -> bool {
 138         self.definitely_has_type_flags(tcx, TypeFlags::HAS_KNOWN_FREE_REGIONS)
 139     }
 140
 141     fn has_erased_regions(&self) -> bool {
 142         self.has_type_flags(TypeFlags::HAS_RE_ERASED)
 143     }
 144
 145     /// True if there are any un-erased free regions.
 146     fn has_erasable_regions(&self, tcx: TyCtxt<'tcx>) -> bool {
 147         self.definitely_has_type_flags(tcx, TypeFlags::HAS_KNOWN_FREE_REGIONS)
 148     }
 149
 150     /// Indicates whether this value definitely references only 'global'
 151     /// generic parameters that are the same regardless of what fn we are
 152     /// in. This is used for caching.
 153     ///
 154     /// Note that this function is pessimistic and may incorrectly return
 155     /// `false`.
 156     fn is_known_global(&self) -> bool {
 157         !self.has_type_flags(TypeFlags::HAS_POTENTIAL_FREE_LOCAL_NAMES)
 158     }
 159
 160     /// Indicates whether this value references only 'global'
 161     /// generic parameters that are the same regardless of what fn we are
 162     /// in. This is used for caching.
 163     fn is_global(&self, tcx: TyCtxt<'tcx>) -> bool {
 164         !self.definitely_has_type_flags(tcx, TypeFlags::HAS_KNOWN_FREE_LOCAL_NAMES)
 165     }
 166
 167     /// True if there are any late-bound regions
 168     fn has_late_bound_regions(&self) -> bool {
 169         self.has_type_flags(TypeFlags::HAS_RE_LATE_BOUND)
 170     }
 171
 172     /// Indicates whether this value still has parameters/placeholders/inference variables
 173     /// which could be replaced later, in a way that would change the results of `impl`
 174     /// specialization.
 175     fn still_further_specializable(&self) -> bool {
 176         self.has_type_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE)
 177     }
 178 }
 179
 180 impl TypeFoldable<'tcx> for hir::Constness {
 181     fn super_fold_with<F: TypeFolder<'tcx>>(self, _: &mut F) -> Self {
 182         self
 183     }
 184     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
 185         ControlFlow::CONTINUE
 186     }
 187 }
 188
 189 /// The `TypeFolder` trait defines the actual *folding*. There is a
 190 /// method defined for every foldable type. Each of these has a
 191 /// default implementation that does an "identity" fold. Within each
 192 /// identity fold, it should invoke `foo.fold_with(self)` to fold each
 193 /// sub-item.
 194 pub trait TypeFolder<'tcx>: Sized {
 195     fn tcx<'a>(&'a self) -> TyCtxt<'tcx>;
 196
 197     fn fold_binder<T>(&mut self, t: Binder<'tcx, T>) -> Binder<'tcx, T>
 198     where
 199         T: TypeFoldable<'tcx>,
 200     {
 201         t.super_fold_with(self)
 202     }
 203
 204     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 205         t.super_fold_with(self)
 206     }
 207
 208     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 209         r.super_fold_with(self)
 210     }
 211
 212     fn fold_const(&mut self, c: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 213         c.super_fold_with(self)
 214     }
 215
 216     fn fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
 217         p.super_fold_with(self)
 218     }
 219
 220     fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
 221         bug!("most type folders should not be folding MIR datastructures: {:?}", c)
 222     }
 223 }
 224
 225 pub trait TypeVisitor<'tcx>: Sized {
 226     type BreakTy = !;
 227     /// Supplies the `tcx` for an unevaluated anonymous constant in case its default substs
 228     /// are not yet supplied.
 229     ///
 230     /// Returning `None` for this method is only recommended if the `TypeVisitor`
 231     /// does not care about default anon const substs, as it ignores generic parameters,
 232     /// and fetching the default substs would cause a query cycle.
 233     ///
 234     /// For visitors which return `None` we completely skip the default substs in `ty::Unevaluated::super_visit_with`.
 235     /// This means that incorrectly returning `None` can very quickly lead to ICE or other critical bugs, so be careful and
 236     /// try to return an actual `tcx` if possible.
 237     fn tcx_for_anon_const_substs(&self) -> Option<TyCtxt<'tcx>>;
 238
 239     fn visit_binder<T: TypeFoldable<'tcx>>(
 240         &mut self,
 241         t: &Binder<'tcx, T>,
 242     ) -> ControlFlow<Self::BreakTy> {
 243         t.super_visit_with(self)
 244     }
 245
 246     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 247         t.super_visit_with(self)
 248     }
 249
 250     fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
 251         r.super_visit_with(self)
 252     }
 253
 254     fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
 255         c.super_visit_with(self)
 256     }
 257
 258     fn visit_unevaluated_const(&mut self, uv: ty::Unevaluated<'tcx>) -> ControlFlow<Self::BreakTy> {
 259         uv.super_visit_with(self)
 260     }
 261
 262     fn visit_predicate(&mut self, p: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
 263         p.super_visit_with(self)
 264     }
 265 }
 266
 267 ///////////////////////////////////////////////////////////////////////////
 268 // Some sample folders
 269
 270 pub struct BottomUpFolder<'tcx, F, G, H>
 271 where
 272     F: FnMut(Ty<'tcx>) -> Ty<'tcx>,
 273     G: FnMut(ty::Region<'tcx>) -> ty::Region<'tcx>,
 274     H: FnMut(&'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx>,
 275 {
 276     pub tcx: TyCtxt<'tcx>,
 277     pub ty_op: F,
 278     pub lt_op: G,
 279     pub ct_op: H,
 280 }
 281
 282 impl<'tcx, F, G, H> TypeFolder<'tcx> for BottomUpFolder<'tcx, F, G, H>
 283 where
 284     F: FnMut(Ty<'tcx>) -> Ty<'tcx>,
 285     G: FnMut(ty::Region<'tcx>) -> ty::Region<'tcx>,
 286     H: FnMut(&'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx>,
 287 {
 288     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 289         self.tcx
 290     }
 291
 292     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 293         let t = ty.super_fold_with(self);
 294         (self.ty_op)(t)
 295     }
 296
 297     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 298         let r = r.super_fold_with(self);
 299         (self.lt_op)(r)
 300     }
 301
 302     fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 303         let ct = ct.super_fold_with(self);
 304         (self.ct_op)(ct)
 305     }
 306 }
 307
 308 ///////////////////////////////////////////////////////////////////////////
 309 // Region folder
 310
 311 impl<'tcx> TyCtxt<'tcx> {
 312     /// Folds the escaping and free regions in `value` using `f`, and
 313     /// sets `skipped_regions` to true if any late-bound region was found
 314     /// and skipped.
 315     pub fn fold_regions<T>(
 316         self,
 317         value: T,
 318         skipped_regions: &mut bool,
 319         mut f: impl FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
 320     ) -> T
 321     where
 322         T: TypeFoldable<'tcx>,
 323     {
 324         value.fold_with(&mut RegionFolder::new(self, skipped_regions, &mut f))
 325     }
 326
 327     /// Invoke `callback` on every region appearing free in `value`.
 328     pub fn for_each_free_region(
 329         self,
 330         value: &impl TypeFoldable<'tcx>,
 331         mut callback: impl FnMut(ty::Region<'tcx>),
 332     ) {
 333         self.any_free_region_meets(value, |r| {
 334             callback(r);
 335             false
 336         });
 337     }
 338
 339     /// Returns `true` if `callback` returns true for every region appearing free in `value`.
 340     pub fn all_free_regions_meet(
 341         self,
 342         value: &impl TypeFoldable<'tcx>,
 343         mut callback: impl FnMut(ty::Region<'tcx>) -> bool,
 344     ) -> bool {
 345         !self.any_free_region_meets(value, |r| !callback(r))
 346     }
 347
 348     /// Returns `true` if `callback` returns true for some region appearing free in `value`.
 349     pub fn any_free_region_meets(
 350         self,
 351         value: &impl TypeFoldable<'tcx>,
 352         callback: impl FnMut(ty::Region<'tcx>) -> bool,
 353     ) -> bool {
 354         struct RegionVisitor<'tcx, F> {
 355             tcx: TyCtxt<'tcx>,
 356             /// The index of a binder *just outside* the things we have
 357             /// traversed. If we encounter a bound region bound by this
 358             /// binder or one outer to it, it appears free. Example:
 359             ///
 360             /// ```
 361             ///    for<'a> fn(for<'b> fn(), T)
 362             /// ^          ^          ^     ^
 363             /// |          |          |     | here, would be shifted in 1
 364             /// |          |          | here, would be shifted in 2
 365             /// |          | here, would be `INNERMOST` shifted in by 1
 366             /// | here, initially, binder would be `INNERMOST`
 367             /// ```
 368             ///
 369             /// You see that, initially, *any* bound value is free,
 370             /// because we've not traversed any binders. As we pass
 371             /// through a binder, we shift the `outer_index` by 1 to
 372             /// account for the new binder that encloses us.
 373             outer_index: ty::DebruijnIndex,
 374             callback: F,
 375         }
 376
 377         impl<'tcx, F> TypeVisitor<'tcx> for RegionVisitor<'tcx, F>
 378         where
 379             F: FnMut(ty::Region<'tcx>) -> bool,
 380         {
 381             type BreakTy = ();
 382
 383             fn tcx_for_anon_const_substs(&self) -> Option<TyCtxt<'tcx>> {
 384                 Some(self.tcx)
 385             }
 386
 387             fn visit_binder<T: TypeFoldable<'tcx>>(
 388                 &mut self,
 389                 t: &Binder<'tcx, T>,
 390             ) -> ControlFlow<Self::BreakTy> {
 391                 self.outer_index.shift_in(1);
 392                 let result = t.as_ref().skip_binder().visit_with(self);
 393                 self.outer_index.shift_out(1);
 394                 result
 395             }
 396
 397             fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
 398                 match *r {
 399                     ty::ReLateBound(debruijn, _) if debruijn < self.outer_index => {
 400                         ControlFlow::CONTINUE
 401                     }
 402                     _ => {
 403                         if (self.callback)(r) {
 404                             ControlFlow::BREAK
 405                         } else {
 406                             ControlFlow::CONTINUE
 407                         }
 408                     }
 409                 }
 410             }
 411
 412             fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 413                 // We're only interested in types involving regions
 414                 if ty.flags().intersects(TypeFlags::HAS_POTENTIAL_FREE_REGIONS) {
 415                     ty.super_visit_with(self)
 416                 } else {
 417                     ControlFlow::CONTINUE
 418                 }
 419             }
 420         }
 421
 422         value
 423             .visit_with(&mut RegionVisitor { tcx: self, outer_index: ty::INNERMOST, callback })
 424             .is_break()
 425     }
 426 }
 427
 428 /// Folds over the substructure of a type, visiting its component
 429 /// types and all regions that occur *free* within it.
 430 ///
 431 /// That is, `Ty` can contain function or method types that bind
 432 /// regions at the call site (`ReLateBound`), and occurrences of
 433 /// regions (aka "lifetimes") that are bound within a type are not
 434 /// visited by this folder; only regions that occur free will be
 435 /// visited by `fld_r`.
 436
 437 pub struct RegionFolder<'a, 'tcx> {
 438     tcx: TyCtxt<'tcx>,
 439     skipped_regions: &'a mut bool,
 440
 441     /// Stores the index of a binder *just outside* the stuff we have
 442     /// visited.  So this begins as INNERMOST; when we pass through a
 443     /// binder, it is incremented (via `shift_in`).
 444     current_index: ty::DebruijnIndex,
 445
 446     /// Callback invokes for each free region. The `DebruijnIndex`
 447     /// points to the binder *just outside* the ones we have passed
 448     /// through.
 449     fold_region_fn:
 450         &'a mut (dyn FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx> + 'a),
 451 }
 452
 453 impl<'a, 'tcx> RegionFolder<'a, 'tcx> {
 454     #[inline]
 455     pub fn new(
 456         tcx: TyCtxt<'tcx>,
 457         skipped_regions: &'a mut bool,
 458         fold_region_fn: &'a mut dyn FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
 459     ) -> RegionFolder<'a, 'tcx> {
 460         RegionFolder { tcx, skipped_regions, current_index: ty::INNERMOST, fold_region_fn }
 461     }
 462 }
 463
 464 impl<'a, 'tcx> TypeFolder<'tcx> for RegionFolder<'a, 'tcx> {
 465     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 466         self.tcx
 467     }
 468
 469     fn fold_binder<T: TypeFoldable<'tcx>>(
 470         &mut self,
 471         t: ty::Binder<'tcx, T>,
 472     ) -> ty::Binder<'tcx, T> {
 473         self.current_index.shift_in(1);
 474         let t = t.super_fold_with(self);
 475         self.current_index.shift_out(1);
 476         t
 477     }
 478
 479     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 480         match *r {
 481             ty::ReLateBound(debruijn, _) if debruijn < self.current_index => {
 482                 debug!(
 483                     "RegionFolder.fold_region({:?}) skipped bound region (current index={:?})",
 484                     r, self.current_index
 485                 );
 486                 *self.skipped_regions = true;
 487                 r
 488             }
 489             _ => {
 490                 debug!(
 491                     "RegionFolder.fold_region({:?}) folding free region (current_index={:?})",
 492                     r, self.current_index
 493                 );
 494                 (self.fold_region_fn)(r, self.current_index)
 495             }
 496         }
 497     }
 498 }
 499
 500 ///////////////////////////////////////////////////////////////////////////
 501 // Bound vars replacer
 502
 503 /// Replaces the escaping bound vars (late bound regions or bound types) in a type.
 504 struct BoundVarReplacer<'a, 'tcx> {
 505     tcx: TyCtxt<'tcx>,
 506
 507     /// As with `RegionFolder`, represents the index of a binder *just outside*
 508     /// the ones we have visited.
 509     current_index: ty::DebruijnIndex,
 510
 511     fld_r: Option<&'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a)>,
 512     fld_t: Option<&'a mut (dyn FnMut(ty::BoundTy) -> Ty<'tcx> + 'a)>,
 513     fld_c: Option<&'a mut (dyn FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx> + 'a)>,
 514 }
 515
 516 impl<'a, 'tcx> BoundVarReplacer<'a, 'tcx> {
 517     fn new(
 518         tcx: TyCtxt<'tcx>,
 519         fld_r: Option<&'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a)>,
 520         fld_t: Option<&'a mut (dyn FnMut(ty::BoundTy) -> Ty<'tcx> + 'a)>,
 521         fld_c: Option<&'a mut (dyn FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx> + 'a)>,
 522     ) -> Self {
 523         BoundVarReplacer { tcx, current_index: ty::INNERMOST, fld_r, fld_t, fld_c }
 524     }
 525 }
 526
 527 impl<'a, 'tcx> TypeFolder<'tcx> for BoundVarReplacer<'a, 'tcx> {
 528     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 529         self.tcx
 530     }
 531
 532     fn fold_binder<T: TypeFoldable<'tcx>>(
 533         &mut self,
 534         t: ty::Binder<'tcx, T>,
 535     ) -> ty::Binder<'tcx, T> {
 536         self.current_index.shift_in(1);
 537         let t = t.super_fold_with(self);
 538         self.current_index.shift_out(1);
 539         t
 540     }
 541
 542     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 543         match *t.kind() {
 544             ty::Bound(debruijn, bound_ty) if debruijn == self.current_index => {
 545                 if let Some(fld_t) = self.fld_t.as_mut() {
 546                     let ty = fld_t(bound_ty);
 547                     return ty::fold::shift_vars(self.tcx, &ty, self.current_index.as_u32());
 548                 }
 549             }
 550             _ if t.has_vars_bound_at_or_above(self.current_index) => {
 551                 return t.super_fold_with(self);
 552             }
 553             _ => {}
 554         }
 555         t
 556     }
 557
 558     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 559         match *r {
 560             ty::ReLateBound(debruijn, br) if debruijn == self.current_index => {
 561                 if let Some(fld_r) = self.fld_r.as_mut() {
 562                     let region = fld_r(br);
 563                     return if let ty::ReLateBound(debruijn1, br) = *region {
 564                         // If the callback returns a late-bound region,
 565                         // that region should always use the INNERMOST
 566                         // debruijn index. Then we adjust it to the
 567                         // correct depth.
 568                         assert_eq!(debruijn1, ty::INNERMOST);
 569                         self.tcx.mk_region(ty::ReLateBound(debruijn, br))
 570                     } else {
 571                         region
 572                     };
 573                 }
 574             }
 575             _ => {}
 576         }
 577         r
 578     }
 579
 580     fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 581         match *ct {
 582             ty::Const { val: ty::ConstKind::Bound(debruijn, bound_const), ty }
 583                 if debruijn == self.current_index =>
 584             {
 585                 if let Some(fld_c) = self.fld_c.as_mut() {
 586                     let ct = fld_c(bound_const, ty);
 587                     return ty::fold::shift_vars(self.tcx, &ct, self.current_index.as_u32());
 588                 }
 589             }
 590             _ if ct.has_vars_bound_at_or_above(self.current_index) => {
 591                 return ct.super_fold_with(self);
 592             }
 593             _ => {}
 594         }
 595         ct
 596     }
 597 }
 598
 599 impl<'tcx> TyCtxt<'tcx> {
 600     /// Replaces all regions bound by the given `Binder` with the
 601     /// results returned by the closure; the closure is expected to
 602     /// return a free region (relative to this binder), and hence the
 603     /// binder is removed in the return type. The closure is invoked
 604     /// once for each unique `BoundRegionKind`; multiple references to the
 605     /// same `BoundRegionKind` will reuse the previous result. A map is
 606     /// returned at the end with each bound region and the free region
 607     /// that replaced it.
 608     ///
 609     /// This method only replaces late bound regions and the result may still
 610     /// contain escaping bound types.
 611     pub fn replace_late_bound_regions<T, F>(
 612         self,
 613         value: Binder<'tcx, T>,
 614         mut fld_r: F,
 615     ) -> (T, BTreeMap<ty::BoundRegion, ty::Region<'tcx>>)
 616     where
 617         F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
 618         T: TypeFoldable<'tcx>,
 619     {
 620         let mut region_map = BTreeMap::new();
 621         let mut real_fld_r =
 622             |br: ty::BoundRegion| *region_map.entry(br).or_insert_with(|| fld_r(br));
 623         let value = value.skip_binder();
 624         let value = if !value.has_escaping_bound_vars() {
 625             value
 626         } else {
 627             let mut replacer = BoundVarReplacer::new(self, Some(&mut real_fld_r), None, None);
 628             value.fold_with(&mut replacer)
 629         };
 630         (value, region_map)
 631     }
 632
 633     /// Replaces all escaping bound vars. The `fld_r` closure replaces escaping
 634     /// bound regions; the `fld_t` closure replaces escaping bound types and the `fld_c`
 635     /// closure replaces escaping bound consts.
 636     pub fn replace_escaping_bound_vars<T, F, G, H>(
 637         self,
 638         value: T,
 639         mut fld_r: F,
 640         mut fld_t: G,
 641         mut fld_c: H,
 642     ) -> T
 643     where
 644         F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
 645         G: FnMut(ty::BoundTy) -> Ty<'tcx>,
 646         H: FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx>,
 647         T: TypeFoldable<'tcx>,
 648     {
 649         if !value.has_escaping_bound_vars() {
 650             value
 651         } else {
 652             let mut replacer =
 653                 BoundVarReplacer::new(self, Some(&mut fld_r), Some(&mut fld_t), Some(&mut fld_c));
 654             value.fold_with(&mut replacer)
 655         }
 656     }
 657
 658     /// Replaces all types or regions bound by the given `Binder`. The `fld_r`
 659     /// closure replaces bound regions while the `fld_t` closure replaces bound
 660     /// types.
 661     pub fn replace_bound_vars<T, F, G, H>(
 662         self,
 663         value: Binder<'tcx, T>,
 664         mut fld_r: F,
 665         fld_t: G,
 666         fld_c: H,
 667     ) -> (T, BTreeMap<ty::BoundRegion, ty::Region<'tcx>>)
 668     where
 669         F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
 670         G: FnMut(ty::BoundTy) -> Ty<'tcx>,
 671         H: FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx>,
 672         T: TypeFoldable<'tcx>,
 673     {
 674         let mut region_map = BTreeMap::new();
 675         let real_fld_r = |br: ty::BoundRegion| *region_map.entry(br).or_insert_with(|| fld_r(br));
 676         let value = self.replace_escaping_bound_vars(value.skip_binder(), real_fld_r, fld_t, fld_c);
 677         (value, region_map)
 678     }
 679
 680     /// Replaces any late-bound regions bound in `value` with
 681     /// free variants attached to `all_outlive_scope`.
 682     pub fn liberate_late_bound_regions<T>(
 683         self,
 684         all_outlive_scope: DefId,
 685         value: ty::Binder<'tcx, T>,
 686     ) -> T
 687     where
 688         T: TypeFoldable<'tcx>,
 689     {
 690         self.replace_late_bound_regions(value, |br| {
 691             self.mk_region(ty::ReFree(ty::FreeRegion {
 692                 scope: all_outlive_scope,
 693                 bound_region: br.kind,
 694             }))
 695         })
 696         .0
 697     }
 698
 699     pub fn shift_bound_var_indices<T>(self, bound_vars: usize, value: T) -> T
 700     where
 701         T: TypeFoldable<'tcx>,
 702     {
 703         self.replace_escaping_bound_vars(
 704             value,
 705             |r| {
 706                 self.mk_region(ty::ReLateBound(
 707                     ty::INNERMOST,
 708                     ty::BoundRegion {
 709                         var: ty::BoundVar::from_usize(r.var.as_usize() + bound_vars),
 710                         kind: r.kind,
 711                     },
 712                 ))
 713             },
 714             |t| {
 715                 self.mk_ty(ty::Bound(
 716                     ty::INNERMOST,
 717                     ty::BoundTy {
 718                         var: ty::BoundVar::from_usize(t.var.as_usize() + bound_vars),
 719                         kind: t.kind,
 720                     },
 721                 ))
 722             },
 723             |c, ty| {
 724                 self.mk_const(ty::Const {
 725                     val: ty::ConstKind::Bound(
 726                         ty::INNERMOST,
 727                         ty::BoundVar::from_usize(c.as_usize() + bound_vars),
 728                     ),
 729                     ty,
 730                 })
 731             },
 732         )
 733     }
 734
 735     /// Returns a set of all late-bound regions that are constrained
 736     /// by `value`, meaning that if we instantiate those LBR with
 737     /// variables and equate `value` with something else, those
 738     /// variables will also be equated.
 739     pub fn collect_constrained_late_bound_regions<T>(
 740         self,
 741         value: &Binder<'tcx, T>,
 742     ) -> FxHashSet<ty::BoundRegionKind>
 743     where
 744         T: TypeFoldable<'tcx>,
 745     {
 746         self.collect_late_bound_regions(value, true)
 747     }
 748
 749     /// Returns a set of all late-bound regions that appear in `value` anywhere.
 750     pub fn collect_referenced_late_bound_regions<T>(
 751         self,
 752         value: &Binder<'tcx, T>,
 753     ) -> FxHashSet<ty::BoundRegionKind>
 754     where
 755         T: TypeFoldable<'tcx>,
 756     {
 757         self.collect_late_bound_regions(value, false)
 758     }
 759
 760     fn collect_late_bound_regions<T>(
 761         self,
 762         value: &Binder<'tcx, T>,
 763         just_constraint: bool,
 764     ) -> FxHashSet<ty::BoundRegionKind>
 765     where
 766         T: TypeFoldable<'tcx>,
 767     {
 768         let mut collector = LateBoundRegionsCollector::new(self, just_constraint);
 769         let result = value.as_ref().skip_binder().visit_with(&mut collector);
 770         assert!(result.is_continue()); // should never have stopped early
 771         collector.regions
 772     }
 773
 774     /// Replaces any late-bound regions bound in `value` with `'erased`. Useful in codegen but also
 775     /// method lookup and a few other places where precise region relationships are not required.
 776     pub fn erase_late_bound_regions<T>(self, value: Binder<'tcx, T>) -> T
 777     where
 778         T: TypeFoldable<'tcx>,
 779     {
 780         self.replace_late_bound_regions(value, |_| self.lifetimes.re_erased).0
 781     }
 782
 783     /// Rewrite any late-bound regions so that they are anonymous. Region numbers are
 784     /// assigned starting at 0 and increasing monotonically in the order traversed
 785     /// by the fold operation.
 786     ///
 787     /// The chief purpose of this function is to canonicalize regions so that two
 788     /// `FnSig`s or `TraitRef`s which are equivalent up to region naming will become
 789     /// structurally identical. For example, `for<'a, 'b> fn(&'a isize, &'b isize)` and
 790     /// `for<'a, 'b> fn(&'b isize, &'a isize)` will become identical after anonymization.
 791     pub fn anonymize_late_bound_regions<T>(self, sig: Binder<'tcx, T>) -> Binder<'tcx, T>
 792     where
 793         T: TypeFoldable<'tcx>,
 794     {
 795         let mut counter = 0;
 796         let inner = self
 797             .replace_late_bound_regions(sig, |_| {
 798                 let br = ty::BoundRegion {
 799                     var: ty::BoundVar::from_u32(counter),
 800                     kind: ty::BrAnon(counter),
 801                 };
 802                 let r = self.mk_region(ty::ReLateBound(ty::INNERMOST, br));
 803                 counter += 1;
 804                 r
 805             })
 806             .0;
 807         let bound_vars = self.mk_bound_variable_kinds(
 808             (0..counter).map(|i| ty::BoundVariableKind::Region(ty::BrAnon(i))),
 809         );
 810         Binder::bind_with_vars(inner, bound_vars)
 811     }
 812 }
 813
 814 pub struct ValidateBoundVars<'tcx> {
 815     bound_vars: &'tcx ty::List<ty::BoundVariableKind>,
 816     binder_index: ty::DebruijnIndex,
 817     // We may encounter the same variable at different levels of binding, so
 818     // this can't just be `Ty`
 819     visited: SsoHashSet<(ty::DebruijnIndex, Ty<'tcx>)>,
 820 }
 821
 822 impl<'tcx> ValidateBoundVars<'tcx> {
 823     pub fn new(bound_vars: &'tcx ty::List<ty::BoundVariableKind>) -> Self {
 824         ValidateBoundVars {
 825             bound_vars,
 826             binder_index: ty::INNERMOST,
 827             visited: SsoHashSet::default(),
 828         }
 829     }
 830 }
 831
 832 impl<'tcx> TypeVisitor<'tcx> for ValidateBoundVars<'tcx> {
 833     type BreakTy = ();
 834
 835     fn tcx_for_anon_const_substs(&self) -> Option<TyCtxt<'tcx>> {
 836         // Anonymous constants do not contain bound vars in their substs by default.
 837         None
 838     }
 839
 840     fn visit_binder<T: TypeFoldable<'tcx>>(
 841         &mut self,
 842         t: &Binder<'tcx, T>,
 843     ) -> ControlFlow<Self::BreakTy> {
 844         self.binder_index.shift_in(1);
 845         let result = t.super_visit_with(self);
 846         self.binder_index.shift_out(1);
 847         result
 848     }
 849
 850     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 851         if t.outer_exclusive_binder < self.binder_index
 852             || !self.visited.insert((self.binder_index, t))
 853         {
 854             return ControlFlow::BREAK;
 855         }
 856         match *t.kind() {
 857             ty::Bound(debruijn, bound_ty) if debruijn == self.binder_index => {
 858                 if self.bound_vars.len() <= bound_ty.var.as_usize() {
 859                     bug!("Not enough bound vars: {:?} not found in {:?}", t, self.bound_vars);
 860                 }
 861                 let list_var = self.bound_vars[bound_ty.var.as_usize()];
 862                 match list_var {
 863                     ty::BoundVariableKind::Ty(kind) => {
 864                         if kind != bound_ty.kind {
 865                             bug!(
 866                                 "Mismatched type kinds: {:?} doesn't var in list {:?}",
 867                                 bound_ty.kind,
 868                                 list_var
 869                             );
 870                         }
 871                     }
 872                     _ => {
 873                         bug!("Mismatched bound variable kinds! Expected type, found {:?}", list_var)
 874                     }
 875                 }
 876             }
 877
 878             _ => (),
 879         };
 880
 881         t.super_visit_with(self)
 882     }
 883
 884     fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
 885         match r {
 886             ty::ReLateBound(index, br) if *index == self.binder_index => {
 887                 if self.bound_vars.len() <= br.var.as_usize() {
 888                     bug!("Not enough bound vars: {:?} not found in {:?}", *br, self.bound_vars);
 889                 }
 890                 let list_var = self.bound_vars[br.var.as_usize()];
 891                 match list_var {
 892                     ty::BoundVariableKind::Region(kind) => {
 893                         if kind != br.kind {
 894                             bug!(
 895                                 "Mismatched region kinds: {:?} doesn't match var ({:?}) in list ({:?})",
 896                                 br.kind,
 897                                 list_var,
 898                                 self.bound_vars
 899                             );
 900                         }
 901                     }
 902                     _ => bug!(
 903                         "Mismatched bound variable kinds! Expected region, found {:?}",
 904                         list_var
 905                     ),
 906                 }
 907             }
 908
 909             _ => (),
 910         };
 911
 912         r.super_visit_with(self)
 913     }
 914 }
 915
 916 ///////////////////////////////////////////////////////////////////////////
 917 // Shifter
 918 //
 919 // Shifts the De Bruijn indices on all escaping bound vars by a
 920 // fixed amount. Useful in substitution or when otherwise introducing
 921 // a binding level that is not intended to capture the existing bound
 922 // vars. See comment on `shift_vars_through_binders` method in
 923 // `subst.rs` for more details.
 924
 925 struct Shifter<'tcx> {
 926     tcx: TyCtxt<'tcx>,
 927     current_index: ty::DebruijnIndex,
 928     amount: u32,
 929 }
 930
 931 impl Shifter<'tcx> {
 932     pub fn new(tcx: TyCtxt<'tcx>, amount: u32) -> Self {
 933         Shifter { tcx, current_index: ty::INNERMOST, amount }
 934     }
 935 }
 936
 937 impl TypeFolder<'tcx> for Shifter<'tcx> {
 938     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 939         self.tcx
 940     }
 941
 942     fn fold_binder<T: TypeFoldable<'tcx>>(
 943         &mut self,
 944         t: ty::Binder<'tcx, T>,
 945     ) -> ty::Binder<'tcx, T> {
 946         self.current_index.shift_in(1);
 947         let t = t.super_fold_with(self);
 948         self.current_index.shift_out(1);
 949         t
 950     }
 951
 952     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 953         match *r {
 954             ty::ReLateBound(debruijn, br) => {
 955                 if self.amount == 0 || debruijn < self.current_index {
 956                     r
 957                 } else {
 958                     let debruijn = debruijn.shifted_in(self.amount);
 959                     let shifted = ty::ReLateBound(debruijn, br);
 960                     self.tcx.mk_region(shifted)
 961                 }
 962             }
 963             _ => r,
 964         }
 965     }
 966
 967     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 968         match *ty.kind() {
 969             ty::Bound(debruijn, bound_ty) => {
 970                 if self.amount == 0 || debruijn < self.current_index {
 971                     ty
 972                 } else {
 973                     let debruijn = debruijn.shifted_in(self.amount);
 974                     self.tcx.mk_ty(ty::Bound(debruijn, bound_ty))
 975                 }
 976             }
 977
 978             _ => ty.super_fold_with(self),
 979         }
 980     }
 981
 982     fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 983         if let ty::Const { val: ty::ConstKind::Bound(debruijn, bound_ct), ty } = *ct {
 984             if self.amount == 0 || debruijn < self.current_index {
 985                 ct
 986             } else {
 987                 let debruijn = debruijn.shifted_in(self.amount);
 988                 self.tcx.mk_const(ty::Const { val: ty::ConstKind::Bound(debruijn, bound_ct), ty })
 989             }
 990         } else {
 991             ct.super_fold_with(self)
 992         }
 993     }
 994 }
 995
 996 pub fn shift_region<'tcx>(
 997     tcx: TyCtxt<'tcx>,
 998     region: ty::Region<'tcx>,
 999     amount: u32,
1000 ) -> ty::Region<'tcx> {
1001     match region {
1002         ty::ReLateBound(debruijn, br) if amount > 0 => {
1003             tcx.mk_region(ty::ReLateBound(debruijn.shifted_in(amount), *br))
1004         }
1005         _ => region,
1006     }
1007 }
1008
1009 pub fn shift_vars<'tcx, T>(tcx: TyCtxt<'tcx>, value: T, amount: u32) -> T
1010 where
1011     T: TypeFoldable<'tcx>,
1012 {
1013     debug!("shift_vars(value={:?}, amount={})", value, amount);
1014
1015     value.fold_with(&mut Shifter::new(tcx, amount))
1016 }
1017
1018 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
1019 struct FoundEscapingVars;
1020
1021 /// An "escaping var" is a bound var whose binder is not part of `t`. A bound var can be a
1022 /// bound region or a bound type.
1023 ///
1024 /// So, for example, consider a type like the following, which has two binders:
1025 ///
1026 ///    for<'a> fn(x: for<'b> fn(&'a isize, &'b isize))
1027 ///    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ outer scope
1028 ///                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~  inner scope
1029 ///
1030 /// This type has *bound regions* (`'a`, `'b`), but it does not have escaping regions, because the
1031 /// binders of both `'a` and `'b` are part of the type itself. However, if we consider the *inner
1032 /// fn type*, that type has an escaping region: `'a`.
1033 ///
1034 /// Note that what I'm calling an "escaping var" is often just called a "free var". However,
1035 /// we already use the term "free var". It refers to the regions or types that we use to represent
1036 /// bound regions or type params on a fn definition while we are type checking its body.
1037 ///
1038 /// To clarify, conceptually there is no particular difference between
1039 /// an "escaping" var and a "free" var. However, there is a big
1040 /// difference in practice. Basically, when "entering" a binding
1041 /// level, one is generally required to do some sort of processing to
1042 /// a bound var, such as replacing it with a fresh/placeholder
1043 /// var, or making an entry in the environment to represent the
1044 /// scope to which it is attached, etc. An escaping var represents
1045 /// a bound var for which this processing has not yet been done.
1046 struct HasEscapingVarsVisitor {
1047     /// Anything bound by `outer_index` or "above" is escaping.
1048     outer_index: ty::DebruijnIndex,
1049 }
1050
1051 impl<'tcx> TypeVisitor<'tcx> for HasEscapingVarsVisitor {
1052     type BreakTy = FoundEscapingVars;
1053
1054     fn tcx_for_anon_const_substs(&self) -> Option<TyCtxt<'tcx>> {
1055         // Anonymous constants do not contain bound vars in their substs by default.
1056         None
1057     }
1058
1059     fn visit_binder<T: TypeFoldable<'tcx>>(
1060         &mut self,
1061         t: &Binder<'tcx, T>,
1062     ) -> ControlFlow<Self::BreakTy> {
1063         self.outer_index.shift_in(1);
1064         let result = t.super_visit_with(self);
1065         self.outer_index.shift_out(1);
1066         result
1067     }
1068
1069     #[inline]
1070     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
1071         // If the outer-exclusive-binder is *strictly greater* than
1072         // `outer_index`, that means that `t` contains some content
1073         // bound at `outer_index` or above (because
1074         // `outer_exclusive_binder` is always 1 higher than the
1075         // content in `t`). Therefore, `t` has some escaping vars.
1076         if t.outer_exclusive_binder > self.outer_index {
1077             ControlFlow::Break(FoundEscapingVars)
1078         } else {
1079             ControlFlow::CONTINUE
1080         }
1081     }
1082
1083     #[inline]
1084     fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
1085         // If the region is bound by `outer_index` or anything outside
1086         // of outer index, then it escapes the binders we have
1087         // visited.
1088         if r.bound_at_or_above_binder(self.outer_index) {
1089             ControlFlow::Break(FoundEscapingVars)
1090         } else {
1091             ControlFlow::CONTINUE
1092         }
1093     }
1094
1095     fn visit_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
1096         // we don't have a `visit_infer_const` callback, so we have to
1097         // hook in here to catch this case (annoying...), but
1098         // otherwise we do want to remember to visit the rest of the
1099         // const, as it has types/regions embedded in a lot of other
1100         // places.
1101         match ct.val {
1102             ty::ConstKind::Bound(debruijn, _) if debruijn >= self.outer_index => {
1103                 ControlFlow::Break(FoundEscapingVars)
1104             }
1105             _ => ct.super_visit_with(self),
1106         }
1107     }
1108
1109     #[inline]
1110     fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
1111         if predicate.inner.outer_exclusive_binder > self.outer_index {
1112             ControlFlow::Break(FoundEscapingVars)
1113         } else {
1114             ControlFlow::CONTINUE
1115         }
1116     }
1117 }
1118
1119 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
1120 struct FoundFlags;
1121
1122 // FIXME: Optimize for checking for infer flags
1123 struct HasTypeFlagsVisitor<'tcx> {
1124     tcx: Option<TyCtxt<'tcx>>,
1125     flags: ty::TypeFlags,
1126 }
1127
1128 impl<'tcx> TypeVisitor<'tcx> for HasTypeFlagsVisitor<'tcx> {
1129     type BreakTy = FoundFlags;
1130     fn tcx_for_anon_const_substs(&self) -> Option<TyCtxt<'tcx>> {
1131         bug!("we shouldn't call this method as we manually look at ct substs");
1132     }
1133
1134     #[inline]
1135     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
1136         let flags = t.flags();
1137         debug!("HasTypeFlagsVisitor: t={:?} flags={:?} self.flags={:?}", t, flags, self.flags);
1138         if flags.intersects(self.flags) {
1139             ControlFlow::Break(FoundFlags)
1140         } else {
1141             match flags.intersects(TypeFlags::HAS_UNKNOWN_DEFAULT_CONST_SUBSTS) {
1142                 true if self.tcx.is_some() => UnknownConstSubstsVisitor::search(&self, t),
1143                 _ => ControlFlow::CONTINUE,
1144             }
1145         }
1146     }
1147
1148     #[inline]
1149     fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
1150         let flags = r.type_flags();
1151         debug!("HasTypeFlagsVisitor: r={:?} r.flags={:?} self.flags={:?}", r, flags, self.flags);
1152         if flags.intersects(self.flags) {
1153             ControlFlow::Break(FoundFlags)
1154         } else {
1155             ControlFlow::CONTINUE
1156         }
1157     }
1158
1159     #[inline]
1160     fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
1161         let flags = FlagComputation::for_const(c);
1162         debug!("HasTypeFlagsVisitor: c={:?} c.flags={:?} self.flags={:?}", c, flags, self.flags);
1163         if flags.intersects(self.flags) {
1164             ControlFlow::Break(FoundFlags)
1165         } else {
1166             match flags.intersects(TypeFlags::HAS_UNKNOWN_DEFAULT_CONST_SUBSTS) {
1167                 true if self.tcx.is_some() => UnknownConstSubstsVisitor::search(&self, c),
1168                 _ => ControlFlow::CONTINUE,
1169             }
1170         }
1171     }
1172
1173     #[inline]
1174     fn visit_unevaluated_const(&mut self, uv: ty::Unevaluated<'tcx>) -> ControlFlow<Self::BreakTy> {
1175         let flags = FlagComputation::for_unevaluated_const(uv);
1176         debug!("HasTypeFlagsVisitor: uv={:?} uv.flags={:?} self.flags={:?}", uv, flags, self.flags);
1177         if flags.intersects(self.flags) {
1178             ControlFlow::Break(FoundFlags)
1179         } else {
1180             match flags.intersects(TypeFlags::HAS_UNKNOWN_DEFAULT_CONST_SUBSTS) {
1181                 true if self.tcx.is_some() => UnknownConstSubstsVisitor::search(&self, uv),
1182                 _ => ControlFlow::CONTINUE,
1183             }
1184         }
1185     }
1186
1187     #[inline]
1188     fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
1189         let flags = predicate.inner.flags;
1190         debug!(
1191             "HasTypeFlagsVisitor: predicate={:?} flags={:?} self.flags={:?}",
1192             predicate, flags, self.flags
1193         );
1194         if flags.intersects(self.flags) {
1195             ControlFlow::Break(FoundFlags)
1196         } else {
1197             match flags.intersects(TypeFlags::HAS_UNKNOWN_DEFAULT_CONST_SUBSTS) {
1198                 true if self.tcx.is_some() => UnknownConstSubstsVisitor::search(&self, predicate),
1199                 _ => ControlFlow::CONTINUE,
1200             }
1201         }
1202     }
1203 }
1204
1205 struct UnknownConstSubstsVisitor<'tcx> {
1206     tcx: TyCtxt<'tcx>,
1207     flags: ty::TypeFlags,
1208 }
1209
1210 impl<'tcx> UnknownConstSubstsVisitor<'tcx> {
1211     /// This is fairly cold and we don't want to
1212     /// bloat the size of the `HasTypeFlagsVisitor`.
1213     #[inline(never)]
1214     pub fn search<T: TypeFoldable<'tcx>>(
1215         visitor: &HasTypeFlagsVisitor<'tcx>,
1216         v: T,
1217     ) -> ControlFlow<FoundFlags> {
1218         if visitor.flags.intersects(TypeFlags::MAY_NEED_DEFAULT_CONST_SUBSTS) {
1219             v.super_visit_with(&mut UnknownConstSubstsVisitor {
1220                 tcx: visitor.tcx.unwrap(),
1221                 flags: visitor.flags,
1222             })
1223         } else {
1224             ControlFlow::CONTINUE
1225         }
1226     }
1227 }
1228
1229 impl<'tcx> TypeVisitor<'tcx> for UnknownConstSubstsVisitor<'tcx> {
1230     type BreakTy = FoundFlags;
1231     fn tcx_for_anon_const_substs(&self) -> Option<TyCtxt<'tcx>> {
1232         bug!("we shouldn't call this method as we manually look at ct substs");
1233     }
1234
1235     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
1236         if t.flags().intersects(TypeFlags::HAS_UNKNOWN_DEFAULT_CONST_SUBSTS) {
1237             t.super_visit_with(self)
1238         } else {
1239             ControlFlow::CONTINUE
1240         }
1241     }
1242
1243     #[inline]
1244     fn visit_unevaluated_const(&mut self, uv: ty::Unevaluated<'tcx>) -> ControlFlow<Self::BreakTy> {
1245         if uv.substs_.is_none() {
1246             self.tcx
1247                 .default_anon_const_substs(uv.def.did)
1248                 .visit_with(&mut HasTypeFlagsVisitor { tcx: Some(self.tcx), flags: self.flags })
1249         } else {
1250             ControlFlow::CONTINUE
1251         }
1252     }
1253
1254     #[inline]
1255     fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
1256         if predicate.inner.flags.intersects(TypeFlags::HAS_UNKNOWN_DEFAULT_CONST_SUBSTS) {
1257             predicate.super_visit_with(self)
1258         } else {
1259             ControlFlow::CONTINUE
1260         }
1261     }
1262 }
1263
1264 impl<'tcx> TyCtxt<'tcx> {
1265     /// This is a HACK(const_generics) and should probably not be needed.
1266     /// Might however be perf relevant, so who knows.
1267     ///
1268     /// FIXME(@lcnr): explain this function a bit more
1269     pub fn expose_default_const_substs<T: TypeFoldable<'tcx>>(self, v: T) -> T {
1270         v.fold_with(&mut ExposeDefaultConstSubstsFolder { tcx: self })
1271     }
1272 }
1273
1274 struct ExposeDefaultConstSubstsFolder<'tcx> {
1275     tcx: TyCtxt<'tcx>,
1276 }
1277
1278 impl<'tcx> TypeFolder<'tcx> for ExposeDefaultConstSubstsFolder<'tcx> {
1279     fn tcx(&self) -> TyCtxt<'tcx> {
1280         self.tcx
1281     }
1282
1283     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1284         if ty.flags().intersects(TypeFlags::HAS_UNKNOWN_DEFAULT_CONST_SUBSTS) {
1285             ty.super_fold_with(self)
1286         } else {
1287             ty
1288         }
1289     }
1290
1291     fn fold_predicate(&mut self, pred: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
1292         if pred.inner.flags.intersects(TypeFlags::HAS_UNKNOWN_DEFAULT_CONST_SUBSTS) {
1293             pred.super_fold_with(self)
1294         } else {
1295             pred
1296         }
1297     }
1298 }
1299
1300 /// Collects all the late-bound regions at the innermost binding level
1301 /// into a hash set.
1302 struct LateBoundRegionsCollector<'tcx> {
1303     tcx: TyCtxt<'tcx>,
1304     current_index: ty::DebruijnIndex,
1305     regions: FxHashSet<ty::BoundRegionKind>,
1306
1307     /// `true` if we only want regions that are known to be
1308     /// "constrained" when you equate this type with another type. In
1309     /// particular, if you have e.g., `&'a u32` and `&'b u32`, equating
1310     /// them constraints `'a == 'b`. But if you have `<&'a u32 as
1311     /// Trait>::Foo` and `<&'b u32 as Trait>::Foo`, normalizing those
1312     /// types may mean that `'a` and `'b` don't appear in the results,
1313     /// so they are not considered *constrained*.
1314     just_constrained: bool,
1315 }
1316
1317 impl LateBoundRegionsCollector<'tcx> {
1318     fn new(tcx: TyCtxt<'tcx>, just_constrained: bool) -> Self {
1319         LateBoundRegionsCollector {
1320             tcx,
1321             current_index: ty::INNERMOST,
1322             regions: Default::default(),
1323             just_constrained,
1324         }
1325     }
1326 }
1327
1328 impl<'tcx> TypeVisitor<'tcx> for LateBoundRegionsCollector<'tcx> {
1329     fn tcx_for_anon_const_substs(&self) -> Option<TyCtxt<'tcx>> {
1330         Some(self.tcx)
1331     }
1332
1333     fn visit_binder<T: TypeFoldable<'tcx>>(
1334         &mut self,
1335         t: &Binder<'tcx, T>,
1336     ) -> ControlFlow<Self::BreakTy> {
1337         self.current_index.shift_in(1);
1338         let result = t.super_visit_with(self);
1339         self.current_index.shift_out(1);
1340         result
1341     }
1342
1343     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
1344         // if we are only looking for "constrained" region, we have to
1345         // ignore the inputs to a projection, as they may not appear
1346         // in the normalized form
1347         if self.just_constrained {
1348             if let ty::Projection(..) | ty::Opaque(..) = t.kind() {
1349                 return ControlFlow::CONTINUE;
1350             }
1351         }
1352
1353         t.super_visit_with(self)
1354     }
1355
1356     fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
1357         // if we are only looking for "constrained" region, we have to
1358         // ignore the inputs of an unevaluated const, as they may not appear
1359         // in the normalized form
1360         if self.just_constrained {
1361             if let ty::ConstKind::Unevaluated(..) = c.val {
1362                 return ControlFlow::CONTINUE;
1363             }
1364         }
1365
1366         c.super_visit_with(self)
1367     }
1368
1369     fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
1370         if let ty::ReLateBound(debruijn, br) = *r {
1371             if debruijn == self.current_index {
1372                 self.regions.insert(br.kind);
1373             }
1374         }
1375         ControlFlow::CONTINUE
1376     }
1377 }