src/librustc/ty/fold.rs

   1 //! Generalized type folding mechanism. The setup is a bit convoluted
   2 //! but allows for convenient usage. Let T be an instance of some
   3 //! "foldable type" (one which implements `TypeFoldable`) and F be an
   4 //! instance of a "folder" (a type which implements `TypeFolder`). Then
   5 //! the setup is intended to be:
   6 //!
   7 //!     T.fold_with(F) --calls--> F.fold_T(T) --calls--> T.super_fold_with(F)
   8 //!
   9 //! This way, when you define a new folder F, you can override
  10 //! `fold_T()` to customize the behavior, and invoke `T.super_fold_with()`
  11 //! to get the original behavior. Meanwhile, to actually fold
  12 //! something, you can just write `T.fold_with(F)`, which is
  13 //! convenient. (Note that `fold_with` will also transparently handle
  14 //! things like a `Vec<T>` where T is foldable and so on.)
  15 //!
  16 //! In this ideal setup, the only function that actually *does*
  17 //! anything is `T.super_fold_with()`, which traverses the type `T`.
  18 //! Moreover, `T.super_fold_with()` should only ever call `T.fold_with()`.
  19 //!
  20 //! In some cases, we follow a degenerate pattern where we do not have
  21 //! a `fold_T` method. Instead, `T.fold_with` traverses the structure directly.
  22 //! This is suboptimal because the behavior cannot be overridden, but it's
  23 //! much less work to implement. If you ever *do* need an override that
  24 //! doesn't exist, it's not hard to convert the degenerate pattern into the
  25 //! proper thing.
  26 //!
  27 //! A `TypeFoldable` T can also be visited by a `TypeVisitor` V using similar setup:
  28 //!
  29 //!     T.visit_with(V) --calls--> V.visit_T(T) --calls--> T.super_visit_with(V).
  30 //!
  31 //! These methods return true to indicate that the visitor has found what it is
  32 //! looking for, and does not need to visit anything else.
  33
  34 use crate::ty::{self, flags::FlagComputation, Binder, Ty, TyCtxt, TypeFlags};
  35 use rustc_hir as hir;
  36 use rustc_hir::def_id::DefId;
  37
  38 use rustc_data_structures::fx::FxHashSet;
  39 use std::collections::BTreeMap;
  40 use std::fmt;
  41
  42 /// This trait is implemented for every type that can be folded.
  43 /// Basically, every type that has a corresponding method in `TypeFolder`.
  44 ///
  45 /// To implement this conveniently, use the derive macro located in librustc_macros.
  46 pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
  47     fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self;
  48     fn fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
  49         self.super_fold_with(folder)
  50     }
  51
  52     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool;
  53     fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
  54         self.super_visit_with(visitor)
  55     }
  56
  57     /// Returns `true` if `self` has any late-bound regions that are either
  58     /// bound by `binder` or bound by some binder outside of `binder`.
  59     /// If `binder` is `ty::INNERMOST`, this indicates whether
  60     /// there are any late-bound regions that appear free.
  61     fn has_vars_bound_at_or_above(&self, binder: ty::DebruijnIndex) -> bool {
  62         self.visit_with(&mut HasEscapingVarsVisitor { outer_index: binder })
  63     }
  64
  65     /// Returns `true` if this `self` has any regions that escape `binder` (and
  66     /// hence are not bound by it).
  67     fn has_vars_bound_above(&self, binder: ty::DebruijnIndex) -> bool {
  68         self.has_vars_bound_at_or_above(binder.shifted_in(1))
  69     }
  70
  71     fn has_escaping_bound_vars(&self) -> bool {
  72         self.has_vars_bound_at_or_above(ty::INNERMOST)
  73     }
  74
  75     fn has_type_flags(&self, flags: TypeFlags) -> bool {
  76         self.visit_with(&mut HasTypeFlagsVisitor { flags })
  77     }
  78     fn has_projections(&self) -> bool {
  79         self.has_type_flags(TypeFlags::HAS_PROJECTION)
  80     }
  81     fn has_opaque_types(&self) -> bool {
  82         self.has_type_flags(TypeFlags::HAS_TY_OPAQUE)
  83     }
  84     fn references_error(&self) -> bool {
  85         self.has_type_flags(TypeFlags::HAS_TY_ERR)
  86     }
  87     fn has_param_types(&self) -> bool {
  88         self.has_type_flags(TypeFlags::HAS_TY_PARAM | TypeFlags::HAS_CT_PARAM)
  89     }
  90     fn has_infer_types(&self) -> bool {
  91         self.has_type_flags(TypeFlags::HAS_TY_INFER)
  92     }
  93     fn has_infer_types_or_consts(&self) -> bool {
  94         self.has_type_flags(TypeFlags::HAS_TY_INFER | TypeFlags::HAS_CT_INFER)
  95     }
  96     fn has_infer_consts(&self) -> bool {
  97         self.has_type_flags(TypeFlags::HAS_CT_INFER)
  98     }
  99     fn has_local_value(&self) -> bool {
 100         self.has_type_flags(TypeFlags::KEEP_IN_LOCAL_TCX)
 101     }
 102     fn needs_infer(&self) -> bool {
 103         self.has_type_flags(TypeFlags::NEEDS_INFER)
 104     }
 105     fn has_placeholders(&self) -> bool {
 106         self.has_type_flags(
 107             TypeFlags::HAS_RE_PLACEHOLDER
 108                 | TypeFlags::HAS_TY_PLACEHOLDER
 109                 | TypeFlags::HAS_CT_PLACEHOLDER,
 110         )
 111     }
 112     fn needs_subst(&self) -> bool {
 113         self.has_type_flags(TypeFlags::NEEDS_SUBST)
 114     }
 115     fn has_re_placeholders(&self) -> bool {
 116         self.has_type_flags(TypeFlags::HAS_RE_PLACEHOLDER)
 117     }
 118     /// "Free" regions in this context means that it has any region
 119     /// that is not (a) erased or (b) late-bound.
 120     fn has_free_regions(&self) -> bool {
 121         self.has_type_flags(TypeFlags::HAS_FREE_REGIONS)
 122     }
 123
 124     fn has_erased_regions(&self) -> bool {
 125         self.has_type_flags(TypeFlags::HAS_RE_ERASED)
 126     }
 127
 128     /// True if there are any un-erased free regions.
 129     fn has_erasable_regions(&self) -> bool {
 130         self.has_type_flags(TypeFlags::HAS_FREE_REGIONS)
 131     }
 132
 133     /// Indicates whether this value references only 'global'
 134     /// generic parameters that are the same regardless of what fn we are
 135     /// in. This is used for caching.
 136     fn is_global(&self) -> bool {
 137         !self.has_type_flags(TypeFlags::HAS_FREE_LOCAL_NAMES)
 138     }
 139
 140     /// True if there are any late-bound regions
 141     fn has_late_bound_regions(&self) -> bool {
 142         self.has_type_flags(TypeFlags::HAS_RE_LATE_BOUND)
 143     }
 144
 145     /// A visitor that does not recurse into types, works like `fn walk_shallow` in `Ty`.
 146     fn visit_tys_shallow(&self, visit: impl FnMut(Ty<'tcx>) -> bool) -> bool {
 147         pub struct Visitor<F>(F);
 148
 149         impl<'tcx, F: FnMut(Ty<'tcx>) -> bool> TypeVisitor<'tcx> for Visitor<F> {
 150             fn visit_ty(&mut self, ty: Ty<'tcx>) -> bool {
 151                 self.0(ty)
 152             }
 153         }
 154
 155         self.visit_with(&mut Visitor(visit))
 156     }
 157 }
 158
 159 impl TypeFoldable<'tcx> for hir::Constness {
 160     fn super_fold_with<F: TypeFolder<'tcx>>(&self, _: &mut F) -> Self {
 161         *self
 162     }
 163     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> bool {
 164         false
 165     }
 166 }
 167
 168 /// The `TypeFolder` trait defines the actual *folding*. There is a
 169 /// method defined for every foldable type. Each of these has a
 170 /// default implementation that does an "identity" fold. Within each
 171 /// identity fold, it should invoke `foo.fold_with(self)` to fold each
 172 /// sub-item.
 173 pub trait TypeFolder<'tcx>: Sized {
 174     fn tcx<'a>(&'a self) -> TyCtxt<'tcx>;
 175
 176     fn fold_binder<T>(&mut self, t: &Binder<T>) -> Binder<T>
 177     where
 178         T: TypeFoldable<'tcx>,
 179     {
 180         t.super_fold_with(self)
 181     }
 182
 183     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 184         t.super_fold_with(self)
 185     }
 186
 187     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 188         r.super_fold_with(self)
 189     }
 190
 191     fn fold_const(&mut self, c: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 192         c.super_fold_with(self)
 193     }
 194 }
 195
 196 pub trait TypeVisitor<'tcx>: Sized {
 197     fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
 198         t.super_visit_with(self)
 199     }
 200
 201     fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
 202         t.super_visit_with(self)
 203     }
 204
 205     fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
 206         r.super_visit_with(self)
 207     }
 208
 209     fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> bool {
 210         c.super_visit_with(self)
 211     }
 212 }
 213
 214 ///////////////////////////////////////////////////////////////////////////
 215 // Some sample folders
 216
 217 pub struct BottomUpFolder<'tcx, F, G, H>
 218 where
 219     F: FnMut(Ty<'tcx>) -> Ty<'tcx>,
 220     G: FnMut(ty::Region<'tcx>) -> ty::Region<'tcx>,
 221     H: FnMut(&'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx>,
 222 {
 223     pub tcx: TyCtxt<'tcx>,
 224     pub ty_op: F,
 225     pub lt_op: G,
 226     pub ct_op: H,
 227 }
 228
 229 impl<'tcx, F, G, H> TypeFolder<'tcx> for BottomUpFolder<'tcx, F, G, H>
 230 where
 231     F: FnMut(Ty<'tcx>) -> Ty<'tcx>,
 232     G: FnMut(ty::Region<'tcx>) -> ty::Region<'tcx>,
 233     H: FnMut(&'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx>,
 234 {
 235     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 236         self.tcx
 237     }
 238
 239     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 240         let t = ty.super_fold_with(self);
 241         (self.ty_op)(t)
 242     }
 243
 244     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 245         let r = r.super_fold_with(self);
 246         (self.lt_op)(r)
 247     }
 248
 249     fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 250         let ct = ct.super_fold_with(self);
 251         (self.ct_op)(ct)
 252     }
 253 }
 254
 255 ///////////////////////////////////////////////////////////////////////////
 256 // Region folder
 257
 258 impl<'tcx> TyCtxt<'tcx> {
 259     /// Collects the free and escaping regions in `value` into `region_set`. Returns
 260     /// whether any late-bound regions were skipped
 261     pub fn collect_regions<T>(self, value: &T, region_set: &mut FxHashSet<ty::Region<'tcx>>) -> bool
 262     where
 263         T: TypeFoldable<'tcx>,
 264     {
 265         let mut have_bound_regions = false;
 266         self.fold_regions(value, &mut have_bound_regions, |r, d| {
 267             region_set.insert(self.mk_region(r.shifted_out_to_binder(d)));
 268             r
 269         });
 270         have_bound_regions
 271     }
 272
 273     /// Folds the escaping and free regions in `value` using `f`, and
 274     /// sets `skipped_regions` to true if any late-bound region was found
 275     /// and skipped.
 276     pub fn fold_regions<T>(
 277         self,
 278         value: &T,
 279         skipped_regions: &mut bool,
 280         mut f: impl FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
 281     ) -> T
 282     where
 283         T: TypeFoldable<'tcx>,
 284     {
 285         value.fold_with(&mut RegionFolder::new(self, skipped_regions, &mut f))
 286     }
 287
 288     /// Invoke `callback` on every region appearing free in `value`.
 289     pub fn for_each_free_region(
 290         self,
 291         value: &impl TypeFoldable<'tcx>,
 292         mut callback: impl FnMut(ty::Region<'tcx>),
 293     ) {
 294         self.any_free_region_meets(value, |r| {
 295             callback(r);
 296             false
 297         });
 298     }
 299
 300     /// Returns `true` if `callback` returns true for every region appearing free in `value`.
 301     pub fn all_free_regions_meet(
 302         self,
 303         value: &impl TypeFoldable<'tcx>,
 304         mut callback: impl FnMut(ty::Region<'tcx>) -> bool,
 305     ) -> bool {
 306         !self.any_free_region_meets(value, |r| !callback(r))
 307     }
 308
 309     /// Returns `true` if `callback` returns true for some region appearing free in `value`.
 310     pub fn any_free_region_meets(
 311         self,
 312         value: &impl TypeFoldable<'tcx>,
 313         callback: impl FnMut(ty::Region<'tcx>) -> bool,
 314     ) -> bool {
 315         return value.visit_with(&mut RegionVisitor { outer_index: ty::INNERMOST, callback });
 316
 317         struct RegionVisitor<F> {
 318             /// The index of a binder *just outside* the things we have
 319             /// traversed. If we encounter a bound region bound by this
 320             /// binder or one outer to it, it appears free. Example:
 321             ///
 322             /// ```
 323             ///    for<'a> fn(for<'b> fn(), T)
 324             /// ^          ^          ^     ^
 325             /// |          |          |     | here, would be shifted in 1
 326             /// |          |          | here, would be shifted in 2
 327             /// |          | here, would be `INNERMOST` shifted in by 1
 328             /// | here, initially, binder would be `INNERMOST`
 329             /// ```
 330             ///
 331             /// You see that, initially, *any* bound value is free,
 332             /// because we've not traversed any binders. As we pass
 333             /// through a binder, we shift the `outer_index` by 1 to
 334             /// account for the new binder that encloses us.
 335             outer_index: ty::DebruijnIndex,
 336             callback: F,
 337         }
 338
 339         impl<'tcx, F> TypeVisitor<'tcx> for RegionVisitor<F>
 340         where
 341             F: FnMut(ty::Region<'tcx>) -> bool,
 342         {
 343             fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
 344                 self.outer_index.shift_in(1);
 345                 let result = t.skip_binder().visit_with(self);
 346                 self.outer_index.shift_out(1);
 347                 result
 348             }
 349
 350             fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
 351                 match *r {
 352                     ty::ReLateBound(debruijn, _) if debruijn < self.outer_index => {
 353                         false // ignore bound regions, keep visiting
 354                     }
 355                     _ => (self.callback)(r),
 356                 }
 357             }
 358
 359             fn visit_ty(&mut self, ty: Ty<'tcx>) -> bool {
 360                 // We're only interested in types involving regions
 361                 if ty.flags.intersects(TypeFlags::HAS_FREE_REGIONS) {
 362                     ty.super_visit_with(self)
 363                 } else {
 364                     false // keep visiting
 365                 }
 366             }
 367         }
 368     }
 369 }
 370
 371 /// Folds over the substructure of a type, visiting its component
 372 /// types and all regions that occur *free* within it.
 373 ///
 374 /// That is, `Ty` can contain function or method types that bind
 375 /// regions at the call site (`ReLateBound`), and occurrences of
 376 /// regions (aka "lifetimes") that are bound within a type are not
 377 /// visited by this folder; only regions that occur free will be
 378 /// visited by `fld_r`.
 379
 380 pub struct RegionFolder<'a, 'tcx> {
 381     tcx: TyCtxt<'tcx>,
 382     skipped_regions: &'a mut bool,
 383
 384     /// Stores the index of a binder *just outside* the stuff we have
 385     /// visited.  So this begins as INNERMOST; when we pass through a
 386     /// binder, it is incremented (via `shift_in`).
 387     current_index: ty::DebruijnIndex,
 388
 389     /// Callback invokes for each free region. The `DebruijnIndex`
 390     /// points to the binder *just outside* the ones we have passed
 391     /// through.
 392     fold_region_fn:
 393         &'a mut (dyn FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx> + 'a),
 394 }
 395
 396 impl<'a, 'tcx> RegionFolder<'a, 'tcx> {
 397     #[inline]
 398     pub fn new(
 399         tcx: TyCtxt<'tcx>,
 400         skipped_regions: &'a mut bool,
 401         fold_region_fn: &'a mut dyn FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
 402     ) -> RegionFolder<'a, 'tcx> {
 403         RegionFolder { tcx, skipped_regions, current_index: ty::INNERMOST, fold_region_fn }
 404     }
 405 }
 406
 407 impl<'a, 'tcx> TypeFolder<'tcx> for RegionFolder<'a, 'tcx> {
 408     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 409         self.tcx
 410     }
 411
 412     fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
 413         self.current_index.shift_in(1);
 414         let t = t.super_fold_with(self);
 415         self.current_index.shift_out(1);
 416         t
 417     }
 418
 419     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 420         match *r {
 421             ty::ReLateBound(debruijn, _) if debruijn < self.current_index => {
 422                 debug!(
 423                     "RegionFolder.fold_region({:?}) skipped bound region (current index={:?})",
 424                     r, self.current_index
 425                 );
 426                 *self.skipped_regions = true;
 427                 r
 428             }
 429             _ => {
 430                 debug!(
 431                     "RegionFolder.fold_region({:?}) folding free region (current_index={:?})",
 432                     r, self.current_index
 433                 );
 434                 (self.fold_region_fn)(r, self.current_index)
 435             }
 436         }
 437     }
 438 }
 439
 440 ///////////////////////////////////////////////////////////////////////////
 441 // Bound vars replacer
 442
 443 /// Replaces the escaping bound vars (late bound regions or bound types) in a type.
 444 struct BoundVarReplacer<'a, 'tcx> {
 445     tcx: TyCtxt<'tcx>,
 446
 447     /// As with `RegionFolder`, represents the index of a binder *just outside*
 448     /// the ones we have visited.
 449     current_index: ty::DebruijnIndex,
 450
 451     fld_r: &'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a),
 452     fld_t: &'a mut (dyn FnMut(ty::BoundTy) -> Ty<'tcx> + 'a),
 453     fld_c: &'a mut (dyn FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx> + 'a),
 454 }
 455
 456 impl<'a, 'tcx> BoundVarReplacer<'a, 'tcx> {
 457     fn new<F, G, H>(tcx: TyCtxt<'tcx>, fld_r: &'a mut F, fld_t: &'a mut G, fld_c: &'a mut H) -> Self
 458     where
 459         F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
 460         G: FnMut(ty::BoundTy) -> Ty<'tcx>,
 461         H: FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx>,
 462     {
 463         BoundVarReplacer { tcx, current_index: ty::INNERMOST, fld_r, fld_t, fld_c }
 464     }
 465 }
 466
 467 impl<'a, 'tcx> TypeFolder<'tcx> for BoundVarReplacer<'a, 'tcx> {
 468     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 469         self.tcx
 470     }
 471
 472     fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<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_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 480         match t.kind {
 481             ty::Bound(debruijn, bound_ty) => {
 482                 if debruijn == self.current_index {
 483                     let fld_t = &mut self.fld_t;
 484                     let ty = fld_t(bound_ty);
 485                     ty::fold::shift_vars(self.tcx, &ty, self.current_index.as_u32())
 486                 } else {
 487                     t
 488                 }
 489             }
 490             _ => {
 491                 if !t.has_vars_bound_at_or_above(self.current_index) {
 492                     // Nothing more to substitute.
 493                     t
 494                 } else {
 495                     t.super_fold_with(self)
 496                 }
 497             }
 498         }
 499     }
 500
 501     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 502         match *r {
 503             ty::ReLateBound(debruijn, br) if debruijn == self.current_index => {
 504                 let fld_r = &mut self.fld_r;
 505                 let region = fld_r(br);
 506                 if let ty::ReLateBound(debruijn1, br) = *region {
 507                     // If the callback returns a late-bound region,
 508                     // that region should always use the INNERMOST
 509                     // debruijn index. Then we adjust it to the
 510                     // correct depth.
 511                     assert_eq!(debruijn1, ty::INNERMOST);
 512                     self.tcx.mk_region(ty::ReLateBound(debruijn, br))
 513                 } else {
 514                     region
 515                 }
 516             }
 517             _ => r,
 518         }
 519     }
 520
 521     fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 522         if let ty::Const { val: ty::ConstKind::Bound(debruijn, bound_const), ty } = *ct {
 523             if debruijn == self.current_index {
 524                 let fld_c = &mut self.fld_c;
 525                 let ct = fld_c(bound_const, ty);
 526                 ty::fold::shift_vars(self.tcx, &ct, self.current_index.as_u32())
 527             } else {
 528                 ct
 529             }
 530         } else {
 531             if !ct.has_vars_bound_at_or_above(self.current_index) {
 532                 // Nothing more to substitute.
 533                 ct
 534             } else {
 535                 ct.super_fold_with(self)
 536             }
 537         }
 538     }
 539 }
 540
 541 impl<'tcx> TyCtxt<'tcx> {
 542     /// Replaces all regions bound by the given `Binder` with the
 543     /// results returned by the closure; the closure is expected to
 544     /// return a free region (relative to this binder), and hence the
 545     /// binder is removed in the return type. The closure is invoked
 546     /// once for each unique `BoundRegion`; multiple references to the
 547     /// same `BoundRegion` will reuse the previous result. A map is
 548     /// returned at the end with each bound region and the free region
 549     /// that replaced it.
 550     ///
 551     /// This method only replaces late bound regions and the result may still
 552     /// contain escaping bound types.
 553     pub fn replace_late_bound_regions<T, F>(
 554         self,
 555         value: &Binder<T>,
 556         fld_r: F,
 557     ) -> (T, BTreeMap<ty::BoundRegion, ty::Region<'tcx>>)
 558     where
 559         F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
 560         T: TypeFoldable<'tcx>,
 561     {
 562         // identity for bound types and consts
 563         let fld_t = |bound_ty| self.mk_ty(ty::Bound(ty::INNERMOST, bound_ty));
 564         let fld_c = |bound_ct, ty| {
 565             self.mk_const(ty::Const { val: ty::ConstKind::Bound(ty::INNERMOST, bound_ct), ty })
 566         };
 567         self.replace_escaping_bound_vars(value.skip_binder(), fld_r, fld_t, fld_c)
 568     }
 569
 570     /// Replaces all escaping bound vars. The `fld_r` closure replaces escaping
 571     /// bound regions; the `fld_t` closure replaces escaping bound types and the `fld_c`
 572     /// closure replaces escaping bound consts.
 573     pub fn replace_escaping_bound_vars<T, F, G, H>(
 574         self,
 575         value: &T,
 576         mut fld_r: F,
 577         mut fld_t: G,
 578         mut fld_c: H,
 579     ) -> (T, BTreeMap<ty::BoundRegion, ty::Region<'tcx>>)
 580     where
 581         F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
 582         G: FnMut(ty::BoundTy) -> Ty<'tcx>,
 583         H: FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx>,
 584         T: TypeFoldable<'tcx>,
 585     {
 586         use rustc_data_structures::fx::FxHashMap;
 587
 588         let mut region_map = BTreeMap::new();
 589         let mut type_map = FxHashMap::default();
 590         let mut const_map = FxHashMap::default();
 591
 592         if !value.has_escaping_bound_vars() {
 593             (value.clone(), region_map)
 594         } else {
 595             let mut real_fld_r = |br| *region_map.entry(br).or_insert_with(|| fld_r(br));
 596
 597             let mut real_fld_t =
 598                 |bound_ty| *type_map.entry(bound_ty).or_insert_with(|| fld_t(bound_ty));
 599
 600             let mut real_fld_c =
 601                 |bound_ct, ty| *const_map.entry(bound_ct).or_insert_with(|| fld_c(bound_ct, ty));
 602
 603             let mut replacer =
 604                 BoundVarReplacer::new(self, &mut real_fld_r, &mut real_fld_t, &mut real_fld_c);
 605             let result = value.fold_with(&mut replacer);
 606             (result, region_map)
 607         }
 608     }
 609
 610     /// Replaces all types or regions bound by the given `Binder`. The `fld_r`
 611     /// closure replaces bound regions while the `fld_t` closure replaces bound
 612     /// types.
 613     pub fn replace_bound_vars<T, F, G, H>(
 614         self,
 615         value: &Binder<T>,
 616         fld_r: F,
 617         fld_t: G,
 618         fld_c: H,
 619     ) -> (T, BTreeMap<ty::BoundRegion, ty::Region<'tcx>>)
 620     where
 621         F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
 622         G: FnMut(ty::BoundTy) -> Ty<'tcx>,
 623         H: FnMut(ty::BoundVar, Ty<'tcx>) -> &'tcx ty::Const<'tcx>,
 624         T: TypeFoldable<'tcx>,
 625     {
 626         self.replace_escaping_bound_vars(value.skip_binder(), fld_r, fld_t, fld_c)
 627     }
 628
 629     /// Replaces any late-bound regions bound in `value` with
 630     /// free variants attached to `all_outlive_scope`.
 631     pub fn liberate_late_bound_regions<T>(
 632         &self,
 633         all_outlive_scope: DefId,
 634         value: &ty::Binder<T>,
 635     ) -> T
 636     where
 637         T: TypeFoldable<'tcx>,
 638     {
 639         self.replace_late_bound_regions(value, |br| {
 640             self.mk_region(ty::ReFree(ty::FreeRegion {
 641                 scope: all_outlive_scope,
 642                 bound_region: br,
 643             }))
 644         })
 645         .0
 646     }
 647
 648     /// Returns a set of all late-bound regions that are constrained
 649     /// by `value`, meaning that if we instantiate those LBR with
 650     /// variables and equate `value` with something else, those
 651     /// variables will also be equated.
 652     pub fn collect_constrained_late_bound_regions<T>(
 653         &self,
 654         value: &Binder<T>,
 655     ) -> FxHashSet<ty::BoundRegion>
 656     where
 657         T: TypeFoldable<'tcx>,
 658     {
 659         self.collect_late_bound_regions(value, true)
 660     }
 661
 662     /// Returns a set of all late-bound regions that appear in `value` anywhere.
 663     pub fn collect_referenced_late_bound_regions<T>(
 664         &self,
 665         value: &Binder<T>,
 666     ) -> FxHashSet<ty::BoundRegion>
 667     where
 668         T: TypeFoldable<'tcx>,
 669     {
 670         self.collect_late_bound_regions(value, false)
 671     }
 672
 673     fn collect_late_bound_regions<T>(
 674         &self,
 675         value: &Binder<T>,
 676         just_constraint: bool,
 677     ) -> FxHashSet<ty::BoundRegion>
 678     where
 679         T: TypeFoldable<'tcx>,
 680     {
 681         let mut collector = LateBoundRegionsCollector::new(just_constraint);
 682         let result = value.skip_binder().visit_with(&mut collector);
 683         assert!(!result); // should never have stopped early
 684         collector.regions
 685     }
 686
 687     /// Replaces any late-bound regions bound in `value` with `'erased`. Useful in codegen but also
 688     /// method lookup and a few other places where precise region relationships are not required.
 689     pub fn erase_late_bound_regions<T>(self, value: &Binder<T>) -> T
 690     where
 691         T: TypeFoldable<'tcx>,
 692     {
 693         self.replace_late_bound_regions(value, |_| self.lifetimes.re_erased).0
 694     }
 695
 696     /// Rewrite any late-bound regions so that they are anonymous. Region numbers are
 697     /// assigned starting at 1 and increasing monotonically in the order traversed
 698     /// by the fold operation.
 699     ///
 700     /// The chief purpose of this function is to canonicalize regions so that two
 701     /// `FnSig`s or `TraitRef`s which are equivalent up to region naming will become
 702     /// structurally identical. For example, `for<'a, 'b> fn(&'a isize, &'b isize)` and
 703     /// `for<'a, 'b> fn(&'b isize, &'a isize)` will become identical after anonymization.
 704     pub fn anonymize_late_bound_regions<T>(self, sig: &Binder<T>) -> Binder<T>
 705     where
 706         T: TypeFoldable<'tcx>,
 707     {
 708         let mut counter = 0;
 709         Binder::bind(
 710             self.replace_late_bound_regions(sig, |_| {
 711                 counter += 1;
 712                 self.mk_region(ty::ReLateBound(ty::INNERMOST, ty::BrAnon(counter)))
 713             })
 714             .0,
 715         )
 716     }
 717 }
 718
 719 ///////////////////////////////////////////////////////////////////////////
 720 // Shifter
 721 //
 722 // Shifts the De Bruijn indices on all escaping bound vars by a
 723 // fixed amount. Useful in substitution or when otherwise introducing
 724 // a binding level that is not intended to capture the existing bound
 725 // vars. See comment on `shift_vars_through_binders` method in
 726 // `subst.rs` for more details.
 727
 728 enum Direction {
 729     In,
 730     Out,
 731 }
 732
 733 struct Shifter<'tcx> {
 734     tcx: TyCtxt<'tcx>,
 735     current_index: ty::DebruijnIndex,
 736     amount: u32,
 737     direction: Direction,
 738 }
 739
 740 impl Shifter<'tcx> {
 741     pub fn new(tcx: TyCtxt<'tcx>, amount: u32, direction: Direction) -> Self {
 742         Shifter { tcx, current_index: ty::INNERMOST, amount, direction }
 743     }
 744 }
 745
 746 impl TypeFolder<'tcx> for Shifter<'tcx> {
 747     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 748         self.tcx
 749     }
 750
 751     fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
 752         self.current_index.shift_in(1);
 753         let t = t.super_fold_with(self);
 754         self.current_index.shift_out(1);
 755         t
 756     }
 757
 758     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 759         match *r {
 760             ty::ReLateBound(debruijn, br) => {
 761                 if self.amount == 0 || debruijn < self.current_index {
 762                     r
 763                 } else {
 764                     let debruijn = match self.direction {
 765                         Direction::In => debruijn.shifted_in(self.amount),
 766                         Direction::Out => {
 767                             assert!(debruijn.as_u32() >= self.amount);
 768                             debruijn.shifted_out(self.amount)
 769                         }
 770                     };
 771                     let shifted = ty::ReLateBound(debruijn, br);
 772                     self.tcx.mk_region(shifted)
 773                 }
 774             }
 775             _ => r,
 776         }
 777     }
 778
 779     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 780         match ty.kind {
 781             ty::Bound(debruijn, bound_ty) => {
 782                 if self.amount == 0 || debruijn < self.current_index {
 783                     ty
 784                 } else {
 785                     let debruijn = match self.direction {
 786                         Direction::In => debruijn.shifted_in(self.amount),
 787                         Direction::Out => {
 788                             assert!(debruijn.as_u32() >= self.amount);
 789                             debruijn.shifted_out(self.amount)
 790                         }
 791                     };
 792                     self.tcx.mk_ty(ty::Bound(debruijn, bound_ty))
 793                 }
 794             }
 795
 796             _ => ty.super_fold_with(self),
 797         }
 798     }
 799
 800     fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 801         if let ty::Const { val: ty::ConstKind::Bound(debruijn, bound_ct), ty } = *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(ty::Const { val: ty::ConstKind::Bound(debruijn, bound_ct), ty })
 813             }
 814         } else {
 815             ct.super_fold_with(self)
 816         }
 817     }
 818 }
 819
 820 pub fn shift_region<'tcx>(
 821     tcx: TyCtxt<'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         _ => region,
 830     }
 831 }
 832
 833 pub fn shift_vars<'tcx, T>(tcx: TyCtxt<'tcx>, value: &T, amount: u32) -> T
 834 where
 835     T: TypeFoldable<'tcx>,
 836 {
 837     debug!("shift_vars(value={:?}, amount={})", value, amount);
 838
 839     value.fold_with(&mut Shifter::new(tcx, amount, Direction::In))
 840 }
 841
 842 pub fn shift_out_vars<'tcx, T>(tcx: TyCtxt<'tcx>, value: &T, amount: u32) -> T
 843 where
 844     T: TypeFoldable<'tcx>,
 845 {
 846     debug!("shift_out_vars(value={:?}, amount={})", value, amount);
 847
 848     value.fold_with(&mut Shifter::new(tcx, amount, Direction::Out))
 849 }
 850
 851 /// An "escaping var" is a bound var whose binder is not part of `t`. A bound var can be a
 852 /// bound region or a bound type.
 853 ///
 854 /// So, for example, consider a type like the following, which has two binders:
 855 ///
 856 ///    for<'a> fn(x: for<'b> fn(&'a isize, &'b isize))
 857 ///    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ outer scope
 858 ///                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~  inner scope
 859 ///
 860 /// This type has *bound regions* (`'a`, `'b`), but it does not have escaping regions, because the
 861 /// binders of both `'a` and `'b` are part of the type itself. However, if we consider the *inner
 862 /// fn type*, that type has an escaping region: `'a`.
 863 ///
 864 /// Note that what I'm calling an "escaping var" is often just called a "free var". However,
 865 /// we already use the term "free var". It refers to the regions or types that we use to represent
 866 /// bound regions or type params on a fn definition while we are type checking its body.
 867 ///
 868 /// To clarify, conceptually there is no particular difference between
 869 /// an "escaping" var and a "free" var. However, there is a big
 870 /// difference in practice. Basically, when "entering" a binding
 871 /// level, one is generally required to do some sort of processing to
 872 /// a bound var, such as replacing it with a fresh/placeholder
 873 /// var, or making an entry in the environment to represent the
 874 /// scope to which it is attached, etc. An escaping var represents
 875 /// a bound var for which this processing has not yet been done.
 876 struct HasEscapingVarsVisitor {
 877     /// Anything bound by `outer_index` or "above" is escaping.
 878     outer_index: ty::DebruijnIndex,
 879 }
 880
 881 impl<'tcx> TypeVisitor<'tcx> for HasEscapingVarsVisitor {
 882     fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
 883         self.outer_index.shift_in(1);
 884         let result = t.super_visit_with(self);
 885         self.outer_index.shift_out(1);
 886         result
 887     }
 888
 889     fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
 890         // If the outer-exclusive-binder is *strictly greater* than
 891         // `outer_index`, that means that `t` contains some content
 892         // bound at `outer_index` or above (because
 893         // `outer_exclusive_binder` is always 1 higher than the
 894         // content in `t`). Therefore, `t` has some escaping vars.
 895         t.outer_exclusive_binder > self.outer_index
 896     }
 897
 898     fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
 899         // If the region is bound by `outer_index` or anything outside
 900         // of outer index, then it escapes the binders we have
 901         // visited.
 902         r.bound_at_or_above_binder(self.outer_index)
 903     }
 904
 905     fn visit_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> bool {
 906         // we don't have a `visit_infer_const` callback, so we have to
 907         // hook in here to catch this case (annoying...), but
 908         // otherwise we do want to remember to visit the rest of the
 909         // const, as it has types/regions embedded in a lot of other
 910         // places.
 911         match ct.val {
 912             ty::ConstKind::Bound(debruijn, _) if debruijn >= self.outer_index => true,
 913             _ => ct.super_visit_with(self),
 914         }
 915     }
 916 }
 917
 918 // FIXME: Optimize for checking for infer flags
 919 struct HasTypeFlagsVisitor {
 920     flags: ty::TypeFlags,
 921 }
 922
 923 impl<'tcx> TypeVisitor<'tcx> for HasTypeFlagsVisitor {
 924     fn visit_ty(&mut self, t: Ty<'_>) -> bool {
 925         debug!("HasTypeFlagsVisitor: t={:?} t.flags={:?} self.flags={:?}", t, t.flags, self.flags);
 926         t.flags.intersects(self.flags)
 927     }
 928
 929     fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
 930         let flags = r.type_flags();
 931         debug!("HasTypeFlagsVisitor: r={:?} r.flags={:?} self.flags={:?}", r, flags, self.flags);
 932         flags.intersects(self.flags)
 933     }
 934
 935     fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> bool {
 936         let flags = FlagComputation::for_const(c);
 937         debug!("HasTypeFlagsVisitor: c={:?} c.flags={:?} self.flags={:?}", c, flags, self.flags);
 938         flags.intersects(self.flags)
 939     }
 940 }
 941
 942 /// Collects all the late-bound regions at the innermost binding level
 943 /// into a hash set.
 944 struct LateBoundRegionsCollector {
 945     current_index: ty::DebruijnIndex,
 946     regions: FxHashSet<ty::BoundRegion>,
 947
 948     /// `true` if we only want regions that are known to be
 949     /// "constrained" when you equate this type with another type. In
 950     /// particular, if you have e.g., `&'a u32` and `&'b u32`, equating
 951     /// them constraints `'a == 'b`. But if you have `<&'a u32 as
 952     /// Trait>::Foo` and `<&'b u32 as Trait>::Foo`, normalizing those
 953     /// types may mean that `'a` and `'b` don't appear in the results,
 954     /// so they are not considered *constrained*.
 955     just_constrained: bool,
 956 }
 957
 958 impl LateBoundRegionsCollector {
 959     fn new(just_constrained: bool) -> Self {
 960         LateBoundRegionsCollector {
 961             current_index: ty::INNERMOST,
 962             regions: Default::default(),
 963             just_constrained,
 964         }
 965     }
 966 }
 967
 968 impl<'tcx> TypeVisitor<'tcx> for LateBoundRegionsCollector {
 969     fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
 970         self.current_index.shift_in(1);
 971         let result = t.super_visit_with(self);
 972         self.current_index.shift_out(1);
 973         result
 974     }
 975
 976     fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
 977         // if we are only looking for "constrained" region, we have to
 978         // ignore the inputs to a projection, as they may not appear
 979         // in the normalized form
 980         if self.just_constrained {
 981             match t.kind {
 982                 ty::Projection(..) | ty::Opaque(..) => {
 983                     return false;
 984                 }
 985                 _ => {}
 986             }
 987         }
 988
 989         t.super_visit_with(self)
 990     }
 991
 992     fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
 993         if let ty::ReLateBound(debruijn, br) = *r {
 994             if debruijn == self.current_index {
 995                 self.regions.insert(br);
 996             }
 997         }
 998         false
 999     }
1000 }