src/librustc_middle/ty/subst.rs

   1 // Type substitutions.
   2
   3 use crate::infer::canonical::Canonical;
   4 use crate::ty::codec::{TyDecoder, TyEncoder};
   5 use crate::ty::fold::{TypeFoldable, TypeFolder, TypeVisitor};
   6 use crate::ty::sty::{ClosureSubsts, GeneratorSubsts};
   7 use crate::ty::{self, Lift, List, ParamConst, Ty, TyCtxt};
   8
   9 use rustc_hir::def_id::DefId;
  10 use rustc_macros::HashStable;
  11 use rustc_serialize::{self, Decodable, Encodable};
  12 use rustc_span::{Span, DUMMY_SP};
  13 use smallvec::SmallVec;
  14
  15 use core::intrinsics;
  16 use std::cmp::Ordering;
  17 use std::fmt;
  18 use std::marker::PhantomData;
  19 use std::mem;
  20 use std::num::NonZeroUsize;
  21
  22 /// An entity in the Rust type system, which can be one of
  23 /// several kinds (types, lifetimes, and consts).
  24 /// To reduce memory usage, a `GenericArg` is a interned pointer,
  25 /// with the lowest 2 bits being reserved for a tag to
  26 /// indicate the type (`Ty`, `Region`, or `Const`) it points to.
  27 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
  28 pub struct GenericArg<'tcx> {
  29     ptr: NonZeroUsize,
  30     marker: PhantomData<(Ty<'tcx>, ty::Region<'tcx>, &'tcx ty::Const<'tcx>)>,
  31 }
  32
  33 const TAG_MASK: usize = 0b11;
  34 const TYPE_TAG: usize = 0b00;
  35 const REGION_TAG: usize = 0b01;
  36 const CONST_TAG: usize = 0b10;
  37
  38 #[derive(Debug, TyEncodable, TyDecodable, PartialEq, Eq, PartialOrd, Ord, HashStable)]
  39 pub enum GenericArgKind<'tcx> {
  40     Lifetime(ty::Region<'tcx>),
  41     Type(Ty<'tcx>),
  42     Const(&'tcx ty::Const<'tcx>),
  43 }
  44
  45 impl<'tcx> GenericArgKind<'tcx> {
  46     fn pack(self) -> GenericArg<'tcx> {
  47         let (tag, ptr) = match self {
  48             GenericArgKind::Lifetime(lt) => {
  49                 // Ensure we can use the tag bits.
  50                 assert_eq!(mem::align_of_val(lt) & TAG_MASK, 0);
  51                 (REGION_TAG, lt as *const _ as usize)
  52             }
  53             GenericArgKind::Type(ty) => {
  54                 // Ensure we can use the tag bits.
  55                 assert_eq!(mem::align_of_val(ty) & TAG_MASK, 0);
  56                 (TYPE_TAG, ty as *const _ as usize)
  57             }
  58             GenericArgKind::Const(ct) => {
  59                 // Ensure we can use the tag bits.
  60                 assert_eq!(mem::align_of_val(ct) & TAG_MASK, 0);
  61                 (CONST_TAG, ct as *const _ as usize)
  62             }
  63         };
  64
  65         GenericArg { ptr: unsafe { NonZeroUsize::new_unchecked(ptr | tag) }, marker: PhantomData }
  66     }
  67 }
  68
  69 impl fmt::Debug for GenericArg<'tcx> {
  70     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  71         match self.unpack() {
  72             GenericArgKind::Lifetime(lt) => lt.fmt(f),
  73             GenericArgKind::Type(ty) => ty.fmt(f),
  74             GenericArgKind::Const(ct) => ct.fmt(f),
  75         }
  76     }
  77 }
  78
  79 impl<'tcx> Ord for GenericArg<'tcx> {
  80     fn cmp(&self, other: &GenericArg<'_>) -> Ordering {
  81         self.unpack().cmp(&other.unpack())
  82     }
  83 }
  84
  85 impl<'tcx> PartialOrd for GenericArg<'tcx> {
  86     fn partial_cmp(&self, other: &GenericArg<'_>) -> Option<Ordering> {
  87         Some(self.cmp(&other))
  88     }
  89 }
  90
  91 impl<'tcx> From<ty::Region<'tcx>> for GenericArg<'tcx> {
  92     fn from(r: ty::Region<'tcx>) -> GenericArg<'tcx> {
  93         GenericArgKind::Lifetime(r).pack()
  94     }
  95 }
  96
  97 impl<'tcx> From<Ty<'tcx>> for GenericArg<'tcx> {
  98     fn from(ty: Ty<'tcx>) -> GenericArg<'tcx> {
  99         GenericArgKind::Type(ty).pack()
 100     }
 101 }
 102
 103 impl<'tcx> From<&'tcx ty::Const<'tcx>> for GenericArg<'tcx> {
 104     fn from(c: &'tcx ty::Const<'tcx>) -> GenericArg<'tcx> {
 105         GenericArgKind::Const(c).pack()
 106     }
 107 }
 108
 109 impl<'tcx> GenericArg<'tcx> {
 110     #[inline]
 111     pub fn unpack(self) -> GenericArgKind<'tcx> {
 112         let ptr = self.ptr.get();
 113         unsafe {
 114             match ptr & TAG_MASK {
 115                 REGION_TAG => GenericArgKind::Lifetime(&*((ptr & !TAG_MASK) as *const _)),
 116                 TYPE_TAG => GenericArgKind::Type(&*((ptr & !TAG_MASK) as *const _)),
 117                 CONST_TAG => GenericArgKind::Const(&*((ptr & !TAG_MASK) as *const _)),
 118                 _ => intrinsics::unreachable(),
 119             }
 120         }
 121     }
 122
 123     /// Unpack the `GenericArg` as a type when it is known certainly to be a type.
 124     /// This is true in cases where `Substs` is used in places where the kinds are known
 125     /// to be limited (e.g. in tuples, where the only parameters are type parameters).
 126     pub fn expect_ty(self) -> Ty<'tcx> {
 127         match self.unpack() {
 128             GenericArgKind::Type(ty) => ty,
 129             _ => bug!("expected a type, but found another kind"),
 130         }
 131     }
 132
 133     /// Unpack the `GenericArg` as a const when it is known certainly to be a const.
 134     pub fn expect_const(self) -> &'tcx ty::Const<'tcx> {
 135         match self.unpack() {
 136             GenericArgKind::Const(c) => c,
 137             _ => bug!("expected a const, but found another kind"),
 138         }
 139     }
 140 }
 141
 142 impl<'a, 'tcx> Lift<'tcx> for GenericArg<'a> {
 143     type Lifted = GenericArg<'tcx>;
 144
 145     fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
 146         match self.unpack() {
 147             GenericArgKind::Lifetime(lt) => tcx.lift(&lt).map(|lt| lt.into()),
 148             GenericArgKind::Type(ty) => tcx.lift(&ty).map(|ty| ty.into()),
 149             GenericArgKind::Const(ct) => tcx.lift(&ct).map(|ct| ct.into()),
 150         }
 151     }
 152 }
 153
 154 impl<'tcx> TypeFoldable<'tcx> for GenericArg<'tcx> {
 155     fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
 156         match self.unpack() {
 157             GenericArgKind::Lifetime(lt) => lt.fold_with(folder).into(),
 158             GenericArgKind::Type(ty) => ty.fold_with(folder).into(),
 159             GenericArgKind::Const(ct) => ct.fold_with(folder).into(),
 160         }
 161     }
 162
 163     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
 164         match self.unpack() {
 165             GenericArgKind::Lifetime(lt) => lt.visit_with(visitor),
 166             GenericArgKind::Type(ty) => ty.visit_with(visitor),
 167             GenericArgKind::Const(ct) => ct.visit_with(visitor),
 168         }
 169     }
 170 }
 171
 172 impl<'tcx, E: TyEncoder<'tcx>> Encodable<E> for GenericArg<'tcx> {
 173     fn encode(&self, e: &mut E) -> Result<(), E::Error> {
 174         self.unpack().encode(e)
 175     }
 176 }
 177
 178 impl<'tcx, D: TyDecoder<'tcx>> Decodable<D> for GenericArg<'tcx> {
 179     fn decode(d: &mut D) -> Result<GenericArg<'tcx>, D::Error> {
 180         Ok(GenericArgKind::decode(d)?.pack())
 181     }
 182 }
 183
 184 /// A substitution mapping generic parameters to new values.
 185 pub type InternalSubsts<'tcx> = List<GenericArg<'tcx>>;
 186
 187 pub type SubstsRef<'tcx> = &'tcx InternalSubsts<'tcx>;
 188
 189 impl<'a, 'tcx> InternalSubsts<'tcx> {
 190     /// Interpret these substitutions as the substitutions of a closure type.
 191     /// Closure substitutions have a particular structure controlled by the
 192     /// compiler that encodes information like the signature and closure kind;
 193     /// see `ty::ClosureSubsts` struct for more comments.
 194     pub fn as_closure(&'a self) -> ClosureSubsts<'a> {
 195         ClosureSubsts { substs: self }
 196     }
 197
 198     /// Interpret these substitutions as the substitutions of a generator type.
 199     /// Closure substitutions have a particular structure controlled by the
 200     /// compiler that encodes information like the signature and generator kind;
 201     /// see `ty::GeneratorSubsts` struct for more comments.
 202     pub fn as_generator(&'tcx self) -> GeneratorSubsts<'tcx> {
 203         GeneratorSubsts { substs: self }
 204     }
 205
 206     /// Creates a `InternalSubsts` that maps each generic parameter to itself.
 207     pub fn identity_for_item(tcx: TyCtxt<'tcx>, def_id: DefId) -> SubstsRef<'tcx> {
 208         Self::for_item(tcx, def_id, |param, _| tcx.mk_param_from_def(param))
 209     }
 210
 211     /// Creates a `InternalSubsts` for generic parameter definitions,
 212     /// by calling closures to obtain each kind.
 213     /// The closures get to observe the `InternalSubsts` as they're
 214     /// being built, which can be used to correctly
 215     /// substitute defaults of generic parameters.
 216     pub fn for_item<F>(tcx: TyCtxt<'tcx>, def_id: DefId, mut mk_kind: F) -> SubstsRef<'tcx>
 217     where
 218         F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
 219     {
 220         let defs = tcx.generics_of(def_id);
 221         let count = defs.count();
 222         let mut substs = SmallVec::with_capacity(count);
 223         Self::fill_item(&mut substs, tcx, defs, &mut mk_kind);
 224         tcx.intern_substs(&substs)
 225     }
 226
 227     pub fn extend_to<F>(&self, tcx: TyCtxt<'tcx>, def_id: DefId, mut mk_kind: F) -> SubstsRef<'tcx>
 228     where
 229         F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
 230     {
 231         Self::for_item(tcx, def_id, |param, substs| {
 232             self.get(param.index as usize).cloned().unwrap_or_else(|| mk_kind(param, substs))
 233         })
 234     }
 235
 236     fn fill_item<F>(
 237         substs: &mut SmallVec<[GenericArg<'tcx>; 8]>,
 238         tcx: TyCtxt<'tcx>,
 239         defs: &ty::Generics,
 240         mk_kind: &mut F,
 241     ) where
 242         F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
 243     {
 244         if let Some(def_id) = defs.parent {
 245             let parent_defs = tcx.generics_of(def_id);
 246             Self::fill_item(substs, tcx, parent_defs, mk_kind);
 247         }
 248         Self::fill_single(substs, defs, mk_kind)
 249     }
 250
 251     fn fill_single<F>(
 252         substs: &mut SmallVec<[GenericArg<'tcx>; 8]>,
 253         defs: &ty::Generics,
 254         mk_kind: &mut F,
 255     ) where
 256         F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
 257     {
 258         substs.reserve(defs.params.len());
 259         for param in &defs.params {
 260             let kind = mk_kind(param, substs);
 261             assert_eq!(param.index as usize, substs.len());
 262             substs.push(kind);
 263         }
 264     }
 265
 266     pub fn is_noop(&self) -> bool {
 267         self.is_empty()
 268     }
 269
 270     #[inline]
 271     pub fn types(&'a self) -> impl DoubleEndedIterator<Item = Ty<'tcx>> + 'a {
 272         self.iter()
 273             .filter_map(|k| if let GenericArgKind::Type(ty) = k.unpack() { Some(ty) } else { None })
 274     }
 275
 276     #[inline]
 277     pub fn regions(&'a self) -> impl DoubleEndedIterator<Item = ty::Region<'tcx>> + 'a {
 278         self.iter().filter_map(|k| {
 279             if let GenericArgKind::Lifetime(lt) = k.unpack() { Some(lt) } else { None }
 280         })
 281     }
 282
 283     #[inline]
 284     pub fn consts(&'a self) -> impl DoubleEndedIterator<Item = &'tcx ty::Const<'tcx>> + 'a {
 285         self.iter().filter_map(|k| {
 286             if let GenericArgKind::Const(ct) = k.unpack() { Some(ct) } else { None }
 287         })
 288     }
 289
 290     #[inline]
 291     pub fn non_erasable_generics(
 292         &'a self,
 293     ) -> impl DoubleEndedIterator<Item = GenericArgKind<'tcx>> + 'a {
 294         self.iter().filter_map(|k| match k.unpack() {
 295             GenericArgKind::Lifetime(_) => None,
 296             generic => Some(generic),
 297         })
 298     }
 299
 300     #[inline]
 301     pub fn type_at(&self, i: usize) -> Ty<'tcx> {
 302         if let GenericArgKind::Type(ty) = self[i].unpack() {
 303             ty
 304         } else {
 305             bug!("expected type for param #{} in {:?}", i, self);
 306         }
 307     }
 308
 309     #[inline]
 310     pub fn region_at(&self, i: usize) -> ty::Region<'tcx> {
 311         if let GenericArgKind::Lifetime(lt) = self[i].unpack() {
 312             lt
 313         } else {
 314             bug!("expected region for param #{} in {:?}", i, self);
 315         }
 316     }
 317
 318     #[inline]
 319     pub fn const_at(&self, i: usize) -> &'tcx ty::Const<'tcx> {
 320         if let GenericArgKind::Const(ct) = self[i].unpack() {
 321             ct
 322         } else {
 323             bug!("expected const for param #{} in {:?}", i, self);
 324         }
 325     }
 326
 327     #[inline]
 328     pub fn type_for_def(&self, def: &ty::GenericParamDef) -> GenericArg<'tcx> {
 329         self.type_at(def.index as usize).into()
 330     }
 331
 332     /// Transform from substitutions for a child of `source_ancestor`
 333     /// (e.g., a trait or impl) to substitutions for the same child
 334     /// in a different item, with `target_substs` as the base for
 335     /// the target impl/trait, with the source child-specific
 336     /// parameters (e.g., method parameters) on top of that base.
 337     ///
 338     /// For example given:
 339     ///
 340     /// ```no_run
 341     /// trait X<S> { fn f<T>(); }
 342     /// impl<U> X<U> for U { fn f<V>() {} }
 343     /// ```
 344     ///
 345     /// * If `self` is `[Self, S, T]`: the identity substs of `f` in the trait.
 346     /// * If `source_ancestor` is the def_id of the trait.
 347     /// * If `target_substs` is `[U]`, the substs for the impl.
 348     /// * Then we will return `[U, T]`, the subst for `f` in the impl that
 349     ///   are needed for it to match the trait.
 350     pub fn rebase_onto(
 351         &self,
 352         tcx: TyCtxt<'tcx>,
 353         source_ancestor: DefId,
 354         target_substs: SubstsRef<'tcx>,
 355     ) -> SubstsRef<'tcx> {
 356         let defs = tcx.generics_of(source_ancestor);
 357         tcx.mk_substs(target_substs.iter().chain(self.iter().skip(defs.params.len())))
 358     }
 359
 360     pub fn truncate_to(&self, tcx: TyCtxt<'tcx>, generics: &ty::Generics) -> SubstsRef<'tcx> {
 361         tcx.mk_substs(self.iter().take(generics.count()))
 362     }
 363 }
 364
 365 impl<'tcx> TypeFoldable<'tcx> for SubstsRef<'tcx> {
 366     fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
 367         // This code is hot enough that it's worth specializing for the most
 368         // common length lists, to avoid the overhead of `SmallVec` creation.
 369         // The match arms are in order of frequency. The 1, 2, and 0 cases are
 370         // typically hit in 90--99.99% of cases. When folding doesn't change
 371         // the substs, it's faster to reuse the existing substs rather than
 372         // calling `intern_substs`.
 373         match self.len() {
 374             1 => {
 375                 let param0 = self[0].fold_with(folder);
 376                 if param0 == self[0] { self } else { folder.tcx().intern_substs(&[param0]) }
 377             }
 378             2 => {
 379                 let param0 = self[0].fold_with(folder);
 380                 let param1 = self[1].fold_with(folder);
 381                 if param0 == self[0] && param1 == self[1] {
 382                     self
 383                 } else {
 384                     folder.tcx().intern_substs(&[param0, param1])
 385                 }
 386             }
 387             0 => self,
 388             _ => {
 389                 let params: SmallVec<[_; 8]> = self.iter().map(|k| k.fold_with(folder)).collect();
 390                 if params[..] == self[..] { self } else { folder.tcx().intern_substs(&params) }
 391             }
 392         }
 393     }
 394
 395     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
 396         self.iter().any(|t| t.visit_with(visitor))
 397     }
 398 }
 399
 400 ///////////////////////////////////////////////////////////////////////////
 401 // Public trait `Subst`
 402 //
 403 // Just call `foo.subst(tcx, substs)` to perform a substitution across
 404 // `foo`. Or use `foo.subst_spanned(tcx, substs, Some(span))` when
 405 // there is more information available (for better errors).
 406
 407 pub trait Subst<'tcx>: Sized {
 408     fn subst(&self, tcx: TyCtxt<'tcx>, substs: &[GenericArg<'tcx>]) -> Self {
 409         self.subst_spanned(tcx, substs, None)
 410     }
 411
 412     fn subst_spanned(
 413         &self,
 414         tcx: TyCtxt<'tcx>,
 415         substs: &[GenericArg<'tcx>],
 416         span: Option<Span>,
 417     ) -> Self;
 418 }
 419
 420 impl<'tcx, T: TypeFoldable<'tcx>> Subst<'tcx> for T {
 421     fn subst_spanned(
 422         &self,
 423         tcx: TyCtxt<'tcx>,
 424         substs: &[GenericArg<'tcx>],
 425         span: Option<Span>,
 426     ) -> T {
 427         let mut folder = SubstFolder { tcx, substs, span, binders_passed: 0 };
 428         (*self).fold_with(&mut folder)
 429     }
 430 }
 431
 432 ///////////////////////////////////////////////////////////////////////////
 433 // The actual substitution engine itself is a type folder.
 434
 435 struct SubstFolder<'a, 'tcx> {
 436     tcx: TyCtxt<'tcx>,
 437     substs: &'a [GenericArg<'tcx>],
 438
 439     /// The location for which the substitution is performed, if available.
 440     span: Option<Span>,
 441
 442     /// Number of region binders we have passed through while doing the substitution
 443     binders_passed: u32,
 444 }
 445
 446 impl<'a, 'tcx> TypeFolder<'tcx> for SubstFolder<'a, 'tcx> {
 447     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 448         self.tcx
 449     }
 450
 451     fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
 452         self.binders_passed += 1;
 453         let t = t.super_fold_with(self);
 454         self.binders_passed -= 1;
 455         t
 456     }
 457
 458     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 459         // Note: This routine only handles regions that are bound on
 460         // type declarations and other outer declarations, not those
 461         // bound in *fn types*. Region substitution of the bound
 462         // regions that appear in a function signature is done using
 463         // the specialized routine `ty::replace_late_regions()`.
 464         match *r {
 465             ty::ReEarlyBound(data) => {
 466                 let rk = self.substs.get(data.index as usize).map(|k| k.unpack());
 467                 match rk {
 468                     Some(GenericArgKind::Lifetime(lt)) => self.shift_region_through_binders(lt),
 469                     _ => {
 470                         let span = self.span.unwrap_or(DUMMY_SP);
 471                         let msg = format!(
 472                             "Region parameter out of range \
 473                              when substituting in region {} (index={})",
 474                             data.name, data.index
 475                         );
 476                         span_bug!(span, "{}", msg);
 477                     }
 478                 }
 479             }
 480             _ => r,
 481         }
 482     }
 483
 484     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 485         if !t.needs_subst() {
 486             return t;
 487         }
 488
 489         match t.kind {
 490             ty::Param(p) => self.ty_for_param(p, t),
 491             _ => t.super_fold_with(self),
 492         }
 493     }
 494
 495     fn fold_const(&mut self, c: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 496         if !c.needs_subst() {
 497             return c;
 498         }
 499
 500         if let ty::ConstKind::Param(p) = c.val {
 501             self.const_for_param(p, c)
 502         } else {
 503             c.super_fold_with(self)
 504         }
 505     }
 506 }
 507
 508 impl<'a, 'tcx> SubstFolder<'a, 'tcx> {
 509     fn ty_for_param(&self, p: ty::ParamTy, source_ty: Ty<'tcx>) -> Ty<'tcx> {
 510         // Look up the type in the substitutions. It really should be in there.
 511         let opt_ty = self.substs.get(p.index as usize).map(|k| k.unpack());
 512         let ty = match opt_ty {
 513             Some(GenericArgKind::Type(ty)) => ty,
 514             Some(kind) => {
 515                 let span = self.span.unwrap_or(DUMMY_SP);
 516                 span_bug!(
 517                     span,
 518                     "expected type for `{:?}` ({:?}/{}) but found {:?} \
 519                      when substituting, substs={:?}",
 520                     p,
 521                     source_ty,
 522                     p.index,
 523                     kind,
 524                     self.substs,
 525                 );
 526             }
 527             None => {
 528                 let span = self.span.unwrap_or(DUMMY_SP);
 529                 span_bug!(
 530                     span,
 531                     "type parameter `{:?}` ({:?}/{}) out of range \
 532                      when substituting, substs={:?}",
 533                     p,
 534                     source_ty,
 535                     p.index,
 536                     self.substs,
 537                 );
 538             }
 539         };
 540
 541         self.shift_vars_through_binders(ty)
 542     }
 543
 544     fn const_for_param(
 545         &self,
 546         p: ParamConst,
 547         source_ct: &'tcx ty::Const<'tcx>,
 548     ) -> &'tcx ty::Const<'tcx> {
 549         // Look up the const in the substitutions. It really should be in there.
 550         let opt_ct = self.substs.get(p.index as usize).map(|k| k.unpack());
 551         let ct = match opt_ct {
 552             Some(GenericArgKind::Const(ct)) => ct,
 553             Some(kind) => {
 554                 let span = self.span.unwrap_or(DUMMY_SP);
 555                 span_bug!(
 556                     span,
 557                     "expected const for `{:?}` ({:?}/{}) but found {:?} \
 558                      when substituting substs={:?}",
 559                     p,
 560                     source_ct,
 561                     p.index,
 562                     kind,
 563                     self.substs,
 564                 );
 565             }
 566             None => {
 567                 let span = self.span.unwrap_or(DUMMY_SP);
 568                 span_bug!(
 569                     span,
 570                     "const parameter `{:?}` ({:?}/{}) out of range \
 571                      when substituting substs={:?}",
 572                     p,
 573                     source_ct,
 574                     p.index,
 575                     self.substs,
 576                 );
 577             }
 578         };
 579
 580         self.shift_vars_through_binders(ct)
 581     }
 582
 583     /// It is sometimes necessary to adjust the De Bruijn indices during substitution. This occurs
 584     /// when we are substituting a type with escaping bound vars into a context where we have
 585     /// passed through binders. That's quite a mouthful. Let's see an example:
 586     ///
 587     /// ```
 588     /// type Func<A> = fn(A);
 589     /// type MetaFunc = for<'a> fn(Func<&'a i32>)
 590     /// ```
 591     ///
 592     /// The type `MetaFunc`, when fully expanded, will be
 593     ///
 594     ///     for<'a> fn(fn(&'a i32))
 595     ///             ^~ ^~ ^~~
 596     ///             |  |  |
 597     ///             |  |  DebruijnIndex of 2
 598     ///             Binders
 599     ///
 600     /// Here the `'a` lifetime is bound in the outer function, but appears as an argument of the
 601     /// inner one. Therefore, that appearance will have a DebruijnIndex of 2, because we must skip
 602     /// over the inner binder (remember that we count De Bruijn indices from 1). However, in the
 603     /// definition of `MetaFunc`, the binder is not visible, so the type `&'a i32` will have a
 604     /// De Bruijn index of 1. It's only during the substitution that we can see we must increase the
 605     /// depth by 1 to account for the binder that we passed through.
 606     ///
 607     /// As a second example, consider this twist:
 608     ///
 609     /// ```
 610     /// type FuncTuple<A> = (A,fn(A));
 611     /// type MetaFuncTuple = for<'a> fn(FuncTuple<&'a i32>)
 612     /// ```
 613     ///
 614     /// Here the final type will be:
 615     ///
 616     ///     for<'a> fn((&'a i32, fn(&'a i32)))
 617     ///                 ^~~         ^~~
 618     ///                 |           |
 619     ///          DebruijnIndex of 1 |
 620     ///                      DebruijnIndex of 2
 621     ///
 622     /// As indicated in the diagram, here the same type `&'a i32` is substituted once, but in the
 623     /// first case we do not increase the De Bruijn index and in the second case we do. The reason
 624     /// is that only in the second case have we passed through a fn binder.
 625     fn shift_vars_through_binders<T: TypeFoldable<'tcx>>(&self, val: T) -> T {
 626         debug!(
 627             "shift_vars(val={:?}, binders_passed={:?}, has_escaping_bound_vars={:?})",
 628             val,
 629             self.binders_passed,
 630             val.has_escaping_bound_vars()
 631         );
 632
 633         if self.binders_passed == 0 || !val.has_escaping_bound_vars() {
 634             return val;
 635         }
 636
 637         let result = ty::fold::shift_vars(self.tcx(), &val, self.binders_passed);
 638         debug!("shift_vars: shifted result = {:?}", result);
 639
 640         result
 641     }
 642
 643     fn shift_region_through_binders(&self, region: ty::Region<'tcx>) -> ty::Region<'tcx> {
 644         if self.binders_passed == 0 || !region.has_escaping_bound_vars() {
 645             return region;
 646         }
 647         ty::fold::shift_region(self.tcx, region, self.binders_passed)
 648     }
 649 }
 650
 651 pub type CanonicalUserSubsts<'tcx> = Canonical<'tcx, UserSubsts<'tcx>>;
 652
 653 /// Stores the user-given substs to reach some fully qualified path
 654 /// (e.g., `<T>::Item` or `<T as Trait>::Item`).
 655 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyEncodable, TyDecodable)]
 656 #[derive(HashStable, TypeFoldable, Lift)]
 657 pub struct UserSubsts<'tcx> {
 658     /// The substitutions for the item as given by the user.
 659     pub substs: SubstsRef<'tcx>,
 660
 661     /// The self type, in the case of a `<T>::Item` path (when applied
 662     /// to an inherent impl). See `UserSelfTy` below.
 663     pub user_self_ty: Option<UserSelfTy<'tcx>>,
 664 }
 665
 666 /// Specifies the user-given self type. In the case of a path that
 667 /// refers to a member in an inherent impl, this self type is
 668 /// sometimes needed to constrain the type parameters on the impl. For
 669 /// example, in this code:
 670 ///
 671 /// ```
 672 /// struct Foo<T> { }
 673 /// impl<A> Foo<A> { fn method() { } }
 674 /// ```
 675 ///
 676 /// when you then have a path like `<Foo<&'static u32>>::method`,
 677 /// this struct would carry the `DefId` of the impl along with the
 678 /// self type `Foo<u32>`. Then we can instantiate the parameters of
 679 /// the impl (with the substs from `UserSubsts`) and apply those to
 680 /// the self type, giving `Foo<?A>`. Finally, we unify that with
 681 /// the self type here, which contains `?A` to be `&'static u32`
 682 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyEncodable, TyDecodable)]
 683 #[derive(HashStable, TypeFoldable, Lift)]
 684 pub struct UserSelfTy<'tcx> {
 685     pub impl_def_id: DefId,
 686     pub self_ty: Ty<'tcx>,
 687 }