src/librustc_middle/ty/subst.rs - third_party/rust - Git at Google

 // Type substitutions.

 use crate::infer::canonical::Canonical;
 use crate::ty::fold::{TypeFoldable, TypeFolder, TypeVisitor};
 use crate::ty::sty::{ClosureSubsts, GeneratorSubsts};
 use crate::ty::{self, Lift, List, ParamConst, Ty, TyCtxt};

 use rustc_hir::def_id::DefId;
 use rustc_macros::HashStable;
 use rustc_serialize::{self, Decodable, Decoder, Encodable, Encoder};
 use rustc_span::{Span, DUMMY_SP};
 use smallvec::SmallVec;

 use core::intrinsics;
 use std::cmp::Ordering;
 use std::fmt;
 use std::marker::PhantomData;
 use std::mem;
 use std::num::NonZeroUsize;

 /// An entity in the Rust type system, which can be one of
 /// several kinds (types, lifetimes, and consts).
 /// To reduce memory usage, a `GenericArg` is a interned pointer,
 /// with the lowest 2 bits being reserved for a tag to
 /// indicate the type (`Ty`, `Region`, or `Const`) it points to.
 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
 pub struct GenericArg<'tcx> {
     ptr: NonZeroUsize,
     marker: PhantomData<(Ty<'tcx>, ty::Region<'tcx>, &'tcx ty::Const<'tcx>)>,
 }

 const TAG_MASK: usize = 0b11;
 const TYPE_TAG: usize = 0b00;
 const REGION_TAG: usize = 0b01;
 const CONST_TAG: usize = 0b10;

 #[derive(Debug, RustcEncodable, RustcDecodable, PartialEq, Eq, PartialOrd, Ord, HashStable)]
 pub enum GenericArgKind<'tcx> {
     Lifetime(ty::Region<'tcx>),
     Type(Ty<'tcx>),
     Const(&'tcx ty::Const<'tcx>),
 }

 impl<'tcx> GenericArgKind<'tcx> {
     fn pack(self) -> GenericArg<'tcx> {
         let (tag, ptr) = match self {
             GenericArgKind::Lifetime(lt) => {
                 // Ensure we can use the tag bits.
                 assert_eq!(mem::align_of_val(lt) & TAG_MASK, 0);
                 (REGION_TAG, lt as *const _ as usize)
             }
             GenericArgKind::Type(ty) => {
                 // Ensure we can use the tag bits.
                 assert_eq!(mem::align_of_val(ty) & TAG_MASK, 0);
                 (TYPE_TAG, ty as *const _ as usize)
             }
             GenericArgKind::Const(ct) => {
                 // Ensure we can use the tag bits.
                 assert_eq!(mem::align_of_val(ct) & TAG_MASK, 0);
                 (CONST_TAG, ct as *const _ as usize)
             }
         };

         GenericArg { ptr: unsafe { NonZeroUsize::new_unchecked(ptr | tag) }, marker: PhantomData }
     }
 }

 impl fmt::Debug for GenericArg<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self.unpack() {
             GenericArgKind::Lifetime(lt) => lt.fmt(f),
             GenericArgKind::Type(ty) => ty.fmt(f),
             GenericArgKind::Const(ct) => ct.fmt(f),
         }
     }
 }

 impl<'tcx> Ord for GenericArg<'tcx> {
     fn cmp(&self, other: &GenericArg<'_>) -> Ordering {
         self.unpack().cmp(&other.unpack())
     }
 }

 impl<'tcx> PartialOrd for GenericArg<'tcx> {
     fn partial_cmp(&self, other: &GenericArg<'_>) -> Option<Ordering> {
         Some(self.cmp(&other))
     }
 }

 impl<'tcx> From<ty::Region<'tcx>> for GenericArg<'tcx> {
     fn from(r: ty::Region<'tcx>) -> GenericArg<'tcx> {
         GenericArgKind::Lifetime(r).pack()
     }
 }

 impl<'tcx> From<Ty<'tcx>> for GenericArg<'tcx> {
     fn from(ty: Ty<'tcx>) -> GenericArg<'tcx> {
         GenericArgKind::Type(ty).pack()
     }
 }

 impl<'tcx> From<&'tcx ty::Const<'tcx>> for GenericArg<'tcx> {
     fn from(c: &'tcx ty::Const<'tcx>) -> GenericArg<'tcx> {
         GenericArgKind::Const(c).pack()
     }
 }

 impl<'tcx> GenericArg<'tcx> {
     #[inline]
     pub fn unpack(self) -> GenericArgKind<'tcx> {
         let ptr = self.ptr.get();
         unsafe {
             match ptr & TAG_MASK {
                 REGION_TAG => GenericArgKind::Lifetime(&*((ptr & !TAG_MASK) as *const _)),
                 TYPE_TAG => GenericArgKind::Type(&*((ptr & !TAG_MASK) as *const _)),
                 CONST_TAG => GenericArgKind::Const(&*((ptr & !TAG_MASK) as *const _)),
                 _ => intrinsics::unreachable(),
             }
         }
     }

     /// Unpack the `GenericArg` as a type when it is known certainly to be a type.
     /// This is true in cases where `Substs` is used in places where the kinds are known
     /// to be limited (e.g. in tuples, where the only parameters are type parameters).
     pub fn expect_ty(self) -> Ty<'tcx> {
         match self.unpack() {
             GenericArgKind::Type(ty) => ty,
             _ => bug!("expected a type, but found another kind"),
         }
     }

     /// Unpack the `GenericArg` as a const when it is known certainly to be a const.
     pub fn expect_const(self) -> &'tcx ty::Const<'tcx> {
         match self.unpack() {
             GenericArgKind::Const(c) => c,
             _ => bug!("expected a const, but found another kind"),
         }
     }
 }

 impl<'a, 'tcx> Lift<'tcx> for GenericArg<'a> {
     type Lifted = GenericArg<'tcx>;

     fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
         match self.unpack() {
             GenericArgKind::Lifetime(lt) => tcx.lift(&lt).map(|lt| lt.into()),
             GenericArgKind::Type(ty) => tcx.lift(&ty).map(|ty| ty.into()),
             GenericArgKind::Const(ct) => tcx.lift(&ct).map(|ct| ct.into()),
         }
     }
 }

 impl<'tcx> TypeFoldable<'tcx> for GenericArg<'tcx> {
     fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
         match self.unpack() {
             GenericArgKind::Lifetime(lt) => lt.fold_with(folder).into(),
             GenericArgKind::Type(ty) => ty.fold_with(folder).into(),
             GenericArgKind::Const(ct) => ct.fold_with(folder).into(),
         }
     }

     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
         match self.unpack() {
             GenericArgKind::Lifetime(lt) => lt.visit_with(visitor),
             GenericArgKind::Type(ty) => ty.visit_with(visitor),
             GenericArgKind::Const(ct) => ct.visit_with(visitor),
         }
     }
 }

 impl<'tcx> Encodable for GenericArg<'tcx> {
     fn encode<E: Encoder>(&self, e: &mut E) -> Result<(), E::Error> {
         self.unpack().encode(e)
     }
 }

 impl<'tcx> Decodable for GenericArg<'tcx> {
     fn decode<D: Decoder>(d: &mut D) -> Result<GenericArg<'tcx>, D::Error> {
         Ok(GenericArgKind::decode(d)?.pack())
     }
 }

 /// A substitution mapping generic parameters to new values.
 pub type InternalSubsts<'tcx> = List<GenericArg<'tcx>>;

 pub type SubstsRef<'tcx> = &'tcx InternalSubsts<'tcx>;

 impl<'a, 'tcx> InternalSubsts<'tcx> {
     /// Interpret these substitutions as the substitutions of a closure type.
     /// Closure substitutions have a particular structure controlled by the
     /// compiler that encodes information like the signature and closure kind;
     /// see `ty::ClosureSubsts` struct for more comments.
     pub fn as_closure(&'a self) -> ClosureSubsts<'a> {
         ClosureSubsts { substs: self }
     }

     /// Interpret these substitutions as the substitutions of a generator type.
     /// Closure substitutions have a particular structure controlled by the
     /// compiler that encodes information like the signature and generator kind;
     /// see `ty::GeneratorSubsts` struct for more comments.
     pub fn as_generator(&'tcx self) -> GeneratorSubsts<'tcx> {
         GeneratorSubsts { substs: self }
     }

     /// Creates a `InternalSubsts` that maps each generic parameter to itself.
     pub fn identity_for_item(tcx: TyCtxt<'tcx>, def_id: DefId) -> SubstsRef<'tcx> {
         Self::for_item(tcx, def_id, |param, _| tcx.mk_param_from_def(param))
     }

     /// Creates a `InternalSubsts` for generic parameter definitions,
     /// by calling closures to obtain each kind.
     /// The closures get to observe the `InternalSubsts` as they're
     /// being built, which can be used to correctly
     /// substitute defaults of generic parameters.
     pub fn for_item<F>(tcx: TyCtxt<'tcx>, def_id: DefId, mut mk_kind: F) -> SubstsRef<'tcx>
     where
         F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
     {
         let defs = tcx.generics_of(def_id);
         let count = defs.count();
         let mut substs = SmallVec::with_capacity(count);
         Self::fill_item(&mut substs, tcx, defs, &mut mk_kind);
         tcx.intern_substs(&substs)
     }

     pub fn extend_to<F>(&self, tcx: TyCtxt<'tcx>, def_id: DefId, mut mk_kind: F) -> SubstsRef<'tcx>
     where
         F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
     {
         Self::for_item(tcx, def_id, |param, substs| {
             self.get(param.index as usize).cloned().unwrap_or_else(|| mk_kind(param, substs))
         })
     }

     fn fill_item<F>(
         substs: &mut SmallVec<[GenericArg<'tcx>; 8]>,
         tcx: TyCtxt<'tcx>,
         defs: &ty::Generics,
         mk_kind: &mut F,
     ) where
         F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
     {
         if let Some(def_id) = defs.parent {
             let parent_defs = tcx.generics_of(def_id);
             Self::fill_item(substs, tcx, parent_defs, mk_kind);
         }
         Self::fill_single(substs, defs, mk_kind)
     }

     fn fill_single<F>(
         substs: &mut SmallVec<[GenericArg<'tcx>; 8]>,
         defs: &ty::Generics,
         mk_kind: &mut F,
     ) where
         F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
     {
         substs.reserve(defs.params.len());
         for param in &defs.params {
             let kind = mk_kind(param, substs);
             assert_eq!(param.index as usize, substs.len());
             substs.push(kind);
         }
     }

     pub fn is_noop(&self) -> bool {
         self.is_empty()
     }

     #[inline]
     pub fn types(&'a self) -> impl DoubleEndedIterator<Item = Ty<'tcx>> + 'a {
         self.iter()
             .filter_map(|k| if let GenericArgKind::Type(ty) = k.unpack() { Some(ty) } else { None })
     }

     #[inline]
     pub fn regions(&'a self) -> impl DoubleEndedIterator<Item = ty::Region<'tcx>> + 'a {
         self.iter().filter_map(|k| {
             if let GenericArgKind::Lifetime(lt) = k.unpack() { Some(lt) } else { None }
         })
     }

     #[inline]
     pub fn consts(&'a self) -> impl DoubleEndedIterator<Item = &'tcx ty::Const<'tcx>> + 'a {
         self.iter().filter_map(|k| {
             if let GenericArgKind::Const(ct) = k.unpack() { Some(ct) } else { None }
         })
     }

     #[inline]
     pub fn non_erasable_generics(
         &'a self,
     ) -> impl DoubleEndedIterator<Item = GenericArgKind<'tcx>> + 'a {
         self.iter().filter_map(|k| match k.unpack() {
             GenericArgKind::Lifetime(_) => None,
             generic => Some(generic),
         })
     }

     #[inline]
     pub fn type_at(&self, i: usize) -> Ty<'tcx> {
         if let GenericArgKind::Type(ty) = self[i].unpack() {
             ty
         } else {
             bug!("expected type for param #{} in {:?}", i, self);
         }
     }

     #[inline]
     pub fn region_at(&self, i: usize) -> ty::Region<'tcx> {
         if let GenericArgKind::Lifetime(lt) = self[i].unpack() {
             lt
         } else {
             bug!("expected region for param #{} in {:?}", i, self);
         }
     }

     #[inline]
     pub fn const_at(&self, i: usize) -> &'tcx ty::Const<'tcx> {
         if let GenericArgKind::Const(ct) = self[i].unpack() {
             ct
         } else {
             bug!("expected const for param #{} in {:?}", i, self);
         }
     }

     #[inline]
     pub fn type_for_def(&self, def: &ty::GenericParamDef) -> GenericArg<'tcx> {
         self.type_at(def.index as usize).into()
     }

     /// Transform from substitutions for a child of `source_ancestor`
     /// (e.g., a trait or impl) to substitutions for the same child
     /// in a different item, with `target_substs` as the base for
     /// the target impl/trait, with the source child-specific
     /// parameters (e.g., method parameters) on top of that base.
     ///
     /// For example given:
     ///
     /// ```no_run
     /// trait X<S> { fn f<T>(); }
     /// impl<U> X<U> for U { fn f<V>() {} }
     /// ```
     ///
     /// * If `self` is `[Self, S, T]`: the identity substs of `f` in the trait.
     /// * If `source_ancestor` is the def_id of the trait.
     /// * If `target_substs` is `[U]`, the substs for the impl.
     /// * Then we will return `[U, T]`, the subst for `f` in the impl that
     ///   are needed for it to match the trait.
     pub fn rebase_onto(
         &self,
         tcx: TyCtxt<'tcx>,
         source_ancestor: DefId,
         target_substs: SubstsRef<'tcx>,
     ) -> SubstsRef<'tcx> {
         let defs = tcx.generics_of(source_ancestor);
         tcx.mk_substs(target_substs.iter().chain(self.iter().skip(defs.params.len())))
     }

     pub fn truncate_to(&self, tcx: TyCtxt<'tcx>, generics: &ty::Generics) -> SubstsRef<'tcx> {
         tcx.mk_substs(self.iter().take(generics.count()))
     }
 }

 impl<'tcx> TypeFoldable<'tcx> for SubstsRef<'tcx> {
     fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
         // This code is hot enough that it's worth specializing for the most
         // common length lists, to avoid the overhead of `SmallVec` creation.
         // The match arms are in order of frequency. The 1, 2, and 0 cases are
         // typically hit in 90--99.99% of cases. When folding doesn't change
         // the substs, it's faster to reuse the existing substs rather than
         // calling `intern_substs`.
         match self.len() {
             1 => {
                 let param0 = self[0].fold_with(folder);
                 if param0 == self[0] { self } else { folder.tcx().intern_substs(&[param0]) }
             }
             2 => {
                 let param0 = self[0].fold_with(folder);
                 let param1 = self[1].fold_with(folder);
                 if param0 == self[0] && param1 == self[1] {
                     self
                 } else {
                     folder.tcx().intern_substs(&[param0, param1])
                 }
             }
             0 => self,
             _ => {
                 let params: SmallVec<[_; 8]> = self.iter().map(|k| k.fold_with(folder)).collect();
                 if params[..] == self[..] { self } else { folder.tcx().intern_substs(&params) }
             }
         }
     }

     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
         self.iter().any(|t| t.visit_with(visitor))
     }
 }

 impl<'tcx> rustc_serialize::UseSpecializedDecodable for SubstsRef<'tcx> {}

 ///////////////////////////////////////////////////////////////////////////
 // Public trait `Subst`
 //
 // Just call `foo.subst(tcx, substs)` to perform a substitution across
 // `foo`. Or use `foo.subst_spanned(tcx, substs, Some(span))` when
 // there is more information available (for better errors).

 pub trait Subst<'tcx>: Sized {
     fn subst(&self, tcx: TyCtxt<'tcx>, substs: &[GenericArg<'tcx>]) -> Self {
         self.subst_spanned(tcx, substs, None)
     }

     fn subst_spanned(
         &self,
         tcx: TyCtxt<'tcx>,
         substs: &[GenericArg<'tcx>],
         span: Option<Span>,
     ) -> Self;
 }

 impl<'tcx, T: TypeFoldable<'tcx>> Subst<'tcx> for T {
     fn subst_spanned(
         &self,
         tcx: TyCtxt<'tcx>,
         substs: &[GenericArg<'tcx>],
         span: Option<Span>,
     ) -> T {
         let mut folder =
             SubstFolder { tcx, substs, span, root_ty: None, ty_stack_depth: 0, binders_passed: 0 };
         (*self).fold_with(&mut folder)
     }
 }

 ///////////////////////////////////////////////////////////////////////////
 // The actual substitution engine itself is a type folder.

 struct SubstFolder<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     substs: &'a [GenericArg<'tcx>],

     /// The location for which the substitution is performed, if available.
     span: Option<Span>,

     /// The root type that is being substituted, if available.
     root_ty: Option<Ty<'tcx>>,

     /// Depth of type stack
     ty_stack_depth: usize,

     /// Number of region binders we have passed through while doing the substitution
     binders_passed: u32,
 }

 impl<'a, 'tcx> TypeFolder<'tcx> for SubstFolder<'a, 'tcx> {
     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
         self.tcx
     }

     fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
         self.binders_passed += 1;
         let t = t.super_fold_with(self);
         self.binders_passed -= 1;
         t
     }

     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
         // Note: This routine only handles regions that are bound on
         // type declarations and other outer declarations, not those
         // bound in *fn types*. Region substitution of the bound
         // regions that appear in a function signature is done using
         // the specialized routine `ty::replace_late_regions()`.
         match *r {
             ty::ReEarlyBound(data) => {
                 let rk = self.substs.get(data.index as usize).map(|k| k.unpack());
                 match rk {
                     Some(GenericArgKind::Lifetime(lt)) => self.shift_region_through_binders(lt),
                     _ => {
                         let span = self.span.unwrap_or(DUMMY_SP);
                         let msg = format!(
                             "Region parameter out of range \
                              when substituting in region {} (root type={:?}) \
                              (index={})",
                             data.name, self.root_ty, data.index
                         );
                         span_bug!(span, "{}", msg);
                     }
                 }
             }
             _ => r,
         }
     }

     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
         if !t.needs_subst() {
             return t;
         }

         // track the root type we were asked to substitute
         let depth = self.ty_stack_depth;
         if depth == 0 {
             self.root_ty = Some(t);
         }
         self.ty_stack_depth += 1;

         let t1 = match t.kind {
             ty::Param(p) => self.ty_for_param(p, t),
             _ => t.super_fold_with(self),
         };

         assert_eq!(depth + 1, self.ty_stack_depth);
         self.ty_stack_depth -= 1;
         if depth == 0 {
             self.root_ty = None;
         }

         t1
     }

     fn fold_const(&mut self, c: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
         if !c.needs_subst() {
             return c;
         }

         if let ty::ConstKind::Param(p) = c.val {
             self.const_for_param(p, c)
         } else {
             c.super_fold_with(self)
         }
     }
 }

 impl<'a, 'tcx> SubstFolder<'a, 'tcx> {
     fn ty_for_param(&self, p: ty::ParamTy, source_ty: Ty<'tcx>) -> Ty<'tcx> {
         // Look up the type in the substitutions. It really should be in there.
         let opt_ty = self.substs.get(p.index as usize).map(|k| k.unpack());
         let ty = match opt_ty {
             Some(GenericArgKind::Type(ty)) => ty,
             Some(kind) => {
                 let span = self.span.unwrap_or(DUMMY_SP);
                 span_bug!(
                     span,
                     "expected type for `{:?}` ({:?}/{}) but found {:?} \
                      when substituting (root type={:?}) substs={:?}",
                     p,
                     source_ty,
                     p.index,
                     kind,
                     self.root_ty,
                     self.substs,
                 );
             }
             None => {
                 let span = self.span.unwrap_or(DUMMY_SP);
                 span_bug!(
                     span,
                     "type parameter `{:?}` ({:?}/{}) out of range \
                      when substituting (root type={:?}) substs={:?}",
                     p,
                     source_ty,
                     p.index,
                     self.root_ty,
                     self.substs,
                 );
             }
         };

         self.shift_vars_through_binders(ty)
     }

     fn const_for_param(
         &self,
         p: ParamConst,
         source_ct: &'tcx ty::Const<'tcx>,
     ) -> &'tcx ty::Const<'tcx> {
         // Look up the const in the substitutions. It really should be in there.
         let opt_ct = self.substs.get(p.index as usize).map(|k| k.unpack());
         let ct = match opt_ct {
             Some(GenericArgKind::Const(ct)) => ct,
             Some(kind) => {
                 let span = self.span.unwrap_or(DUMMY_SP);
                 span_bug!(
                     span,
                     "expected const for `{:?}` ({:?}/{}) but found {:?} \
                      when substituting substs={:?}",
                     p,
                     source_ct,
                     p.index,
                     kind,
                     self.substs,
                 );
             }
             None => {
                 let span = self.span.unwrap_or(DUMMY_SP);
                 span_bug!(
                     span,
                     "const parameter `{:?}` ({:?}/{}) out of range \
                      when substituting substs={:?}",
                     p,
                     source_ct,
                     p.index,
                     self.substs,
                 );
             }
         };

         self.shift_vars_through_binders(ct)
     }

     /// It is sometimes necessary to adjust the De Bruijn indices during substitution. This occurs
     /// when we are substituting a type with escaping bound vars into a context where we have
     /// passed through binders. That's quite a mouthful. Let's see an example:
     ///
     /// ```
     /// type Func<A> = fn(A);
     /// type MetaFunc = for<'a> fn(Func<&'a i32>)
     /// ```
     ///
     /// The type `MetaFunc`, when fully expanded, will be
     ///
     ///     for<'a> fn(fn(&'a i32))
     ///             ^~ ^~ ^~~
     ///             |  |  |
     ///             |  |  DebruijnIndex of 2
     ///             Binders
     ///
     /// Here the `'a` lifetime is bound in the outer function, but appears as an argument of the
     /// inner one. Therefore, that appearance will have a DebruijnIndex of 2, because we must skip
     /// over the inner binder (remember that we count De Bruijn indices from 1). However, in the
     /// definition of `MetaFunc`, the binder is not visible, so the type `&'a i32` will have a
     /// De Bruijn index of 1. It's only during the substitution that we can see we must increase the
     /// depth by 1 to account for the binder that we passed through.
     ///
     /// As a second example, consider this twist:
     ///
     /// ```
     /// type FuncTuple<A> = (A,fn(A));
     /// type MetaFuncTuple = for<'a> fn(FuncTuple<&'a i32>)
     /// ```
     ///
     /// Here the final type will be:
     ///
     ///     for<'a> fn((&'a i32, fn(&'a i32)))
     ///                 ^~~         ^~~
     ///                 |           |
     ///          DebruijnIndex of 1 |
     ///                      DebruijnIndex of 2
     ///
     /// As indicated in the diagram, here the same type `&'a i32` is substituted once, but in the
     /// first case we do not increase the De Bruijn index and in the second case we do. The reason
     /// is that only in the second case have we passed through a fn binder.
     fn shift_vars_through_binders<T: TypeFoldable<'tcx>>(&self, val: T) -> T {
         debug!(
             "shift_vars(val={:?}, binders_passed={:?}, has_escaping_bound_vars={:?})",
             val,
             self.binders_passed,
             val.has_escaping_bound_vars()
         );

         if self.binders_passed == 0 || !val.has_escaping_bound_vars() {
             return val;
         }

         let result = ty::fold::shift_vars(self.tcx(), &val, self.binders_passed);
         debug!("shift_vars: shifted result = {:?}", result);

         result
     }

     fn shift_region_through_binders(&self, region: ty::Region<'tcx>) -> ty::Region<'tcx> {
         if self.binders_passed == 0 || !region.has_escaping_bound_vars() {
             return region;
         }
         ty::fold::shift_region(self.tcx, region, self.binders_passed)
     }
 }

 pub type CanonicalUserSubsts<'tcx> = Canonical<'tcx, UserSubsts<'tcx>>;

 /// Stores the user-given substs to reach some fully qualified path
 /// (e.g., `<T>::Item` or `<T as Trait>::Item`).
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable)]
 #[derive(HashStable, TypeFoldable, Lift)]
 pub struct UserSubsts<'tcx> {
     /// The substitutions for the item as given by the user.
     pub substs: SubstsRef<'tcx>,

     /// The self type, in the case of a `<T>::Item` path (when applied
     /// to an inherent impl). See `UserSelfTy` below.
     pub user_self_ty: Option<UserSelfTy<'tcx>>,
 }

 /// Specifies the user-given self type. In the case of a path that
 /// refers to a member in an inherent impl, this self type is
 /// sometimes needed to constrain the type parameters on the impl. For
 /// example, in this code:
 ///
 /// ```
 /// struct Foo<T> { }
 /// impl<A> Foo<A> { fn method() { } }
 /// ```
 ///
 /// when you then have a path like `<Foo<&'static u32>>::method`,
 /// this struct would carry the `DefId` of the impl along with the
 /// self type `Foo<u32>`. Then we can instantiate the parameters of
 /// the impl (with the substs from `UserSubsts`) and apply those to
 /// the self type, giving `Foo<?A>`. Finally, we unify that with
 /// the self type here, which contains `?A` to be `&'static u32`
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable)]
 #[derive(HashStable, TypeFoldable, Lift)]
 pub struct UserSelfTy<'tcx> {
     pub impl_def_id: DefId,
     pub self_ty: Ty<'tcx>,
 }
	// Type substitutions.

	use crate::infer::canonical::Canonical;
	use crate::ty::fold::{TypeFoldable, TypeFolder, TypeVisitor};
	use crate::ty::sty::{ClosureSubsts, GeneratorSubsts};
	use crate::ty::{self, Lift, List, ParamConst, Ty, TyCtxt};

	use rustc_hir::def_id::DefId;
	use rustc_macros::HashStable;
	use rustc_serialize::{self, Decodable, Decoder, Encodable, Encoder};
	use rustc_span::{Span, DUMMY_SP};
	use smallvec::SmallVec;

	use core::intrinsics;
	use std::cmp::Ordering;
	use std::fmt;
	use std::marker::PhantomData;
	use std::mem;
	use std::num::NonZeroUsize;

	/// An entity in the Rust type system, which can be one of
	/// several kinds (types, lifetimes, and consts).
	/// To reduce memory usage, a `GenericArg` is a interned pointer,
	/// with the lowest 2 bits being reserved for a tag to
	/// indicate the type (`Ty`, `Region`, or `Const`) it points to.
	#[derive(Copy, Clone, PartialEq, Eq, Hash)]
	pub struct GenericArg<'tcx> {
	ptr: NonZeroUsize,
	marker: PhantomData<(Ty<'tcx>, ty::Region<'tcx>, &'tcx ty::Const<'tcx>)>,
	}

	const TAG_MASK: usize = 0b11;
	const TYPE_TAG: usize = 0b00;
	const REGION_TAG: usize = 0b01;
	const CONST_TAG: usize = 0b10;

	#[derive(Debug, RustcEncodable, RustcDecodable, PartialEq, Eq, PartialOrd, Ord, HashStable)]
	pub enum GenericArgKind<'tcx> {
	Lifetime(ty::Region<'tcx>),
	Type(Ty<'tcx>),
	Const(&'tcx ty::Const<'tcx>),
	}

	impl<'tcx> GenericArgKind<'tcx> {
	fn pack(self) -> GenericArg<'tcx> {
	let (tag, ptr) = match self {
	GenericArgKind::Lifetime(lt) => {
	// Ensure we can use the tag bits.
	assert_eq!(mem::align_of_val(lt) & TAG_MASK, 0);
	(REGION_TAG, lt as *const _ as usize)
	}
	GenericArgKind::Type(ty) => {
	// Ensure we can use the tag bits.
	assert_eq!(mem::align_of_val(ty) & TAG_MASK, 0);
	(TYPE_TAG, ty as *const _ as usize)
	}
	GenericArgKind::Const(ct) => {
	// Ensure we can use the tag bits.
	assert_eq!(mem::align_of_val(ct) & TAG_MASK, 0);
	(CONST_TAG, ct as *const _ as usize)
	}
	};

	GenericArg { ptr: unsafe { NonZeroUsize::new_unchecked(ptr \| tag) }, marker: PhantomData }
	}
	}

	impl fmt::Debug for GenericArg<'tcx> {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	match self.unpack() {
	GenericArgKind::Lifetime(lt) => lt.fmt(f),
	GenericArgKind::Type(ty) => ty.fmt(f),
	GenericArgKind::Const(ct) => ct.fmt(f),
	}
	}
	}

	impl<'tcx> Ord for GenericArg<'tcx> {
	fn cmp(&self, other: &GenericArg<'_>) -> Ordering {
	self.unpack().cmp(&other.unpack())
	}
	}

	impl<'tcx> PartialOrd for GenericArg<'tcx> {
	fn partial_cmp(&self, other: &GenericArg<'_>) -> Option<Ordering> {
	Some(self.cmp(&other))
	}
	}

	impl<'tcx> From<ty::Region<'tcx>> for GenericArg<'tcx> {
	fn from(r: ty::Region<'tcx>) -> GenericArg<'tcx> {
	GenericArgKind::Lifetime(r).pack()
	}
	}

	impl<'tcx> From<Ty<'tcx>> for GenericArg<'tcx> {
	fn from(ty: Ty<'tcx>) -> GenericArg<'tcx> {
	GenericArgKind::Type(ty).pack()
	}
	}

	impl<'tcx> From<&'tcx ty::Const<'tcx>> for GenericArg<'tcx> {
	fn from(c: &'tcx ty::Const<'tcx>) -> GenericArg<'tcx> {
	GenericArgKind::Const(c).pack()
	}
	}

	impl<'tcx> GenericArg<'tcx> {
	#[inline]
	pub fn unpack(self) -> GenericArgKind<'tcx> {
	let ptr = self.ptr.get();
	unsafe {
	match ptr & TAG_MASK {
	REGION_TAG => GenericArgKind::Lifetime(&((ptr & !TAG_MASK) as const _)),
	TYPE_TAG => GenericArgKind::Type(&((ptr & !TAG_MASK) as const _)),
	CONST_TAG => GenericArgKind::Const(&((ptr & !TAG_MASK) as const _)),
	_ => intrinsics::unreachable(),
	}
	}
	}

	/// Unpack the `GenericArg` as a type when it is known certainly to be a type.
	/// This is true in cases where `Substs` is used in places where the kinds are known
	/// to be limited (e.g. in tuples, where the only parameters are type parameters).
	pub fn expect_ty(self) -> Ty<'tcx> {
	match self.unpack() {
	GenericArgKind::Type(ty) => ty,
	_ => bug!("expected a type, but found another kind"),
	}
	}

	/// Unpack the `GenericArg` as a const when it is known certainly to be a const.
	pub fn expect_const(self) -> &'tcx ty::Const<'tcx> {
	match self.unpack() {
	GenericArgKind::Const(c) => c,
	_ => bug!("expected a const, but found another kind"),
	}
	}
	}

	impl<'a, 'tcx> Lift<'tcx> for GenericArg<'a> {
	type Lifted = GenericArg<'tcx>;

	fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
	match self.unpack() {
	GenericArgKind::Lifetime(lt) => tcx.lift(&lt).map(\|lt\| lt.into()),
	GenericArgKind::Type(ty) => tcx.lift(&ty).map(\|ty\| ty.into()),
	GenericArgKind::Const(ct) => tcx.lift(&ct).map(\|ct\| ct.into()),
	}
	}
	}

	impl<'tcx> TypeFoldable<'tcx> for GenericArg<'tcx> {
	fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
	match self.unpack() {
	GenericArgKind::Lifetime(lt) => lt.fold_with(folder).into(),
	GenericArgKind::Type(ty) => ty.fold_with(folder).into(),
	GenericArgKind::Const(ct) => ct.fold_with(folder).into(),
	}
	}

	fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
	match self.unpack() {
	GenericArgKind::Lifetime(lt) => lt.visit_with(visitor),
	GenericArgKind::Type(ty) => ty.visit_with(visitor),
	GenericArgKind::Const(ct) => ct.visit_with(visitor),
	}
	}
	}

	impl<'tcx> Encodable for GenericArg<'tcx> {
	fn encode<E: Encoder>(&self, e: &mut E) -> Result<(), E::Error> {
	self.unpack().encode(e)
	}
	}

	impl<'tcx> Decodable for GenericArg<'tcx> {
	fn decode<D: Decoder>(d: &mut D) -> Result<GenericArg<'tcx>, D::Error> {
	Ok(GenericArgKind::decode(d)?.pack())
	}
	}

	/// A substitution mapping generic parameters to new values.
	pub type InternalSubsts<'tcx> = List<GenericArg<'tcx>>;

	pub type SubstsRef<'tcx> = &'tcx InternalSubsts<'tcx>;

	impl<'a, 'tcx> InternalSubsts<'tcx> {
	/// Interpret these substitutions as the substitutions of a closure type.
	/// Closure substitutions have a particular structure controlled by the
	/// compiler that encodes information like the signature and closure kind;
	/// see `ty::ClosureSubsts` struct for more comments.
	pub fn as_closure(&'a self) -> ClosureSubsts<'a> {
	ClosureSubsts { substs: self }
	}

	/// Interpret these substitutions as the substitutions of a generator type.
	/// Closure substitutions have a particular structure controlled by the
	/// compiler that encodes information like the signature and generator kind;
	/// see `ty::GeneratorSubsts` struct for more comments.
	pub fn as_generator(&'tcx self) -> GeneratorSubsts<'tcx> {
	GeneratorSubsts { substs: self }
	}

	/// Creates a `InternalSubsts` that maps each generic parameter to itself.
	pub fn identity_for_item(tcx: TyCtxt<'tcx>, def_id: DefId) -> SubstsRef<'tcx> {
	Self::for_item(tcx, def_id, \|param, _\| tcx.mk_param_from_def(param))
	}

	/// Creates a `InternalSubsts` for generic parameter definitions,
	/// by calling closures to obtain each kind.
	/// The closures get to observe the `InternalSubsts` as they're
	/// being built, which can be used to correctly
	/// substitute defaults of generic parameters.
	pub fn for_item<F>(tcx: TyCtxt<'tcx>, def_id: DefId, mut mk_kind: F) -> SubstsRef<'tcx>
	where
	F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
	{
	let defs = tcx.generics_of(def_id);
	let count = defs.count();
	let mut substs = SmallVec::with_capacity(count);
	Self::fill_item(&mut substs, tcx, defs, &mut mk_kind);
	tcx.intern_substs(&substs)
	}

	pub fn extend_to<F>(&self, tcx: TyCtxt<'tcx>, def_id: DefId, mut mk_kind: F) -> SubstsRef<'tcx>
	where
	F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
	{
	Self::for_item(tcx, def_id, \|param, substs\| {
	self.get(param.index as usize).cloned().unwrap_or_else(\|\| mk_kind(param, substs))
	})
	}

	fn fill_item<F>(
	substs: &mut SmallVec<[GenericArg<'tcx>; 8]>,
	tcx: TyCtxt<'tcx>,
	defs: &ty::Generics,
	mk_kind: &mut F,
	) where
	F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
	{
	if let Some(def_id) = defs.parent {
	let parent_defs = tcx.generics_of(def_id);
	Self::fill_item(substs, tcx, parent_defs, mk_kind);
	}
	Self::fill_single(substs, defs, mk_kind)
	}

	fn fill_single<F>(
	substs: &mut SmallVec<[GenericArg<'tcx>; 8]>,
	defs: &ty::Generics,
	mk_kind: &mut F,
	) where
	F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
	{
	substs.reserve(defs.params.len());
	for param in &defs.params {
	let kind = mk_kind(param, substs);
	assert_eq!(param.index as usize, substs.len());
	substs.push(kind);
	}
	}

	pub fn is_noop(&self) -> bool {
	self.is_empty()
	}

	#[inline]
	pub fn types(&'a self) -> impl DoubleEndedIterator<Item = Ty<'tcx>> + 'a {
	self.iter()
	.filter_map(\|k\| if let GenericArgKind::Type(ty) = k.unpack() { Some(ty) } else { None })
	}

	#[inline]
	pub fn regions(&'a self) -> impl DoubleEndedIterator<Item = ty::Region<'tcx>> + 'a {
	self.iter().filter_map(\|k\| {
	if let GenericArgKind::Lifetime(lt) = k.unpack() { Some(lt) } else { None }
	})
	}

	#[inline]
	pub fn consts(&'a self) -> impl DoubleEndedIterator<Item = &'tcx ty::Const<'tcx>> + 'a {
	self.iter().filter_map(\|k\| {
	if let GenericArgKind::Const(ct) = k.unpack() { Some(ct) } else { None }
	})
	}

	#[inline]
	pub fn non_erasable_generics(
	&'a self,
	) -> impl DoubleEndedIterator<Item = GenericArgKind<'tcx>> + 'a {
	self.iter().filter_map(\|k\| match k.unpack() {
	GenericArgKind::Lifetime(_) => None,
	generic => Some(generic),
	})
	}

	#[inline]
	pub fn type_at(&self, i: usize) -> Ty<'tcx> {
	if let GenericArgKind::Type(ty) = self[i].unpack() {
	ty
	} else {
	bug!("expected type for param #{} in {:?}", i, self);
	}
	}

	#[inline]
	pub fn region_at(&self, i: usize) -> ty::Region<'tcx> {
	if let GenericArgKind::Lifetime(lt) = self[i].unpack() {
	lt
	} else {
	bug!("expected region for param #{} in {:?}", i, self);
	}
	}

	#[inline]
	pub fn const_at(&self, i: usize) -> &'tcx ty::Const<'tcx> {
	if let GenericArgKind::Const(ct) = self[i].unpack() {
	ct
	} else {
	bug!("expected const for param #{} in {:?}", i, self);
	}
	}

	#[inline]
	pub fn type_for_def(&self, def: &ty::GenericParamDef) -> GenericArg<'tcx> {
	self.type_at(def.index as usize).into()
	}

	/// Transform from substitutions for a child of `source_ancestor`
	/// (e.g., a trait or impl) to substitutions for the same child
	/// in a different item, with `target_substs` as the base for
	/// the target impl/trait, with the source child-specific
	/// parameters (e.g., method parameters) on top of that base.
	///
	/// For example given:
	///
	/// ```no_run
	/// trait X<S> { fn f<T>(); }
	/// impl<U> X<U> for U { fn f<V>() {} }
	/// ```
	///
	/// * If `self` is `[Self, S, T]`: the identity substs of `f` in the trait.
	/// * If `source_ancestor` is the def_id of the trait.
	/// * If `target_substs` is `[U]`, the substs for the impl.
	/// * Then we will return `[U, T]`, the subst for `f` in the impl that
	/// are needed for it to match the trait.
	pub fn rebase_onto(
	&self,
	tcx: TyCtxt<'tcx>,
	source_ancestor: DefId,
	target_substs: SubstsRef<'tcx>,
	) -> SubstsRef<'tcx> {
	let defs = tcx.generics_of(source_ancestor);
	tcx.mk_substs(target_substs.iter().chain(self.iter().skip(defs.params.len())))
	}

	pub fn truncate_to(&self, tcx: TyCtxt<'tcx>, generics: &ty::Generics) -> SubstsRef<'tcx> {
	tcx.mk_substs(self.iter().take(generics.count()))
	}
	}

	impl<'tcx> TypeFoldable<'tcx> for SubstsRef<'tcx> {
	fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
	// This code is hot enough that it's worth specializing for the most
	// common length lists, to avoid the overhead of `SmallVec` creation.
	// The match arms are in order of frequency. The 1, 2, and 0 cases are
	// typically hit in 90--99.99% of cases. When folding doesn't change
	// the substs, it's faster to reuse the existing substs rather than
	// calling `intern_substs`.
	match self.len() {
	1 => {
	let param0 = self[0].fold_with(folder);
	if param0 == self[0] { self } else { folder.tcx().intern_substs(&[param0]) }
	}
	2 => {
	let param0 = self[0].fold_with(folder);
	let param1 = self[1].fold_with(folder);
	if param0 == self[0] && param1 == self[1] {
	self
	} else {
	folder.tcx().intern_substs(&[param0, param1])
	}
	}
	0 => self,
	_ => {
	let params: SmallVec<[_; 8]> = self.iter().map(\|k\| k.fold_with(folder)).collect();
	if params[..] == self[..] { self } else { folder.tcx().intern_substs(&params) }
	}
	}
	}

	fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
	self.iter().any(\|t\| t.visit_with(visitor))
	}
	}

	impl<'tcx> rustc_serialize::UseSpecializedDecodable for SubstsRef<'tcx> {}

	///////////////////////////////////////////////////////////////////////////
	// Public trait `Subst`
	//
	// Just call `foo.subst(tcx, substs)` to perform a substitution across
	// `foo`. Or use `foo.subst_spanned(tcx, substs, Some(span))` when
	// there is more information available (for better errors).

	pub trait Subst<'tcx>: Sized {
	fn subst(&self, tcx: TyCtxt<'tcx>, substs: &[GenericArg<'tcx>]) -> Self {
	self.subst_spanned(tcx, substs, None)
	}

	fn subst_spanned(
	&self,
	tcx: TyCtxt<'tcx>,
	substs: &[GenericArg<'tcx>],
	span: Option<Span>,
	) -> Self;
	}

	impl<'tcx, T: TypeFoldable<'tcx>> Subst<'tcx> for T {
	fn subst_spanned(
	&self,
	tcx: TyCtxt<'tcx>,
	substs: &[GenericArg<'tcx>],
	span: Option<Span>,
	) -> T {
	let mut folder =
	SubstFolder { tcx, substs, span, root_ty: None, ty_stack_depth: 0, binders_passed: 0 };
	(*self).fold_with(&mut folder)
	}
	}

	///////////////////////////////////////////////////////////////////////////
	// The actual substitution engine itself is a type folder.

	struct SubstFolder<'a, 'tcx> {
	tcx: TyCtxt<'tcx>,
	substs: &'a [GenericArg<'tcx>],

	/// The location for which the substitution is performed, if available.
	span: Option<Span>,

	/// The root type that is being substituted, if available.
	root_ty: Option<Ty<'tcx>>,

	/// Depth of type stack
	ty_stack_depth: usize,

	/// Number of region binders we have passed through while doing the substitution
	binders_passed: u32,
	}

	impl<'a, 'tcx> TypeFolder<'tcx> for SubstFolder<'a, 'tcx> {
	fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
	self.tcx
	}

	fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
	self.binders_passed += 1;
	let t = t.super_fold_with(self);
	self.binders_passed -= 1;
	t
	}

	fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
	// Note: This routine only handles regions that are bound on
	// type declarations and other outer declarations, not those
	// bound in fn types. Region substitution of the bound
	// regions that appear in a function signature is done using
	// the specialized routine `ty::replace_late_regions()`.
	match *r {
	ty::ReEarlyBound(data) => {
	let rk = self.substs.get(data.index as usize).map(\|k\| k.unpack());
	match rk {
	Some(GenericArgKind::Lifetime(lt)) => self.shift_region_through_binders(lt),
	_ => {
	let span = self.span.unwrap_or(DUMMY_SP);
	let msg = format!(
	"Region parameter out of range \
	when substituting in region {} (root type={:?}) \
	(index={})",
	data.name, self.root_ty, data.index
	);
	span_bug!(span, "{}", msg);
	}
	}
	}
	_ => r,
	}
	}

	fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
	if !t.needs_subst() {
	return t;
	}

	// track the root type we were asked to substitute
	let depth = self.ty_stack_depth;
	if depth == 0 {
	self.root_ty = Some(t);
	}
	self.ty_stack_depth += 1;

	let t1 = match t.kind {
	ty::Param(p) => self.ty_for_param(p, t),
	_ => t.super_fold_with(self),
	};

	assert_eq!(depth + 1, self.ty_stack_depth);
	self.ty_stack_depth -= 1;
	if depth == 0 {
	self.root_ty = None;
	}

	t1
	}

	fn fold_const(&mut self, c: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
	if !c.needs_subst() {
	return c;
	}

	if let ty::ConstKind::Param(p) = c.val {
	self.const_for_param(p, c)
	} else {
	c.super_fold_with(self)
	}
	}
	}

	impl<'a, 'tcx> SubstFolder<'a, 'tcx> {
	fn ty_for_param(&self, p: ty::ParamTy, source_ty: Ty<'tcx>) -> Ty<'tcx> {
	// Look up the type in the substitutions. It really should be in there.
	let opt_ty = self.substs.get(p.index as usize).map(\|k\| k.unpack());
	let ty = match opt_ty {
	Some(GenericArgKind::Type(ty)) => ty,
	Some(kind) => {
	let span = self.span.unwrap_or(DUMMY_SP);
	span_bug!(
	span,
	"expected type for `{:?}` ({:?}/{}) but found {:?} \
	when substituting (root type={:?}) substs={:?}",
	p,
	source_ty,
	p.index,
	kind,
	self.root_ty,
	self.substs,
	);
	}
	None => {
	let span = self.span.unwrap_or(DUMMY_SP);
	span_bug!(
	span,
	"type parameter `{:?}` ({:?}/{}) out of range \
	when substituting (root type={:?}) substs={:?}",
	p,
	source_ty,
	p.index,
	self.root_ty,
	self.substs,
	);
	}
	};

	self.shift_vars_through_binders(ty)
	}

	fn const_for_param(
	&self,
	p: ParamConst,
	source_ct: &'tcx ty::Const<'tcx>,
	) -> &'tcx ty::Const<'tcx> {
	// Look up the const in the substitutions. It really should be in there.
	let opt_ct = self.substs.get(p.index as usize).map(\|k\| k.unpack());
	let ct = match opt_ct {
	Some(GenericArgKind::Const(ct)) => ct,
	Some(kind) => {
	let span = self.span.unwrap_or(DUMMY_SP);
	span_bug!(
	span,
	"expected const for `{:?}` ({:?}/{}) but found {:?} \
	when substituting substs={:?}",
	p,
	source_ct,
	p.index,
	kind,
	self.substs,
	);
	}
	None => {
	let span = self.span.unwrap_or(DUMMY_SP);
	span_bug!(
	span,
	"const parameter `{:?}` ({:?}/{}) out of range \
	when substituting substs={:?}",
	p,
	source_ct,
	p.index,
	self.substs,
	);
	}
	};

	self.shift_vars_through_binders(ct)
	}

	/// It is sometimes necessary to adjust the De Bruijn indices during substitution. This occurs
	/// when we are substituting a type with escaping bound vars into a context where we have
	/// passed through binders. That's quite a mouthful. Let's see an example:
	///
	/// ```
	/// type Func<A> = fn(A);
	/// type MetaFunc = for<'a> fn(Func<&'a i32>)
	/// ```
	///
	/// The type `MetaFunc`, when fully expanded, will be
	///
	/// for<'a> fn(fn(&'a i32))
	/// ^~ ^~ ^~~
	/// \| \| \|
	/// \| \| DebruijnIndex of 2
	/// Binders
	///
	/// Here the `'a` lifetime is bound in the outer function, but appears as an argument of the
	/// inner one. Therefore, that appearance will have a DebruijnIndex of 2, because we must skip
	/// over the inner binder (remember that we count De Bruijn indices from 1). However, in the
	/// definition of `MetaFunc`, the binder is not visible, so the type `&'a i32` will have a
	/// De Bruijn index of 1. It's only during the substitution that we can see we must increase the
	/// depth by 1 to account for the binder that we passed through.
	///
	/// As a second example, consider this twist:
	///
	/// ```
	/// type FuncTuple<A> = (A,fn(A));
	/// type MetaFuncTuple = for<'a> fn(FuncTuple<&'a i32>)
	/// ```
	///
	/// Here the final type will be:
	///
	/// for<'a> fn((&'a i32, fn(&'a i32)))
	/// ^~~ ^~~
	/// \| \|
	/// DebruijnIndex of 1 \|
	/// DebruijnIndex of 2
	///
	/// As indicated in the diagram, here the same type `&'a i32` is substituted once, but in the
	/// first case we do not increase the De Bruijn index and in the second case we do. The reason
	/// is that only in the second case have we passed through a fn binder.
	fn shift_vars_through_binders<T: TypeFoldable<'tcx>>(&self, val: T) -> T {
	debug!(
	"shift_vars(val={:?}, binders_passed={:?}, has_escaping_bound_vars={:?})",
	val,
	self.binders_passed,
	val.has_escaping_bound_vars()
	);

	if self.binders_passed == 0 \|\| !val.has_escaping_bound_vars() {
	return val;
	}

	let result = ty::fold::shift_vars(self.tcx(), &val, self.binders_passed);
	debug!("shift_vars: shifted result = {:?}", result);

	result
	}

	fn shift_region_through_binders(&self, region: ty::Region<'tcx>) -> ty::Region<'tcx> {
	if self.binders_passed == 0 \|\| !region.has_escaping_bound_vars() {
	return region;
	}
	ty::fold::shift_region(self.tcx, region, self.binders_passed)
	}
	}

	pub type CanonicalUserSubsts<'tcx> = Canonical<'tcx, UserSubsts<'tcx>>;

	/// Stores the user-given substs to reach some fully qualified path
	/// (e.g., `<T>::Item` or `<T as Trait>::Item`).
	#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable)]
	#[derive(HashStable, TypeFoldable, Lift)]
	pub struct UserSubsts<'tcx> {
	/// The substitutions for the item as given by the user.
	pub substs: SubstsRef<'tcx>,

	/// The self type, in the case of a `<T>::Item` path (when applied
	/// to an inherent impl). See `UserSelfTy` below.
	pub user_self_ty: Option<UserSelfTy<'tcx>>,
	}

	/// Specifies the user-given self type. In the case of a path that
	/// refers to a member in an inherent impl, this self type is
	/// sometimes needed to constrain the type parameters on the impl. For
	/// example, in this code:
	///
	/// ```
	/// struct Foo<T> { }
	/// impl<A> Foo<A> { fn method() { } }
	/// ```
	///
	/// when you then have a path like `<Foo<&'static u32>>::method`,
	/// this struct would carry the `DefId` of the impl along with the
	/// self type `Foo<u32>`. Then we can instantiate the parameters of
	/// the impl (with the substs from `UserSubsts`) and apply those to
	/// the self type, giving `Foo<?A>`. Finally, we unify that with
	/// the self type here, which contains `?A` to be `&'static u32`
	#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable)]
	#[derive(HashStable, TypeFoldable, Lift)]
	pub struct UserSelfTy<'tcx> {
	pub impl_def_id: DefId,
	pub self_ty: Ty<'tcx>,
	}