mod.rs

// ignore-tidy-filelength

pub use self::Variance::*;
pub use self::AssocItemContainer::*;
pub use self::BorrowKind::*;
pub use self::IntVarValue::*;
pub use self::fold::TypeFoldable;

use crate::hir::{map as hir_map, GlobMap, TraitMap};
use crate::hir::Node;
use crate::hir::def::{Res, DefKind, CtorOf, CtorKind, ExportMap};
use crate::hir::def_id::{CrateNum, DefId, LocalDefId, CRATE_DEF_INDEX, LOCAL_CRATE};
use rustc_data_structures::svh::Svh;
use rustc_macros::HashStable;
use crate::ich::Fingerprint;
use crate::ich::StableHashingContext;
use crate::infer::canonical::Canonical;
use crate::middle::lang_items::{FnTraitLangItem, FnMutTraitLangItem, FnOnceTraitLangItem};
use crate::middle::resolve_lifetime::ObjectLifetimeDefault;
use crate::mir::Body;
use crate::mir::interpret::{GlobalId, ErrorHandled};
use crate::mir::GeneratorLayout;
use crate::session::CrateDisambiguator;
use crate::traits::{self, Reveal};
use crate::ty;
use crate::ty::layout::VariantIdx;
use crate::ty::subst::{Subst, InternalSubsts, SubstsRef};
use crate::ty::util::{IntTypeExt, Discr};
use crate::ty::walk::TypeWalker;
use crate::util::captures::Captures;
use crate::util::nodemap::{NodeSet, DefIdMap, FxHashMap};
use arena::SyncDroplessArena;
use crate::session::DataTypeKind;

use rustc_serialize::{self, Encodable, Encoder};
use rustc_target::abi::Align;
use std::cell::RefCell;
use std::cmp::{self, Ordering};
use std::fmt;
use std::hash::{Hash, Hasher};
use std::ops::Deref;
use rustc_data_structures::sync::{self, Lrc, ParallelIterator, par_iter};
use std::slice;
use std::{mem, ptr};
use std::ops::Range;
use syntax::ast::{self, Name, Ident, NodeId};
use syntax::attr;
use syntax::ext::hygiene::ExpnId;
use syntax::symbol::{kw, sym, Symbol, LocalInternedString, InternedString};
use syntax_pos::Span;

use smallvec;
use rustc_data_structures::fx::FxIndexMap;
use rustc_data_structures::indexed_vec::{Idx, IndexVec};
use rustc_data_structures::stable_hasher::{StableHasher, StableHasherResult,
                                           HashStable};

use crate::hir;

pub use self::sty::{Binder, BoundTy, BoundTyKind, BoundVar, DebruijnIndex, INNERMOST};
pub use self::sty::{FnSig, GenSig, CanonicalPolyFnSig, PolyFnSig, PolyGenSig};
pub use self::sty::{InferTy, ParamTy, ParamConst, InferConst, ProjectionTy, ExistentialPredicate};
pub use self::sty::{ClosureSubsts, GeneratorSubsts, UpvarSubsts, TypeAndMut};
pub use self::sty::{TraitRef, TyKind, PolyTraitRef};
pub use self::sty::{ExistentialTraitRef, PolyExistentialTraitRef};
pub use self::sty::{ExistentialProjection, PolyExistentialProjection, Const};
pub use self::sty::{BoundRegion, EarlyBoundRegion, FreeRegion, Region};
pub use self::sty::RegionKind;
pub use self::sty::{TyVid, IntVid, FloatVid, ConstVid, RegionVid};
pub use self::sty::BoundRegion::*;
pub use self::sty::InferTy::*;
pub use self::sty::RegionKind::*;
pub use self::sty::TyKind::*;

pub use self::binding::BindingMode;
pub use self::binding::BindingMode::*;

pub use self::context::{TyCtxt, FreeRegionInfo, AllArenas, tls, keep_local};
pub use self::context::{Lift, TypeckTables, CtxtInterners, GlobalCtxt};
pub use self::context::{
    UserTypeAnnotationIndex, UserType, CanonicalUserType,
    CanonicalUserTypeAnnotation, CanonicalUserTypeAnnotations, ResolvedOpaqueTy,
};

pub use self::instance::{Instance, InstanceDef};

pub use self::trait_def::TraitDef;

pub use self::query::queries;

pub mod adjustment;
pub mod binding;
pub mod cast;
#[macro_use]
pub mod codec;
mod constness;
pub mod error;
mod erase_regions;
pub mod fast_reject;
pub mod flags;
pub mod fold;
pub mod inhabitedness;
pub mod layout;
pub mod _match;
pub mod outlives;
pub mod print;
pub mod query;
pub mod relate;
pub mod steal;
pub mod subst;
pub mod trait_def;
pub mod walk;
pub mod wf;
pub mod util;

mod context;
mod instance;
mod structural_impls;
mod sty;

// Data types

#[derive(Clone)]
pub struct Resolutions {
    pub trait_map: TraitMap,
    pub maybe_unused_trait_imports: NodeSet,
    pub maybe_unused_extern_crates: Vec<(NodeId, Span)>,
    pub export_map: ExportMap<NodeId>,
    pub glob_map: GlobMap,
    /// Extern prelude entries. The value is `true` if the entry was introduced
    /// via `extern crate` item and not `--extern` option or compiler built-in.
    pub extern_prelude: FxHashMap<Name, bool>,
}

#[derive(Clone, Copy, PartialEq, Eq, Debug, HashStable)]
pub enum AssocItemContainer {
    TraitContainer(DefId),
    ImplContainer(DefId),
}

impl AssocItemContainer {
    /// Asserts that this is the `DefId` of an associated item declared
    /// in a trait, and returns the trait `DefId`.
    pub fn assert_trait(&self) -> DefId {
        match *self {
            TraitContainer(id) => id,
            _ => bug!("associated item has wrong container type: {:?}", self)
        }
    }

    pub fn id(&self) -> DefId {
        match *self {
            TraitContainer(id) => id,
            ImplContainer(id) => id,
        }
    }
}

/// The "header" of an impl is everything outside the body: a Self type, a trait
/// ref (in the case of a trait impl), and a set of predicates (from the
/// bounds / where-clauses).
#[derive(Clone, PartialEq, Eq, Hash, Debug)]
pub struct ImplHeader<'tcx> {
    pub impl_def_id: DefId,
    pub self_ty: Ty<'tcx>,
    pub trait_ref: Option<TraitRef<'tcx>>,
    pub predicates: Vec<Predicate<'tcx>>,
}

#[derive(Copy, Clone, Debug, PartialEq, HashStable)]
pub struct AssocItem {
    pub def_id: DefId,
    #[stable_hasher(project(name))]
    pub ident: Ident,
    pub kind: AssocKind,
    pub vis: Visibility,
    pub defaultness: hir::Defaultness,
    pub container: AssocItemContainer,

    /// Whether this is a method with an explicit self
    /// as its first argument, allowing method calls.
    pub method_has_self_argument: bool,
}

#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash, RustcEncodable, RustcDecodable, HashStable)]
pub enum AssocKind {
    Const,
    Method,
    OpaqueTy,
    Type
}

impl AssocItem {
    pub fn def_kind(&self) -> DefKind {
        match self.kind {
            AssocKind::Const => DefKind::AssocConst,
            AssocKind::Method => DefKind::Method,
            AssocKind::Type => DefKind::AssocTy,
            AssocKind::OpaqueTy => DefKind::AssocOpaqueTy,
        }
    }

    /// Tests whether the associated item admits a non-trivial implementation
    /// for !
    pub fn relevant_for_never(&self) -> bool {
        match self.kind {
            AssocKind::OpaqueTy |
            AssocKind::Const |
            AssocKind::Type => true,
            // FIXME(canndrew): Be more thorough here, check if any argument is uninhabited.
            AssocKind::Method => !self.method_has_self_argument,
        }
    }

    pub fn signature(&self, tcx: TyCtxt<'_>) -> String {
        match self.kind {
            ty::AssocKind::Method => {
                // We skip the binder here because the binder would deanonymize all
                // late-bound regions, and we don't want method signatures to show up
                // `as for<'r> fn(&'r MyType)`.  Pretty-printing handles late-bound
                // regions just fine, showing `fn(&MyType)`.
                tcx.fn_sig(self.def_id).skip_binder().to_string()
            }
            ty::AssocKind::Type => format!("type {};", self.ident),
            // FIXME(type_alias_impl_trait): we should print bounds here too.
            ty::AssocKind::OpaqueTy => format!("type {};", self.ident),
            ty::AssocKind::Const => {
                format!("const {}: {:?};", self.ident, tcx.type_of(self.def_id))
            }
        }
    }
}

#[derive(Clone, Debug, PartialEq, Eq, Copy, RustcEncodable, RustcDecodable, HashStable)]
pub enum Visibility {
    /// Visible everywhere (including in other crates).
    Public,
    /// Visible only in the given crate-local module.
    Restricted(DefId),
    /// Not visible anywhere in the local crate. This is the visibility of private external items.
    Invisible,
}

pub trait DefIdTree: Copy {
    fn parent(self, id: DefId) -> Option<DefId>;

    fn is_descendant_of(self, mut descendant: DefId, ancestor: DefId) -> bool {
        if descendant.krate != ancestor.krate {
            return false;
        }

        while descendant != ancestor {
            match self.parent(descendant) {
                Some(parent) => descendant = parent,
                None => return false,
            }
        }
        true
    }
}

impl<'tcx> DefIdTree for TyCtxt<'tcx> {
    fn parent(self, id: DefId) -> Option<DefId> {
        self.def_key(id).parent.map(|index| DefId { index: index, ..id })
    }
}

impl Visibility {
    pub fn from_hir(visibility: &hir::Visibility, id: hir::HirId, tcx: TyCtxt<'_>) -> Self {
        match visibility.node {
            hir::VisibilityKind::Public => Visibility::Public,
            hir::VisibilityKind::Crate(_) => Visibility::Restricted(DefId::local(CRATE_DEF_INDEX)),
            hir::VisibilityKind::Restricted { ref path, .. } => match path.res {
                // If there is no resolution, `resolve` will have already reported an error, so
                // assume that the visibility is public to avoid reporting more privacy errors.
                Res::Err => Visibility::Public,
                def => Visibility::Restricted(def.def_id()),
            },
            hir::VisibilityKind::Inherited => {
                Visibility::Restricted(tcx.hir().get_module_parent(id))
            }
        }
    }

    /// Returns `true` if an item with this visibility is accessible from the given block.
    pub fn is_accessible_from<T: DefIdTree>(self, module: DefId, tree: T) -> bool {
        let restriction = match self {
            // Public items are visible everywhere.
            Visibility::Public => return true,
            // Private items from other crates are visible nowhere.
            Visibility::Invisible => return false,
            // Restricted items are visible in an arbitrary local module.
            Visibility::Restricted(other) if other.krate != module.krate => return false,
            Visibility::Restricted(module) => module,
        };

        tree.is_descendant_of(module, restriction)
    }

    /// Returns `true` if this visibility is at least as accessible as the given visibility
    pub fn is_at_least<T: DefIdTree>(self, vis: Visibility, tree: T) -> bool {
        let vis_restriction = match vis {
            Visibility::Public => return self == Visibility::Public,
            Visibility::Invisible => return true,
            Visibility::Restricted(module) => module,
        };

        self.is_accessible_from(vis_restriction, tree)
    }

    // Returns `true` if this item is visible anywhere in the local crate.
    pub fn is_visible_locally(self) -> bool {
        match self {
            Visibility::Public => true,
            Visibility::Restricted(def_id) => def_id.is_local(),
            Visibility::Invisible => false,
        }
    }
}

#[derive(Copy, Clone, PartialEq, Eq, RustcDecodable, RustcEncodable, Hash, HashStable)]
pub enum Variance {
    Covariant,      // T<A> <: T<B> iff A <: B -- e.g., function return type
    Invariant,      // T<A> <: T<B> iff B == A -- e.g., type of mutable cell
    Contravariant,  // T<A> <: T<B> iff B <: A -- e.g., function param type
    Bivariant,      // T<A> <: T<B>            -- e.g., unused type parameter
}

/// The crate variances map is computed during typeck and contains the
/// variance of every item in the local crate. You should not use it
/// directly, because to do so will make your pass dependent on the
/// HIR of every item in the local crate. Instead, use
/// `tcx.variances_of()` to get the variance for a *particular*
/// item.
#[derive(HashStable)]
pub struct CrateVariancesMap<'tcx> {
    /// For each item with generics, maps to a vector of the variance
    /// of its generics. If an item has no generics, it will have no
    /// entry.
    pub variances: FxHashMap<DefId, &'tcx [ty::Variance]>,
}

impl Variance {
    /// `a.xform(b)` combines the variance of a context with the
    /// variance of a type with the following meaning. If we are in a
    /// context with variance `a`, and we encounter a type argument in
    /// a position with variance `b`, then `a.xform(b)` is the new
    /// variance with which the argument appears.
    ///
    /// Example 1:
    ///
    ///     *mut Vec<i32>
    ///
    /// Here, the "ambient" variance starts as covariant. `*mut T` is
    /// invariant with respect to `T`, so the variance in which the
    /// `Vec<i32>` appears is `Covariant.xform(Invariant)`, which
    /// yields `Invariant`. Now, the type `Vec<T>` is covariant with
    /// respect to its type argument `T`, and hence the variance of
    /// the `i32` here is `Invariant.xform(Covariant)`, which results
    /// (again) in `Invariant`.
    ///
    /// Example 2:
    ///
    ///     fn(*const Vec<i32>, *mut Vec<i32)
    ///
    /// The ambient variance is covariant. A `fn` type is
    /// contravariant with respect to its parameters, so the variance
    /// within which both pointer types appear is
    /// `Covariant.xform(Contravariant)`, or `Contravariant`. `*const
    /// T` is covariant with respect to `T`, so the variance within
    /// which the first `Vec<i32>` appears is
    /// `Contravariant.xform(Covariant)` or `Contravariant`. The same
    /// is true for its `i32` argument. In the `*mut T` case, the
    /// variance of `Vec<i32>` is `Contravariant.xform(Invariant)`,
    /// and hence the outermost type is `Invariant` with respect to
    /// `Vec<i32>` (and its `i32` argument).
    ///
    /// Source: Figure 1 of "Taming the Wildcards:
    /// Combining Definition- and Use-Site Variance" published in PLDI'11.
    pub fn xform(self, v: ty::Variance) -> ty::Variance {
        match (self, v) {
            // Figure 1, column 1.
            (ty::Covariant, ty::Covariant) => ty::Covariant,
            (ty::Covariant, ty::Contravariant) => ty::Contravariant,
            (ty::Covariant, ty::Invariant) => ty::Invariant,
            (ty::Covariant, ty::Bivariant) => ty::Bivariant,

            // Figure 1, column 2.
            (ty::Contravariant, ty::Covariant) => ty::Contravariant,
            (ty::Contravariant, ty::Contravariant) => ty::Covariant,
            (ty::Contravariant, ty::Invariant) => ty::Invariant,
            (ty::Contravariant, ty::Bivariant) => ty::Bivariant,

            // Figure 1, column 3.
            (ty::Invariant, _) => ty::Invariant,

            // Figure 1, column 4.
            (ty::Bivariant, _) => ty::Bivariant,
        }
    }
}

// Contains information needed to resolve types and (in the future) look up
// the types of AST nodes.
#[derive(Copy, Clone, PartialEq, Eq, Hash)]
pub struct CReaderCacheKey {
    pub cnum: CrateNum,
    pub pos: usize,
}

// Flags that we track on types. These flags are propagated upwards
// through the type during type construction, so that we can quickly
// check whether the type has various kinds of types in it without
// recursing over the type itself.
bitflags! {
    pub struct TypeFlags: u32 {
        const HAS_PARAMS         = 1 << 0;
        const HAS_TY_INFER       = 1 << 1;
        const HAS_RE_INFER       = 1 << 2;
        const HAS_RE_PLACEHOLDER = 1 << 3;

        /// Does this have any `ReEarlyBound` regions? Used to
        /// determine whether substitition is required, since those
        /// represent regions that are bound in a `ty::Generics` and
        /// hence may be substituted.
        const HAS_RE_EARLY_BOUND = 1 << 4;

        /// Does this have any region that "appears free" in the type?
        /// Basically anything but `ReLateBound` and `ReErased`.
        const HAS_FREE_REGIONS   = 1 << 5;

        /// Is an error type reachable?
        const HAS_TY_ERR         = 1 << 6;
        const HAS_PROJECTION     = 1 << 7;

        // FIXME: Rename this to the actual property since it's used for generators too
        const HAS_TY_CLOSURE     = 1 << 8;

        /// `true` if there are "names" of types and regions and so forth
        /// that are local to a particular fn
        const HAS_FREE_LOCAL_NAMES    = 1 << 9;

        /// Present if the type belongs in a local type context.
        /// Only set for Infer other than Fresh.
        const KEEP_IN_LOCAL_TCX  = 1 << 10;

        /// Does this have any `ReLateBound` regions? Used to check
        /// if a global bound is safe to evaluate.
        const HAS_RE_LATE_BOUND = 1 << 11;

        const HAS_TY_PLACEHOLDER = 1 << 12;

        const HAS_CT_INFER = 1 << 13;
        const HAS_CT_PLACEHOLDER = 1 << 14;

        const NEEDS_SUBST        = TypeFlags::HAS_PARAMS.bits |
                                   TypeFlags::HAS_RE_EARLY_BOUND.bits;

        /// Flags representing the nominal content of a type,
        /// computed by FlagsComputation. If you add a new nominal
        /// flag, it should be added here too.
        const NOMINAL_FLAGS     = TypeFlags::HAS_PARAMS.bits |
                                  TypeFlags::HAS_TY_INFER.bits |
                                  TypeFlags::HAS_RE_INFER.bits |
                                  TypeFlags::HAS_RE_PLACEHOLDER.bits |
                                  TypeFlags::HAS_RE_EARLY_BOUND.bits |
                                  TypeFlags::HAS_FREE_REGIONS.bits |
                                  TypeFlags::HAS_TY_ERR.bits |
                                  TypeFlags::HAS_PROJECTION.bits |
                                  TypeFlags::HAS_TY_CLOSURE.bits |
                                  TypeFlags::HAS_FREE_LOCAL_NAMES.bits |
                                  TypeFlags::KEEP_IN_LOCAL_TCX.bits |
                                  TypeFlags::HAS_RE_LATE_BOUND.bits |
                                  TypeFlags::HAS_TY_PLACEHOLDER.bits |
                                  TypeFlags::HAS_CT_INFER.bits |
                                  TypeFlags::HAS_CT_PLACEHOLDER.bits;
    }
}

#[allow(rustc::usage_of_ty_tykind)]
pub struct TyS<'tcx> {
    pub sty: TyKind<'tcx>,
    pub flags: TypeFlags,

    /// This is a kind of confusing thing: it stores the smallest
    /// binder such that
    ///
    /// (a) the binder itself captures nothing but
    /// (b) all the late-bound things within the type are captured
    ///     by some sub-binder.
    ///
    /// So, for a type without any late-bound things, like `u32`, this
    /// will be *innermost*, because that is the innermost binder that
    /// captures nothing. But for a type `&'D u32`, where `'D` is a
    /// late-bound region with De Bruijn index `D`, this would be `D + 1`
    /// -- the binder itself does not capture `D`, but `D` is captured
    /// by an inner binder.
    ///
    /// We call this concept an "exclusive" binder `D` because all
    /// De Bruijn indices within the type are contained within `0..D`
    /// (exclusive).
    outer_exclusive_binder: ty::DebruijnIndex,
}

// `TyS` is used a lot. Make sure it doesn't unintentionally get bigger.
#[cfg(target_arch = "x86_64")]
static_assert_size!(TyS<'_>, 32);

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

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

impl<'tcx> PartialEq for TyS<'tcx> {
    #[inline]
    fn eq(&self, other: &TyS<'tcx>) -> bool {
        ptr::eq(self, other)
    }
}
impl<'tcx> Eq for TyS<'tcx> {}

impl<'tcx> Hash for TyS<'tcx> {
    fn hash<H: Hasher>(&self, s: &mut H) {
        (self as *const TyS<'_>).hash(s)
    }
}

impl<'tcx> TyS<'tcx> {
    pub fn is_primitive_ty(&self) -> bool {
        match self.sty {
            Bool |
            Char |
            Int(_) |
            Uint(_) |
            Float(_) |
            Infer(InferTy::IntVar(_)) |
            Infer(InferTy::FloatVar(_)) |
            Infer(InferTy::FreshIntTy(_)) |
            Infer(InferTy::FreshFloatTy(_)) => true,
            Ref(_, x, _) => x.is_primitive_ty(),
            _ => false,
        }
    }

    pub fn is_suggestable(&self) -> bool {
        match self.sty {
            Opaque(..) |
            FnDef(..) |
            FnPtr(..) |
            Dynamic(..) |
            Closure(..) |
            Infer(..) |
            Projection(..) => false,
            _ => true,
        }
    }
}

impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for ty::TyS<'tcx> {
    fn hash_stable<W: StableHasherResult>(&self,
                                          hcx: &mut StableHashingContext<'a>,
                                          hasher: &mut StableHasher<W>) {
        let ty::TyS {
            ref sty,

            // The other fields just provide fast access to information that is
            // also contained in `sty`, so no need to hash them.
            flags: _,

            outer_exclusive_binder: _,
        } = *self;

        sty.hash_stable(hcx, hasher);
    }
}

pub type Ty<'tcx> = &'tcx TyS<'tcx>;

impl<'tcx> rustc_serialize::UseSpecializedEncodable for Ty<'tcx> {}
impl<'tcx> rustc_serialize::UseSpecializedDecodable for Ty<'tcx> {}

pub type CanonicalTy<'tcx> = Canonical<'tcx, Ty<'tcx>>;

extern {
    /// A dummy type used to force List to by unsized without requiring fat pointers
    type OpaqueListContents;
}

/// A wrapper for slices with the additional invariant
/// that the slice is interned and no other slice with
/// the same contents can exist in the same context.
/// This means we can use pointer for both
/// equality comparisons and hashing.
/// Note: `Slice` was already taken by the `Ty`.
#[repr(C)]
pub struct List<T> {
    len: usize,
    data: [T; 0],
    opaque: OpaqueListContents,
}

unsafe impl<T: Sync> Sync for List<T> {}

impl<T: Copy> List<T> {
    #[inline]
    fn from_arena<'tcx>(arena: &'tcx SyncDroplessArena, slice: &[T]) -> &'tcx List<T> {
        assert!(!mem::needs_drop::<T>());
        assert!(mem::size_of::<T>() != 0);
        assert!(slice.len() != 0);

        // Align up the size of the len (usize) field
        let align = mem::align_of::<T>();
        let align_mask = align - 1;
        let offset = mem::size_of::<usize>();
        let offset = (offset + align_mask) & !align_mask;

        let size = offset + slice.len() * mem::size_of::<T>();

        let mem = arena.alloc_raw(
            size,
            cmp::max(mem::align_of::<T>(), mem::align_of::<usize>()));
        unsafe {
            let result = &mut *(mem.as_mut_ptr() as *mut List<T>);
            // Write the length
            result.len = slice.len();

            // Write the elements
            let arena_slice = slice::from_raw_parts_mut(result.data.as_mut_ptr(), result.len);
            arena_slice.copy_from_slice(slice);

            result
        }
    }
}

impl<T: fmt::Debug> fmt::Debug for List<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

impl<T: Encodable> Encodable for List<T> {
    #[inline]
    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
        (**self).encode(s)
    }
}

impl<T> Ord for List<T> where T: Ord {
    fn cmp(&self, other: &List<T>) -> Ordering {
        if self == other { Ordering::Equal } else {
            <[T] as Ord>::cmp(&**self, &**other)
        }
    }
}

impl<T> PartialOrd for List<T> where T: PartialOrd {
    fn partial_cmp(&self, other: &List<T>) -> Option<Ordering> {
        if self == other { Some(Ordering::Equal) } else {
            <[T] as PartialOrd>::partial_cmp(&**self, &**other)
        }
    }
}

impl<T: PartialEq> PartialEq for List<T> {
    #[inline]
    fn eq(&self, other: &List<T>) -> bool {
        ptr::eq(self, other)
    }
}
impl<T: Eq> Eq for List<T> {}

impl<T> Hash for List<T> {
    #[inline]
    fn hash<H: Hasher>(&self, s: &mut H) {
        (self as *const List<T>).hash(s)
    }
}

impl<T> Deref for List<T> {
    type Target = [T];
    #[inline(always)]
    fn deref(&self) -> &[T] {
        unsafe {
            slice::from_raw_parts(self.data.as_ptr(), self.len)
        }
    }
}

impl<'a, T> IntoIterator for &'a List<T> {
    type Item = &'a T;
    type IntoIter = <&'a [T] as IntoIterator>::IntoIter;
    #[inline(always)]
    fn into_iter(self) -> Self::IntoIter {
        self[..].iter()
    }
}

impl<'tcx> rustc_serialize::UseSpecializedDecodable for &'tcx List<Ty<'tcx>> {}

impl<T> List<T> {
    #[inline(always)]
    pub fn empty<'a>() -> &'a List<T> {
        #[repr(align(64), C)]
        struct EmptySlice([u8; 64]);
        static EMPTY_SLICE: EmptySlice = EmptySlice([0; 64]);
        assert!(mem::align_of::<T>() <= 64);
        unsafe {
            &*(&EMPTY_SLICE as *const _ as *const List<T>)
        }
    }
}

#[derive(Clone, Copy, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, HashStable)]
pub struct UpvarPath {
    pub hir_id: hir::HirId,
}

/// Upvars do not get their own `NodeId`. Instead, we use the pair of
/// the original var ID (that is, the root variable that is referenced
/// by the upvar) and the ID of the closure expression.
#[derive(Clone, Copy, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, HashStable)]
pub struct UpvarId {
    pub var_path: UpvarPath,
    pub closure_expr_id: LocalDefId,
}

#[derive(Clone, PartialEq, Eq, Hash, Debug, RustcEncodable, RustcDecodable, Copy, HashStable)]
pub enum BorrowKind {
    /// Data must be immutable and is aliasable.
    ImmBorrow,

    /// Data must be immutable but not aliasable. This kind of borrow
    /// cannot currently be expressed by the user and is used only in
    /// implicit closure bindings. It is needed when the closure
    /// is borrowing or mutating a mutable referent, e.g.:
    ///
    ///    let x: &mut isize = ...;
    ///    let y = || *x += 5;
    ///
    /// If we were to try to translate this closure into a more explicit
    /// form, we'd encounter an error with the code as written:
    ///
    ///    struct Env { x: & &mut isize }
    ///    let x: &mut isize = ...;
    ///    let y = (&mut Env { &x }, fn_ptr);  // Closure is pair of env and fn
    ///    fn fn_ptr(env: &mut Env) { **env.x += 5; }
    ///
    /// This is then illegal because you cannot mutate a `&mut` found
    /// in an aliasable location. To solve, you'd have to translate with
    /// an `&mut` borrow:
    ///
    ///    struct Env { x: & &mut isize }
    ///    let x: &mut isize = ...;
    ///    let y = (&mut Env { &mut x }, fn_ptr); // changed from &x to &mut x
    ///    fn fn_ptr(env: &mut Env) { **env.x += 5; }
    ///
    /// Now the assignment to `**env.x` is legal, but creating a
    /// mutable pointer to `x` is not because `x` is not mutable. We
    /// could fix this by declaring `x` as `let mut x`. This is ok in
    /// user code, if awkward, but extra weird for closures, since the
    /// borrow is hidden.
    ///
    /// So we introduce a "unique imm" borrow -- the referent is
    /// immutable, but not aliasable. This solves the problem. For
    /// simplicity, we don't give users the way to express this
    /// borrow, it's just used when translating closures.
    UniqueImmBorrow,

    /// Data is mutable and not aliasable.
    MutBorrow
}

/// Information describing the capture of an upvar. This is computed
/// during `typeck`, specifically by `regionck`.
#[derive(PartialEq, Clone, Debug, Copy, RustcEncodable, RustcDecodable, HashStable)]
pub enum UpvarCapture<'tcx> {
    /// Upvar is captured by value. This is always true when the
    /// closure is labeled `move`, but can also be true in other cases
    /// depending on inference.
    ByValue,

    /// Upvar is captured by reference.
    ByRef(UpvarBorrow<'tcx>),
}

#[derive(PartialEq, Clone, Copy, RustcEncodable, RustcDecodable, HashStable)]
pub struct UpvarBorrow<'tcx> {
    /// The kind of borrow: by-ref upvars have access to shared
    /// immutable borrows, which are not part of the normal language
    /// syntax.
    pub kind: BorrowKind,

    /// Region of the resulting reference.
    pub region: ty::Region<'tcx>,
}

pub type UpvarListMap = FxHashMap<DefId, FxIndexMap<hir::HirId, UpvarId>>;
pub type UpvarCaptureMap<'tcx> = FxHashMap<UpvarId, UpvarCapture<'tcx>>;

#[derive(Copy, Clone)]
pub struct ClosureUpvar<'tcx> {
    pub res: Res,
    pub span: Span,
    pub ty: Ty<'tcx>,
}

#[derive(Clone, Copy, PartialEq, Eq)]
pub enum IntVarValue {
    IntType(ast::IntTy),
    UintType(ast::UintTy),
}

#[derive(Clone, Copy, PartialEq, Eq)]
pub struct FloatVarValue(pub ast::FloatTy);

impl ty::EarlyBoundRegion {
    pub fn to_bound_region(&self) -> ty::BoundRegion {
        ty::BoundRegion::BrNamed(self.def_id, self.name)
    }

    /// Does this early bound region have a name? Early bound regions normally
    /// always have names except when using anonymous lifetimes (`'_`).
    pub fn has_name(&self) -> bool {
        self.name != kw::UnderscoreLifetime.as_interned_str()
    }
}

#[derive(Clone, Debug, RustcEncodable, RustcDecodable, HashStable)]
pub enum GenericParamDefKind {
    Lifetime,
    Type {
        has_default: bool,
        object_lifetime_default: ObjectLifetimeDefault,
        synthetic: Option<hir::SyntheticTyParamKind>,
    },
    Const,
}

#[derive(Clone, RustcEncodable, RustcDecodable, HashStable)]
pub struct GenericParamDef {
    pub name: InternedString,
    pub def_id: DefId,
    pub index: u32,

    /// `pure_wrt_drop`, set by the (unsafe) `#[may_dangle]` attribute
    /// on generic parameter `'a`/`T`, asserts data behind the parameter
    /// `'a`/`T` won't be accessed during the parent type's `Drop` impl.
    pub pure_wrt_drop: bool,

    pub kind: GenericParamDefKind,
}

impl GenericParamDef {
    pub fn to_early_bound_region_data(&self) -> ty::EarlyBoundRegion {
        if let GenericParamDefKind::Lifetime = self.kind {
            ty::EarlyBoundRegion {
                def_id: self.def_id,
                index: self.index,
                name: self.name,
            }
        } else {
            bug!("cannot convert a non-lifetime parameter def to an early bound region")
        }
    }

    pub fn to_bound_region(&self) -> ty::BoundRegion {
        if let GenericParamDefKind::Lifetime = self.kind {
            self.to_early_bound_region_data().to_bound_region()
        } else {
            bug!("cannot convert a non-lifetime parameter def to an early bound region")
        }
    }
}

#[derive(Default)]
pub struct GenericParamCount {
    pub lifetimes: usize,
    pub types: usize,
    pub consts: usize,
}

/// Information about the formal type/lifetime parameters associated
/// with an item or method. Analogous to `hir::Generics`.
///
/// The ordering of parameters is the same as in `Subst` (excluding child generics):
/// `Self` (optionally), `Lifetime` params..., `Type` params...
#[derive(Clone, Debug, RustcEncodable, RustcDecodable, HashStable)]
pub struct Generics {
    pub parent: Option<DefId>,
    pub parent_count: usize,
    pub params: Vec<GenericParamDef>,

    /// Reverse map to the `index` field of each `GenericParamDef`.
    #[stable_hasher(ignore)]
    pub param_def_id_to_index: FxHashMap<DefId, u32>,

    pub has_self: bool,
    pub has_late_bound_regions: Option<Span>,
}

impl<'tcx> Generics {
    pub fn count(&self) -> usize {
        self.parent_count + self.params.len()
    }

    pub fn own_counts(&self) -> GenericParamCount {
        // We could cache this as a property of `GenericParamCount`, but
        // the aim is to refactor this away entirely eventually and the
        // presence of this method will be a constant reminder.
        let mut own_counts: GenericParamCount = Default::default();

        for param in &self.params {
            match param.kind {
                GenericParamDefKind::Lifetime => own_counts.lifetimes += 1,
                GenericParamDefKind::Type { .. } => own_counts.types += 1,
                GenericParamDefKind::Const => own_counts.consts += 1,
            };
        }

        own_counts
    }

    pub fn requires_monomorphization(&self, tcx: TyCtxt<'tcx>) -> bool {
        if self.own_requires_monomorphization() {
            return true;
        }

        if let Some(parent_def_id) = self.parent {
            let parent = tcx.generics_of(parent_def_id);
            parent.requires_monomorphization(tcx)
        } else {
            false
        }
    }

    pub fn own_requires_monomorphization(&self) -> bool {
        for param in &self.params {
            match param.kind {
                GenericParamDefKind::Type { .. } | GenericParamDefKind::Const => return true,
                GenericParamDefKind::Lifetime => {}
            }
        }
        false
    }

    pub fn region_param(
        &'tcx self,
        param: &EarlyBoundRegion,
        tcx: TyCtxt<'tcx>,
    ) -> &'tcx GenericParamDef {
        if let Some(index) = param.index.checked_sub(self.parent_count as u32) {
            let param = &self.params[index as usize];
            match param.kind {
                GenericParamDefKind::Lifetime => param,
                _ => bug!("expected lifetime parameter, but found another generic parameter")
            }
        } else {
            tcx.generics_of(self.parent.expect("parent_count > 0 but no parent?"))
               .region_param(param, tcx)
        }
    }

    /// Returns the `GenericParamDef` associated with this `ParamTy`.
    pub fn type_param(&'tcx self, param: &ParamTy, tcx: TyCtxt<'tcx>) -> &'tcx GenericParamDef {
        if let Some(index) = param.index.checked_sub(self.parent_count as u32) {
            let param = &self.params[index as usize];
            match param.kind {
                GenericParamDefKind::Type { .. } => param,
                _ => bug!("expected type parameter, but found another generic parameter")
            }
        } else {
            tcx.generics_of(self.parent.expect("parent_count > 0 but no parent?"))
               .type_param(param, tcx)
        }
    }

    /// Returns the `ConstParameterDef` associated with this `ParamConst`.
    pub fn const_param(&'tcx self, param: &ParamConst, tcx: TyCtxt<'tcx>) -> &GenericParamDef {
        if let Some(index) = param.index.checked_sub(self.parent_count as u32) {
            let param = &self.params[index as usize];
            match param.kind {
                GenericParamDefKind::Const => param,
                _ => bug!("expected const parameter, but found another generic parameter")
            }
        } else {
            tcx.generics_of(self.parent.expect("parent_count>0 but no parent?"))
                .const_param(param, tcx)
        }
    }
}

/// Bounds on generics.
#[derive(Clone, Default, Debug, HashStable)]
pub struct GenericPredicates<'tcx> {
    pub parent: Option<DefId>,
    pub predicates: Vec<(Predicate<'tcx>, Span)>,
}

impl<'tcx> rustc_serialize::UseSpecializedEncodable for GenericPredicates<'tcx> {}
impl<'tcx> rustc_serialize::UseSpecializedDecodable for GenericPredicates<'tcx> {}

impl<'tcx> GenericPredicates<'tcx> {
    pub fn instantiate(
        &self,
        tcx: TyCtxt<'tcx>,
        substs: SubstsRef<'tcx>,
    ) -> InstantiatedPredicates<'tcx> {
        let mut instantiated = InstantiatedPredicates::empty();
        self.instantiate_into(tcx, &mut instantiated, substs);
        instantiated
    }

    pub fn instantiate_own(
        &self,
        tcx: TyCtxt<'tcx>,
        substs: SubstsRef<'tcx>,
    ) -> InstantiatedPredicates<'tcx> {
        InstantiatedPredicates {
            predicates: self.predicates.iter().map(|(p, _)| p.subst(tcx, substs)).collect(),
        }
    }

    fn instantiate_into(
        &self,
        tcx: TyCtxt<'tcx>,
        instantiated: &mut InstantiatedPredicates<'tcx>,
        substs: SubstsRef<'tcx>,
    ) {
        if let Some(def_id) = self.parent {
            tcx.predicates_of(def_id).instantiate_into(tcx, instantiated, substs);
        }
        instantiated.predicates.extend(
            self.predicates.iter().map(|(p, _)| p.subst(tcx, substs)),
        );
    }

    pub fn instantiate_identity(&self, tcx: TyCtxt<'tcx>) -> InstantiatedPredicates<'tcx> {
        let mut instantiated = InstantiatedPredicates::empty();
        self.instantiate_identity_into(tcx, &mut instantiated);
        instantiated
    }

    fn instantiate_identity_into(
        &self,
        tcx: TyCtxt<'tcx>,
        instantiated: &mut InstantiatedPredicates<'tcx>,
    ) {
        if let Some(def_id) = self.parent {
            tcx.predicates_of(def_id).instantiate_identity_into(tcx, instantiated);
        }
        instantiated.predicates.extend(self.predicates.iter().map(|&(p, _)| p))
    }

    pub fn instantiate_supertrait(
        &self,
        tcx: TyCtxt<'tcx>,
        poly_trait_ref: &ty::PolyTraitRef<'tcx>,
    ) -> InstantiatedPredicates<'tcx> {
        assert_eq!(self.parent, None);
        InstantiatedPredicates {
            predicates: self.predicates.iter().map(|(pred, _)| {
                pred.subst_supertrait(tcx, poly_trait_ref)
            }).collect()
        }
    }
}

#[derive(Clone, Copy, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, HashStable)]
pub enum Predicate<'tcx> {
    /// Corresponds to `where Foo: Bar<A, B, C>`. `Foo` here would be
    /// the `Self` type of the trait reference and `A`, `B`, and `C`
    /// would be the type parameters.
    Trait(PolyTraitPredicate<'tcx>),

    /// `where 'a: 'b`
    RegionOutlives(PolyRegionOutlivesPredicate<'tcx>),

    /// `where T: 'a`
    TypeOutlives(PolyTypeOutlivesPredicate<'tcx>),

    /// `where <T as TraitRef>::Name == X`, approximately.
    /// See the `ProjectionPredicate` struct for details.
    Projection(PolyProjectionPredicate<'tcx>),

    /// No syntax: `T` well-formed.
    WellFormed(Ty<'tcx>),

    /// Trait must be object-safe.
    ObjectSafe(DefId),

    /// No direct syntax. May be thought of as `where T: FnFoo<...>`
    /// for some substitutions `...` and `T` being a closure type.
    /// Satisfied (or refuted) once we know the closure's kind.
    ClosureKind(DefId, ClosureSubsts<'tcx>, ClosureKind),

    /// `T1 <: T2`
    Subtype(PolySubtypePredicate<'tcx>),

    /// Constant initializer must evaluate successfully.
    ConstEvaluatable(DefId, SubstsRef<'tcx>),
}

/// The crate outlives map is computed during typeck and contains the
/// outlives of every item in the local crate. You should not use it
/// directly, because to do so will make your pass dependent on the
/// HIR of every item in the local crate. Instead, use
/// `tcx.inferred_outlives_of()` to get the outlives for a *particular*
/// item.
#[derive(HashStable)]
pub struct CratePredicatesMap<'tcx> {
    /// For each struct with outlive bounds, maps to a vector of the
    /// predicate of its outlive bounds. If an item has no outlives
    /// bounds, it will have no entry.
    pub predicates: FxHashMap<DefId, &'tcx [ty::Predicate<'tcx>]>,
}

impl<'tcx> AsRef<Predicate<'tcx>> for Predicate<'tcx> {
    fn as_ref(&self) -> &Predicate<'tcx> {
        self
    }
}

impl<'tcx> Predicate<'tcx> {
    /// Performs a substitution suitable for going from a
    /// poly-trait-ref to supertraits that must hold if that
    /// poly-trait-ref holds. This is slightly different from a normal
    /// substitution in terms of what happens with bound regions. See
    /// lengthy comment below for details.
    pub fn subst_supertrait(
        &self,
        tcx: TyCtxt<'tcx>,
        trait_ref: &ty::PolyTraitRef<'tcx>,
    ) -> ty::Predicate<'tcx> {
        // The interaction between HRTB and supertraits is not entirely
        // obvious. Let me walk you (and myself) through an example.
        //
        // Let's start with an easy case. Consider two traits:
        //
        //     trait Foo<'a>: Bar<'a,'a> { }
        //     trait Bar<'b,'c> { }
        //
        // Now, if we have a trait reference `for<'x> T: Foo<'x>`, then
        // we can deduce that `for<'x> T: Bar<'x,'x>`. Basically, if we
        // knew that `Foo<'x>` (for any 'x) then we also know that
        // `Bar<'x,'x>` (for any 'x). This more-or-less falls out from
        // normal substitution.
        //
        // In terms of why this is sound, the idea is that whenever there
        // is an impl of `T:Foo<'a>`, it must show that `T:Bar<'a,'a>`
        // holds.  So if there is an impl of `T:Foo<'a>` that applies to
        // all `'a`, then we must know that `T:Bar<'a,'a>` holds for all
        // `'a`.
        //
        // Another example to be careful of is this:
        //
        //     trait Foo1<'a>: for<'b> Bar1<'a,'b> { }
        //     trait Bar1<'b,'c> { }
        //
        // Here, if we have `for<'x> T: Foo1<'x>`, then what do we know?
        // The answer is that we know `for<'x,'b> T: Bar1<'x,'b>`. The
        // reason is similar to the previous example: any impl of
        // `T:Foo1<'x>` must show that `for<'b> T: Bar1<'x, 'b>`.  So
        // basically we would want to collapse the bound lifetimes from
        // the input (`trait_ref`) and the supertraits.
        //
        // To achieve this in practice is fairly straightforward. Let's
        // consider the more complicated scenario:
        //
        // - We start out with `for<'x> T: Foo1<'x>`. In this case, `'x`
        //   has a De Bruijn index of 1. We want to produce `for<'x,'b> T: Bar1<'x,'b>`,
        //   where both `'x` and `'b` would have a DB index of 1.
        //   The substitution from the input trait-ref is therefore going to be
        //   `'a => 'x` (where `'x` has a DB index of 1).
        // - The super-trait-ref is `for<'b> Bar1<'a,'b>`, where `'a` is an
        //   early-bound parameter and `'b' is a late-bound parameter with a
        //   DB index of 1.
        // - If we replace `'a` with `'x` from the input, it too will have
        //   a DB index of 1, and thus we'll have `for<'x,'b> Bar1<'x,'b>`
        //   just as we wanted.
        //
        // There is only one catch. If we just apply the substitution `'a
        // => 'x` to `for<'b> Bar1<'a,'b>`, the substitution code will
        // adjust the DB index because we substituting into a binder (it
        // tries to be so smart...) resulting in `for<'x> for<'b>
        // Bar1<'x,'b>` (we have no syntax for this, so use your
        // imagination). Basically the 'x will have DB index of 2 and 'b
        // will have DB index of 1. Not quite what we want. So we apply
        // the substitution to the *contents* of the trait reference,
        // rather than the trait reference itself (put another way, the
        // substitution code expects equal binding levels in the values
        // from the substitution and the value being substituted into, and
        // this trick achieves that).

        let substs = &trait_ref.skip_binder().substs;
        match *self {
            Predicate::Trait(ref binder) =>
                Predicate::Trait(binder.map_bound(|data| data.subst(tcx, substs))),
            Predicate::Subtype(ref binder) =>
                Predicate::Subtype(binder.map_bound(|data| data.subst(tcx, substs))),
            Predicate::RegionOutlives(ref binder) =>
                Predicate::RegionOutlives(binder.map_bound(|data| data.subst(tcx, substs))),
            Predicate::TypeOutlives(ref binder) =>
                Predicate::TypeOutlives(binder.map_bound(|data| data.subst(tcx, substs))),
            Predicate::Projection(ref binder) =>
                Predicate::Projection(binder.map_bound(|data| data.subst(tcx, substs))),
            Predicate::WellFormed(data) =>
                Predicate::WellFormed(data.subst(tcx, substs)),
            Predicate::ObjectSafe(trait_def_id) =>
                Predicate::ObjectSafe(trait_def_id),
            Predicate::ClosureKind(closure_def_id, closure_substs, kind) =>
                Predicate::ClosureKind(closure_def_id, closure_substs.subst(tcx, substs), kind),
            Predicate::ConstEvaluatable(def_id, const_substs) =>
                Predicate::ConstEvaluatable(def_id, const_substs.subst(tcx, substs)),
        }
    }
}

#[derive(Clone, Copy, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, HashStable)]
pub struct TraitPredicate<'tcx> {
    pub trait_ref: TraitRef<'tcx>
}

pub type PolyTraitPredicate<'tcx> = ty::Binder<TraitPredicate<'tcx>>;

impl<'tcx> TraitPredicate<'tcx> {
    pub fn def_id(&self) -> DefId {
        self.trait_ref.def_id
    }

    pub fn input_types<'a>(&'a self) -> impl DoubleEndedIterator<Item = Ty<'tcx>> + 'a {
        self.trait_ref.input_types()
    }

    pub fn self_ty(&self) -> Ty<'tcx> {
        self.trait_ref.self_ty()
    }
}

impl<'tcx> PolyTraitPredicate<'tcx> {
    pub fn def_id(&self) -> DefId {
        // Ok to skip binder since trait `DefId` does not care about regions.
        self.skip_binder().def_id()
    }
}

#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord,
         Hash, Debug, RustcEncodable, RustcDecodable, HashStable)]
pub struct OutlivesPredicate<A, B>(pub A, pub B); // `A: B`
pub type PolyOutlivesPredicate<A, B> = ty::Binder<OutlivesPredicate<A, B>>;
pub type RegionOutlivesPredicate<'tcx> = OutlivesPredicate<ty::Region<'tcx>, ty::Region<'tcx>>;
pub type TypeOutlivesPredicate<'tcx> = OutlivesPredicate<Ty<'tcx>, ty::Region<'tcx>>;
pub type PolyRegionOutlivesPredicate<'tcx> = ty::Binder<RegionOutlivesPredicate<'tcx>>;
pub type PolyTypeOutlivesPredicate<'tcx> = ty::Binder<TypeOutlivesPredicate<'tcx>>;

#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug, RustcEncodable, RustcDecodable, HashStable)]
pub struct SubtypePredicate<'tcx> {
    pub a_is_expected: bool,
    pub a: Ty<'tcx>,
    pub b: Ty<'tcx>
}
pub type PolySubtypePredicate<'tcx> = ty::Binder<SubtypePredicate<'tcx>>;

/// This kind of predicate has no *direct* correspondent in the
/// syntax, but it roughly corresponds to the syntactic forms:
///
/// 1. `T: TraitRef<..., Item = Type>`
/// 2. `<T as TraitRef<...>>::Item == Type` (NYI)
///
/// In particular, form #1 is "desugared" to the combination of a
/// normal trait predicate (`T: TraitRef<...>`) and one of these
/// predicates. Form #2 is a broader form in that it also permits
/// equality between arbitrary types. Processing an instance of
/// Form #2 eventually yields one of these `ProjectionPredicate`
/// instances to normalize the LHS.
#[derive(Copy, Clone, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, HashStable)]
pub struct ProjectionPredicate<'tcx> {
    pub projection_ty: ProjectionTy<'tcx>,
    pub ty: Ty<'tcx>,
}

pub type PolyProjectionPredicate<'tcx> = Binder<ProjectionPredicate<'tcx>>;

impl<'tcx> PolyProjectionPredicate<'tcx> {
    /// Returns the `DefId` of the associated item being projected.
    pub fn item_def_id(&self) -> DefId {
        self.skip_binder().projection_ty.item_def_id
    }

    #[inline]
    pub fn to_poly_trait_ref(&self, tcx: TyCtxt<'_>) -> PolyTraitRef<'tcx> {
        // Note: unlike with `TraitRef::to_poly_trait_ref()`,
        // `self.0.trait_ref` is permitted to have escaping regions.
        // This is because here `self` has a `Binder` and so does our
        // return value, so we are preserving the number of binding
        // levels.
        self.map_bound(|predicate| predicate.projection_ty.trait_ref(tcx))
    }

    pub fn ty(&self) -> Binder<Ty<'tcx>> {
        self.map_bound(|predicate| predicate.ty)
    }

    /// The `DefId` of the `TraitItem` for the associated type.
    ///
    /// Note that this is not the `DefId` of the `TraitRef` containing this
    /// associated type, which is in `tcx.associated_item(projection_def_id()).container`.
    pub fn projection_def_id(&self) -> DefId {
        // Ok to skip binder since trait `DefId` does not care about regions.
        self.skip_binder().projection_ty.item_def_id
    }
}

pub trait ToPolyTraitRef<'tcx> {
    fn to_poly_trait_ref(&self) -> PolyTraitRef<'tcx>;
}

impl<'tcx> ToPolyTraitRef<'tcx> for TraitRef<'tcx> {
    fn to_poly_trait_ref(&self) -> PolyTraitRef<'tcx> {
        ty::Binder::dummy(self.clone())
    }
}

impl<'tcx> ToPolyTraitRef<'tcx> for PolyTraitPredicate<'tcx> {
    fn to_poly_trait_ref(&self) -> PolyTraitRef<'tcx> {
        self.map_bound_ref(|trait_pred| trait_pred.trait_ref)
    }
}

pub trait ToPredicate<'tcx> {
    fn to_predicate(&self) -> Predicate<'tcx>;
}

impl<'tcx> ToPredicate<'tcx> for TraitRef<'tcx> {
    fn to_predicate(&self) -> Predicate<'tcx> {
        ty::Predicate::Trait(ty::Binder::dummy(ty::TraitPredicate {
            trait_ref: self.clone()
        }))
    }
}

impl<'tcx> ToPredicate<'tcx> for PolyTraitRef<'tcx> {
    fn to_predicate(&self) -> Predicate<'tcx> {
        ty::Predicate::Trait(self.to_poly_trait_predicate())
    }
}

impl<'tcx> ToPredicate<'tcx> for PolyRegionOutlivesPredicate<'tcx> {
    fn to_predicate(&self) -> Predicate<'tcx> {
        Predicate::RegionOutlives(self.clone())
    }
}

impl<'tcx> ToPredicate<'tcx> for PolyTypeOutlivesPredicate<'tcx> {
    fn to_predicate(&self) -> Predicate<'tcx> {
        Predicate::TypeOutlives(self.clone())
    }
}

impl<'tcx> ToPredicate<'tcx> for PolyProjectionPredicate<'tcx> {
    fn to_predicate(&self) -> Predicate<'tcx> {
        Predicate::Projection(self.clone())
    }
}

// A custom iterator used by `Predicate::walk_tys`.
enum WalkTysIter<'tcx, I, J, K>
    where I: Iterator<Item = Ty<'tcx>>,
          J: Iterator<Item = Ty<'tcx>>,
          K: Iterator<Item = Ty<'tcx>>
{
    None,
    One(Ty<'tcx>),
    Two(Ty<'tcx>, Ty<'tcx>),
    Types(I),
    InputTypes(J),
    ProjectionTypes(K)
}

impl<'tcx, I, J, K> Iterator for WalkTysIter<'tcx, I, J, K>
    where I: Iterator<Item = Ty<'tcx>>,
          J: Iterator<Item = Ty<'tcx>>,
          K: Iterator<Item = Ty<'tcx>>
{
    type Item = Ty<'tcx>;

    fn next(&mut self) -> Option<Ty<'tcx>> {
        match *self {
            WalkTysIter::None => None,
            WalkTysIter::One(item) => {
                *self = WalkTysIter::None;
                Some(item)
            },
            WalkTysIter::Two(item1, item2) => {
                *self = WalkTysIter::One(item2);
                Some(item1)
            },
            WalkTysIter::Types(ref mut iter) => {
                iter.next()
            },
            WalkTysIter::InputTypes(ref mut iter) => {
                iter.next()
            },
            WalkTysIter::ProjectionTypes(ref mut iter) => {
                iter.next()
            }
        }
    }
}

impl<'tcx> Predicate<'tcx> {
    /// Iterates over the types in this predicate. Note that in all
    /// cases this is skipping over a binder, so late-bound regions
    /// with depth 0 are bound by the predicate.
    pub fn walk_tys(&'a self) -> impl Iterator<Item = Ty<'tcx>> + 'a {
        match *self {
            ty::Predicate::Trait(ref data) => {
                WalkTysIter::InputTypes(data.skip_binder().input_types())
            }
            ty::Predicate::Subtype(binder) => {
                let SubtypePredicate { a, b, a_is_expected: _ } = binder.skip_binder();
                WalkTysIter::Two(a, b)
            }
            ty::Predicate::TypeOutlives(binder) => {
                WalkTysIter::One(binder.skip_binder().0)
            }
            ty::Predicate::RegionOutlives(..) => {
                WalkTysIter::None
            }
            ty::Predicate::Projection(ref data) => {
                let inner = data.skip_binder();
                WalkTysIter::ProjectionTypes(
                    inner.projection_ty.substs.types().chain(Some(inner.ty)))
            }
            ty::Predicate::WellFormed(data) => {
                WalkTysIter::One(data)
            }
            ty::Predicate::ObjectSafe(_trait_def_id) => {
                WalkTysIter::None
            }
            ty::Predicate::ClosureKind(_closure_def_id, closure_substs, _kind) => {
                WalkTysIter::Types(closure_substs.substs.types())
            }
            ty::Predicate::ConstEvaluatable(_, substs) => {
                WalkTysIter::Types(substs.types())
            }
        }
    }

    pub fn to_opt_poly_trait_ref(&self) -> Option<PolyTraitRef<'tcx>> {
        match *self {
            Predicate::Trait(ref t) => {
                Some(t.to_poly_trait_ref())
            }
            Predicate::Projection(..) |
            Predicate::Subtype(..) |
            Predicate::RegionOutlives(..) |
            Predicate::WellFormed(..) |
            Predicate::ObjectSafe(..) |
            Predicate::ClosureKind(..) |
            Predicate::TypeOutlives(..) |
            Predicate::ConstEvaluatable(..) => {
                None
            }
        }
    }

    pub fn to_opt_type_outlives(&self) -> Option<PolyTypeOutlivesPredicate<'tcx>> {
        match *self {
            Predicate::TypeOutlives(data) => {
                Some(data)
            }
            Predicate::Trait(..) |
            Predicate::Projection(..) |
            Predicate::Subtype(..) |
            Predicate::RegionOutlives(..) |
            Predicate::WellFormed(..) |
            Predicate::ObjectSafe(..) |
            Predicate::ClosureKind(..) |
            Predicate::ConstEvaluatable(..) => {
                None
            }
        }
    }
}

/// Represents the bounds declared on a particular set of type
/// parameters. Should eventually be generalized into a flag list of
/// where-clauses. You can obtain a `InstantiatedPredicates` list from a
/// `GenericPredicates` by using the `instantiate` method. Note that this method
/// reflects an important semantic invariant of `InstantiatedPredicates`: while
/// the `GenericPredicates` are expressed in terms of the bound type
/// parameters of the impl/trait/whatever, an `InstantiatedPredicates` instance
/// represented a set of bounds for some particular instantiation,
/// meaning that the generic parameters have been substituted with
/// their values.
///
/// Example:
///
///     struct Foo<T, U: Bar<T>> { ... }
///
/// Here, the `GenericPredicates` for `Foo` would contain a list of bounds like
/// `[[], [U:Bar<T>]]`. Now if there were some particular reference
/// like `Foo<isize,usize>`, then the `InstantiatedPredicates` would be `[[],
/// [usize:Bar<isize>]]`.
#[derive(Clone, Debug)]
pub struct InstantiatedPredicates<'tcx> {
    pub predicates: Vec<Predicate<'tcx>>,
}

impl<'tcx> InstantiatedPredicates<'tcx> {
    pub fn empty() -> InstantiatedPredicates<'tcx> {
        InstantiatedPredicates { predicates: vec![] }
    }

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

newtype_index! {
    /// "Universes" are used during type- and trait-checking in the
    /// presence of `for<..>` binders to control what sets of names are
    /// visible. Universes are arranged into a tree: the root universe
    /// contains names that are always visible. Each child then adds a new
    /// set of names that are visible, in addition to those of its parent.
    /// We say that the child universe "extends" the parent universe with
    /// new names.
    ///
    /// To make this more concrete, consider this program:
    ///
    /// ```
    /// struct Foo { }
    /// fn bar<T>(x: T) {
    ///   let y: for<'a> fn(&'a u8, Foo) = ...;
    /// }
    /// ```
    ///
    /// The struct name `Foo` is in the root universe U0. But the type
    /// parameter `T`, introduced on `bar`, is in an extended universe U1
    /// -- i.e., within `bar`, we can name both `T` and `Foo`, but outside
    /// of `bar`, we cannot name `T`. Then, within the type of `y`, the
    /// region `'a` is in a universe U2 that extends U1, because we can
    /// name it inside the fn type but not outside.
    ///
    /// Universes are used to do type- and trait-checking around these
    /// "forall" binders (also called **universal quantification**). The
    /// idea is that when, in the body of `bar`, we refer to `T` as a
    /// type, we aren't referring to any type in particular, but rather a
    /// kind of "fresh" type that is distinct from all other types we have
    /// actually declared. This is called a **placeholder** type, and we
    /// use universes to talk about this. In other words, a type name in
    /// universe 0 always corresponds to some "ground" type that the user
    /// declared, but a type name in a non-zero universe is a placeholder
    /// type -- an idealized representative of "types in general" that we
    /// use for checking generic functions.
    pub struct UniverseIndex {
        DEBUG_FORMAT = "U{}",
    }
}

impl_stable_hash_for!(struct UniverseIndex { private });

impl UniverseIndex {
    pub const ROOT: UniverseIndex = UniverseIndex::from_u32_const(0);

    /// Returns the "next" universe index in order -- this new index
    /// is considered to extend all previous universes. This
    /// corresponds to entering a `forall` quantifier. So, for
    /// example, suppose we have this type in universe `U`:
    ///
    /// ```
    /// for<'a> fn(&'a u32)
    /// ```
    ///
    /// Once we "enter" into this `for<'a>` quantifier, we are in a
    /// new universe that extends `U` -- in this new universe, we can
    /// name the region `'a`, but that region was not nameable from
    /// `U` because it was not in scope there.
    pub fn next_universe(self) -> UniverseIndex {
        UniverseIndex::from_u32(self.private.checked_add(1).unwrap())
    }

    /// Returns `true` if `self` can name a name from `other` -- in other words,
    /// if the set of names in `self` is a superset of those in
    /// `other` (`self >= other`).
    pub fn can_name(self, other: UniverseIndex) -> bool {
        self.private >= other.private
    }

    /// Returns `true` if `self` cannot name some names from `other` -- in other
    /// words, if the set of names in `self` is a strict subset of
    /// those in `other` (`self < other`).
    pub fn cannot_name(self, other: UniverseIndex) -> bool {
        self.private < other.private
    }
}

/// The "placeholder index" fully defines a placeholder region.
/// Placeholder regions are identified by both a **universe** as well
/// as a "bound-region" within that universe. The `bound_region` is
/// basically a name -- distinct bound regions within the same
/// universe are just two regions with an unknown relationship to one
/// another.
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, PartialOrd, Ord)]
pub struct Placeholder<T> {
    pub universe: UniverseIndex,
    pub name: T,
}

impl<'a, T> HashStable<StableHashingContext<'a>> for Placeholder<T>
where
    T: HashStable<StableHashingContext<'a>>,
{
    fn hash_stable<W: StableHasherResult>(
        &self,
        hcx: &mut StableHashingContext<'a>,
        hasher: &mut StableHasher<W>
    ) {
        self.universe.hash_stable(hcx, hasher);
        self.name.hash_stable(hcx, hasher);
    }
}

pub type PlaceholderRegion = Placeholder<BoundRegion>;

pub type PlaceholderType = Placeholder<BoundVar>;

pub type PlaceholderConst = Placeholder<BoundVar>;

/// When type checking, we use the `ParamEnv` to track
/// details about the set of where-clauses that are in scope at this
/// particular point.
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable)]
pub struct ParamEnv<'tcx> {
    /// `Obligation`s that the caller must satisfy. This is basically
    /// the set of bounds on the in-scope type parameters, translated
    /// into `Obligation`s, and elaborated and normalized.
    pub caller_bounds: &'tcx List<ty::Predicate<'tcx>>,

    /// Typically, this is `Reveal::UserFacing`, but during codegen we
    /// want `Reveal::All` -- note that this is always paired with an
    /// empty environment. To get that, use `ParamEnv::reveal()`.
    pub reveal: traits::Reveal,

    /// If this `ParamEnv` comes from a call to `tcx.param_env(def_id)`,
    /// register that `def_id` (useful for transitioning to the chalk trait
    /// solver).
    pub def_id: Option<DefId>,
}

impl<'tcx> ParamEnv<'tcx> {
    /// Construct a trait environment suitable for contexts where
    /// there are no where-clauses in scope. Hidden types (like `impl
    /// Trait`) are left hidden, so this is suitable for ordinary
    /// type-checking.
    #[inline]
    pub fn empty() -> Self {
        Self::new(List::empty(), Reveal::UserFacing, None)
    }

    /// Construct a trait environment with no where-clauses in scope
    /// where the values of all `impl Trait` and other hidden types
    /// are revealed. This is suitable for monomorphized, post-typeck
    /// environments like codegen or doing optimizations.
    ///
    /// N.B., if you want to have predicates in scope, use `ParamEnv::new`,
    /// or invoke `param_env.with_reveal_all()`.
    #[inline]
    pub fn reveal_all() -> Self {
        Self::new(List::empty(), Reveal::All, None)
    }

    /// Construct a trait environment with the given set of predicates.
    #[inline]
    pub fn new(
        caller_bounds: &'tcx List<ty::Predicate<'tcx>>,
        reveal: Reveal,
        def_id: Option<DefId>
    ) -> Self {
        ty::ParamEnv { caller_bounds, reveal, def_id }
    }

    /// Returns a new parameter environment with the same clauses, but
    /// which "reveals" the true results of projections in all cases
    /// (even for associated types that are specializable). This is
    /// the desired behavior during codegen and certain other special
    /// contexts; normally though we want to use `Reveal::UserFacing`,
    /// which is the default.
    pub fn with_reveal_all(self) -> Self {
        ty::ParamEnv { reveal: Reveal::All, ..self }
    }

    /// Returns this same environment but with no caller bounds.
    pub fn without_caller_bounds(self) -> Self {
        ty::ParamEnv { caller_bounds: List::empty(), ..self }
    }

    /// Creates a suitable environment in which to perform trait
    /// queries on the given value. When type-checking, this is simply
    /// the pair of the environment plus value. But when reveal is set to
    /// All, then if `value` does not reference any type parameters, we will
    /// pair it with the empty environment. This improves caching and is generally
    /// invisible.
    ///
    /// N.B., we preserve the environment when type-checking because it
    /// is possible for the user to have wacky where-clauses like
    /// `where Box<u32>: Copy`, which are clearly never
    /// satisfiable. We generally want to behave as if they were true,
    /// although the surrounding function is never reachable.
    pub fn and<T: TypeFoldable<'tcx>>(self, value: T) -> ParamEnvAnd<'tcx, T> {
        match self.reveal {
            Reveal::UserFacing => {
                ParamEnvAnd {
                    param_env: self,
                    value,
                }
            }

            Reveal::All => {
                if value.has_placeholders()
                    || value.needs_infer()
                    || value.has_param_types()
                {
                    ParamEnvAnd {
                        param_env: self,
                        value,
                    }
                } else {
                    ParamEnvAnd {
                        param_env: self.without_caller_bounds(),
                        value,
                    }
                }
            }
        }
    }
}

#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
pub struct ParamEnvAnd<'tcx, T> {
    pub param_env: ParamEnv<'tcx>,
    pub value: T,
}

impl<'tcx, T> ParamEnvAnd<'tcx, T> {
    pub fn into_parts(self) -> (ParamEnv<'tcx>, T) {
        (self.param_env, self.value)
    }
}

impl<'a, 'tcx, T> HashStable<StableHashingContext<'a>> for ParamEnvAnd<'tcx, T>
where
    T: HashStable<StableHashingContext<'a>>,
{
    fn hash_stable<W: StableHasherResult>(&self,
                                          hcx: &mut StableHashingContext<'a>,
                                          hasher: &mut StableHasher<W>) {
        let ParamEnvAnd {
            ref param_env,
            ref value
        } = *self;

        param_env.hash_stable(hcx, hasher);
        value.hash_stable(hcx, hasher);
    }
}

#[derive(Copy, Clone, Debug, HashStable)]
pub struct Destructor {
    /// The `DefId` of the destructor method
    pub did: DefId,
}

bitflags! {
    #[derive(HashStable)]
    pub struct AdtFlags: u32 {
        const NO_ADT_FLAGS        = 0;
        /// Indicates whether the ADT is an enum.
        const IS_ENUM             = 1 << 0;
        /// Indicates whether the ADT is a union.
        const IS_UNION            = 1 << 1;
        /// Indicates whether the ADT is a struct.
        const IS_STRUCT           = 1 << 2;
        /// Indicates whether the ADT is a struct and has a constructor.
        const HAS_CTOR            = 1 << 3;
        /// Indicates whether the type is a `PhantomData`.
        const IS_PHANTOM_DATA     = 1 << 4;
        /// Indicates whether the type has a `#[fundamental]` attribute.
        const IS_FUNDAMENTAL      = 1 << 5;
        /// Indicates whether the type is a `Box`.
        const IS_BOX              = 1 << 6;
        /// Indicates whether the type is an `Arc`.
        const IS_ARC              = 1 << 7;
        /// Indicates whether the type is an `Rc`.
        const IS_RC               = 1 << 8;
        /// Indicates whether the variant list of this ADT is `#[non_exhaustive]`.
        /// (i.e., this flag is never set unless this ADT is an enum).
        const IS_VARIANT_LIST_NON_EXHAUSTIVE = 1 << 9;
    }
}

bitflags! {
    #[derive(HashStable)]
    pub struct VariantFlags: u32 {
        const NO_VARIANT_FLAGS        = 0;
        /// Indicates whether the field list of this variant is `#[non_exhaustive]`.
        const IS_FIELD_LIST_NON_EXHAUSTIVE = 1 << 0;
    }
}

/// Definition of a variant -- a struct's fields or a enum variant.
#[derive(Debug)]
pub struct VariantDef {
    /// `DefId` that identifies the variant itself.
    /// If this variant belongs to a struct or union, then this is a copy of its `DefId`.
    pub def_id: DefId,
    /// `DefId` that identifies the variant's constructor.
    /// If this variant is a struct variant, then this is `None`.
    pub ctor_def_id: Option<DefId>,
    /// Variant or struct name.
    pub ident: Ident,
    /// Discriminant of this variant.
    pub discr: VariantDiscr,
    /// Fields of this variant.
    pub fields: Vec<FieldDef>,
    /// Type of constructor of variant.
    pub ctor_kind: CtorKind,
    /// Flags of the variant (e.g. is field list non-exhaustive)?
    flags: VariantFlags,
    /// Variant is obtained as part of recovering from a syntactic error.
    /// May be incomplete or bogus.
    pub recovered: bool,
}

impl<'tcx> VariantDef {
    /// Creates a new `VariantDef`.
    ///
    /// `variant_did` is the `DefId` that identifies the enum variant (if this `VariantDef`
    /// represents an enum variant).
    ///
    /// `ctor_did` is the `DefId` that identifies the constructor of unit or
    /// tuple-variants/structs. If this is a `struct`-variant then this should be `None`.
    ///
    /// `parent_did` is the `DefId` of the `AdtDef` representing the enum or struct that
    /// owns this variant. It is used for checking if a struct has `#[non_exhaustive]` w/out having
    /// to go through the redirect of checking the ctor's attributes - but compiling a small crate
    /// requires loading the `AdtDef`s for all the structs in the universe (e.g., coherence for any
    /// built-in trait), and we do not want to load attributes twice.
    ///
    /// If someone speeds up attribute loading to not be a performance concern, they can
    /// remove this hack and use the constructor `DefId` everywhere.
    pub fn new(
        tcx: TyCtxt<'tcx>,
        ident: Ident,
        variant_did: Option<DefId>,
        ctor_def_id: Option<DefId>,
        discr: VariantDiscr,
        fields: Vec<FieldDef>,
        ctor_kind: CtorKind,
        adt_kind: AdtKind,
        parent_did: DefId,
        recovered: bool,
    ) -> Self {
        debug!(
            "VariantDef::new(ident = {:?}, variant_did = {:?}, ctor_def_id = {:?}, discr = {:?},
             fields = {:?}, ctor_kind = {:?}, adt_kind = {:?}, parent_did = {:?})",
             ident, variant_did, ctor_def_id, discr, fields, ctor_kind, adt_kind, parent_did,
        );

        let mut flags = VariantFlags::NO_VARIANT_FLAGS;
        if adt_kind == AdtKind::Struct && tcx.has_attr(parent_did, sym::non_exhaustive) {
            debug!("found non-exhaustive field list for {:?}", parent_did);
            flags = flags | VariantFlags::IS_FIELD_LIST_NON_EXHAUSTIVE;
        } else if let Some(variant_did) = variant_did {
            if tcx.has_attr(variant_did, sym::non_exhaustive) {
                debug!("found non-exhaustive field list for {:?}", variant_did);
                flags = flags | VariantFlags::IS_FIELD_LIST_NON_EXHAUSTIVE;
            }
        }

        VariantDef {
            def_id: variant_did.unwrap_or(parent_did),
            ctor_def_id,
            ident,
            discr,
            fields,
            ctor_kind,
            flags,
            recovered,
        }
    }

    /// Is this field list non-exhaustive?
    #[inline]
    pub fn is_field_list_non_exhaustive(&self) -> bool {
        self.flags.intersects(VariantFlags::IS_FIELD_LIST_NON_EXHAUSTIVE)
    }
}

impl_stable_hash_for!(struct VariantDef {
    def_id,
    ctor_def_id,
    ident -> (ident.name),
    discr,
    fields,
    ctor_kind,
    flags,
    recovered
});

#[derive(Copy, Clone, Debug, PartialEq, Eq, RustcEncodable, RustcDecodable, HashStable)]
pub enum VariantDiscr {
    /// Explicit value for this variant, i.e., `X = 123`.
    /// The `DefId` corresponds to the embedded constant.
    Explicit(DefId),

    /// The previous variant's discriminant plus one.
    /// For efficiency reasons, the distance from the
    /// last `Explicit` discriminant is being stored,
    /// or `0` for the first variant, if it has none.
    Relative(u32),
}

#[derive(Debug, HashStable)]
pub struct FieldDef {
    pub did: DefId,
    #[stable_hasher(project(name))]
    pub ident: Ident,
    pub vis: Visibility,
}

/// The definition of an abstract data type -- a struct or enum.
///
/// These are all interned (by `intern_adt_def`) into the `adt_defs` table.
pub struct AdtDef {
    /// `DefId` of the struct, enum or union item.
    pub did: DefId,
    /// Variants of the ADT. If this is a struct or union, then there will be a single variant.
    pub variants: IndexVec<self::layout::VariantIdx, VariantDef>,
    /// Flags of the ADT (e.g. is this a struct? is this non-exhaustive?)
    flags: AdtFlags,
    /// Repr options provided by the user.
    pub repr: ReprOptions,
}

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

/// There should be only one AdtDef for each `did`, therefore
/// it is fine to implement `Ord` only based on `did`.
impl Ord for AdtDef {
    fn cmp(&self, other: &AdtDef) -> Ordering {
        self.did.cmp(&other.did)
    }
}

impl PartialEq for AdtDef {
    // AdtDef are always interned and this is part of TyS equality
    #[inline]
    fn eq(&self, other: &Self) -> bool { ptr::eq(self, other) }
}

impl Eq for AdtDef {}

impl Hash for AdtDef {
    #[inline]
    fn hash<H: Hasher>(&self, s: &mut H) {
        (self as *const AdtDef).hash(s)
    }
}

impl<'tcx> rustc_serialize::UseSpecializedEncodable for &'tcx AdtDef {
    fn default_encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
        self.did.encode(s)
    }
}

impl<'tcx> rustc_serialize::UseSpecializedDecodable for &'tcx AdtDef {}


impl<'a> HashStable<StableHashingContext<'a>> for AdtDef {
    fn hash_stable<W: StableHasherResult>(&self,
                                          hcx: &mut StableHashingContext<'a>,
                                          hasher: &mut StableHasher<W>) {
        thread_local! {
            static CACHE: RefCell<FxHashMap<usize, Fingerprint>> = Default::default();
        }

        let hash: Fingerprint = CACHE.with(|cache| {
            let addr = self as *const AdtDef as usize;
            *cache.borrow_mut().entry(addr).or_insert_with(|| {
                let ty::AdtDef {
                    did,
                    ref variants,
                    ref flags,
                    ref repr,
                } = *self;

                let mut hasher = StableHasher::new();
                did.hash_stable(hcx, &mut hasher);
                variants.hash_stable(hcx, &mut hasher);
                flags.hash_stable(hcx, &mut hasher);
                repr.hash_stable(hcx, &mut hasher);

                hasher.finish()
           })
        });

        hash.hash_stable(hcx, hasher);
    }
}

#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
pub enum AdtKind { Struct, Union, Enum }

impl Into<DataTypeKind> for AdtKind {
    fn into(self) -> DataTypeKind {
        match self {
            AdtKind::Struct => DataTypeKind::Struct,
            AdtKind::Union => DataTypeKind::Union,
            AdtKind::Enum => DataTypeKind::Enum,
        }
    }
}

bitflags! {
    #[derive(RustcEncodable, RustcDecodable, Default)]
    pub struct ReprFlags: u8 {
        const IS_C               = 1 << 0;
        const IS_SIMD            = 1 << 1;
        const IS_TRANSPARENT     = 1 << 2;
        // Internal only for now. If true, don't reorder fields.
        const IS_LINEAR          = 1 << 3;

        // Any of these flags being set prevent field reordering optimisation.
        const IS_UNOPTIMISABLE   = ReprFlags::IS_C.bits |
                                   ReprFlags::IS_SIMD.bits |
                                   ReprFlags::IS_LINEAR.bits;
    }
}

impl_stable_hash_for!(struct ReprFlags {
    bits
});

/// Represents the repr options provided by the user,
#[derive(Copy, Clone, Debug, Eq, PartialEq, RustcEncodable, RustcDecodable, Default)]
pub struct ReprOptions {
    pub int: Option<attr::IntType>,
    pub align: Option<Align>,
    pub pack: Option<Align>,
    pub flags: ReprFlags,
}

impl_stable_hash_for!(struct ReprOptions {
    align,
    pack,
    int,
    flags
});

impl ReprOptions {
    pub fn new(tcx: TyCtxt<'_>, did: DefId) -> ReprOptions {
        let mut flags = ReprFlags::empty();
        let mut size = None;
        let mut max_align: Option<Align> = None;
        let mut min_pack: Option<Align> = None;
        for attr in tcx.get_attrs(did).iter() {
            for r in attr::find_repr_attrs(&tcx.sess.parse_sess, attr) {
                flags.insert(match r {
                    attr::ReprC => ReprFlags::IS_C,
                    attr::ReprPacked(pack) => {
                        let pack = Align::from_bytes(pack as u64).unwrap();
                        min_pack = Some(if let Some(min_pack) = min_pack {
                            min_pack.min(pack)
                        } else {
                            pack
                        });
                        ReprFlags::empty()
                    },
                    attr::ReprTransparent => ReprFlags::IS_TRANSPARENT,
                    attr::ReprSimd => ReprFlags::IS_SIMD,
                    attr::ReprInt(i) => {
                        size = Some(i);
                        ReprFlags::empty()
                    },
                    attr::ReprAlign(align) => {
                        max_align = max_align.max(Some(Align::from_bytes(align as u64).unwrap()));
                        ReprFlags::empty()
                    },
                });
            }
        }

        // This is here instead of layout because the choice must make it into metadata.
        if !tcx.consider_optimizing(|| format!("Reorder fields of {:?}", tcx.def_path_str(did))) {
            flags.insert(ReprFlags::IS_LINEAR);
        }
        ReprOptions { int: size, align: max_align, pack: min_pack, flags: flags }
    }

    #[inline]
    pub fn simd(&self) -> bool { self.flags.contains(ReprFlags::IS_SIMD) }
    #[inline]
    pub fn c(&self) -> bool { self.flags.contains(ReprFlags::IS_C) }
    #[inline]
    pub fn packed(&self) -> bool { self.pack.is_some() }
    #[inline]
    pub fn transparent(&self) -> bool { self.flags.contains(ReprFlags::IS_TRANSPARENT) }
    #[inline]
    pub fn linear(&self) -> bool { self.flags.contains(ReprFlags::IS_LINEAR) }

    pub fn discr_type(&self) -> attr::IntType {
        self.int.unwrap_or(attr::SignedInt(ast::IntTy::Isize))
    }

    /// Returns `true` if this `#[repr()]` should inhabit "smart enum
    /// layout" optimizations, such as representing `Foo<&T>` as a
    /// single pointer.
    pub fn inhibit_enum_layout_opt(&self) -> bool {
        self.c() || self.int.is_some()
    }

    /// Returns `true` if this `#[repr()]` should inhibit struct field reordering
    /// optimizations, such as with `repr(C)`, `repr(packed(1))`, or `repr(<int>)`.
    pub fn inhibit_struct_field_reordering_opt(&self) -> bool {
        if let Some(pack) = self.pack {
            if pack.bytes() == 1 {
                return true;
            }
        }
        self.flags.intersects(ReprFlags::IS_UNOPTIMISABLE) || self.int.is_some()
    }

    /// Returns `true` if this `#[repr()]` should inhibit union ABI optimisations.
    pub fn inhibit_union_abi_opt(&self) -> bool {
        self.c()
    }
}

impl<'tcx> AdtDef {
    /// Creates a new `AdtDef`.
    fn new(
        tcx: TyCtxt<'_>,
        did: DefId,
        kind: AdtKind,
        variants: IndexVec<VariantIdx, VariantDef>,
        repr: ReprOptions,
    ) -> Self {
        debug!("AdtDef::new({:?}, {:?}, {:?}, {:?})", did, kind, variants, repr);
        let mut flags = AdtFlags::NO_ADT_FLAGS;

        if kind == AdtKind::Enum && tcx.has_attr(did, sym::non_exhaustive) {
            debug!("found non-exhaustive variant list for {:?}", did);
            flags = flags | AdtFlags::IS_VARIANT_LIST_NON_EXHAUSTIVE;
        }

        flags |= match kind {
            AdtKind::Enum => AdtFlags::IS_ENUM,
            AdtKind::Union => AdtFlags::IS_UNION,
            AdtKind::Struct => AdtFlags::IS_STRUCT,
        };

        if kind == AdtKind::Struct && variants[VariantIdx::new(0)].ctor_def_id.is_some() {
            flags |= AdtFlags::HAS_CTOR;
        }

        let attrs = tcx.get_attrs(did);
        if attr::contains_name(&attrs, sym::fundamental) {
            flags |= AdtFlags::IS_FUNDAMENTAL;
        }
        if Some(did) == tcx.lang_items().phantom_data() {
            flags |= AdtFlags::IS_PHANTOM_DATA;
        }
        if Some(did) == tcx.lang_items().owned_box() {
            flags |= AdtFlags::IS_BOX;
        }
        if Some(did) == tcx.lang_items().arc() {
            flags |= AdtFlags::IS_ARC;
        }
        if Some(did) == tcx.lang_items().rc() {
            flags |= AdtFlags::IS_RC;
        }

        AdtDef {
            did,
            variants,
            flags,
            repr,
        }
    }

    /// Returns `true` if this is a struct.
    #[inline]
    pub fn is_struct(&self) -> bool {
        self.flags.contains(AdtFlags::IS_STRUCT)
    }

    /// Returns `true` if this is a union.
    #[inline]
    pub fn is_union(&self) -> bool {
        self.flags.contains(AdtFlags::IS_UNION)
    }

    /// Returns `true` if this is a enum.
    #[inline]
    pub fn is_enum(&self) -> bool {
        self.flags.contains(AdtFlags::IS_ENUM)
    }

    /// Returns `true` if the variant list of this ADT is `#[non_exhaustive]`.
    #[inline]
    pub fn is_variant_list_non_exhaustive(&self) -> bool {
        self.flags.contains(AdtFlags::IS_VARIANT_LIST_NON_EXHAUSTIVE)
    }

    /// Returns the kind of the ADT.
    #[inline]
    pub fn adt_kind(&self) -> AdtKind {
        if self.is_enum() {
            AdtKind::Enum
        } else if self.is_union() {
            AdtKind::Union
        } else {
            AdtKind::Struct
        }
    }

    /// Returns a description of this abstract data type.
    pub fn descr(&self) -> &'static str {
        match self.adt_kind() {
            AdtKind::Struct => "struct",
            AdtKind::Union => "union",
            AdtKind::Enum => "enum",
        }
    }

    /// Returns a description of a variant of this abstract data type.
    #[inline]
    pub fn variant_descr(&self) -> &'static str {
        match self.adt_kind() {
            AdtKind::Struct => "struct",
            AdtKind::Union => "union",
            AdtKind::Enum => "variant",
        }
    }

    /// If this function returns `true`, it implies that `is_struct` must return `true`.
    #[inline]
    pub fn has_ctor(&self) -> bool {
        self.flags.contains(AdtFlags::HAS_CTOR)
    }

    /// Returns `true` if this type is `#[fundamental]` for the purposes
    /// of coherence checking.
    #[inline]
    pub fn is_fundamental(&self) -> bool {
        self.flags.contains(AdtFlags::IS_FUNDAMENTAL)
    }

    /// Returns `true` if this is `PhantomData<T>`.
    #[inline]
    pub fn is_phantom_data(&self) -> bool {
        self.flags.contains(AdtFlags::IS_PHANTOM_DATA)
    }

    /// Returns `true` if this is `Arc<T>`.
    pub fn is_arc(&self) -> bool {
        self.flags.contains(AdtFlags::IS_ARC)
    }

    /// Returns `true` if this is `Rc<T>`.
    pub fn is_rc(&self) -> bool {
        self.flags.contains(AdtFlags::IS_RC)
    }

    /// Returns `true` if this is Box<T>.
    #[inline]
    pub fn is_box(&self) -> bool {
        self.flags.contains(AdtFlags::IS_BOX)
    }

    /// Returns `true` if this type has a destructor.
    pub fn has_dtor(&self, tcx: TyCtxt<'tcx>) -> bool {
        self.destructor(tcx).is_some()
    }

    /// Asserts this is a struct or union and returns its unique variant.
    pub fn non_enum_variant(&self) -> &VariantDef {
        assert!(self.is_struct() || self.is_union());
        &self.variants[VariantIdx::new(0)]
    }

    #[inline]
    pub fn predicates(&self, tcx: TyCtxt<'tcx>) -> &'tcx GenericPredicates<'tcx> {
        tcx.predicates_of(self.did)
    }

    /// Returns an iterator over all fields contained
    /// by this ADT.
    #[inline]
    pub fn all_fields(&self) -> impl Iterator<Item=&FieldDef> + Clone {
        self.variants.iter().flat_map(|v| v.fields.iter())
    }

    pub fn is_payloadfree(&self) -> bool {
        !self.variants.is_empty() &&
            self.variants.iter().all(|v| v.fields.is_empty())
    }

    /// Return a `VariantDef` given a variant id.
    pub fn variant_with_id(&self, vid: DefId) -> &VariantDef {
        self.variants.iter().find(|v| v.def_id == vid)
            .expect("variant_with_id: unknown variant")
    }

    /// Return a `VariantDef` given a constructor id.
    pub fn variant_with_ctor_id(&self, cid: DefId) -> &VariantDef {
        self.variants.iter().find(|v| v.ctor_def_id == Some(cid))
            .expect("variant_with_ctor_id: unknown variant")
    }

    /// Return the index of `VariantDef` given a variant id.
    pub fn variant_index_with_id(&self, vid: DefId) -> VariantIdx {
        self.variants.iter_enumerated().find(|(_, v)| v.def_id == vid)
            .expect("variant_index_with_id: unknown variant").0
    }

    /// Return the index of `VariantDef` given a constructor id.
    pub fn variant_index_with_ctor_id(&self, cid: DefId) -> VariantIdx {
        self.variants.iter_enumerated().find(|(_, v)| v.ctor_def_id == Some(cid))
            .expect("variant_index_with_ctor_id: unknown variant").0
    }

    pub fn variant_of_res(&self, res: Res) -> &VariantDef {
        match res {
            Res::Def(DefKind::Variant, vid) => self.variant_with_id(vid),
            Res::Def(DefKind::Ctor(..), cid) => self.variant_with_ctor_id(cid),
            Res::Def(DefKind::Struct, _) | Res::Def(DefKind::Union, _) |
            Res::Def(DefKind::TyAlias, _) | Res::Def(DefKind::AssocTy, _) | Res::SelfTy(..) |
            Res::SelfCtor(..) => self.non_enum_variant(),
            _ => bug!("unexpected res {:?} in variant_of_res", res)
        }
    }

    #[inline]
    pub fn eval_explicit_discr(&self, tcx: TyCtxt<'tcx>, expr_did: DefId) -> Option<Discr<'tcx>> {
        let param_env = tcx.param_env(expr_did);
        let repr_type = self.repr.discr_type();
        let substs = InternalSubsts::identity_for_item(tcx.global_tcx(), expr_did);
        let instance = ty::Instance::new(expr_did, substs);
        let cid = GlobalId {
            instance,
            promoted: None
        };
        match tcx.const_eval(param_env.and(cid)) {
            Ok(val) => {
                // FIXME: Find the right type and use it instead of `val.ty` here
                if let Some(b) = val.try_eval_bits(tcx.global_tcx(), param_env, val.ty) {
                    trace!("discriminants: {} ({:?})", b, repr_type);
                    Some(Discr {
                        val: b,
                        ty: val.ty,
                    })
                } else {
                    info!("invalid enum discriminant: {:#?}", val);
                    crate::mir::interpret::struct_error(
                        tcx.at(tcx.def_span(expr_did)),
                        "constant evaluation of enum discriminant resulted in non-integer",
                    ).emit();
                    None
                }
            }
            Err(ErrorHandled::Reported) => {
                if !expr_did.is_local() {
                    span_bug!(tcx.def_span(expr_did),
                        "variant discriminant evaluation succeeded \
                         in its crate but failed locally");
                }
                None
            }
            Err(ErrorHandled::TooGeneric) => span_bug!(
                tcx.def_span(expr_did),
                "enum discriminant depends on generic arguments",
            ),
        }
    }

    #[inline]
    pub fn discriminants(
        &'tcx self,
        tcx: TyCtxt<'tcx>,
    ) -> impl Iterator<Item = (VariantIdx, Discr<'tcx>)> + Captures<'tcx> {
        let repr_type = self.repr.discr_type();
        let initial = repr_type.initial_discriminant(tcx.global_tcx());
        let mut prev_discr = None::<Discr<'tcx>>;
        self.variants.iter_enumerated().map(move |(i, v)| {
            let mut discr = prev_discr.map_or(initial, |d| d.wrap_incr(tcx));
            if let VariantDiscr::Explicit(expr_did) = v.discr {
                if let Some(new_discr) = self.eval_explicit_discr(tcx, expr_did) {
                    discr = new_discr;
                }
            }
            prev_discr = Some(discr);

            (i, discr)
        })
    }

    #[inline]
    pub fn variant_range(&self) -> Range<VariantIdx> {
        (VariantIdx::new(0)..VariantIdx::new(self.variants.len()))
    }

    /// Computes the discriminant value used by a specific variant.
    /// Unlike `discriminants`, this is (amortized) constant-time,
    /// only doing at most one query for evaluating an explicit
    /// discriminant (the last one before the requested variant),
    /// assuming there are no constant-evaluation errors there.
    #[inline]
    pub fn discriminant_for_variant(
        &self,
        tcx: TyCtxt<'tcx>,
        variant_index: VariantIdx,
    ) -> Discr<'tcx> {
        let (val, offset) = self.discriminant_def_for_variant(variant_index);
        let explicit_value = val
            .and_then(|expr_did| self.eval_explicit_discr(tcx, expr_did))
            .unwrap_or_else(|| self.repr.discr_type().initial_discriminant(tcx.global_tcx()));
        explicit_value.checked_add(tcx, offset as u128).0
    }

    /// Yields a `DefId` for the discriminant and an offset to add to it
    /// Alternatively, if there is no explicit discriminant, returns the
    /// inferred discriminant directly.
    pub fn discriminant_def_for_variant(
        &self,
        variant_index: VariantIdx,
    ) -> (Option<DefId>, u32) {
        let mut explicit_index = variant_index.as_u32();
        let expr_did;
        loop {
            match self.variants[VariantIdx::from_u32(explicit_index)].discr {
                ty::VariantDiscr::Relative(0) => {
                    expr_did = None;
                    break;
                },
                ty::VariantDiscr::Relative(distance) => {
                    explicit_index -= distance;
                }
                ty::VariantDiscr::Explicit(did) => {
                    expr_did = Some(did);
                    break;
                }
            }
        }
        (expr_did, variant_index.as_u32() - explicit_index)
    }

    pub fn destructor(&self, tcx: TyCtxt<'tcx>) -> Option<Destructor> {
        tcx.adt_destructor(self.did)
    }

    /// Returns a list of types such that `Self: Sized` if and only
    /// if that type is `Sized`, or `TyErr` if this type is recursive.
    ///
    /// Oddly enough, checking that the sized-constraint is `Sized` is
    /// actually more expressive than checking all members:
    /// the `Sized` trait is inductive, so an associated type that references
    /// `Self` would prevent its containing ADT from being `Sized`.
    ///
    /// Due to normalization being eager, this applies even if
    /// the associated type is behind a pointer (e.g., issue #31299).
    pub fn sized_constraint(&self, tcx: TyCtxt<'tcx>) -> &'tcx [Ty<'tcx>] {
        tcx.adt_sized_constraint(self.did).0
    }

    fn sized_constraint_for_ty(&self, tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Vec<Ty<'tcx>> {
        let result = match ty.sty {
            Bool | Char | Int(..) | Uint(..) | Float(..) |
            RawPtr(..) | Ref(..) | FnDef(..) | FnPtr(_) |
            Array(..) | Closure(..) | Generator(..) | Never => {
                vec![]
            }

            Str |
            Dynamic(..) |
            Slice(_) |
            Foreign(..) |
            Error |
            GeneratorWitness(..) => {
                // these are never sized - return the target type
                vec![ty]
            }

            Tuple(ref tys) => {
                match tys.last() {
                    None => vec![],
                    Some(ty) => self.sized_constraint_for_ty(tcx, ty.expect_ty()),
                }
            }

            Adt(adt, substs) => {
                // recursive case
                let adt_tys = adt.sized_constraint(tcx);
                debug!("sized_constraint_for_ty({:?}) intermediate = {:?}",
                       ty, adt_tys);
                adt_tys.iter()
                       .map(|ty| ty.subst(tcx, substs))
                       .flat_map(|ty| self.sized_constraint_for_ty(tcx, ty))
                       .collect()
            }

            Projection(..) | Opaque(..) => {
                // must calculate explicitly.
                // FIXME: consider special-casing always-Sized projections
                vec![ty]
            }

            UnnormalizedProjection(..) => bug!("only used with chalk-engine"),

            Param(..) => {
                // perf hack: if there is a `T: Sized` bound, then
                // we know that `T` is Sized and do not need to check
                // it on the impl.

                let sized_trait = match tcx.lang_items().sized_trait() {
                    Some(x) => x,
                    _ => return vec![ty]
                };
                let sized_predicate = Binder::dummy(TraitRef {
                    def_id: sized_trait,
                    substs: tcx.mk_substs_trait(ty, &[])
                }).to_predicate();
                let predicates = &tcx.predicates_of(self.did).predicates;
                if predicates.iter().any(|(p, _)| *p == sized_predicate) {
                    vec![]
                } else {
                    vec![ty]
                }
            }

            Placeholder(..) |
            Bound(..) |
            Infer(..) => {
                bug!("unexpected type `{:?}` in sized_constraint_for_ty",
                     ty)
            }
        };
        debug!("sized_constraint_for_ty({:?}) = {:?}", ty, result);
        result
    }
}

impl<'tcx> FieldDef {
    /// Returns the type of this field. The `subst` is typically obtained
    /// via the second field of `TyKind::AdtDef`.
    pub fn ty(&self, tcx: TyCtxt<'tcx>, subst: SubstsRef<'tcx>) -> Ty<'tcx> {
        tcx.type_of(self.did).subst(tcx, subst)
    }
}

/// Represents the various closure traits in the language. This
/// will determine the type of the environment (`self`, in the
/// desugaring) argument that the closure expects.
///
/// You can get the environment type of a closure using
/// `tcx.closure_env_ty()`.
#[derive(Clone, Copy, PartialOrd, Ord, PartialEq, Eq, Hash, Debug,
         RustcEncodable, RustcDecodable, HashStable)]
pub enum ClosureKind {
    // Warning: Ordering is significant here! The ordering is chosen
    // because the trait Fn is a subtrait of FnMut and so in turn, and
    // hence we order it so that Fn < FnMut < FnOnce.
    Fn,
    FnMut,
    FnOnce,
}

impl<'tcx> ClosureKind {
    // This is the initial value used when doing upvar inference.
    pub const LATTICE_BOTTOM: ClosureKind = ClosureKind::Fn;

    pub fn trait_did(&self, tcx: TyCtxt<'tcx>) -> DefId {
        match *self {
            ClosureKind::Fn => tcx.require_lang_item(FnTraitLangItem, None),
            ClosureKind::FnMut => {
                tcx.require_lang_item(FnMutTraitLangItem, None)
            }
            ClosureKind::FnOnce => {
                tcx.require_lang_item(FnOnceTraitLangItem, None)
            }
        }
    }

    /// Returns `true` if this a type that impls this closure kind
    /// must also implement `other`.
    pub fn extends(self, other: ty::ClosureKind) -> bool {
        match (self, other) {
            (ClosureKind::Fn, ClosureKind::Fn) => true,
            (ClosureKind::Fn, ClosureKind::FnMut) => true,
            (ClosureKind::Fn, ClosureKind::FnOnce) => true,
            (ClosureKind::FnMut, ClosureKind::FnMut) => true,
            (ClosureKind::FnMut, ClosureKind::FnOnce) => true,
            (ClosureKind::FnOnce, ClosureKind::FnOnce) => true,
            _ => false,
        }
    }

    /// Returns the representative scalar type for this closure kind.
    /// See `TyS::to_opt_closure_kind` for more details.
    pub fn to_ty(self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
        match self {
            ty::ClosureKind::Fn => tcx.types.i8,
            ty::ClosureKind::FnMut => tcx.types.i16,
            ty::ClosureKind::FnOnce => tcx.types.i32,
        }
    }
}

impl<'tcx> TyS<'tcx> {
    /// Iterator that walks `self` and any types reachable from
    /// `self`, in depth-first order. Note that just walks the types
    /// that appear in `self`, it does not descend into the fields of
    /// structs or variants. For example:
    ///
    /// ```notrust
    /// isize => { isize }
    /// Foo<Bar<isize>> => { Foo<Bar<isize>>, Bar<isize>, isize }
    /// [isize] => { [isize], isize }
    /// ```
    pub fn walk(&'tcx self) -> TypeWalker<'tcx> {
        TypeWalker::new(self)
    }

    /// Iterator that walks the immediate children of `self`. Hence
    /// `Foo<Bar<i32>, u32>` yields the sequence `[Bar<i32>, u32]`
    /// (but not `i32`, like `walk`).
    pub fn walk_shallow(&'tcx self) -> smallvec::IntoIter<walk::TypeWalkerArray<'tcx>> {
        walk::walk_shallow(self)
    }

    /// Walks `ty` and any types appearing within `ty`, invoking the
    /// callback `f` on each type. If the callback returns `false`, then the
    /// children of the current type are ignored.
    ///
    /// Note: prefer `ty.walk()` where possible.
    pub fn maybe_walk<F>(&'tcx self, mut f: F)
        where F: FnMut(Ty<'tcx>) -> bool
    {
        let mut walker = self.walk();
        while let Some(ty) = walker.next() {
            if !f(ty) {
                walker.skip_current_subtree();
            }
        }
    }
}

impl BorrowKind {
    pub fn from_mutbl(m: hir::Mutability) -> BorrowKind {
        match m {
            hir::MutMutable => MutBorrow,
            hir::MutImmutable => ImmBorrow,
        }
    }

    /// Returns a mutability `m` such that an `&m T` pointer could be used to obtain this borrow
    /// kind. Because borrow kinds are richer than mutabilities, we sometimes have to pick a
    /// mutability that is stronger than necessary so that it at least *would permit* the borrow in
    /// question.
    pub fn to_mutbl_lossy(self) -> hir::Mutability {
        match self {
            MutBorrow => hir::MutMutable,
            ImmBorrow => hir::MutImmutable,

            // We have no type corresponding to a unique imm borrow, so
            // use `&mut`. It gives all the capabilities of an `&uniq`
            // and hence is a safe "over approximation".
            UniqueImmBorrow => hir::MutMutable,
        }
    }

    pub fn to_user_str(&self) -> &'static str {
        match *self {
            MutBorrow => "mutable",
            ImmBorrow => "immutable",
            UniqueImmBorrow => "uniquely immutable",
        }
    }
}

#[derive(Debug, Clone)]
pub enum Attributes<'tcx> {
    Owned(Lrc<[ast::Attribute]>),
    Borrowed(&'tcx [ast::Attribute]),
}

impl<'tcx> ::std::ops::Deref for Attributes<'tcx> {
    type Target = [ast::Attribute];

    fn deref(&self) -> &[ast::Attribute] {
        match self {
            &Attributes::Owned(ref data) => &data,
            &Attributes::Borrowed(data) => data
        }
    }
}

#[derive(Debug, PartialEq, Eq)]
pub enum ImplOverlapKind {
    /// These impls are always allowed to overlap.
    Permitted,
    /// These impls are allowed to overlap, but that raises
    /// an issue #33140 future-compatibility warning.
    ///
    /// Some background: in Rust 1.0, the trait-object types `Send + Sync` (today's
    /// `dyn Send + Sync`) and `Sync + Send` (now `dyn Sync + Send`) were different.
    ///
    /// The widely-used version 0.1.0 of the crate `traitobject` had accidentally relied
    /// that difference, making what reduces to the following set of impls:
    ///
    /// ```
    /// trait Trait {}
    /// impl Trait for dyn Send + Sync {}
    /// impl Trait for dyn Sync + Send {}
    /// ```
    ///
    /// Obviously, once we made these types be identical, that code causes a coherence
    /// error and a fairly big headache for us. However, luckily for us, the trait
    /// `Trait` used in this case is basically a marker trait, and therefore having
    /// overlapping impls for it is sound.
    ///
    /// To handle this, we basically regard the trait as a marker trait, with an additional
    /// future-compatibility warning. To avoid accidentally "stabilizing" this feature,
    /// it has the following restrictions:
    ///
    /// 1. The trait must indeed be a marker-like trait (i.e., no items), and must be
    /// positive impls.
    /// 2. The trait-ref of both impls must be equal.
    /// 3. The trait-ref of both impls must be a trait object type consisting only of
    /// marker traits.
    /// 4. Neither of the impls can have any where-clauses.
    ///
    /// Once `traitobject` 0.1.0 is no longer an active concern, this hack can be removed.
    Issue33140
}

impl<'tcx> TyCtxt<'tcx> {
    pub fn body_tables(self, body: hir::BodyId) -> &'tcx TypeckTables<'tcx> {
        self.typeck_tables_of(self.hir().body_owner_def_id(body))
    }

    /// Returns an iterator of the `DefId`s for all body-owners in this
    /// crate. If you would prefer to iterate over the bodies
    /// themselves, you can do `self.hir().krate().body_ids.iter()`.
    pub fn body_owners(self) -> impl Iterator<Item = DefId> + Captures<'tcx> + 'tcx {
        self.hir().krate()
                  .body_ids
                  .iter()
                  .map(move |&body_id| self.hir().body_owner_def_id(body_id))
    }

    pub fn par_body_owners<F: Fn(DefId) + sync::Sync + sync::Send>(self, f: F) {
        par_iter(&self.hir().krate().body_ids).for_each(|&body_id| {
            f(self.hir().body_owner_def_id(body_id))
        });
    }

    pub fn provided_trait_methods(self, id: DefId) -> Vec<AssocItem> {
        self.associated_items(id)
            .filter(|item| item.kind == AssocKind::Method && item.defaultness.has_value())
            .collect()
    }

    pub fn trait_relevant_for_never(self, did: DefId) -> bool {
        self.associated_items(did).any(|item| {
            item.relevant_for_never()
        })
    }

    pub fn opt_associated_item(self, def_id: DefId) -> Option<AssocItem> {
        let is_associated_item = if let Some(hir_id) = self.hir().as_local_hir_id(def_id) {
            match self.hir().get(hir_id) {
                Node::TraitItem(_) | Node::ImplItem(_) => true,
                _ => false,
            }
        } else {
            match self.def_kind(def_id).expect("no def for `DefId`") {
                DefKind::AssocConst
                | DefKind::Method
                | DefKind::AssocTy => true,
                _ => false,
            }
        };

        if is_associated_item {
            Some(self.associated_item(def_id))
        } else {
            None
        }
    }

    fn associated_item_from_trait_item_ref(self,
                                           parent_def_id: DefId,
                                           parent_vis: &hir::Visibility,
                                           trait_item_ref: &hir::TraitItemRef)
                                           -> AssocItem {
        let def_id = self.hir().local_def_id(trait_item_ref.id.hir_id);
        let (kind, has_self) = match trait_item_ref.kind {
            hir::AssocItemKind::Const => (ty::AssocKind::Const, false),
            hir::AssocItemKind::Method { has_self } => {
                (ty::AssocKind::Method, has_self)
            }
            hir::AssocItemKind::Type => (ty::AssocKind::Type, false),
            hir::AssocItemKind::OpaqueTy => bug!("only impls can have opaque types"),
        };

        AssocItem {
            ident: trait_item_ref.ident,
            kind,
            // Visibility of trait items is inherited from their traits.
            vis: Visibility::from_hir(parent_vis, trait_item_ref.id.hir_id, self),
            defaultness: trait_item_ref.defaultness,
            def_id,
            container: TraitContainer(parent_def_id),
            method_has_self_argument: has_self
        }
    }

    fn associated_item_from_impl_item_ref(self,
                                          parent_def_id: DefId,
                                          impl_item_ref: &hir::ImplItemRef)
                                          -> AssocItem {
        let def_id = self.hir().local_def_id(impl_item_ref.id.hir_id);
        let (kind, has_self) = match impl_item_ref.kind {
            hir::AssocItemKind::Const => (ty::AssocKind::Const, false),
            hir::AssocItemKind::Method { has_self } => {
                (ty::AssocKind::Method, has_self)
            }
            hir::AssocItemKind::Type => (ty::AssocKind::Type, false),
            hir::AssocItemKind::OpaqueTy => (ty::AssocKind::OpaqueTy, false),
        };

        AssocItem {
            ident: impl_item_ref.ident,
            kind,
            // Visibility of trait impl items doesn't matter.
            vis: ty::Visibility::from_hir(&impl_item_ref.vis, impl_item_ref.id.hir_id, self),
            defaultness: impl_item_ref.defaultness,
            def_id,
            container: ImplContainer(parent_def_id),
            method_has_self_argument: has_self
        }
    }

    pub fn field_index(self, hir_id: hir::HirId, tables: &TypeckTables<'_>) -> usize {
        tables.field_indices().get(hir_id).cloned().expect("no index for a field")
    }

    pub fn find_field_index(self, ident: Ident, variant: &VariantDef) -> Option<usize> {
        variant.fields.iter().position(|field| {
            self.hygienic_eq(ident, field.ident, variant.def_id)
        })
    }

    pub fn associated_items(self, def_id: DefId) -> AssocItemsIterator<'tcx> {
        // Ideally, we would use `-> impl Iterator` here, but it falls
        // afoul of the conservative "capture [restrictions]" we put
        // in place, so we use a hand-written iterator.
        //
        // [restrictions]: https://github.com/rust-lang/rust/issues/34511#issuecomment-373423999
        AssocItemsIterator {
            tcx: self,
            def_ids: self.associated_item_def_ids(def_id),
            next_index: 0,
        }
    }

    /// Returns `true` if the impls are the same polarity and the trait either
    /// has no items or is annotated #[marker] and prevents item overrides.
    pub fn impls_are_allowed_to_overlap(self, def_id1: DefId, def_id2: DefId)
                                        -> Option<ImplOverlapKind>
    {
        let is_legit = if self.features().overlapping_marker_traits {
            let trait1_is_empty = self.impl_trait_ref(def_id1)
                .map_or(false, |trait_ref| {
                    self.associated_item_def_ids(trait_ref.def_id).is_empty()
                });
            let trait2_is_empty = self.impl_trait_ref(def_id2)
                .map_or(false, |trait_ref| {
                    self.associated_item_def_ids(trait_ref.def_id).is_empty()
                });
            self.impl_polarity(def_id1) == self.impl_polarity(def_id2)
                && trait1_is_empty
                && trait2_is_empty
        } else {
            let is_marker_impl = |def_id: DefId| -> bool {
                let trait_ref = self.impl_trait_ref(def_id);
                trait_ref.map_or(false, |tr| self.trait_def(tr.def_id).is_marker)
            };
            self.impl_polarity(def_id1) == self.impl_polarity(def_id2)
                && is_marker_impl(def_id1)
                && is_marker_impl(def_id2)
        };

        if is_legit {
            debug!("impls_are_allowed_to_overlap({:?}, {:?}) = Some(Permitted)",
                  def_id1, def_id2);
            Some(ImplOverlapKind::Permitted)
        } else {
            if let Some(self_ty1) = self.issue33140_self_ty(def_id1) {
                if let Some(self_ty2) = self.issue33140_self_ty(def_id2) {
                    if self_ty1 == self_ty2 {
                        debug!("impls_are_allowed_to_overlap({:?}, {:?}) - issue #33140 HACK",
                               def_id1, def_id2);
                        return Some(ImplOverlapKind::Issue33140);
                    } else {
                        debug!("impls_are_allowed_to_overlap({:?}, {:?}) - found {:?} != {:?}",
                              def_id1, def_id2, self_ty1, self_ty2);
                    }
                }
            }

            debug!("impls_are_allowed_to_overlap({:?}, {:?}) = None",
                  def_id1, def_id2);
            None
        }
    }

    /// Returns `ty::VariantDef` if `res` refers to a struct,
    /// or variant or their constructors, panics otherwise.
    pub fn expect_variant_res(self, res: Res) -> &'tcx VariantDef {
        match res {
            Res::Def(DefKind::Variant, did) => {
                let enum_did = self.parent(did).unwrap();
                self.adt_def(enum_did).variant_with_id(did)
            }
            Res::Def(DefKind::Struct, did) | Res::Def(DefKind::Union, did) => {
                self.adt_def(did).non_enum_variant()
            }
            Res::Def(DefKind::Ctor(CtorOf::Variant, ..), variant_ctor_did) => {
                let variant_did = self.parent(variant_ctor_did).unwrap();
                let enum_did = self.parent(variant_did).unwrap();
                self.adt_def(enum_did).variant_with_ctor_id(variant_ctor_did)
            }
            Res::Def(DefKind::Ctor(CtorOf::Struct, ..), ctor_did) => {
                let struct_did = self.parent(ctor_did).expect("struct ctor has no parent");
                self.adt_def(struct_did).non_enum_variant()
            }
            _ => bug!("expect_variant_res used with unexpected res {:?}", res)
        }
    }

    pub fn item_name(self, id: DefId) -> Symbol {
        if id.index == CRATE_DEF_INDEX {
            self.original_crate_name(id.krate)
        } else {
            let def_key = self.def_key(id);
            match def_key.disambiguated_data.data {
                // The name of a constructor is that of its parent.
                hir_map::DefPathData::Ctor =>
                    self.item_name(DefId {
                        krate: id.krate,
                        index: def_key.parent.unwrap()
                    }),
                _ => def_key.disambiguated_data.data.get_opt_name().unwrap_or_else(|| {
                    bug!("item_name: no name for {:?}", self.def_path(id));
                }).as_symbol(),
            }
        }
    }

    /// Returns the possibly-auto-generated MIR of a `(DefId, Subst)` pair.
    pub fn instance_mir(self, instance: ty::InstanceDef<'tcx>) -> &'tcx Body<'tcx> {
        match instance {
            ty::InstanceDef::Item(did) => {
                self.optimized_mir(did)
            }
            ty::InstanceDef::VtableShim(..) |
            ty::InstanceDef::Intrinsic(..) |
            ty::InstanceDef::FnPtrShim(..) |
            ty::InstanceDef::Virtual(..) |
            ty::InstanceDef::ClosureOnceShim { .. } |
            ty::InstanceDef::DropGlue(..) |
            ty::InstanceDef::CloneShim(..) => {
                self.mir_shims(instance)
            }
        }
    }

    /// Gets the attributes of a definition.
    pub fn get_attrs(self, did: DefId) -> Attributes<'tcx> {
        if let Some(id) = self.hir().as_local_hir_id(did) {
            Attributes::Borrowed(self.hir().attrs(id))
        } else {
            Attributes::Owned(self.item_attrs(did))
        }
    }

    /// Determines whether an item is annotated with an attribute.
    pub fn has_attr(self, did: DefId, attr: Symbol) -> bool {
        attr::contains_name(&self.get_attrs(did), attr)
    }

    /// Returns `true` if this is an `auto trait`.
    pub fn trait_is_auto(self, trait_def_id: DefId) -> bool {
        self.trait_def(trait_def_id).has_auto_impl
    }

    pub fn generator_layout(self, def_id: DefId) -> &'tcx GeneratorLayout<'tcx> {
        self.optimized_mir(def_id).generator_layout.as_ref().unwrap()
    }

    /// Given the `DefId` of an impl, returns the `DefId` of the trait it implements.
    /// If it implements no trait, returns `None`.
    pub fn trait_id_of_impl(self, def_id: DefId) -> Option<DefId> {
        self.impl_trait_ref(def_id).map(|tr| tr.def_id)
    }

    /// If the given defid describes a method belonging to an impl, returns the
    /// `DefId` of the impl that the method belongs to; otherwise, returns `None`.
    pub fn impl_of_method(self, def_id: DefId) -> Option<DefId> {
        let item = if def_id.krate != LOCAL_CRATE {
            if let Some(DefKind::Method) = self.def_kind(def_id) {
                Some(self.associated_item(def_id))
            } else {
                None
            }
        } else {
            self.opt_associated_item(def_id)
        };

        item.and_then(|trait_item|
            match trait_item.container {
                TraitContainer(_) => None,
                ImplContainer(def_id) => Some(def_id),
            }
        )
    }

    /// Looks up the span of `impl_did` if the impl is local; otherwise returns `Err`
    /// with the name of the crate containing the impl.
    pub fn span_of_impl(self, impl_did: DefId) -> Result<Span, Symbol> {
        if impl_did.is_local() {
            let hir_id = self.hir().as_local_hir_id(impl_did).unwrap();
            Ok(self.hir().span(hir_id))
        } else {
            Err(self.crate_name(impl_did.krate))
        }
    }

    /// Hygienically compares a use-site name (`use_name`) for a field or an associated item with
    /// its supposed definition name (`def_name`). The method also needs `DefId` of the supposed
    /// definition's parent/scope to perform comparison.
    pub fn hygienic_eq(self, use_name: Ident, def_name: Ident, def_parent_def_id: DefId) -> bool {
        // We could use `Ident::eq` here, but we deliberately don't. The name
        // comparison fails frequently, and we want to avoid the expensive
        // `modern()` calls required for the span comparison whenever possible.
        use_name.name == def_name.name &&
        use_name.span.ctxt().hygienic_eq(def_name.span.ctxt(),
                                         self.expansion_that_defined(def_parent_def_id))
    }

    fn expansion_that_defined(self, scope: DefId) -> ExpnId {
        match scope.krate {
            LOCAL_CRATE => self.hir().definitions().expansion_that_defined(scope.index),
            _ => ExpnId::root(),
        }
    }

    pub fn adjust_ident(self, mut ident: Ident, scope: DefId) -> Ident {
        ident.span.modernize_and_adjust(self.expansion_that_defined(scope));
        ident
    }

    pub fn adjust_ident_and_get_scope(self, mut ident: Ident, scope: DefId, block: hir::HirId)
                                      -> (Ident, DefId) {
        let scope = match ident.span.modernize_and_adjust(self.expansion_that_defined(scope)) {
            Some(actual_expansion) =>
                self.hir().definitions().parent_module_of_macro_def(actual_expansion),
            None => self.hir().get_module_parent(block),
        };
        (ident, scope)
    }
}

pub struct AssocItemsIterator<'tcx> {
    tcx: TyCtxt<'tcx>,
    def_ids: &'tcx [DefId],
    next_index: usize,
}

impl Iterator for AssocItemsIterator<'_> {
    type Item = AssocItem;

    fn next(&mut self) -> Option<AssocItem> {
        let def_id = self.def_ids.get(self.next_index)?;
        self.next_index += 1;
        Some(self.tcx.associated_item(*def_id))
    }
}

fn associated_item(tcx: TyCtxt<'_>, def_id: DefId) -> AssocItem {
    let id = tcx.hir().as_local_hir_id(def_id).unwrap();
    let parent_id = tcx.hir().get_parent_item(id);
    let parent_def_id = tcx.hir().local_def_id(parent_id);
    let parent_item = tcx.hir().expect_item(parent_id);
    match parent_item.node {
        hir::ItemKind::Impl(.., ref impl_item_refs) => {
            if let Some(impl_item_ref) = impl_item_refs.iter().find(|i| i.id.hir_id == id) {
                let assoc_item = tcx.associated_item_from_impl_item_ref(parent_def_id,
                                                                        impl_item_ref);
                debug_assert_eq!(assoc_item.def_id, def_id);
                return assoc_item;
            }
        }

        hir::ItemKind::Trait(.., ref trait_item_refs) => {
            if let Some(trait_item_ref) = trait_item_refs.iter().find(|i| i.id.hir_id == id) {
                let assoc_item = tcx.associated_item_from_trait_item_ref(parent_def_id,
                                                                         &parent_item.vis,
                                                                         trait_item_ref);
                debug_assert_eq!(assoc_item.def_id, def_id);
                return assoc_item;
            }
        }

        _ => { }
    }

    span_bug!(parent_item.span,
              "unexpected parent of trait or impl item or item not found: {:?}",
              parent_item.node)
}

#[derive(Clone, HashStable)]
pub struct AdtSizedConstraint<'tcx>(pub &'tcx [Ty<'tcx>]);

/// Calculates the `Sized` constraint.
///
/// In fact, there are only a few options for the types in the constraint:
///     - an obviously-unsized type
///     - a type parameter or projection whose Sizedness can't be known
///     - a tuple of type parameters or projections, if there are multiple
///       such.
///     - a Error, if a type contained itself. The representability
///       check should catch this case.
fn adt_sized_constraint(tcx: TyCtxt<'_>, def_id: DefId) -> AdtSizedConstraint<'_> {
    let def = tcx.adt_def(def_id);

    let result = tcx.mk_type_list(def.variants.iter().flat_map(|v| {
        v.fields.last()
    }).flat_map(|f| {
        def.sized_constraint_for_ty(tcx, tcx.type_of(f.did))
    }));

    debug!("adt_sized_constraint: {:?} => {:?}", def, result);

    AdtSizedConstraint(result)
}

fn associated_item_def_ids(tcx: TyCtxt<'_>, def_id: DefId) -> &[DefId] {
    let id = tcx.hir().as_local_hir_id(def_id).unwrap();
    let item = tcx.hir().expect_item(id);
    match item.node {
        hir::ItemKind::Trait(.., ref trait_item_refs) => {
            tcx.arena.alloc_from_iter(
                trait_item_refs.iter()
                               .map(|trait_item_ref| trait_item_ref.id)
                               .map(|id| tcx.hir().local_def_id(id.hir_id))
            )
        }
        hir::ItemKind::Impl(.., ref impl_item_refs) => {
            tcx.arena.alloc_from_iter(
                impl_item_refs.iter()
                              .map(|impl_item_ref| impl_item_ref.id)
                              .map(|id| tcx.hir().local_def_id(id.hir_id))
            )
        }
        hir::ItemKind::TraitAlias(..) => &[],
        _ => span_bug!(item.span, "associated_item_def_ids: not impl or trait")
    }
}

fn def_span(tcx: TyCtxt<'_>, def_id: DefId) -> Span {
    tcx.hir().span_if_local(def_id).unwrap()
}

/// If the given `DefId` describes an item belonging to a trait,
/// returns the `DefId` of the trait that the trait item belongs to;
/// otherwise, returns `None`.
fn trait_of_item(tcx: TyCtxt<'_>, def_id: DefId) -> Option<DefId> {
    tcx.opt_associated_item(def_id)
        .and_then(|associated_item| {
            match associated_item.container {
                TraitContainer(def_id) => Some(def_id),
                ImplContainer(_) => None
            }
        })
}

/// Yields the parent function's `DefId` if `def_id` is an `impl Trait` definition.
pub fn is_impl_trait_defn(tcx: TyCtxt<'_>, def_id: DefId) -> Option<DefId> {
    if let Some(hir_id) = tcx.hir().as_local_hir_id(def_id) {
        if let Node::Item(item) = tcx.hir().get(hir_id) {
            if let hir::ItemKind::OpaqueTy(ref opaque_ty) = item.node {
                return opaque_ty.impl_trait_fn;
            }
        }
    }
    None
}

/// See `ParamEnv` struct definition for details.
fn param_env(tcx: TyCtxt<'_>, def_id: DefId) -> ParamEnv<'_> {
    // The param_env of an impl Trait type is its defining function's param_env
    if let Some(parent) = is_impl_trait_defn(tcx, def_id) {
        return param_env(tcx, parent);
    }
    // Compute the bounds on Self and the type parameters.

    let InstantiatedPredicates { predicates } =
        tcx.predicates_of(def_id).instantiate_identity(tcx);

    // Finally, we have to normalize the bounds in the environment, in
    // case they contain any associated type projections. This process
    // can yield errors if the put in illegal associated types, like
    // `<i32 as Foo>::Bar` where `i32` does not implement `Foo`. We
    // report these errors right here; this doesn't actually feel
    // right to me, because constructing the environment feels like a
    // kind of a "idempotent" action, but I'm not sure where would be
    // a better place. In practice, we construct environments for
    // every fn once during type checking, and we'll abort if there
    // are any errors at that point, so after type checking you can be
    // sure that this will succeed without errors anyway.

    let unnormalized_env = ty::ParamEnv::new(
        tcx.intern_predicates(&predicates),
        traits::Reveal::UserFacing,
        if tcx.sess.opts.debugging_opts.chalk { Some(def_id) } else { None }
    );

    let body_id = tcx.hir().as_local_hir_id(def_id).map_or(hir::DUMMY_HIR_ID, |id| {
        tcx.hir().maybe_body_owned_by(id).map_or(id, |body| body.hir_id)
    });
    let cause = traits::ObligationCause::misc(tcx.def_span(def_id), body_id);
    traits::normalize_param_env_or_error(tcx, def_id, unnormalized_env, cause)
}

fn crate_disambiguator(tcx: TyCtxt<'_>, crate_num: CrateNum) -> CrateDisambiguator {
    assert_eq!(crate_num, LOCAL_CRATE);
    tcx.sess.local_crate_disambiguator()
}

fn original_crate_name(tcx: TyCtxt<'_>, crate_num: CrateNum) -> Symbol {
    assert_eq!(crate_num, LOCAL_CRATE);
    tcx.crate_name.clone()
}

fn crate_hash(tcx: TyCtxt<'_>, crate_num: CrateNum) -> Svh {
    assert_eq!(crate_num, LOCAL_CRATE);
    tcx.hir().crate_hash
}

fn instance_def_size_estimate<'tcx>(tcx: TyCtxt<'tcx>, instance_def: InstanceDef<'tcx>) -> usize {
    match instance_def {
        InstanceDef::Item(..) |
        InstanceDef::DropGlue(..) => {
            let mir = tcx.instance_mir(instance_def);
            mir.basic_blocks().iter().map(|bb| bb.statements.len()).sum()
        },
        // Estimate the size of other compiler-generated shims to be 1.
        _ => 1
    }
}

/// If `def_id` is an issue 33140 hack impl, returns its self type; otherwise, returns `None`.
///
/// See [`ImplOverlapKind::Issue33140`] for more details.
fn issue33140_self_ty(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Ty<'_>> {
    debug!("issue33140_self_ty({:?})", def_id);

    let trait_ref = tcx.impl_trait_ref(def_id).unwrap_or_else(|| {
        bug!("issue33140_self_ty called on inherent impl {:?}", def_id)
    });

    debug!("issue33140_self_ty({:?}), trait-ref={:?}", def_id, trait_ref);

    let is_marker_like =
        tcx.impl_polarity(def_id) == hir::ImplPolarity::Positive &&
        tcx.associated_item_def_ids(trait_ref.def_id).is_empty();

    // Check whether these impls would be ok for a marker trait.
    if !is_marker_like {
        debug!("issue33140_self_ty - not marker-like!");
        return None;
    }

    // impl must be `impl Trait for dyn Marker1 + Marker2 + ...`
    if trait_ref.substs.len() != 1 {
        debug!("issue33140_self_ty - impl has substs!");
        return None;
    }

    let predicates = tcx.predicates_of(def_id);
    if predicates.parent.is_some() || !predicates.predicates.is_empty() {
        debug!("issue33140_self_ty - impl has predicates {:?}!", predicates);
        return None;
    }

    let self_ty = trait_ref.self_ty();
    let self_ty_matches = match self_ty.sty {
        ty::Dynamic(ref data, ty::ReStatic) => data.principal().is_none(),
        _ => false
    };

    if self_ty_matches {
        debug!("issue33140_self_ty - MATCHES!");
        Some(self_ty)
    } else {
        debug!("issue33140_self_ty - non-matching self type");
        None
    }
}

pub fn provide(providers: &mut ty::query::Providers<'_>) {
    context::provide(providers);
    erase_regions::provide(providers);
    layout::provide(providers);
    util::provide(providers);
    constness::provide(providers);
    *providers = ty::query::Providers {
        associated_item,
        associated_item_def_ids,
        adt_sized_constraint,
        def_span,
        param_env,
        trait_of_item,
        crate_disambiguator,
        original_crate_name,
        crate_hash,
        trait_impls_of: trait_def::trait_impls_of_provider,
        instance_def_size_estimate,
        issue33140_self_ty,
        ..*providers
    };
}

/// A map for the local crate mapping each type to a vector of its
/// inherent impls. This is not meant to be used outside of coherence;
/// rather, you should request the vector for a specific type via
/// `tcx.inherent_impls(def_id)` so as to minimize your dependencies
/// (constructing this map requires touching the entire crate).
#[derive(Clone, Debug, Default, HashStable)]
pub struct CrateInherentImpls {
    pub inherent_impls: DefIdMap<Vec<DefId>>,
}

#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, RustcEncodable, RustcDecodable)]
pub struct SymbolName {
    // FIXME: we don't rely on interning or equality here - better have
    // this be a `&'tcx str`.
    pub name: InternedString
}

impl_stable_hash_for!(struct self::SymbolName {
    name
});

impl SymbolName {
    pub fn new(name: &str) -> SymbolName {
        SymbolName {
            name: InternedString::intern(name)
        }
    }

    pub fn as_str(&self) -> LocalInternedString {
        self.name.as_str()
    }
}

impl fmt::Display for SymbolName {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&self.name, fmt)
    }
}

impl fmt::Debug for SymbolName {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&self.name, fmt)
    }
}