[rust.git] / src / librustc_middle / ty / structural_impls.rs

//! This module contains implements of the `Lift` and `TypeFoldable`
//! traits for various types in the Rust compiler. Most are written by
//! hand, though we've recently added some macros and proc-macros to help with the tedium.

use crate::mir::interpret;
use crate::mir::ProjectionKind;
use crate::ty::fold::{TypeFoldable, TypeFolder, TypeVisitor};
use crate::ty::print::{FmtPrinter, Printer};
use crate::ty::{self, InferConst, Lift, Ty, TyCtxt};
use rustc_hir as hir;
use rustc_hir::def::Namespace;
use rustc_hir::def_id::CRATE_DEF_INDEX;
use rustc_index::vec::{Idx, IndexVec};

use smallvec::SmallVec;
use std::fmt;
use std::rc::Rc;
use std::sync::Arc;

impl fmt::Debug for ty::TraitDef {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        ty::tls::with(|tcx| {
            FmtPrinter::new(tcx, f, Namespace::TypeNS).print_def_path(self.def_id, &[])?;
            Ok(())
        })
    }
}

impl fmt::Debug for ty::AdtDef {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        ty::tls::with(|tcx| {
            FmtPrinter::new(tcx, f, Namespace::TypeNS).print_def_path(self.did, &[])?;
            Ok(())
        })
    }
}

impl fmt::Debug for ty::UpvarId {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let name = ty::tls::with(|tcx| tcx.hir().name(self.var_path.hir_id));
        write!(f, "UpvarId({:?};`{}`;{:?})", self.var_path.hir_id, name, self.closure_expr_id)
    }
}

impl fmt::Debug for ty::UpvarBorrow<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "UpvarBorrow({:?}, {:?})", self.kind, self.region)
    }
}

impl fmt::Debug for ty::ExistentialTraitRef<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(self, f)
    }
}

impl fmt::Debug for ty::adjustment::Adjustment<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{:?} -> {}", self.kind, self.target)
    }
}

impl fmt::Debug for ty::BoundRegion {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            ty::BrAnon(n) => write!(f, "BrAnon({:?})", n),
            ty::BrNamed(did, name) => {
                if did.index == CRATE_DEF_INDEX {
                    write!(f, "BrNamed({})", name)
                } else {
                    write!(f, "BrNamed({:?}, {})", did, name)
                }
            }
            ty::BrEnv => write!(f, "BrEnv"),
        }
    }
}

impl fmt::Debug for ty::RegionKind {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            ty::ReEarlyBound(ref data) => write!(f, "ReEarlyBound({}, {})", data.index, data.name),

            ty::ReLateBound(binder_id, ref bound_region) => {
                write!(f, "ReLateBound({:?}, {:?})", binder_id, bound_region)
            }

            ty::ReFree(ref fr) => fr.fmt(f),

            ty::ReStatic => write!(f, "ReStatic"),

            ty::ReVar(ref vid) => vid.fmt(f),

            ty::RePlaceholder(placeholder) => write!(f, "RePlaceholder({:?})", placeholder),

            ty::ReEmpty(ui) => write!(f, "ReEmpty({:?})", ui),

            ty::ReErased => write!(f, "ReErased"),
        }
    }
}

impl fmt::Debug for ty::FreeRegion {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "ReFree({:?}, {:?})", self.scope, self.bound_region)
    }
}

impl fmt::Debug for ty::Variance {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(match *self {
            ty::Covariant => "+",
            ty::Contravariant => "-",
            ty::Invariant => "o",
            ty::Bivariant => "*",
        })
    }
}

impl fmt::Debug for ty::FnSig<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "({:?}; c_variadic: {})->{:?}", self.inputs(), self.c_variadic, self.output())
    }
}

impl fmt::Debug for ty::TyVid {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "_#{}t", self.index)
    }
}

impl<'tcx> fmt::Debug for ty::ConstVid<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "_#{}c", self.index)
    }
}

impl fmt::Debug for ty::IntVid {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "_#{}i", self.index)
    }
}

impl fmt::Debug for ty::FloatVid {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "_#{}f", self.index)
    }
}

impl fmt::Debug for ty::RegionVid {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "'_#{}r", self.index())
    }
}

impl fmt::Debug for ty::InferTy {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            ty::TyVar(ref v) => v.fmt(f),
            ty::IntVar(ref v) => v.fmt(f),
            ty::FloatVar(ref v) => v.fmt(f),
            ty::FreshTy(v) => write!(f, "FreshTy({:?})", v),
            ty::FreshIntTy(v) => write!(f, "FreshIntTy({:?})", v),
            ty::FreshFloatTy(v) => write!(f, "FreshFloatTy({:?})", v),
        }
    }
}

impl fmt::Debug for ty::IntVarValue {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            ty::IntType(ref v) => v.fmt(f),
            ty::UintType(ref v) => v.fmt(f),
        }
    }
}

impl fmt::Debug for ty::FloatVarValue {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.0.fmt(f)
    }
}

impl fmt::Debug for ty::TraitRef<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(self, f)
    }
}

impl fmt::Debug for Ty<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(self, f)
    }
}

impl fmt::Debug for ty::ParamTy {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}/#{}", self.name, self.index)
    }
}

impl fmt::Debug for ty::ParamConst {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}/#{}", self.name, self.index)
    }
}

impl fmt::Debug for ty::TraitPredicate<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "TraitPredicate({:?})", self.trait_ref)
    }
}

impl fmt::Debug for ty::ProjectionPredicate<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "ProjectionPredicate({:?}, {:?})", self.projection_ty, self.ty)
    }
}

impl fmt::Debug for ty::Predicate<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{:?}", self.kind())
    }
}

impl fmt::Debug for ty::PredicateKind<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            ty::PredicateKind::Trait(ref a, constness) => {
                if let hir::Constness::Const = constness {
                    write!(f, "const ")?;
                }
                a.fmt(f)
            }
            ty::PredicateKind::Subtype(ref pair) => pair.fmt(f),
            ty::PredicateKind::RegionOutlives(ref pair) => pair.fmt(f),
            ty::PredicateKind::TypeOutlives(ref pair) => pair.fmt(f),
            ty::PredicateKind::Projection(ref pair) => pair.fmt(f),
            ty::PredicateKind::WellFormed(data) => write!(f, "WellFormed({:?})", data),
            ty::PredicateKind::ObjectSafe(trait_def_id) => {
                write!(f, "ObjectSafe({:?})", trait_def_id)
            }
            ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
                write!(f, "ClosureKind({:?}, {:?}, {:?})", closure_def_id, closure_substs, kind)
            }
            ty::PredicateKind::ConstEvaluatable(def_id, substs) => {
                write!(f, "ConstEvaluatable({:?}, {:?})", def_id, substs)
            }
            ty::PredicateKind::ConstEquate(c1, c2) => write!(f, "ConstEquate({:?}, {:?})", c1, c2),
            ty::PredicateKind::ForAll(binder) => write!(f, "ForAll({:?})", binder),
        }
    }
}

///////////////////////////////////////////////////////////////////////////
// Atomic structs
//
// For things that don't carry any arena-allocated data (and are
// copy...), just add them to this list.

CloneTypeFoldableAndLiftImpls! {
    (),
    bool,
    usize,
    ::rustc_target::abi::VariantIdx,
    u64,
    String,
    crate::middle::region::Scope,
    ::rustc_ast::ast::FloatTy,
    ::rustc_ast::ast::InlineAsmOptions,
    ::rustc_ast::ast::InlineAsmTemplatePiece,
    ::rustc_ast::ast::NodeId,
    ::rustc_span::symbol::Symbol,
    ::rustc_hir::def::Res,
    ::rustc_hir::def_id::DefId,
    ::rustc_hir::def_id::LocalDefId,
    ::rustc_hir::LlvmInlineAsmInner,
    ::rustc_hir::MatchSource,
    ::rustc_hir::Mutability,
    ::rustc_hir::Unsafety,
    ::rustc_target::asm::InlineAsmRegOrRegClass,
    ::rustc_target::spec::abi::Abi,
    crate::mir::Local,
    crate::mir::Promoted,
    crate::traits::Reveal,
    crate::ty::adjustment::AutoBorrowMutability,
    crate::ty::AdtKind,
    // Including `BoundRegion` is a *bit* dubious, but direct
    // references to bound region appear in `ty::Error`, and aren't
    // really meant to be folded. In general, we can only fold a fully
    // general `Region`.
    crate::ty::BoundRegion,
    crate::ty::Placeholder<crate::ty::BoundRegion>,
    crate::ty::ClosureKind,
    crate::ty::FreeRegion,
    crate::ty::InferTy,
    crate::ty::IntVarValue,
    crate::ty::ParamConst,
    crate::ty::ParamTy,
    crate::ty::adjustment::PointerCast,
    crate::ty::RegionVid,
    crate::ty::UniverseIndex,
    crate::ty::Variance,
    ::rustc_span::Span,
}

///////////////////////////////////////////////////////////////////////////
// Lift implementations

// FIXME(eddyb) replace all the uses of `Option::map` with `?`.
impl<'tcx, A: Lift<'tcx>, B: Lift<'tcx>> Lift<'tcx> for (A, B) {
    type Lifted = (A::Lifted, B::Lifted);
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&self.0).and_then(|a| tcx.lift(&self.1).map(|b| (a, b)))
    }
}

impl<'tcx, A: Lift<'tcx>, B: Lift<'tcx>, C: Lift<'tcx>> Lift<'tcx> for (A, B, C) {
    type Lifted = (A::Lifted, B::Lifted, C::Lifted);
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&self.0)
            .and_then(|a| tcx.lift(&self.1).and_then(|b| tcx.lift(&self.2).map(|c| (a, b, c))))
    }
}

impl<'tcx, T: Lift<'tcx>> Lift<'tcx> for Option<T> {
    type Lifted = Option<T::Lifted>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        match *self {
            Some(ref x) => tcx.lift(x).map(Some),
            None => Some(None),
        }
    }
}

impl<'tcx, T: Lift<'tcx>, E: Lift<'tcx>> Lift<'tcx> for Result<T, E> {
    type Lifted = Result<T::Lifted, E::Lifted>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        match *self {
            Ok(ref x) => tcx.lift(x).map(Ok),
            Err(ref e) => tcx.lift(e).map(Err),
        }
    }
}

impl<'tcx, T: Lift<'tcx>> Lift<'tcx> for Box<T> {
    type Lifted = Box<T::Lifted>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&**self).map(Box::new)
    }
}

impl<'tcx, T: Lift<'tcx>> Lift<'tcx> for Rc<T> {
    type Lifted = Rc<T::Lifted>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&**self).map(Rc::new)
    }
}

impl<'tcx, T: Lift<'tcx>> Lift<'tcx> for Arc<T> {
    type Lifted = Arc<T::Lifted>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&**self).map(Arc::new)
    }
}

impl<'tcx, T: Lift<'tcx>> Lift<'tcx> for [T] {
    type Lifted = Vec<T::Lifted>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        // type annotation needed to inform `projection_must_outlive`
        let mut result: Vec<<T as Lift<'tcx>>::Lifted> = Vec::with_capacity(self.len());
        for x in self {
            if let Some(value) = tcx.lift(x) {
                result.push(value);
            } else {
                return None;
            }
        }
        Some(result)
    }
}

impl<'tcx, T: Lift<'tcx>> Lift<'tcx> for Vec<T> {
    type Lifted = Vec<T::Lifted>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&self[..])
    }
}

impl<'tcx, I: Idx, T: Lift<'tcx>> Lift<'tcx> for IndexVec<I, T> {
    type Lifted = IndexVec<I, T::Lifted>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        self.iter().map(|e| tcx.lift(e)).collect()
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::TraitRef<'a> {
    type Lifted = ty::TraitRef<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&self.substs).map(|substs| ty::TraitRef { def_id: self.def_id, substs })
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::ExistentialTraitRef<'a> {
    type Lifted = ty::ExistentialTraitRef<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&self.substs).map(|substs| ty::ExistentialTraitRef { def_id: self.def_id, substs })
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::ExistentialPredicate<'a> {
    type Lifted = ty::ExistentialPredicate<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        match self {
            ty::ExistentialPredicate::Trait(x) => tcx.lift(x).map(ty::ExistentialPredicate::Trait),
            ty::ExistentialPredicate::Projection(x) => {
                tcx.lift(x).map(ty::ExistentialPredicate::Projection)
            }
            ty::ExistentialPredicate::AutoTrait(def_id) => {
                Some(ty::ExistentialPredicate::AutoTrait(*def_id))
            }
        }
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::TraitPredicate<'a> {
    type Lifted = ty::TraitPredicate<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<ty::TraitPredicate<'tcx>> {
        tcx.lift(&self.trait_ref).map(|trait_ref| ty::TraitPredicate { trait_ref })
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::SubtypePredicate<'a> {
    type Lifted = ty::SubtypePredicate<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<ty::SubtypePredicate<'tcx>> {
        tcx.lift(&(self.a, self.b)).map(|(a, b)| ty::SubtypePredicate {
            a_is_expected: self.a_is_expected,
            a,
            b,
        })
    }
}

impl<'tcx, A: Copy + Lift<'tcx>, B: Copy + Lift<'tcx>> Lift<'tcx> for ty::OutlivesPredicate<A, B> {
    type Lifted = ty::OutlivesPredicate<A::Lifted, B::Lifted>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&(self.0, self.1)).map(|(a, b)| ty::OutlivesPredicate(a, b))
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::ProjectionTy<'a> {
    type Lifted = ty::ProjectionTy<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<ty::ProjectionTy<'tcx>> {
        tcx.lift(&self.substs)
            .map(|substs| ty::ProjectionTy { item_def_id: self.item_def_id, substs })
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::ProjectionPredicate<'a> {
    type Lifted = ty::ProjectionPredicate<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<ty::ProjectionPredicate<'tcx>> {
        tcx.lift(&(self.projection_ty, self.ty))
            .map(|(projection_ty, ty)| ty::ProjectionPredicate { projection_ty, ty })
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::ExistentialProjection<'a> {
    type Lifted = ty::ExistentialProjection<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&self.substs).map(|substs| ty::ExistentialProjection {
            substs,
            ty: tcx.lift(&self.ty).expect("type must lift when substs do"),
            item_def_id: self.item_def_id,
        })
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::PredicateKind<'a> {
    type Lifted = ty::PredicateKind<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        match *self {
            ty::PredicateKind::Trait(ref data, constness) => {
                tcx.lift(data).map(|data| ty::PredicateKind::Trait(data, constness))
            }
            ty::PredicateKind::Subtype(ref data) => tcx.lift(data).map(ty::PredicateKind::Subtype),
            ty::PredicateKind::RegionOutlives(ref data) => {
                tcx.lift(data).map(ty::PredicateKind::RegionOutlives)
            }
            ty::PredicateKind::TypeOutlives(ref data) => {
                tcx.lift(data).map(ty::PredicateKind::TypeOutlives)
            }
            ty::PredicateKind::Projection(ref data) => {
                tcx.lift(data).map(ty::PredicateKind::Projection)
            }
            ty::PredicateKind::WellFormed(ty) => tcx.lift(&ty).map(ty::PredicateKind::WellFormed),
            ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
                tcx.lift(&closure_substs).map(|closure_substs| {
                    ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind)
                })
            }
            ty::PredicateKind::ObjectSafe(trait_def_id) => {
                Some(ty::PredicateKind::ObjectSafe(trait_def_id))
            }
            ty::PredicateKind::ConstEvaluatable(def_id, substs) => {
                tcx.lift(&substs).map(|substs| ty::PredicateKind::ConstEvaluatable(def_id, substs))
            }
            ty::PredicateKind::ConstEquate(c1, c2) => {
                tcx.lift(&(c1, c2)).map(|(c1, c2)| ty::PredicateKind::ConstEquate(c1, c2))
            }
            ty::PredicateKind::ForAll(ref binder) => {
                tcx.lift(binder).map(ty::PredicateKind::ForAll)
            }
        }
    }
}

impl<'tcx, T: Lift<'tcx>> Lift<'tcx> for ty::Binder<T> {
    type Lifted = ty::Binder<T::Lifted>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(self.as_ref().skip_binder()).map(ty::Binder::bind)
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::ParamEnv<'a> {
    type Lifted = ty::ParamEnv<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&self.caller_bounds())
            .map(|caller_bounds| ty::ParamEnv::new(caller_bounds, self.reveal(), self.def_id))
    }
}

impl<'a, 'tcx, T: Lift<'tcx>> Lift<'tcx> for ty::ParamEnvAnd<'a, T> {
    type Lifted = ty::ParamEnvAnd<'tcx, T::Lifted>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&self.param_env).and_then(|param_env| {
            tcx.lift(&self.value).map(|value| ty::ParamEnvAnd { param_env, value })
        })
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::ClosureSubsts<'a> {
    type Lifted = ty::ClosureSubsts<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&self.substs).map(|substs| ty::ClosureSubsts { substs })
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::GeneratorSubsts<'a> {
    type Lifted = ty::GeneratorSubsts<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&self.substs).map(|substs| ty::GeneratorSubsts { substs })
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::adjustment::Adjustment<'a> {
    type Lifted = ty::adjustment::Adjustment<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&self.kind).and_then(|kind| {
            tcx.lift(&self.target).map(|target| ty::adjustment::Adjustment { kind, target })
        })
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::adjustment::Adjust<'a> {
    type Lifted = ty::adjustment::Adjust<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        match *self {
            ty::adjustment::Adjust::NeverToAny => Some(ty::adjustment::Adjust::NeverToAny),
            ty::adjustment::Adjust::Pointer(ptr) => Some(ty::adjustment::Adjust::Pointer(ptr)),
            ty::adjustment::Adjust::Deref(ref overloaded) => {
                tcx.lift(overloaded).map(ty::adjustment::Adjust::Deref)
            }
            ty::adjustment::Adjust::Borrow(ref autoref) => {
                tcx.lift(autoref).map(ty::adjustment::Adjust::Borrow)
            }
        }
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::adjustment::OverloadedDeref<'a> {
    type Lifted = ty::adjustment::OverloadedDeref<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&self.region)
            .map(|region| ty::adjustment::OverloadedDeref { region, mutbl: self.mutbl })
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::adjustment::AutoBorrow<'a> {
    type Lifted = ty::adjustment::AutoBorrow<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        match *self {
            ty::adjustment::AutoBorrow::Ref(r, m) => {
                tcx.lift(&r).map(|r| ty::adjustment::AutoBorrow::Ref(r, m))
            }
            ty::adjustment::AutoBorrow::RawPtr(m) => Some(ty::adjustment::AutoBorrow::RawPtr(m)),
        }
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::GenSig<'a> {
    type Lifted = ty::GenSig<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&(self.resume_ty, self.yield_ty, self.return_ty))
            .map(|(resume_ty, yield_ty, return_ty)| ty::GenSig { resume_ty, yield_ty, return_ty })
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::FnSig<'a> {
    type Lifted = ty::FnSig<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&self.inputs_and_output).map(|x| ty::FnSig {
            inputs_and_output: x,
            c_variadic: self.c_variadic,
            unsafety: self.unsafety,
            abi: self.abi,
        })
    }
}

impl<'tcx, T: Lift<'tcx>> Lift<'tcx> for ty::error::ExpectedFound<T> {
    type Lifted = ty::error::ExpectedFound<T::Lifted>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        tcx.lift(&self.expected).and_then(|expected| {
            tcx.lift(&self.found).map(|found| ty::error::ExpectedFound { expected, found })
        })
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::error::TypeError<'a> {
    type Lifted = ty::error::TypeError<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        use crate::ty::error::TypeError::*;

        Some(match *self {
            Mismatch => Mismatch,
            UnsafetyMismatch(x) => UnsafetyMismatch(x),
            AbiMismatch(x) => AbiMismatch(x),
            Mutability => Mutability,
            TupleSize(x) => TupleSize(x),
            FixedArraySize(x) => FixedArraySize(x),
            ArgCount => ArgCount,
            RegionsDoesNotOutlive(a, b) => {
                return tcx.lift(&(a, b)).map(|(a, b)| RegionsDoesNotOutlive(a, b));
            }
            RegionsInsufficientlyPolymorphic(a, b) => {
                return tcx.lift(&b).map(|b| RegionsInsufficientlyPolymorphic(a, b));
            }
            RegionsOverlyPolymorphic(a, b) => {
                return tcx.lift(&b).map(|b| RegionsOverlyPolymorphic(a, b));
            }
            RegionsPlaceholderMismatch => RegionsPlaceholderMismatch,
            IntMismatch(x) => IntMismatch(x),
            FloatMismatch(x) => FloatMismatch(x),
            Traits(x) => Traits(x),
            VariadicMismatch(x) => VariadicMismatch(x),
            CyclicTy(t) => return tcx.lift(&t).map(|t| CyclicTy(t)),
            ProjectionMismatched(x) => ProjectionMismatched(x),
            Sorts(ref x) => return tcx.lift(x).map(Sorts),
            ExistentialMismatch(ref x) => return tcx.lift(x).map(ExistentialMismatch),
            ConstMismatch(ref x) => return tcx.lift(x).map(ConstMismatch),
            IntrinsicCast => IntrinsicCast,
            TargetFeatureCast(ref x) => TargetFeatureCast(*x),
            ObjectUnsafeCoercion(ref x) => return tcx.lift(x).map(ObjectUnsafeCoercion),
        })
    }
}

impl<'a, 'tcx> Lift<'tcx> for ty::InstanceDef<'a> {
    type Lifted = ty::InstanceDef<'tcx>;
    fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
        match *self {
            ty::InstanceDef::Item(def_id) => Some(ty::InstanceDef::Item(def_id)),
            ty::InstanceDef::VtableShim(def_id) => Some(ty::InstanceDef::VtableShim(def_id)),
            ty::InstanceDef::ReifyShim(def_id) => Some(ty::InstanceDef::ReifyShim(def_id)),
            ty::InstanceDef::Intrinsic(def_id) => Some(ty::InstanceDef::Intrinsic(def_id)),
            ty::InstanceDef::FnPtrShim(def_id, ref ty) => {
                Some(ty::InstanceDef::FnPtrShim(def_id, tcx.lift(ty)?))
            }
            ty::InstanceDef::Virtual(def_id, n) => Some(ty::InstanceDef::Virtual(def_id, n)),
            ty::InstanceDef::ClosureOnceShim { call_once } => {
                Some(ty::InstanceDef::ClosureOnceShim { call_once })
            }
            ty::InstanceDef::DropGlue(def_id, ref ty) => {
                Some(ty::InstanceDef::DropGlue(def_id, tcx.lift(ty)?))
            }
            ty::InstanceDef::CloneShim(def_id, ref ty) => {
                Some(ty::InstanceDef::CloneShim(def_id, tcx.lift(ty)?))
            }
        }
    }
}

///////////////////////////////////////////////////////////////////////////
// TypeFoldable implementations.
//
// Ideally, each type should invoke `folder.fold_foo(self)` and
// nothing else. In some cases, though, we haven't gotten around to
// adding methods on the `folder` yet, and thus the folding is
// hard-coded here. This is less-flexible, because folders cannot
// override the behavior, but there are a lot of random types and one
// can easily refactor the folding into the TypeFolder trait as
// needed.

/// AdtDefs are basically the same as a DefId.
impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::AdtDef {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, _folder: &mut F) -> Self {
        *self
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> bool {
        false
    }
}

impl<'tcx, T: TypeFoldable<'tcx>, U: TypeFoldable<'tcx>> TypeFoldable<'tcx> for (T, U) {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> (T, U) {
        (self.0.fold_with(folder), self.1.fold_with(folder))
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        self.0.visit_with(visitor) || self.1.visit_with(visitor)
    }
}

impl<'tcx, A: TypeFoldable<'tcx>, B: TypeFoldable<'tcx>, C: TypeFoldable<'tcx>> TypeFoldable<'tcx>
    for (A, B, C)
{
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> (A, B, C) {
        (self.0.fold_with(folder), self.1.fold_with(folder), self.2.fold_with(folder))
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        self.0.visit_with(visitor) || self.1.visit_with(visitor) || self.2.visit_with(visitor)
    }
}

EnumTypeFoldableImpl! {
    impl<'tcx, T> TypeFoldable<'tcx> for Option<T> {
        (Some)(a),
        (None),
    } where T: TypeFoldable<'tcx>
}

EnumTypeFoldableImpl! {
    impl<'tcx, T, E> TypeFoldable<'tcx> for Result<T, E> {
        (Ok)(a),
        (Err)(a),
    } where T: TypeFoldable<'tcx>, E: TypeFoldable<'tcx>,
}

impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Rc<T> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        Rc::new((**self).fold_with(folder))
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        (**self).visit_with(visitor)
    }
}

impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Arc<T> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        Arc::new((**self).fold_with(folder))
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        (**self).visit_with(visitor)
    }
}

impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Box<T> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        let content: T = (**self).fold_with(folder);
        box content
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        (**self).visit_with(visitor)
    }
}

impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Vec<T> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        self.iter().map(|t| t.fold_with(folder)).collect()
    }

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

impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Box<[T]> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        self.iter().map(|t| t.fold_with(folder)).collect::<Vec<_>>().into_boxed_slice()
    }

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

impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for ty::Binder<T> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        self.map_bound_ref(|ty| ty.fold_with(folder))
    }

    fn fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        folder.fold_binder(self)
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        self.as_ref().skip_binder().visit_with(visitor)
    }

    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        visitor.visit_binder(self)
    }
}

impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ty::ExistentialPredicate<'tcx>> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        fold_list(*self, folder, |tcx, v| tcx.intern_existential_predicates(v))
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        self.iter().any(|p| p.visit_with(visitor))
    }
}

impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<Ty<'tcx>> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        fold_list(*self, folder, |tcx, v| tcx.intern_type_list(v))
    }

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

impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ProjectionKind> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        fold_list(*self, folder, |tcx, v| tcx.intern_projs(v))
    }

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

impl<'tcx> TypeFoldable<'tcx> for ty::instance::Instance<'tcx> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        use crate::ty::InstanceDef::*;
        Self {
            substs: self.substs.fold_with(folder),
            def: match self.def {
                Item(def) => Item(def.fold_with(folder)),
                VtableShim(did) => VtableShim(did.fold_with(folder)),
                ReifyShim(did) => ReifyShim(did.fold_with(folder)),
                Intrinsic(did) => Intrinsic(did.fold_with(folder)),
                FnPtrShim(did, ty) => FnPtrShim(did.fold_with(folder), ty.fold_with(folder)),
                Virtual(did, i) => Virtual(did.fold_with(folder), i),
                ClosureOnceShim { call_once } => {
                    ClosureOnceShim { call_once: call_once.fold_with(folder) }
                }
                DropGlue(did, ty) => DropGlue(did.fold_with(folder), ty.fold_with(folder)),
                CloneShim(did, ty) => CloneShim(did.fold_with(folder), ty.fold_with(folder)),
            },
        }
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        use crate::ty::InstanceDef::*;
        self.substs.visit_with(visitor)
            || match self.def {
                Item(def) => def.visit_with(visitor),
                VtableShim(did) | ReifyShim(did) | Intrinsic(did) | Virtual(did, _) => {
                    did.visit_with(visitor)
                }
                FnPtrShim(did, ty) | CloneShim(did, ty) => {
                    did.visit_with(visitor) || ty.visit_with(visitor)
                }
                DropGlue(did, ty) => did.visit_with(visitor) || ty.visit_with(visitor),
                ClosureOnceShim { call_once } => call_once.visit_with(visitor),
            }
    }
}

impl<'tcx> TypeFoldable<'tcx> for interpret::GlobalId<'tcx> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        Self { instance: self.instance.fold_with(folder), promoted: self.promoted }
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        self.instance.visit_with(visitor)
    }
}

impl<'tcx> TypeFoldable<'tcx> for Ty<'tcx> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        let kind = match self.kind {
            ty::RawPtr(tm) => ty::RawPtr(tm.fold_with(folder)),
            ty::Array(typ, sz) => ty::Array(typ.fold_with(folder), sz.fold_with(folder)),
            ty::Slice(typ) => ty::Slice(typ.fold_with(folder)),
            ty::Adt(tid, substs) => ty::Adt(tid, substs.fold_with(folder)),
            ty::Dynamic(ref trait_ty, ref region) => {
                ty::Dynamic(trait_ty.fold_with(folder), region.fold_with(folder))
            }
            ty::Tuple(ts) => ty::Tuple(ts.fold_with(folder)),
            ty::FnDef(def_id, substs) => ty::FnDef(def_id, substs.fold_with(folder)),
            ty::FnPtr(f) => ty::FnPtr(f.fold_with(folder)),
            ty::Ref(ref r, ty, mutbl) => ty::Ref(r.fold_with(folder), ty.fold_with(folder), mutbl),
            ty::Generator(did, substs, movability) => {
                ty::Generator(did, substs.fold_with(folder), movability)
            }
            ty::GeneratorWitness(types) => ty::GeneratorWitness(types.fold_with(folder)),
            ty::Closure(did, substs) => ty::Closure(did, substs.fold_with(folder)),
            ty::Projection(ref data) => ty::Projection(data.fold_with(folder)),
            ty::Opaque(did, substs) => ty::Opaque(did, substs.fold_with(folder)),

            ty::Bool
            | ty::Char
            | ty::Str
            | ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Error(_)
            | ty::Infer(_)
            | ty::Param(..)
            | ty::Bound(..)
            | ty::Placeholder(..)
            | ty::Never
            | ty::Foreign(..) => return self,
        };

        if self.kind == kind { self } else { folder.tcx().mk_ty(kind) }
    }

    fn fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        folder.fold_ty(*self)
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        match self.kind {
            ty::RawPtr(ref tm) => tm.visit_with(visitor),
            ty::Array(typ, sz) => typ.visit_with(visitor) || sz.visit_with(visitor),
            ty::Slice(typ) => typ.visit_with(visitor),
            ty::Adt(_, substs) => substs.visit_with(visitor),
            ty::Dynamic(ref trait_ty, ref reg) => {
                trait_ty.visit_with(visitor) || reg.visit_with(visitor)
            }
            ty::Tuple(ts) => ts.visit_with(visitor),
            ty::FnDef(_, substs) => substs.visit_with(visitor),
            ty::FnPtr(ref f) => f.visit_with(visitor),
            ty::Ref(r, ty, _) => r.visit_with(visitor) || ty.visit_with(visitor),
            ty::Generator(_did, ref substs, _) => substs.visit_with(visitor),
            ty::GeneratorWitness(ref types) => types.visit_with(visitor),
            ty::Closure(_did, ref substs) => substs.visit_with(visitor),
            ty::Projection(ref data) => data.visit_with(visitor),
            ty::Opaque(_, ref substs) => substs.visit_with(visitor),

            ty::Bool
            | ty::Char
            | ty::Str
            | ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Error(_)
            | ty::Infer(_)
            | ty::Bound(..)
            | ty::Placeholder(..)
            | ty::Param(..)
            | ty::Never
            | ty::Foreign(..) => false,
        }
    }

    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        visitor.visit_ty(self)
    }
}

impl<'tcx> TypeFoldable<'tcx> for ty::Region<'tcx> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, _folder: &mut F) -> Self {
        *self
    }

    fn fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        folder.fold_region(*self)
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> bool {
        false
    }

    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        visitor.visit_region(*self)
    }
}

impl<'tcx> TypeFoldable<'tcx> for ty::Predicate<'tcx> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        let new = ty::PredicateKind::super_fold_with(&self.inner.kind, folder);
        folder.tcx().reuse_or_mk_predicate(*self, new)
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        ty::PredicateKind::super_visit_with(&self.inner.kind, visitor)
    }

    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        visitor.visit_predicate(*self)
    }

    fn has_vars_bound_at_or_above(&self, binder: ty::DebruijnIndex) -> bool {
        self.inner.outer_exclusive_binder > binder
    }

    fn has_type_flags(&self, flags: ty::TypeFlags) -> bool {
        self.inner.flags.intersects(flags)
    }
}

pub(super) trait PredicateVisitor<'tcx>: TypeVisitor<'tcx> {
    fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> bool;
}

impl<T: TypeVisitor<'tcx>> PredicateVisitor<'tcx> for T {
    default fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> bool {
        predicate.super_visit_with(self)
    }
}

impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ty::Predicate<'tcx>> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        fold_list(*self, folder, |tcx, v| tcx.intern_predicates(v))
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        self.iter().any(|p| p.visit_with(visitor))
    }
}

impl<'tcx, T: TypeFoldable<'tcx>, I: Idx> TypeFoldable<'tcx> for IndexVec<I, T> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        self.iter().map(|x| x.fold_with(folder)).collect()
    }

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

impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::Const<'tcx> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        let ty = self.ty.fold_with(folder);
        let val = self.val.fold_with(folder);
        if ty != self.ty || val != self.val {
            folder.tcx().mk_const(ty::Const { ty, val })
        } else {
            *self
        }
    }

    fn fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        folder.fold_const(*self)
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        self.ty.visit_with(visitor) || self.val.visit_with(visitor)
    }

    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        visitor.visit_const(self)
    }
}

impl<'tcx> TypeFoldable<'tcx> for ty::ConstKind<'tcx> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
        match *self {
            ty::ConstKind::Infer(ic) => ty::ConstKind::Infer(ic.fold_with(folder)),
            ty::ConstKind::Param(p) => ty::ConstKind::Param(p.fold_with(folder)),
            ty::ConstKind::Unevaluated(did, substs, promoted) => {
                ty::ConstKind::Unevaluated(did, substs.fold_with(folder), promoted)
            }
            ty::ConstKind::Value(_)
            | ty::ConstKind::Bound(..)
            | ty::ConstKind::Placeholder(..)
            | ty::ConstKind::Error(_) => *self,
        }
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
        match *self {
            ty::ConstKind::Infer(ic) => ic.visit_with(visitor),
            ty::ConstKind::Param(p) => p.visit_with(visitor),
            ty::ConstKind::Unevaluated(_, substs, _) => substs.visit_with(visitor),
            ty::ConstKind::Value(_)
            | ty::ConstKind::Bound(..)
            | ty::ConstKind::Placeholder(_)
            | ty::ConstKind::Error(_) => false,
        }
    }
}

impl<'tcx> TypeFoldable<'tcx> for InferConst<'tcx> {
    fn super_fold_with<F: TypeFolder<'tcx>>(&self, _folder: &mut F) -> Self {
        *self
    }

    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> bool {
        false
    }
}

// Does the equivalent of
// ```
// let v = self.iter().map(|p| p.fold_with(folder)).collect::<SmallVec<[_; 8]>>();
// folder.tcx().intern_*(&v)
// ```
fn fold_list<'tcx, F, T>(
    list: &'tcx ty::List<T>,
    folder: &mut F,
    intern: impl FnOnce(TyCtxt<'tcx>, &[T]) -> &'tcx ty::List<T>,
) -> &'tcx ty::List<T>
where
    F: TypeFolder<'tcx>,
    T: TypeFoldable<'tcx> + PartialEq + Copy,
{
    let mut iter = list.iter();
    // Look for the first element that changed
    if let Some((i, new_t)) = iter.by_ref().enumerate().find_map(|(i, t)| {
        let new_t = t.fold_with(folder);
        if new_t == t { None } else { Some((i, new_t)) }
    }) {
        // An element changed, prepare to intern the resulting list
        let mut new_list = SmallVec::<[_; 8]>::with_capacity(list.len());
        new_list.extend_from_slice(&list[..i]);
        new_list.push(new_t);
        new_list.extend(iter.map(|t| t.fold_with(folder)));
        intern(folder.tcx(), &new_list)
    } else {
        list
    }
}