fold.rs

//! A generalized traversal mechanism for complex data structures that contain
//! type information.
//!
//! There are two types of traversal.
//! - Folding. This is a modifying traversal. It consumes the data structure,
//!   producing a (possibly) modified version of it. Both fallible and
//!   infallible versions are available. The name is potentially
//!   confusing, because this traversal is more like `Iterator::map` than
//!   `Iterator::fold`.
//! - Visiting. This is a read-only traversal of the data structure.
//!
//! These traversals have limited flexibility. Only a small number of "types of
//! interest" within the complex data structures can receive custom
//! modification (when folding) or custom visitation (when visiting). These are
//! the ones containing the most important type-related information, such as
//! `Ty`, `Predicate`, `Region`, and `Const`.
//!
//! There are two traits involved in each traversal type.
//! - The first trait is `TypeFoldable`, which is implemented once for many
//!   types. This includes both (a) types of interest, and (b) all other
//!   relevant types, including generic containers like `Vec` and `Option`. It
//!   defines a "skeleton" of how they should be traversed, for both folding
//!   and visiting.
//! - The second trait is `TypeFolder`/`FallibleTypeFolder` (for
//!   infallible/fallible folding traversals) or `TypeVisitor` (for visiting
//!   traversals). One of these is implemented for each folder/visitor. This
//!   defines how types of interest are handled.
//!
//! This means each traversal is a mixture of (a) generic traversal operations,
//! and (b) custom fold/visit operations that are specific to the
//! folder/visitor.
//! - The `TypeFoldable` impls handle most of the traversal, and call into
//!   `TypeFolder`/`FallibleTypeFolder`/`TypeVisitor` when they encounter a
//!   type of interest.
//! - A `TypeFolder`/`FallibleTypeFolder`/`TypeVisitor` may also call back into
//!   a `TypeFoldable` impl, because (a) the types of interest are recursive
//!   and can contain other types of interest, and (b) each folder/visitor
//!   might provide custom handling only for some types of interest, or only
//!   for some variants of each type of interest, and then use default
//!   traversal for the remaining cases.
//!
//! For example, if you have `struct S(Ty, U)` where `S: TypeFoldable` and `U:
//! TypeFoldable`, and an instance `S(ty, u)`, it would be visited like so:
//! ```
//! s.visit_with(visitor) calls
//! - s.super_visit_with(visitor) calls
//!   - ty.visit_with(visitor) calls
//!     - visitor.visit_ty(ty) may call
//!       - ty.super_visit_with(visitor)
//!   - u.visit_with(visitor)
//! ```
use crate::mir;
use crate::ty::{self, flags::FlagComputation, Binder, Ty, TyCtxt, TypeFlags};
use rustc_hir::def_id::DefId;

use rustc_data_structures::fx::FxHashSet;
use rustc_data_structures::sso::SsoHashSet;
use std::collections::BTreeMap;
use std::fmt;
use std::ops::ControlFlow;

/// This trait is implemented for every type that can be folded/visited,
/// providing the skeleton of the traversal.
///
/// To implement this conveniently, use the derive macro located in
/// `rustc_macros`.
pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
    /// The main entry point for folding. To fold a value `t` with a folder `f`
    /// call: `t.try_fold_with(f)`.
    ///
    /// For types of interest (such as `Ty`), this default is overridden with a
    /// method that calls a folder method specifically for that type (such as
    /// `F::try_fold_ty`). This is where control transfers from `TypeFoldable`
    /// to `TypeFolder`.
    ///
    /// For other types, this default is used.
    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
        self.try_super_fold_with(folder)
    }

    /// A convenient alternative to `try_fold_with` for use with infallible
    /// folders. Do not override this method, to ensure coherence with
    /// `try_fold_with`.
    fn fold_with<F: TypeFolder<'tcx, Error = !>>(self, folder: &mut F) -> Self {
        self.try_fold_with(folder).into_ok()
    }

    /// Traverses the type in question, typically by calling `try_fold_with` on
    /// each field/element. This is true even for types of interest such as
    /// `Ty`. This should only be called within `TypeFolder` methods, when
    /// non-custom traversals are desired for types of interest.
    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
        self,
        folder: &mut F,
    ) -> Result<Self, F::Error>;

    /// A convenient alternative to `try_super_fold_with` for use with
    /// infallible folders. Do not override this method, to ensure coherence
    /// with `try_super_fold_with`.
    fn super_fold_with<F: TypeFolder<'tcx, Error = !>>(self, folder: &mut F) -> Self {
        self.try_super_fold_with(folder).into_ok()
    }

    /// The entry point for visiting. To visit a value `t` with a visitor `v`
    /// call: `t.visit_with(v)`.
    ///
    /// For types of interest (such as `Ty`), this default is overridden with a
    /// method that calls a visitor method specifically for that type (such as
    /// `V::visit_ty`). This is where control transfers from `TypeFoldable` to
    /// `TypeVisitor`.
    ///
    /// For other types, this default is used.
    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
        self.super_visit_with(visitor)
    }

    /// Traverses the type in question, typically by calling `visit_with` on
    /// each field/element. This is true even for types of interest such as
    /// `Ty`. This should only be called within `TypeVisitor` methods, when
    /// non-custom traversals are desired for types of interest.
    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy>;

    /// Returns `true` if `self` has any late-bound regions that are either
    /// bound by `binder` or bound by some binder outside of `binder`.
    /// If `binder` is `ty::INNERMOST`, this indicates whether
    /// there are any late-bound regions that appear free.
    fn has_vars_bound_at_or_above(&self, binder: ty::DebruijnIndex) -> bool {
        self.visit_with(&mut HasEscapingVarsVisitor { outer_index: binder }).is_break()
    }

    /// Returns `true` if this `self` has any regions that escape `binder` (and
    /// hence are not bound by it).
    fn has_vars_bound_above(&self, binder: ty::DebruijnIndex) -> bool {
        self.has_vars_bound_at_or_above(binder.shifted_in(1))
    }

    fn has_escaping_bound_vars(&self) -> bool {
        self.has_vars_bound_at_or_above(ty::INNERMOST)
    }

    #[instrument(level = "trace")]
    fn has_type_flags(&self, flags: TypeFlags) -> bool {
        self.visit_with(&mut HasTypeFlagsVisitor { flags }).break_value() == Some(FoundFlags)
    }
    fn has_projections(&self) -> bool {
        self.has_type_flags(TypeFlags::HAS_PROJECTION)
    }
    fn has_opaque_types(&self) -> bool {
        self.has_type_flags(TypeFlags::HAS_TY_OPAQUE)
    }
    fn references_error(&self) -> bool {
        self.has_type_flags(TypeFlags::HAS_ERROR)
    }
    fn has_param_types_or_consts(&self) -> bool {
        self.has_type_flags(TypeFlags::HAS_TY_PARAM | TypeFlags::HAS_CT_PARAM)
    }
    fn has_infer_regions(&self) -> bool {
        self.has_type_flags(TypeFlags::HAS_RE_INFER)
    }
    fn has_infer_types(&self) -> bool {
        self.has_type_flags(TypeFlags::HAS_TY_INFER)
    }
    fn has_infer_types_or_consts(&self) -> bool {
        self.has_type_flags(TypeFlags::HAS_TY_INFER | TypeFlags::HAS_CT_INFER)
    }
    fn needs_infer(&self) -> bool {
        self.has_type_flags(TypeFlags::NEEDS_INFER)
    }
    fn has_placeholders(&self) -> bool {
        self.has_type_flags(
            TypeFlags::HAS_RE_PLACEHOLDER
                | TypeFlags::HAS_TY_PLACEHOLDER
                | TypeFlags::HAS_CT_PLACEHOLDER,
        )
    }
    fn needs_subst(&self) -> bool {
        self.has_type_flags(TypeFlags::NEEDS_SUBST)
    }
    /// "Free" regions in this context means that it has any region
    /// that is not (a) erased or (b) late-bound.
    fn has_free_regions(&self) -> bool {
        self.has_type_flags(TypeFlags::HAS_FREE_REGIONS)
    }

    fn has_erased_regions(&self) -> bool {
        self.has_type_flags(TypeFlags::HAS_RE_ERASED)
    }

    /// True if there are any un-erased free regions.
    fn has_erasable_regions(&self) -> bool {
        self.has_type_flags(TypeFlags::HAS_FREE_REGIONS)
    }

    /// Indicates whether this value references only 'global'
    /// generic parameters that are the same regardless of what fn we are
    /// in. This is used for caching.
    fn is_global(&self) -> bool {
        !self.has_type_flags(TypeFlags::HAS_FREE_LOCAL_NAMES)
    }

    /// True if there are any late-bound regions
    fn has_late_bound_regions(&self) -> bool {
        self.has_type_flags(TypeFlags::HAS_RE_LATE_BOUND)
    }

    /// Indicates whether this value still has parameters/placeholders/inference variables
    /// which could be replaced later, in a way that would change the results of `impl`
    /// specialization.
    fn still_further_specializable(&self) -> bool {
        self.has_type_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE)
    }
}

/// This trait is implemented for every folding traversal. There is a fold
/// method defined for every type of interest. Each such method has a default
/// that does an "identity" fold.
///
/// If this folder is fallible (and therefore its [`Error`][`TypeFolder::Error`]
/// associated type is something other than the default `!`) then
/// [`FallibleTypeFolder`] should be implemented manually. Otherwise,
/// a blanket implementation of [`FallibleTypeFolder`] will defer to
/// the infallible methods of this trait to ensure that the two APIs
/// are coherent.
pub trait TypeFolder<'tcx>: Sized {
    type Error = !;

    fn tcx<'a>(&'a self) -> TyCtxt<'tcx>;

    fn fold_binder<T>(&mut self, t: Binder<'tcx, T>) -> Binder<'tcx, T>
    where
        T: TypeFoldable<'tcx>,
        Self: TypeFolder<'tcx, Error = !>,
    {
        t.super_fold_with(self)
    }

    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx>
    where
        Self: TypeFolder<'tcx, Error = !>,
    {
        t.super_fold_with(self)
    }

    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx>
    where
        Self: TypeFolder<'tcx, Error = !>,
    {
        r.super_fold_with(self)
    }

    fn fold_const(&mut self, c: ty::Const<'tcx>) -> ty::Const<'tcx>
    where
        Self: TypeFolder<'tcx, Error = !>,
    {
        c.super_fold_with(self)
    }

    fn fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> ty::Predicate<'tcx>
    where
        Self: TypeFolder<'tcx, Error = !>,
    {
        p.super_fold_with(self)
    }

    fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx>
    where
        Self: TypeFolder<'tcx, Error = !>,
    {
        bug!("most type folders should not be folding MIR datastructures: {:?}", c)
    }
}

/// This trait is implemented for every folding traversal. There is a fold
/// method defined for every type of interest. Each such method has a default
/// that does an "identity" fold.
///
/// A blanket implementation of this trait (that defers to the relevant
/// method of [`TypeFolder`]) is provided for all infallible folders in
/// order to ensure the two APIs are coherent.
pub trait FallibleTypeFolder<'tcx>: TypeFolder<'tcx> {
    fn try_fold_binder<T>(&mut self, t: Binder<'tcx, T>) -> Result<Binder<'tcx, T>, Self::Error>
    where
        T: TypeFoldable<'tcx>,
    {
        t.try_super_fold_with(self)
    }

    fn try_fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
        t.try_super_fold_with(self)
    }

    fn try_fold_region(&mut self, r: ty::Region<'tcx>) -> Result<ty::Region<'tcx>, Self::Error> {
        r.try_super_fold_with(self)
    }

    fn try_fold_const(&mut self, c: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, Self::Error> {
        c.try_super_fold_with(self)
    }

    fn try_fold_predicate(
        &mut self,
        p: ty::Predicate<'tcx>,
    ) -> Result<ty::Predicate<'tcx>, Self::Error> {
        p.try_super_fold_with(self)
    }

    fn try_fold_mir_const(
        &mut self,
        c: mir::ConstantKind<'tcx>,
    ) -> Result<mir::ConstantKind<'tcx>, Self::Error> {
        bug!("most type folders should not be folding MIR datastructures: {:?}", c)
    }
}

// This blanket implementation of the fallible trait for infallible folders
// delegates to infallible methods to ensure coherence.
impl<'tcx, F> FallibleTypeFolder<'tcx> for F
where
    F: TypeFolder<'tcx, Error = !>,
{
    fn try_fold_binder<T>(&mut self, t: Binder<'tcx, T>) -> Result<Binder<'tcx, T>, Self::Error>
    where
        T: TypeFoldable<'tcx>,
    {
        Ok(self.fold_binder(t))
    }

    fn try_fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
        Ok(self.fold_ty(t))
    }

    fn try_fold_region(&mut self, r: ty::Region<'tcx>) -> Result<ty::Region<'tcx>, Self::Error> {
        Ok(self.fold_region(r))
    }

    fn try_fold_const(&mut self, c: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, Self::Error> {
        Ok(self.fold_const(c))
    }

    fn try_fold_predicate(
        &mut self,
        p: ty::Predicate<'tcx>,
    ) -> Result<ty::Predicate<'tcx>, Self::Error> {
        Ok(self.fold_predicate(p))
    }

    fn try_fold_mir_const(
        &mut self,
        c: mir::ConstantKind<'tcx>,
    ) -> Result<mir::ConstantKind<'tcx>, Self::Error> {
        Ok(self.fold_mir_const(c))
    }
}

/// This trait is implemented for every visiting traversal. There is a visit
/// method defined for every type of interest. Each such method has a default
/// that recurses into the type's fields in a non-custom fashion.
pub trait TypeVisitor<'tcx>: Sized {
    type BreakTy = !;

    fn visit_binder<T: TypeFoldable<'tcx>>(
        &mut self,
        t: &Binder<'tcx, T>,
    ) -> ControlFlow<Self::BreakTy> {
        t.super_visit_with(self)
    }

    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
        t.super_visit_with(self)
    }

    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
        r.super_visit_with(self)
    }

    fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
        c.super_visit_with(self)
    }

    fn visit_unevaluated_const(&mut self, uv: ty::Unevaluated<'tcx>) -> ControlFlow<Self::BreakTy> {
        uv.super_visit_with(self)
    }

    fn visit_predicate(&mut self, p: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
        p.super_visit_with(self)
    }
}

///////////////////////////////////////////////////////////////////////////
// Some sample folders

pub struct BottomUpFolder<'tcx, F, G, H>
where
    F: FnMut(Ty<'tcx>) -> Ty<'tcx>,
    G: FnMut(ty::Region<'tcx>) -> ty::Region<'tcx>,
    H: FnMut(ty::Const<'tcx>) -> ty::Const<'tcx>,
{
    pub tcx: TyCtxt<'tcx>,
    pub ty_op: F,
    pub lt_op: G,
    pub ct_op: H,
}

impl<'tcx, F, G, H> TypeFolder<'tcx> for BottomUpFolder<'tcx, F, G, H>
where
    F: FnMut(Ty<'tcx>) -> Ty<'tcx>,
    G: FnMut(ty::Region<'tcx>) -> ty::Region<'tcx>,
    H: FnMut(ty::Const<'tcx>) -> ty::Const<'tcx>,
{
    fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
        self.tcx
    }

    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
        let t = ty.super_fold_with(self);
        (self.ty_op)(t)
    }

    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
        let r = r.super_fold_with(self);
        (self.lt_op)(r)
    }

    fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
        let ct = ct.super_fold_with(self);
        (self.ct_op)(ct)
    }
}

///////////////////////////////////////////////////////////////////////////
// Region folder

impl<'tcx> TyCtxt<'tcx> {
    /// Folds the escaping and free regions in `value` using `f`, and
    /// sets `skipped_regions` to true if any late-bound region was found
    /// and skipped.
    pub fn fold_regions<T>(
        self,
        value: T,
        skipped_regions: &mut bool,
        mut f: impl FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
    ) -> T
    where
        T: TypeFoldable<'tcx>,
    {
        value.fold_with(&mut RegionFolder::new(self, skipped_regions, &mut f))
    }

    /// Invoke `callback` on every region appearing free in `value`.
    pub fn for_each_free_region(
        self,
        value: &impl TypeFoldable<'tcx>,
        mut callback: impl FnMut(ty::Region<'tcx>),
    ) {
        self.any_free_region_meets(value, |r| {
            callback(r);
            false
        });
    }

    /// Returns `true` if `callback` returns true for every region appearing free in `value`.
    pub fn all_free_regions_meet(
        self,
        value: &impl TypeFoldable<'tcx>,
        mut callback: impl FnMut(ty::Region<'tcx>) -> bool,
    ) -> bool {
        !self.any_free_region_meets(value, |r| !callback(r))
    }

    /// Returns `true` if `callback` returns true for some region appearing free in `value`.
    pub fn any_free_region_meets(
        self,
        value: &impl TypeFoldable<'tcx>,
        callback: impl FnMut(ty::Region<'tcx>) -> bool,
    ) -> bool {
        struct RegionVisitor<F> {
            /// The index of a binder *just outside* the things we have
            /// traversed. If we encounter a bound region bound by this
            /// binder or one outer to it, it appears free. Example:
            ///
            /// ```
            ///    for<'a> fn(for<'b> fn(), T)
            /// ^          ^          ^     ^
            /// |          |          |     | here, would be shifted in 1
            /// |          |          | here, would be shifted in 2
            /// |          | here, would be `INNERMOST` shifted in by 1
            /// | here, initially, binder would be `INNERMOST`
            /// ```
            ///
            /// You see that, initially, *any* bound value is free,
            /// because we've not traversed any binders. As we pass
            /// through a binder, we shift the `outer_index` by 1 to
            /// account for the new binder that encloses us.
            outer_index: ty::DebruijnIndex,
            callback: F,
        }

        impl<'tcx, F> TypeVisitor<'tcx> for RegionVisitor<F>
        where
            F: FnMut(ty::Region<'tcx>) -> bool,
        {
            type BreakTy = ();

            fn visit_binder<T: TypeFoldable<'tcx>>(
                &mut self,
                t: &Binder<'tcx, T>,
            ) -> ControlFlow<Self::BreakTy> {
                self.outer_index.shift_in(1);
                let result = t.as_ref().skip_binder().visit_with(self);
                self.outer_index.shift_out(1);
                result
            }

            fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
                match *r {
                    ty::ReLateBound(debruijn, _) if debruijn < self.outer_index => {
                        ControlFlow::CONTINUE
                    }
                    _ => {
                        if (self.callback)(r) {
                            ControlFlow::BREAK
                        } else {
                            ControlFlow::CONTINUE
                        }
                    }
                }
            }

            fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
                // We're only interested in types involving regions
                if ty.flags().intersects(TypeFlags::HAS_FREE_REGIONS) {
                    ty.super_visit_with(self)
                } else {
                    ControlFlow::CONTINUE
                }
            }
        }

        value.visit_with(&mut RegionVisitor { outer_index: ty::INNERMOST, callback }).is_break()
    }
}

/// Folds over the substructure of a type, visiting its component
/// types and all regions that occur *free* within it.
///
/// That is, `Ty` can contain function or method types that bind
/// regions at the call site (`ReLateBound`), and occurrences of
/// regions (aka "lifetimes") that are bound within a type are not
/// visited by this folder; only regions that occur free will be
/// visited by `fld_r`.

pub struct RegionFolder<'a, 'tcx> {
    tcx: TyCtxt<'tcx>,
    skipped_regions: &'a mut bool,

    /// Stores the index of a binder *just outside* the stuff we have
    /// visited.  So this begins as INNERMOST; when we pass through a
    /// binder, it is incremented (via `shift_in`).
    current_index: ty::DebruijnIndex,

    /// Callback invokes for each free region. The `DebruijnIndex`
    /// points to the binder *just outside* the ones we have passed
    /// through.
    fold_region_fn:
        &'a mut (dyn FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx> + 'a),
}

impl<'a, 'tcx> RegionFolder<'a, 'tcx> {
    #[inline]
    pub fn new(
        tcx: TyCtxt<'tcx>,
        skipped_regions: &'a mut bool,
        fold_region_fn: &'a mut dyn FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
    ) -> RegionFolder<'a, 'tcx> {
        RegionFolder { tcx, skipped_regions, current_index: ty::INNERMOST, fold_region_fn }
    }
}

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

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

    #[instrument(skip(self), level = "debug")]
    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
        match *r {
            ty::ReLateBound(debruijn, _) if debruijn < self.current_index => {
                debug!(?self.current_index, "skipped bound region");
                *self.skipped_regions = true;
                r
            }
            _ => {
                debug!(?self.current_index, "folding free region");
                (self.fold_region_fn)(r, self.current_index)
            }
        }
    }
}

///////////////////////////////////////////////////////////////////////////
// Bound vars replacer

/// Replaces the escaping bound vars (late bound regions or bound types) in a type.
struct BoundVarReplacer<'a, 'tcx> {
    tcx: TyCtxt<'tcx>,

    /// As with `RegionFolder`, represents the index of a binder *just outside*
    /// the ones we have visited.
    current_index: ty::DebruijnIndex,

    fld_r: Option<&'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a)>,
    fld_t: Option<&'a mut (dyn FnMut(ty::BoundTy) -> Ty<'tcx> + 'a)>,
    fld_c: Option<&'a mut (dyn FnMut(ty::BoundVar, Ty<'tcx>) -> ty::Const<'tcx> + 'a)>,
}

impl<'a, 'tcx> BoundVarReplacer<'a, 'tcx> {
    fn new(
        tcx: TyCtxt<'tcx>,
        fld_r: Option<&'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a)>,
        fld_t: Option<&'a mut (dyn FnMut(ty::BoundTy) -> Ty<'tcx> + 'a)>,
        fld_c: Option<&'a mut (dyn FnMut(ty::BoundVar, Ty<'tcx>) -> ty::Const<'tcx> + 'a)>,
    ) -> Self {
        BoundVarReplacer { tcx, current_index: ty::INNERMOST, fld_r, fld_t, fld_c }
    }
}

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

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

    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
        match *t.kind() {
            ty::Bound(debruijn, bound_ty) if debruijn == self.current_index => {
                if let Some(fld_t) = self.fld_t.as_mut() {
                    let ty = fld_t(bound_ty);
                    return ty::fold::shift_vars(self.tcx, ty, self.current_index.as_u32());
                }
            }
            _ if t.has_vars_bound_at_or_above(self.current_index) => {
                return t.super_fold_with(self);
            }
            _ => {}
        }
        t
    }

    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
        match *r {
            ty::ReLateBound(debruijn, br) if debruijn == self.current_index => {
                if let Some(fld_r) = self.fld_r.as_mut() {
                    let region = fld_r(br);
                    return if let ty::ReLateBound(debruijn1, br) = *region {
                        // If the callback returns a late-bound region,
                        // that region should always use the INNERMOST
                        // debruijn index. Then we adjust it to the
                        // correct depth.
                        assert_eq!(debruijn1, ty::INNERMOST);
                        self.tcx.mk_region(ty::ReLateBound(debruijn, br))
                    } else {
                        region
                    };
                }
            }
            _ => {}
        }
        r
    }

    fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
        match ct.val() {
            ty::ConstKind::Bound(debruijn, bound_const) if debruijn == self.current_index => {
                if let Some(fld_c) = self.fld_c.as_mut() {
                    let ct = fld_c(bound_const, ct.ty());
                    return ty::fold::shift_vars(self.tcx, ct, self.current_index.as_u32());
                }
            }
            _ if ct.has_vars_bound_at_or_above(self.current_index) => {
                return ct.super_fold_with(self);
            }
            _ => {}
        }
        ct
    }
}

impl<'tcx> TyCtxt<'tcx> {
    /// Replaces all regions bound by the given `Binder` with the
    /// results returned by the closure; the closure is expected to
    /// return a free region (relative to this binder), and hence the
    /// binder is removed in the return type. The closure is invoked
    /// once for each unique `BoundRegionKind`; multiple references to the
    /// same `BoundRegionKind` will reuse the previous result. A map is
    /// returned at the end with each bound region and the free region
    /// that replaced it.
    ///
    /// This method only replaces late bound regions and the result may still
    /// contain escaping bound types.
    pub fn replace_late_bound_regions<T, F>(
        self,
        value: Binder<'tcx, T>,
        mut fld_r: F,
    ) -> (T, BTreeMap<ty::BoundRegion, ty::Region<'tcx>>)
    where
        F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
        T: TypeFoldable<'tcx>,
    {
        let mut region_map = BTreeMap::new();
        let mut real_fld_r =
            |br: ty::BoundRegion| *region_map.entry(br).or_insert_with(|| fld_r(br));
        let value = value.skip_binder();
        let value = if !value.has_escaping_bound_vars() {
            value
        } else {
            let mut replacer = BoundVarReplacer::new(self, Some(&mut real_fld_r), None, None);
            value.fold_with(&mut replacer)
        };
        (value, region_map)
    }

    /// Replaces all escaping bound vars. The `fld_r` closure replaces escaping
    /// bound regions; the `fld_t` closure replaces escaping bound types and the `fld_c`
    /// closure replaces escaping bound consts.
    pub fn replace_escaping_bound_vars<T, F, G, H>(
        self,
        value: T,
        mut fld_r: F,
        mut fld_t: G,
        mut fld_c: H,
    ) -> T
    where
        F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
        G: FnMut(ty::BoundTy) -> Ty<'tcx>,
        H: FnMut(ty::BoundVar, Ty<'tcx>) -> ty::Const<'tcx>,
        T: TypeFoldable<'tcx>,
    {
        if !value.has_escaping_bound_vars() {
            value
        } else {
            let mut replacer =
                BoundVarReplacer::new(self, Some(&mut fld_r), Some(&mut fld_t), Some(&mut fld_c));
            value.fold_with(&mut replacer)
        }
    }

    /// Replaces all types or regions bound by the given `Binder`. The `fld_r`
    /// closure replaces bound regions while the `fld_t` closure replaces bound
    /// types.
    pub fn replace_bound_vars<T, F, G, H>(
        self,
        value: Binder<'tcx, T>,
        mut fld_r: F,
        fld_t: G,
        fld_c: H,
    ) -> (T, BTreeMap<ty::BoundRegion, ty::Region<'tcx>>)
    where
        F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
        G: FnMut(ty::BoundTy) -> Ty<'tcx>,
        H: FnMut(ty::BoundVar, Ty<'tcx>) -> ty::Const<'tcx>,
        T: TypeFoldable<'tcx>,
    {
        let mut region_map = BTreeMap::new();
        let real_fld_r = |br: ty::BoundRegion| *region_map.entry(br).or_insert_with(|| fld_r(br));
        let value = self.replace_escaping_bound_vars(value.skip_binder(), real_fld_r, fld_t, fld_c);
        (value, region_map)
    }

    /// Replaces any late-bound regions bound in `value` with
    /// free variants attached to `all_outlive_scope`.
    pub fn liberate_late_bound_regions<T>(
        self,
        all_outlive_scope: DefId,
        value: ty::Binder<'tcx, T>,
    ) -> T
    where
        T: TypeFoldable<'tcx>,
    {
        self.replace_late_bound_regions(value, |br| {
            self.mk_region(ty::ReFree(ty::FreeRegion {
                scope: all_outlive_scope,
                bound_region: br.kind,
            }))
        })
        .0
    }

    pub fn shift_bound_var_indices<T>(self, bound_vars: usize, value: T) -> T
    where
        T: TypeFoldable<'tcx>,
    {
        self.replace_escaping_bound_vars(
            value,
            |r| {
                self.mk_region(ty::ReLateBound(
                    ty::INNERMOST,
                    ty::BoundRegion {
                        var: ty::BoundVar::from_usize(r.var.as_usize() + bound_vars),
                        kind: r.kind,
                    },
                ))
            },
            |t| {
                self.mk_ty(ty::Bound(
                    ty::INNERMOST,
                    ty::BoundTy {
                        var: ty::BoundVar::from_usize(t.var.as_usize() + bound_vars),
                        kind: t.kind,
                    },
                ))
            },
            |c, ty| {
                self.mk_const(ty::ConstS {
                    val: ty::ConstKind::Bound(
                        ty::INNERMOST,
                        ty::BoundVar::from_usize(c.as_usize() + bound_vars),
                    ),
                    ty,
                })
            },
        )
    }

    /// Returns a set of all late-bound regions that are constrained
    /// by `value`, meaning that if we instantiate those LBR with
    /// variables and equate `value` with something else, those
    /// variables will also be equated.
    pub fn collect_constrained_late_bound_regions<T>(
        self,
        value: &Binder<'tcx, T>,
    ) -> FxHashSet<ty::BoundRegionKind>
    where
        T: TypeFoldable<'tcx>,
    {
        self.collect_late_bound_regions(value, true)
    }

    /// Returns a set of all late-bound regions that appear in `value` anywhere.
    pub fn collect_referenced_late_bound_regions<T>(
        self,
        value: &Binder<'tcx, T>,
    ) -> FxHashSet<ty::BoundRegionKind>
    where
        T: TypeFoldable<'tcx>,
    {
        self.collect_late_bound_regions(value, false)
    }

    fn collect_late_bound_regions<T>(
        self,
        value: &Binder<'tcx, T>,
        just_constraint: bool,
    ) -> FxHashSet<ty::BoundRegionKind>
    where
        T: TypeFoldable<'tcx>,
    {
        let mut collector = LateBoundRegionsCollector::new(just_constraint);
        let result = value.as_ref().skip_binder().visit_with(&mut collector);
        assert!(result.is_continue()); // should never have stopped early
        collector.regions
    }

    /// Replaces any late-bound regions bound in `value` with `'erased`. Useful in codegen but also
    /// method lookup and a few other places where precise region relationships are not required.
    pub fn erase_late_bound_regions<T>(self, value: Binder<'tcx, T>) -> T
    where
        T: TypeFoldable<'tcx>,
    {
        self.replace_late_bound_regions(value, |_| self.lifetimes.re_erased).0
    }

    /// Rewrite any late-bound regions so that they are anonymous. Region numbers are
    /// assigned starting at 0 and increasing monotonically in the order traversed
    /// by the fold operation.
    ///
    /// The chief purpose of this function is to canonicalize regions so that two
    /// `FnSig`s or `TraitRef`s which are equivalent up to region naming will become
    /// structurally identical. For example, `for<'a, 'b> fn(&'a isize, &'b isize)` and
    /// `for<'a, 'b> fn(&'b isize, &'a isize)` will become identical after anonymization.
    pub fn anonymize_late_bound_regions<T>(self, sig: Binder<'tcx, T>) -> Binder<'tcx, T>
    where
        T: TypeFoldable<'tcx>,
    {
        let mut counter = 0;
        let inner = self
            .replace_late_bound_regions(sig, |_| {
                let br = ty::BoundRegion {
                    var: ty::BoundVar::from_u32(counter),
                    kind: ty::BrAnon(counter),
                };
                let r = self.mk_region(ty::ReLateBound(ty::INNERMOST, br));
                counter += 1;
                r
            })
            .0;
        let bound_vars = self.mk_bound_variable_kinds(
            (0..counter).map(|i| ty::BoundVariableKind::Region(ty::BrAnon(i))),
        );
        Binder::bind_with_vars(inner, bound_vars)
    }
}

pub struct ValidateBoundVars<'tcx> {
    bound_vars: &'tcx ty::List<ty::BoundVariableKind>,
    binder_index: ty::DebruijnIndex,
    // We may encounter the same variable at different levels of binding, so
    // this can't just be `Ty`
    visited: SsoHashSet<(ty::DebruijnIndex, Ty<'tcx>)>,
}

impl<'tcx> ValidateBoundVars<'tcx> {
    pub fn new(bound_vars: &'tcx ty::List<ty::BoundVariableKind>) -> Self {
        ValidateBoundVars {
            bound_vars,
            binder_index: ty::INNERMOST,
            visited: SsoHashSet::default(),
        }
    }
}

impl<'tcx> TypeVisitor<'tcx> for ValidateBoundVars<'tcx> {
    type BreakTy = ();

    fn visit_binder<T: TypeFoldable<'tcx>>(
        &mut self,
        t: &Binder<'tcx, T>,
    ) -> ControlFlow<Self::BreakTy> {
        self.binder_index.shift_in(1);
        let result = t.super_visit_with(self);
        self.binder_index.shift_out(1);
        result
    }

    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
        if t.outer_exclusive_binder() < self.binder_index
            || !self.visited.insert((self.binder_index, t))
        {
            return ControlFlow::BREAK;
        }
        match *t.kind() {
            ty::Bound(debruijn, bound_ty) if debruijn == self.binder_index => {
                if self.bound_vars.len() <= bound_ty.var.as_usize() {
                    bug!("Not enough bound vars: {:?} not found in {:?}", t, self.bound_vars);
                }
                let list_var = self.bound_vars[bound_ty.var.as_usize()];
                match list_var {
                    ty::BoundVariableKind::Ty(kind) => {
                        if kind != bound_ty.kind {
                            bug!(
                                "Mismatched type kinds: {:?} doesn't var in list {:?}",
                                bound_ty.kind,
                                list_var
                            );
                        }
                    }
                    _ => {
                        bug!("Mismatched bound variable kinds! Expected type, found {:?}", list_var)
                    }
                }
            }

            _ => (),
        };

        t.super_visit_with(self)
    }

    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
        match *r {
            ty::ReLateBound(index, br) if index == self.binder_index => {
                if self.bound_vars.len() <= br.var.as_usize() {
                    bug!("Not enough bound vars: {:?} not found in {:?}", br, self.bound_vars);
                }
                let list_var = self.bound_vars[br.var.as_usize()];
                match list_var {
                    ty::BoundVariableKind::Region(kind) => {
                        if kind != br.kind {
                            bug!(
                                "Mismatched region kinds: {:?} doesn't match var ({:?}) in list ({:?})",
                                br.kind,
                                list_var,
                                self.bound_vars
                            );
                        }
                    }
                    _ => bug!(
                        "Mismatched bound variable kinds! Expected region, found {:?}",
                        list_var
                    ),
                }
            }

            _ => (),
        };

        r.super_visit_with(self)
    }
}

///////////////////////////////////////////////////////////////////////////
// Shifter
//
// Shifts the De Bruijn indices on all escaping bound vars by a
// fixed amount. Useful in substitution or when otherwise introducing
// a binding level that is not intended to capture the existing bound
// vars. See comment on `shift_vars_through_binders` method in
// `subst.rs` for more details.

struct Shifter<'tcx> {
    tcx: TyCtxt<'tcx>,
    current_index: ty::DebruijnIndex,
    amount: u32,
}

impl<'tcx> Shifter<'tcx> {
    pub fn new(tcx: TyCtxt<'tcx>, amount: u32) -> Self {
        Shifter { tcx, current_index: ty::INNERMOST, amount }
    }
}

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

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

    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
        match *r {
            ty::ReLateBound(debruijn, br) => {
                if self.amount == 0 || debruijn < self.current_index {
                    r
                } else {
                    let debruijn = debruijn.shifted_in(self.amount);
                    let shifted = ty::ReLateBound(debruijn, br);
                    self.tcx.mk_region(shifted)
                }
            }
            _ => r,
        }
    }

    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
        match *ty.kind() {
            ty::Bound(debruijn, bound_ty) => {
                if self.amount == 0 || debruijn < self.current_index {
                    ty
                } else {
                    let debruijn = debruijn.shifted_in(self.amount);
                    self.tcx.mk_ty(ty::Bound(debruijn, bound_ty))
                }
            }

            _ => ty.super_fold_with(self),
        }
    }

    fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
        if let ty::ConstKind::Bound(debruijn, bound_ct) = ct.val() {
            if self.amount == 0 || debruijn < self.current_index {
                ct
            } else {
                let debruijn = debruijn.shifted_in(self.amount);
                self.tcx.mk_const(ty::ConstS {
                    val: ty::ConstKind::Bound(debruijn, bound_ct),
                    ty: ct.ty(),
                })
            }
        } else {
            ct.super_fold_with(self)
        }
    }
}

pub fn shift_region<'tcx>(
    tcx: TyCtxt<'tcx>,
    region: ty::Region<'tcx>,
    amount: u32,
) -> ty::Region<'tcx> {
    match *region {
        ty::ReLateBound(debruijn, br) if amount > 0 => {
            tcx.mk_region(ty::ReLateBound(debruijn.shifted_in(amount), br))
        }
        _ => region,
    }
}

pub fn shift_vars<'tcx, T>(tcx: TyCtxt<'tcx>, value: T, amount: u32) -> T
where
    T: TypeFoldable<'tcx>,
{
    debug!("shift_vars(value={:?}, amount={})", value, amount);

    value.fold_with(&mut Shifter::new(tcx, amount))
}

#[derive(Debug, PartialEq, Eq, Copy, Clone)]
struct FoundEscapingVars;

/// An "escaping var" is a bound var whose binder is not part of `t`. A bound var can be a
/// bound region or a bound type.
///
/// So, for example, consider a type like the following, which has two binders:
///
///    for<'a> fn(x: for<'b> fn(&'a isize, &'b isize))
///    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ outer scope
///                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~  inner scope
///
/// This type has *bound regions* (`'a`, `'b`), but it does not have escaping regions, because the
/// binders of both `'a` and `'b` are part of the type itself. However, if we consider the *inner
/// fn type*, that type has an escaping region: `'a`.
///
/// Note that what I'm calling an "escaping var" is often just called a "free var". However,
/// we already use the term "free var". It refers to the regions or types that we use to represent
/// bound regions or type params on a fn definition while we are type checking its body.
///
/// To clarify, conceptually there is no particular difference between
/// an "escaping" var and a "free" var. However, there is a big
/// difference in practice. Basically, when "entering" a binding
/// level, one is generally required to do some sort of processing to
/// a bound var, such as replacing it with a fresh/placeholder
/// var, or making an entry in the environment to represent the
/// scope to which it is attached, etc. An escaping var represents
/// a bound var for which this processing has not yet been done.
struct HasEscapingVarsVisitor {
    /// Anything bound by `outer_index` or "above" is escaping.
    outer_index: ty::DebruijnIndex,
}

impl<'tcx> TypeVisitor<'tcx> for HasEscapingVarsVisitor {
    type BreakTy = FoundEscapingVars;

    fn visit_binder<T: TypeFoldable<'tcx>>(
        &mut self,
        t: &Binder<'tcx, T>,
    ) -> ControlFlow<Self::BreakTy> {
        self.outer_index.shift_in(1);
        let result = t.super_visit_with(self);
        self.outer_index.shift_out(1);
        result
    }

    #[inline]
    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
        // If the outer-exclusive-binder is *strictly greater* than
        // `outer_index`, that means that `t` contains some content
        // bound at `outer_index` or above (because
        // `outer_exclusive_binder` is always 1 higher than the
        // content in `t`). Therefore, `t` has some escaping vars.
        if t.outer_exclusive_binder() > self.outer_index {
            ControlFlow::Break(FoundEscapingVars)
        } else {
            ControlFlow::CONTINUE
        }
    }

    #[inline]
    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
        // If the region is bound by `outer_index` or anything outside
        // of outer index, then it escapes the binders we have
        // visited.
        if r.bound_at_or_above_binder(self.outer_index) {
            ControlFlow::Break(FoundEscapingVars)
        } else {
            ControlFlow::CONTINUE
        }
    }

    fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
        // we don't have a `visit_infer_const` callback, so we have to
        // hook in here to catch this case (annoying...), but
        // otherwise we do want to remember to visit the rest of the
        // const, as it has types/regions embedded in a lot of other
        // places.
        match ct.val() {
            ty::ConstKind::Bound(debruijn, _) if debruijn >= self.outer_index => {
                ControlFlow::Break(FoundEscapingVars)
            }
            _ => ct.super_visit_with(self),
        }
    }

    #[inline]
    fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
        if predicate.outer_exclusive_binder() > self.outer_index {
            ControlFlow::Break(FoundEscapingVars)
        } else {
            ControlFlow::CONTINUE
        }
    }
}

#[derive(Debug, PartialEq, Eq, Copy, Clone)]
struct FoundFlags;

// FIXME: Optimize for checking for infer flags
struct HasTypeFlagsVisitor {
    flags: ty::TypeFlags,
}

impl std::fmt::Debug for HasTypeFlagsVisitor {
    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        self.flags.fmt(fmt)
    }
}

impl<'tcx> TypeVisitor<'tcx> for HasTypeFlagsVisitor {
    type BreakTy = FoundFlags;

    #[inline]
    #[instrument(level = "trace")]
    fn visit_ty(&mut self, t: Ty<'_>) -> ControlFlow<Self::BreakTy> {
        debug!(
            "HasTypeFlagsVisitor: t={:?} t.flags={:?} self.flags={:?}",
            t,
            t.flags(),
            self.flags
        );
        if t.flags().intersects(self.flags) {
            ControlFlow::Break(FoundFlags)
        } else {
            ControlFlow::CONTINUE
        }
    }

    #[inline]
    #[instrument(skip(self), level = "trace")]
    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
        let flags = r.type_flags();
        trace!(r.flags=?flags);
        if flags.intersects(self.flags) {
            ControlFlow::Break(FoundFlags)
        } else {
            ControlFlow::CONTINUE
        }
    }

    #[inline]
    #[instrument(level = "trace")]
    fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
        let flags = FlagComputation::for_const(c);
        trace!(r.flags=?flags);
        if flags.intersects(self.flags) {
            ControlFlow::Break(FoundFlags)
        } else {
            ControlFlow::CONTINUE
        }
    }

    #[inline]
    #[instrument(level = "trace")]
    fn visit_unevaluated_const(&mut self, uv: ty::Unevaluated<'tcx>) -> ControlFlow<Self::BreakTy> {
        let flags = FlagComputation::for_unevaluated_const(uv);
        trace!(r.flags=?flags);
        if flags.intersects(self.flags) {
            ControlFlow::Break(FoundFlags)
        } else {
            ControlFlow::CONTINUE
        }
    }

    #[inline]
    #[instrument(level = "trace")]
    fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
        debug!(
            "HasTypeFlagsVisitor: predicate={:?} predicate.flags={:?} self.flags={:?}",
            predicate,
            predicate.flags(),
            self.flags
        );
        if predicate.flags().intersects(self.flags) {
            ControlFlow::Break(FoundFlags)
        } else {
            ControlFlow::CONTINUE
        }
    }
}

/// Collects all the late-bound regions at the innermost binding level
/// into a hash set.
struct LateBoundRegionsCollector {
    current_index: ty::DebruijnIndex,
    regions: FxHashSet<ty::BoundRegionKind>,

    /// `true` if we only want regions that are known to be
    /// "constrained" when you equate this type with another type. In
    /// particular, if you have e.g., `&'a u32` and `&'b u32`, equating
    /// them constraints `'a == 'b`. But if you have `<&'a u32 as
    /// Trait>::Foo` and `<&'b u32 as Trait>::Foo`, normalizing those
    /// types may mean that `'a` and `'b` don't appear in the results,
    /// so they are not considered *constrained*.
    just_constrained: bool,
}

impl LateBoundRegionsCollector {
    fn new(just_constrained: bool) -> Self {
        LateBoundRegionsCollector {
            current_index: ty::INNERMOST,
            regions: Default::default(),
            just_constrained,
        }
    }
}

impl<'tcx> TypeVisitor<'tcx> for LateBoundRegionsCollector {
    fn visit_binder<T: TypeFoldable<'tcx>>(
        &mut self,
        t: &Binder<'tcx, T>,
    ) -> ControlFlow<Self::BreakTy> {
        self.current_index.shift_in(1);
        let result = t.super_visit_with(self);
        self.current_index.shift_out(1);
        result
    }

    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
        // if we are only looking for "constrained" region, we have to
        // ignore the inputs to a projection, as they may not appear
        // in the normalized form
        if self.just_constrained {
            if let ty::Projection(..) | ty::Opaque(..) = t.kind() {
                return ControlFlow::CONTINUE;
            }
        }

        t.super_visit_with(self)
    }

    fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
        // if we are only looking for "constrained" region, we have to
        // ignore the inputs of an unevaluated const, as they may not appear
        // in the normalized form
        if self.just_constrained {
            if let ty::ConstKind::Unevaluated(..) = c.val() {
                return ControlFlow::CONTINUE;
            }
        }

        c.super_visit_with(self)
    }

    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
        if let ty::ReLateBound(debruijn, br) = *r {
            if debruijn == self.current_index {
                self.regions.insert(br.kind);
            }
        }
        ControlFlow::CONTINUE
    }
}