Merging upstream version 1.67.1+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

16 files changed, 8459 insertions, 0 deletions

diff --git a/vendor/chalk-ir/src/cast.rs b/vendor/chalk-ir/src/cast.rs
new file mode 100644
index 000000000..0c6b682ca
--- /dev/null
+++ b/vendor/chalk-ir/src/cast.rs
@@ -0,0 +1,364 @@
+//! Upcasts, to avoid writing out wrapper types.
+
+use crate::*;
+use std::marker::PhantomData;
+
+/// The `Cast` trait is used to make annoying upcasts between
+/// logically equivalent types that imply wrappers. For example, one
+/// could convert a `DomainGoal` into a `Goal` by doing:
+///
+/// ```ignore
+/// let goal: Goal = domain_goal.cast();
+/// ```
+///
+/// This is equivalent to the more explicit:
+///
+/// ```ignore
+/// let goal: Goal = Goal::DomainGoal(domain_goal)
+/// ```
+///
+/// Another useful trick is the `casted()` iterator adapter, which
+/// casts each element in the iterator as it is produced (you must
+/// have the `Caster` trait in scope for that).
+///
+/// # Invariant
+///
+/// `Cast` imposes a key invariant. You can only implement `T:
+/// Cast<U>` if both `T` and `U` have the same semantic meaning. Also,
+/// as part of this, they should always use the same set of free
+/// variables (the `Canonical` implementation, for example, relies on
+/// that).
+///
+/// # Iterators
+///
+/// If you import the `Caster` trait, you can also write `.casted()` on an
+/// iterator chain to cast every instance within.
+///
+/// # Implementing Cast
+///
+/// Do not implement `Cast` directly. Instead, implement `CastTo`.
+/// This split setup allows us to write `foo.cast::<T>()` to mean
+/// "cast to T".
+pub trait Cast: Sized {
+    /// Cast a value to type `U` using `CastTo`.
+    fn cast<U>(self, interner: U::Interner) -> U
+    where
+        Self: CastTo<U>,
+        U: HasInterner,
+    {
+        self.cast_to(interner)
+    }
+}
+
+impl<T> Cast for T {}
+
+/// The "helper" trait for `cast` that actually implements the
+/// transformations. You can also use this if you want to have
+/// functions that take (e.g.) an `impl CastTo<Goal<_>>` or something
+/// like that.
+pub trait CastTo<T: HasInterner>: Sized {
+    /// Cast a value to type `T`.
+    fn cast_to(self, interner: T::Interner) -> T;
+}
+
+macro_rules! reflexive_impl {
+    (for($($t:tt)*) $u:ty) => {
+        impl<$($t)*> CastTo<$u> for $u {
+            fn cast_to(self, _interner: <$u as HasInterner>::Interner) -> $u {
+                self
+            }
+        }
+    };
+    ($u:ty) => {
+        impl CastTo<$u> for $u {
+            fn cast_to(self, interner: <$u as HasInterner>::Interner) -> $u {
+                self
+            }
+        }
+    };
+}
+
+reflexive_impl!(for(I: Interner) TyKind<I>);
+reflexive_impl!(for(I: Interner) LifetimeData<I>);
+reflexive_impl!(for(I: Interner) ConstData<I>);
+reflexive_impl!(for(I: Interner) TraitRef<I>);
+reflexive_impl!(for(I: Interner) DomainGoal<I>);
+reflexive_impl!(for(I: Interner) Goal<I>);
+reflexive_impl!(for(I: Interner) WhereClause<I>);
+reflexive_impl!(for(I: Interner) ProgramClause<I>);
+reflexive_impl!(for(I: Interner) QuantifiedWhereClause<I>);
+reflexive_impl!(for(I: Interner) VariableKind<I>);
+reflexive_impl!(for(I: Interner) VariableKinds<I>);
+reflexive_impl!(for(I: Interner) CanonicalVarKind<I>);
+reflexive_impl!(for(I: Interner) CanonicalVarKinds<I>);
+reflexive_impl!(for(I: Interner) Constraint<I>);
+
+impl<I: Interner> CastTo<WhereClause<I>> for TraitRef<I> {
+    fn cast_to(self, _interner: I) -> WhereClause<I> {
+        WhereClause::Implemented(self)
+    }
+}
+
+impl<I: Interner> CastTo<WhereClause<I>> for AliasEq<I> {
+    fn cast_to(self, _interner: I) -> WhereClause<I> {
+        WhereClause::AliasEq(self)
+    }
+}
+
+impl<I: Interner> CastTo<WhereClause<I>> for LifetimeOutlives<I> {
+    fn cast_to(self, _interner: I) -> WhereClause<I> {
+        WhereClause::LifetimeOutlives(self)
+    }
+}
+
+impl<I: Interner> CastTo<WhereClause<I>> for TypeOutlives<I> {
+    fn cast_to(self, _interner: I) -> WhereClause<I> {
+        WhereClause::TypeOutlives(self)
+    }
+}
+
+impl<T, I> CastTo<DomainGoal<I>> for T
+where
+    T: CastTo<WhereClause<I>>,
+    I: Interner,
+{
+    fn cast_to(self, interner: I) -> DomainGoal<I> {
+        DomainGoal::Holds(self.cast(interner))
+    }
+}
+
+impl<T, I: Interner> CastTo<Goal<I>> for T
+where
+    T: CastTo<DomainGoal<I>>,
+{
+    fn cast_to(self, interner: I) -> Goal<I> {
+        GoalData::DomainGoal(self.cast(interner)).intern(interner)
+    }
+}
+
+impl<I: Interner> CastTo<DomainGoal<I>> for Normalize<I> {
+    fn cast_to(self, _interner: I) -> DomainGoal<I> {
+        DomainGoal::Normalize(self)
+    }
+}
+
+impl<I: Interner> CastTo<DomainGoal<I>> for WellFormed<I> {
+    fn cast_to(self, _interner: I) -> DomainGoal<I> {
+        DomainGoal::WellFormed(self)
+    }
+}
+
+impl<I: Interner> CastTo<DomainGoal<I>> for FromEnv<I> {
+    fn cast_to(self, _interner: I) -> DomainGoal<I> {
+        DomainGoal::FromEnv(self)
+    }
+}
+
+impl<I: Interner> CastTo<Goal<I>> for EqGoal<I> {
+    fn cast_to(self, interner: I) -> Goal<I> {
+        GoalData::EqGoal(self).intern(interner)
+    }
+}
+
+impl<I: Interner> CastTo<Goal<I>> for SubtypeGoal<I> {
+    fn cast_to(self, interner: I) -> Goal<I> {
+        GoalData::SubtypeGoal(self).intern(interner)
+    }
+}
+
+impl<I: Interner, T: HasInterner<Interner = I> + CastTo<Goal<I>>> CastTo<Goal<I>> for Binders<T> {
+    fn cast_to(self, interner: I) -> Goal<I> {
+        GoalData::Quantified(
+            QuantifierKind::ForAll,
+            self.map(|bound| bound.cast(interner)),
+        )
+        .intern(interner)
+    }
+}
+
+impl<I: Interner> CastTo<TyKind<I>> for AliasTy<I> {
+    fn cast_to(self, _interner: I) -> TyKind<I> {
+        TyKind::Alias(self)
+    }
+}
+
+impl<I: Interner> CastTo<GenericArg<I>> for Ty<I> {
+    fn cast_to(self, interner: I) -> GenericArg<I> {
+        GenericArg::new(interner, GenericArgData::Ty(self))
+    }
+}
+
+impl<I: Interner> CastTo<GenericArg<I>> for Lifetime<I> {
+    fn cast_to(self, interner: I) -> GenericArg<I> {
+        GenericArg::new(interner, GenericArgData::Lifetime(self))
+    }
+}
+
+impl<I: Interner> CastTo<GenericArg<I>> for Const<I> {
+    fn cast_to(self, interner: I) -> GenericArg<I> {
+        GenericArg::new(interner, GenericArgData::Const(self))
+    }
+}
+
+impl<I: Interner> CastTo<GenericArg<I>> for GenericArg<I> {
+    fn cast_to(self, _interner: I) -> GenericArg<I> {
+        self
+    }
+}
+
+impl<T, I> CastTo<ProgramClause<I>> for T
+where
+    T: CastTo<DomainGoal<I>>,
+    I: Interner,
+{
+    fn cast_to(self, interner: I) -> ProgramClause<I> {
+        let implication = ProgramClauseImplication {
+            consequence: self.cast(interner),
+            conditions: Goals::empty(interner),
+            constraints: Constraints::empty(interner),
+            priority: ClausePriority::High,
+        };
+
+        ProgramClauseData(Binders::empty(interner, implication.shifted_in(interner)))
+            .intern(interner)
+    }
+}
+
+impl<I, T> CastTo<ProgramClause<I>> for Binders<T>
+where
+    I: Interner,
+    T: HasInterner<Interner = I> + CastTo<DomainGoal<I>>,
+{
+    fn cast_to(self, interner: I) -> ProgramClause<I> {
+        ProgramClauseData(self.map(|bound| ProgramClauseImplication {
+            consequence: bound.cast(interner),
+            conditions: Goals::empty(interner),
+            constraints: Constraints::empty(interner),
+            priority: ClausePriority::High,
+        }))
+        .intern(interner)
+    }
+}
+
+impl<T, U> CastTo<Option<U>> for Option<T>
+where
+    T: CastTo<U>,
+    U: HasInterner,
+{
+    fn cast_to(self, interner: U::Interner) -> Option<U> {
+        self.map(|v| v.cast(interner))
+    }
+}
+
+impl<T, U, I> CastTo<InEnvironment<U>> for InEnvironment<T>
+where
+    T: HasInterner<Interner = I> + CastTo<U>,
+    U: HasInterner<Interner = I>,
+    I: Interner,
+{
+    fn cast_to(self, interner: U::Interner) -> InEnvironment<U> {
+        self.map(|v| v.cast(interner))
+    }
+}
+
+impl<T, U, E> CastTo<Result<U, E>> for Result<T, E>
+where
+    T: CastTo<U>,
+    U: HasInterner,
+{
+    fn cast_to(self, interner: U::Interner) -> Result<U, E> {
+        self.map(|v| v.cast(interner))
+    }
+}
+
+impl<T> HasInterner for Option<T>
+where
+    T: HasInterner,
+{
+    type Interner = T::Interner;
+}
+
+impl<T, E> HasInterner for Result<T, E>
+where
+    T: HasInterner,
+{
+    type Interner = T::Interner;
+}
+
+impl<T, U> CastTo<Canonical<U>> for Canonical<T>
+where
+    T: CastTo<U> + HasInterner,
+    U: HasInterner<Interner = T::Interner>,
+{
+    fn cast_to(self, interner: T::Interner) -> Canonical<U> {
+        // Subtle point: It should be ok to re-use the binders here,
+        // because `cast()` never introduces new inference variables,
+        // nor changes the "substance" of the type we are working
+        // with. It just introduces new wrapper types.
+        Canonical {
+            value: self.value.cast(interner),
+            binders: self.binders.cast(interner),
+        }
+    }
+}
+
+impl<T, U> CastTo<Vec<U>> for Vec<T>
+where
+    T: CastTo<U> + HasInterner,
+    U: HasInterner,
+{
+    fn cast_to(self, interner: U::Interner) -> Vec<U> {
+        self.into_iter().casted(interner).collect()
+    }
+}
+
+impl<T> CastTo<T> for &T
+where
+    T: Clone + HasInterner,
+{
+    fn cast_to(self, _interner: T::Interner) -> T {
+        self.clone()
+    }
+}
+
+/// An iterator that casts each element to some other type.
+pub struct Casted<IT, U: HasInterner> {
+    interner: U::Interner,
+    iterator: IT,
+    _cast: PhantomData<U>,
+}
+
+impl<IT: Iterator, U> Iterator for Casted<IT, U>
+where
+    IT::Item: CastTo<U>,
+    U: HasInterner,
+{
+    type Item = U;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.iterator.next().map(|item| item.cast_to(self.interner))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iterator.size_hint()
+    }
+}
+
+/// An iterator adapter that casts each element we are iterating over
+/// to some other type.
+pub trait Caster: Iterator + Sized {
+    /// Cast each element in this iterator.
+    fn casted<U>(self, interner: U::Interner) -> Casted<Self, U>
+    where
+        Self::Item: CastTo<U>,
+        U: HasInterner,
+    {
+        Casted {
+            interner,
+            iterator: self,
+            _cast: PhantomData,
+        }
+    }
+}
+
+impl<I> Caster for I where I: Iterator {}
diff --git a/vendor/chalk-ir/src/could_match.rs b/vendor/chalk-ir/src/could_match.rs
new file mode 100644
index 000000000..9f94ff47a
--- /dev/null
+++ b/vendor/chalk-ir/src/could_match.rs
@@ -0,0 +1,213 @@
+//! Fast matching check for zippable values.
+
+use crate::interner::HasInterner;
+use crate::zip::{Zip, Zipper};
+use crate::*;
+
+/// A fast check to see whether two things could ever possibly match.
+pub trait CouldMatch<T: ?Sized + HasInterner> {
+    /// Checks whether `self` and `other` could possibly match.
+    fn could_match(
+        &self,
+        interner: T::Interner,
+        db: &dyn UnificationDatabase<T::Interner>,
+        other: &T,
+    ) -> bool;
+}
+
+#[allow(unreachable_code, unused_variables)]
+impl<T, I> CouldMatch<T> for T
+where
+    T: Zip<I> + ?Sized + HasInterner<Interner = I>,
+    I: Interner,
+{
+    fn could_match(&self, interner: I, db: &dyn UnificationDatabase<I>, other: &T) -> bool {
+        return Zip::zip_with(
+            &mut MatchZipper { interner, db },
+            Variance::Invariant,
+            self,
+            other,
+        )
+        .is_ok();
+
+        struct MatchZipper<'i, I> {
+            interner: I,
+            db: &'i dyn UnificationDatabase<I>,
+        }
+
+        impl<'i, I: Interner> Zipper<I> for MatchZipper<'i, I> {
+            fn zip_tys(&mut self, variance: Variance, a: &Ty<I>, b: &Ty<I>) -> Fallible<()> {
+                let interner = self.interner;
+                let matches = |a: &Substitution<I>, b: &Substitution<I>| {
+                    a.iter(interner)
+                        .zip(b.iter(interner))
+                        .all(|(p_a, p_b)| p_a.could_match(interner, self.db, p_b))
+                };
+                let could_match = match (a.kind(interner), b.kind(interner)) {
+                    (TyKind::Adt(id_a, substitution_a), TyKind::Adt(id_b, substitution_b)) => {
+                        id_a == id_b
+                            && self
+                                .zip_substs(
+                                    variance,
+                                    Some(self.unification_database().adt_variance(*id_a)),
+                                    substitution_a.as_slice(interner),
+                                    substitution_b.as_slice(interner),
+                                )
+                                .is_ok()
+                    }
+                    (
+                        TyKind::AssociatedType(assoc_ty_a, substitution_a),
+                        TyKind::AssociatedType(assoc_ty_b, substitution_b),
+                    ) => assoc_ty_a == assoc_ty_b && matches(substitution_a, substitution_b),
+                    (TyKind::Scalar(scalar_a), TyKind::Scalar(scalar_b)) => scalar_a == scalar_b,
+                    (TyKind::Str, TyKind::Str) => true,
+                    (
+                        TyKind::Tuple(arity_a, substitution_a),
+                        TyKind::Tuple(arity_b, substitution_b),
+                    ) => arity_a == arity_b && matches(substitution_a, substitution_b),
+                    (
+                        TyKind::OpaqueType(opaque_ty_a, substitution_a),
+                        TyKind::OpaqueType(opaque_ty_b, substitution_b),
+                    ) => opaque_ty_a == opaque_ty_b && matches(substitution_a, substitution_b),
+                    (TyKind::Slice(ty_a), TyKind::Slice(ty_b)) => {
+                        ty_a.could_match(interner, self.db, ty_b)
+                    }
+                    (
+                        TyKind::FnDef(fn_def_a, substitution_a),
+                        TyKind::FnDef(fn_def_b, substitution_b),
+                    ) => {
+                        fn_def_a == fn_def_b
+                            && self
+                                .zip_substs(
+                                    variance,
+                                    Some(self.unification_database().fn_def_variance(*fn_def_a)),
+                                    substitution_a.as_slice(interner),
+                                    substitution_b.as_slice(interner),
+                                )
+                                .is_ok()
+                    }
+                    (
+                        TyKind::Ref(mutability_a, lifetime_a, ty_a),
+                        TyKind::Ref(mutability_b, lifetime_b, ty_b),
+                    ) => {
+                        mutability_a == mutability_b
+                            && lifetime_a.could_match(interner, self.db, lifetime_b)
+                            && ty_a.could_match(interner, self.db, ty_b)
+                    }
+                    (TyKind::Raw(mutability_a, ty_a), TyKind::Raw(mutability_b, ty_b)) => {
+                        mutability_a == mutability_b && ty_a.could_match(interner, self.db, ty_b)
+                    }
+                    (TyKind::Never, TyKind::Never) => true,
+                    (TyKind::Array(ty_a, const_a), TyKind::Array(ty_b, const_b)) => {
+                        ty_a.could_match(interner, self.db, ty_b)
+                            && const_a.could_match(interner, self.db, const_b)
+                    }
+                    (
+                        TyKind::Closure(id_a, substitution_a),
+                        TyKind::Closure(id_b, substitution_b),
+                    ) => id_a == id_b && matches(substitution_a, substitution_b),
+                    (
+                        TyKind::Generator(generator_a, substitution_a),
+                        TyKind::Generator(generator_b, substitution_b),
+                    ) => {
+                        generator_a == generator_b
+                            && self
+                                .zip_substs(
+                                    variance,
+                                    None,
+                                    substitution_a.as_slice(interner),
+                                    substitution_b.as_slice(interner),
+                                )
+                                .is_ok()
+                    }
+                    (
+                        TyKind::GeneratorWitness(generator_a, substitution_a),
+                        TyKind::GeneratorWitness(generator_b, substitution_b),
+                    ) => {
+                        generator_a == generator_b
+                            && self
+                                .zip_substs(
+                                    variance,
+                                    None,
+                                    substitution_a.as_slice(interner),
+                                    substitution_b.as_slice(interner),
+                                )
+                                .is_ok()
+                    }
+                    (TyKind::Foreign(foreign_ty_a), TyKind::Foreign(foreign_ty_b)) => {
+                        foreign_ty_a == foreign_ty_b
+                    }
+                    (TyKind::Error, TyKind::Error) => true,
+
+                    _ => true,
+                };
+
+                if could_match {
+                    Ok(())
+                } else {
+                    Err(NoSolution)
+                }
+            }
+
+            fn zip_lifetimes(
+                &mut self,
+                variance: Variance,
+                _: &Lifetime<I>,
+                _: &Lifetime<I>,
+            ) -> Fallible<()> {
+                Ok(())
+            }
+
+            fn zip_consts(
+                &mut self,
+                variance: Variance,
+                _: &Const<I>,
+                _: &Const<I>,
+            ) -> Fallible<()> {
+                Ok(())
+            }
+
+            fn zip_binders<T>(
+                &mut self,
+                variance: Variance,
+                a: &Binders<T>,
+                b: &Binders<T>,
+            ) -> Fallible<()>
+            where
+                T: HasInterner + Zip<I>,
+            {
+                Zip::zip_with(self, variance, &a.value, &b.value)
+            }
+
+            fn interner(&self) -> I {
+                self.interner
+            }
+
+            fn unification_database(&self) -> &dyn UnificationDatabase<I> {
+                self.db
+            }
+        }
+    }
+}
+
+impl<I: Interner> CouldMatch<DomainGoal<I>> for ProgramClauseData<I> {
+    fn could_match(
+        &self,
+        interner: I,
+        db: &dyn UnificationDatabase<I>,
+        other: &DomainGoal<I>,
+    ) -> bool {
+        self.0.value.consequence.could_match(interner, db, other)
+    }
+}
+
+impl<I: Interner> CouldMatch<DomainGoal<I>> for ProgramClause<I> {
+    fn could_match(
+        &self,
+        interner: I,
+        db: &dyn UnificationDatabase<I>,
+        other: &DomainGoal<I>,
+    ) -> bool {
+        self.data(interner).could_match(interner, db, other)
+    }
+}
diff --git a/vendor/chalk-ir/src/debug.rs b/vendor/chalk-ir/src/debug.rs
new file mode 100644
index 000000000..76d66edf5
--- /dev/null
+++ b/vendor/chalk-ir/src/debug.rs
@@ -0,0 +1,1016 @@
+//! Debug impls for types.
+
+use std::fmt::{self, Debug, Display, Error, Formatter};
+
+use super::*;
+
+/// Wrapper to allow forwarding to `Display::fmt`, `Debug::fmt`, etc.
+pub struct Fmt<F>(pub F)
+where
+    F: Fn(&mut fmt::Formatter<'_>) -> fmt::Result;
+
+impl<F> fmt::Display for Fmt<F>
+where
+    F: Fn(&mut fmt::Formatter<'_>) -> fmt::Result,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (self.0)(f)
+    }
+}
+
+impl<I: Interner> Debug for TraitId<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_trait_id(*self, fmt).unwrap_or_else(|| write!(fmt, "TraitId({:?})", self.0))
+    }
+}
+
+impl<I: Interner> Debug for AdtId<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_adt_id(*self, fmt).unwrap_or_else(|| write!(fmt, "AdtId({:?})", self.0))
+    }
+}
+
+impl<I: Interner> Debug for AssocTypeId<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_assoc_type_id(*self, fmt)
+            .unwrap_or_else(|| write!(fmt, "AssocTypeId({:?})", self.0))
+    }
+}
+
+impl<I: Interner> Debug for FnDefId<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> std::fmt::Result {
+        I::debug_fn_def_id(*self, fmt).unwrap_or_else(|| write!(fmt, "FnDefId({:?})", self.0))
+    }
+}
+
+impl<I: Interner> Debug for ClosureId<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> std::fmt::Result {
+        I::debug_closure_id(*self, fmt).unwrap_or_else(|| write!(fmt, "ClosureId({:?})", self.0))
+    }
+}
+
+impl<I: Interner> Debug for GeneratorId<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> std::fmt::Result {
+        I::debug_generator_id(*self, fmt)
+            .unwrap_or_else(|| write!(fmt, "GeneratorId({:?})", self.0))
+    }
+}
+
+impl<I: Interner> Debug for ForeignDefId<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> std::fmt::Result {
+        I::debug_foreign_def_id(*self, fmt)
+            .unwrap_or_else(|| write!(fmt, "ForeignDefId({:?})", self.0))
+    }
+}
+
+impl<I: Interner> Debug for Ty<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_ty(self, fmt).unwrap_or_else(|| write!(fmt, "{:?}", self.interned))
+    }
+}
+
+impl<I: Interner> Debug for Lifetime<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_lifetime(self, fmt).unwrap_or_else(|| write!(fmt, "{:?}", self.interned))
+    }
+}
+
+impl<I: Interner> Debug for Const<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_const(self, fmt).unwrap_or_else(|| write!(fmt, "{:?}", self.interned))
+    }
+}
+
+impl<I: Interner> Debug for ConcreteConst<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        write!(fmt, "{:?}", self.interned)
+    }
+}
+
+impl<I: Interner> Debug for GenericArg<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_generic_arg(self, fmt).unwrap_or_else(|| write!(fmt, "{:?}", self.interned))
+    }
+}
+
+impl<I: Interner> Debug for Goal<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_goal(self, fmt).unwrap_or_else(|| write!(fmt, "{:?}", self.interned))
+    }
+}
+
+impl<I: Interner> Debug for Goals<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_goals(self, fmt).unwrap_or_else(|| write!(fmt, "{:?}", self.interned))
+    }
+}
+
+impl<I: Interner> Debug for ProgramClauseImplication<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_program_clause_implication(self, fmt)
+            .unwrap_or_else(|| write!(fmt, "ProgramClauseImplication(?)"))
+    }
+}
+
+impl<I: Interner> Debug for ProgramClause<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_program_clause(self, fmt).unwrap_or_else(|| write!(fmt, "{:?}", self.interned))
+    }
+}
+
+impl<I: Interner> Debug for ProgramClauses<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_program_clauses(self, fmt).unwrap_or_else(|| write!(fmt, "{:?}", self.interned))
+    }
+}
+
+impl<I: Interner> Debug for Constraints<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_constraints(self, fmt).unwrap_or_else(|| write!(fmt, "{:?}", self.interned))
+    }
+}
+
+impl<I: Interner> Debug for SeparatorTraitRef<'_, I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_separator_trait_ref(self, fmt)
+            .unwrap_or_else(|| write!(fmt, "SeparatorTraitRef(?)"))
+    }
+}
+
+impl<I: Interner> Debug for AliasTy<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_alias(self, fmt).unwrap_or_else(|| write!(fmt, "AliasTy(?)"))
+    }
+}
+
+impl<I: Interner> Debug for QuantifiedWhereClauses<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_quantified_where_clauses(self, fmt)
+            .unwrap_or_else(|| write!(fmt, "{:?}", self.interned))
+    }
+}
+
+impl<I: Interner> Debug for ProjectionTy<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_projection_ty(self, fmt).unwrap_or_else(|| {
+            unimplemented!("cannot format ProjectionTy without setting Program in tls")
+        })
+    }
+}
+
+impl<I: Interner> Debug for OpaqueTy<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_opaque_ty(self, fmt).unwrap_or_else(|| {
+            unimplemented!("cannot format OpaqueTy without setting Program in tls")
+        })
+    }
+}
+
+impl<I: Interner> Display for Substitution<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_substitution(self, fmt).unwrap_or_else(|| write!(fmt, "{:?}", self.interned))
+    }
+}
+
+impl<I: Interner> Debug for OpaqueTyId<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_opaque_ty_id(*self, fmt).unwrap_or_else(|| write!(fmt, "OpaqueTyId({:?})", self.0))
+    }
+}
+
+impl Display for UniverseIndex {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        write!(fmt, "U{}", self.counter)
+    }
+}
+
+impl Debug for UniverseIndex {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        write!(fmt, "U{}", self.counter)
+    }
+}
+
+impl<I: Interner> Debug for TyData<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        self.kind.fmt(fmt)
+    }
+}
+
+impl<I: Interner> Debug for TyKind<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        match self {
+            TyKind::BoundVar(db) => write!(fmt, "{:?}", db),
+            TyKind::Dyn(clauses) => write!(fmt, "{:?}", clauses),
+            TyKind::InferenceVar(var, TyVariableKind::General) => write!(fmt, "{:?}", var),
+            TyKind::InferenceVar(var, TyVariableKind::Integer) => write!(fmt, "{:?}i", var),
+            TyKind::InferenceVar(var, TyVariableKind::Float) => write!(fmt, "{:?}f", var),
+            TyKind::Alias(alias) => write!(fmt, "{:?}", alias),
+            TyKind::Placeholder(index) => write!(fmt, "{:?}", index),
+            TyKind::Function(function) => write!(fmt, "{:?}", function),
+            TyKind::Adt(id, substitution) => write!(fmt, "{:?}<{:?}>", id, substitution),
+            TyKind::AssociatedType(assoc_ty, substitution) => {
+                write!(fmt, "{:?}<{:?}>", assoc_ty, substitution)
+            }
+            TyKind::Scalar(scalar) => write!(fmt, "{:?}", scalar),
+            TyKind::Str => write!(fmt, "Str"),
+            TyKind::Tuple(arity, substitution) => write!(fmt, "{:?}<{:?}>", arity, substitution),
+            TyKind::OpaqueType(opaque_ty, substitution) => {
+                write!(fmt, "!{:?}<{:?}>", opaque_ty, substitution)
+            }
+            TyKind::Slice(substitution) => write!(fmt, "{{slice}}<{:?}>", substitution),
+            TyKind::FnDef(fn_def, substitution) => write!(fmt, "{:?}<{:?}>", fn_def, substitution),
+            TyKind::Ref(mutability, lifetime, ty) => match mutability {
+                Mutability::Mut => write!(fmt, "(&{:?} mut {:?})", lifetime, ty),
+                Mutability::Not => write!(fmt, "(&{:?} {:?})", lifetime, ty),
+            },
+            TyKind::Raw(mutability, ty) => match mutability {
+                Mutability::Mut => write!(fmt, "(*mut {:?})", ty),
+                Mutability::Not => write!(fmt, "(*const {:?})", ty),
+            },
+            TyKind::Never => write!(fmt, "Never"),
+            TyKind::Array(ty, const_) => write!(fmt, "[{:?}; {:?}]", ty, const_),
+            TyKind::Closure(id, substitution) => {
+                write!(fmt, "{{closure:{:?}}}<{:?}>", id, substitution)
+            }
+            TyKind::Generator(generator, substitution) => {
+                write!(fmt, "{:?}<{:?}>", generator, substitution)
+            }
+            TyKind::GeneratorWitness(witness, substitution) => {
+                write!(fmt, "{:?}<{:?}>", witness, substitution)
+            }
+            TyKind::Foreign(foreign_ty) => write!(fmt, "{:?}", foreign_ty),
+            TyKind::Error => write!(fmt, "{{error}}"),
+        }
+    }
+}
+
+impl Debug for BoundVar {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        let BoundVar { debruijn, index } = self;
+        write!(fmt, "{:?}.{:?}", debruijn, index)
+    }
+}
+
+impl Debug for DebruijnIndex {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        let DebruijnIndex { depth } = self;
+        write!(fmt, "^{}", depth)
+    }
+}
+
+impl<I: Interner> Debug for DynTy<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        let DynTy { bounds, lifetime } = self;
+        write!(fmt, "dyn {:?} + {:?}", bounds, lifetime)
+    }
+}
+
+impl Debug for InferenceVar {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        write!(fmt, "?{}", self.index)
+    }
+}
+
+impl<I: Interner> Debug for FnSubst<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        write!(fmt, "{:?}", self.0)
+    }
+}
+
+impl<I: Interner> Debug for FnPointer<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        // FIXME -- we should introduce some names or something here
+        let FnPointer {
+            num_binders,
+            substitution,
+            sig,
+        } = self;
+        write!(
+            fmt,
+            "{}{:?} for<{}> {:?}",
+            match sig.safety {
+                Safety::Unsafe => "unsafe ",
+                Safety::Safe => "",
+            },
+            sig.abi,
+            num_binders,
+            substitution
+        )
+    }
+}
+
+impl<I: Interner> Debug for LifetimeData<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        match self {
+            LifetimeData::BoundVar(db) => write!(fmt, "'{:?}", db),
+            LifetimeData::InferenceVar(var) => write!(fmt, "'{:?}", var),
+            LifetimeData::Placeholder(index) => write!(fmt, "'{:?}", index),
+            LifetimeData::Static => write!(fmt, "'static"),
+            LifetimeData::Erased => write!(fmt, "'<erased>"),
+            LifetimeData::Phantom(..) => unreachable!(),
+        }
+    }
+}
+
+impl<I: Interner> VariableKinds<I> {
+    fn debug(&self) -> VariableKindsDebug<'_, I> {
+        VariableKindsDebug(self)
+    }
+
+    /// Helper method for debugging variable kinds.
+    pub fn inner_debug(&self, interner: I) -> VariableKindsInnerDebug<'_, I> {
+        VariableKindsInnerDebug {
+            variable_kinds: self,
+            interner,
+        }
+    }
+}
+
+struct VariableKindsDebug<'a, I: Interner>(&'a VariableKinds<I>);
+
+impl<'a, I: Interner> Debug for VariableKindsDebug<'a, I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_variable_kinds_with_angles(self.0, fmt)
+            .unwrap_or_else(|| write!(fmt, "{:?}", self.0.interned))
+    }
+}
+
+/// Helper struct for showing debug output for `VariableKinds`.
+pub struct VariableKindsInnerDebug<'a, I: Interner> {
+    variable_kinds: &'a VariableKinds<I>,
+    interner: I,
+}
+
+impl<'a, I: Interner> Debug for VariableKindsInnerDebug<'a, I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        // NB: We print variable kinds as a list delimited by `<>`,
+        // like `<K1, K2, ..>`. This is because variable kind lists
+        // are always associated with binders like `forall<type> {
+        // ... }`.
+        write!(fmt, "<")?;
+        for (index, binder) in self.variable_kinds.iter(self.interner).enumerate() {
+            if index > 0 {
+                write!(fmt, ", ")?;
+            }
+            match binder {
+                VariableKind::Ty(TyVariableKind::General) => write!(fmt, "type")?,
+                VariableKind::Ty(TyVariableKind::Integer) => write!(fmt, "integer type")?,
+                VariableKind::Ty(TyVariableKind::Float) => write!(fmt, "float type")?,
+                VariableKind::Lifetime => write!(fmt, "lifetime")?,
+                VariableKind::Const(ty) => write!(fmt, "const: {:?}", ty)?,
+            }
+        }
+        write!(fmt, ">")
+    }
+}
+
+impl<I: Interner> Debug for ConstData<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        match &self.value {
+            ConstValue::BoundVar(db) => write!(fmt, "{:?}", db),
+            ConstValue::InferenceVar(var) => write!(fmt, "{:?}", var),
+            ConstValue::Placeholder(index) => write!(fmt, "{:?}", index),
+            ConstValue::Concrete(evaluated) => write!(fmt, "{:?}", evaluated),
+        }
+    }
+}
+
+impl<I: Interner> Debug for GoalData<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        match self {
+            GoalData::Quantified(qkind, ref subgoal) => write!(
+                fmt,
+                "{:?}{:?} {{ {:?} }}",
+                qkind,
+                subgoal.binders.debug(),
+                subgoal.value
+            ),
+            GoalData::Implies(ref wc, ref g) => write!(fmt, "if ({:?}) {{ {:?} }}", wc, g),
+            GoalData::All(ref goals) => write!(fmt, "all{:?}", goals),
+            GoalData::Not(ref g) => write!(fmt, "not {{ {:?} }}", g),
+            GoalData::EqGoal(ref wc) => write!(fmt, "{:?}", wc),
+            GoalData::SubtypeGoal(ref wc) => write!(fmt, "{:?}", wc),
+            GoalData::DomainGoal(ref wc) => write!(fmt, "{:?}", wc),
+            GoalData::CannotProve => write!(fmt, r"¯\_(ツ)_/¯"),
+        }
+    }
+}
+
+/// Helper struct for showing debug output for `Goals`.
+pub struct GoalsDebug<'a, I: Interner> {
+    goals: &'a Goals<I>,
+    interner: I,
+}
+
+impl<'a, I: Interner> Debug for GoalsDebug<'a, I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        write!(fmt, "(")?;
+        for (goal, index) in self.goals.iter(self.interner).zip(0..) {
+            if index > 0 {
+                write!(fmt, ", ")?;
+            }
+            write!(fmt, "{:?}", goal)?;
+        }
+        write!(fmt, ")")?;
+        Ok(())
+    }
+}
+
+impl<I: Interner> Goals<I> {
+    /// Show debug output for `Goals`.
+    pub fn debug(&self, interner: I) -> GoalsDebug<'_, I> {
+        GoalsDebug {
+            goals: self,
+            interner,
+        }
+    }
+}
+
+/// Helper struct for showing debug output for `GenericArgData`.
+pub struct GenericArgDataInnerDebug<'a, I: Interner>(&'a GenericArgData<I>);
+
+impl<'a, I: Interner> Debug for GenericArgDataInnerDebug<'a, I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        match self.0 {
+            GenericArgData::Ty(n) => write!(fmt, "{:?}", n),
+            GenericArgData::Lifetime(n) => write!(fmt, "{:?}", n),
+            GenericArgData::Const(n) => write!(fmt, "{:?}", n),
+        }
+    }
+}
+
+impl<I: Interner> GenericArgData<I> {
+    /// Helper method for debugging `GenericArgData`.
+    pub fn inner_debug(&self) -> GenericArgDataInnerDebug<'_, I> {
+        GenericArgDataInnerDebug(self)
+    }
+}
+
+/// Helper struct for showing debug output for program clause implications.
+pub struct ProgramClauseImplicationDebug<'a, I: Interner> {
+    pci: &'a ProgramClauseImplication<I>,
+    interner: I,
+}
+
+impl<'a, I: Interner> Debug for ProgramClauseImplicationDebug<'a, I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        let ProgramClauseImplicationDebug { pci, interner } = self;
+        write!(fmt, "{:?}", pci.consequence)?;
+
+        let conditions = pci.conditions.as_slice(*interner);
+
+        let conds = conditions.len();
+        if conds == 0 {
+            return Ok(());
+        }
+
+        write!(fmt, " :- ")?;
+        for cond in &conditions[..conds - 1] {
+            write!(fmt, "{:?}, ", cond)?;
+        }
+        write!(fmt, "{:?}", conditions[conds - 1])
+    }
+}
+
+impl<I: Interner> ProgramClauseImplication<I> {
+    /// Show debug output for the program clause implication.
+    pub fn debug(&self, interner: I) -> ProgramClauseImplicationDebug<'_, I> {
+        ProgramClauseImplicationDebug {
+            pci: self,
+            interner,
+        }
+    }
+}
+
+/// Helper struct for showing debug output for application types.
+pub struct TyKindDebug<'a, I: Interner> {
+    ty: &'a TyKind<I>,
+    interner: I,
+}
+
+impl<'a, I: Interner> Debug for TyKindDebug<'a, I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        let interner = self.interner;
+        match self.ty {
+            TyKind::BoundVar(db) => write!(fmt, "{:?}", db),
+            TyKind::Dyn(clauses) => write!(fmt, "{:?}", clauses),
+            TyKind::InferenceVar(var, TyVariableKind::General) => write!(fmt, "{:?}", var),
+            TyKind::InferenceVar(var, TyVariableKind::Integer) => write!(fmt, "{:?}i", var),
+            TyKind::InferenceVar(var, TyVariableKind::Float) => write!(fmt, "{:?}f", var),
+            TyKind::Alias(alias) => write!(fmt, "{:?}", alias),
+            TyKind::Placeholder(index) => write!(fmt, "{:?}", index),
+            TyKind::Function(function) => write!(fmt, "{:?}", function),
+            TyKind::Adt(id, substitution) => {
+                write!(fmt, "{:?}{:?}", id, substitution.with_angle(interner))
+            }
+            TyKind::AssociatedType(assoc_ty, substitution) => {
+                write!(fmt, "{:?}{:?}", assoc_ty, substitution.with_angle(interner))
+            }
+            TyKind::Scalar(scalar) => write!(fmt, "{:?}", scalar),
+            TyKind::Str => write!(fmt, "Str"),
+            TyKind::Tuple(arity, substitution) => {
+                write!(fmt, "{:?}{:?}", arity, substitution.with_angle(interner))
+            }
+            TyKind::OpaqueType(opaque_ty, substitution) => write!(
+                fmt,
+                "!{:?}{:?}",
+                opaque_ty,
+                substitution.with_angle(interner)
+            ),
+            TyKind::Slice(ty) => write!(fmt, "[{:?}]", ty),
+            TyKind::FnDef(fn_def, substitution) => {
+                write!(fmt, "{:?}{:?}", fn_def, substitution.with_angle(interner))
+            }
+            TyKind::Ref(mutability, lifetime, ty) => match mutability {
+                Mutability::Mut => write!(fmt, "(&{:?} mut {:?})", lifetime, ty),
+                Mutability::Not => write!(fmt, "(&{:?} {:?})", lifetime, ty),
+            },
+            TyKind::Raw(mutability, ty) => match mutability {
+                Mutability::Mut => write!(fmt, "(*mut {:?})", ty),
+                Mutability::Not => write!(fmt, "(*const {:?})", ty),
+            },
+            TyKind::Never => write!(fmt, "Never"),
+            TyKind::Array(ty, const_) => write!(fmt, "[{:?}; {:?}]", ty, const_),
+            TyKind::Closure(id, substitution) => write!(
+                fmt,
+                "{{closure:{:?}}}{:?}",
+                id,
+                substitution.with_angle(interner)
+            ),
+            TyKind::Generator(generator, substitution) => write!(
+                fmt,
+                "{:?}{:?}",
+                generator,
+                substitution.with_angle(interner)
+            ),
+            TyKind::GeneratorWitness(witness, substitution) => {
+                write!(fmt, "{:?}{:?}", witness, substitution.with_angle(interner))
+            }
+            TyKind::Foreign(foreign_ty) => write!(fmt, "{:?}", foreign_ty,),
+            TyKind::Error => write!(fmt, "{{error}}"),
+        }
+    }
+}
+
+impl<I: Interner> TyKind<I> {
+    /// Show debug output for the application type.
+    pub fn debug(&self, interner: I) -> TyKindDebug<'_, I> {
+        TyKindDebug { ty: self, interner }
+    }
+}
+
+/// Helper struct for showing debug output for substitutions.
+pub struct SubstitutionDebug<'a, I: Interner> {
+    substitution: &'a Substitution<I>,
+    interner: I,
+}
+
+impl<'a, I: Interner> Debug for SubstitutionDebug<'a, I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        let SubstitutionDebug {
+            substitution,
+            interner,
+        } = self;
+        let mut first = true;
+
+        write!(fmt, "[")?;
+
+        for (index, value) in substitution.iter(*interner).enumerate() {
+            if first {
+                first = false;
+            } else {
+                write!(fmt, ", ")?;
+            }
+
+            write!(fmt, "?{} := {:?}", index, value)?;
+        }
+
+        write!(fmt, "]")?;
+
+        Ok(())
+    }
+}
+
+impl<I: Interner> Substitution<I> {
+    /// Show debug output for the substitution.
+    pub fn debug(&self, interner: I) -> SubstitutionDebug<'_, I> {
+        SubstitutionDebug {
+            substitution: self,
+            interner,
+        }
+    }
+}
+
+impl Debug for PlaceholderIndex {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        let PlaceholderIndex { ui, idx } = self;
+        write!(fmt, "!{}_{}", ui.counter, idx)
+    }
+}
+
+impl<I: Interner> TraitRef<I> {
+    /// Returns a "Debuggable" type that prints like `P0 as Trait<P1..>`.
+    pub fn with_as(&self) -> impl std::fmt::Debug + '_ {
+        SeparatorTraitRef {
+            trait_ref: self,
+            separator: " as ",
+        }
+    }
+
+    /// Returns a "Debuggable" type that prints like `P0: Trait<P1..>`.
+    pub fn with_colon(&self) -> impl std::fmt::Debug + '_ {
+        SeparatorTraitRef {
+            trait_ref: self,
+            separator: ": ",
+        }
+    }
+}
+
+impl<I: Interner> Debug for TraitRef<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        Debug::fmt(&self.with_as(), fmt)
+    }
+}
+
+/// Trait ref with associated separator used for debug output.
+pub struct SeparatorTraitRef<'me, I: Interner> {
+    /// The `TraitRef` itself.
+    pub trait_ref: &'me TraitRef<I>,
+
+    /// The separator used for displaying the `TraitRef`.
+    pub separator: &'me str,
+}
+
+/// Helper struct for showing debug output for the `SeperatorTraitRef`.
+pub struct SeparatorTraitRefDebug<'a, 'me, I: Interner> {
+    separator_trait_ref: &'a SeparatorTraitRef<'me, I>,
+    interner: I,
+}
+
+impl<'a, 'me, I: Interner> Debug for SeparatorTraitRefDebug<'a, 'me, I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        let SeparatorTraitRefDebug {
+            separator_trait_ref,
+            interner,
+        } = self;
+        let parameters = separator_trait_ref
+            .trait_ref
+            .substitution
+            .as_slice(*interner);
+        write!(
+            fmt,
+            "{:?}{}{:?}{:?}",
+            parameters[0],
+            separator_trait_ref.separator,
+            separator_trait_ref.trait_ref.trait_id,
+            Angle(&parameters[1..])
+        )
+    }
+}
+
+impl<'me, I: Interner> SeparatorTraitRef<'me, I> {
+    /// Show debug output for the `SeperatorTraitRef`.
+    pub fn debug<'a>(&'a self, interner: I) -> SeparatorTraitRefDebug<'a, 'me, I> {
+        SeparatorTraitRefDebug {
+            separator_trait_ref: self,
+            interner,
+        }
+    }
+}
+
+impl<I: Interner> Debug for LifetimeOutlives<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        write!(fmt, "{:?}: {:?}", self.a, self.b)
+    }
+}
+
+impl<I: Interner> Debug for TypeOutlives<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        write!(fmt, "{:?}: {:?}", self.ty, self.lifetime)
+    }
+}
+
+/// Helper struct for showing debug output for projection types.
+pub struct ProjectionTyDebug<'a, I: Interner> {
+    projection_ty: &'a ProjectionTy<I>,
+    interner: I,
+}
+
+impl<'a, I: Interner> Debug for ProjectionTyDebug<'a, I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        let ProjectionTyDebug {
+            projection_ty,
+            interner,
+        } = self;
+        write!(
+            fmt,
+            "({:?}){:?}",
+            projection_ty.associated_ty_id,
+            projection_ty.substitution.with_angle(*interner)
+        )
+    }
+}
+
+impl<I: Interner> ProjectionTy<I> {
+    /// Show debug output for the projection type.
+    pub fn debug(&self, interner: I) -> ProjectionTyDebug<'_, I> {
+        ProjectionTyDebug {
+            projection_ty: self,
+            interner,
+        }
+    }
+}
+
+/// Helper struct for showing debug output for opaque types.
+pub struct OpaqueTyDebug<'a, I: Interner> {
+    opaque_ty: &'a OpaqueTy<I>,
+    interner: I,
+}
+
+impl<'a, I: Interner> Debug for OpaqueTyDebug<'a, I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        let OpaqueTyDebug {
+            opaque_ty,
+            interner,
+        } = self;
+        write!(
+            fmt,
+            "{:?}{:?}",
+            opaque_ty.opaque_ty_id,
+            opaque_ty.substitution.with_angle(*interner)
+        )
+    }
+}
+
+impl<I: Interner> OpaqueTy<I> {
+    /// Show debug output for the opaque type.
+    pub fn debug(&self, interner: I) -> OpaqueTyDebug<'_, I> {
+        OpaqueTyDebug {
+            opaque_ty: self,
+            interner,
+        }
+    }
+}
+
+/// Wraps debug output in angle brackets (`<>`).
+pub struct Angle<'a, T>(pub &'a [T]);
+
+impl<'a, T: Debug> Debug for Angle<'a, T> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        if !self.0.is_empty() {
+            write!(fmt, "<")?;
+            for (index, elem) in self.0.iter().enumerate() {
+                if index > 0 {
+                    write!(fmt, ", {:?}", elem)?;
+                } else {
+                    write!(fmt, "{:?}", elem)?;
+                }
+            }
+            write!(fmt, ">")?;
+        }
+        Ok(())
+    }
+}
+
+impl<I: Interner> Debug for Normalize<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        write!(fmt, "Normalize({:?} -> {:?})", self.alias, self.ty)
+    }
+}
+
+impl<I: Interner> Debug for AliasEq<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        write!(fmt, "AliasEq({:?} = {:?})", self.alias, self.ty)
+    }
+}
+
+impl<I: Interner> Debug for WhereClause<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        match self {
+            WhereClause::Implemented(tr) => write!(fmt, "Implemented({:?})", tr.with_colon()),
+            WhereClause::AliasEq(a) => write!(fmt, "{:?}", a),
+            WhereClause::LifetimeOutlives(l_o) => write!(fmt, "{:?}", l_o),
+            WhereClause::TypeOutlives(t_o) => write!(fmt, "{:?}", t_o),
+        }
+    }
+}
+
+impl<I: Interner> Debug for FromEnv<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        match self {
+            FromEnv::Trait(t) => write!(fmt, "FromEnv({:?})", t.with_colon()),
+            FromEnv::Ty(t) => write!(fmt, "FromEnv({:?})", t),
+        }
+    }
+}
+
+impl<I: Interner> Debug for WellFormed<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        match self {
+            WellFormed::Trait(t) => write!(fmt, "WellFormed({:?})", t.with_colon()),
+            WellFormed::Ty(t) => write!(fmt, "WellFormed({:?})", t),
+        }
+    }
+}
+
+impl<I: Interner> Debug for DomainGoal<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        match self {
+            DomainGoal::Holds(n) => write!(fmt, "{:?}", n),
+            DomainGoal::WellFormed(n) => write!(fmt, "{:?}", n),
+            DomainGoal::FromEnv(n) => write!(fmt, "{:?}", n),
+            DomainGoal::Normalize(n) => write!(fmt, "{:?}", n),
+            DomainGoal::IsLocal(n) => write!(fmt, "IsLocal({:?})", n),
+            DomainGoal::IsUpstream(n) => write!(fmt, "IsUpstream({:?})", n),
+            DomainGoal::IsFullyVisible(n) => write!(fmt, "IsFullyVisible({:?})", n),
+            DomainGoal::LocalImplAllowed(tr) => {
+                write!(fmt, "LocalImplAllowed({:?})", tr.with_colon(),)
+            }
+            DomainGoal::Compatible => write!(fmt, "Compatible"),
+            DomainGoal::DownstreamType(n) => write!(fmt, "DownstreamType({:?})", n),
+            DomainGoal::Reveal => write!(fmt, "Reveal"),
+            DomainGoal::ObjectSafe(n) => write!(fmt, "ObjectSafe({:?})", n),
+        }
+    }
+}
+
+impl<I: Interner> Debug for EqGoal<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        write!(fmt, "({:?} = {:?})", self.a, self.b)
+    }
+}
+
+impl<I: Interner> Debug for SubtypeGoal<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        write!(fmt, "({:?} <: {:?})", self.a, self.b)
+    }
+}
+
+impl<T: HasInterner + Debug> Debug for Binders<T> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        let Binders {
+            ref binders,
+            ref value,
+        } = *self;
+        write!(fmt, "for{:?} ", binders.debug())?;
+        Debug::fmt(value, fmt)
+    }
+}
+
+impl<I: Interner> Debug for ProgramClauseData<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        write!(fmt, "{:?}", self.0)
+    }
+}
+
+impl<I: Interner> Debug for Environment<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        write!(fmt, "Env({:?})", self.clauses)
+    }
+}
+
+impl<I: Interner> Debug for CanonicalVarKinds<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_canonical_var_kinds(self, fmt)
+            .unwrap_or_else(|| write!(fmt, "{:?}", self.interned))
+    }
+}
+
+impl<T: HasInterner + Display> Canonical<T> {
+    /// Display the canonicalized item.
+    pub fn display(&self, interner: T::Interner) -> CanonicalDisplay<'_, T> {
+        CanonicalDisplay {
+            canonical: self,
+            interner,
+        }
+    }
+}
+
+/// Helper struct for displaying canonicalized items.
+pub struct CanonicalDisplay<'a, T: HasInterner> {
+    canonical: &'a Canonical<T>,
+    interner: T::Interner,
+}
+
+impl<'a, T: HasInterner + Display> Display for CanonicalDisplay<'a, T> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error> {
+        let Canonical { binders, value } = self.canonical;
+        let interner = self.interner;
+        let binders = binders.as_slice(interner);
+        if binders.is_empty() {
+            // Ordinarily, we try to print all binder levels, if they
+            // are empty, but we can skip in this *particular* case
+            // because we know that `Canonical` terms are never
+            // supposed to contain free variables.  In other words,
+            // all "bound variables" that appear inside the canonical
+            // value must reference values that appear in `binders`.
+            write!(f, "{}", value)?;
+        } else {
+            write!(f, "for<")?;
+
+            for (i, pk) in binders.iter().enumerate() {
+                if i > 0 {
+                    write!(f, ",")?;
+                }
+                write!(f, "?{}", pk.skip_kind())?;
+            }
+
+            write!(f, "> {{ {} }}", value)?;
+        }
+
+        Ok(())
+    }
+}
+
+impl<I: Interner> Debug for GenericArgData<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        match self {
+            GenericArgData::Ty(t) => write!(fmt, "Ty({:?})", t),
+            GenericArgData::Lifetime(l) => write!(fmt, "Lifetime({:?})", l),
+            GenericArgData::Const(c) => write!(fmt, "Const({:?})", c),
+        }
+    }
+}
+
+impl<I: Interner> Debug for VariableKind<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        match self {
+            VariableKind::Ty(TyVariableKind::General) => write!(fmt, "type"),
+            VariableKind::Ty(TyVariableKind::Integer) => write!(fmt, "integer type"),
+            VariableKind::Ty(TyVariableKind::Float) => write!(fmt, "float type"),
+            VariableKind::Lifetime => write!(fmt, "lifetime"),
+            VariableKind::Const(ty) => write!(fmt, "const: {:?}", ty),
+        }
+    }
+}
+
+impl<I: Interner, T: Debug> Debug for WithKind<I, T> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        let value = self.skip_kind();
+        match &self.kind {
+            VariableKind::Ty(TyVariableKind::General) => write!(fmt, "{:?} with kind type", value),
+            VariableKind::Ty(TyVariableKind::Integer) => {
+                write!(fmt, "{:?} with kind integer type", value)
+            }
+            VariableKind::Ty(TyVariableKind::Float) => {
+                write!(fmt, "{:?} with kind float type", value)
+            }
+            VariableKind::Lifetime => write!(fmt, "{:?} with kind lifetime", value),
+            VariableKind::Const(ty) => write!(fmt, "{:?} with kind {:?}", value, ty),
+        }
+    }
+}
+
+impl<I: Interner> Debug for Constraint<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        match self {
+            Constraint::LifetimeOutlives(a, b) => write!(fmt, "{:?}: {:?}", a, b),
+            Constraint::TypeOutlives(ty, lifetime) => write!(fmt, "{:?}: {:?}", ty, lifetime),
+        }
+    }
+}
+
+impl<I: Interner> Display for ConstrainedSubst<I> {
+    #[rustfmt::skip]
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error> {
+        let ConstrainedSubst { subst, constraints } = self;
+
+        let mut first = true;
+
+        let subst = format!("{}", Fmt(|f| Display::fmt(subst, f)));
+        if subst != "[]" {
+            write!(f, "substitution {}", subst)?;
+            first = false;
+        }
+
+        let constraints = format!("{}", Fmt(|f| Debug::fmt(constraints, f)));
+        if constraints != "[]" {
+            if !first { write!(f, ", ")?; }
+            write!(f, "lifetime constraints {}", constraints)?;
+            first = false;
+        }
+
+        let _ = first;
+        Ok(())
+    }
+}
+
+impl<I: Interner> Substitution<I> {
+    /// Displays the substitution in the form `< P0, .. Pn >`, or (if
+    /// the substitution is empty) as an empty string.
+    pub fn with_angle(&self, interner: I) -> Angle<'_, GenericArg<I>> {
+        Angle(self.as_slice(interner))
+    }
+}
+
+impl<I: Interner> Debug for Substitution<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        Display::fmt(self, fmt)
+    }
+}
+
+impl<I: Interner> Debug for Variances<I> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error> {
+        I::debug_variances(self, fmt).unwrap_or_else(|| write!(fmt, "{:?}", self.interned))
+    }
+}
diff --git a/vendor/chalk-ir/src/fold.rs b/vendor/chalk-ir/src/fold.rs
new file mode 100644
index 000000000..b15cbff7a
--- /dev/null
+++ b/vendor/chalk-ir/src/fold.rs
@@ -0,0 +1,920 @@
+//! Traits for transforming bits of IR.
+
+use crate::*;
+use std::convert::Infallible;
+use std::fmt::Debug;
+
+mod binder_impls;
+mod boring_impls;
+mod in_place;
+pub mod shift;
+mod subst;
+
+pub use self::shift::Shift;
+pub use self::subst::Subst;
+
+/// A "folder" is a transformer that can be used to make a copy of
+/// some term -- that is, some bit of IR, such as a `Goal` -- with
+/// certain changes applied. The idea is that it contains methods that
+/// let you swap types/lifetimes for new types/lifetimes; meanwhile,
+/// each bit of IR implements the `TypeFoldable` trait which, given a
+/// `FallibleTypeFolder`, will reconstruct itself, invoking the folder's
+/// methods to transform each of the types/lifetimes embedded within.
+///
+/// As the name suggests, folds performed by `FallibleTypeFolder` can
+/// fail (with type `Error`); if the folder cannot fail, consider
+/// implementing `TypeFolder` instead (which is an infallible, but
+/// otherwise equivalent, trait).
+///
+/// # Usage patterns
+///
+/// ## Substituting for free variables
+///
+/// Most of the time, though, we are not interested in adjust
+/// arbitrary types/lifetimes, but rather just free variables (even
+/// more often, just free existential variables) that appear within
+/// the term.
+///
+/// For this reason, the `FallibleTypeFolder` trait extends two other
+/// traits that contain methods that are invoked when just those particular
+///
+/// In particular, folders can intercept references to free variables
+/// (either existentially or universally quantified) and replace them
+/// with other types/lifetimes as appropriate.
+///
+/// To create a folder `F`, one never implements `FallibleTypeFolder`
+/// directly, but instead implements one of each of these three sub-traits:
+///
+/// - `FreeVarFolder` -- folds `BoundVar` instances that appear free
+///   in the term being folded (use `DefaultFreeVarFolder` to
+///   ignore/forbid these altogether)
+/// - `InferenceFolder` -- folds existential `InferenceVar` instances
+///   that appear in the term being folded (use
+///   `DefaultInferenceFolder` to ignore/forbid these altogether)
+/// - `PlaceholderFolder` -- folds universal `Placeholder` instances
+///   that appear in the term being folded (use
+///   `DefaultPlaceholderFolder` to ignore/forbid these altogether)
+///
+/// To **apply** a folder, use the `TypeFoldable::try_fold_with` method,
+/// like so
+///
+/// ```rust,ignore
+/// let x = x.try_fold_with(&mut folder, 0);
+/// ```
+pub trait FallibleTypeFolder<I: Interner> {
+    /// The type this folder returns when folding fails. This is
+    /// commonly [`NoSolution`].
+    type Error;
+
+    /// Creates a `dyn` value from this folder. Unfortunately, this
+    /// must be added manually to each impl of FallibleTypeFolder; it
+    /// permits the default implements below to create a
+    /// `&mut dyn FallibleTypeFolder` from `Self` without knowing what
+    /// `Self` is (by invoking this method). Effectively, this limits
+    /// impls of `FallibleTypeFolder` to types for which we are able to
+    /// create a dyn value (i.e., not `[T]` types).
+    fn as_dyn(&mut self) -> &mut dyn FallibleTypeFolder<I, Error = Self::Error>;
+
+    /// Top-level callback: invoked for each `Ty<I>` that is
+    /// encountered when folding. By default, invokes
+    /// `try_super_fold_with`, which will in turn invoke the more
+    /// specialized folding methods below, like `try_fold_free_var_ty`.
+    fn try_fold_ty(
+        &mut self,
+        ty: Ty<I>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Ty<I>, Self::Error> {
+        ty.try_super_fold_with(self.as_dyn(), outer_binder)
+    }
+
+    /// Top-level callback: invoked for each `Lifetime<I>` that is
+    /// encountered when folding. By default, invokes
+    /// `try_super_fold_with`, which will in turn invoke the more
+    /// specialized folding methods below, like `try_fold_free_var_lifetime`.
+    fn try_fold_lifetime(
+        &mut self,
+        lifetime: Lifetime<I>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Lifetime<I>, Self::Error> {
+        lifetime.try_super_fold_with(self.as_dyn(), outer_binder)
+    }
+
+    /// Top-level callback: invoked for each `Const<I>` that is
+    /// encountered when folding. By default, invokes
+    /// `try_super_fold_with`, which will in turn invoke the more
+    /// specialized folding methods below, like `try_fold_free_var_const`.
+    fn try_fold_const(
+        &mut self,
+        constant: Const<I>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Const<I>, Self::Error> {
+        constant.try_super_fold_with(self.as_dyn(), outer_binder)
+    }
+
+    /// Invoked for every program clause. By default, recursively folds the goals contents.
+    fn try_fold_program_clause(
+        &mut self,
+        clause: ProgramClause<I>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<ProgramClause<I>, Self::Error> {
+        clause.try_super_fold_with(self.as_dyn(), outer_binder)
+    }
+
+    /// Invoked for every goal. By default, recursively folds the goals contents.
+    fn try_fold_goal(
+        &mut self,
+        goal: Goal<I>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Goal<I>, Self::Error> {
+        goal.try_super_fold_with(self.as_dyn(), outer_binder)
+    }
+
+    /// If overridden to return true, then folding will panic if a
+    /// free variable is encountered. This should be done if free
+    /// type/lifetime variables are not expected.
+    fn forbid_free_vars(&self) -> bool {
+        false
+    }
+
+    /// Invoked for `TyKind::BoundVar` instances that are not bound
+    /// within the type being folded over:
+    ///
+    /// - `depth` is the depth of the `TyKind::BoundVar`; this has
+    ///   been adjusted to account for binders in scope.
+    /// - `binders` is the number of binders in scope.
+    ///
+    /// This should return a type suitable for a context with
+    /// `binders` in scope.
+    fn try_fold_free_var_ty(
+        &mut self,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Ty<I>, Self::Error> {
+        if self.forbid_free_vars() {
+            panic!(
+                "unexpected free variable with depth `{:?}` with outer binder {:?}",
+                bound_var, outer_binder
+            )
+        } else {
+            let bound_var = bound_var.shifted_in_from(outer_binder);
+            Ok(TyKind::<I>::BoundVar(bound_var).intern(self.interner()))
+        }
+    }
+
+    /// As `try_fold_free_var_ty`, but for lifetimes.
+    fn try_fold_free_var_lifetime(
+        &mut self,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Lifetime<I>, Self::Error> {
+        if self.forbid_free_vars() {
+            panic!(
+                "unexpected free variable with depth `{:?}` with outer binder {:?}",
+                bound_var, outer_binder
+            )
+        } else {
+            let bound_var = bound_var.shifted_in_from(outer_binder);
+            Ok(LifetimeData::<I>::BoundVar(bound_var).intern(self.interner()))
+        }
+    }
+
+    /// As `try_fold_free_var_ty`, but for constants.
+    fn try_fold_free_var_const(
+        &mut self,
+        ty: Ty<I>,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Const<I>, Self::Error> {
+        if self.forbid_free_vars() {
+            panic!(
+                "unexpected free variable with depth `{:?}` with outer binder {:?}",
+                bound_var, outer_binder
+            )
+        } else {
+            let bound_var = bound_var.shifted_in_from(outer_binder);
+            Ok(ConstData {
+                ty: ty.try_fold_with(self.as_dyn(), outer_binder)?,
+                value: ConstValue::<I>::BoundVar(bound_var),
+            }
+            .intern(self.interner()))
+        }
+    }
+
+    /// If overridden to return true, we will panic when a free
+    /// placeholder type/lifetime/const is encountered.
+    fn forbid_free_placeholders(&self) -> bool {
+        false
+    }
+
+    /// Invoked for each occurrence of a placeholder type; these are
+    /// used when we instantiate binders universally. Returns a type
+    /// to use instead, which should be suitably shifted to account
+    /// for `binders`.
+    ///
+    /// - `universe` is the universe of the `TypeName::ForAll` that was found
+    /// - `binders` is the number of binders in scope
+    #[allow(unused_variables)]
+    fn try_fold_free_placeholder_ty(
+        &mut self,
+        universe: PlaceholderIndex,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Ty<I>, Self::Error> {
+        if self.forbid_free_placeholders() {
+            panic!("unexpected placeholder type `{:?}`", universe)
+        } else {
+            Ok(universe.to_ty::<I>(self.interner()))
+        }
+    }
+
+    /// As with `try_fold_free_placeholder_ty`, but for lifetimes.
+    #[allow(unused_variables)]
+    fn try_fold_free_placeholder_lifetime(
+        &mut self,
+        universe: PlaceholderIndex,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Lifetime<I>, Self::Error> {
+        if self.forbid_free_placeholders() {
+            panic!("unexpected placeholder lifetime `{:?}`", universe)
+        } else {
+            Ok(universe.to_lifetime(self.interner()))
+        }
+    }
+
+    /// As with `try_fold_free_placeholder_ty`, but for constants.
+    #[allow(unused_variables)]
+    fn try_fold_free_placeholder_const(
+        &mut self,
+        ty: Ty<I>,
+        universe: PlaceholderIndex,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Const<I>, Self::Error> {
+        if self.forbid_free_placeholders() {
+            panic!("unexpected placeholder const `{:?}`", universe)
+        } else {
+            Ok(universe.to_const(
+                self.interner(),
+                ty.try_fold_with(self.as_dyn(), outer_binder)?,
+            ))
+        }
+    }
+
+    /// If overridden to return true, inference variables will trigger
+    /// panics when folded. Used when inference variables are
+    /// unexpected.
+    fn forbid_inference_vars(&self) -> bool {
+        false
+    }
+
+    /// Invoked for each occurrence of a inference type; these are
+    /// used when we instantiate binders universally. Returns a type
+    /// to use instead, which should be suitably shifted to account
+    /// for `binders`.
+    ///
+    /// - `universe` is the universe of the `TypeName::ForAll` that was found
+    /// - `binders` is the number of binders in scope
+    #[allow(unused_variables)]
+    fn try_fold_inference_ty(
+        &mut self,
+        var: InferenceVar,
+        kind: TyVariableKind,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Ty<I>, Self::Error> {
+        if self.forbid_inference_vars() {
+            panic!("unexpected inference type `{:?}`", var)
+        } else {
+            Ok(var.to_ty(self.interner(), kind))
+        }
+    }
+
+    /// As with `try_fold_inference_ty`, but for lifetimes.
+    #[allow(unused_variables)]
+    fn try_fold_inference_lifetime(
+        &mut self,
+        var: InferenceVar,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Lifetime<I>, Self::Error> {
+        if self.forbid_inference_vars() {
+            panic!("unexpected inference lifetime `'{:?}`", var)
+        } else {
+            Ok(var.to_lifetime(self.interner()))
+        }
+    }
+
+    /// As with `try_fold_inference_ty`, but for constants.
+    #[allow(unused_variables)]
+    fn try_fold_inference_const(
+        &mut self,
+        ty: Ty<I>,
+        var: InferenceVar,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Const<I>, Self::Error> {
+        if self.forbid_inference_vars() {
+            panic!("unexpected inference const `{:?}`", var)
+        } else {
+            Ok(var.to_const(
+                self.interner(),
+                ty.try_fold_with(self.as_dyn(), outer_binder)?,
+            ))
+        }
+    }
+
+    /// Gets the interner that is being folded from.
+    fn interner(&self) -> I;
+}
+
+/// A "folder" is a transformer that can be used to make a copy of
+/// some term -- that is, some bit of IR, such as a `Goal` -- with
+/// certain changes applied. The idea is that it contains methods that
+/// let you swap types/lifetimes for new types/lifetimes; meanwhile,
+/// each bit of IR implements the `TypeFoldable` trait which, given a
+/// `TypeFolder`, will reconstruct itself, invoking the folder's methods
+/// to transform each of the types/lifetimes embedded within.
+///
+/// Folds performed by `TypeFolder` cannot fail.  If folds might fail,
+/// consider implementing `FallibleTypeFolder` instead (which is a
+/// fallible, but otherwise equivalent, trait).
+///
+/// # Usage patterns
+///
+/// ## Substituting for free variables
+///
+/// Most of the time, though, we are not interested in adjust
+/// arbitrary types/lifetimes, but rather just free variables (even
+/// more often, just free existential variables) that appear within
+/// the term.
+///
+/// For this reason, the `TypeFolder` trait extends two other traits that
+/// contain methods that are invoked when just those particular
+///
+/// In particular, folders can intercept references to free variables
+/// (either existentially or universally quantified) and replace them
+/// with other types/lifetimes as appropriate.
+///
+/// To create a folder `F`, one never implements `TypeFolder` directly, but instead
+/// implements one of each of these three sub-traits:
+///
+/// - `FreeVarFolder` -- folds `BoundVar` instances that appear free
+///   in the term being folded (use `DefaultFreeVarFolder` to
+///   ignore/forbid these altogether)
+/// - `InferenceFolder` -- folds existential `InferenceVar` instances
+///   that appear in the term being folded (use
+///   `DefaultInferenceFolder` to ignore/forbid these altogether)
+/// - `PlaceholderFolder` -- folds universal `Placeholder` instances
+///   that appear in the term being folded (use
+///   `DefaultPlaceholderFolder` to ignore/forbid these altogether)
+///
+/// To **apply** a folder, use the `TypeFoldable::fold_with` method, like so
+///
+/// ```rust,ignore
+/// let x = x.fold_with(&mut folder, 0);
+/// ```
+pub trait TypeFolder<I: Interner>: FallibleTypeFolder<I, Error = Infallible> {
+    /// Creates a `dyn` value from this folder. Unfortunately, this
+    /// must be added manually to each impl of TypeFolder; it permits the
+    /// default implements below to create a `&mut dyn TypeFolder` from
+    /// `Self` without knowing what `Self` is (by invoking this
+    /// method). Effectively, this limits impls of `TypeFolder` to types
+    /// for which we are able to create a dyn value (i.e., not `[T]`
+    /// types).
+    fn as_dyn(&mut self) -> &mut dyn TypeFolder<I>;
+
+    /// Top-level callback: invoked for each `Ty<I>` that is
+    /// encountered when folding. By default, invokes
+    /// `super_fold_with`, which will in turn invoke the more
+    /// specialized folding methods below, like `fold_free_var_ty`.
+    fn fold_ty(&mut self, ty: Ty<I>, outer_binder: DebruijnIndex) -> Ty<I> {
+        ty.super_fold_with(TypeFolder::as_dyn(self), outer_binder)
+    }
+
+    /// Top-level callback: invoked for each `Lifetime<I>` that is
+    /// encountered when folding. By default, invokes
+    /// `super_fold_with`, which will in turn invoke the more
+    /// specialized folding methods below, like `fold_free_var_lifetime`.
+    fn fold_lifetime(&mut self, lifetime: Lifetime<I>, outer_binder: DebruijnIndex) -> Lifetime<I> {
+        lifetime.super_fold_with(TypeFolder::as_dyn(self), outer_binder)
+    }
+
+    /// Top-level callback: invoked for each `Const<I>` that is
+    /// encountered when folding. By default, invokes
+    /// `super_fold_with`, which will in turn invoke the more
+    /// specialized folding methods below, like `fold_free_var_const`.
+    fn fold_const(&mut self, constant: Const<I>, outer_binder: DebruijnIndex) -> Const<I> {
+        constant.super_fold_with(TypeFolder::as_dyn(self), outer_binder)
+    }
+
+    /// Invoked for every program clause. By default, recursively folds the goals contents.
+    fn fold_program_clause(
+        &mut self,
+        clause: ProgramClause<I>,
+        outer_binder: DebruijnIndex,
+    ) -> ProgramClause<I> {
+        clause.super_fold_with(TypeFolder::as_dyn(self), outer_binder)
+    }
+
+    /// Invoked for every goal. By default, recursively folds the goals contents.
+    fn fold_goal(&mut self, goal: Goal<I>, outer_binder: DebruijnIndex) -> Goal<I> {
+        goal.super_fold_with(TypeFolder::as_dyn(self), outer_binder)
+    }
+
+    /// If overridden to return true, then folding will panic if a
+    /// free variable is encountered. This should be done if free
+    /// type/lifetime variables are not expected.
+    fn forbid_free_vars(&self) -> bool {
+        false
+    }
+
+    /// Invoked for `TyKind::BoundVar` instances that are not bound
+    /// within the type being folded over:
+    ///
+    /// - `depth` is the depth of the `TyKind::BoundVar`; this has
+    ///   been adjusted to account for binders in scope.
+    /// - `binders` is the number of binders in scope.
+    ///
+    /// This should return a type suitable for a context with
+    /// `binders` in scope.
+    fn fold_free_var_ty(&mut self, bound_var: BoundVar, outer_binder: DebruijnIndex) -> Ty<I> {
+        if TypeFolder::forbid_free_vars(self) {
+            panic!(
+                "unexpected free variable with depth `{:?}` with outer binder {:?}",
+                bound_var, outer_binder
+            )
+        } else {
+            let bound_var = bound_var.shifted_in_from(outer_binder);
+            TyKind::<I>::BoundVar(bound_var).intern(TypeFolder::interner(self))
+        }
+    }
+
+    /// As `fold_free_var_ty`, but for lifetimes.
+    fn fold_free_var_lifetime(
+        &mut self,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> Lifetime<I> {
+        if TypeFolder::forbid_free_vars(self) {
+            panic!(
+                "unexpected free variable with depth `{:?}` with outer binder {:?}",
+                bound_var, outer_binder
+            )
+        } else {
+            let bound_var = bound_var.shifted_in_from(outer_binder);
+            LifetimeData::<I>::BoundVar(bound_var).intern(TypeFolder::interner(self))
+        }
+    }
+
+    /// As `fold_free_var_ty`, but for constants.
+    fn fold_free_var_const(
+        &mut self,
+        ty: Ty<I>,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> Const<I> {
+        if TypeFolder::forbid_free_vars(self) {
+            panic!(
+                "unexpected free variable with depth `{:?}` with outer binder {:?}",
+                bound_var, outer_binder
+            )
+        } else {
+            let bound_var = bound_var.shifted_in_from(outer_binder);
+            ConstData {
+                ty: ty.fold_with(TypeFolder::as_dyn(self), outer_binder),
+                value: ConstValue::<I>::BoundVar(bound_var),
+            }
+            .intern(TypeFolder::interner(self))
+        }
+    }
+
+    /// If overridden to return true, we will panic when a free
+    /// placeholder type/lifetime/const is encountered.
+    fn forbid_free_placeholders(&self) -> bool {
+        false
+    }
+
+    /// Invoked for each occurrence of a placeholder type; these are
+    /// used when we instantiate binders universally. Returns a type
+    /// to use instead, which should be suitably shifted to account
+    /// for `binders`.
+    ///
+    /// - `universe` is the universe of the `TypeName::ForAll` that was found
+    /// - `binders` is the number of binders in scope
+    #[allow(unused_variables)]
+    fn fold_free_placeholder_ty(
+        &mut self,
+        universe: PlaceholderIndex,
+        outer_binder: DebruijnIndex,
+    ) -> Ty<I> {
+        if TypeFolder::forbid_free_placeholders(self) {
+            panic!("unexpected placeholder type `{:?}`", universe)
+        } else {
+            universe.to_ty::<I>(TypeFolder::interner(self))
+        }
+    }
+
+    /// As with `fold_free_placeholder_ty`, but for lifetimes.
+    #[allow(unused_variables)]
+    fn fold_free_placeholder_lifetime(
+        &mut self,
+        universe: PlaceholderIndex,
+        outer_binder: DebruijnIndex,
+    ) -> Lifetime<I> {
+        if TypeFolder::forbid_free_placeholders(self) {
+            panic!("unexpected placeholder lifetime `{:?}`", universe)
+        } else {
+            universe.to_lifetime(TypeFolder::interner(self))
+        }
+    }
+
+    /// As with `fold_free_placeholder_ty`, but for constants.
+    #[allow(unused_variables)]
+    fn fold_free_placeholder_const(
+        &mut self,
+        ty: Ty<I>,
+        universe: PlaceholderIndex,
+        outer_binder: DebruijnIndex,
+    ) -> Const<I> {
+        if TypeFolder::forbid_free_placeholders(self) {
+            panic!("unexpected placeholder const `{:?}`", universe)
+        } else {
+            universe.to_const(
+                TypeFolder::interner(self),
+                ty.fold_with(TypeFolder::as_dyn(self), outer_binder),
+            )
+        }
+    }
+
+    /// If overridden to return true, inference variables will trigger
+    /// panics when folded. Used when inference variables are
+    /// unexpected.
+    fn forbid_inference_vars(&self) -> bool {
+        false
+    }
+
+    /// Invoked for each occurrence of a inference type; these are
+    /// used when we instantiate binders universally. Returns a type
+    /// to use instead, which should be suitably shifted to account
+    /// for `binders`.
+    ///
+    /// - `universe` is the universe of the `TypeName::ForAll` that was found
+    /// - `binders` is the number of binders in scope
+    #[allow(unused_variables)]
+    fn fold_inference_ty(
+        &mut self,
+        var: InferenceVar,
+        kind: TyVariableKind,
+        outer_binder: DebruijnIndex,
+    ) -> Ty<I> {
+        if TypeFolder::forbid_inference_vars(self) {
+            panic!("unexpected inference type `{:?}`", var)
+        } else {
+            var.to_ty(TypeFolder::interner(self), kind)
+        }
+    }
+
+    /// As with `fold_inference_ty`, but for lifetimes.
+    #[allow(unused_variables)]
+    fn fold_inference_lifetime(
+        &mut self,
+        var: InferenceVar,
+        outer_binder: DebruijnIndex,
+    ) -> Lifetime<I> {
+        if TypeFolder::forbid_inference_vars(self) {
+            panic!("unexpected inference lifetime `'{:?}`", var)
+        } else {
+            var.to_lifetime(TypeFolder::interner(self))
+        }
+    }
+
+    /// As with `fold_inference_ty`, but for constants.
+    #[allow(unused_variables)]
+    fn fold_inference_const(
+        &mut self,
+        ty: Ty<I>,
+        var: InferenceVar,
+        outer_binder: DebruijnIndex,
+    ) -> Const<I> {
+        if TypeFolder::forbid_inference_vars(self) {
+            panic!("unexpected inference const `{:?}`", var)
+        } else {
+            var.to_const(
+                TypeFolder::interner(self),
+                ty.fold_with(TypeFolder::as_dyn(self), outer_binder),
+            )
+        }
+    }
+
+    /// Gets the interner that is being folded from.
+    fn interner(&self) -> I;
+}
+
+/// Applies the given `TypeFolder` to a value, producing a folded result
+/// of type `Self::Result`. The result type is typically the same as
+/// the source type, but in some cases we convert from borrowed
+/// to owned as well (e.g., the folder for `&T` will fold to a fresh
+/// `T`; well, actually `T::Result`).
+pub trait TypeFoldable<I: Interner>: Debug + Sized {
+    /// Apply the given folder `folder` to `self`; `binders` is the
+    /// number of binders that are in scope when beginning the
+    /// folder. Typically `binders` starts as 0, but is adjusted when
+    /// we encounter `Binders<T>` in the IR or other similar
+    /// constructs.
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E>;
+
+    /// 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(self, folder: &mut dyn TypeFolder<I>, outer_binder: DebruijnIndex) -> Self {
+        self.try_fold_with(FallibleTypeFolder::as_dyn(folder), outer_binder)
+            .unwrap()
+    }
+}
+
+/// For types where "fold" invokes a callback on the `TypeFolder`, the
+/// `TypeSuperFoldable` trait captures the recursive behavior that folds all
+/// the contents of the type.
+pub trait TypeSuperFoldable<I: Interner>: TypeFoldable<I> {
+    /// Recursively folds the value.
+    fn try_super_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E>;
+
+    /// 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(self, folder: &mut dyn TypeFolder<I>, outer_binder: DebruijnIndex) -> Self {
+        self.try_super_fold_with(FallibleTypeFolder::as_dyn(folder), outer_binder)
+            .unwrap()
+    }
+}
+
+/// "Folding" a type invokes the `try_fold_ty` method on the folder; this
+/// usually (in turn) invokes `try_super_fold_ty` to fold the individual
+/// parts.
+impl<I: Interner> TypeFoldable<I> for Ty<I> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        folder.try_fold_ty(self, outer_binder)
+    }
+}
+
+/// "Super fold" for a type invokes te more detailed callbacks on the type
+impl<I> TypeSuperFoldable<I> for Ty<I>
+where
+    I: Interner,
+{
+    fn try_super_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Ty<I>, E> {
+        let interner = folder.interner();
+        Ok(match self.kind(interner) {
+            TyKind::BoundVar(bound_var) => {
+                if let Some(bound_var1) = bound_var.shifted_out_to(outer_binder) {
+                    // This variable was bound outside of the binders
+                    // that we have traversed during folding;
+                    // therefore, it is free. Let the folder have a
+                    // crack at it.
+                    folder.try_fold_free_var_ty(bound_var1, outer_binder)?
+                } else {
+                    // This variable was bound within the binders that
+                    // we folded over, so just return a bound
+                    // variable.
+                    self
+                }
+            }
+            TyKind::Dyn(clauses) => {
+                TyKind::Dyn(clauses.clone().try_fold_with(folder, outer_binder)?)
+                    .intern(folder.interner())
+            }
+            TyKind::InferenceVar(var, kind) => {
+                folder.try_fold_inference_ty(*var, *kind, outer_binder)?
+            }
+            TyKind::Placeholder(ui) => folder.try_fold_free_placeholder_ty(*ui, outer_binder)?,
+            TyKind::Alias(proj) => TyKind::Alias(proj.clone().try_fold_with(folder, outer_binder)?)
+                .intern(folder.interner()),
+            TyKind::Function(fun) => {
+                TyKind::Function(fun.clone().try_fold_with(folder, outer_binder)?)
+                    .intern(folder.interner())
+            }
+            TyKind::Adt(id, substitution) => TyKind::Adt(
+                id.try_fold_with(folder, outer_binder)?,
+                substitution.clone().try_fold_with(folder, outer_binder)?,
+            )
+            .intern(folder.interner()),
+            TyKind::AssociatedType(assoc_ty, substitution) => TyKind::AssociatedType(
+                assoc_ty.try_fold_with(folder, outer_binder)?,
+                substitution.clone().try_fold_with(folder, outer_binder)?,
+            )
+            .intern(folder.interner()),
+            TyKind::Scalar(scalar) => TyKind::Scalar(scalar.try_fold_with(folder, outer_binder)?)
+                .intern(folder.interner()),
+            TyKind::Str => TyKind::Str.intern(folder.interner()),
+            TyKind::Tuple(arity, substitution) => TyKind::Tuple(
+                *arity,
+                substitution.clone().try_fold_with(folder, outer_binder)?,
+            )
+            .intern(folder.interner()),
+            TyKind::OpaqueType(opaque_ty, substitution) => TyKind::OpaqueType(
+                opaque_ty.try_fold_with(folder, outer_binder)?,
+                substitution.clone().try_fold_with(folder, outer_binder)?,
+            )
+            .intern(folder.interner()),
+            TyKind::Slice(substitution) => {
+                TyKind::Slice(substitution.clone().try_fold_with(folder, outer_binder)?)
+                    .intern(folder.interner())
+            }
+            TyKind::FnDef(fn_def, substitution) => TyKind::FnDef(
+                fn_def.try_fold_with(folder, outer_binder)?,
+                substitution.clone().try_fold_with(folder, outer_binder)?,
+            )
+            .intern(folder.interner()),
+            TyKind::Ref(mutability, lifetime, ty) => TyKind::Ref(
+                mutability.try_fold_with(folder, outer_binder)?,
+                lifetime.clone().try_fold_with(folder, outer_binder)?,
+                ty.clone().try_fold_with(folder, outer_binder)?,
+            )
+            .intern(folder.interner()),
+            TyKind::Raw(mutability, ty) => TyKind::Raw(
+                mutability.try_fold_with(folder, outer_binder)?,
+                ty.clone().try_fold_with(folder, outer_binder)?,
+            )
+            .intern(folder.interner()),
+            TyKind::Never => TyKind::Never.intern(folder.interner()),
+            TyKind::Array(ty, const_) => TyKind::Array(
+                ty.clone().try_fold_with(folder, outer_binder)?,
+                const_.clone().try_fold_with(folder, outer_binder)?,
+            )
+            .intern(folder.interner()),
+            TyKind::Closure(id, substitution) => TyKind::Closure(
+                id.try_fold_with(folder, outer_binder)?,
+                substitution.clone().try_fold_with(folder, outer_binder)?,
+            )
+            .intern(folder.interner()),
+            TyKind::Generator(id, substitution) => TyKind::Generator(
+                id.try_fold_with(folder, outer_binder)?,
+                substitution.clone().try_fold_with(folder, outer_binder)?,
+            )
+            .intern(folder.interner()),
+            TyKind::GeneratorWitness(id, substitution) => TyKind::GeneratorWitness(
+                id.try_fold_with(folder, outer_binder)?,
+                substitution.clone().try_fold_with(folder, outer_binder)?,
+            )
+            .intern(folder.interner()),
+            TyKind::Foreign(id) => {
+                TyKind::Foreign(id.try_fold_with(folder, outer_binder)?).intern(folder.interner())
+            }
+            TyKind::Error => TyKind::Error.intern(folder.interner()),
+        })
+    }
+}
+
+/// "Folding" a lifetime invokes the `fold_lifetime` method on the folder; this
+/// usually (in turn) invokes `super_fold_lifetime` to fold the individual
+/// parts.
+impl<I: Interner> TypeFoldable<I> for Lifetime<I> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        folder.try_fold_lifetime(self, outer_binder)
+    }
+}
+
+impl<I> TypeSuperFoldable<I> for Lifetime<I>
+where
+    I: Interner,
+{
+    fn try_super_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Lifetime<I>, E> {
+        let interner = folder.interner();
+        match self.data(interner) {
+            LifetimeData::BoundVar(bound_var) => {
+                if let Some(bound_var1) = bound_var.shifted_out_to(outer_binder) {
+                    // This variable was bound outside of the binders
+                    // that we have traversed during folding;
+                    // therefore, it is free. Let the folder have a
+                    // crack at it.
+                    folder.try_fold_free_var_lifetime(bound_var1, outer_binder)
+                } else {
+                    // This variable was bound within the binders that
+                    // we folded over, so just return a bound
+                    // variable.
+                    Ok(self)
+                }
+            }
+            LifetimeData::InferenceVar(var) => {
+                folder.try_fold_inference_lifetime(*var, outer_binder)
+            }
+            LifetimeData::Placeholder(universe) => {
+                folder.try_fold_free_placeholder_lifetime(*universe, outer_binder)
+            }
+            LifetimeData::Static => Ok(LifetimeData::<I>::Static.intern(folder.interner())),
+            LifetimeData::Erased => Ok(LifetimeData::<I>::Erased.intern(folder.interner())),
+            LifetimeData::Phantom(void, ..) => match *void {},
+        }
+    }
+}
+
+/// "Folding" a const invokes the `fold_const` method on the folder; this
+/// usually (in turn) invokes `super_fold_const` to fold the individual
+/// parts.
+impl<I: Interner> TypeFoldable<I> for Const<I> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        folder.try_fold_const(self, outer_binder)
+    }
+}
+
+impl<I> TypeSuperFoldable<I> for Const<I>
+where
+    I: Interner,
+{
+    fn try_super_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Const<I>, E> {
+        let interner = folder.interner();
+        let ConstData { ref ty, ref value } = self.data(interner);
+        let mut fold_ty = || ty.clone().try_fold_with(folder, outer_binder);
+        match value {
+            ConstValue::BoundVar(bound_var) => {
+                if let Some(bound_var1) = bound_var.shifted_out_to(outer_binder) {
+                    folder.try_fold_free_var_const(ty.clone(), bound_var1, outer_binder)
+                } else {
+                    Ok(self)
+                }
+            }
+            ConstValue::InferenceVar(var) => {
+                folder.try_fold_inference_const(ty.clone(), *var, outer_binder)
+            }
+            ConstValue::Placeholder(universe) => {
+                folder.try_fold_free_placeholder_const(ty.clone(), *universe, outer_binder)
+            }
+            ConstValue::Concrete(ev) => Ok(ConstData {
+                ty: fold_ty()?,
+                value: ConstValue::Concrete(ConcreteConst {
+                    interned: ev.interned.clone(),
+                }),
+            }
+            .intern(folder.interner())),
+        }
+    }
+}
+
+/// Folding a goal invokes the `fold_goal` callback (which will, by
+/// default, invoke super-fold).
+impl<I: Interner> TypeFoldable<I> for Goal<I> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        folder.try_fold_goal(self, outer_binder)
+    }
+}
+
+/// Superfold folds recursively.
+impl<I: Interner> TypeSuperFoldable<I> for Goal<I> {
+    fn try_super_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        let interner = folder.interner();
+        Ok(Goal::new(
+            interner,
+            self.data(interner)
+                .clone()
+                .try_fold_with(folder, outer_binder)?,
+        ))
+    }
+}
+
+/// Folding a program clause invokes the `fold_program_clause`
+/// callback on the folder (which will, by default, invoke the
+/// `super_fold_with` method on the program clause).
+impl<I: Interner> TypeFoldable<I> for ProgramClause<I> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        folder.try_fold_program_clause(self, outer_binder)
+    }
+}
diff --git a/vendor/chalk-ir/src/fold/binder_impls.rs b/vendor/chalk-ir/src/fold/binder_impls.rs
new file mode 100644
index 000000000..1f44c1629
--- /dev/null
+++ b/vendor/chalk-ir/src/fold/binder_impls.rs
@@ -0,0 +1,73 @@
+//! This module contains impls of `TypeFoldable` for those types that
+//! introduce binders.
+//!
+//! The more interesting impls of `TypeFoldable` remain in the `fold` module.
+
+use crate::*;
+
+impl<I: Interner> TypeFoldable<I> for FnPointer<I> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        let FnPointer {
+            num_binders,
+            substitution,
+            sig,
+        } = self;
+        Ok(FnPointer {
+            num_binders,
+            substitution: substitution.try_fold_with(folder, outer_binder.shifted_in())?,
+            sig: FnSig {
+                abi: sig.abi,
+                safety: sig.safety,
+                variadic: sig.variadic,
+            },
+        })
+    }
+}
+
+impl<T, I: Interner> TypeFoldable<I> for Binders<T>
+where
+    T: HasInterner<Interner = I> + TypeFoldable<I>,
+    I: Interner,
+{
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        let Binders {
+            binders: self_binders,
+            value: self_value,
+        } = self;
+        let value = self_value.try_fold_with(folder, outer_binder.shifted_in())?;
+        let binders = VariableKinds {
+            interned: self_binders.interned().clone(),
+        };
+        Ok(Binders::new(binders, value))
+    }
+}
+
+impl<I, T> TypeFoldable<I> for Canonical<T>
+where
+    I: Interner,
+    T: HasInterner<Interner = I> + TypeFoldable<I>,
+{
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        let Canonical {
+            binders: self_binders,
+            value: self_value,
+        } = self;
+        let value = self_value.try_fold_with(folder, outer_binder.shifted_in())?;
+        let binders = CanonicalVarKinds {
+            interned: self_binders.interned().clone(),
+        };
+        Ok(Canonical { binders, value })
+    }
+}
diff --git a/vendor/chalk-ir/src/fold/boring_impls.rs b/vendor/chalk-ir/src/fold/boring_impls.rs
new file mode 100644
index 000000000..fb4e3e30b
--- /dev/null
+++ b/vendor/chalk-ir/src/fold/boring_impls.rs
@@ -0,0 +1,244 @@
+//! This module contains "rote and uninteresting" impls of `TypeFoldable` for
+//! various types. In general, we prefer to derive `TypeFoldable`, but
+//! sometimes that doesn't work for whatever reason.
+//!
+//! The more interesting impls of `TypeFoldable` remain in the `fold` module.
+
+use super::in_place;
+use crate::*;
+use std::marker::PhantomData;
+
+impl<T: TypeFoldable<I>, I: Interner> TypeFoldable<I> for Vec<T> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        in_place::fallible_map_vec(self, |e| e.try_fold_with(folder, outer_binder))
+    }
+}
+
+impl<T: TypeFoldable<I>, I: Interner> TypeFoldable<I> for Box<T> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        in_place::fallible_map_box(self, |e| e.try_fold_with(folder, outer_binder))
+    }
+}
+
+macro_rules! tuple_fold {
+    ($($n:ident),*) => {
+        impl<$($n: TypeFoldable<I>,)* I: Interner> TypeFoldable<I> for ($($n,)*) {
+            fn try_fold_with<Error>(self, folder: &mut dyn FallibleTypeFolder<I, Error = Error>, outer_binder: DebruijnIndex) -> Result<Self, Error>
+            {
+                #[allow(non_snake_case)]
+                let ($($n),*) = self;
+                Ok(($($n.try_fold_with(folder, outer_binder)?,)*))
+            }
+        }
+    }
+}
+
+tuple_fold!(A, B);
+tuple_fold!(A, B, C);
+tuple_fold!(A, B, C, D);
+tuple_fold!(A, B, C, D, E);
+
+impl<T: TypeFoldable<I>, I: Interner> TypeFoldable<I> for Option<T> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        match self {
+            None => Ok(None),
+            Some(e) => Ok(Some(e.try_fold_with(folder, outer_binder)?)),
+        }
+    }
+}
+
+impl<I: Interner> TypeFoldable<I> for GenericArg<I> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        let interner = folder.interner();
+
+        let data = self
+            .data(interner)
+            .clone()
+            .try_fold_with(folder, outer_binder)?;
+        Ok(GenericArg::new(interner, data))
+    }
+}
+
+impl<I: Interner> TypeFoldable<I> for Substitution<I> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        let interner = folder.interner();
+
+        let folded = self
+            .iter(interner)
+            .cloned()
+            .map(|p| p.try_fold_with(folder, outer_binder));
+        Substitution::from_fallible(interner, folded)
+    }
+}
+
+impl<I: Interner> TypeFoldable<I> for Goals<I> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        let interner = folder.interner();
+        let folded = self
+            .iter(interner)
+            .cloned()
+            .map(|p| p.try_fold_with(folder, outer_binder));
+        Goals::from_fallible(interner, folded)
+    }
+}
+
+impl<I: Interner> TypeFoldable<I> for ProgramClauses<I> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        let interner = folder.interner();
+        let folded = self
+            .iter(interner)
+            .cloned()
+            .map(|p| p.try_fold_with(folder, outer_binder));
+        ProgramClauses::from_fallible(interner, folded)
+    }
+}
+
+impl<I: Interner> TypeFoldable<I> for QuantifiedWhereClauses<I> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        let interner = folder.interner();
+        let folded = self
+            .iter(interner)
+            .cloned()
+            .map(|p| p.try_fold_with(folder, outer_binder));
+        QuantifiedWhereClauses::from_fallible(interner, folded)
+    }
+}
+
+impl<I: Interner> TypeFoldable<I> for Constraints<I> {
+    fn try_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> Result<Self, E> {
+        let interner = folder.interner();
+        let folded = self
+            .iter(interner)
+            .cloned()
+            .map(|p| p.try_fold_with(folder, outer_binder));
+        Constraints::from_fallible(interner, folded)
+    }
+}
+
+#[doc(hidden)]
+#[macro_export]
+macro_rules! copy_fold {
+    ($t:ty) => {
+        impl<I: Interner> $crate::fold::TypeFoldable<I> for $t {
+            fn try_fold_with<E>(
+                self,
+                _folder: &mut dyn ($crate::fold::FallibleTypeFolder<I, Error = E>),
+                _outer_binder: DebruijnIndex,
+            ) -> ::std::result::Result<Self, E> {
+                Ok(self)
+            }
+        }
+    };
+}
+
+copy_fold!(bool);
+copy_fold!(usize);
+copy_fold!(UniverseIndex);
+copy_fold!(PlaceholderIndex);
+copy_fold!(QuantifierKind);
+copy_fold!(DebruijnIndex);
+copy_fold!(());
+copy_fold!(UintTy);
+copy_fold!(IntTy);
+copy_fold!(FloatTy);
+copy_fold!(Scalar);
+copy_fold!(ClausePriority);
+copy_fold!(Mutability);
+copy_fold!(Safety);
+
+#[doc(hidden)]
+#[macro_export]
+macro_rules! id_fold {
+    ($t:ident) => {
+        impl<I: Interner> $crate::fold::TypeFoldable<I> for $t<I> {
+            fn try_fold_with<E>(
+                self,
+                _folder: &mut dyn ($crate::fold::FallibleTypeFolder<I, Error = E>),
+                _outer_binder: DebruijnIndex,
+            ) -> ::std::result::Result<Self, E> {
+                Ok(self)
+            }
+        }
+    };
+}
+
+id_fold!(ImplId);
+id_fold!(AdtId);
+id_fold!(TraitId);
+id_fold!(AssocTypeId);
+id_fold!(OpaqueTyId);
+id_fold!(FnDefId);
+id_fold!(ClosureId);
+id_fold!(GeneratorId);
+id_fold!(ForeignDefId);
+
+impl<I: Interner> TypeSuperFoldable<I> for ProgramClauseData<I> {
+    fn try_super_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> ::std::result::Result<Self, E> {
+        Ok(ProgramClauseData(
+            self.0.try_fold_with(folder, outer_binder)?,
+        ))
+    }
+}
+
+impl<I: Interner> TypeSuperFoldable<I> for ProgramClause<I> {
+    fn try_super_fold_with<E>(
+        self,
+        folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        outer_binder: DebruijnIndex,
+    ) -> ::std::result::Result<Self, E> {
+        let clause = self.data(folder.interner()).clone();
+        Ok(clause
+            .try_super_fold_with(folder, outer_binder)?
+            .intern(folder.interner()))
+    }
+}
+
+impl<I: Interner> TypeFoldable<I> for PhantomData<I> {
+    fn try_fold_with<E>(
+        self,
+        _folder: &mut dyn FallibleTypeFolder<I, Error = E>,
+        _outer_binder: DebruijnIndex,
+    ) -> ::std::result::Result<Self, E> {
+        Ok(PhantomData)
+    }
+}
diff --git a/vendor/chalk-ir/src/fold/in_place.rs b/vendor/chalk-ir/src/fold/in_place.rs
new file mode 100644
index 000000000..263da617d
--- /dev/null
+++ b/vendor/chalk-ir/src/fold/in_place.rs
@@ -0,0 +1,263 @@
+//! Subroutines to help implementers of `TypeFoldable` avoid unnecessary heap allocations.
+
+use std::marker::PhantomData;
+use std::{mem, ptr};
+
+fn is_zst<T>() -> bool {
+    mem::size_of::<T>() == 0
+}
+
+fn is_layout_identical<T, U>() -> bool {
+    mem::size_of::<T>() == mem::size_of::<U>() && mem::align_of::<T>() == mem::align_of::<U>()
+}
+
+/// Maps a `Box<T>` to a `Box<U>`, reusing the underlying storage if possible.
+pub(super) fn fallible_map_box<T, U, E>(
+    b: Box<T>,
+    map: impl FnOnce(T) -> Result<U, E>,
+) -> Result<Box<U>, E> {
+    // This optimization is only valid when `T` and `U` have the same size/alignment and is not
+    // useful for ZSTs.
+    if !is_layout_identical::<T, U>() || is_zst::<T>() {
+        return map(*b).map(Box::new);
+    }
+
+    let raw = Box::into_raw(b);
+    unsafe {
+        let val = ptr::read(raw);
+
+        // Box<T> -> Box<MaybeUninit<U>>
+        let mut raw: Box<mem::MaybeUninit<U>> = Box::from_raw(raw.cast());
+
+        // If `map` panics or returns an error, `raw` will free the memory associated with `b`, but
+        // not drop the boxed value itself since it is wrapped in `MaybeUninit`. This is what we
+        // want since the boxed value was moved into `map`.
+        let mapped_val = map(val)?;
+        ptr::write(raw.as_mut_ptr(), mapped_val);
+
+        // Box<MaybeUninit<U>> -> Box<U>
+        Ok(Box::from_raw(Box::into_raw(raw).cast()))
+    }
+}
+
+/// Maps a `Vec<T>` to a `Vec<U>`, reusing the underlying storage if possible.
+pub(super) fn fallible_map_vec<T, U, E>(
+    vec: Vec<T>,
+    mut map: impl FnMut(T) -> Result<U, E>,
+) -> Result<Vec<U>, E> {
+    // This optimization is only valid when `T` and `U` have the same size/alignment and is not
+    // useful for ZSTs.
+    if !is_layout_identical::<T, U>() || is_zst::<T>() {
+        return vec.into_iter().map(map).collect();
+    }
+
+    let mut vec = VecMappedInPlace::<T, U>::new(vec);
+
+    unsafe {
+        for i in 0..vec.len {
+            let place = vec.ptr.add(i);
+            let val = ptr::read(place);
+
+            // Set `map_in_progress` so the drop impl for `VecMappedInPlace` can handle the other
+            // elements correctly in case `map` panics or returns an error.
+            vec.map_in_progress = i;
+            let mapped_val = map(val)?;
+
+            ptr::write(place as *mut U, mapped_val);
+        }
+
+        Ok(vec.finish())
+    }
+}
+
+/// Takes ownership of a `Vec` that is being mapped in place, cleaning up if the map fails.
+struct VecMappedInPlace<T, U> {
+    ptr: *mut T,
+    len: usize,
+    cap: usize,
+
+    map_in_progress: usize,
+    _elem_tys: PhantomData<(T, U)>,
+}
+
+impl<T, U> VecMappedInPlace<T, U> {
+    fn new(mut vec: Vec<T>) -> Self {
+        assert!(is_layout_identical::<T, U>());
+
+        // FIXME: This is just `Vec::into_raw_parts`. Use that instead when it is stabilized.
+        let ptr = vec.as_mut_ptr();
+        let len = vec.len();
+        let cap = vec.capacity();
+        mem::forget(vec);
+
+        VecMappedInPlace {
+            ptr,
+            len,
+            cap,
+
+            map_in_progress: 0,
+            _elem_tys: PhantomData,
+        }
+    }
+
+    /// Converts back into a `Vec` once the map is complete.
+    unsafe fn finish(self) -> Vec<U> {
+        let this = mem::ManuallyDrop::new(self);
+        Vec::from_raw_parts(this.ptr as *mut U, this.len, this.cap)
+    }
+}
+
+/// `VecMappedInPlace` drops everything but the element that was passed to `map` when it panicked or
+/// returned an error. Everything before that index in the vector has type `U` (it has been mapped)
+/// and everything after it has type `T` (it has not been mapped).
+///
+/// ```text
+///  mapped
+///  |      not yet mapped
+///  |----| |-----|
+/// [UUUU UxTT TTTT]
+///        ^
+///    `map_in_progress` (not dropped)
+/// ```
+impl<T, U> Drop for VecMappedInPlace<T, U> {
+    fn drop(&mut self) {
+        // Drop mapped elements of type `U`.
+        for i in 0..self.map_in_progress {
+            unsafe {
+                ptr::drop_in_place(self.ptr.add(i) as *mut U);
+            }
+        }
+
+        // Drop unmapped elements of type `T`.
+        for i in (self.map_in_progress + 1)..self.len {
+            unsafe {
+                ptr::drop_in_place(self.ptr.add(i));
+            }
+        }
+
+        // Free the underlying storage for the `Vec`.
+        // `len` is 0 because the elements were handled above.
+        unsafe {
+            Vec::from_raw_parts(self.ptr, 0, self.cap);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::fmt;
+    use std::sync::{Arc, Mutex};
+
+    /// A wrapper around `T` that records when it is dropped.
+    struct RecordDrop<T: fmt::Display> {
+        id: T,
+        drops: Arc<Mutex<Vec<String>>>,
+    }
+
+    impl<T: fmt::Display> RecordDrop<T> {
+        fn new(id: T, drops: &Arc<Mutex<Vec<String>>>) -> Self {
+            RecordDrop {
+                id,
+                drops: drops.clone(),
+            }
+        }
+    }
+
+    impl RecordDrop<u8> {
+        fn map_to_char(self) -> RecordDrop<char> {
+            let this = std::mem::ManuallyDrop::new(self);
+            RecordDrop {
+                id: (this.id + b'A') as char,
+                drops: this.drops.clone(),
+            }
+        }
+    }
+
+    impl<T: fmt::Display> Drop for RecordDrop<T> {
+        fn drop(&mut self) {
+            self.drops.lock().unwrap().push(format!("{}", self.id));
+        }
+    }
+
+    #[test]
+    fn vec_no_cleanup_after_success() {
+        let drops = Arc::new(Mutex::new(Vec::new()));
+        let to_fold = (0u8..5).map(|i| RecordDrop::new(i, &drops)).collect();
+
+        let res: Result<_, ()> = super::fallible_map_vec(to_fold, |x| Ok(x.map_to_char()));
+
+        assert!(res.is_ok());
+        assert!(drops.lock().unwrap().is_empty());
+    }
+
+    #[test]
+    fn vec_cleanup_after_panic() {
+        let drops = Arc::new(Mutex::new(Vec::new()));
+        let to_fold = (0u8..5).map(|i| RecordDrop::new(i, &drops)).collect();
+
+        let res = std::panic::catch_unwind(|| {
+            let _: Result<_, ()> = super::fallible_map_vec(to_fold, |x| {
+                if x.id == 3 {
+                    panic!();
+                }
+
+                Ok(x.map_to_char())
+            });
+        });
+
+        assert!(res.is_err());
+        assert_eq!(*drops.lock().unwrap(), &["3", "A", "B", "C", "4"]);
+    }
+
+    #[test]
+    fn vec_cleanup_after_early_return() {
+        let drops = Arc::new(Mutex::new(Vec::new()));
+        let to_fold = (0u8..5).map(|i| RecordDrop::new(i, &drops)).collect();
+
+        let res = super::fallible_map_vec(to_fold, |x| {
+            if x.id == 2 {
+                return Err(());
+            }
+
+            Ok(x.map_to_char())
+        });
+
+        assert!(res.is_err());
+        assert_eq!(*drops.lock().unwrap(), &["2", "A", "B", "3", "4"]);
+    }
+
+    #[test]
+    fn box_no_cleanup_after_success() {
+        let drops = Arc::new(Mutex::new(Vec::new()));
+        let to_fold = Box::new(RecordDrop::new(0, &drops));
+
+        let res: Result<Box<_>, ()> = super::fallible_map_box(to_fold, |x| Ok(x.map_to_char()));
+
+        assert!(res.is_ok());
+        assert!(drops.lock().unwrap().is_empty());
+    }
+
+    #[test]
+    fn box_cleanup_after_panic() {
+        let drops = Arc::new(Mutex::new(Vec::new()));
+        let to_fold = Box::new(RecordDrop::new(0, &drops));
+
+        let res = std::panic::catch_unwind(|| {
+            let _: Result<Box<()>, ()> = super::fallible_map_box(to_fold, |_| panic!());
+        });
+
+        assert!(res.is_err());
+        assert_eq!(*drops.lock().unwrap(), &["0"]);
+    }
+
+    #[test]
+    fn box_cleanup_after_early_return() {
+        let drops = Arc::new(Mutex::new(Vec::new()));
+        let to_fold = Box::new(RecordDrop::new(0, &drops));
+
+        let res: Result<Box<()>, _> = super::fallible_map_box(to_fold, |_| Err(()));
+
+        assert!(res.is_err());
+        assert_eq!(*drops.lock().unwrap(), &["0"]);
+    }
+}
diff --git a/vendor/chalk-ir/src/fold/shift.rs b/vendor/chalk-ir/src/fold/shift.rs
new file mode 100644
index 000000000..f7e5e4a4a
--- /dev/null
+++ b/vendor/chalk-ir/src/fold/shift.rs
@@ -0,0 +1,177 @@
+//! Shifting of debruijn indices
+
+use crate::*;
+
+/// Methods for converting debruijn indices to move values into or out
+/// of binders.
+pub trait Shift<I: Interner>: TypeFoldable<I> {
+    /// Shifts this term in one level of binders.
+    fn shifted_in(self, interner: I) -> Self;
+
+    /// Shifts a term valid at `outer_binder` so that it is
+    /// valid at the innermost binder. See [`DebruijnIndex::shifted_in_from`]
+    /// for a detailed explanation.
+    fn shifted_in_from(self, interner: I, source_binder: DebruijnIndex) -> Self;
+
+    /// Shifts this term out one level of binders.
+    fn shifted_out(self, interner: I) -> Fallible<Self>;
+
+    /// Shifts a term valid at the innermost binder so that it is
+    /// valid at `outer_binder`. See [`DebruijnIndex::shifted_out_to`]
+    /// for a detailed explanation.
+    fn shifted_out_to(self, interner: I, target_binder: DebruijnIndex) -> Fallible<Self>;
+}
+
+impl<T: TypeFoldable<I>, I: Interner> Shift<I> for T {
+    fn shifted_in(self, interner: I) -> Self {
+        self.shifted_in_from(interner, DebruijnIndex::ONE)
+    }
+
+    fn shifted_in_from(self, interner: I, source_binder: DebruijnIndex) -> T {
+        self.try_fold_with(
+            &mut Shifter {
+                source_binder,
+                interner,
+            },
+            DebruijnIndex::INNERMOST,
+        )
+        .unwrap()
+    }
+
+    fn shifted_out_to(self, interner: I, target_binder: DebruijnIndex) -> Fallible<T> {
+        self.try_fold_with(
+            &mut DownShifter {
+                target_binder,
+                interner,
+            },
+            DebruijnIndex::INNERMOST,
+        )
+    }
+
+    fn shifted_out(self, interner: I) -> Fallible<Self> {
+        self.shifted_out_to(interner, DebruijnIndex::ONE)
+    }
+}
+
+/// A folder that adjusts debruijn indices by a certain amount.
+#[derive(FallibleTypeFolder)]
+struct Shifter<I: Interner> {
+    source_binder: DebruijnIndex,
+    interner: I,
+}
+
+impl<I: Interner> Shifter<I> {
+    /// Given a free variable at `depth`, shifts that depth to `depth
+    /// + self.adjustment`, and then wraps *that* within the internal
+    /// set `binders`.
+    fn adjust(&self, bound_var: BoundVar, outer_binder: DebruijnIndex) -> BoundVar {
+        bound_var
+            .shifted_in_from(self.source_binder)
+            .shifted_in_from(outer_binder)
+    }
+}
+
+impl<I: Interner> TypeFolder<I> for Shifter<I> {
+    fn as_dyn(&mut self) -> &mut dyn TypeFolder<I> {
+        self
+    }
+
+    fn fold_free_var_ty(&mut self, bound_var: BoundVar, outer_binder: DebruijnIndex) -> Ty<I> {
+        TyKind::<I>::BoundVar(self.adjust(bound_var, outer_binder))
+            .intern(TypeFolder::interner(self))
+    }
+
+    fn fold_free_var_lifetime(
+        &mut self,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> Lifetime<I> {
+        LifetimeData::<I>::BoundVar(self.adjust(bound_var, outer_binder))
+            .intern(TypeFolder::interner(self))
+    }
+
+    fn fold_free_var_const(
+        &mut self,
+        ty: Ty<I>,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> Const<I> {
+        // const types don't have free variables, so we can skip folding `ty`
+        self.adjust(bound_var, outer_binder)
+            .to_const(TypeFolder::interner(self), ty)
+    }
+
+    fn interner(&self) -> I {
+        self.interner
+    }
+}
+
+//---------------------------------------------------------------------------
+
+/// A shifter that reduces debruijn indices -- in other words, which lifts a value
+/// *out* from binders. Consider this example:
+///
+struct DownShifter<I> {
+    target_binder: DebruijnIndex,
+    interner: I,
+}
+
+impl<I> DownShifter<I> {
+    /// Given a reference to a free variable at depth `depth`
+    /// (appearing within `binders` internal binders), attempts to
+    /// lift that free variable out from `adjustment` levels of
+    /// binders (i.e., convert it to depth `depth -
+    /// self.adjustment`). If the free variable is bound by one of
+    /// those internal binders (i.e., `depth < self.adjustment`) the
+    /// this will fail with `Err`. Otherwise, returns the variable at
+    /// this new depth (but adjusted to appear within `binders`).
+    fn adjust(&self, bound_var: BoundVar, outer_binder: DebruijnIndex) -> Fallible<BoundVar> {
+        match bound_var.shifted_out_to(self.target_binder) {
+            Some(bound_var1) => Ok(bound_var1.shifted_in_from(outer_binder)),
+            None => Err(NoSolution),
+        }
+    }
+}
+
+impl<I: Interner> FallibleTypeFolder<I> for DownShifter<I> {
+    type Error = NoSolution;
+
+    fn as_dyn(&mut self) -> &mut dyn FallibleTypeFolder<I, Error = Self::Error> {
+        self
+    }
+
+    fn try_fold_free_var_ty(
+        &mut self,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> Fallible<Ty<I>> {
+        Ok(TyKind::<I>::BoundVar(self.adjust(bound_var, outer_binder)?).intern(self.interner()))
+    }
+
+    fn try_fold_free_var_lifetime(
+        &mut self,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> Fallible<Lifetime<I>> {
+        Ok(
+            LifetimeData::<I>::BoundVar(self.adjust(bound_var, outer_binder)?)
+                .intern(self.interner()),
+        )
+    }
+
+    fn try_fold_free_var_const(
+        &mut self,
+        ty: Ty<I>,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> Fallible<Const<I>> {
+        // const types don't have free variables, so we can skip folding `ty`
+        Ok(self
+            .adjust(bound_var, outer_binder)?
+            .to_const(self.interner(), ty))
+    }
+
+    fn interner(&self) -> I {
+        self.interner
+    }
+}
diff --git a/vendor/chalk-ir/src/fold/subst.rs b/vendor/chalk-ir/src/fold/subst.rs
new file mode 100644
index 000000000..7cff8d89c
--- /dev/null
+++ b/vendor/chalk-ir/src/fold/subst.rs
@@ -0,0 +1,118 @@
+use super::*;
+use crate::fold::shift::Shift;
+
+/// Substitution used during folding
+#[derive(FallibleTypeFolder)]
+pub struct Subst<'s, I: Interner> {
+    /// Values to substitute. A reference to a free variable with
+    /// index `i` will be mapped to `parameters[i]` -- if `i >
+    /// parameters.len()`, then we will leave the variable untouched.
+    parameters: &'s [GenericArg<I>],
+    interner: I,
+}
+
+impl<I: Interner> Subst<'_, I> {
+    /// Applies the substitution by folding
+    pub fn apply<T: TypeFoldable<I>>(interner: I, parameters: &[GenericArg<I>], value: T) -> T {
+        value
+            .try_fold_with(
+                &mut Subst {
+                    parameters,
+                    interner,
+                },
+                DebruijnIndex::INNERMOST,
+            )
+            .unwrap()
+    }
+}
+
+impl<I: Interner> TypeFolder<I> for Subst<'_, I> {
+    fn as_dyn(&mut self) -> &mut dyn TypeFolder<I> {
+        self
+    }
+
+    /// We are eliminating one binder, but binders outside of that get preserved.
+    ///
+    /// So e.g. consider this:
+    ///
+    /// ```notrust
+    /// for<A, B> { for<C> { [A, C] } }
+    /// //          ^ the binder we are substituing with `[u32]`
+    /// ```
+    ///
+    /// Here, `A` would be `^1.0` and `C` would be `^0.0`. We will replace `^0.0` with the
+    /// 0th index from the list (`u32`). We will convert `^1.0` (A) to `^0.0` -- i.e., shift
+    /// it **out** of one level of binder (the `for<C>` binder we are eliminating).
+    ///
+    /// This gives us as a result:
+    ///
+    /// ```notrust
+    /// for<A, B> { [A, u32] }
+    ///              ^ represented as `^0.0`
+    /// ```
+    fn fold_free_var_ty(&mut self, bound_var: BoundVar, outer_binder: DebruijnIndex) -> Ty<I> {
+        if let Some(index) = bound_var.index_if_innermost() {
+            match self.parameters[index].data(TypeFolder::interner(self)) {
+                GenericArgData::Ty(t) => t
+                    .clone()
+                    .shifted_in_from(TypeFolder::interner(self), outer_binder),
+                _ => panic!("mismatched kinds in substitution"),
+            }
+        } else {
+            bound_var
+                .shifted_out()
+                .expect("cannot fail because this is not the innermost")
+                .shifted_in_from(outer_binder)
+                .to_ty(TypeFolder::interner(self))
+        }
+    }
+
+    /// see `fold_free_var_ty`
+    fn fold_free_var_lifetime(
+        &mut self,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> Lifetime<I> {
+        if let Some(index) = bound_var.index_if_innermost() {
+            match self.parameters[index].data(TypeFolder::interner(self)) {
+                GenericArgData::Lifetime(l) => l
+                    .clone()
+                    .shifted_in_from(TypeFolder::interner(self), outer_binder),
+                _ => panic!("mismatched kinds in substitution"),
+            }
+        } else {
+            bound_var
+                .shifted_out()
+                .unwrap()
+                .shifted_in_from(outer_binder)
+                .to_lifetime(TypeFolder::interner(self))
+        }
+    }
+
+    /// see `fold_free_var_ty`
+    fn fold_free_var_const(
+        &mut self,
+        ty: Ty<I>,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> Const<I> {
+        if let Some(index) = bound_var.index_if_innermost() {
+            match self.parameters[index].data(TypeFolder::interner(self)) {
+                GenericArgData::Const(c) => c
+                    .clone()
+                    .shifted_in_from(TypeFolder::interner(self), outer_binder),
+                _ => panic!("mismatched kinds in substitution"),
+            }
+        } else {
+            bound_var
+                .shifted_out()
+                .unwrap()
+                .shifted_in_from(outer_binder)
+                .to_const(TypeFolder::interner(self), ty)
+        }
+    }
+
+    fn interner(&self) -> I {
+        self.interner
+    }
+}
diff --git a/vendor/chalk-ir/src/interner.rs b/vendor/chalk-ir/src/interner.rs
new file mode 100644
index 000000000..4959e4b74
--- /dev/null
+++ b/vendor/chalk-ir/src/interner.rs
@@ -0,0 +1,702 @@
+//! Encapsulates the concrete representation of core types such as types and goals.
+use crate::AliasTy;
+use crate::AssocTypeId;
+use crate::CanonicalVarKind;
+use crate::CanonicalVarKinds;
+use crate::ClosureId;
+use crate::Constraint;
+use crate::Constraints;
+use crate::FnDefId;
+use crate::ForeignDefId;
+use crate::GeneratorId;
+use crate::GenericArg;
+use crate::GenericArgData;
+use crate::Goal;
+use crate::GoalData;
+use crate::Goals;
+use crate::InEnvironment;
+use crate::Lifetime;
+use crate::LifetimeData;
+use crate::OpaqueTy;
+use crate::OpaqueTyId;
+use crate::ProgramClause;
+use crate::ProgramClauseData;
+use crate::ProgramClauseImplication;
+use crate::ProgramClauses;
+use crate::ProjectionTy;
+use crate::QuantifiedWhereClause;
+use crate::QuantifiedWhereClauses;
+use crate::SeparatorTraitRef;
+use crate::Substitution;
+use crate::TraitId;
+use crate::Ty;
+use crate::TyData;
+use crate::VariableKind;
+use crate::VariableKinds;
+use crate::Variance;
+use crate::Variances;
+use crate::{AdtId, TyKind};
+use crate::{Const, ConstData};
+use std::fmt::{self, Debug};
+use std::hash::Hash;
+use std::marker::PhantomData;
+use std::sync::Arc;
+
+/// A "interner" encapsulates the concrete representation of
+/// certain "core types" from chalk-ir. All the types in chalk-ir are
+/// parameterized by a `I: Interner`, and so (e.g.) if they want to
+/// store a type, they don't store a `Ty<I>` instance directly, but
+/// rather prefer a `Ty<I>`. You can think of `I::Type` as the
+/// interned representation (and, indeed, it may well be an interned
+/// pointer, e.g. in rustc).
+///
+/// Type families allow chalk to be embedded in different contexts
+/// where the concrete representation of core types varies. They also
+/// allow us to write generic code that reasons about multiple
+/// distinct sets of types by using distinct generic type parameters
+/// (e.g., `SourceI` and `TargetI`) -- even if those type parameters
+/// wind up being mapped to the same underlying type families in the
+/// end.
+pub trait Interner: Debug + Copy + Eq + Hash + Sized {
+    /// "Interned" representation of types.  In normal user code,
+    /// `Self::InternedType` is not referenced. Instead, we refer to
+    /// `Ty<Self>`, which wraps this type.
+    ///
+    /// An `InternedType` must be something that can be created from a
+    /// `TyKind` (by the [`intern_ty`][Self::intern_ty] method) and then later
+    /// converted back (by the [`ty_data`][Self::ty_data] method). The interned form
+    /// must also introduce indirection, either via a `Box`, `&`, or
+    /// other pointer type.
+    type InternedType: Debug + Clone + Eq + Hash;
+
+    /// "Interned" representation of lifetimes.  In normal user code,
+    /// `Self::InternedLifetime` is not referenced. Instead, we refer to
+    /// `Lifetime<Self>`, which wraps this type.
+    ///
+    /// An `InternedLifetime` must be something that can be created
+    /// from a `LifetimeData` (by the [`intern_lifetime`][Self::intern_lifetime] method) and
+    /// then later converted back (by the [`lifetime_data`][Self::lifetime_data] method).
+    type InternedLifetime: Debug + Clone + Eq + Hash;
+
+    /// "Interned" representation of const expressions. In normal user code,
+    /// `Self::InternedConst` is not referenced. Instead, we refer to
+    /// `Const<Self>`, which wraps this type.
+    ///
+    /// An `InternedConst` must be something that can be created
+    /// from a `ConstData` (by the [`intern_const`][Self::intern_const] method) and
+    /// then later converted back (by the [`const_data`][Self::const_data] method).
+    type InternedConst: Debug + Clone + Eq + Hash;
+
+    /// "Interned" representation of an evaluated const value.
+    /// `Self::InternedConcreteConst` is not referenced. Instead,
+    /// we refer to `ConcreteConst<Self>`, which wraps this type.
+    ///
+    /// `InternedConcreteConst` instances are not created by chalk,
+    /// it can only make a query asking about equality of two
+    /// evaluated consts.
+    type InternedConcreteConst: Debug + Clone + Eq + Hash;
+
+    /// "Interned" representation of a "generic parameter", which can
+    /// be either a type or a lifetime.  In normal user code,
+    /// `Self::InternedGenericArg` is not referenced. Instead, we refer to
+    /// `GenericArg<Self>`, which wraps this type.
+    ///
+    /// An `InternedType` is created by `intern_generic_arg` and can be
+    /// converted back to its underlying data via `generic_arg_data`.
+    type InternedGenericArg: Debug + Clone + Eq + Hash;
+
+    /// "Interned" representation of a "goal".  In normal user code,
+    /// `Self::InternedGoal` is not referenced. Instead, we refer to
+    /// `Goal<Self>`, which wraps this type.
+    ///
+    /// An `InternedGoal` is created by `intern_goal` and can be
+    /// converted back to its underlying data via `goal_data`.
+    type InternedGoal: Debug + Clone + Eq + Hash;
+
+    /// "Interned" representation of a list of goals.  In normal user code,
+    /// `Self::InternedGoals` is not referenced. Instead, we refer to
+    /// `Goals<Self>`, which wraps this type.
+    ///
+    /// An `InternedGoals` is created by `intern_goals` and can be
+    /// converted back to its underlying data via `goals_data`.
+    type InternedGoals: Debug + Clone + Eq + Hash;
+
+    /// "Interned" representation of a "substitution".  In normal user code,
+    /// `Self::InternedSubstitution` is not referenced. Instead, we refer to
+    /// `Substitution<Self>`, which wraps this type.
+    ///
+    /// An `InternedSubstitution` is created by `intern_substitution` and can be
+    /// converted back to its underlying data via `substitution_data`.
+    type InternedSubstitution: Debug + Clone + Eq + Hash;
+
+    /// "Interned" representation of a list of program clauses.  In normal user code,
+    /// `Self::InternedProgramClauses` is not referenced. Instead, we refer to
+    /// `ProgramClauses<Self>`, which wraps this type.
+    ///
+    /// An `InternedProgramClauses` is created by `intern_program_clauses` and can be
+    /// converted back to its underlying data via `program_clauses_data`.
+    type InternedProgramClauses: Debug + Clone + Eq + Hash;
+
+    /// "Interned" representation of a "program clause".  In normal user code,
+    /// `Self::InternedProgramClause` is not referenced. Instead, we refer to
+    /// `ProgramClause<Self>`, which wraps this type.
+    ///
+    /// An `InternedProgramClause` is created by `intern_program_clause` and can be
+    /// converted back to its underlying data via `program_clause_data`.
+    type InternedProgramClause: Debug + Clone + Eq + Hash;
+
+    /// "Interned" representation of a list of quantified where clauses.
+    /// In normal user code, `Self::InternedQuantifiedWhereClauses` is not referenced.
+    /// Instead, we refer to `QuantifiedWhereClauses<Self>`, which wraps this type.
+    ///
+    /// An `InternedQuantifiedWhereClauses` is created by `intern_quantified_where_clauses`
+    /// and can be converted back to its underlying data via `quantified_where_clauses_data`.
+    type InternedQuantifiedWhereClauses: Debug + Clone + Eq + Hash;
+
+    /// "Interned" representation of a list of variable kinds.
+    /// In normal user code, `Self::InternedVariableKinds` is not referenced.
+    /// Instead, we refer to `VariableKinds<Self>`, which wraps this type.
+    ///
+    /// An `InternedVariableKinds` is created by `intern_generic_arg_kinds`
+    /// and can be converted back to its underlying data via `variable_kinds_data`.
+    type InternedVariableKinds: Debug + Clone + Eq + Hash;
+
+    /// "Interned" representation of a list of variable kinds with universe index.
+    /// In normal user code, `Self::InternedCanonicalVarKinds` is not referenced.
+    /// Instead, we refer to `CanonicalVarKinds<Self>`, which wraps this type.
+    ///
+    /// An `InternedCanonicalVarKinds` is created by
+    /// `intern_canonical_var_kinds` and can be converted back
+    /// to its underlying data via `canonical_var_kinds_data`.
+    type InternedCanonicalVarKinds: Debug + Clone + Eq + Hash;
+
+    /// "Interned" representation of a list of region constraints.
+    /// In normal user code, `Self::InternedConstraints` is not referenced.
+    /// Instead, we refer to `Constraints<Self>`, which wraps this type.
+    ///
+    /// An `InternedConstraints` is created by `intern_constraints`
+    /// and can be converted back to its underlying data via `constraints_data`.
+    type InternedConstraints: Debug + Clone + Eq + Hash;
+
+    /// "Interned" representation of a list of `chalk_ir::Variance`.
+    /// In normal user code, `Self::InternedVariances` is not referenced.
+    /// Instead, we refer to `Variances<Self>`, which wraps this type.
+    ///
+    /// An `InternedVariances` is created by
+    /// `intern_variances` and can be converted back
+    /// to its underlying data via `variances_data`.
+    type InternedVariances: Debug + Clone + Eq + Hash;
+
+    /// The core "id" type used for trait-ids and the like.
+    type DefId: Debug + Copy + Eq + Hash;
+
+    /// The ID type for ADTs
+    type InternedAdtId: Debug + Copy + Eq + Hash;
+
+    /// Representation of identifiers.
+    type Identifier: Debug + Clone + Eq + Hash;
+
+    /// Representation of function ABI (e.g. calling convention).
+    type FnAbi: Debug + Copy + Eq + Hash;
+
+    /// Prints the debug representation of a type-kind-id.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_adt_id(adt_id: AdtId<Self>, fmt: &mut fmt::Formatter<'_>) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a type-kind-id.
+    /// Returns `None` to fallback to the default debug output (e.g.,
+    /// if no info about current program is available from TLS).
+    #[allow(unused_variables)]
+    fn debug_trait_id(
+        trait_id: TraitId<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a type-kind-id.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_assoc_type_id(
+        type_id: AssocTypeId<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of an opaque type.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_opaque_ty_id(
+        opaque_ty_id: OpaqueTyId<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a function-def-id.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_fn_def_id(
+        fn_def_id: FnDefId<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a closure id.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_closure_id(
+        fn_def_id: ClosureId<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a foreign-def-id.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_foreign_def_id(
+        foreign_def_id: ForeignDefId<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of an alias.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_generator_id(
+        generator_id: GeneratorId<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of an alias. To get good
+    /// results, this requires inspecting TLS, and is difficult to
+    /// code without reference to a specific interner (and hence
+    /// fully known types).
+    ///
+    /// Returns `None` to fallback to the default debug output (e.g.,
+    /// if no info about current program is available from TLS).
+    #[allow(unused_variables)]
+    fn debug_alias(alias: &AliasTy<Self>, fmt: &mut fmt::Formatter<'_>) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a ProjectionTy.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_projection_ty(
+        projection_ty: &ProjectionTy<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of an OpaqueTy.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_opaque_ty(
+        opaque_ty: &OpaqueTy<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a type.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_ty(ty: &Ty<Self>, fmt: &mut fmt::Formatter<'_>) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a lifetime.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_lifetime(
+        lifetime: &Lifetime<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a const.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_const(constant: &Const<Self>, fmt: &mut fmt::Formatter<'_>) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of an parameter.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_generic_arg(
+        generic_arg: &GenericArg<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a parameter kinds list.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_variable_kinds(
+        variable_kinds: &VariableKinds<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a parameter kinds list, with angle brackets.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_variable_kinds_with_angles(
+        variable_kinds: &VariableKinds<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of an parameter kinds list with universe index.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_canonical_var_kinds(
+        canonical_var_kinds: &CanonicalVarKinds<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of an goal.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_goal(goal: &Goal<Self>, fmt: &mut fmt::Formatter<'_>) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a list of goals.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_goals(goals: &Goals<Self>, fmt: &mut fmt::Formatter<'_>) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a ProgramClauseImplication.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_program_clause_implication(
+        pci: &ProgramClauseImplication<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a ProgramClause.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_program_clause(
+        clause: &ProgramClause<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a ProgramClauses.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_program_clauses(
+        clauses: &ProgramClauses<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a Substitution.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_substitution(
+        substitution: &Substitution<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a SeparatorTraitRef.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_separator_trait_ref(
+        separator_trait_ref: &SeparatorTraitRef<'_, Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a QuantifiedWhereClauses.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_quantified_where_clauses(
+        clauses: &QuantifiedWhereClauses<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a Constraints.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_constraints(
+        clauses: &Constraints<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Prints the debug representation of a Variances.
+    /// Returns `None` to fallback to the default debug output.
+    #[allow(unused_variables)]
+    fn debug_variances(
+        variances: &Variances<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        None
+    }
+
+    /// Create an "interned" type from `ty`. This is not normally
+    /// invoked directly; instead, you invoke `TyKind::intern` (which
+    /// will ultimately call this method).
+    fn intern_ty(self, kind: TyKind<Self>) -> Self::InternedType;
+
+    /// Lookup the `TyKind` from an interned type.
+    fn ty_data(self, ty: &Self::InternedType) -> &TyData<Self>;
+
+    /// Create an "interned" lifetime from `lifetime`. This is not
+    /// normally invoked directly; instead, you invoke
+    /// `LifetimeData::intern` (which will ultimately call this
+    /// method).
+    fn intern_lifetime(self, lifetime: LifetimeData<Self>) -> Self::InternedLifetime;
+
+    /// Lookup the `LifetimeData` that was interned to create a `InternedLifetime`.
+    fn lifetime_data(self, lifetime: &Self::InternedLifetime) -> &LifetimeData<Self>;
+
+    /// Create an "interned" const from `const`. This is not
+    /// normally invoked directly; instead, you invoke
+    /// `ConstData::intern` (which will ultimately call this
+    /// method).
+    fn intern_const(self, constant: ConstData<Self>) -> Self::InternedConst;
+
+    /// Lookup the `ConstData` that was interned to create a `InternedConst`.
+    fn const_data(self, constant: &Self::InternedConst) -> &ConstData<Self>;
+
+    /// Determine whether two concrete const values are equal.
+    fn const_eq(
+        self,
+        ty: &Self::InternedType,
+        c1: &Self::InternedConcreteConst,
+        c2: &Self::InternedConcreteConst,
+    ) -> bool;
+
+    /// Create an "interned" parameter from `data`. This is not
+    /// normally invoked directly; instead, you invoke
+    /// `GenericArgData::intern` (which will ultimately call this
+    /// method).
+    fn intern_generic_arg(self, data: GenericArgData<Self>) -> Self::InternedGenericArg;
+
+    /// Lookup the `LifetimeData` that was interned to create a `InternedLifetime`.
+    fn generic_arg_data(self, lifetime: &Self::InternedGenericArg) -> &GenericArgData<Self>;
+
+    /// Create an "interned" goal from `data`. This is not
+    /// normally invoked directly; instead, you invoke
+    /// `GoalData::intern` (which will ultimately call this
+    /// method).
+    fn intern_goal(self, data: GoalData<Self>) -> Self::InternedGoal;
+
+    /// Lookup the `GoalData` that was interned to create a `InternedGoal`.
+    fn goal_data(self, goal: &Self::InternedGoal) -> &GoalData<Self>;
+
+    /// Create an "interned" goals from `data`. This is not
+    /// normally invoked directly; instead, you invoke
+    /// `GoalsData::intern` (which will ultimately call this
+    /// method).
+    fn intern_goals<E>(
+        self,
+        data: impl IntoIterator<Item = Result<Goal<Self>, E>>,
+    ) -> Result<Self::InternedGoals, E>;
+
+    /// Lookup the `GoalsData` that was interned to create a `InternedGoals`.
+    fn goals_data(self, goals: &Self::InternedGoals) -> &[Goal<Self>];
+
+    /// Create an "interned" substitution from `data`. This is not
+    /// normally invoked directly; instead, you invoke
+    /// `SubstitutionData::intern` (which will ultimately call this
+    /// method).
+    fn intern_substitution<E>(
+        self,
+        data: impl IntoIterator<Item = Result<GenericArg<Self>, E>>,
+    ) -> Result<Self::InternedSubstitution, E>;
+
+    /// Lookup the `SubstitutionData` that was interned to create a `InternedSubstitution`.
+    fn substitution_data(self, substitution: &Self::InternedSubstitution) -> &[GenericArg<Self>];
+
+    /// Create an "interned" program clause from `data`. This is not
+    /// normally invoked directly; instead, you invoke
+    /// `ProgramClauseData::intern` (which will ultimately call this
+    /// method).
+    fn intern_program_clause(self, data: ProgramClauseData<Self>) -> Self::InternedProgramClause;
+
+    /// Lookup the `ProgramClauseData` that was interned to create a `ProgramClause`.
+    fn program_clause_data(self, clause: &Self::InternedProgramClause) -> &ProgramClauseData<Self>;
+
+    /// Create an "interned" program clauses from `data`. This is not
+    /// normally invoked directly; instead, you invoke
+    /// `ProgramClauses::from_iter` (which will ultimately call this
+    /// method).
+    fn intern_program_clauses<E>(
+        self,
+        data: impl IntoIterator<Item = Result<ProgramClause<Self>, E>>,
+    ) -> Result<Self::InternedProgramClauses, E>;
+
+    /// Lookup the `ProgramClauseData` that was interned to create a `ProgramClause`.
+    fn program_clauses_data(self, clauses: &Self::InternedProgramClauses)
+        -> &[ProgramClause<Self>];
+
+    /// Create an "interned" quantified where clauses from `data`. This is not
+    /// normally invoked directly; instead, you invoke
+    /// `QuantifiedWhereClauses::from_iter` (which will ultimately call this
+    /// method).
+    fn intern_quantified_where_clauses<E>(
+        self,
+        data: impl IntoIterator<Item = Result<QuantifiedWhereClause<Self>, E>>,
+    ) -> Result<Self::InternedQuantifiedWhereClauses, E>;
+
+    /// Lookup the slice of `QuantifiedWhereClause` that was interned to
+    /// create a `QuantifiedWhereClauses`.
+    fn quantified_where_clauses_data(
+        self,
+        clauses: &Self::InternedQuantifiedWhereClauses,
+    ) -> &[QuantifiedWhereClause<Self>];
+
+    /// Create an "interned" parameter kinds from `data`. This is not
+    /// normally invoked directly; instead, you invoke
+    /// `VariableKinds::from_iter` (which will ultimately call this
+    /// method).
+    fn intern_generic_arg_kinds<E>(
+        self,
+        data: impl IntoIterator<Item = Result<VariableKind<Self>, E>>,
+    ) -> Result<Self::InternedVariableKinds, E>;
+
+    /// Lookup the slice of `VariableKinds` that was interned to
+    /// create a `VariableKinds`.
+    fn variable_kinds_data(
+        self,
+        variable_kinds: &Self::InternedVariableKinds,
+    ) -> &[VariableKind<Self>];
+
+    /// Create "interned" variable kinds with universe index from `data`. This is not
+    /// normally invoked directly; instead, you invoke
+    /// `CanonicalVarKinds::from_iter` (which will ultimately call this
+    /// method).
+    fn intern_canonical_var_kinds<E>(
+        self,
+        data: impl IntoIterator<Item = Result<CanonicalVarKind<Self>, E>>,
+    ) -> Result<Self::InternedCanonicalVarKinds, E>;
+
+    /// Lookup the slice of `CanonicalVariableKind` that was interned to
+    /// create a `CanonicalVariableKinds`.
+    fn canonical_var_kinds_data(
+        self,
+        canonical_var_kinds: &Self::InternedCanonicalVarKinds,
+    ) -> &[CanonicalVarKind<Self>];
+
+    /// Create "interned" constraints from `data`. This is not
+    /// normally invoked dirctly; instead, you invoke
+    /// `Constraints::from_iter` (which will ultimately call this
+    /// method).
+    fn intern_constraints<E>(
+        self,
+        data: impl IntoIterator<Item = Result<InEnvironment<Constraint<Self>>, E>>,
+    ) -> Result<Self::InternedConstraints, E>;
+
+    /// Lookup the slice of `Constraint` that was interned to
+    /// create a `Constraints`.
+    fn constraints_data(
+        self,
+        constraints: &Self::InternedConstraints,
+    ) -> &[InEnvironment<Constraint<Self>>];
+
+    /// Create "interned" variances from `data`. This is not
+    /// normally invoked directly; instead, you invoke
+    /// `Variances::from` (which will ultimately call this
+    /// method).
+    fn intern_variances<E>(
+        self,
+        data: impl IntoIterator<Item = Result<Variance, E>>,
+    ) -> Result<Self::InternedVariances, E>;
+
+    /// Lookup the slice of `Variance` that was interned to
+    /// create a `Variances`.
+    fn variances_data(self, variances: &Self::InternedVariances) -> &[Variance];
+}
+
+/// Implemented by types that have an associated interner (which
+/// are virtually all of the types in chalk-ir, for example).
+/// This lets us map from a type like `Ty<I>` to the parameter `I`.
+///
+/// It's particularly useful for writing `TypeFoldable` impls for generic types like
+/// `Binder<T>`, since it allows us to figure out the interner of `T`.
+pub trait HasInterner {
+    /// The interner associated with the type.
+    type Interner: Interner;
+}
+
+impl<T: HasInterner> HasInterner for [T] {
+    type Interner = T::Interner;
+}
+
+impl<T: HasInterner> HasInterner for Vec<T> {
+    type Interner = T::Interner;
+}
+
+impl<T: HasInterner> HasInterner for Box<T> {
+    type Interner = T::Interner;
+}
+
+impl<T: HasInterner> HasInterner for Arc<T> {
+    type Interner = T::Interner;
+}
+
+impl<T: HasInterner + ?Sized> HasInterner for &T {
+    type Interner = T::Interner;
+}
+
+impl<I: Interner> HasInterner for PhantomData<I> {
+    type Interner = I;
+}
+
+impl<A, B, I> HasInterner for (A, B)
+where
+    A: HasInterner<Interner = I>,
+    B: HasInterner<Interner = I>,
+    I: Interner,
+{
+    type Interner = I;
+}
+
+impl<A, B, C, I> HasInterner for (A, B, C)
+where
+    A: HasInterner<Interner = I>,
+    B: HasInterner<Interner = I>,
+    C: HasInterner<Interner = I>,
+    I: Interner,
+{
+    type Interner = I;
+}
+
+impl<'a, T: HasInterner> HasInterner for std::slice::Iter<'a, T> {
+    type Interner = T::Interner;
+}
diff --git a/vendor/chalk-ir/src/lib.rs b/vendor/chalk-ir/src/lib.rs
new file mode 100644
index 000000000..5cf44b0c3
--- /dev/null
+++ b/vendor/chalk-ir/src/lib.rs
@@ -0,0 +1,3055 @@
+//! Defines the IR for types and logical predicates.
+
+#![deny(rust_2018_idioms)]
+#![warn(missing_docs)]
+
+// Allows macros to refer to this crate as `::chalk_ir`
+extern crate self as chalk_ir;
+
+use crate::cast::{Cast, CastTo, Caster};
+use crate::fold::shift::Shift;
+use crate::fold::{FallibleTypeFolder, Subst, TypeFoldable, TypeFolder, TypeSuperFoldable};
+use crate::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitor, VisitExt};
+use chalk_derive::{
+    FallibleTypeFolder, HasInterner, TypeFoldable, TypeSuperVisitable, TypeVisitable, Zip,
+};
+use std::marker::PhantomData;
+use std::ops::ControlFlow;
+
+pub use crate::debug::SeparatorTraitRef;
+#[macro_use(bitflags)]
+extern crate bitflags;
+/// Uninhabited (empty) type, used in combination with `PhantomData`.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum Void {}
+
+/// Many of our internal operations (e.g., unification) are an attempt
+/// to perform some operation which may not complete.
+pub type Fallible<T> = Result<T, NoSolution>;
+
+/// A combination of `Fallible` and `Floundered`.
+pub enum FallibleOrFloundered<T> {
+    /// Success
+    Ok(T),
+    /// No solution. See `chalk_ir::NoSolution`.
+    NoSolution,
+    /// Floundered. See `chalk_ir::Floundered`.
+    Floundered,
+}
+
+/// Indicates that the attempted operation has "no solution" -- i.e.,
+/// cannot be performed.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct NoSolution;
+
+/// Indicates that the complete set of program clauses for this goal
+/// cannot be enumerated.
+pub struct Floundered;
+
+macro_rules! impl_debugs {
+    ($($id:ident), *) => {
+        $(
+            impl<I: Interner> std::fmt::Debug for $id<I> {
+                fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
+                    write!(fmt, "{}({:?})", stringify!($id), self.0)
+                }
+            }
+        )*
+    };
+}
+
+#[macro_use]
+pub mod zip;
+
+#[macro_use]
+pub mod fold;
+
+#[macro_use]
+pub mod visit;
+
+pub mod cast;
+
+pub mod interner;
+use interner::{HasInterner, Interner};
+
+pub mod could_match;
+pub mod debug;
+
+/// Variance
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub enum Variance {
+    /// a <: b
+    Covariant,
+    /// a == b
+    Invariant,
+    /// b <: a
+    Contravariant,
+}
+
+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:
+    ///
+    /// ```ignore
+    /// *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:
+    ///
+    /// ```ignore
+    /// 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.
+    /// (Doc from rustc)
+    pub fn xform(self, other: Variance) -> Variance {
+        match (self, other) {
+            (Variance::Invariant, _) => Variance::Invariant,
+            (_, Variance::Invariant) => Variance::Invariant,
+            (_, Variance::Covariant) => self,
+            (Variance::Covariant, Variance::Contravariant) => Variance::Contravariant,
+            (Variance::Contravariant, Variance::Contravariant) => Variance::Covariant,
+        }
+    }
+
+    /// Converts `Covariant` into `Contravariant` and vice-versa. `Invariant`
+    /// stays the same.
+    pub fn invert(self) -> Variance {
+        match self {
+            Variance::Invariant => Variance::Invariant,
+            Variance::Covariant => Variance::Contravariant,
+            Variance::Contravariant => Variance::Covariant,
+        }
+    }
+}
+
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner)]
+/// The set of assumptions we've made so far, and the current number of
+/// universal (forall) quantifiers we're within.
+pub struct Environment<I: Interner> {
+    /// The clauses in the environment.
+    pub clauses: ProgramClauses<I>,
+}
+
+impl<I: Interner> Copy for Environment<I> where I::InternedProgramClauses: Copy {}
+
+impl<I: Interner> Environment<I> {
+    /// Creates a new environment.
+    pub fn new(interner: I) -> Self {
+        Environment {
+            clauses: ProgramClauses::empty(interner),
+        }
+    }
+
+    /// Adds (an iterator of) clauses to the environment.
+    pub fn add_clauses<II>(&self, interner: I, clauses: II) -> Self
+    where
+        II: IntoIterator<Item = ProgramClause<I>>,
+    {
+        let mut env = self.clone();
+        env.clauses =
+            ProgramClauses::from_iter(interner, env.clauses.iter(interner).cloned().chain(clauses));
+        env
+    }
+
+    /// True if any of the clauses in the environment have a consequence of `Compatible`.
+    /// Panics if the conditions or constraints of that clause are not empty.
+    pub fn has_compatible_clause(&self, interner: I) -> bool {
+        self.clauses.as_slice(interner).iter().any(|c| {
+            let ProgramClauseData(implication) = c.data(interner);
+            match implication.skip_binders().consequence {
+                DomainGoal::Compatible => {
+                    // We currently don't generate `Compatible` with any conditions or constraints
+                    // If this was needed, for whatever reason, then a third "yes, but must evaluate"
+                    // return value would have to be added.
+                    assert!(implication.skip_binders().conditions.is_empty(interner));
+                    assert!(implication.skip_binders().constraints.is_empty(interner));
+                    true
+                }
+                _ => false,
+            }
+        })
+    }
+}
+
+/// A goal with an environment to solve it in.
+#[derive(Clone, Debug, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable)]
+#[allow(missing_docs)]
+pub struct InEnvironment<G: HasInterner> {
+    pub environment: Environment<G::Interner>,
+    pub goal: G,
+}
+
+impl<G: HasInterner<Interner = I> + Copy, I: Interner> Copy for InEnvironment<G> where
+    I::InternedProgramClauses: Copy
+{
+}
+
+impl<G: HasInterner> InEnvironment<G> {
+    /// Creates a new environment/goal pair.
+    pub fn new(environment: &Environment<G::Interner>, goal: G) -> Self {
+        InEnvironment {
+            environment: environment.clone(),
+            goal,
+        }
+    }
+
+    /// Maps the goal without touching the environment.
+    pub fn map<OP, H>(self, op: OP) -> InEnvironment<H>
+    where
+        OP: FnOnce(G) -> H,
+        H: HasInterner<Interner = G::Interner>,
+    {
+        InEnvironment {
+            environment: self.environment,
+            goal: op(self.goal),
+        }
+    }
+}
+
+impl<G: HasInterner> HasInterner for InEnvironment<G> {
+    type Interner = G::Interner;
+}
+
+/// Different signed int types.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[allow(missing_docs)]
+pub enum IntTy {
+    Isize,
+    I8,
+    I16,
+    I32,
+    I64,
+    I128,
+}
+
+/// Different unsigned int types.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[allow(missing_docs)]
+pub enum UintTy {
+    Usize,
+    U8,
+    U16,
+    U32,
+    U64,
+    U128,
+}
+
+/// Different kinds of float types.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[allow(missing_docs)]
+pub enum FloatTy {
+    F32,
+    F64,
+}
+
+/// Types of scalar values.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[allow(missing_docs)]
+pub enum Scalar {
+    Bool,
+    Char,
+    Int(IntTy),
+    Uint(UintTy),
+    Float(FloatTy),
+}
+
+/// Whether a function is safe or not.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum Safety {
+    /// Safe
+    Safe,
+    /// Unsafe
+    Unsafe,
+}
+
+/// Whether a type is mutable or not.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum Mutability {
+    /// Mutable
+    Mut,
+    /// Immutable
+    Not,
+}
+
+/// An universe index is how a universally quantified parameter is
+/// represented when it's binder is moved into the environment.
+/// An example chain of transformations would be:
+/// `forall<T> { Goal(T) }` (syntactical representation)
+/// `forall { Goal(?0) }` (used a DeBruijn index)
+/// `Goal(!U1)` (the quantifier was moved to the environment and replaced with a universe index)
+/// See <https://rustc-dev-guide.rust-lang.org/borrow_check/region_inference.html#placeholders-and-universes> for more.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct UniverseIndex {
+    /// The counter for the universe index, starts with 0.
+    pub counter: usize,
+}
+
+impl UniverseIndex {
+    /// Root universe index (0).
+    pub const ROOT: UniverseIndex = UniverseIndex { counter: 0 };
+
+    /// Root universe index (0).
+    pub fn root() -> UniverseIndex {
+        Self::ROOT
+    }
+
+    /// Whether one universe can "see" another.
+    pub fn can_see(self, ui: UniverseIndex) -> bool {
+        self.counter >= ui.counter
+    }
+
+    /// Increases the index counter.
+    pub fn next(self) -> UniverseIndex {
+        UniverseIndex {
+            counter: self.counter + 1,
+        }
+    }
+}
+
+/// Maps the universes found in the `u_canonicalize` result (the
+/// "canonical" universes) to the universes found in the original
+/// value (and vice versa). When used as a folder -- i.e., from
+/// outside this module -- converts from "canonical" universes to the
+/// original (but see the `UMapToCanonical` folder).
+#[derive(Clone, Debug)]
+pub struct UniverseMap {
+    /// A reverse map -- for each universe Ux that appears in
+    /// `quantified`, the corresponding universe in the original was
+    /// `universes[x]`.
+    pub universes: Vec<UniverseIndex>,
+}
+
+impl UniverseMap {
+    /// Creates a new universe map.
+    pub fn new() -> Self {
+        UniverseMap {
+            universes: vec![UniverseIndex::root()],
+        }
+    }
+
+    /// Number of canonical universes.
+    pub fn num_canonical_universes(&self) -> usize {
+        self.universes.len()
+    }
+}
+
+/// The id for an Abstract Data Type (i.e. structs, unions and enums).
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct AdtId<I: Interner>(pub I::InternedAdtId);
+
+/// The id of a trait definition; could be used to load the trait datum by
+/// invoking the [`trait_datum`] method.
+///
+/// [`trait_datum`]: ../chalk_solve/trait.RustIrDatabase.html#tymethod.trait_datum
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct TraitId<I: Interner>(pub I::DefId);
+
+/// The id for an impl.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ImplId<I: Interner>(pub I::DefId);
+
+/// Id for a specific clause.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ClauseId<I: Interner>(pub I::DefId);
+
+/// The id for the associated type member of a trait. The details of the type
+/// can be found by invoking the [`associated_ty_data`] method.
+///
+/// [`associated_ty_data`]: ../chalk_solve/trait.RustIrDatabase.html#tymethod.associated_ty_data
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct AssocTypeId<I: Interner>(pub I::DefId);
+
+/// Id for an opaque type.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct OpaqueTyId<I: Interner>(pub I::DefId);
+
+/// Function definition id.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct FnDefId<I: Interner>(pub I::DefId);
+
+/// Id for Rust closures.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ClosureId<I: Interner>(pub I::DefId);
+
+/// Id for Rust generators.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct GeneratorId<I: Interner>(pub I::DefId);
+
+/// Id for foreign types.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ForeignDefId<I: Interner>(pub I::DefId);
+
+impl_debugs!(ImplId, ClauseId);
+
+/// A Rust type. The actual type data is stored in `TyKind`.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, HasInterner)]
+pub struct Ty<I: Interner> {
+    interned: I::InternedType,
+}
+
+impl<I: Interner> Ty<I> {
+    /// Creates a type from `TyKind`.
+    pub fn new(interner: I, data: impl CastTo<TyKind<I>>) -> Self {
+        let ty_kind = data.cast(interner);
+        Ty {
+            interned: I::intern_ty(interner, ty_kind),
+        }
+    }
+
+    /// Gets the interned type.
+    pub fn interned(&self) -> &I::InternedType {
+        &self.interned
+    }
+
+    /// Gets the underlying type data.
+    pub fn data(&self, interner: I) -> &TyData<I> {
+        I::ty_data(interner, &self.interned)
+    }
+
+    /// Gets the underlying type kind.
+    pub fn kind(&self, interner: I) -> &TyKind<I> {
+        &I::ty_data(interner, &self.interned).kind
+    }
+
+    /// Creates a `FromEnv` constraint using this type.
+    pub fn from_env(&self) -> FromEnv<I> {
+        FromEnv::Ty(self.clone())
+    }
+
+    /// Creates a WF-constraint for this type.
+    pub fn well_formed(&self) -> WellFormed<I> {
+        WellFormed::Ty(self.clone())
+    }
+
+    /// Creates a domain goal `FromEnv(T)` where `T` is this type.
+    pub fn into_from_env_goal(self, interner: I) -> DomainGoal<I> {
+        self.from_env().cast(interner)
+    }
+
+    /// If this is a `TyKind::BoundVar(d)`, returns `Some(d)` else `None`.
+    pub fn bound_var(&self, interner: I) -> Option<BoundVar> {
+        if let TyKind::BoundVar(bv) = self.kind(interner) {
+            Some(*bv)
+        } else {
+            None
+        }
+    }
+
+    /// If this is a `TyKind::InferenceVar(d)`, returns `Some(d)` else `None`.
+    pub fn inference_var(&self, interner: I) -> Option<InferenceVar> {
+        if let TyKind::InferenceVar(depth, _) = self.kind(interner) {
+            Some(*depth)
+        } else {
+            None
+        }
+    }
+
+    /// Returns true if this is a `BoundVar` or an `InferenceVar` of `TyVariableKind::General`.
+    pub fn is_general_var(&self, interner: I, binders: &CanonicalVarKinds<I>) -> bool {
+        match self.kind(interner) {
+            TyKind::BoundVar(bv)
+                if bv.debruijn == DebruijnIndex::INNERMOST
+                    && binders.at(interner, bv.index).kind
+                        == VariableKind::Ty(TyVariableKind::General) =>
+            {
+                true
+            }
+            TyKind::InferenceVar(_, TyVariableKind::General) => true,
+            _ => false,
+        }
+    }
+
+    /// Returns true if this is an `Alias`.
+    pub fn is_alias(&self, interner: I) -> bool {
+        matches!(self.kind(interner), TyKind::Alias(..))
+    }
+
+    /// Returns true if this is an `IntTy` or `UintTy`.
+    pub fn is_integer(&self, interner: I) -> bool {
+        matches!(
+            self.kind(interner),
+            TyKind::Scalar(Scalar::Int(_) | Scalar::Uint(_))
+        )
+    }
+
+    /// Returns true if this is a `FloatTy`.
+    pub fn is_float(&self, interner: I) -> bool {
+        matches!(self.kind(interner), TyKind::Scalar(Scalar::Float(_)))
+    }
+
+    /// Returns `Some(adt_id)` if this is an ADT, `None` otherwise
+    pub fn adt_id(&self, interner: I) -> Option<AdtId<I>> {
+        match self.kind(interner) {
+            TyKind::Adt(adt_id, _) => Some(*adt_id),
+            _ => None,
+        }
+    }
+
+    /// True if this type contains "bound" types/lifetimes, and hence
+    /// needs to be shifted across binders. This is a very inefficient
+    /// check, intended only for debug assertions, because I am lazy.
+    pub fn needs_shift(&self, interner: I) -> bool {
+        self.has_free_vars(interner)
+    }
+}
+
+/// Contains the data for a Ty
+#[derive(Clone, PartialEq, Eq, Hash, HasInterner)]
+pub struct TyData<I: Interner> {
+    /// The kind
+    pub kind: TyKind<I>,
+    /// Type flags
+    pub flags: TypeFlags,
+}
+
+bitflags! {
+    /// Contains flags indicating various properties of a Ty
+    pub struct TypeFlags : u16 {
+        /// Does the type contain an InferenceVar
+        const HAS_TY_INFER                = 1;
+        /// Does the type contain a lifetime with an InferenceVar
+        const HAS_RE_INFER                = 1 << 1;
+        /// Does the type contain a ConstValue with an InferenceVar
+        const HAS_CT_INFER                = 1 << 2;
+        /// Does the type contain a Placeholder TyKind
+        const HAS_TY_PLACEHOLDER          = 1 << 3;
+        /// Does the type contain a lifetime with a Placeholder
+        const HAS_RE_PLACEHOLDER          = 1 << 4;
+        /// Does the type contain a ConstValue Placeholder
+        const HAS_CT_PLACEHOLDER          = 1 << 5;
+        /// True when the type has free lifetimes related to a local context
+        const HAS_FREE_LOCAL_REGIONS      = 1 << 6;
+        /// Does the type contain a projection of an associated type
+        const HAS_TY_PROJECTION           = 1 << 7;
+        /// Does the type contain an opaque type
+        const HAS_TY_OPAQUE               = 1 << 8;
+        /// Does the type contain an unevaluated const projection
+        const HAS_CT_PROJECTION           = 1 << 9;
+        /// Does the type contain an error
+        const HAS_ERROR                   = 1 << 10;
+        /// Does the type contain any free lifetimes
+        const HAS_FREE_REGIONS            = 1 << 11;
+        /// True when the type contains lifetimes that will be substituted when function is called
+        const HAS_RE_LATE_BOUND           = 1 << 12;
+        /// True when the type contains an erased lifetime
+        const HAS_RE_ERASED               = 1 << 13;
+        /// Does the type contain placeholders or inference variables that could be replaced later
+        const STILL_FURTHER_SPECIALIZABLE = 1 << 14;
+
+        /// True when the type contains free names local to a particular context
+        const HAS_FREE_LOCAL_NAMES        = TypeFlags::HAS_TY_INFER.bits
+                                          | TypeFlags::HAS_CT_INFER.bits
+                                          | TypeFlags::HAS_TY_PLACEHOLDER.bits
+                                          | TypeFlags::HAS_CT_PLACEHOLDER.bits
+                                          | TypeFlags::HAS_FREE_LOCAL_REGIONS.bits;
+
+        /// Does the type contain any form of projection
+        const HAS_PROJECTION              = TypeFlags::HAS_TY_PROJECTION.bits
+                                          | TypeFlags::HAS_TY_OPAQUE.bits
+                                          | TypeFlags::HAS_CT_PROJECTION.bits;
+    }
+}
+/// Type data, which holds the actual type information.
+#[derive(Clone, PartialEq, Eq, Hash, HasInterner)]
+pub enum TyKind<I: Interner> {
+    /// Abstract data types, i.e., structs, unions, or enumerations.
+    /// For example, a type like `Vec<T>`.
+    Adt(AdtId<I>, Substitution<I>),
+
+    /// an associated type like `Iterator::Item`; see `AssociatedType` for details
+    AssociatedType(AssocTypeId<I>, Substitution<I>),
+
+    /// a scalar type like `bool` or `u32`
+    Scalar(Scalar),
+
+    /// a tuple of the given arity
+    Tuple(usize, Substitution<I>),
+
+    /// an array type like `[T; N]`
+    Array(Ty<I>, Const<I>),
+
+    /// a slice type like `[T]`
+    Slice(Ty<I>),
+
+    /// a raw pointer type like `*const T` or `*mut T`
+    Raw(Mutability, Ty<I>),
+
+    /// a reference type like `&T` or `&mut T`
+    Ref(Mutability, Lifetime<I>, Ty<I>),
+
+    /// a placeholder for opaque types like `impl Trait`
+    OpaqueType(OpaqueTyId<I>, Substitution<I>),
+
+    /// a function definition
+    FnDef(FnDefId<I>, Substitution<I>),
+
+    /// the string primitive type
+    Str,
+
+    /// the never type `!`
+    Never,
+
+    /// A closure.
+    Closure(ClosureId<I>, Substitution<I>),
+
+    /// A generator.
+    Generator(GeneratorId<I>, Substitution<I>),
+
+    /// A generator witness.
+    GeneratorWitness(GeneratorId<I>, Substitution<I>),
+
+    /// foreign types
+    Foreign(ForeignDefId<I>),
+
+    /// This can be used to represent an error, e.g. during name resolution of a type.
+    /// Chalk itself will not produce this, just pass it through when given.
+    Error,
+
+    /// instantiated from a universally quantified type, e.g., from
+    /// `forall<T> { .. }`. Stands in as a representative of "some
+    /// unknown type".
+    Placeholder(PlaceholderIndex),
+
+    /// A "dyn" type is a trait object type created via the "dyn Trait" syntax.
+    /// In the chalk parser, the traits that the object represents is parsed as
+    /// a QuantifiedInlineBound, and is then changed to a list of where clauses
+    /// during lowering.
+    ///
+    /// See the `Opaque` variant for a discussion about the use of
+    /// binders here.
+    Dyn(DynTy<I>),
+
+    /// An "alias" type represents some form of type alias, such as:
+    /// - An associated type projection like `<T as Iterator>::Item`
+    /// - `impl Trait` types
+    /// - Named type aliases like `type Foo<X> = Vec<X>`
+    Alias(AliasTy<I>),
+
+    /// A function type such as `for<'a> fn(&'a u32)`.
+    /// Note that "higher-ranked" types (starting with `for<>`) are either
+    /// function types or dyn types, and do not appear otherwise in Rust
+    /// surface syntax.
+    Function(FnPointer<I>),
+
+    /// References the binding at the given depth. The index is a [de
+    /// Bruijn index], so it counts back through the in-scope binders.
+    BoundVar(BoundVar),
+
+    /// Inference variable defined in the current inference context.
+    InferenceVar(InferenceVar, TyVariableKind),
+}
+
+impl<I: Interner> Copy for TyKind<I>
+where
+    I::InternedLifetime: Copy,
+    I::InternedSubstitution: Copy,
+    I::InternedVariableKinds: Copy,
+    I::InternedQuantifiedWhereClauses: Copy,
+    I::InternedType: Copy,
+    I::InternedConst: Copy,
+{
+}
+
+impl<I: Interner> TyKind<I> {
+    /// Casts the type data to a type.
+    pub fn intern(self, interner: I) -> Ty<I> {
+        Ty::new(interner, self)
+    }
+
+    /// Compute type flags for a TyKind
+    pub fn compute_flags(&self, interner: I) -> TypeFlags {
+        match self {
+            TyKind::Adt(_, substitution)
+            | TyKind::AssociatedType(_, substitution)
+            | TyKind::Tuple(_, substitution)
+            | TyKind::Closure(_, substitution)
+            | TyKind::Generator(_, substitution)
+            | TyKind::GeneratorWitness(_, substitution)
+            | TyKind::FnDef(_, substitution)
+            | TyKind::OpaqueType(_, substitution) => substitution.compute_flags(interner),
+            TyKind::Scalar(_) | TyKind::Str | TyKind::Never | TyKind::Foreign(_) => {
+                TypeFlags::empty()
+            }
+            TyKind::Error => TypeFlags::HAS_ERROR,
+            TyKind::Slice(ty) | TyKind::Raw(_, ty) => ty.data(interner).flags,
+            TyKind::Ref(_, lifetime, ty) => {
+                lifetime.compute_flags(interner) | ty.data(interner).flags
+            }
+            TyKind::Array(ty, const_ty) => {
+                let flags = ty.data(interner).flags;
+                let const_data = const_ty.data(interner);
+                flags
+                    | const_data.ty.data(interner).flags
+                    | match const_data.value {
+                        ConstValue::BoundVar(_) | ConstValue::Concrete(_) => TypeFlags::empty(),
+                        ConstValue::InferenceVar(_) => {
+                            TypeFlags::HAS_CT_INFER | TypeFlags::STILL_FURTHER_SPECIALIZABLE
+                        }
+                        ConstValue::Placeholder(_) => {
+                            TypeFlags::HAS_CT_PLACEHOLDER | TypeFlags::STILL_FURTHER_SPECIALIZABLE
+                        }
+                    }
+            }
+            TyKind::Placeholder(_) => TypeFlags::HAS_TY_PLACEHOLDER,
+            TyKind::Dyn(dyn_ty) => {
+                let lifetime_flags = dyn_ty.lifetime.compute_flags(interner);
+                let mut dyn_flags = TypeFlags::empty();
+                for var_kind in dyn_ty.bounds.skip_binders().iter(interner) {
+                    match &(var_kind.skip_binders()) {
+                        WhereClause::Implemented(trait_ref) => {
+                            dyn_flags |= trait_ref.substitution.compute_flags(interner)
+                        }
+                        WhereClause::AliasEq(alias_eq) => {
+                            dyn_flags |= alias_eq.alias.compute_flags(interner);
+                            dyn_flags |= alias_eq.ty.data(interner).flags;
+                        }
+                        WhereClause::LifetimeOutlives(lifetime_outlives) => {
+                            dyn_flags |= lifetime_outlives.a.compute_flags(interner)
+                                | lifetime_outlives.b.compute_flags(interner);
+                        }
+                        WhereClause::TypeOutlives(type_outlives) => {
+                            dyn_flags |= type_outlives.ty.data(interner).flags;
+                            dyn_flags |= type_outlives.lifetime.compute_flags(interner);
+                        }
+                    }
+                }
+                lifetime_flags | dyn_flags
+            }
+            TyKind::Alias(alias_ty) => alias_ty.compute_flags(interner),
+            TyKind::BoundVar(_) => TypeFlags::empty(),
+            TyKind::InferenceVar(_, _) => TypeFlags::HAS_TY_INFER,
+            TyKind::Function(fn_pointer) => fn_pointer.substitution.0.compute_flags(interner),
+        }
+    }
+}
+
+/// Identifies a particular bound variable within a binder.
+/// Variables are identified by the combination of a [`DebruijnIndex`],
+/// which identifies the *binder*, and an index within that binder.
+///
+/// Consider this case:
+///
+/// ```ignore
+/// forall<'a, 'b> { forall<'c, 'd> { ... } }
+/// ```
+///
+/// Within the `...` term:
+///
+/// * the variable `'a` have a debruijn index of 1 and index 0
+/// * the variable `'b` have a debruijn index of 1 and index 1
+/// * the variable `'c` have a debruijn index of 0 and index 0
+/// * the variable `'d` have a debruijn index of 0 and index 1
+///
+/// The variables `'a` and `'b` both have debruijn index of 1 because,
+/// counting out, they are the 2nd binder enclosing `...`. The indices
+/// identify the location *within* that binder.
+///
+/// The variables `'c` and `'d` both have debruijn index of 0 because
+/// they appear in the *innermost* binder enclosing the `...`. The
+/// indices identify the location *within* that binder.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub struct BoundVar {
+    /// Debruijn index, which identifies the binder.
+    pub debruijn: DebruijnIndex,
+    /// Index within the binder.
+    pub index: usize,
+}
+
+impl BoundVar {
+    /// Creates a new bound variable.
+    pub fn new(debruijn: DebruijnIndex, index: usize) -> Self {
+        Self { debruijn, index }
+    }
+
+    /// Casts the bound variable to a type.
+    pub fn to_ty<I: Interner>(self, interner: I) -> Ty<I> {
+        TyKind::<I>::BoundVar(self).intern(interner)
+    }
+
+    /// Wrap the bound variable in a lifetime.
+    pub fn to_lifetime<I: Interner>(self, interner: I) -> Lifetime<I> {
+        LifetimeData::<I>::BoundVar(self).intern(interner)
+    }
+
+    /// Wraps the bound variable in a constant.
+    pub fn to_const<I: Interner>(self, interner: I, ty: Ty<I>) -> Const<I> {
+        ConstData {
+            ty,
+            value: ConstValue::<I>::BoundVar(self),
+        }
+        .intern(interner)
+    }
+
+    /// True if this variable is bound within the `amount` innermost binders.
+    pub fn bound_within(self, outer_binder: DebruijnIndex) -> bool {
+        self.debruijn.within(outer_binder)
+    }
+
+    /// Adjusts the debruijn index (see [`DebruijnIndex::shifted_in`]).
+    #[must_use]
+    pub fn shifted_in(self) -> Self {
+        BoundVar::new(self.debruijn.shifted_in(), self.index)
+    }
+
+    /// Adjusts the debruijn index (see [`DebruijnIndex::shifted_in`]).
+    #[must_use]
+    pub fn shifted_in_from(self, outer_binder: DebruijnIndex) -> Self {
+        BoundVar::new(self.debruijn.shifted_in_from(outer_binder), self.index)
+    }
+
+    /// Adjusts the debruijn index (see [`DebruijnIndex::shifted_in`]).
+    #[must_use]
+    pub fn shifted_out(self) -> Option<Self> {
+        self.debruijn
+            .shifted_out()
+            .map(|db| BoundVar::new(db, self.index))
+    }
+
+    /// Adjusts the debruijn index (see [`DebruijnIndex::shifted_in`]).
+    #[must_use]
+    pub fn shifted_out_to(self, outer_binder: DebruijnIndex) -> Option<Self> {
+        self.debruijn
+            .shifted_out_to(outer_binder)
+            .map(|db| BoundVar::new(db, self.index))
+    }
+
+    /// Return the index of the bound variable, but only if it is bound
+    /// at the innermost binder. Otherwise, returns `None`.
+    pub fn index_if_innermost(self) -> Option<usize> {
+        self.index_if_bound_at(DebruijnIndex::INNERMOST)
+    }
+
+    /// Return the index of the bound variable, but only if it is bound
+    /// at the innermost binder. Otherwise, returns `None`.
+    pub fn index_if_bound_at(self, debruijn: DebruijnIndex) -> Option<usize> {
+        if self.debruijn == debruijn {
+            Some(self.index)
+        } else {
+            None
+        }
+    }
+}
+
+/// References the binder at the given depth. The index is a [de
+/// Bruijn index], so it counts back through the in-scope binders,
+/// with 0 being the innermost binder. This is used in impls and
+/// the like. For example, if we had a rule like `for<T> { (T:
+/// Clone) :- (T: Copy) }`, then `T` would be represented as a
+/// `BoundVar(0)` (as the `for` is the innermost binder).
+///
+/// [de Bruijn index]: https://en.wikipedia.org/wiki/De_Bruijn_index
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub struct DebruijnIndex {
+    depth: u32,
+}
+
+impl DebruijnIndex {
+    /// Innermost index.
+    pub const INNERMOST: DebruijnIndex = DebruijnIndex { depth: 0 };
+    /// One level higher than the innermost index.
+    pub const ONE: DebruijnIndex = DebruijnIndex { depth: 1 };
+
+    /// Creates a new de Bruijn index with a given depth.
+    pub fn new(depth: u32) -> Self {
+        DebruijnIndex { depth }
+    }
+
+    /// Depth of the De Bruijn index, counting from 0 starting with
+    /// the innermost binder.
+    pub fn depth(self) -> u32 {
+        self.depth
+    }
+
+    /// True if the binder identified by this index is within the
+    /// binder identified by the index `outer_binder`.
+    ///
+    /// # Example
+    ///
+    /// Imagine you have the following binders in scope
+    ///
+    /// ```ignore
+    /// forall<a> forall<b> forall<c>
+    /// ```
+    ///
+    /// then the Debruijn index for `c` would be `0`, the index for
+    /// `b` would be 1, and so on. Now consider the following calls:
+    ///
+    /// * `c.within(a) = true`
+    /// * `b.within(a) = true`
+    /// * `a.within(a) = false`
+    /// * `a.within(c) = false`
+    pub fn within(self, outer_binder: DebruijnIndex) -> bool {
+        self < outer_binder
+    }
+
+    /// Returns the resulting index when this value is moved into
+    /// through one binder.
+    #[must_use]
+    pub fn shifted_in(self) -> DebruijnIndex {
+        self.shifted_in_from(DebruijnIndex::ONE)
+    }
+
+    /// Update this index in place by shifting it "in" through
+    /// `amount` number of binders.
+    pub fn shift_in(&mut self) {
+        *self = self.shifted_in();
+    }
+
+    /// Adds `outer_binder` levels to the `self` index. Intuitively, this
+    /// shifts the `self` index, which was valid at the outer binder,
+    /// so that it is valid at the innermost binder.
+    ///
+    /// Example: Assume that the following binders are in scope:
+    ///
+    /// ```ignore
+    /// for<A> for<B> for<C> for<D>
+    ///            ^ outer binder
+    /// ```
+    ///
+    /// Assume further that the `outer_binder` argument is 2,
+    /// which means that it is referring to the `for<B>` binder
+    /// (since `D` would be the innermost binder).
+    ///
+    /// This means that `self` is relative to the binder `B` -- so
+    /// if `self` is 0 (`INNERMOST`), then it refers to `B`,
+    /// and if `self` is 1, then it refers to `A`.
+    ///
+    /// We will return as follows:
+    ///
+    /// * `0.shifted_in_from(2) = 2` -- i.e., `B`, when shifted in to the binding level `D`, has index 2
+    /// * `1.shifted_in_from(2) = 3` -- i.e., `A`, when shifted in to the binding level `D`, has index 3
+    /// * `2.shifted_in_from(1) = 3` -- here, we changed the `outer_binder`  to refer to `C`.
+    ///   Therefore `2` (relative to `C`) refers to `A`, so the result is still 3 (since `A`, relative to the
+    ///   innermost binder, has index 3).
+    #[must_use]
+    pub fn shifted_in_from(self, outer_binder: DebruijnIndex) -> DebruijnIndex {
+        DebruijnIndex::new(self.depth() + outer_binder.depth())
+    }
+
+    /// Returns the resulting index when this value is moved out from
+    /// `amount` number of new binders.
+    #[must_use]
+    pub fn shifted_out(self) -> Option<DebruijnIndex> {
+        self.shifted_out_to(DebruijnIndex::ONE)
+    }
+
+    /// Update in place by shifting out from `amount` binders.
+    pub fn shift_out(&mut self) {
+        *self = self.shifted_out().unwrap();
+    }
+
+    /// Subtracts `outer_binder` levels from the `self` index. Intuitively, this
+    /// shifts the `self` index, which was valid at the innermost
+    /// binder, to one that is valid at the binder `outer_binder`.
+    ///
+    /// This will return `None` if the `self` index is internal to the
+    /// outer binder (i.e., if `self < outer_binder`).
+    ///
+    /// Example: Assume that the following binders are in scope:
+    ///
+    /// ```ignore
+    /// for<A> for<B> for<C> for<D>
+    ///            ^ outer binder
+    /// ```
+    ///
+    /// Assume further that the `outer_binder` argument is 2,
+    /// which means that it is referring to the `for<B>` binder
+    /// (since `D` would be the innermost binder).
+    ///
+    /// This means that the result is relative to the binder `B` -- so
+    /// if `self` is 0 (`INNERMOST`), then it refers to `B`,
+    /// and if `self` is 1, then it refers to `A`.
+    ///
+    /// We will return as follows:
+    ///
+    /// * `1.shifted_out_to(2) = None` -- i.e., the binder for `C` can't be named from the binding level `B`
+    /// * `3.shifted_out_to(2) = Some(1)` -- i.e., `A`, when shifted out to the binding level `B`, has index 1
+    pub fn shifted_out_to(self, outer_binder: DebruijnIndex) -> Option<DebruijnIndex> {
+        if self.within(outer_binder) {
+            None
+        } else {
+            Some(DebruijnIndex::new(self.depth() - outer_binder.depth()))
+        }
+    }
+}
+
+/// A "DynTy" represents a trait object (`dyn Trait`). Trait objects
+/// are conceptually very related to an "existential type" of the form
+/// `exists<T> { T: Trait }` (another example of such type is `impl Trait`).
+/// `DynTy` represents the bounds on that type.
+///
+/// The "bounds" here represents the unknown self type. So, a type like
+/// `dyn for<'a> Fn(&'a u32)` would be represented with two-levels of
+/// binder, as "depicted" here:
+///
+/// ```notrust
+/// exists<type> {
+///    vec![
+///        // A QuantifiedWhereClause:
+///        forall<region> { ^1.0: Fn(&^0.0 u32) }
+///    ]
+/// }
+/// ```
+///
+/// The outer `exists<type>` binder indicates that there exists
+/// some type that meets the criteria within, but that type is not
+/// known. It is referenced within the type using `^1.0`, indicating
+/// a bound type with debruijn index 1 (i.e., skipping through one
+/// level of binder).
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner)]
+pub struct DynTy<I: Interner> {
+    /// The unknown self type.
+    pub bounds: Binders<QuantifiedWhereClauses<I>>,
+    /// Lifetime of the `DynTy`.
+    pub lifetime: Lifetime<I>,
+}
+
+impl<I: Interner> Copy for DynTy<I>
+where
+    I::InternedLifetime: Copy,
+    I::InternedQuantifiedWhereClauses: Copy,
+    I::InternedVariableKinds: Copy,
+{
+}
+
+/// A type, lifetime or constant whose value is being inferred.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub struct InferenceVar {
+    index: u32,
+}
+
+impl From<u32> for InferenceVar {
+    fn from(index: u32) -> InferenceVar {
+        InferenceVar { index }
+    }
+}
+
+impl InferenceVar {
+    /// Gets the underlying index value.
+    pub fn index(self) -> u32 {
+        self.index
+    }
+
+    /// Wraps the inference variable in a type.
+    pub fn to_ty<I: Interner>(self, interner: I, kind: TyVariableKind) -> Ty<I> {
+        TyKind::<I>::InferenceVar(self, kind).intern(interner)
+    }
+
+    /// Wraps the inference variable in a lifetime.
+    pub fn to_lifetime<I: Interner>(self, interner: I) -> Lifetime<I> {
+        LifetimeData::<I>::InferenceVar(self).intern(interner)
+    }
+
+    /// Wraps the inference variable in a constant.
+    pub fn to_const<I: Interner>(self, interner: I, ty: Ty<I>) -> Const<I> {
+        ConstData {
+            ty,
+            value: ConstValue::<I>::InferenceVar(self),
+        }
+        .intern(interner)
+    }
+}
+
+/// A function signature.
+#[derive(Clone, Copy, PartialEq, Eq, Hash, HasInterner, Debug)]
+#[allow(missing_docs)]
+pub struct FnSig<I: Interner> {
+    pub abi: I::FnAbi,
+    pub safety: Safety,
+    pub variadic: bool,
+}
+/// A wrapper for the substs on a Fn.
+#[derive(Clone, PartialEq, Eq, Hash, HasInterner, TypeFoldable, TypeVisitable)]
+pub struct FnSubst<I: Interner>(pub Substitution<I>);
+
+impl<I: Interner> Copy for FnSubst<I> where I::InternedSubstitution: Copy {}
+
+/// for<'a...'z> X -- all binders are instantiated at once,
+/// and we use deBruijn indices within `self.ty`
+#[derive(Clone, PartialEq, Eq, Hash, HasInterner)]
+#[allow(missing_docs)]
+pub struct FnPointer<I: Interner> {
+    pub num_binders: usize,
+    pub sig: FnSig<I>,
+    pub substitution: FnSubst<I>,
+}
+
+impl<I: Interner> Copy for FnPointer<I> where I::InternedSubstitution: Copy {}
+
+impl<I: Interner> FnPointer<I> {
+    /// Represent the current `Fn` as if it was wrapped in `Binders`
+    pub fn into_binders(self, interner: I) -> Binders<FnSubst<I>> {
+        Binders::new(
+            VariableKinds::from_iter(
+                interner,
+                (0..self.num_binders).map(|_| VariableKind::Lifetime),
+            ),
+            self.substitution,
+        )
+    }
+
+    /// Represent the current `Fn` as if it was wrapped in `Binders`
+    pub fn as_binders(&self, interner: I) -> Binders<&FnSubst<I>> {
+        Binders::new(
+            VariableKinds::from_iter(
+                interner,
+                (0..self.num_binders).map(|_| VariableKind::Lifetime),
+            ),
+            &self.substitution,
+        )
+    }
+}
+
+/// Constants.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, HasInterner)]
+pub struct Const<I: Interner> {
+    interned: I::InternedConst,
+}
+
+impl<I: Interner> Const<I> {
+    /// Create a `Const` using something that can be cast to const data.
+    pub fn new(interner: I, data: impl CastTo<ConstData<I>>) -> Self {
+        Const {
+            interned: I::intern_const(interner, data.cast(interner)),
+        }
+    }
+
+    /// Gets the interned constant.
+    pub fn interned(&self) -> &I::InternedConst {
+        &self.interned
+    }
+
+    /// Gets the constant data from the interner.
+    pub fn data(&self, interner: I) -> &ConstData<I> {
+        I::const_data(interner, &self.interned)
+    }
+
+    /// If this is a `ConstData::BoundVar(d)`, returns `Some(d)` else `None`.
+    pub fn bound_var(&self, interner: I) -> Option<BoundVar> {
+        if let ConstValue::BoundVar(bv) = &self.data(interner).value {
+            Some(*bv)
+        } else {
+            None
+        }
+    }
+
+    /// If this is a `ConstData::InferenceVar(d)`, returns `Some(d)` else `None`.
+    pub fn inference_var(&self, interner: I) -> Option<InferenceVar> {
+        if let ConstValue::InferenceVar(iv) = &self.data(interner).value {
+            Some(*iv)
+        } else {
+            None
+        }
+    }
+
+    /// True if this const is a "bound" const, and hence
+    /// needs to be shifted across binders. Meant for debug assertions.
+    pub fn needs_shift(&self, interner: I) -> bool {
+        match &self.data(interner).value {
+            ConstValue::BoundVar(_) => true,
+            ConstValue::InferenceVar(_) => false,
+            ConstValue::Placeholder(_) => false,
+            ConstValue::Concrete(_) => false,
+        }
+    }
+}
+
+/// Constant data, containing the constant's type and value.
+#[derive(Clone, PartialEq, Eq, Hash, HasInterner)]
+pub struct ConstData<I: Interner> {
+    /// Type that holds the constant.
+    pub ty: Ty<I>,
+    /// The value of the constant.
+    pub value: ConstValue<I>,
+}
+
+/// A constant value, not necessarily concrete.
+#[derive(Clone, PartialEq, Eq, Hash, HasInterner)]
+pub enum ConstValue<I: Interner> {
+    /// Bound var (e.g. a parameter).
+    BoundVar(BoundVar),
+    /// Constant whose value is being inferred.
+    InferenceVar(InferenceVar),
+    /// Lifetime on some yet-unknown placeholder.
+    Placeholder(PlaceholderIndex),
+    /// Concrete constant value.
+    Concrete(ConcreteConst<I>),
+}
+
+impl<I: Interner> Copy for ConstValue<I> where I::InternedConcreteConst: Copy {}
+
+impl<I: Interner> ConstData<I> {
+    /// Wraps the constant data in a `Const`.
+    pub fn intern(self, interner: I) -> Const<I> {
+        Const::new(interner, self)
+    }
+}
+
+/// Concrete constant, whose value is known (as opposed to
+/// inferred constants and placeholders).
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, HasInterner)]
+pub struct ConcreteConst<I: Interner> {
+    /// The interned constant.
+    pub interned: I::InternedConcreteConst,
+}
+
+impl<I: Interner> ConcreteConst<I> {
+    /// Checks whether two concrete constants are equal.
+    pub fn const_eq(&self, ty: &Ty<I>, other: &ConcreteConst<I>, interner: I) -> bool {
+        interner.const_eq(&ty.interned, &self.interned, &other.interned)
+    }
+}
+
+/// A Rust lifetime.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, HasInterner)]
+pub struct Lifetime<I: Interner> {
+    interned: I::InternedLifetime,
+}
+
+impl<I: Interner> Lifetime<I> {
+    /// Create a lifetime from lifetime data
+    /// (or something that can be cast to lifetime data).
+    pub fn new(interner: I, data: impl CastTo<LifetimeData<I>>) -> Self {
+        Lifetime {
+            interned: I::intern_lifetime(interner, data.cast(interner)),
+        }
+    }
+
+    /// Gets the interned value.
+    pub fn interned(&self) -> &I::InternedLifetime {
+        &self.interned
+    }
+
+    /// Gets the lifetime data.
+    pub fn data(&self, interner: I) -> &LifetimeData<I> {
+        I::lifetime_data(interner, &self.interned)
+    }
+
+    /// If this is a `Lifetime::BoundVar(d)`, returns `Some(d)` else `None`.
+    pub fn bound_var(&self, interner: I) -> Option<BoundVar> {
+        if let LifetimeData::BoundVar(bv) = self.data(interner) {
+            Some(*bv)
+        } else {
+            None
+        }
+    }
+
+    /// If this is a `Lifetime::InferenceVar(d)`, returns `Some(d)` else `None`.
+    pub fn inference_var(&self, interner: I) -> Option<InferenceVar> {
+        if let LifetimeData::InferenceVar(depth) = self.data(interner) {
+            Some(*depth)
+        } else {
+            None
+        }
+    }
+
+    /// True if this lifetime is a "bound" lifetime, and hence
+    /// needs to be shifted across binders. Meant for debug assertions.
+    pub fn needs_shift(&self, interner: I) -> bool {
+        match self.data(interner) {
+            LifetimeData::BoundVar(_) => true,
+            LifetimeData::InferenceVar(_) => false,
+            LifetimeData::Placeholder(_) => false,
+            LifetimeData::Static => false,
+            LifetimeData::Erased => false,
+            LifetimeData::Phantom(..) => unreachable!(),
+        }
+    }
+
+    ///compute type flags for Lifetime
+    fn compute_flags(&self, interner: I) -> TypeFlags {
+        match self.data(interner) {
+            LifetimeData::InferenceVar(_) => {
+                TypeFlags::HAS_RE_INFER
+                    | TypeFlags::HAS_FREE_LOCAL_REGIONS
+                    | TypeFlags::HAS_FREE_REGIONS
+            }
+            LifetimeData::Placeholder(_) => {
+                TypeFlags::HAS_RE_PLACEHOLDER
+                    | TypeFlags::HAS_FREE_LOCAL_REGIONS
+                    | TypeFlags::HAS_FREE_REGIONS
+            }
+            LifetimeData::Static => TypeFlags::HAS_FREE_REGIONS,
+            LifetimeData::Phantom(_, _) => TypeFlags::empty(),
+            LifetimeData::BoundVar(_) => TypeFlags::HAS_RE_LATE_BOUND,
+            LifetimeData::Erased => TypeFlags::HAS_RE_ERASED,
+        }
+    }
+}
+
+/// Lifetime data, including what kind of lifetime it is and what it points to.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, HasInterner)]
+pub enum LifetimeData<I: Interner> {
+    /// See TyKind::BoundVar.
+    BoundVar(BoundVar),
+    /// Lifetime whose value is being inferred.
+    InferenceVar(InferenceVar),
+    /// Lifetime on some yet-unknown placeholder.
+    Placeholder(PlaceholderIndex),
+    /// Static lifetime
+    Static,
+    /// An erased lifetime, used by rustc to improve caching when we doesn't
+    /// care about lifetimes
+    Erased,
+    /// Lifetime on phantom data.
+    Phantom(Void, PhantomData<I>),
+}
+
+impl<I: Interner> LifetimeData<I> {
+    /// Wrap the lifetime data in a lifetime.
+    pub fn intern(self, interner: I) -> Lifetime<I> {
+        Lifetime::new(interner, self)
+    }
+}
+
+/// Index of an universally quantified parameter in the environment.
+/// Two indexes are required, the one of the universe itself
+/// and the relative index inside the universe.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct PlaceholderIndex {
+    /// Index *of* the universe.
+    pub ui: UniverseIndex,
+    /// Index *in* the universe.
+    pub idx: usize,
+}
+
+impl PlaceholderIndex {
+    /// Wrap the placeholder instance in a lifetime.
+    pub fn to_lifetime<I: Interner>(self, interner: I) -> Lifetime<I> {
+        LifetimeData::<I>::Placeholder(self).intern(interner)
+    }
+
+    /// Create an interned type.
+    pub fn to_ty<I: Interner>(self, interner: I) -> Ty<I> {
+        TyKind::Placeholder(self).intern(interner)
+    }
+
+    /// Wrap the placeholder index in a constant.
+    pub fn to_const<I: Interner>(self, interner: I, ty: Ty<I>) -> Const<I> {
+        ConstData {
+            ty,
+            value: ConstValue::Placeholder(self),
+        }
+        .intern(interner)
+    }
+}
+/// Represents some extra knowledge we may have about the type variable.
+/// ```ignore
+/// let x: &[u32];
+/// let i = 1;
+/// x[i]
+/// ```
+/// In this example, `i` is known to be some type of integer. We can infer that
+/// it is `usize` because that is the only integer type that slices have an
+/// `Index` impl for. `i` would have a `TyVariableKind` of `Integer` to guide the
+/// inference process.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+#[allow(missing_docs)]
+pub enum TyVariableKind {
+    General,
+    Integer,
+    Float,
+}
+
+/// The "kind" of variable. Type, lifetime or constant.
+#[derive(Clone, PartialEq, Eq, Hash)]
+#[allow(missing_docs)]
+pub enum VariableKind<I: Interner> {
+    Ty(TyVariableKind),
+    Lifetime,
+    Const(Ty<I>),
+}
+
+impl<I: Interner> interner::HasInterner for VariableKind<I> {
+    type Interner = I;
+}
+
+impl<I: Interner> Copy for VariableKind<I> where I::InternedType: Copy {}
+
+impl<I: Interner> VariableKind<I> {
+    fn to_bound_variable(&self, interner: I, bound_var: BoundVar) -> GenericArg<I> {
+        match self {
+            VariableKind::Ty(_) => {
+                GenericArgData::Ty(TyKind::BoundVar(bound_var).intern(interner)).intern(interner)
+            }
+            VariableKind::Lifetime => {
+                GenericArgData::Lifetime(LifetimeData::BoundVar(bound_var).intern(interner))
+                    .intern(interner)
+            }
+            VariableKind::Const(ty) => GenericArgData::Const(
+                ConstData {
+                    ty: ty.clone(),
+                    value: ConstValue::BoundVar(bound_var),
+                }
+                .intern(interner),
+            )
+            .intern(interner),
+        }
+    }
+}
+
+/// A generic argument, see `GenericArgData` for more information.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, HasInterner)]
+pub struct GenericArg<I: Interner> {
+    interned: I::InternedGenericArg,
+}
+
+impl<I: Interner> GenericArg<I> {
+    /// Constructs a generic argument using `GenericArgData`.
+    pub fn new(interner: I, data: GenericArgData<I>) -> Self {
+        let interned = I::intern_generic_arg(interner, data);
+        GenericArg { interned }
+    }
+
+    /// Gets the interned value.
+    pub fn interned(&self) -> &I::InternedGenericArg {
+        &self.interned
+    }
+
+    /// Gets the underlying data.
+    pub fn data(&self, interner: I) -> &GenericArgData<I> {
+        I::generic_arg_data(interner, &self.interned)
+    }
+
+    /// Asserts that this is a type argument.
+    pub fn assert_ty_ref(&self, interner: I) -> &Ty<I> {
+        self.ty(interner).unwrap()
+    }
+
+    /// Asserts that this is a lifetime argument.
+    pub fn assert_lifetime_ref(&self, interner: I) -> &Lifetime<I> {
+        self.lifetime(interner).unwrap()
+    }
+
+    /// Asserts that this is a constant argument.
+    pub fn assert_const_ref(&self, interner: I) -> &Const<I> {
+        self.constant(interner).unwrap()
+    }
+
+    /// Checks whether the generic argument is a type.
+    pub fn is_ty(&self, interner: I) -> bool {
+        match self.data(interner) {
+            GenericArgData::Ty(_) => true,
+            GenericArgData::Lifetime(_) => false,
+            GenericArgData::Const(_) => false,
+        }
+    }
+
+    /// Returns the type if it is one, `None` otherwise.
+    pub fn ty(&self, interner: I) -> Option<&Ty<I>> {
+        match self.data(interner) {
+            GenericArgData::Ty(t) => Some(t),
+            _ => None,
+        }
+    }
+
+    /// Returns the lifetime if it is one, `None` otherwise.
+    pub fn lifetime(&self, interner: I) -> Option<&Lifetime<I>> {
+        match self.data(interner) {
+            GenericArgData::Lifetime(t) => Some(t),
+            _ => None,
+        }
+    }
+
+    /// Returns the constant if it is one, `None` otherwise.
+    pub fn constant(&self, interner: I) -> Option<&Const<I>> {
+        match self.data(interner) {
+            GenericArgData::Const(c) => Some(c),
+            _ => None,
+        }
+    }
+
+    /// Compute type flags for GenericArg<I>
+    fn compute_flags(&self, interner: I) -> TypeFlags {
+        match self.data(interner) {
+            GenericArgData::Ty(ty) => ty.data(interner).flags,
+            GenericArgData::Lifetime(lifetime) => lifetime.compute_flags(interner),
+            GenericArgData::Const(constant) => {
+                let data = constant.data(interner);
+                let flags = data.ty.data(interner).flags;
+                match data.value {
+                    ConstValue::BoundVar(_) => flags,
+                    ConstValue::InferenceVar(_) => {
+                        flags | TypeFlags::HAS_CT_INFER | TypeFlags::STILL_FURTHER_SPECIALIZABLE
+                    }
+                    ConstValue::Placeholder(_) => {
+                        flags
+                            | TypeFlags::HAS_CT_PLACEHOLDER
+                            | TypeFlags::STILL_FURTHER_SPECIALIZABLE
+                    }
+                    ConstValue::Concrete(_) => flags,
+                }
+            }
+        }
+    }
+}
+
+/// Generic arguments data.
+#[derive(Clone, PartialEq, Eq, Hash, TypeVisitable, TypeFoldable, Zip)]
+pub enum GenericArgData<I: Interner> {
+    /// Type argument
+    Ty(Ty<I>),
+    /// Lifetime argument
+    Lifetime(Lifetime<I>),
+    /// Constant argument
+    Const(Const<I>),
+}
+
+impl<I: Interner> Copy for GenericArgData<I>
+where
+    I::InternedType: Copy,
+    I::InternedLifetime: Copy,
+    I::InternedConst: Copy,
+{
+}
+
+impl<I: Interner> GenericArgData<I> {
+    /// Create an interned type.
+    pub fn intern(self, interner: I) -> GenericArg<I> {
+        GenericArg::new(interner, self)
+    }
+}
+
+/// A value with an associated variable kind.
+#[derive(Clone, PartialEq, Eq, Hash)]
+pub struct WithKind<I: Interner, T> {
+    /// The associated variable kind.
+    pub kind: VariableKind<I>,
+    /// The wrapped value.
+    value: T,
+}
+
+impl<I: Interner, T: Copy> Copy for WithKind<I, T> where I::InternedType: Copy {}
+
+impl<I: Interner, T> HasInterner for WithKind<I, T> {
+    type Interner = I;
+}
+
+impl<I: Interner, T> From<WithKind<I, T>> for (VariableKind<I>, T) {
+    fn from(with_kind: WithKind<I, T>) -> Self {
+        (with_kind.kind, with_kind.value)
+    }
+}
+
+impl<I: Interner, T> WithKind<I, T> {
+    /// Creates a `WithKind` from a variable kind and a value.
+    pub fn new(kind: VariableKind<I>, value: T) -> Self {
+        Self { kind, value }
+    }
+
+    /// Maps the value in `WithKind`.
+    pub fn map<U, OP>(self, op: OP) -> WithKind<I, U>
+    where
+        OP: FnOnce(T) -> U,
+    {
+        WithKind {
+            kind: self.kind,
+            value: op(self.value),
+        }
+    }
+
+    /// Maps a function taking `WithKind<I, &T>` over `&WithKind<I, T>`.
+    pub fn map_ref<U, OP>(&self, op: OP) -> WithKind<I, U>
+    where
+        OP: FnOnce(&T) -> U,
+    {
+        WithKind {
+            kind: self.kind.clone(),
+            value: op(&self.value),
+        }
+    }
+
+    /// Extract the value, ignoring the variable kind.
+    pub fn skip_kind(&self) -> &T {
+        &self.value
+    }
+}
+
+/// A variable kind with universe index.
+#[allow(type_alias_bounds)]
+pub type CanonicalVarKind<I: Interner> = WithKind<I, UniverseIndex>;
+
+/// An alias, which is a trait indirection such as a projection or opaque type.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner, Zip)]
+pub enum AliasTy<I: Interner> {
+    /// An associated type projection.
+    Projection(ProjectionTy<I>),
+    /// An opaque type.
+    Opaque(OpaqueTy<I>),
+}
+
+impl<I: Interner> Copy for AliasTy<I> where I::InternedSubstitution: Copy {}
+
+impl<I: Interner> AliasTy<I> {
+    /// Create an interned type for this alias.
+    pub fn intern(self, interner: I) -> Ty<I> {
+        Ty::new(interner, self)
+    }
+
+    /// Compute type flags for aliases
+    fn compute_flags(&self, interner: I) -> TypeFlags {
+        match self {
+            AliasTy::Projection(projection_ty) => {
+                TypeFlags::HAS_TY_PROJECTION | projection_ty.substitution.compute_flags(interner)
+            }
+            AliasTy::Opaque(opaque_ty) => {
+                TypeFlags::HAS_TY_OPAQUE | opaque_ty.substitution.compute_flags(interner)
+            }
+        }
+    }
+}
+
+/// A projection `<P0 as TraitName<P1..Pn>>::AssocItem<Pn+1..Pm>`.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner)]
+pub struct ProjectionTy<I: Interner> {
+    /// The id for the associated type member.
+    pub associated_ty_id: AssocTypeId<I>,
+    /// The substitution for the projection.
+    pub substitution: Substitution<I>,
+}
+
+impl<I: Interner> Copy for ProjectionTy<I> where I::InternedSubstitution: Copy {}
+
+/// An opaque type `opaque type T<..>: Trait = HiddenTy`.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner)]
+pub struct OpaqueTy<I: Interner> {
+    /// The id for the opaque type.
+    pub opaque_ty_id: OpaqueTyId<I>,
+    /// The substitution for the opaque type.
+    pub substitution: Substitution<I>,
+}
+
+impl<I: Interner> Copy for OpaqueTy<I> where I::InternedSubstitution: Copy {}
+
+/// A trait reference describes the relationship between a type and a trait.
+/// This can be used in two forms:
+/// - `P0: Trait<P1..Pn>` (e.g. `i32: Copy`), which mentions that the type
+///   implements the trait.
+/// - `<P0 as Trait<P1..Pn>>` (e.g. `i32 as Copy`), which casts the type to
+///   that specific trait.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner)]
+pub struct TraitRef<I: Interner> {
+    /// The trait id.
+    pub trait_id: TraitId<I>,
+    /// The substitution, containing both the `Self` type and the parameters.
+    pub substitution: Substitution<I>,
+}
+
+impl<I: Interner> Copy for TraitRef<I> where I::InternedSubstitution: Copy {}
+
+impl<I: Interner> TraitRef<I> {
+    /// Gets all type parameters in this trait ref, including `Self`.
+    pub fn type_parameters(&self, interner: I) -> impl Iterator<Item = Ty<I>> + '_ {
+        self.substitution
+            .iter(interner)
+            .filter_map(move |p| p.ty(interner))
+            .cloned()
+    }
+
+    /// Gets the type parameters of the `Self` type in this trait ref.
+    pub fn self_type_parameter(&self, interner: I) -> Ty<I> {
+        self.type_parameters(interner).next().unwrap()
+    }
+
+    /// Construct a `FromEnv` using this trait ref.
+    pub fn from_env(self) -> FromEnv<I> {
+        FromEnv::Trait(self)
+    }
+
+    /// Construct a `WellFormed` using this trait ref.
+    pub fn well_formed(self) -> WellFormed<I> {
+        WellFormed::Trait(self)
+    }
+}
+
+/// Lifetime outlives, which for `'a: 'b`` checks that the lifetime `'a`
+/// is a superset of the value of `'b`.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner, Zip)]
+#[allow(missing_docs)]
+pub struct LifetimeOutlives<I: Interner> {
+    pub a: Lifetime<I>,
+    pub b: Lifetime<I>,
+}
+
+impl<I: Interner> Copy for LifetimeOutlives<I> where I::InternedLifetime: Copy {}
+
+/// Type outlives, which for `T: 'a` checks that the type `T`
+/// lives at least as long as the lifetime `'a`
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner, Zip)]
+pub struct TypeOutlives<I: Interner> {
+    /// The type which must outlive the given lifetime.
+    pub ty: Ty<I>,
+    /// The lifetime which the type must outlive.
+    pub lifetime: Lifetime<I>,
+}
+
+impl<I: Interner> Copy for TypeOutlives<I>
+where
+    I::InternedLifetime: Copy,
+    I::InternedType: Copy,
+{
+}
+
+/// Where clauses that can be written by a Rust programmer.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeSuperVisitable, HasInterner, Zip)]
+pub enum WhereClause<I: Interner> {
+    /// Type implements a trait.
+    Implemented(TraitRef<I>),
+    /// Type is equal to an alias.
+    AliasEq(AliasEq<I>),
+    /// One lifetime outlives another.
+    LifetimeOutlives(LifetimeOutlives<I>),
+    /// Type outlives a lifetime.
+    TypeOutlives(TypeOutlives<I>),
+}
+
+impl<I: Interner> Copy for WhereClause<I>
+where
+    I::InternedSubstitution: Copy,
+    I::InternedLifetime: Copy,
+    I::InternedType: Copy,
+{
+}
+
+/// Checks whether a type or trait ref is well-formed.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner, Zip)]
+pub enum WellFormed<I: Interner> {
+    /// A predicate which is true when some trait ref is well-formed.
+    /// For example, given the following trait definitions:
+    ///
+    /// ```notrust
+    /// trait Clone { ... }
+    /// trait Copy where Self: Clone { ... }
+    /// ```
+    ///
+    /// then we have the following rule:
+    ///
+    /// ```notrust
+    /// WellFormed(?Self: Copy) :- ?Self: Copy, WellFormed(?Self: Clone)
+    /// ```
+    Trait(TraitRef<I>),
+
+    /// A predicate which is true when some type is well-formed.
+    /// For example, given the following type definition:
+    ///
+    /// ```notrust
+    /// struct Set<K> where K: Hash {
+    ///     ...
+    /// }
+    /// ```
+    ///
+    /// then we have the following rule: `WellFormedTy(Set<K>) :- Implemented(K: Hash)`.
+    Ty(Ty<I>),
+}
+
+impl<I: Interner> Copy for WellFormed<I>
+where
+    I::InternedType: Copy,
+    I::InternedSubstitution: Copy,
+{
+}
+
+/// Checks whether a type or trait ref can be derived from the contents of the environment.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner, Zip)]
+pub enum FromEnv<I: Interner> {
+    /// A predicate which enables deriving everything which should be true if we *know* that
+    /// some trait ref is well-formed. For example given the above trait definitions, we can use
+    /// `FromEnv(T: Copy)` to derive that `T: Clone`, like in:
+    ///
+    /// ```notrust
+    /// forall<T> {
+    ///     if (FromEnv(T: Copy)) {
+    ///         T: Clone
+    ///     }
+    /// }
+    /// ```
+    Trait(TraitRef<I>),
+
+    /// A predicate which enables deriving everything which should be true if we *know* that
+    /// some type is well-formed. For example given the above type definition, we can use
+    /// `FromEnv(Set<K>)` to derive that `K: Hash`, like in:
+    ///
+    /// ```notrust
+    /// forall<K> {
+    ///     if (FromEnv(Set<K>)) {
+    ///         K: Hash
+    ///     }
+    /// }
+    /// ```
+    Ty(Ty<I>),
+}
+
+impl<I: Interner> Copy for FromEnv<I>
+where
+    I::InternedType: Copy,
+    I::InternedSubstitution: Copy,
+{
+}
+
+/// A "domain goal" is a goal that is directly about Rust, rather than a pure
+/// logical statement. As much as possible, the Chalk solver should avoid
+/// decomposing this enum, and instead treat its values opaquely.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeSuperVisitable, HasInterner, Zip)]
+pub enum DomainGoal<I: Interner> {
+    /// Simple goal that is true if the where clause is true.
+    Holds(WhereClause<I>),
+
+    /// True if the type or trait ref is well-formed.
+    WellFormed(WellFormed<I>),
+
+    /// True if the trait ref can be derived from in-scope where clauses.
+    FromEnv(FromEnv<I>),
+
+    /// True if the alias type can be normalized to some other type
+    Normalize(Normalize<I>),
+
+    /// True if a type is considered to have been "defined" by the current crate. This is true for
+    /// a `struct Foo { }` but false for a `#[upstream] struct Foo { }`. However, for fundamental types
+    /// like `Box<T>`, it is true if `T` is local.
+    IsLocal(Ty<I>),
+
+    /// True if a type is *not* considered to have been "defined" by the current crate. This is
+    /// false for a `struct Foo { }` but true for a `#[upstream] struct Foo { }`. However, for
+    /// fundamental types like `Box<T>`, it is true if `T` is upstream.
+    IsUpstream(Ty<I>),
+
+    /// True if a type and its input types are fully visible, known types. That is, there are no
+    /// unknown type parameters anywhere in this type.
+    ///
+    /// More formally, for each struct S<P0..Pn>:
+    /// forall<P0..Pn> {
+    ///     IsFullyVisible(S<P0...Pn>) :-
+    ///         IsFullyVisible(P0),
+    ///         ...
+    ///         IsFullyVisible(Pn)
+    /// }
+    ///
+    /// Note that any of these types can have lifetimes in their parameters too, but we only
+    /// consider type parameters.
+    IsFullyVisible(Ty<I>),
+
+    /// Used to dictate when trait impls are allowed in the current (local) crate based on the
+    /// orphan rules.
+    ///
+    /// `LocalImplAllowed(T: Trait)` is true if the type T is allowed to impl trait Trait in
+    /// the current crate. Under the current rules, this is unconditionally true for all types if
+    /// the Trait is considered to be "defined" in the current crate. If that is not the case, then
+    /// `LocalImplAllowed(T: Trait)` can still be true if `IsLocal(T)` is true.
+    LocalImplAllowed(TraitRef<I>),
+
+    /// Used to activate the "compatible modality" rules. Rules that introduce predicates that have
+    /// to do with "all compatible universes" should depend on this clause so that they only apply
+    /// if this is present.
+    Compatible,
+
+    /// Used to indicate that a given type is in a downstream crate. Downstream crates contain the
+    /// current crate at some level of their dependencies.
+    ///
+    /// Since chalk does not actually see downstream types, this is usually introduced with
+    /// implication on a fresh, universally quantified type.
+    ///
+    /// forall<T> { if (DownstreamType(T)) { /* ... */ } }
+    ///
+    /// This makes a new type `T` available and makes `DownstreamType(T)` provable for that type.
+    DownstreamType(Ty<I>),
+
+    /// Used to activate the "reveal mode", in which opaque (`impl Trait`) types can be equated
+    /// to their actual type.
+    Reveal,
+
+    /// Used to indicate that a trait is object safe.
+    ObjectSafe(TraitId<I>),
+}
+
+impl<I: Interner> Copy for DomainGoal<I>
+where
+    I::InternedSubstitution: Copy,
+    I::InternedLifetime: Copy,
+    I::InternedType: Copy,
+{
+}
+
+/// A where clause that can contain `forall<>` or `exists<>` quantifiers.
+pub type QuantifiedWhereClause<I> = Binders<WhereClause<I>>;
+
+impl<I: Interner> WhereClause<I> {
+    /// Turn a where clause into the WF version of it i.e.:
+    /// * `Implemented(T: Trait)` maps to `WellFormed(T: Trait)`
+    /// * `ProjectionEq(<T as Trait>::Item = Foo)` maps to `WellFormed(<T as Trait>::Item = Foo)`
+    /// * any other clause maps to itself
+    pub fn into_well_formed_goal(self, interner: I) -> DomainGoal<I> {
+        match self {
+            WhereClause::Implemented(trait_ref) => WellFormed::Trait(trait_ref).cast(interner),
+            wc => wc.cast(interner),
+        }
+    }
+
+    /// Same as `into_well_formed_goal` but with the `FromEnv` predicate instead of `WellFormed`.
+    pub fn into_from_env_goal(self, interner: I) -> DomainGoal<I> {
+        match self {
+            WhereClause::Implemented(trait_ref) => FromEnv::Trait(trait_ref).cast(interner),
+            wc => wc.cast(interner),
+        }
+    }
+
+    /// If where clause is a `TraitRef`, returns its trait id.
+    pub fn trait_id(&self) -> Option<TraitId<I>> {
+        match self {
+            WhereClause::Implemented(trait_ref) => Some(trait_ref.trait_id),
+            WhereClause::AliasEq(_) => None,
+            WhereClause::LifetimeOutlives(_) => None,
+            WhereClause::TypeOutlives(_) => None,
+        }
+    }
+}
+
+impl<I: Interner> QuantifiedWhereClause<I> {
+    /// As with `WhereClause::into_well_formed_goal`, but for a
+    /// quantified where clause. For example, `forall<T> {
+    /// Implemented(T: Trait)}` would map to `forall<T> {
+    /// WellFormed(T: Trait) }`.
+    pub fn into_well_formed_goal(self, interner: I) -> Binders<DomainGoal<I>> {
+        self.map(|wc| wc.into_well_formed_goal(interner))
+    }
+
+    /// As with `WhereClause::into_from_env_goal`, but mapped over any
+    /// binders. For example, `forall<T> {
+    /// Implemented(T: Trait)}` would map to `forall<T> {
+    /// FromEnv(T: Trait) }`.
+    pub fn into_from_env_goal(self, interner: I) -> Binders<DomainGoal<I>> {
+        self.map(|wc| wc.into_from_env_goal(interner))
+    }
+
+    /// If the underlying where clause is a `TraitRef`, returns its trait id.
+    pub fn trait_id(&self) -> Option<TraitId<I>> {
+        self.skip_binders().trait_id()
+    }
+}
+
+impl<I: Interner> DomainGoal<I> {
+    /// Convert `Implemented(...)` into `FromEnv(...)`, but leave other
+    /// goals unchanged.
+    pub fn into_from_env_goal(self, interner: I) -> DomainGoal<I> {
+        match self {
+            DomainGoal::Holds(wc) => wc.into_from_env_goal(interner),
+            goal => goal,
+        }
+    }
+
+    /// Lists generic arguments that are inputs to this domain goal.
+    pub fn inputs(&self, interner: I) -> Vec<GenericArg<I>> {
+        match self {
+            DomainGoal::Holds(WhereClause::AliasEq(alias_eq)) => {
+                vec![GenericArgData::Ty(alias_eq.alias.clone().intern(interner)).intern(interner)]
+            }
+            _ => Vec::new(),
+        }
+    }
+}
+
+/// Equality goal: tries to prove that two values are equal.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, Zip)]
+#[allow(missing_docs)]
+pub struct EqGoal<I: Interner> {
+    pub a: GenericArg<I>,
+    pub b: GenericArg<I>,
+}
+
+impl<I: Interner> Copy for EqGoal<I> where I::InternedGenericArg: Copy {}
+
+/// Subtype goal: tries to prove that `a` is a subtype of `b`
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, Zip)]
+#[allow(missing_docs)]
+pub struct SubtypeGoal<I: Interner> {
+    pub a: Ty<I>,
+    pub b: Ty<I>,
+}
+
+impl<I: Interner> Copy for SubtypeGoal<I> where I::InternedType: Copy {}
+
+/// Proves that the given type alias **normalizes** to the given
+/// type. A projection `T::Foo` normalizes to the type `U` if we can
+/// **match it to an impl** and that impl has a `type Foo = V` where
+/// `U = V`.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, Zip)]
+#[allow(missing_docs)]
+pub struct Normalize<I: Interner> {
+    pub alias: AliasTy<I>,
+    pub ty: Ty<I>,
+}
+
+impl<I: Interner> Copy for Normalize<I>
+where
+    I::InternedSubstitution: Copy,
+    I::InternedType: Copy,
+{
+}
+
+/// Proves **equality** between an alias and a type.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, Zip)]
+#[allow(missing_docs)]
+pub struct AliasEq<I: Interner> {
+    pub alias: AliasTy<I>,
+    pub ty: Ty<I>,
+}
+
+impl<I: Interner> Copy for AliasEq<I>
+where
+    I::InternedSubstitution: Copy,
+    I::InternedType: Copy,
+{
+}
+
+impl<I: Interner> HasInterner for AliasEq<I> {
+    type Interner = I;
+}
+
+/// Indicates that the `value` is universally quantified over `N`
+/// parameters of the given kinds, where `N == self.binders.len()`. A
+/// variable with depth `i < N` refers to the value at
+/// `self.binders[i]`. Variables with depth `>= N` are free.
+///
+/// (IOW, we use deBruijn indices, where binders are introduced in reverse order
+/// of `self.binders`.)
+#[derive(Clone, PartialEq, Eq, Hash)]
+pub struct Binders<T: HasInterner> {
+    /// The binders that quantify over the value.
+    pub binders: VariableKinds<T::Interner>,
+
+    /// The value being quantified over.
+    value: T,
+}
+
+impl<T: HasInterner + Copy> Copy for Binders<T> where
+    <T::Interner as Interner>::InternedVariableKinds: Copy
+{
+}
+
+impl<T: HasInterner> HasInterner for Binders<T> {
+    type Interner = T::Interner;
+}
+
+impl<T: Clone + HasInterner> Binders<&T> {
+    /// Converts a `Binders<&T>` to a `Binders<T>` by cloning `T`.
+    pub fn cloned(self) -> Binders<T> {
+        self.map(Clone::clone)
+    }
+}
+
+impl<T: HasInterner> Binders<T> {
+    /// Create new binders.
+    pub fn new(binders: VariableKinds<T::Interner>, value: T) -> Self {
+        Self { binders, value }
+    }
+
+    /// Wraps the given value in a binder without variables, i.e. `for<>
+    /// (value)`. Since our deBruijn indices count binders, not variables, this
+    /// is sometimes useful.
+    pub fn empty(interner: T::Interner, value: T) -> Self {
+        let binders = VariableKinds::empty(interner);
+        Self { binders, value }
+    }
+
+    /// Skips the binder and returns the "bound" value. This is a
+    /// risky thing to do because it's easy to get confused about
+    /// De Bruijn indices and the like. `skip_binder` is only valid
+    /// when you are either extracting data that has nothing to
+    /// do with bound vars, or you are being very careful about
+    /// your depth accounting.
+    ///
+    /// Some examples where `skip_binder` is reasonable:
+    ///
+    /// - extracting the `TraitId` from a TraitRef;
+    /// - checking if there are any fields in a StructDatum
+    pub fn skip_binders(&self) -> &T {
+        &self.value
+    }
+
+    /// Skips the binder and returns the "bound" value as well as the skipped free variables. This
+    /// is just as risky as [`skip_binders`][Self::skip_binders].
+    pub fn into_value_and_skipped_binders(self) -> (T, VariableKinds<T::Interner>) {
+        (self.value, self.binders)
+    }
+
+    /// Converts `&Binders<T>` to `Binders<&T>`. Produces new `Binders`
+    /// with cloned quantifiers containing a reference to the original
+    /// value, leaving the original in place.
+    pub fn as_ref(&self) -> Binders<&T> {
+        Binders {
+            binders: self.binders.clone(),
+            value: &self.value,
+        }
+    }
+
+    /// Maps the binders by applying a function.
+    pub fn map<U, OP>(self, op: OP) -> Binders<U>
+    where
+        OP: FnOnce(T) -> U,
+        U: HasInterner<Interner = T::Interner>,
+    {
+        let value = op(self.value);
+        Binders {
+            binders: self.binders,
+            value,
+        }
+    }
+
+    /// Transforms the inner value according to the given function; returns
+    /// `None` if the function returns `None`.
+    pub fn filter_map<U, OP>(self, op: OP) -> Option<Binders<U>>
+    where
+        OP: FnOnce(T) -> Option<U>,
+        U: HasInterner<Interner = T::Interner>,
+    {
+        let value = op(self.value)?;
+        Some(Binders {
+            binders: self.binders,
+            value,
+        })
+    }
+
+    /// Maps a function taking `Binders<&T>` over `&Binders<T>`.
+    pub fn map_ref<'a, U, OP>(&'a self, op: OP) -> Binders<U>
+    where
+        OP: FnOnce(&'a T) -> U,
+        U: HasInterner<Interner = T::Interner>,
+    {
+        self.as_ref().map(op)
+    }
+
+    /// Creates a `Substitution` containing bound vars such that applying this
+    /// substitution will not change the value, i.e. `^0.0, ^0.1, ^0.2` and so
+    /// on.
+    pub fn identity_substitution(&self, interner: T::Interner) -> Substitution<T::Interner> {
+        Substitution::from_iter(
+            interner,
+            self.binders
+                .iter(interner)
+                .enumerate()
+                .map(|p| p.to_generic_arg(interner)),
+        )
+    }
+
+    /// Creates a fresh binders that contains a single type
+    /// variable. The result of the closure will be embedded in this
+    /// binder. Note that you should be careful with what you return
+    /// from the closure to account for the binder that will be added.
+    ///
+    /// XXX FIXME -- this is potentially a pretty footgun-y function.
+    pub fn with_fresh_type_var(
+        interner: T::Interner,
+        op: impl FnOnce(Ty<T::Interner>) -> T,
+    ) -> Binders<T> {
+        // The new variable is at the front and everything afterwards is shifted up by 1
+        let new_var = TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, 0)).intern(interner);
+        let value = op(new_var);
+        let binders = VariableKinds::from1(interner, VariableKind::Ty(TyVariableKind::General));
+        Binders { binders, value }
+    }
+
+    /// Returns the number of binders.
+    pub fn len(&self, interner: T::Interner) -> usize {
+        self.binders.len(interner)
+    }
+}
+
+impl<T, I> Binders<Binders<T>>
+where
+    T: TypeFoldable<I> + HasInterner<Interner = I>,
+    I: Interner,
+{
+    /// This turns two levels of binders (`for<A> for<B>`) into one level (`for<A, B>`).
+    pub fn fuse_binders(self, interner: T::Interner) -> Binders<T> {
+        let num_binders = self.len(interner);
+        // generate a substitution to shift the indexes of the inner binder:
+        let subst = Substitution::from_iter(
+            interner,
+            self.value
+                .binders
+                .iter(interner)
+                .enumerate()
+                .map(|(i, pk)| (i + num_binders, pk).to_generic_arg(interner)),
+        );
+        let binders = VariableKinds::from_iter(
+            interner,
+            self.binders
+                .iter(interner)
+                .chain(self.value.binders.iter(interner))
+                .cloned(),
+        );
+        let value = self.value.substitute(interner, &subst);
+        Binders { binders, value }
+    }
+}
+
+impl<T: HasInterner> From<Binders<T>> for (VariableKinds<T::Interner>, T) {
+    fn from(binders: Binders<T>) -> Self {
+        (binders.binders, binders.value)
+    }
+}
+
+impl<T, I> Binders<T>
+where
+    T: TypeFoldable<I> + HasInterner<Interner = I>,
+    I: Interner,
+{
+    /// Substitute `parameters` for the variables introduced by these
+    /// binders. So if the binders represent (e.g.) `<X, Y> { T }` and
+    /// parameters is the slice `[A, B]`, then returns `[X => A, Y =>
+    /// B] T`.
+    pub fn substitute(self, interner: I, parameters: &(impl AsParameters<I> + ?Sized)) -> T {
+        let parameters = parameters.as_parameters(interner);
+        assert_eq!(self.binders.len(interner), parameters.len());
+        Subst::apply(interner, parameters, self.value)
+    }
+}
+
+/// Allows iterating over a Binders<Vec<T>>, for instance.
+/// Each element will include the same set of parameter bounds.
+impl<V, U> IntoIterator for Binders<V>
+where
+    V: HasInterner + IntoIterator<Item = U>,
+    U: HasInterner<Interner = V::Interner>,
+{
+    type Item = Binders<U>;
+    type IntoIter = BindersIntoIterator<V>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        BindersIntoIterator {
+            iter: self.value.into_iter(),
+            binders: self.binders,
+        }
+    }
+}
+
+/// `IntoIterator` for binders.
+pub struct BindersIntoIterator<V: HasInterner + IntoIterator> {
+    iter: <V as IntoIterator>::IntoIter,
+    binders: VariableKinds<V::Interner>,
+}
+
+impl<V> Iterator for BindersIntoIterator<V>
+where
+    V: HasInterner + IntoIterator,
+    <V as IntoIterator>::Item: HasInterner<Interner = V::Interner>,
+{
+    type Item = Binders<<V as IntoIterator>::Item>;
+    fn next(&mut self) -> Option<Self::Item> {
+        self.iter
+            .next()
+            .map(|v| Binders::new(self.binders.clone(), v))
+    }
+}
+
+/// Represents one clause of the form `consequence :- conditions` where
+/// `conditions = cond_1 && cond_2 && ...` is the conjunction of the individual
+/// conditions.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner, Zip)]
+pub struct ProgramClauseImplication<I: Interner> {
+    /// The consequence of the clause, which holds if the conditions holds.
+    pub consequence: DomainGoal<I>,
+
+    /// The condition goals that should hold.
+    pub conditions: Goals<I>,
+
+    /// The lifetime constraints that should be proven.
+    pub constraints: Constraints<I>,
+
+    /// The relative priority of the implication.
+    pub priority: ClausePriority,
+}
+
+/// Specifies how important an implication is.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
+pub enum ClausePriority {
+    /// High priority, the solver should prioritize this.
+    High,
+
+    /// Low priority, this implication has lower chance to be relevant to the goal.
+    Low,
+}
+
+impl std::ops::BitAnd for ClausePriority {
+    type Output = ClausePriority;
+    fn bitand(self, rhs: ClausePriority) -> Self::Output {
+        match (self, rhs) {
+            (ClausePriority::High, ClausePriority::High) => ClausePriority::High,
+            _ => ClausePriority::Low,
+        }
+    }
+}
+
+/// Contains the data for a program clause.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, HasInterner, Zip)]
+pub struct ProgramClauseData<I: Interner>(pub Binders<ProgramClauseImplication<I>>);
+
+impl<I: Interner> ProgramClauseImplication<I> {
+    /// Change the implication into an application holding a `FromEnv` goal.
+    pub fn into_from_env_clause(self, interner: I) -> ProgramClauseImplication<I> {
+        if self.conditions.is_empty(interner) {
+            ProgramClauseImplication {
+                consequence: self.consequence.into_from_env_goal(interner),
+                conditions: self.conditions.clone(),
+                constraints: self.constraints.clone(),
+                priority: self.priority,
+            }
+        } else {
+            self
+        }
+    }
+}
+
+impl<I: Interner> ProgramClauseData<I> {
+    /// Change the program clause data into a `FromEnv` program clause.
+    pub fn into_from_env_clause(self, interner: I) -> ProgramClauseData<I> {
+        ProgramClauseData(self.0.map(|i| i.into_from_env_clause(interner)))
+    }
+
+    /// Intern the program clause data.
+    pub fn intern(self, interner: I) -> ProgramClause<I> {
+        ProgramClause {
+            interned: interner.intern_program_clause(self),
+        }
+    }
+}
+
+/// A program clause is a logic expression used to describe a part of the program.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, HasInterner)]
+pub struct ProgramClause<I: Interner> {
+    interned: I::InternedProgramClause,
+}
+
+impl<I: Interner> ProgramClause<I> {
+    /// Create a new program clause using `ProgramClauseData`.
+    pub fn new(interner: I, clause: ProgramClauseData<I>) -> Self {
+        let interned = interner.intern_program_clause(clause);
+        Self { interned }
+    }
+
+    /// Change the clause into a `FromEnv` clause.
+    pub fn into_from_env_clause(self, interner: I) -> ProgramClause<I> {
+        let program_clause_data = self.data(interner);
+        let new_clause = program_clause_data.clone().into_from_env_clause(interner);
+        Self::new(interner, new_clause)
+    }
+
+    /// Get the interned program clause.
+    pub fn interned(&self) -> &I::InternedProgramClause {
+        &self.interned
+    }
+
+    /// Get the program clause data.
+    pub fn data(&self, interner: I) -> &ProgramClauseData<I> {
+        interner.program_clause_data(&self.interned)
+    }
+}
+
+/// Wraps a "canonicalized item". Items are canonicalized as follows:
+///
+/// All unresolved existential variables are "renumbered" according to their
+/// first appearance; the kind/universe of the variable is recorded in the
+/// `binders` field.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct Canonical<T: HasInterner> {
+    /// The item that is canonicalized.
+    pub value: T,
+
+    /// The kind/universe of the variable.
+    pub binders: CanonicalVarKinds<T::Interner>,
+}
+
+impl<T: HasInterner> HasInterner for Canonical<T> {
+    type Interner = T::Interner;
+}
+
+/// A "universe canonical" value. This is a wrapper around a
+/// `Canonical`, indicating that the universes within have been
+/// "renumbered" to start from 0 and collapse unimportant
+/// distinctions.
+///
+/// To produce one of these values, use the `u_canonicalize` method.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct UCanonical<T: HasInterner> {
+    /// The wrapped `Canonical`.
+    pub canonical: Canonical<T>,
+
+    /// The number of universes that have been collapsed.
+    pub universes: usize,
+}
+
+impl<T: HasInterner> UCanonical<T> {
+    /// Checks whether the universe canonical value is a trivial
+    /// substitution (e.g. an identity substitution).
+    pub fn is_trivial_substitution(
+        &self,
+        interner: T::Interner,
+        canonical_subst: &Canonical<AnswerSubst<T::Interner>>,
+    ) -> bool {
+        let subst = &canonical_subst.value.subst;
+        assert_eq!(
+            self.canonical.binders.len(interner),
+            subst.as_slice(interner).len()
+        );
+        subst.is_identity_subst(interner)
+    }
+
+    /// Creates an identity substitution.
+    pub fn trivial_substitution(&self, interner: T::Interner) -> Substitution<T::Interner> {
+        let binders = &self.canonical.binders;
+        Substitution::from_iter(
+            interner,
+            binders
+                .iter(interner)
+                .enumerate()
+                .map(|(index, pk)| {
+                    let bound_var = BoundVar::new(DebruijnIndex::INNERMOST, index);
+                    match &pk.kind {
+                        VariableKind::Ty(_) => {
+                            GenericArgData::Ty(TyKind::BoundVar(bound_var).intern(interner))
+                                .intern(interner)
+                        }
+                        VariableKind::Lifetime => GenericArgData::Lifetime(
+                            LifetimeData::BoundVar(bound_var).intern(interner),
+                        )
+                        .intern(interner),
+                        VariableKind::Const(ty) => GenericArgData::Const(
+                            ConstData {
+                                ty: ty.clone(),
+                                value: ConstValue::BoundVar(bound_var),
+                            }
+                            .intern(interner),
+                        )
+                        .intern(interner),
+                    }
+                })
+                .collect::<Vec<_>>(),
+        )
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, HasInterner)]
+/// A general goal; this is the full range of questions you can pose to Chalk.
+pub struct Goal<I: Interner> {
+    interned: I::InternedGoal,
+}
+
+impl<I: Interner> Goal<I> {
+    /// Create a new goal using `GoalData`.
+    pub fn new(interner: I, interned: GoalData<I>) -> Self {
+        let interned = I::intern_goal(interner, interned);
+        Self { interned }
+    }
+
+    /// Gets the interned goal.
+    pub fn interned(&self) -> &I::InternedGoal {
+        &self.interned
+    }
+
+    /// Gets the interned goal data.
+    pub fn data(&self, interner: I) -> &GoalData<I> {
+        interner.goal_data(&self.interned)
+    }
+
+    /// Create a goal using a `forall` or `exists` quantifier.
+    pub fn quantify(self, interner: I, kind: QuantifierKind, binders: VariableKinds<I>) -> Goal<I> {
+        GoalData::Quantified(kind, Binders::new(binders, self)).intern(interner)
+    }
+
+    /// Takes a goal `G` and turns it into `not { G }`.
+    pub fn negate(self, interner: I) -> Self {
+        GoalData::Not(self).intern(interner)
+    }
+
+    /// Takes a goal `G` and turns it into `compatible { G }`.
+    pub fn compatible(self, interner: I) -> Self {
+        // compatible { G } desugars into: forall<T> { if (Compatible, DownstreamType(T)) { G } }
+        // This activates the compatible modality rules and introduces an anonymous downstream type
+        GoalData::Quantified(
+            QuantifierKind::ForAll,
+            Binders::with_fresh_type_var(interner, |ty| {
+                GoalData::Implies(
+                    ProgramClauses::from_iter(
+                        interner,
+                        vec![DomainGoal::Compatible, DomainGoal::DownstreamType(ty)],
+                    ),
+                    self.shifted_in(interner),
+                )
+                .intern(interner)
+            }),
+        )
+        .intern(interner)
+    }
+
+    /// Create an implication goal that holds if the predicates are true.
+    pub fn implied_by(self, interner: I, predicates: ProgramClauses<I>) -> Goal<I> {
+        GoalData::Implies(predicates, self).intern(interner)
+    }
+
+    /// True if this goal is "trivially true" -- i.e., no work is
+    /// required to prove it.
+    pub fn is_trivially_true(&self, interner: I) -> bool {
+        match self.data(interner) {
+            GoalData::All(goals) => goals.is_empty(interner),
+            _ => false,
+        }
+    }
+}
+
+impl<I> Goal<I>
+where
+    I: Interner,
+{
+    /// Creates a single goal that only holds if a list of goals holds.
+    pub fn all<II>(interner: I, iter: II) -> Self
+    where
+        II: IntoIterator<Item = Goal<I>>,
+    {
+        let mut iter = iter.into_iter();
+        if let Some(goal0) = iter.next() {
+            if let Some(goal1) = iter.next() {
+                // More than one goal to prove
+                let goals = Goals::from_iter(
+                    interner,
+                    Some(goal0).into_iter().chain(Some(goal1)).chain(iter),
+                );
+                GoalData::All(goals).intern(interner)
+            } else {
+                // One goal to prove
+                goal0
+            }
+        } else {
+            // No goals to prove, always true
+            GoalData::All(Goals::empty(interner)).intern(interner)
+        }
+    }
+}
+
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner, Zip)]
+/// A general goal; this is the full range of questions you can pose to Chalk.
+pub enum GoalData<I: Interner> {
+    /// Introduces a binding at depth 0, shifting other bindings up
+    /// (deBruijn index).
+    Quantified(QuantifierKind, Binders<Goal<I>>),
+
+    /// A goal that holds given some clauses (like an if-statement).
+    Implies(ProgramClauses<I>, Goal<I>),
+
+    /// List of goals that all should hold.
+    All(Goals<I>),
+
+    /// Negation: the inner goal should not hold.
+    Not(Goal<I>),
+
+    /// Make two things equal; the rules for doing so are well known to the logic
+    EqGoal(EqGoal<I>),
+
+    /// Make one thing a subtype of another; the rules for doing so are well known to the logic
+    SubtypeGoal(SubtypeGoal<I>),
+
+    /// A "domain goal" indicates some base sort of goal that can be
+    /// proven via program clauses
+    DomainGoal(DomainGoal<I>),
+
+    /// Indicates something that cannot be proven to be true or false
+    /// definitively. This can occur with overflow but also with
+    /// unifications of skolemized variables like `forall<X,Y> { X = Y
+    /// }`. Of course, that statement is false, as there exist types
+    /// X, Y where `X = Y` is not true. But we treat it as "cannot
+    /// prove" so that `forall<X,Y> { not { X = Y } }` also winds up
+    /// as cannot prove.
+    CannotProve,
+}
+
+impl<I: Interner> Copy for GoalData<I>
+where
+    I::InternedType: Copy,
+    I::InternedLifetime: Copy,
+    I::InternedGenericArg: Copy,
+    I::InternedSubstitution: Copy,
+    I::InternedGoal: Copy,
+    I::InternedGoals: Copy,
+    I::InternedProgramClauses: Copy,
+    I::InternedVariableKinds: Copy,
+{
+}
+
+impl<I: Interner> GoalData<I> {
+    /// Create an interned goal.
+    pub fn intern(self, interner: I) -> Goal<I> {
+        Goal::new(interner, self)
+    }
+}
+
+/// Kinds of quantifiers in the logic, such as `forall` and `exists`.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub enum QuantifierKind {
+    /// Universal quantifier `ForAll`.
+    ///
+    /// A formula with the universal quantifier `forall(x). P(x)` is satisfiable
+    /// if and only if the subformula `P(x)` is true for all possible values for x.
+    ForAll,
+
+    /// Existential quantifier `Exists`.
+    ///
+    /// A formula with the existential quantifier `exists(x). P(x)` is satisfiable
+    /// if and only if there exists at least one value for all possible values of x
+    /// which satisfies the subformula `P(x)`.
+
+    /// In the context of chalk, the existential quantifier usually demands the
+    /// existence of exactly one instance (i.e. type) that satisfies the formula
+    /// (i.e. type constraints). More than one instance means that the result is ambiguous.
+    Exists,
+}
+
+/// A constraint on lifetimes.
+///
+/// When we search for solutions within the trait system, we essentially ignore
+/// lifetime constraints, instead gathering them up to return with our solution
+/// for later checking. This allows for decoupling between type and region
+/// checking in the compiler.
+#[derive(Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner, Zip)]
+pub enum Constraint<I: Interner> {
+    /// Outlives constraint `'a: 'b`, indicating that the value of `'a` must be
+    /// a superset of the value of `'b`.
+    LifetimeOutlives(Lifetime<I>, Lifetime<I>),
+
+    /// Type outlives constraint `T: 'a`, indicating that the type `T` must live
+    /// at least as long as the value of `'a`.
+    TypeOutlives(Ty<I>, Lifetime<I>),
+}
+
+impl<I: Interner> Copy for Constraint<I>
+where
+    I::InternedLifetime: Copy,
+    I::InternedType: Copy,
+{
+}
+
+impl<I: Interner> Substitution<I> {
+    /// A substitution is an **identity substitution** if it looks
+    /// like this
+    ///
+    /// ```text
+    /// ?0 := ?0
+    /// ?1 := ?1
+    /// ?2 := ?2
+    /// ...
+    /// ```
+    ///
+    /// Basically, each value is mapped to a type or lifetime with its
+    /// same index.
+    pub fn is_identity_subst(&self, interner: I) -> bool {
+        self.iter(interner).zip(0..).all(|(generic_arg, index)| {
+            let index_db = BoundVar::new(DebruijnIndex::INNERMOST, index);
+            match generic_arg.data(interner) {
+                GenericArgData::Ty(ty) => match ty.kind(interner) {
+                    TyKind::BoundVar(depth) => index_db == *depth,
+                    _ => false,
+                },
+                GenericArgData::Lifetime(lifetime) => match lifetime.data(interner) {
+                    LifetimeData::BoundVar(depth) => index_db == *depth,
+                    _ => false,
+                },
+                GenericArgData::Const(constant) => match &constant.data(interner).value {
+                    ConstValue::BoundVar(depth) => index_db == *depth,
+                    _ => false,
+                },
+            }
+        })
+    }
+
+    /// Apply the substitution to a value.
+    pub fn apply<T>(&self, value: T, interner: I) -> T
+    where
+        T: TypeFoldable<I>,
+    {
+        Substitute::apply(self, value, interner)
+    }
+
+    /// Gets an iterator of all type parameters.
+    pub fn type_parameters(&self, interner: I) -> impl Iterator<Item = Ty<I>> + '_ {
+        self.iter(interner)
+            .filter_map(move |p| p.ty(interner))
+            .cloned()
+    }
+
+    /// Compute type flags for Substitution<I>
+    fn compute_flags(&self, interner: I) -> TypeFlags {
+        let mut flags = TypeFlags::empty();
+        for generic_arg in self.iter(interner) {
+            flags |= generic_arg.compute_flags(interner);
+        }
+        flags
+    }
+}
+
+#[derive(FallibleTypeFolder)]
+struct SubstFolder<'i, I: Interner, A: AsParameters<I>> {
+    interner: I,
+    subst: &'i A,
+}
+
+impl<I: Interner, A: AsParameters<I>> SubstFolder<'_, I, A> {
+    /// Index into the list of parameters.
+    pub fn at(&self, index: usize) -> &GenericArg<I> {
+        let interner = self.interner;
+        &self.subst.as_parameters(interner)[index]
+    }
+}
+
+/// Convert a value to a list of parameters.
+pub trait AsParameters<I: Interner> {
+    /// Convert the current value to parameters.
+    fn as_parameters(&self, interner: I) -> &[GenericArg<I>];
+}
+
+impl<I: Interner> AsParameters<I> for Substitution<I> {
+    #[allow(unreachable_code, unused_variables)]
+    fn as_parameters(&self, interner: I) -> &[GenericArg<I>] {
+        self.as_slice(interner)
+    }
+}
+
+impl<I: Interner> AsParameters<I> for [GenericArg<I>] {
+    fn as_parameters(&self, _interner: I) -> &[GenericArg<I>] {
+        self
+    }
+}
+
+impl<I: Interner> AsParameters<I> for [GenericArg<I>; 1] {
+    fn as_parameters(&self, _interner: I) -> &[GenericArg<I>] {
+        self
+    }
+}
+
+impl<I: Interner> AsParameters<I> for Vec<GenericArg<I>> {
+    fn as_parameters(&self, _interner: I) -> &[GenericArg<I>] {
+        self
+    }
+}
+
+impl<T, I: Interner> AsParameters<I> for &T
+where
+    T: ?Sized + AsParameters<I>,
+{
+    fn as_parameters(&self, interner: I) -> &[GenericArg<I>] {
+        T::as_parameters(self, interner)
+    }
+}
+
+/// An extension trait to anything that can be represented as list of `GenericArg`s that signifies
+/// that it can applied as a substituion to a value
+pub trait Substitute<I: Interner>: AsParameters<I> {
+    /// Apply the substitution to a value.
+    fn apply<T: TypeFoldable<I>>(&self, value: T, interner: I) -> T;
+}
+
+impl<I: Interner, A: AsParameters<I>> Substitute<I> for A {
+    fn apply<T>(&self, value: T, interner: I) -> T
+    where
+        T: TypeFoldable<I>,
+    {
+        value
+            .try_fold_with(
+                &mut SubstFolder {
+                    interner,
+                    subst: self,
+                },
+                DebruijnIndex::INNERMOST,
+            )
+            .unwrap()
+    }
+}
+
+/// Utility for converting a list of all the binders into scope
+/// into references to those binders. Simply pair the binders with
+/// the indices, and invoke `to_generic_arg()` on the `(binder,
+/// index)` pair. The result will be a reference to a bound
+/// variable of appropriate kind at the corresponding index.
+pub trait ToGenericArg<I: Interner> {
+    /// Converts the binders in scope to references to those binders.
+    fn to_generic_arg(&self, interner: I) -> GenericArg<I> {
+        self.to_generic_arg_at_depth(interner, DebruijnIndex::INNERMOST)
+    }
+
+    /// Converts the binders at the specified depth to references to those binders.
+    fn to_generic_arg_at_depth(&self, interner: I, debruijn: DebruijnIndex) -> GenericArg<I>;
+}
+
+impl<'a, I: Interner> ToGenericArg<I> for (usize, &'a VariableKind<I>) {
+    fn to_generic_arg_at_depth(&self, interner: I, debruijn: DebruijnIndex) -> GenericArg<I> {
+        let &(index, binder) = self;
+        let bound_var = BoundVar::new(debruijn, index);
+        binder.to_bound_variable(interner, bound_var)
+    }
+}
+
+impl<'i, I: Interner, A: AsParameters<I>> TypeFolder<I> for SubstFolder<'i, I, A> {
+    fn as_dyn(&mut self) -> &mut dyn TypeFolder<I> {
+        self
+    }
+
+    fn fold_free_var_ty(&mut self, bound_var: BoundVar, outer_binder: DebruijnIndex) -> Ty<I> {
+        assert_eq!(bound_var.debruijn, DebruijnIndex::INNERMOST);
+        let ty = self.at(bound_var.index);
+        let ty = ty.assert_ty_ref(TypeFolder::interner(self));
+        ty.clone()
+            .shifted_in_from(TypeFolder::interner(self), outer_binder)
+    }
+
+    fn fold_free_var_lifetime(
+        &mut self,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> Lifetime<I> {
+        assert_eq!(bound_var.debruijn, DebruijnIndex::INNERMOST);
+        let l = self.at(bound_var.index);
+        let l = l.assert_lifetime_ref(TypeFolder::interner(self));
+        l.clone()
+            .shifted_in_from(TypeFolder::interner(self), outer_binder)
+    }
+
+    fn fold_free_var_const(
+        &mut self,
+        _ty: Ty<I>,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> Const<I> {
+        assert_eq!(bound_var.debruijn, DebruijnIndex::INNERMOST);
+        let c = self.at(bound_var.index);
+        let c = c.assert_const_ref(TypeFolder::interner(self));
+        c.clone()
+            .shifted_in_from(TypeFolder::interner(self), outer_binder)
+    }
+
+    fn interner(&self) -> I {
+        self.interner
+    }
+}
+
+macro_rules! interned_slice_common {
+    ($seq:ident, $data:ident => $elem:ty, $intern:ident => $interned:ident) => {
+        /// List of interned elements.
+        #[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, HasInterner)]
+        pub struct $seq<I: Interner> {
+            interned: I::$interned,
+        }
+
+        impl<I: Interner> $seq<I> {
+            /// Get the interned elements.
+            pub fn interned(&self) -> &I::$interned {
+                &self.interned
+            }
+
+            /// Returns a slice containing the elements.
+            pub fn as_slice(&self, interner: I) -> &[$elem] {
+                Interner::$data(interner, &self.interned)
+            }
+
+            /// Index into the sequence.
+            pub fn at(&self, interner: I, index: usize) -> &$elem {
+                &self.as_slice(interner)[index]
+            }
+
+            /// Create an empty sequence.
+            pub fn empty(interner: I) -> Self {
+                Self::from_iter(interner, None::<$elem>)
+            }
+
+            /// Check whether this is an empty sequence.
+            pub fn is_empty(&self, interner: I) -> bool {
+                self.as_slice(interner).is_empty()
+            }
+
+            /// Get an iterator over the elements of the sequence.
+            pub fn iter(&self, interner: I) -> std::slice::Iter<'_, $elem> {
+                self.as_slice(interner).iter()
+            }
+
+            /// Get the length of the sequence.
+            pub fn len(&self, interner: I) -> usize {
+                self.as_slice(interner).len()
+            }
+        }
+    };
+}
+
+macro_rules! interned_slice {
+    ($seq:ident, $data:ident => $elem:ty, $intern:ident => $interned:ident) => {
+        interned_slice_common!($seq, $data => $elem, $intern => $interned);
+
+        impl<I: Interner> $seq<I> {
+            /// Tries to create a sequence using an iterator of element-like things.
+            pub fn from_fallible<E>(
+                interner: I,
+                elements: impl IntoIterator<Item = Result<impl CastTo<$elem>, E>>,
+            ) -> Result<Self, E> {
+                Ok(Self {
+                    interned: I::$intern(interner, elements.into_iter().casted(interner))?,
+                })
+            }
+
+            /// Create a sequence from elements
+            pub fn from_iter(
+                interner: I,
+                elements: impl IntoIterator<Item = impl CastTo<$elem>>,
+            ) -> Self {
+                Self::from_fallible(
+                    interner,
+                    elements
+                        .into_iter()
+                        .map(|el| -> Result<$elem, ()> { Ok(el.cast(interner)) }),
+                )
+                .unwrap()
+            }
+
+            /// Create a sequence from a single element.
+            pub fn from1(interner: I, element: impl CastTo<$elem>) -> Self {
+                Self::from_iter(interner, Some(element))
+            }
+        }
+    };
+}
+
+interned_slice!(
+    QuantifiedWhereClauses,
+    quantified_where_clauses_data => QuantifiedWhereClause<I>,
+    intern_quantified_where_clauses => InternedQuantifiedWhereClauses
+);
+
+interned_slice!(
+    ProgramClauses,
+    program_clauses_data => ProgramClause<I>,
+    intern_program_clauses => InternedProgramClauses
+);
+
+interned_slice!(
+    VariableKinds,
+    variable_kinds_data => VariableKind<I>,
+    intern_generic_arg_kinds => InternedVariableKinds
+);
+
+interned_slice!(
+    CanonicalVarKinds,
+    canonical_var_kinds_data => CanonicalVarKind<I>,
+    intern_canonical_var_kinds => InternedCanonicalVarKinds
+);
+
+interned_slice!(Goals, goals_data => Goal<I>, intern_goals => InternedGoals);
+
+interned_slice!(
+    Constraints,
+    constraints_data => InEnvironment<Constraint<I>>,
+    intern_constraints => InternedConstraints
+);
+
+interned_slice!(
+    Substitution,
+    substitution_data => GenericArg<I>,
+    intern_substitution => InternedSubstitution
+);
+
+interned_slice_common!(
+    Variances,
+    variances_data => Variance,
+    intern_variance => InternedVariances
+);
+
+impl<I: Interner> Variances<I> {
+    /// Tries to create a list of canonical variable kinds using an iterator.
+    pub fn from_fallible<E>(
+        interner: I,
+        variances: impl IntoIterator<Item = Result<Variance, E>>,
+    ) -> Result<Self, E> {
+        Ok(Variances {
+            interned: I::intern_variances(interner, variances.into_iter())?,
+        })
+    }
+
+    /// Creates a list of canonical variable kinds using an iterator.
+    pub fn from_iter(interner: I, variances: impl IntoIterator<Item = Variance>) -> Self {
+        Self::from_fallible(
+            interner,
+            variances
+                .into_iter()
+                .map(|p| -> Result<Variance, ()> { Ok(p) }),
+        )
+        .unwrap()
+    }
+
+    /// Creates a list of canonical variable kinds from a single canonical variable kind.
+    pub fn from1(interner: I, variance: Variance) -> Self {
+        Self::from_iter(interner, Some(variance))
+    }
+}
+
+/// Combines a substitution (`subst`) with a set of region constraints
+/// (`constraints`). This represents the result of a query; the
+/// substitution stores the values for the query's unknown variables,
+/// and the constraints represents any region constraints that must
+/// additionally be solved.
+#[derive(Clone, Debug, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner)]
+pub struct ConstrainedSubst<I: Interner> {
+    /// The substitution that is being constrained.
+    ///
+    /// NB: The `is_trivial` routine relies on the fact that `subst` is folded first.
+    pub subst: Substitution<I>,
+
+    /// Region constraints that constrain the substitution.
+    pub constraints: Constraints<I>,
+}
+
+/// The resulting substitution after solving a goal.
+#[derive(Clone, Debug, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable, HasInterner)]
+pub struct AnswerSubst<I: Interner> {
+    /// The substitution result.
+    ///
+    /// NB: The `is_trivial` routine relies on the fact that `subst` is folded first.
+    pub subst: Substitution<I>,
+
+    /// List of constraints that are part of the answer.
+    pub constraints: Constraints<I>,
+
+    /// Delayed subgoals, used when the solver answered with an (incomplete) `Answer` (instead of a `CompleteAnswer`).
+    pub delayed_subgoals: Vec<InEnvironment<Goal<I>>>,
+}
+
+/// Logic to decide the Variance for a given subst
+pub trait UnificationDatabase<I>
+where
+    Self: std::fmt::Debug,
+    I: Interner,
+{
+    /// Gets the variances for the substitution of a fn def
+    fn fn_def_variance(&self, fn_def_id: FnDefId<I>) -> Variances<I>;
+
+    /// Gets the variances for the substitution of a adt
+    fn adt_variance(&self, adt_id: AdtId<I>) -> Variances<I>;
+}
diff --git a/vendor/chalk-ir/src/visit.rs b/vendor/chalk-ir/src/visit.rs
new file mode 100644
index 000000000..c761ab46f
--- /dev/null
+++ b/vendor/chalk-ir/src/visit.rs
@@ -0,0 +1,422 @@
+//! Traits for visiting bits of IR.
+use std::fmt::Debug;
+use std::ops::ControlFlow;
+
+use crate::{
+    BoundVar, Const, ConstValue, DebruijnIndex, DomainGoal, Goal, InferenceVar, Interner, Lifetime,
+    LifetimeData, PlaceholderIndex, ProgramClause, Ty, TyKind, WhereClause,
+};
+
+mod binder_impls;
+mod boring_impls;
+pub mod visitors;
+
+pub use visitors::VisitExt;
+
+/// Unwraps a `ControlFlow` or propagates its `Break` value.
+/// This replaces the `Try` implementation that would be used
+/// with `std::ops::ControlFlow`.
+#[macro_export]
+macro_rules! try_break {
+    ($expr:expr) => {
+        match $expr {
+            std::ops::ControlFlow::Continue(c) => c,
+            std::ops::ControlFlow::Break(b) => return std::ops::ControlFlow::Break(b),
+        }
+    };
+}
+
+/// A "visitor" recursively folds some term -- that is, some bit of IR,
+/// such as a `Goal`, and computes a value as a result.
+///
+///
+/// To **apply** a visitor, use the `TypeVisitable::visit_with` method, like so
+///
+/// ```rust,ignore
+/// let result = x.visit_with(&mut visitor, 0);
+/// ```
+pub trait TypeVisitor<I: Interner> {
+    /// The "break type" of the visitor, often `()`. It represents the result
+    /// the visitor yields when it stops visiting.
+    type BreakTy;
+
+    /// Creates a `dyn` value from this visitor. Unfortunately, this
+    /// must be added manually to each impl of visitor; it permits the
+    /// default implements below to create a `&mut dyn TypeVisitor` from
+    /// `Self` without knowing what `Self` is (by invoking this
+    /// method). Effectively, this limits impls of `visitor` to types
+    /// for which we are able to create a dyn value (i.e., not `[T]`
+    /// types).
+    fn as_dyn(&mut self) -> &mut dyn TypeVisitor<I, BreakTy = Self::BreakTy>;
+
+    /// Top-level callback: invoked for each `Ty<I>` that is
+    /// encountered when visiting. By default, invokes
+    /// `super_visit_with`, which will in turn invoke the more
+    /// specialized visiting methods below, like `visit_free_var`.
+    fn visit_ty(&mut self, ty: &Ty<I>, outer_binder: DebruijnIndex) -> ControlFlow<Self::BreakTy> {
+        ty.super_visit_with(self.as_dyn(), outer_binder)
+    }
+
+    /// Top-level callback: invoked for each `Lifetime<I>` that is
+    /// encountered when visiting. By default, invokes
+    /// `super_visit_with`, which will in turn invoke the more
+    /// specialized visiting methods below, like `visit_free_var`.
+    fn visit_lifetime(
+        &mut self,
+        lifetime: &Lifetime<I>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<Self::BreakTy> {
+        lifetime.super_visit_with(self.as_dyn(), outer_binder)
+    }
+
+    /// Top-level callback: invoked for each `Const<I>` that is
+    /// encountered when visiting. By default, invokes
+    /// `super_visit_with`, which will in turn invoke the more
+    /// specialized visiting methods below, like `visit_free_var`.
+    fn visit_const(
+        &mut self,
+        constant: &Const<I>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<Self::BreakTy> {
+        constant.super_visit_with(self.as_dyn(), outer_binder)
+    }
+
+    /// Invoked for every program clause. By default, recursively visits the goals contents.
+    fn visit_program_clause(
+        &mut self,
+        clause: &ProgramClause<I>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<Self::BreakTy> {
+        clause.super_visit_with(self.as_dyn(), outer_binder)
+    }
+
+    /// Invoked for every goal. By default, recursively visits the goals contents.
+    fn visit_goal(
+        &mut self,
+        goal: &Goal<I>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<Self::BreakTy> {
+        goal.super_visit_with(self.as_dyn(), outer_binder)
+    }
+
+    /// Invoked for each domain goal.
+    fn visit_domain_goal(
+        &mut self,
+        domain_goal: &DomainGoal<I>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<Self::BreakTy> {
+        domain_goal.super_visit_with(self.as_dyn(), outer_binder)
+    }
+
+    /// If overridden to return true, then visiting will panic if a
+    /// free variable is encountered. This should be done if free
+    /// type/lifetime/const variables are not expected.
+    fn forbid_free_vars(&self) -> bool {
+        false
+    }
+
+    /// Invoked for `BoundVar` instances that are not bound
+    /// within the type being visited over:
+    fn visit_free_var(
+        &mut self,
+        bound_var: BoundVar,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<Self::BreakTy> {
+        if self.forbid_free_vars() {
+            panic!(
+                "unexpected free variable `{:?}` with outer binder {:?}",
+                bound_var, outer_binder
+            )
+        } else {
+            ControlFlow::Continue(())
+        }
+    }
+
+    /// If overridden to return true, we will panic when a free
+    /// placeholder type/lifetime is encountered.
+    fn forbid_free_placeholders(&self) -> bool {
+        false
+    }
+
+    /// Invoked for each occurrence of a placeholder type; these are
+    /// used when we instantiate binders universally.
+    fn visit_free_placeholder(
+        &mut self,
+        universe: PlaceholderIndex,
+        _outer_binder: DebruijnIndex,
+    ) -> ControlFlow<Self::BreakTy> {
+        if self.forbid_free_placeholders() {
+            panic!("unexpected placeholder type `{:?}`", universe)
+        } else {
+            ControlFlow::Continue(())
+        }
+    }
+
+    /// Invoked for each where clause.
+    fn visit_where_clause(
+        &mut self,
+        where_clause: &WhereClause<I>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<Self::BreakTy> {
+        where_clause.super_visit_with(self.as_dyn(), outer_binder)
+    }
+
+    /// If overridden to return true, inference variables will trigger
+    /// panics when visited. Used when inference variables are
+    /// unexpected.
+    fn forbid_inference_vars(&self) -> bool {
+        false
+    }
+
+    /// Invoked for each occurrence of a inference type; these are
+    /// used when we instantiate binders universally.
+    fn visit_inference_var(
+        &mut self,
+        var: InferenceVar,
+        _outer_binder: DebruijnIndex,
+    ) -> ControlFlow<Self::BreakTy> {
+        if self.forbid_inference_vars() {
+            panic!("unexpected inference type `{:?}`", var)
+        } else {
+            ControlFlow::Continue(())
+        }
+    }
+
+    /// Gets the visitor's interner.
+    fn interner(&self) -> I;
+}
+
+/// Applies the given `visitor` to a value, producing a visited result
+/// of type `TypeVisitor::Result`.
+pub trait TypeVisitable<I: Interner>: Debug {
+    /// Apply the given visitor `visitor` to `self`; `binders` is the
+    /// number of binders that are in scope when beginning the
+    /// visitor. Typically `binders` starts as 0, but is adjusted when
+    /// we encounter `Binders<T>` in the IR or other similar
+    /// constructs.
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B>;
+}
+
+/// For types where "visit" invokes a callback on the `visitor`, the
+/// `TypeSuperVisitable` trait captures the recursive behavior that visits all
+/// the contents of the type.
+pub trait TypeSuperVisitable<I: Interner>: TypeVisitable<I> {
+    /// Recursively visits the type contents.
+    fn super_visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B>;
+}
+
+/// "visiting" a type invokes the `visit_ty` method on the visitor; this
+/// usually (in turn) invokes `super_visit_ty` to visit the individual
+/// parts.
+impl<I: Interner> TypeVisitable<I> for Ty<I> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        visitor.visit_ty(self, outer_binder)
+    }
+}
+
+/// "Super visit" for a type invokes the more detailed callbacks on the type
+impl<I> TypeSuperVisitable<I> for Ty<I>
+where
+    I: Interner,
+{
+    fn super_visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        let interner = visitor.interner();
+        match self.kind(interner) {
+            TyKind::BoundVar(bound_var) => {
+                if bound_var.shifted_out_to(outer_binder).is_some() {
+                    visitor.visit_free_var(*bound_var, outer_binder)
+                } else {
+                    ControlFlow::Continue(())
+                }
+            }
+            TyKind::Dyn(clauses) => clauses.visit_with(visitor, outer_binder),
+            TyKind::InferenceVar(var, _) => visitor.visit_inference_var(*var, outer_binder),
+            TyKind::Placeholder(ui) => visitor.visit_free_placeholder(*ui, outer_binder),
+            TyKind::Alias(proj) => proj.visit_with(visitor, outer_binder),
+            TyKind::Function(fun) => fun.visit_with(visitor, outer_binder),
+            TyKind::Adt(_id, substitution) => substitution.visit_with(visitor, outer_binder),
+            TyKind::AssociatedType(_assoc_ty, substitution) => {
+                substitution.visit_with(visitor, outer_binder)
+            }
+            TyKind::Scalar(scalar) => scalar.visit_with(visitor, outer_binder),
+            TyKind::Str => ControlFlow::Continue(()),
+            TyKind::Tuple(arity, substitution) => {
+                try_break!(arity.visit_with(visitor, outer_binder));
+                substitution.visit_with(visitor, outer_binder)
+            }
+            TyKind::OpaqueType(opaque_ty, substitution) => {
+                try_break!(opaque_ty.visit_with(visitor, outer_binder));
+                substitution.visit_with(visitor, outer_binder)
+            }
+            TyKind::Slice(substitution) => substitution.visit_with(visitor, outer_binder),
+            TyKind::FnDef(fn_def, substitution) => {
+                try_break!(fn_def.visit_with(visitor, outer_binder));
+                substitution.visit_with(visitor, outer_binder)
+            }
+            TyKind::Ref(mutability, lifetime, ty) => {
+                try_break!(mutability.visit_with(visitor, outer_binder));
+                try_break!(lifetime.visit_with(visitor, outer_binder));
+                ty.visit_with(visitor, outer_binder)
+            }
+            TyKind::Raw(mutability, ty) => {
+                try_break!(mutability.visit_with(visitor, outer_binder));
+                ty.visit_with(visitor, outer_binder)
+            }
+            TyKind::Never => ControlFlow::Continue(()),
+            TyKind::Array(ty, const_) => {
+                try_break!(ty.visit_with(visitor, outer_binder));
+                const_.visit_with(visitor, outer_binder)
+            }
+            TyKind::Closure(id, substitution) => {
+                try_break!(id.visit_with(visitor, outer_binder));
+                substitution.visit_with(visitor, outer_binder)
+            }
+            TyKind::Generator(generator, substitution) => {
+                try_break!(generator.visit_with(visitor, outer_binder));
+                substitution.visit_with(visitor, outer_binder)
+            }
+            TyKind::GeneratorWitness(witness, substitution) => {
+                try_break!(witness.visit_with(visitor, outer_binder));
+                substitution.visit_with(visitor, outer_binder)
+            }
+            TyKind::Foreign(foreign_ty) => foreign_ty.visit_with(visitor, outer_binder),
+            TyKind::Error => ControlFlow::Continue(()),
+        }
+    }
+}
+
+impl<I: Interner> TypeVisitable<I> for Lifetime<I> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        visitor.visit_lifetime(self, outer_binder)
+    }
+}
+
+impl<I: Interner> TypeSuperVisitable<I> for Lifetime<I> {
+    fn super_visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        let interner = visitor.interner();
+        match self.data(interner) {
+            LifetimeData::BoundVar(bound_var) => {
+                if bound_var.shifted_out_to(outer_binder).is_some() {
+                    visitor.visit_free_var(*bound_var, outer_binder)
+                } else {
+                    ControlFlow::Continue(())
+                }
+            }
+            LifetimeData::InferenceVar(var) => visitor.visit_inference_var(*var, outer_binder),
+            LifetimeData::Placeholder(universe) => {
+                visitor.visit_free_placeholder(*universe, outer_binder)
+            }
+            LifetimeData::Static | LifetimeData::Erased => ControlFlow::Continue(()),
+            LifetimeData::Phantom(void, ..) => match *void {},
+        }
+    }
+}
+
+impl<I: Interner> TypeVisitable<I> for Const<I> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        visitor.visit_const(self, outer_binder)
+    }
+}
+
+impl<I: Interner> TypeSuperVisitable<I> for Const<I> {
+    fn super_visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        let interner = visitor.interner();
+        match &self.data(interner).value {
+            ConstValue::BoundVar(bound_var) => {
+                if bound_var.shifted_out_to(outer_binder).is_some() {
+                    visitor.visit_free_var(*bound_var, outer_binder)
+                } else {
+                    ControlFlow::Continue(())
+                }
+            }
+            ConstValue::InferenceVar(var) => visitor.visit_inference_var(*var, outer_binder),
+            ConstValue::Placeholder(universe) => {
+                visitor.visit_free_placeholder(*universe, outer_binder)
+            }
+            ConstValue::Concrete(_) => ControlFlow::Continue(()),
+        }
+    }
+}
+
+impl<I: Interner> TypeVisitable<I> for Goal<I> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        visitor.visit_goal(self, outer_binder)
+    }
+}
+
+impl<I: Interner> TypeSuperVisitable<I> for Goal<I> {
+    fn super_visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        let interner = visitor.interner();
+        self.data(interner).visit_with(visitor, outer_binder)
+    }
+}
+
+impl<I: Interner> TypeVisitable<I> for ProgramClause<I> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        visitor.visit_program_clause(self, outer_binder)
+    }
+}
+
+impl<I: Interner> TypeVisitable<I> for WhereClause<I> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        visitor.visit_where_clause(self, outer_binder)
+    }
+}
+
+impl<I: Interner> TypeVisitable<I> for DomainGoal<I> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        visitor.visit_domain_goal(self, outer_binder)
+    }
+}
diff --git a/vendor/chalk-ir/src/visit/binder_impls.rs b/vendor/chalk-ir/src/visit/binder_impls.rs
new file mode 100644
index 000000000..709f99044
--- /dev/null
+++ b/vendor/chalk-ir/src/visit/binder_impls.rs
@@ -0,0 +1,47 @@
+//! This module contains impls of `TypeVisitable` for those types that
+//! introduce binders.
+//!
+//! The more interesting impls of `TypeVisitable` remain in the `visit` module.
+
+use crate::interner::HasInterner;
+use crate::{
+    Binders, Canonical, ControlFlow, DebruijnIndex, FnPointer, Interner, TypeVisitable, TypeVisitor,
+};
+
+impl<I: Interner> TypeVisitable<I> for FnPointer<I> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        self.substitution
+            .visit_with(visitor, outer_binder.shifted_in())
+    }
+}
+
+impl<T, I: Interner> TypeVisitable<I> for Binders<T>
+where
+    T: HasInterner + TypeVisitable<I>,
+{
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        self.value.visit_with(visitor, outer_binder.shifted_in())
+    }
+}
+
+impl<I, T> TypeVisitable<I> for Canonical<T>
+where
+    I: Interner,
+    T: HasInterner<Interner = I> + TypeVisitable<I>,
+{
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        self.value.visit_with(visitor, outer_binder.shifted_in())
+    }
+}
diff --git a/vendor/chalk-ir/src/visit/boring_impls.rs b/vendor/chalk-ir/src/visit/boring_impls.rs
new file mode 100644
index 000000000..512d9ecd8
--- /dev/null
+++ b/vendor/chalk-ir/src/visit/boring_impls.rs
@@ -0,0 +1,261 @@
+//! This module contains "rote and uninteresting" impls of `TypeVisitable` for
+//! various types. In general, we prefer to derive `TypeVisitable`, but
+//! sometimes that doesn't work for whatever reason.
+//!
+//! The more interesting impls of `TypeVisitable` remain in the `visit` module.
+
+use crate::{
+    try_break, AdtId, AssocTypeId, ClausePriority, ClosureId, Constraints, ControlFlow,
+    DebruijnIndex, FloatTy, FnDefId, ForeignDefId, GeneratorId, GenericArg, Goals, ImplId, IntTy,
+    Interner, Mutability, OpaqueTyId, PlaceholderIndex, ProgramClause, ProgramClauses,
+    QuantifiedWhereClauses, QuantifierKind, Safety, Scalar, Substitution, TraitId,
+    TypeSuperVisitable, TypeVisitable, TypeVisitor, UintTy, UniverseIndex,
+};
+use std::{marker::PhantomData, sync::Arc};
+
+/// Convenience function to visit all the items in the iterator it.
+pub fn visit_iter<'i, T, I, B>(
+    it: impl Iterator<Item = T>,
+    visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+    outer_binder: DebruijnIndex,
+) -> ControlFlow<B>
+where
+    T: TypeVisitable<I>,
+    I: 'i + Interner,
+{
+    for e in it {
+        try_break!(e.visit_with(visitor, outer_binder));
+    }
+    ControlFlow::Continue(())
+}
+
+impl<T: TypeVisitable<I>, I: Interner> TypeVisitable<I> for &T {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        T::visit_with(self, visitor, outer_binder)
+    }
+}
+
+impl<T: TypeVisitable<I>, I: Interner> TypeVisitable<I> for Vec<T> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        visit_iter(self.iter(), visitor, outer_binder)
+    }
+}
+
+impl<T: TypeVisitable<I>, I: Interner> TypeVisitable<I> for &[T] {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        visit_iter(self.iter(), visitor, outer_binder)
+    }
+}
+
+impl<T: TypeVisitable<I>, I: Interner> TypeVisitable<I> for Box<T> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        T::visit_with(self, visitor, outer_binder)
+    }
+}
+
+impl<T: TypeVisitable<I>, I: Interner> TypeVisitable<I> for Arc<T> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        T::visit_with(self, visitor, outer_binder)
+    }
+}
+
+macro_rules! tuple_visit {
+    ($($n:ident),*) => {
+        impl<$($n: TypeVisitable<I>,)* I: Interner> TypeVisitable<I> for ($($n,)*) {
+            fn visit_with<BT>(&self, visitor: &mut dyn TypeVisitor<I, BreakTy = BT>, outer_binder: DebruijnIndex) -> ControlFlow<BT> {
+                #[allow(non_snake_case)]
+                let &($(ref $n),*) = self;
+                $(
+                    try_break!($n.visit_with(visitor, outer_binder));
+                )*
+                ControlFlow::Continue(())
+            }
+        }
+    }
+}
+
+tuple_visit!(A, B);
+tuple_visit!(A, B, C);
+tuple_visit!(A, B, C, D);
+tuple_visit!(A, B, C, D, E);
+
+impl<T: TypeVisitable<I>, I: Interner> TypeVisitable<I> for Option<T> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        match self {
+            Some(e) => e.visit_with(visitor, outer_binder),
+            None => ControlFlow::Continue(()),
+        }
+    }
+}
+
+impl<I: Interner> TypeVisitable<I> for GenericArg<I> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        let interner = visitor.interner();
+        self.data(interner).visit_with(visitor, outer_binder)
+    }
+}
+
+impl<I: Interner> TypeVisitable<I> for Substitution<I> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        let interner = visitor.interner();
+        visit_iter(self.iter(interner), visitor, outer_binder)
+    }
+}
+
+impl<I: Interner> TypeVisitable<I> for Goals<I> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        let interner = visitor.interner();
+        visit_iter(self.iter(interner), visitor, outer_binder)
+    }
+}
+
+#[doc(hidden)]
+#[macro_export]
+macro_rules! const_visit {
+    ($t:ty) => {
+        impl<I: Interner> $crate::visit::TypeVisitable<I> for $t {
+            fn visit_with<B>(
+                &self,
+                _visitor: &mut dyn ($crate::visit::TypeVisitor<I, BreakTy = B>),
+                _outer_binder: DebruijnIndex,
+            ) -> ControlFlow<B> {
+                ControlFlow::Continue(())
+            }
+        }
+    };
+}
+
+const_visit!(bool);
+const_visit!(usize);
+const_visit!(UniverseIndex);
+const_visit!(PlaceholderIndex);
+const_visit!(QuantifierKind);
+const_visit!(DebruijnIndex);
+const_visit!(ClausePriority);
+const_visit!(());
+const_visit!(Scalar);
+const_visit!(UintTy);
+const_visit!(IntTy);
+const_visit!(FloatTy);
+const_visit!(Mutability);
+const_visit!(Safety);
+
+#[doc(hidden)]
+#[macro_export]
+macro_rules! id_visit {
+    ($t:ident) => {
+        impl<I: Interner> $crate::visit::TypeVisitable<I> for $t<I> {
+            fn visit_with<B>(
+                &self,
+                _visitor: &mut dyn ($crate::visit::TypeVisitor<I, BreakTy = B>),
+                _outer_binder: DebruijnIndex,
+            ) -> ControlFlow<B> {
+                ControlFlow::Continue(())
+            }
+        }
+    };
+}
+
+id_visit!(ImplId);
+id_visit!(AdtId);
+id_visit!(TraitId);
+id_visit!(OpaqueTyId);
+id_visit!(AssocTypeId);
+id_visit!(FnDefId);
+id_visit!(ClosureId);
+id_visit!(GeneratorId);
+id_visit!(ForeignDefId);
+
+impl<I: Interner> TypeSuperVisitable<I> for ProgramClause<I> {
+    fn super_visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        let interner = visitor.interner();
+
+        self.data(interner).0.visit_with(visitor, outer_binder)
+    }
+}
+
+impl<I: Interner> TypeVisitable<I> for ProgramClauses<I> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        let interner = visitor.interner();
+
+        visit_iter(self.iter(interner), visitor, outer_binder)
+    }
+}
+
+impl<I: Interner> TypeVisitable<I> for Constraints<I> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        let interner = visitor.interner();
+
+        visit_iter(self.iter(interner), visitor, outer_binder)
+    }
+}
+
+impl<I: Interner> TypeVisitable<I> for QuantifiedWhereClauses<I> {
+    fn visit_with<B>(
+        &self,
+        visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        let interner = visitor.interner();
+
+        visit_iter(self.iter(interner), visitor, outer_binder)
+    }
+}
+
+impl<I: Interner> TypeVisitable<I> for PhantomData<I> {
+    fn visit_with<B>(
+        &self,
+        _visitor: &mut dyn TypeVisitor<I, BreakTy = B>,
+        _outer_binder: DebruijnIndex,
+    ) -> ControlFlow<B> {
+        ControlFlow::Continue(())
+    }
+}
diff --git a/vendor/chalk-ir/src/visit/visitors.rs b/vendor/chalk-ir/src/visit/visitors.rs
new file mode 100644
index 000000000..51c087840
--- /dev/null
+++ b/vendor/chalk-ir/src/visit/visitors.rs
@@ -0,0 +1,41 @@
+//! TypeVisitor helpers
+
+use crate::{BoundVar, ControlFlow, DebruijnIndex, Interner, TypeVisitable, TypeVisitor};
+
+/// TypeVisitor extensions.
+pub trait VisitExt<I: Interner>: TypeVisitable<I> {
+    /// Check whether there are free (non-bound) variables.
+    fn has_free_vars(&self, interner: I) -> bool {
+        let flow = self.visit_with(
+            &mut FindFreeVarsVisitor { interner },
+            DebruijnIndex::INNERMOST,
+        );
+        matches!(flow, ControlFlow::Break(_))
+    }
+}
+
+impl<T, I: Interner> VisitExt<I> for T where T: TypeVisitable<I> {}
+
+struct FindFreeVarsVisitor<I: Interner> {
+    interner: I,
+}
+
+impl<I: Interner> TypeVisitor<I> for FindFreeVarsVisitor<I> {
+    type BreakTy = ();
+
+    fn as_dyn(&mut self) -> &mut dyn TypeVisitor<I, BreakTy = Self::BreakTy> {
+        self
+    }
+
+    fn interner(&self) -> I {
+        self.interner
+    }
+
+    fn visit_free_var(
+        &mut self,
+        _bound_var: BoundVar,
+        _outer_binder: DebruijnIndex,
+    ) -> ControlFlow<()> {
+        ControlFlow::Break(())
+    }
+}
diff --git a/vendor/chalk-ir/src/zip.rs b/vendor/chalk-ir/src/zip.rs
new file mode 100644
index 000000000..afa4a13cb
--- /dev/null
+++ b/vendor/chalk-ir/src/zip.rs
@@ -0,0 +1,543 @@
+//! Traits for "zipping" types, walking through two structures and checking that they match.
+
+use crate::fold::TypeFoldable;
+use crate::*;
+use std::fmt::Debug;
+use std::sync::Arc;
+
+/// When we zip types, we basically traverse the structure, ensuring
+/// that it matches.  When we come to types/lifetimes, we invoke the
+/// callback methods in the zipper to match them up. Primarily used
+/// during unification or similar operations.
+///
+/// So e.g. if you had `A: Eq<B>` zipped with `X: Eq<Y>`, then the zipper
+/// would get two callbacks, one pairing `A` and `X`, and the other pairing
+/// `B` and `Y`.
+///
+/// For things other than types/lifetimes, the zip impls will
+/// guarantee equality. So e.g. if you have `A: Eq<B>` zipped with `X:
+/// Ord<Y>`, you would wind up with an error, no matter what zipper
+/// you are using. This is because the traits `Eq` and `Ord` are
+/// represented by two distinct `ItemId` values, and the impl for
+/// `ItemId` requires that all `ItemId` in the two zipped values match
+/// up.
+pub trait Zipper<I: Interner> {
+    /// Indicates that the two types `a` and `b` were found in matching spots.
+    fn zip_tys(&mut self, variance: Variance, a: &Ty<I>, b: &Ty<I>) -> Fallible<()>;
+
+    /// Indicates that the two lifetimes `a` and `b` were found in matching spots.
+    fn zip_lifetimes(
+        &mut self,
+        variance: Variance,
+        a: &Lifetime<I>,
+        b: &Lifetime<I>,
+    ) -> Fallible<()>;
+
+    /// Indicates that the two consts `a` and `b` were found in matching spots.
+    fn zip_consts(&mut self, variance: Variance, a: &Const<I>, b: &Const<I>) -> Fallible<()>;
+
+    /// Zips two values appearing beneath binders.
+    fn zip_binders<T>(
+        &mut self,
+        variance: Variance,
+        a: &Binders<T>,
+        b: &Binders<T>,
+    ) -> Fallible<()>
+    where
+        T: Clone + HasInterner<Interner = I> + Zip<I> + TypeFoldable<I>;
+
+    /// Zips two substs
+    fn zip_substs(
+        &mut self,
+        ambient: Variance,
+        variances: Option<Variances<I>>,
+        a: &[GenericArg<I>],
+        b: &[GenericArg<I>],
+    ) -> Fallible<()>
+    where
+        Self: Sized,
+    {
+        for (i, (a, b)) in a.iter().zip(b.iter()).enumerate() {
+            let variance = variances
+                .as_ref()
+                .map(|v| v.as_slice(self.interner())[i])
+                .unwrap_or(Variance::Invariant);
+            Zip::zip_with(self, ambient.xform(variance), a, b)?;
+        }
+        Ok(())
+    }
+
+    /// Retrieves the interner from the underlying zipper object
+    fn interner(&self) -> I;
+
+    /// Retrieves the `UnificationDatabase` from the underlying zipper object
+    fn unification_database(&self) -> &dyn UnificationDatabase<I>;
+}
+
+impl<'f, Z, I> Zipper<I> for &'f mut Z
+where
+    I: Interner,
+    Z: Zipper<I>,
+{
+    fn zip_tys(&mut self, variance: Variance, a: &Ty<I>, b: &Ty<I>) -> Fallible<()> {
+        (**self).zip_tys(variance, a, b)
+    }
+
+    fn zip_lifetimes(
+        &mut self,
+        variance: Variance,
+        a: &Lifetime<I>,
+        b: &Lifetime<I>,
+    ) -> Fallible<()> {
+        (**self).zip_lifetimes(variance, a, b)
+    }
+
+    fn zip_consts(&mut self, variance: Variance, a: &Const<I>, b: &Const<I>) -> Fallible<()> {
+        (**self).zip_consts(variance, a, b)
+    }
+
+    fn zip_binders<T>(&mut self, variance: Variance, a: &Binders<T>, b: &Binders<T>) -> Fallible<()>
+    where
+        T: Clone + HasInterner<Interner = I> + Zip<I> + TypeFoldable<I>,
+    {
+        (**self).zip_binders(variance, a, b)
+    }
+
+    fn interner(&self) -> I {
+        Z::interner(*self)
+    }
+
+    fn unification_database(&self) -> &dyn UnificationDatabase<I> {
+        (**self).unification_database()
+    }
+}
+
+/// The `Zip` trait walks two values, invoking the `Zipper` methods where
+/// appropriate, but otherwise requiring strict equality.
+///
+/// See `Zipper` trait for more details.
+///
+/// To implement the trait, typically you would use one of the macros
+/// like `eq_zip!`, `struct_zip!`, or `enum_zip!`.
+pub trait Zip<I>: Debug
+where
+    I: Interner,
+{
+    /// Uses the zipper to walk through two values, ensuring that they match.
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()>;
+}
+
+impl<'a, T: ?Sized + Zip<I>, I: Interner> Zip<I> for &'a T {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        <T as Zip<I>>::zip_with(zipper, variance, a, b)
+    }
+}
+
+impl<I: Interner> Zip<I> for () {
+    fn zip_with<Z: Zipper<I>>(_: &mut Z, _: Variance, _: &Self, _: &Self) -> Fallible<()> {
+        Ok(())
+    }
+}
+
+impl<T: Zip<I>, I: Interner> Zip<I> for Vec<T> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        <[T] as Zip<I>>::zip_with(zipper, variance, a, b)
+    }
+}
+
+impl<T: Zip<I>, I: Interner> Zip<I> for [T] {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        if a.len() != b.len() {
+            return Err(NoSolution);
+        }
+
+        for (a_elem, b_elem) in a.iter().zip(b) {
+            Zip::zip_with(zipper, variance, a_elem, b_elem)?;
+        }
+
+        Ok(())
+    }
+}
+
+impl<T: Zip<I>, I: Interner> Zip<I> for Arc<T> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        <T as Zip<I>>::zip_with(zipper, variance, a, b)
+    }
+}
+
+impl<T: Zip<I>, I: Interner> Zip<I> for Box<T> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        <T as Zip<I>>::zip_with(zipper, variance, a, b)
+    }
+}
+
+impl<T: Zip<I>, U: Zip<I>, I: Interner> Zip<I> for (T, U) {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        Zip::zip_with(zipper, variance, &a.0, &b.0)?;
+        Zip::zip_with(zipper, variance, &a.1, &b.1)?;
+        Ok(())
+    }
+}
+
+impl<I: Interner> Zip<I> for Ty<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        zipper.zip_tys(variance, a, b)
+    }
+}
+
+impl<I: Interner> Zip<I> for Lifetime<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        zipper.zip_lifetimes(variance, a, b)
+    }
+}
+
+impl<I: Interner> Zip<I> for Const<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        zipper.zip_consts(variance, a, b)
+    }
+}
+impl<I: Interner, T> Zip<I> for Binders<T>
+where
+    T: Clone + HasInterner<Interner = I> + Zip<I> + TypeFoldable<I>,
+{
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        zipper.zip_binders(variance, a, b)
+    }
+}
+
+/// Generates a Zip impl that requires the two values be
+/// equal. Suitable for atomic, scalar values.
+macro_rules! eq_zip {
+    ($I:ident => $t:ty) => {
+        impl<$I: Interner> Zip<$I> for $t {
+            fn zip_with<Z: Zipper<$I>>(
+                _zipper: &mut Z,
+                _variance: Variance,
+                a: &Self,
+                b: &Self,
+            ) -> Fallible<()> {
+                if a != b {
+                    return Err(NoSolution);
+                }
+                Ok(())
+            }
+        }
+    };
+}
+
+eq_zip!(I => AdtId<I>);
+eq_zip!(I => TraitId<I>);
+eq_zip!(I => AssocTypeId<I>);
+eq_zip!(I => OpaqueTyId<I>);
+eq_zip!(I => GeneratorId<I>);
+eq_zip!(I => ForeignDefId<I>);
+eq_zip!(I => FnDefId<I>);
+eq_zip!(I => ClosureId<I>);
+eq_zip!(I => QuantifierKind);
+eq_zip!(I => PhantomData<I>);
+eq_zip!(I => PlaceholderIndex);
+eq_zip!(I => ClausePriority);
+eq_zip!(I => Mutability);
+eq_zip!(I => Scalar);
+
+impl<T: HasInterner<Interner = I> + Zip<I>, I: Interner> Zip<I> for InEnvironment<T> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        Zip::zip_with(zipper, variance, &a.environment, &b.environment)?;
+        Zip::zip_with(zipper, variance, &a.goal, &b.goal)?;
+        Ok(())
+    }
+}
+
+impl<I: Interner> Zip<I> for Environment<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        let interner = zipper.interner();
+        assert_eq!(a.clauses.len(interner), b.clauses.len(interner)); // or different numbers of clauses
+        Zip::zip_with(
+            zipper,
+            variance,
+            a.clauses.as_slice(interner),
+            b.clauses.as_slice(interner),
+        )?;
+        Ok(())
+    }
+}
+
+impl<I: Interner> Zip<I> for Goals<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        let interner = zipper.interner();
+        Zip::zip_with(zipper, variance, a.as_slice(interner), b.as_slice(interner))?;
+        Ok(())
+    }
+}
+
+impl<I: Interner> Zip<I> for ProgramClauses<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        let interner = zipper.interner();
+        Zip::zip_with(zipper, variance, a.as_slice(interner), b.as_slice(interner))?;
+        Ok(())
+    }
+}
+
+impl<I: Interner> Zip<I> for Constraints<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        let interner = zipper.interner();
+        Zip::zip_with(zipper, variance, a.as_slice(interner), b.as_slice(interner))?;
+        Ok(())
+    }
+}
+
+impl<I: Interner> Zip<I> for QuantifiedWhereClauses<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        let interner = zipper.interner();
+        Zip::zip_with(zipper, variance, a.as_slice(interner), b.as_slice(interner))?;
+        Ok(())
+    }
+}
+
+// Annoyingly, Goal cannot use `enum_zip` because some variants have
+// two parameters, and I'm too lazy to make the macro account for the
+// relevant name mangling.
+impl<I: Interner> Zip<I> for Goal<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        let interner = zipper.interner();
+        Zip::zip_with(zipper, variance, a.data(interner), b.data(interner))
+    }
+}
+
+// I'm too lazy to make `enum_zip` support type parameters.
+impl<I: Interner> Zip<I> for VariableKind<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        match (a, b) {
+            (VariableKind::Ty(a), VariableKind::Ty(b)) if a == b => Ok(()),
+            (VariableKind::Lifetime, VariableKind::Lifetime) => Ok(()),
+            (VariableKind::Const(ty_a), VariableKind::Const(ty_b)) => {
+                Zip::zip_with(zipper, variance, ty_a, ty_b)
+            }
+            (VariableKind::Ty(_), _)
+            | (VariableKind::Lifetime, _)
+            | (VariableKind::Const(_), _) => panic!("zipping things of mixed kind"),
+        }
+    }
+}
+
+impl<I: Interner> Zip<I> for GenericArg<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        let interner = zipper.interner();
+        Zip::zip_with(zipper, variance, a.data(interner), b.data(interner))
+    }
+}
+
+impl<I: Interner> Zip<I> for ProgramClause<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        let interner = zipper.interner();
+        Zip::zip_with(zipper, variance, a.data(interner), b.data(interner))
+    }
+}
+
+impl<I: Interner> Zip<I> for TraitRef<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        let interner = zipper.interner();
+        Zip::zip_with(zipper, variance, &a.trait_id, &b.trait_id)?;
+        zipper.zip_substs(
+            variance,
+            None,
+            a.substitution.as_slice(interner),
+            b.substitution.as_slice(interner),
+        )
+    }
+}
+
+impl<I: Interner> Zip<I> for ProjectionTy<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        let interner = zipper.interner();
+        Zip::zip_with(zipper, variance, &a.associated_ty_id, &b.associated_ty_id)?;
+        zipper.zip_substs(
+            variance,
+            None,
+            a.substitution.as_slice(interner),
+            b.substitution.as_slice(interner),
+        )
+    }
+}
+
+impl<I: Interner> Zip<I> for OpaqueTy<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        let interner = zipper.interner();
+        Zip::zip_with(zipper, variance, &a.opaque_ty_id, &b.opaque_ty_id)?;
+        zipper.zip_substs(
+            variance,
+            None,
+            a.substitution.as_slice(interner),
+            b.substitution.as_slice(interner),
+        )
+    }
+}
+
+impl<I: Interner> Zip<I> for DynTy<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        Zip::zip_with(
+            zipper,
+            variance.xform(Variance::Invariant),
+            &a.bounds,
+            &b.bounds,
+        )?;
+        Zip::zip_with(
+            zipper,
+            variance.xform(Variance::Contravariant),
+            &a.lifetime,
+            &b.lifetime,
+        )?;
+        Ok(())
+    }
+}
+
+impl<I: Interner> Zip<I> for FnSubst<I> {
+    fn zip_with<Z: Zipper<I>>(
+        zipper: &mut Z,
+        variance: Variance,
+        a: &Self,
+        b: &Self,
+    ) -> Fallible<()> {
+        let interner = zipper.interner();
+        // Parameters
+        Zip::zip_with(
+            zipper,
+            variance.xform(Variance::Contravariant),
+            &a.0.as_slice(interner)[..a.0.len(interner) - 1],
+            &b.0.as_slice(interner)[..b.0.len(interner) - 1],
+        )?;
+        // Return type
+        Zip::zip_with(
+            zipper,
+            variance,
+            a.0.iter(interner).last().unwrap(),
+            b.0.iter(interner).last().unwrap(),
+        )?;
+        Ok(())
+    }
+}