diff --git a/compiler/rustc_middle/src/infer/canonical.rs b/compiler/rustc_middle/src/infer/canonical.rs
index 64b63f4c5eb4f..a4e8acf3d667e 100644
--- a/compiler/rustc_middle/src/infer/canonical.rs
+++ b/compiler/rustc_middle/src/infer/canonical.rs
@@ -23,6 +23,8 @@
 
 use rustc_macros::HashStable;
 use rustc_type_ir::Canonical as IrCanonical;
+use rustc_type_ir::CanonicalVarInfo as IrCanonicalVarInfo;
+pub use rustc_type_ir::{CanonicalTyVarKind, CanonicalVarKind};
 use smallvec::SmallVec;
 use std::ops::Index;
 
@@ -33,6 +35,8 @@ use crate::ty::{self, BoundVar, List, Region, Ty, TyCtxt};
 
 pub type Canonical<'tcx, V> = IrCanonical<TyCtxt<'tcx>, V>;
 
+pub type CanonicalVarInfo<'tcx> = IrCanonicalVarInfo<TyCtxt<'tcx>>;
+
 pub type CanonicalVarInfos<'tcx> = &'tcx List<CanonicalVarInfo<'tcx>>;
 
 impl<'tcx> ty::TypeFoldable<TyCtxt<'tcx>> for CanonicalVarInfos<'tcx> {
@@ -138,158 +142,6 @@ impl<'tcx> Default for OriginalQueryValues<'tcx> {
     }
 }
 
-/// Information about a canonical variable that is included with the
-/// canonical value. This is sufficient information for code to create
-/// a copy of the canonical value in some other inference context,
-/// with fresh inference variables replacing the canonical values.
-#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyDecodable, TyEncodable, HashStable)]
-#[derive(TypeFoldable, TypeVisitable)]
-pub struct CanonicalVarInfo<'tcx> {
-    pub kind: CanonicalVarKind<'tcx>,
-}
-
-impl<'tcx> CanonicalVarInfo<'tcx> {
-    pub fn universe(&self) -> ty::UniverseIndex {
-        self.kind.universe()
-    }
-
-    #[must_use]
-    pub fn with_updated_universe(self, ui: ty::UniverseIndex) -> CanonicalVarInfo<'tcx> {
-        CanonicalVarInfo { kind: self.kind.with_updated_universe(ui) }
-    }
-
-    pub fn is_existential(&self) -> bool {
-        match self.kind {
-            CanonicalVarKind::Ty(_) => true,
-            CanonicalVarKind::PlaceholderTy(_) => false,
-            CanonicalVarKind::Region(_) => true,
-            CanonicalVarKind::PlaceholderRegion(..) => false,
-            CanonicalVarKind::Const(..) => true,
-            CanonicalVarKind::PlaceholderConst(_, _) => false,
-            CanonicalVarKind::Effect => true,
-        }
-    }
-
-    pub fn is_region(&self) -> bool {
-        match self.kind {
-            CanonicalVarKind::Region(_) | CanonicalVarKind::PlaceholderRegion(_) => true,
-            CanonicalVarKind::Ty(_)
-            | CanonicalVarKind::PlaceholderTy(_)
-            | CanonicalVarKind::Const(_, _)
-            | CanonicalVarKind::PlaceholderConst(_, _)
-            | CanonicalVarKind::Effect => false,
-        }
-    }
-
-    pub fn expect_placeholder_index(self) -> usize {
-        match self.kind {
-            CanonicalVarKind::Ty(_)
-            | CanonicalVarKind::Region(_)
-            | CanonicalVarKind::Const(_, _)
-            | CanonicalVarKind::Effect => bug!("expected placeholder: {self:?}"),
-
-            CanonicalVarKind::PlaceholderRegion(placeholder) => placeholder.bound.var.as_usize(),
-            CanonicalVarKind::PlaceholderTy(placeholder) => placeholder.bound.var.as_usize(),
-            CanonicalVarKind::PlaceholderConst(placeholder, _) => placeholder.bound.as_usize(),
-        }
-    }
-}
-
-/// Describes the "kind" of the canonical variable. This is a "kind"
-/// in the type-theory sense of the term -- i.e., a "meta" type system
-/// that analyzes type-like values.
-#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyDecodable, TyEncodable, HashStable)]
-#[derive(TypeFoldable, TypeVisitable)]
-pub enum CanonicalVarKind<'tcx> {
-    /// Some kind of type inference variable.
-    Ty(CanonicalTyVarKind),
-
-    /// A "placeholder" that represents "any type".
-    PlaceholderTy(ty::PlaceholderType),
-
-    /// Region variable `'?R`.
-    Region(ty::UniverseIndex),
-
-    /// A "placeholder" that represents "any region". Created when you
-    /// are solving a goal like `for<'a> T: Foo<'a>` to represent the
-    /// bound region `'a`.
-    PlaceholderRegion(ty::PlaceholderRegion),
-
-    /// Some kind of const inference variable.
-    Const(ty::UniverseIndex, Ty<'tcx>),
-
-    /// Effect variable `'?E`.
-    Effect,
-
-    /// A "placeholder" that represents "any const".
-    PlaceholderConst(ty::PlaceholderConst, Ty<'tcx>),
-}
-
-impl<'tcx> CanonicalVarKind<'tcx> {
-    pub fn universe(self) -> ty::UniverseIndex {
-        match self {
-            CanonicalVarKind::Ty(CanonicalTyVarKind::General(ui)) => ui,
-            CanonicalVarKind::Ty(CanonicalTyVarKind::Float | CanonicalTyVarKind::Int) => {
-                ty::UniverseIndex::ROOT
-            }
-            CanonicalVarKind::Effect => ty::UniverseIndex::ROOT,
-            CanonicalVarKind::PlaceholderTy(placeholder) => placeholder.universe,
-            CanonicalVarKind::Region(ui) => ui,
-            CanonicalVarKind::PlaceholderRegion(placeholder) => placeholder.universe,
-            CanonicalVarKind::Const(ui, _) => ui,
-            CanonicalVarKind::PlaceholderConst(placeholder, _) => placeholder.universe,
-        }
-    }
-
-    /// Replaces the universe of this canonical variable with `ui`.
-    ///
-    /// In case this is a float or int variable, this causes an ICE if
-    /// the updated universe is not the root.
-    pub fn with_updated_universe(self, ui: ty::UniverseIndex) -> CanonicalVarKind<'tcx> {
-        match self {
-            CanonicalVarKind::Ty(CanonicalTyVarKind::General(_)) => {
-                CanonicalVarKind::Ty(CanonicalTyVarKind::General(ui))
-            }
-            CanonicalVarKind::Ty(CanonicalTyVarKind::Int | CanonicalTyVarKind::Float)
-            | CanonicalVarKind::Effect => {
-                assert_eq!(ui, ty::UniverseIndex::ROOT);
-                self
-            }
-            CanonicalVarKind::PlaceholderTy(placeholder) => {
-                CanonicalVarKind::PlaceholderTy(ty::Placeholder { universe: ui, ..placeholder })
-            }
-            CanonicalVarKind::Region(_) => CanonicalVarKind::Region(ui),
-            CanonicalVarKind::PlaceholderRegion(placeholder) => {
-                CanonicalVarKind::PlaceholderRegion(ty::Placeholder { universe: ui, ..placeholder })
-            }
-            CanonicalVarKind::Const(_, ty) => CanonicalVarKind::Const(ui, ty),
-            CanonicalVarKind::PlaceholderConst(placeholder, ty) => {
-                CanonicalVarKind::PlaceholderConst(
-                    ty::Placeholder { universe: ui, ..placeholder },
-                    ty,
-                )
-            }
-        }
-    }
-}
-
-/// Rust actually has more than one category of type variables;
-/// notably, the type variables we create for literals (e.g., 22 or
-/// 22.) can only be instantiated with integral/float types (e.g.,
-/// usize or f32). In order to faithfully reproduce a type, we need to
-/// know what set of types a given type variable can be unified with.
-#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyDecodable, TyEncodable, HashStable)]
-pub enum CanonicalTyVarKind {
-    /// General type variable `?T` that can be unified with arbitrary types.
-    General(ty::UniverseIndex),
-
-    /// Integral type variable `?I` (that can only be unified with integral types).
-    Int,
-
-    /// Floating-point type variable `?F` (that can only be unified with float types).
-    Float,
-}
-
 /// After we execute a query with a canonicalized key, we get back a
 /// `Canonical<QueryResponse<..>>`. You can use
 /// `instantiate_query_result` to access the data in this result.
@@ -366,7 +218,6 @@ pub type QueryOutlivesConstraint<'tcx> =
 
 TrivialTypeTraversalImpls! {
     crate::infer::canonical::Certainty,
-    crate::infer::canonical::CanonicalTyVarKind,
 }
 
 impl<'tcx> CanonicalVarValues<'tcx> {
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index c33803925de99..24b6c15488b62 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -1518,8 +1518,36 @@ pub struct Placeholder<T> {
 
 pub type PlaceholderRegion = Placeholder<BoundRegion>;
 
+impl rustc_type_ir::Placeholder for PlaceholderRegion {
+    fn universe(&self) -> UniverseIndex {
+        self.universe
+    }
+
+    fn var(&self) -> BoundVar {
+        self.bound.var
+    }
+
+    fn with_updated_universe(self, ui: UniverseIndex) -> Self {
+        Placeholder { universe: ui, ..self }
+    }
+}
+
 pub type PlaceholderType = Placeholder<BoundTy>;
 
+impl rustc_type_ir::Placeholder for PlaceholderType {
+    fn universe(&self) -> UniverseIndex {
+        self.universe
+    }
+
+    fn var(&self) -> BoundVar {
+        self.bound.var
+    }
+
+    fn with_updated_universe(self, ui: UniverseIndex) -> Self {
+        Placeholder { universe: ui, ..self }
+    }
+}
+
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable)]
 #[derive(TyEncodable, TyDecodable, PartialOrd, Ord)]
 pub struct BoundConst<'tcx> {
@@ -1529,6 +1557,20 @@ pub struct BoundConst<'tcx> {
 
 pub type PlaceholderConst = Placeholder<BoundVar>;
 
+impl rustc_type_ir::Placeholder for PlaceholderConst {
+    fn universe(&self) -> UniverseIndex {
+        self.universe
+    }
+
+    fn var(&self) -> BoundVar {
+        self.bound
+    }
+
+    fn with_updated_universe(self, ui: UniverseIndex) -> Self {
+        Placeholder { universe: ui, ..self }
+    }
+}
+
 /// When type checking, we use the `ParamEnv` to track
 /// details about the set of where-clauses that are in scope at this
 /// particular point.
diff --git a/compiler/rustc_type_ir/src/canonical.rs b/compiler/rustc_type_ir/src/canonical.rs
index 08a52865fa67b..1fdcf2f8faeb8 100644
--- a/compiler/rustc_type_ir/src/canonical.rs
+++ b/compiler/rustc_type_ir/src/canonical.rs
@@ -4,7 +4,7 @@ use std::ops::ControlFlow;
 
 use crate::fold::{FallibleTypeFolder, TypeFoldable};
 use crate::visit::{TypeVisitable, TypeVisitor};
-use crate::{Interner, UniverseIndex};
+use crate::{Interner, Placeholder, UniverseIndex};
 
 /// A "canonicalized" type `V` is one where all free inference
 /// variables have been rewritten to "canonical vars". These are
@@ -113,3 +113,257 @@ where
         self.variables.visit_with(folder)
     }
 }
+
+/// Information about a canonical variable that is included with the
+/// canonical value. This is sufficient information for code to create
+/// a copy of the canonical value in some other inference context,
+/// with fresh inference variables replacing the canonical values.
+#[derive(derivative::Derivative)]
+#[derivative(
+    Clone(bound = ""),
+    Hash(bound = ""),
+    Copy(bound = "CanonicalVarKind<I>: Copy"),
+    Debug(bound = "")
+)]
+#[derive(TyDecodable, TyEncodable, HashStable_NoContext)]
+pub struct CanonicalVarInfo<I: Interner> {
+    pub kind: CanonicalVarKind<I>,
+}
+
+impl<I: Interner> PartialEq for CanonicalVarInfo<I> {
+    fn eq(&self, other: &Self) -> bool {
+        self.kind == other.kind
+    }
+}
+
+impl<I: Interner> Eq for CanonicalVarInfo<I> {}
+
+impl<I: Interner> TypeVisitable<I> for CanonicalVarInfo<I>
+where
+    I::Ty: TypeVisitable<I>,
+{
+    fn visit_with<V: TypeVisitor<I>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.kind.visit_with(visitor)
+    }
+}
+
+impl<I: Interner> TypeFoldable<I> for CanonicalVarInfo<I>
+where
+    I::Ty: TypeFoldable<I>,
+{
+    fn try_fold_with<F: FallibleTypeFolder<I>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        Ok(CanonicalVarInfo { kind: self.kind.try_fold_with(folder)? })
+    }
+}
+
+impl<I: Interner> CanonicalVarInfo<I> {
+    pub fn universe(&self) -> UniverseIndex {
+        self.kind.universe()
+    }
+
+    #[must_use]
+    pub fn with_updated_universe(self, ui: UniverseIndex) -> CanonicalVarInfo<I> {
+        CanonicalVarInfo { kind: self.kind.with_updated_universe(ui) }
+    }
+
+    pub fn is_existential(&self) -> bool {
+        match self.kind {
+            CanonicalVarKind::Ty(_) => true,
+            CanonicalVarKind::PlaceholderTy(_) => false,
+            CanonicalVarKind::Region(_) => true,
+            CanonicalVarKind::PlaceholderRegion(..) => false,
+            CanonicalVarKind::Const(..) => true,
+            CanonicalVarKind::PlaceholderConst(_, _) => false,
+            CanonicalVarKind::Effect => true,
+        }
+    }
+
+    pub fn is_region(&self) -> bool {
+        match self.kind {
+            CanonicalVarKind::Region(_) | CanonicalVarKind::PlaceholderRegion(_) => true,
+            CanonicalVarKind::Ty(_)
+            | CanonicalVarKind::PlaceholderTy(_)
+            | CanonicalVarKind::Const(_, _)
+            | CanonicalVarKind::PlaceholderConst(_, _)
+            | CanonicalVarKind::Effect => false,
+        }
+    }
+
+    pub fn expect_placeholder_index(self) -> usize {
+        match self.kind {
+            CanonicalVarKind::Ty(_)
+            | CanonicalVarKind::Region(_)
+            | CanonicalVarKind::Const(_, _)
+            | CanonicalVarKind::Effect => panic!("expected placeholder: {self:?}"),
+
+            CanonicalVarKind::PlaceholderRegion(placeholder) => placeholder.var().as_usize(),
+            CanonicalVarKind::PlaceholderTy(placeholder) => placeholder.var().as_usize(),
+            CanonicalVarKind::PlaceholderConst(placeholder, _) => placeholder.var().as_usize(),
+        }
+    }
+}
+
+/// Describes the "kind" of the canonical variable. This is a "kind"
+/// in the type-theory sense of the term -- i.e., a "meta" type system
+/// that analyzes type-like values.
+#[derive(derivative::Derivative)]
+#[derivative(Clone(bound = ""), Hash(bound = ""), Debug(bound = ""))]
+#[derive(TyDecodable, TyEncodable, HashStable_NoContext)]
+pub enum CanonicalVarKind<I: Interner> {
+    /// Some kind of type inference variable.
+    Ty(CanonicalTyVarKind),
+
+    /// A "placeholder" that represents "any type".
+    PlaceholderTy(I::PlaceholderTy),
+
+    /// Region variable `'?R`.
+    Region(UniverseIndex),
+
+    /// A "placeholder" that represents "any region". Created when you
+    /// are solving a goal like `for<'a> T: Foo<'a>` to represent the
+    /// bound region `'a`.
+    PlaceholderRegion(I::PlaceholderRegion),
+
+    /// Some kind of const inference variable.
+    Const(UniverseIndex, I::Ty),
+
+    /// Effect variable `'?E`.
+    Effect,
+
+    /// A "placeholder" that represents "any const".
+    PlaceholderConst(I::PlaceholderConst, I::Ty),
+}
+
+impl<I: Interner> Copy for CanonicalVarKind<I>
+where
+    I::PlaceholderTy: Copy,
+    I::PlaceholderRegion: Copy,
+    I::PlaceholderConst: Copy,
+    I::Ty: Copy,
+{
+}
+
+impl<I: Interner> PartialEq for CanonicalVarKind<I> {
+    fn eq(&self, other: &Self) -> bool {
+        match (self, other) {
+            (Self::Ty(l0), Self::Ty(r0)) => l0 == r0,
+            (Self::PlaceholderTy(l0), Self::PlaceholderTy(r0)) => l0 == r0,
+            (Self::Region(l0), Self::Region(r0)) => l0 == r0,
+            (Self::PlaceholderRegion(l0), Self::PlaceholderRegion(r0)) => l0 == r0,
+            (Self::Const(l0, l1), Self::Const(r0, r1)) => l0 == r0 && l1 == r1,
+            (Self::PlaceholderConst(l0, l1), Self::PlaceholderConst(r0, r1)) => {
+                l0 == r0 && l1 == r1
+            }
+            _ => std::mem::discriminant(self) == std::mem::discriminant(other),
+        }
+    }
+}
+
+impl<I: Interner> Eq for CanonicalVarKind<I> {}
+
+impl<I: Interner> TypeVisitable<I> for CanonicalVarKind<I>
+where
+    I::Ty: TypeVisitable<I>,
+{
+    fn visit_with<V: TypeVisitor<I>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        match self {
+            CanonicalVarKind::Ty(_)
+            | CanonicalVarKind::PlaceholderTy(_)
+            | CanonicalVarKind::Region(_)
+            | CanonicalVarKind::PlaceholderRegion(_)
+            | CanonicalVarKind::Effect => ControlFlow::Continue(()),
+            CanonicalVarKind::Const(_, ty) | CanonicalVarKind::PlaceholderConst(_, ty) => {
+                ty.visit_with(visitor)
+            }
+        }
+    }
+}
+
+impl<I: Interner> TypeFoldable<I> for CanonicalVarKind<I>
+where
+    I::Ty: TypeFoldable<I>,
+{
+    fn try_fold_with<F: FallibleTypeFolder<I>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        Ok(match self {
+            CanonicalVarKind::Ty(kind) => CanonicalVarKind::Ty(kind),
+            CanonicalVarKind::Region(kind) => CanonicalVarKind::Region(kind),
+            CanonicalVarKind::Const(kind, ty) => {
+                CanonicalVarKind::Const(kind, ty.try_fold_with(folder)?)
+            }
+            CanonicalVarKind::PlaceholderTy(placeholder) => {
+                CanonicalVarKind::PlaceholderTy(placeholder)
+            }
+            CanonicalVarKind::PlaceholderRegion(placeholder) => {
+                CanonicalVarKind::PlaceholderRegion(placeholder)
+            }
+            CanonicalVarKind::PlaceholderConst(placeholder, ty) => {
+                CanonicalVarKind::PlaceholderConst(placeholder, ty.try_fold_with(folder)?)
+            }
+            CanonicalVarKind::Effect => CanonicalVarKind::Effect,
+        })
+    }
+}
+
+impl<I: Interner> CanonicalVarKind<I> {
+    pub fn universe(&self) -> UniverseIndex {
+        match self {
+            CanonicalVarKind::Ty(CanonicalTyVarKind::General(ui)) => *ui,
+            CanonicalVarKind::Region(ui) => *ui,
+            CanonicalVarKind::Const(ui, _) => *ui,
+            CanonicalVarKind::PlaceholderTy(placeholder) => placeholder.universe(),
+            CanonicalVarKind::PlaceholderRegion(placeholder) => placeholder.universe(),
+            CanonicalVarKind::PlaceholderConst(placeholder, _) => placeholder.universe(),
+            CanonicalVarKind::Ty(CanonicalTyVarKind::Float | CanonicalTyVarKind::Int) => {
+                UniverseIndex::ROOT
+            }
+            CanonicalVarKind::Effect => UniverseIndex::ROOT,
+        }
+    }
+
+    /// Replaces the universe of this canonical variable with `ui`.
+    ///
+    /// In case this is a float or int variable, this causes an ICE if
+    /// the updated universe is not the root.
+    pub fn with_updated_universe(self, ui: UniverseIndex) -> CanonicalVarKind<I> {
+        match self {
+            CanonicalVarKind::Ty(CanonicalTyVarKind::General(_)) => {
+                CanonicalVarKind::Ty(CanonicalTyVarKind::General(ui))
+            }
+            CanonicalVarKind::Region(_) => CanonicalVarKind::Region(ui),
+            CanonicalVarKind::Const(_, ty) => CanonicalVarKind::Const(ui, ty),
+
+            CanonicalVarKind::PlaceholderTy(placeholder) => {
+                CanonicalVarKind::PlaceholderTy(placeholder.with_updated_universe(ui))
+            }
+            CanonicalVarKind::PlaceholderRegion(placeholder) => {
+                CanonicalVarKind::PlaceholderRegion(placeholder.with_updated_universe(ui))
+            }
+            CanonicalVarKind::PlaceholderConst(placeholder, ty) => {
+                CanonicalVarKind::PlaceholderConst(placeholder.with_updated_universe(ui), ty)
+            }
+            CanonicalVarKind::Ty(CanonicalTyVarKind::Int | CanonicalTyVarKind::Float)
+            | CanonicalVarKind::Effect => {
+                assert_eq!(ui, UniverseIndex::ROOT);
+                self
+            }
+        }
+    }
+}
+
+/// Rust actually has more than one category of type variables;
+/// notably, the type variables we create for literals (e.g., 22 or
+/// 22.) can only be instantiated with integral/float types (e.g.,
+/// usize or f32). In order to faithfully reproduce a type, we need to
+/// know what set of types a given type variable can be unified with.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+#[derive(Decodable, Encodable, HashStable_NoContext)]
+pub enum CanonicalTyVarKind {
+    /// General type variable `?T` that can be unified with arbitrary types.
+    General(UniverseIndex),
+
+    /// Integral type variable `?I` (that can only be unified with integral types).
+    Int,
+
+    /// Floating-point type variable `?F` (that can only be unified with float types).
+    Float,
+}
diff --git a/compiler/rustc_type_ir/src/interner.rs b/compiler/rustc_type_ir/src/interner.rs
index 7f75e5b35a204..b3b58957eb003 100644
--- a/compiler/rustc_type_ir/src/interner.rs
+++ b/compiler/rustc_type_ir/src/interner.rs
@@ -2,7 +2,7 @@ use smallvec::SmallVec;
 use std::fmt::Debug;
 use std::hash::Hash;
 
-use crate::{DebugWithInfcx, Mutability};
+use crate::{BoundVar, DebugWithInfcx, Mutability, UniverseIndex};
 
 pub trait Interner: Sized {
     type DefId: Clone + Debug + Hash + Ord;
@@ -26,7 +26,7 @@ pub trait Interner: Sized {
     type AliasTy: Clone + DebugWithInfcx<Self> + Hash + Ord;
     type ParamTy: Clone + Debug + Hash + Ord;
     type BoundTy: Clone + Debug + Hash + Ord;
-    type PlaceholderTy: Clone + Debug + Hash + Ord;
+    type PlaceholderTy: Clone + Debug + Hash + Ord + Placeholder;
     type InferTy: Clone + DebugWithInfcx<Self> + Hash + Ord;
 
     // Things stored inside of tys
@@ -39,7 +39,7 @@ pub trait Interner: Sized {
     type Const: Clone + DebugWithInfcx<Self> + Hash + Ord;
     type InferConst: Clone + DebugWithInfcx<Self> + Hash + Ord;
     type AliasConst: Clone + DebugWithInfcx<Self> + Hash + Ord;
-    type PlaceholderConst: Clone + Debug + Hash + Ord;
+    type PlaceholderConst: Clone + Debug + Hash + Ord + Placeholder;
     type ParamConst: Clone + Debug + Hash + Ord;
     type BoundConst: Clone + Debug + Hash + Ord;
     type ValueConst: Clone + Debug + Hash + Ord;
@@ -51,7 +51,7 @@ pub trait Interner: Sized {
     type BoundRegion: Clone + Debug + Hash + Ord;
     type FreeRegion: Clone + Debug + Hash + Ord;
     type InferRegion: Clone + DebugWithInfcx<Self> + Hash + Ord;
-    type PlaceholderRegion: Clone + Debug + Hash + Ord;
+    type PlaceholderRegion: Clone + Debug + Hash + Ord + Placeholder;
 
     // Predicates
     type Predicate: Clone + Debug + Hash + Eq;
@@ -66,6 +66,14 @@ pub trait Interner: Sized {
     fn ty_and_mut_to_parts(ty_and_mut: Self::TypeAndMut) -> (Self::Ty, Mutability);
 }
 
+/// Common capabilities of placeholder kinds
+pub trait Placeholder {
+    fn universe(&self) -> UniverseIndex;
+    fn var(&self) -> BoundVar;
+
+    fn with_updated_universe(self, ui: UniverseIndex) -> Self;
+}
+
 /// Imagine you have a function `F: FnOnce(&[T]) -> R`, plus an iterator `iter`
 /// that produces `T` items. You could combine them with
 /// `f(&iter.collect::<Vec<_>>())`, but this requires allocating memory for the