Uplift `ExistentialTraitRef`, `ExistentialProjection`, `ProjectionPredicate`

12 files changed, 428 insertions, 325 deletions

diff --git a/compiler/rustc_errors/src/diagnostic_impls.rs b/compiler/rustc_errors/src/diagnostic_impls.rs
index 40560a5ad79..60a2f5469fe 100644
--- a/compiler/rustc_errors/src/diagnostic_impls.rs
+++ b/compiler/rustc_errors/src/diagnostic_impls.rs
@@ -100,6 +100,14 @@ impl<I: rustc_type_ir::Interner> IntoDiagArg for rustc_type_ir::TraitRef<I> {
     }
 }
 
+
+
+impl<I: rustc_type_ir::Interner> IntoDiagArg for rustc_type_ir::ExistentialTraitRef<I> {
+    fn into_diag_arg(self) -> DiagArgValue {
+        self.to_string().into_diag_arg()
+    }
+}
+
 into_diag_arg_for_number!(i8, u8, i16, u16, i32, u32, i64, u64, i128, u128, isize, usize);
 
 impl IntoDiagArg for bool {
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 0464be2df06..93fba3fbc86 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -99,17 +99,17 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
     type CanonicalVars = CanonicalVarInfos<'tcx>;
 
     type Ty = Ty<'tcx>;
-    type Pat = Pattern<'tcx>;
     type Tys = &'tcx List<Ty<'tcx>>;
     type AliasTy = ty::AliasTy<'tcx>;
     type ParamTy = ParamTy;
     type BoundTy = ty::BoundTy;
     type PlaceholderTy = ty::PlaceholderType;
-
     type ErrorGuaranteed = ErrorGuaranteed;
+
     type BoundExistentialPredicates = &'tcx List<PolyExistentialPredicate<'tcx>>;
     type PolyFnSig = PolyFnSig<'tcx>;
     type AllocId = crate::mir::interpret::AllocId;
+    type Pat = Pattern<'tcx>;
 
     type Const = ty::Const<'tcx>;
     type AliasConst = ty::UnevaluatedConst<'tcx>;
@@ -121,8 +121,8 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
 
     type Region = Region<'tcx>;
     type EarlyParamRegion = ty::EarlyParamRegion;
-    type BoundRegion = ty::BoundRegion;
     type LateParamRegion = ty::LateParamRegion;
+    type BoundRegion = ty::BoundRegion;
     type InferRegion = ty::RegionVid;
     type PlaceholderRegion = ty::PlaceholderRegion;
 
@@ -146,6 +146,10 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
         self.generics_of(def_id)
     }
 
+    fn mk_args(self, args: &[Self::GenericArg]) -> Self::GenericArgs {
+        self.mk_args(args)
+    }
+    
     fn check_and_mk_args(
         self,
         def_id: DefId,
@@ -153,6 +157,10 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
     ) -> ty::GenericArgsRef<'tcx> {
         self.check_and_mk_args(def_id, args)
     }
+    
+    fn parent(self, def_id: Self::DefId) -> Self::DefId {
+        self.parent(def_id)
+    }
 }
 
 type InternedSet<'tcx, T> = ShardedHashMap<InternedInSet<'tcx, T>, ()>;
diff --git a/compiler/rustc_middle/src/ty/predicate.rs b/compiler/rustc_middle/src/ty/predicate.rs
index 80a9499e862..bc5f75f58c3 100644
--- a/compiler/rustc_middle/src/ty/predicate.rs
+++ b/compiler/rustc_middle/src/ty/predicate.rs
@@ -1,21 +1,25 @@
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::intern::Interned;
-use rustc_errors::{DiagArgValue, IntoDiagArg};
 use rustc_hir::def_id::DefId;
 use rustc_macros::{HashStable, Lift, TyDecodable, TyEncodable, TypeFoldable, TypeVisitable};
 use rustc_type_ir::ClauseKind as IrClauseKind;
 use rustc_type_ir::PredicateKind as IrPredicateKind;
 use rustc_type_ir::TraitPredicate as IrTraitPredicate;
 use rustc_type_ir::TraitRef as IrTraitRef;
+use rustc_type_ir::ProjectionPredicate as IrProjectionPredicate;
+use rustc_type_ir::ExistentialTraitRef as IrExistentialTraitRef;
+use rustc_type_ir::ExistentialProjection as IrExistentialProjection;
 use std::cmp::Ordering;
 
-use crate::ty::visit::TypeVisitableExt;
 use crate::ty::{
-    self, AliasTy, Binder, DebruijnIndex, DebugWithInfcx, EarlyBinder, GenericArgsRef,
+    self, AliasTy, Binder, DebruijnIndex, DebugWithInfcx, EarlyBinder,
     PredicatePolarity, Term, Ty, TyCtxt, TypeFlags, WithCachedTypeInfo,
 };
 
 pub type TraitRef<'tcx> = IrTraitRef<TyCtxt<'tcx>>;
+pub type ProjectionPredicate<'tcx> = IrProjectionPredicate<TyCtxt<'tcx>>;
+pub type ExistentialTraitRef<'tcx> = IrExistentialTraitRef<TyCtxt<'tcx>>;
+pub type ExistentialProjection<'tcx> = IrExistentialProjection<TyCtxt<'tcx>>;
 pub type TraitPredicate<'tcx> = IrTraitPredicate<TyCtxt<'tcx>>;
 pub type ClauseKind<'tcx> = IrClauseKind<TyCtxt<'tcx>>;
 pub type PredicateKind<'tcx> = IrPredicateKind<TyCtxt<'tcx>>;
@@ -342,52 +346,6 @@ impl<'tcx> PolyTraitRef<'tcx> {
     }
 }
 
-/// An existential reference to a trait, where `Self` is erased.
-/// For example, the trait object `Trait<'a, 'b, X, Y>` is:
-/// ```ignore (illustrative)
-/// exists T. T: Trait<'a, 'b, X, Y>
-/// ```
-/// The generic parameters don't include the erased `Self`, only trait
-/// type and lifetime parameters (`[X, Y]` and `['a, 'b]` above).
-#[derive(Copy, Clone, PartialEq, Eq, Hash, TyEncodable, TyDecodable)]
-#[derive(HashStable, TypeFoldable, TypeVisitable, Lift)]
-pub struct ExistentialTraitRef<'tcx> {
-    pub def_id: DefId,
-    pub args: GenericArgsRef<'tcx>,
-}
-
-impl<'tcx> ExistentialTraitRef<'tcx> {
-    pub fn erase_self_ty(
-        tcx: TyCtxt<'tcx>,
-        trait_ref: ty::TraitRef<'tcx>,
-    ) -> ty::ExistentialTraitRef<'tcx> {
-        // Assert there is a Self.
-        trait_ref.args.type_at(0);
-
-        ty::ExistentialTraitRef {
-            def_id: trait_ref.def_id,
-            args: tcx.mk_args(&trait_ref.args[1..]),
-        }
-    }
-
-    /// Object types don't have a self type specified. Therefore, when
-    /// we convert the principal trait-ref into a normal trait-ref,
-    /// you must give *some* self type. A common choice is `mk_err()`
-    /// or some placeholder type.
-    pub fn with_self_ty(&self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> ty::TraitRef<'tcx> {
-        // otherwise the escaping vars would be captured by the binder
-        // debug_assert!(!self_ty.has_escaping_bound_vars());
-
-        ty::TraitRef::new(tcx, self.def_id, [self_ty.into()].into_iter().chain(self.args.iter()))
-    }
-}
-
-impl<'tcx> IntoDiagArg for ExistentialTraitRef<'tcx> {
-    fn into_diag_arg(self) -> DiagArgValue {
-        self.to_string().into_diag_arg()
-    }
-}
-
 pub type PolyExistentialTraitRef<'tcx> = ty::Binder<'tcx, ExistentialTraitRef<'tcx>>;
 
 impl<'tcx> PolyExistentialTraitRef<'tcx> {
@@ -404,62 +362,8 @@ impl<'tcx> PolyExistentialTraitRef<'tcx> {
     }
 }
 
-/// A `ProjectionPredicate` for an `ExistentialTraitRef`.
-#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug, TyEncodable, TyDecodable)]
-#[derive(HashStable, TypeFoldable, TypeVisitable, Lift)]
-pub struct ExistentialProjection<'tcx> {
-    pub def_id: DefId,
-    pub args: GenericArgsRef<'tcx>,
-    pub term: Term<'tcx>,
-}
-
 pub type PolyExistentialProjection<'tcx> = ty::Binder<'tcx, ExistentialProjection<'tcx>>;
 
-impl<'tcx> ExistentialProjection<'tcx> {
-    /// Extracts the underlying existential trait reference from this projection.
-    /// For example, if this is a projection of `exists T. <T as Iterator>::Item == X`,
-    /// then this function would return an `exists T. T: Iterator` existential trait
-    /// reference.
-    pub fn trait_ref(&self, tcx: TyCtxt<'tcx>) -> ty::ExistentialTraitRef<'tcx> {
-        let def_id = tcx.parent(self.def_id);
-        let args_count = tcx.generics_of(def_id).count() - 1;
-        let args = tcx.mk_args(&self.args[..args_count]);
-        ty::ExistentialTraitRef { def_id, args }
-    }
-
-    pub fn with_self_ty(
-        &self,
-        tcx: TyCtxt<'tcx>,
-        self_ty: Ty<'tcx>,
-    ) -> ty::ProjectionPredicate<'tcx> {
-        // otherwise the escaping regions would be captured by the binders
-        debug_assert!(!self_ty.has_escaping_bound_vars());
-
-        ty::ProjectionPredicate {
-            projection_ty: AliasTy::new(
-                tcx,
-                self.def_id,
-                [self_ty.into()].into_iter().chain(self.args),
-            ),
-            term: self.term,
-        }
-    }
-
-    pub fn erase_self_ty(
-        tcx: TyCtxt<'tcx>,
-        projection_predicate: ty::ProjectionPredicate<'tcx>,
-    ) -> Self {
-        // Assert there is a Self.
-        projection_predicate.projection_ty.args.type_at(0);
-
-        Self {
-            def_id: projection_predicate.projection_ty.def_id,
-            args: tcx.mk_args(&projection_predicate.projection_ty.args[1..]),
-            term: projection_predicate.term,
-        }
-    }
-}
-
 impl<'tcx> PolyExistentialProjection<'tcx> {
     pub fn with_self_ty(
         &self,
@@ -628,43 +532,6 @@ pub struct CoercePredicate<'tcx> {
 }
 pub type PolyCoercePredicate<'tcx> = ty::Binder<'tcx, CoercePredicate<'tcx>>;
 
-/// This kind of predicate has no *direct* correspondent in the
-/// syntax, but it roughly corresponds to the syntactic forms:
-///
-/// 1. `T: TraitRef<..., Item = Type>`
-/// 2. `<T as TraitRef<...>>::Item == Type` (NYI)
-///
-/// In particular, form #1 is "desugared" to the combination of a
-/// normal trait predicate (`T: TraitRef<...>`) and one of these
-/// predicates. Form #2 is a broader form in that it also permits
-/// equality between arbitrary types. Processing an instance of
-/// Form #2 eventually yields one of these `ProjectionPredicate`
-/// instances to normalize the LHS.
-#[derive(Copy, Clone, PartialEq, Eq, Hash, TyEncodable, TyDecodable)]
-#[derive(HashStable, TypeFoldable, TypeVisitable, Lift)]
-pub struct ProjectionPredicate<'tcx> {
-    pub projection_ty: AliasTy<'tcx>,
-    pub term: Term<'tcx>,
-}
-
-impl<'tcx> ProjectionPredicate<'tcx> {
-    pub fn self_ty(self) -> Ty<'tcx> {
-        self.projection_ty.self_ty()
-    }
-
-    pub fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> ProjectionPredicate<'tcx> {
-        Self { projection_ty: self.projection_ty.with_self_ty(tcx, self_ty), ..self }
-    }
-
-    pub fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId {
-        self.projection_ty.trait_def_id(tcx)
-    }
-
-    pub fn def_id(self) -> DefId {
-        self.projection_ty.def_id
-    }
-}
-
 pub type PolyProjectionPredicate<'tcx> = Binder<'tcx, ProjectionPredicate<'tcx>>;
 
 impl<'tcx> PolyProjectionPredicate<'tcx> {
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index fc42ba6fcd3..9e5edb97fe2 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -3087,14 +3087,6 @@ define_print! {
             ty::PredicateKind::AliasRelate(t1, t2, dir) => p!(print(t1), write(" {} ", dir), print(t2)),
         }
     }
-}
-
-define_print_and_forward_display! {
-    (self, cx):
-
-    &'tcx ty::List<Ty<'tcx>> {
-        p!("{{", comma_sep(self.iter()), "}}")
-    }
 
     ty::ExistentialTraitRef<'tcx> {
         // Use a type that can't appear in defaults of type parameters.
@@ -3108,6 +3100,20 @@ define_print_and_forward_display! {
         p!(write("{} = ", name), print(self.term))
     }
 
+    ty::ProjectionPredicate<'tcx> {
+        p!(print(self.projection_ty), " == ");
+        cx.reset_type_limit();
+        p!(print(self.term))
+    }
+}
+
+define_print_and_forward_display! {
+    (self, cx):
+
+    &'tcx ty::List<Ty<'tcx>> {
+        p!("{{", comma_sep(self.iter()), "}}")
+    }
+
     ty::ExistentialPredicate<'tcx> {
         match *self {
             ty::ExistentialPredicate::Trait(x) => p!(print(x)),
@@ -3186,12 +3192,6 @@ define_print_and_forward_display! {
         p!(print(self.b))
     }
 
-    ty::ProjectionPredicate<'tcx> {
-        p!(print(self.projection_ty), " == ");
-        cx.reset_type_limit();
-        p!(print(self.term))
-    }
-
     ty::NormalizesTo<'tcx> {
         p!(print(self.alias), " normalizes-to ");
         cx.reset_type_limit();
diff --git a/compiler/rustc_middle/src/ty/structural_impls.rs b/compiler/rustc_middle/src/ty/structural_impls.rs
index e0fbcb1a7f0..b08ae4e5890 100644
--- a/compiler/rustc_middle/src/ty/structural_impls.rs
+++ b/compiler/rustc_middle/src/ty/structural_impls.rs
@@ -55,12 +55,6 @@ impl fmt::Debug for ty::UpvarId {
     }
 }
 
-impl<'tcx> fmt::Debug for ty::ExistentialTraitRef<'tcx> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        with_no_trimmed_paths!(fmt::Display::fmt(self, f))
-    }
-}
-
 impl<'tcx> fmt::Debug for ty::adjustment::Adjustment<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "{:?} -> {}", self.kind, self.target)
@@ -158,12 +152,6 @@ impl fmt::Debug for ty::ParamConst {
     }
 }
 
-impl<'tcx> fmt::Debug for ty::ProjectionPredicate<'tcx> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "ProjectionPredicate({:?}, {:?})", self.projection_ty, self.term)
-    }
-}
-
 impl<'tcx> fmt::Debug for ty::NormalizesTo<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "NormalizesTo({:?}, {:?})", self.alias, self.term)
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index a97244dda9a..1fc8eefd65f 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -1142,6 +1142,36 @@ pub struct AliasTy<'tcx> {
     _use_alias_ty_new_instead: (),
 }
 
+impl<'tcx> rustc_type_ir::inherent::AliasTy<TyCtxt<'tcx>> for AliasTy<'tcx> {
+    fn new(
+        interner: TyCtxt<'tcx>,
+        trait_def_id: DefId,
+        args: impl IntoIterator<Item: Into<ty::GenericArg<'tcx>>>,
+    ) -> Self {
+        AliasTy::new(interner, trait_def_id, args)
+    }
+
+    fn def_id(self) -> DefId {
+        self.def_id
+    }
+
+    fn args(self) -> ty::GenericArgsRef<'tcx> {
+        self.args
+    }
+
+    fn trait_def_id(self, interner: TyCtxt<'tcx>) -> DefId {
+        self.trait_def_id(interner)
+    }
+
+    fn self_ty(self) -> Ty<'tcx> {
+        self.self_ty()
+    }
+
+    fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
+        self.with_self_ty(tcx, self_ty)
+    }
+}
+
 impl<'tcx> AliasTy<'tcx> {
     pub fn new(
         tcx: TyCtxt<'tcx>,
diff --git a/compiler/rustc_type_ir/src/inherent.rs b/compiler/rustc_type_ir/src/inherent.rs
index 0fd34e0a65f..cf67f82efd7 100644
--- a/compiler/rustc_type_ir/src/inherent.rs
+++ b/compiler/rustc_type_ir/src/inherent.rs
@@ -1,5 +1,6 @@
 use std::fmt::Debug;
 use std::hash::Hash;
+use std::ops::Deref;
 
 use crate::fold::TypeSuperFoldable;
 use crate::visit::{Flags, TypeSuperVisitable};
@@ -50,7 +51,12 @@ pub trait GenericsOf<I: Interner<GenericsOf = Self>> {
 }
 
 pub trait GenericArgs<I: Interner<GenericArgs = Self>>:
-    Copy + DebugWithInfcx<I> + Hash + Eq + IntoIterator<Item = I::GenericArg>
+    Copy
+    + DebugWithInfcx<I>
+    + Hash
+    + Eq
+    + IntoIterator<Item = I::GenericArg>
+    + Deref<Target: Deref<Target = [I::GenericArg]>>
 {
     fn type_at(self, i: usize) -> I::Ty;
 
@@ -83,3 +89,23 @@ pub trait BoundVars<I: Interner> {
 
     fn has_no_bound_vars(&self) -> bool;
 }
+
+// TODO: Uplift `AliasTy`
+pub trait AliasTy<I: Interner>: Copy + DebugWithInfcx<I> + Hash + Eq + Sized {
+    fn new(
+        interner: I,
+        trait_def_id: I::DefId,
+        args: impl IntoIterator<Item: Into<I::GenericArg>>,
+    ) -> Self;
+
+    fn def_id(self) -> I::DefId;
+
+    fn args(self) -> I::GenericArgs;
+
+    fn trait_def_id(self, interner: I) -> I::DefId;
+
+    fn self_ty(self) -> I::Ty;
+
+    fn with_self_ty(self, tcx: I, self_ty: I::Ty) -> Self;
+
+}
diff --git a/compiler/rustc_type_ir/src/interner.rs b/compiler/rustc_type_ir/src/interner.rs
index fe14836b1d5..bfa769237a1 100644
--- a/compiler/rustc_type_ir/src/interner.rs
+++ b/compiler/rustc_type_ir/src/interner.rs
@@ -5,9 +5,20 @@ use std::hash::Hash;
 use crate::inherent::*;
 use crate::ir_print::IrPrint;
 use crate::visit::{Flags, TypeSuperVisitable, TypeVisitable};
-use crate::{CanonicalVarInfo, DebugWithInfcx, TraitPredicate, TraitRef};
-
-pub trait Interner: Sized + Copy + IrPrint<TraitRef<Self>> + IrPrint<TraitPredicate<Self>> {
+use crate::{
+    CanonicalVarInfo, DebugWithInfcx, ExistentialProjection, ExistentialTraitRef,
+    ProjectionPredicate, TraitPredicate, TraitRef,
+};
+
+pub trait Interner:
+    Sized
+    + Copy
+    + IrPrint<TraitRef<Self>>
+    + IrPrint<TraitPredicate<Self>>
+    + IrPrint<ExistentialTraitRef<Self>>
+    + IrPrint<ExistentialProjection<Self>>
+    + IrPrint<ProjectionPredicate<Self>>
+{
     type DefId: Copy + Debug + Hash + Eq;
     type DefiningOpaqueTypes: Copy + Debug + Hash + Default + Eq + TypeVisitable<Self>;
     type AdtDef: Copy + Debug + Hash + Eq;
@@ -25,7 +36,7 @@ pub trait Interner: Sized + Copy + IrPrint<TraitRef<Self>> + IrPrint<TraitPredic
     // Kinds of tys
     type Ty: Ty<Self>;
     type Tys: Copy + Debug + Hash + Eq + IntoIterator<Item = Self::Ty>;
-    type AliasTy: Copy + DebugWithInfcx<Self> + Hash + Eq;
+    type AliasTy: AliasTy<Self>;
     type ParamTy: Copy + Debug + Hash + Eq;
     type BoundTy: Copy + Debug + Hash + Eq;
     type PlaceholderTy: PlaceholderLike;
@@ -71,11 +82,15 @@ pub trait Interner: Sized + Copy + IrPrint<TraitRef<Self>> + IrPrint<TraitPredic
     type GenericsOf: GenericsOf<Self>;
     fn generics_of(self, def_id: Self::DefId) -> Self::GenericsOf;
 
+    fn mk_args(self, args: &[Self::GenericArg]) -> Self::GenericArgs;
+
     fn check_and_mk_args(
         self,
         def_id: Self::DefId,
         args: impl IntoIterator<Item: Into<Self::GenericArg>>,
     ) -> Self::GenericArgs;
+
+    fn parent(self, def_id: Self::DefId) -> Self::DefId;
 }
 
 /// Imagine you have a function `F: FnOnce(&[T]) -> R`, plus an iterator `iter`
diff --git a/compiler/rustc_type_ir/src/ir_print.rs b/compiler/rustc_type_ir/src/ir_print.rs
index 715fb317669..9b72764b4d1 100644
--- a/compiler/rustc_type_ir/src/ir_print.rs
+++ b/compiler/rustc_type_ir/src/ir_print.rs
@@ -1,6 +1,9 @@
 use std::fmt;
 
-use crate::{Interner, TraitPredicate, TraitRef};
+use crate::{
+    ExistentialProjection, ExistentialTraitRef, Interner, ProjectionPredicate, TraitPredicate,
+    TraitRef,
+};
 
 pub trait IrPrint<T> {
     fn print(t: &T, fmt: &mut fmt::Formatter<'_>) -> fmt::Result;
@@ -31,6 +34,12 @@ macro_rules! define_debug_via_print {
     }
 }
 
-define_display_via_print!(TraitRef, TraitPredicate,);
+define_display_via_print!(
+    TraitRef,
+    TraitPredicate,
+    ExistentialTraitRef,
+    ExistentialProjection,
+    ProjectionPredicate
+);
 
-define_debug_via_print!(TraitRef,);
+define_debug_via_print!(TraitRef, ExistentialTraitRef, ExistentialProjection);
diff --git a/compiler/rustc_type_ir/src/lib.rs b/compiler/rustc_type_ir/src/lib.rs
index 62efa32c9f2..184b882c2b7 100644
--- a/compiler/rustc_type_ir/src/lib.rs
+++ b/compiler/rustc_type_ir/src/lib.rs
@@ -39,7 +39,7 @@ mod infcx;
 mod interner;
 mod predicate_kind;
 mod region_kind;
-mod trait_ref;
+mod predicate;
 
 pub use canonical::*;
 #[cfg(feature = "nightly")]
@@ -51,7 +51,7 @@ pub use infcx::InferCtxtLike;
 pub use interner::*;
 pub use predicate_kind::*;
 pub use region_kind::*;
-pub use trait_ref::*;
+pub use predicate::*;
 pub use ty_info::*;
 pub use ty_kind::*;
 pub use AliasKind::*;
diff --git a/compiler/rustc_type_ir/src/predicate.rs b/compiler/rustc_type_ir/src/predicate.rs
new file mode 100644
index 00000000000..9cd9319a1b1
--- /dev/null
+++ b/compiler/rustc_type_ir/src/predicate.rs
@@ -0,0 +1,298 @@
+use std::fmt;
+
+use rustc_macros::{HashStable_NoContext, TyDecodable, TyEncodable};
+use rustc_type_ir_macros::{Lift_Generic, TypeFoldable_Generic, TypeVisitable_Generic};
+
+use crate::inherent::*;
+use crate::visit::TypeVisitableExt as _;
+use crate::Interner;
+
+/// A complete reference to a trait. These take numerous guises in syntax,
+/// but perhaps the most recognizable form is in a where-clause:
+/// ```ignore (illustrative)
+/// T: Foo<U>
+/// ```
+/// This would be represented by a trait-reference where the `DefId` is the
+/// `DefId` for the trait `Foo` and the args define `T` as parameter 0,
+/// and `U` as parameter 1.
+///
+/// Trait references also appear in object types like `Foo<U>`, but in
+/// that case the `Self` parameter is absent from the generic parameters.
+#[derive(derivative::Derivative)]
+#[derivative(
+    Clone(bound = ""),
+    Copy(bound = ""),
+    Hash(bound = ""),
+    PartialEq(bound = ""),
+    Eq(bound = "")
+)]
+#[derive(TypeVisitable_Generic, TypeFoldable_Generic, Lift_Generic)]
+#[cfg_attr(feature = "nightly", derive(TyDecodable, TyEncodable, HashStable_NoContext))]
+pub struct TraitRef<I: Interner> {
+    pub def_id: I::DefId,
+    pub args: I::GenericArgs,
+    /// This field exists to prevent the creation of `TraitRef` without
+    /// calling [`TraitRef::new`].
+    _use_trait_ref_new_instead: (),
+}
+
+impl<I: Interner> TraitRef<I> {
+    pub fn new(
+        interner: I,
+        trait_def_id: I::DefId,
+        args: impl IntoIterator<Item: Into<I::GenericArg>>,
+    ) -> Self {
+        let args = interner.check_and_mk_args(trait_def_id, args);
+        Self { def_id: trait_def_id, args, _use_trait_ref_new_instead: () }
+    }
+
+    pub fn from_method(interner: I, trait_id: I::DefId, args: I::GenericArgs) -> TraitRef<I> {
+        let generics = interner.generics_of(trait_id);
+        TraitRef::new(interner, trait_id, args.into_iter().take(generics.count()))
+    }
+
+    /// Returns a `TraitRef` of the form `P0: Foo<P1..Pn>` where `Pi`
+    /// are the parameters defined on trait.
+    pub fn identity(interner: I, def_id: I::DefId) -> TraitRef<I> {
+        TraitRef::new(interner, def_id, I::GenericArgs::identity_for_item(interner, def_id))
+    }
+
+    pub fn with_self_ty(self, interner: I, self_ty: I::Ty) -> Self {
+        TraitRef::new(
+            interner,
+            self.def_id,
+            [self_ty.into()].into_iter().chain(self.args.into_iter().skip(1)),
+        )
+    }
+
+    #[inline]
+    pub fn self_ty(&self) -> I::Ty {
+        self.args.type_at(0)
+    }
+}
+
+#[derive(derivative::Derivative)]
+#[derivative(
+    Clone(bound = ""),
+    Copy(bound = ""),
+    Hash(bound = ""),
+    PartialEq(bound = ""),
+    Eq(bound = "")
+)]
+#[derive(TypeVisitable_Generic, TypeFoldable_Generic, Lift_Generic)]
+#[cfg_attr(feature = "nightly", derive(TyDecodable, TyEncodable, HashStable_NoContext))]
+pub struct TraitPredicate<I: Interner> {
+    pub trait_ref: TraitRef<I>,
+
+    /// If polarity is Positive: we are proving that the trait is implemented.
+    ///
+    /// If polarity is Negative: we are proving that a negative impl of this trait
+    /// exists. (Note that coherence also checks whether negative impls of supertraits
+    /// exist via a series of predicates.)
+    ///
+    /// If polarity is Reserved: that's a bug.
+    pub polarity: PredicatePolarity,
+}
+
+impl<I: Interner> TraitPredicate<I> {
+    pub fn with_self_ty(self, interner: I, self_ty: I::Ty) -> Self {
+        Self { trait_ref: self.trait_ref.with_self_ty(interner, self_ty), polarity: self.polarity }
+    }
+
+    pub fn def_id(self) -> I::DefId {
+        self.trait_ref.def_id
+    }
+
+    pub fn self_ty(self) -> I::Ty {
+        self.trait_ref.self_ty()
+    }
+}
+
+impl<I: Interner> fmt::Debug for TraitPredicate<I> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // FIXME(effects) printing?
+        write!(f, "TraitPredicate({:?}, polarity:{:?})", self.trait_ref, self.polarity)
+    }
+}
+
+/// Polarity for a trait predicate. May either be negative or positive.
+/// Distinguished from [`ImplPolarity`] since we never compute goals with
+/// "reservation" level.
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
+#[cfg_attr(feature = "nightly", derive(TyDecodable, TyEncodable, HashStable_NoContext))]
+pub enum PredicatePolarity {
+    /// `Type: Trait`
+    Positive,
+    /// `Type: !Trait`
+    Negative,
+}
+
+impl PredicatePolarity {
+    /// Flips polarity by turning `Positive` into `Negative` and `Negative` into `Positive`.
+    pub fn flip(&self) -> PredicatePolarity {
+        match self {
+            PredicatePolarity::Positive => PredicatePolarity::Negative,
+            PredicatePolarity::Negative => PredicatePolarity::Positive,
+        }
+    }
+}
+
+impl fmt::Display for PredicatePolarity {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::Positive => f.write_str("positive"),
+            Self::Negative => f.write_str("negative"),
+        }
+    }
+}
+
+/// An existential reference to a trait, where `Self` is erased.
+/// For example, the trait object `Trait<'a, 'b, X, Y>` is:
+/// ```ignore (illustrative)
+/// exists T. T: Trait<'a, 'b, X, Y>
+/// ```
+/// The generic parameters don't include the erased `Self`, only trait
+/// type and lifetime parameters (`[X, Y]` and `['a, 'b]` above).
+#[derive(derivative::Derivative)]
+#[derivative(
+    Clone(bound = ""),
+    Copy(bound = ""),
+    Hash(bound = ""),
+    PartialEq(bound = ""),
+    Eq(bound = "")
+)]
+#[derive(TypeVisitable_Generic, TypeFoldable_Generic, Lift_Generic)]
+#[cfg_attr(feature = "nightly", derive(TyDecodable, TyEncodable, HashStable_NoContext))]
+pub struct ExistentialTraitRef<I: Interner> {
+    pub def_id: I::DefId,
+    pub args: I::GenericArgs,
+}
+
+impl<I: Interner> ExistentialTraitRef<I> {
+    pub fn erase_self_ty(interner: I, trait_ref: TraitRef<I>) -> ExistentialTraitRef<I> {
+        // Assert there is a Self.
+        trait_ref.args.type_at(0);
+
+        ExistentialTraitRef {
+            def_id: trait_ref.def_id,
+            args: interner.mk_args(&trait_ref.args[1..]),
+        }
+    }
+
+    /// Object types don't have a self type specified. Therefore, when
+    /// we convert the principal trait-ref into a normal trait-ref,
+    /// you must give *some* self type. A common choice is `mk_err()`
+    /// or some placeholder type.
+    pub fn with_self_ty(self, interner: I, self_ty: I::Ty) -> TraitRef<I> {
+        // otherwise the escaping vars would be captured by the binder
+        // debug_assert!(!self_ty.has_escaping_bound_vars());
+
+        TraitRef::new(interner, self.def_id, [self_ty.into()].into_iter().chain(self.args.into_iter()))
+    }
+}
+
+/// A `ProjectionPredicate` for an `ExistentialTraitRef`.
+#[derive(derivative::Derivative)]
+#[derivative(
+    Clone(bound = ""),
+    Copy(bound = ""),
+    Hash(bound = ""),
+    PartialEq(bound = ""),
+    Eq(bound = "")
+)]
+#[derive(TypeVisitable_Generic, TypeFoldable_Generic, Lift_Generic)]
+#[cfg_attr(feature = "nightly", derive(TyDecodable, TyEncodable, HashStable_NoContext))]
+pub struct ExistentialProjection<I: Interner> {
+    pub def_id: I::DefId,
+    pub args: I::GenericArgs,
+    pub term: I::Term,
+}
+
+impl<I: Interner> ExistentialProjection<I> {
+    /// Extracts the underlying existential trait reference from this projection.
+    /// For example, if this is a projection of `exists T. <T as Iterator>::Item == X`,
+    /// then this function would return an `exists T. T: Iterator` existential trait
+    /// reference.
+    pub fn trait_ref(&self, tcx: I) -> ExistentialTraitRef<I> {
+        let def_id = tcx.parent(self.def_id);
+        let args_count = tcx.generics_of(def_id).count() - 1;
+        let args = tcx.mk_args(&self.args[..args_count]);
+        ExistentialTraitRef { def_id, args }
+    }
+
+    pub fn with_self_ty(&self, tcx: I, self_ty: I::Ty) -> ProjectionPredicate<I> {
+        // otherwise the escaping regions would be captured by the binders
+        debug_assert!(!self_ty.has_escaping_bound_vars());
+
+        ProjectionPredicate {
+            projection_ty: I::AliasTy::new(
+                tcx,
+                self.def_id,
+                [self_ty.into()].into_iter().chain(self.args),
+            ),
+            term: self.term,
+        }
+    }
+
+    pub fn erase_self_ty(tcx: I, projection_predicate: ProjectionPredicate<I>) -> Self {
+        // Assert there is a Self.
+        projection_predicate.projection_ty.args().type_at(0);
+
+        Self {
+            def_id: projection_predicate.projection_ty.def_id(),
+            args: tcx.mk_args(&projection_predicate.projection_ty.args()[1..]),
+            term: projection_predicate.term,
+        }
+    }
+}
+
+/// This kind of predicate has no *direct* correspondent in the
+/// syntax, but it roughly corresponds to the syntactic forms:
+///
+/// 1. `T: TraitRef<..., Item = Type>`
+/// 2. `<T as TraitRef<...>>::Item == Type` (NYI)
+///
+/// In particular, form #1 is "desugared" to the combination of a
+/// normal trait predicate (`T: TraitRef<...>`) and one of these
+/// predicates. Form #2 is a broader form in that it also permits
+/// equality between arbitrary types. Processing an instance of
+/// Form #2 eventually yields one of these `ProjectionPredicate`
+/// instances to normalize the LHS.
+#[derive(derivative::Derivative)]
+#[derivative(
+    Clone(bound = ""),
+    Copy(bound = ""),
+    Hash(bound = ""),
+    PartialEq(bound = ""),
+    Eq(bound = "")
+)]
+#[derive(TypeVisitable_Generic, TypeFoldable_Generic, Lift_Generic)]
+#[cfg_attr(feature = "nightly", derive(TyDecodable, TyEncodable, HashStable_NoContext))]
+pub struct ProjectionPredicate<I: Interner> {
+    pub projection_ty: I::AliasTy,
+    pub term: I::Term,
+}
+
+impl<I: Interner> ProjectionPredicate<I> {
+    pub fn self_ty(self) -> I::Ty {
+        self.projection_ty.self_ty()
+    }
+
+    pub fn with_self_ty(self, tcx: I, self_ty: I::Ty) -> ProjectionPredicate<I> {
+        Self { projection_ty: self.projection_ty.with_self_ty(tcx, self_ty), ..self }
+    }
+
+    pub fn trait_def_id(self, tcx: I) -> I::DefId {
+        self.projection_ty.trait_def_id(tcx)
+    }
+
+    pub fn def_id(self) -> I::DefId {
+        self.projection_ty.def_id()
+    }
+}
+
+impl<I: Interner> fmt::Debug for ProjectionPredicate<I> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "ProjectionPredicate({:?}, {:?})", self.projection_ty, self.term)
+    }
+}
diff --git a/compiler/rustc_type_ir/src/trait_ref.rs b/compiler/rustc_type_ir/src/trait_ref.rs
deleted file mode 100644
index 0aae75ebf43..00000000000
--- a/compiler/rustc_type_ir/src/trait_ref.rs
+++ /dev/null
@@ -1,146 +0,0 @@
-use std::fmt;
-
-use rustc_macros::{HashStable_NoContext, TyDecodable, TyEncodable};
-use rustc_type_ir_macros::{Lift_Generic, TypeFoldable_Generic, TypeVisitable_Generic};
-
-use crate::inherent::*;
-use crate::Interner;
-
-/// A complete reference to a trait. These take numerous guises in syntax,
-/// but perhaps the most recognizable form is in a where-clause:
-/// ```ignore (illustrative)
-/// T: Foo<U>
-/// ```
-/// This would be represented by a trait-reference where the `DefId` is the
-/// `DefId` for the trait `Foo` and the args define `T` as parameter 0,
-/// and `U` as parameter 1.
-///
-/// Trait references also appear in object types like `Foo<U>`, but in
-/// that case the `Self` parameter is absent from the generic parameters.
-#[derive(derivative::Derivative)]
-#[derivative(
-    Clone(bound = ""),
-    Copy(bound = ""),
-    Hash(bound = ""),
-    PartialEq(bound = ""),
-    Eq(bound = "")
-)]
-#[derive(TypeVisitable_Generic, TypeFoldable_Generic, Lift_Generic)]
-#[cfg_attr(feature = "nightly", derive(TyDecodable, TyEncodable, HashStable_NoContext))]
-pub struct TraitRef<I: Interner> {
-    pub def_id: I::DefId,
-    pub args: I::GenericArgs,
-    /// This field exists to prevent the creation of `TraitRef` without
-    /// calling [`TraitRef::new`].
-    _use_trait_ref_new_instead: (),
-}
-
-impl<I: Interner> TraitRef<I> {
-    pub fn new(
-        interner: I,
-        trait_def_id: I::DefId,
-        args: impl IntoIterator<Item: Into<I::GenericArg>>,
-    ) -> Self {
-        let args = interner.check_and_mk_args(trait_def_id, args);
-        Self { def_id: trait_def_id, args, _use_trait_ref_new_instead: () }
-    }
-
-    pub fn from_method(interner: I, trait_id: I::DefId, args: I::GenericArgs) -> TraitRef<I> {
-        let generics = interner.generics_of(trait_id);
-        TraitRef::new(interner, trait_id, args.into_iter().take(generics.count()))
-    }
-
-    /// Returns a `TraitRef` of the form `P0: Foo<P1..Pn>` where `Pi`
-    /// are the parameters defined on trait.
-    pub fn identity(interner: I, def_id: I::DefId) -> TraitRef<I> {
-        TraitRef::new(interner, def_id, I::GenericArgs::identity_for_item(interner, def_id))
-    }
-
-    pub fn with_self_ty(self, interner: I, self_ty: I::Ty) -> Self {
-        TraitRef::new(
-            interner,
-            self.def_id,
-            [self_ty.into()].into_iter().chain(self.args.into_iter().skip(1)),
-        )
-    }
-
-    #[inline]
-    pub fn self_ty(&self) -> I::Ty {
-        self.args.type_at(0)
-    }
-}
-
-#[derive(derivative::Derivative)]
-#[derivative(
-    Clone(bound = ""),
-    Copy(bound = ""),
-    Hash(bound = ""),
-    PartialEq(bound = ""),
-    Eq(bound = "")
-)]
-#[derive(TypeVisitable_Generic, TypeFoldable_Generic, Lift_Generic)]
-#[cfg_attr(feature = "nightly", derive(TyDecodable, TyEncodable, HashStable_NoContext))]
-pub struct TraitPredicate<I: Interner> {
-    pub trait_ref: TraitRef<I>,
-
-    /// If polarity is Positive: we are proving that the trait is implemented.
-    ///
-    /// If polarity is Negative: we are proving that a negative impl of this trait
-    /// exists. (Note that coherence also checks whether negative impls of supertraits
-    /// exist via a series of predicates.)
-    ///
-    /// If polarity is Reserved: that's a bug.
-    pub polarity: PredicatePolarity,
-}
-
-impl<I: Interner> TraitPredicate<I> {
-    pub fn with_self_ty(self, interner: I, self_ty: I::Ty) -> Self {
-        Self { trait_ref: self.trait_ref.with_self_ty(interner, self_ty), polarity: self.polarity }
-    }
-
-    pub fn def_id(self) -> I::DefId {
-        self.trait_ref.def_id
-    }
-
-    pub fn self_ty(self) -> I::Ty {
-        self.trait_ref.self_ty()
-    }
-}
-
-impl<I: Interner> fmt::Debug for TraitPredicate<I> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        // FIXME(effects) printing?
-        write!(f, "TraitPredicate({:?}, polarity:{:?})", self.trait_ref, self.polarity)
-    }
-}
-
-/// Polarity for a trait predicate. May either be negative or positive.
-/// Distinguished from [`ImplPolarity`] since we never compute goals with
-/// "reservation" level.
-#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
-#[cfg_attr(feature = "nightly", derive(TyDecodable, TyEncodable, HashStable_NoContext))]
-pub enum PredicatePolarity {
-    /// `Type: Trait`
-    Positive,
-    /// `Type: !Trait`
-    Negative,
-}
-
-impl PredicatePolarity {
-    /// Flips polarity by turning `Positive` into `Negative` and `Negative` into `Positive`.
-    pub fn flip(&self) -> PredicatePolarity {
-        match self {
-            PredicatePolarity::Positive => PredicatePolarity::Negative,
-            PredicatePolarity::Negative => PredicatePolarity::Positive,
-        }
-    }
-}
-
-impl fmt::Display for PredicatePolarity {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self {
-            Self::Positive => f.write_str("positive"),
-            Self::Negative => f.write_str("negative"),
-        }
-    }
-}