Rollup merge of #109470 - compiler-errors:gat-normalize-bound, r=jackh726

Correctly substitute GAT's type used in `normalize_param_env` in `check_type_bounds`

Given:

```rust
trait Foo {
    type Assoc<T>: PartialEq<Self::Assoc<i32>>;
}

impl Foo for () {
    type Assoc<T> = Wrapper<T>;
}

struct Wrapper<T>(T);

impl<T> PartialEq<Wrapper<i32>> for Wrapper<T> { }
```

We add an additional predicate in the `normalize_param_env` in `check_type_bounds` that is used to normalize the GAT's bounds to check them in the impl. Problematically, though, that predicate is constructed to be `for<^0> <() as Foo>::Assoc<^0> => Wrapper<T>`, instead of `for<^0> <() as Foo>::Assoc<^0> => Wrapper<^0>`.

That means `Self::Assoc<i32>` in the bounds that we're checking normalizes to `Wrapper<T>`, instead of `Wrapper<i32>`, and so the bound `Self::Assoc<T>: PartialEq<Self::Assoc<i32>>` normalizes to `Wrapper<T>: PartialEq<Wrapper<T>>`, which does not hold.

Fixes this by properly substituting the RHS of that normalizes predicate that we add to the `normalize_param_env`. That means the bound is properly normalized to `Wrapper<T>: PartialEq<Wrapper<i32>>`, which *does* hold.

---

The second commit in this PR just cleans up some substs stuff and some naming.

r? `@jackh726` cc #87900

2 files changed, 67 insertions, 53 deletions

diff --git a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
index 49665525967..17abb6ef75a 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
@@ -1876,14 +1876,17 @@ pub(super) fn check_type_bounds<'tcx>(
     impl_ty: ty::AssocItem,
     impl_trait_ref: ty::TraitRef<'tcx>,
 ) -> Result<(), ErrorGuaranteed> {
+    let param_env = tcx.param_env(impl_ty.def_id);
+    let container_id = impl_ty.container_id(tcx);
     // Given
     //
     // impl<A, B> Foo<u32> for (A, B) {
-    //     type Bar<C> =...
+    //     type Bar<C> = Wrapper<A, B, C>
     // }
     //
     // - `impl_trait_ref` would be `<(A, B) as Foo<u32>>`
-    // - `impl_ty_substs` would be `[A, B, ^0.0]` (`^0.0` here is the bound var with db 0 and index 0)
+    // - `normalize_impl_ty_substs` would be `[A, B, ^0.0]` (`^0.0` here is the bound var with db 0 and index 0)
+    // - `normalize_impl_ty` would be `Wrapper<A, B, ^0.0>`
     // - `rebased_substs` would be `[(A, B), u32, ^0.0]`, combining the substs from
     //    the *trait* with the generic associated type parameters (as bound vars).
     //
@@ -1912,56 +1915,46 @@ pub(super) fn check_type_bounds<'tcx>(
     // Member<C: Eq> = .... That type would fail a well-formedness check that we ought to be doing
     // elsewhere, which would check that any <T as Family>::Member<X> meets the bounds declared in
     // the trait (notably, that X: Eq and T: Family).
-    let defs: &ty::Generics = tcx.generics_of(impl_ty.def_id);
-    let mut substs = smallvec::SmallVec::with_capacity(defs.count());
-    if let Some(def_id) = defs.parent {
-        let parent_defs = tcx.generics_of(def_id);
-        InternalSubsts::fill_item(&mut substs, tcx, parent_defs, &mut |param, _| {
-            tcx.mk_param_from_def(param)
-        });
-    }
     let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
-        smallvec::SmallVec::with_capacity(defs.count());
-    InternalSubsts::fill_single(&mut substs, defs, &mut |param, _| match param.kind {
-        GenericParamDefKind::Type { .. } => {
-            let kind = ty::BoundTyKind::Param(param.def_id, param.name);
-            let bound_var = ty::BoundVariableKind::Ty(kind);
-            bound_vars.push(bound_var);
-            tcx.mk_bound(
-                ty::INNERMOST,
-                ty::BoundTy { var: ty::BoundVar::from_usize(bound_vars.len() - 1), kind },
-            )
-            .into()
-        }
-        GenericParamDefKind::Lifetime => {
-            let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
-            let bound_var = ty::BoundVariableKind::Region(kind);
-            bound_vars.push(bound_var);
-            tcx.mk_re_late_bound(
-                ty::INNERMOST,
-                ty::BoundRegion { var: ty::BoundVar::from_usize(bound_vars.len() - 1), kind },
-            )
-            .into()
-        }
-        GenericParamDefKind::Const { .. } => {
-            let bound_var = ty::BoundVariableKind::Const;
-            bound_vars.push(bound_var);
-            tcx.mk_const(
-                ty::ConstKind::Bound(ty::INNERMOST, ty::BoundVar::from_usize(bound_vars.len() - 1)),
-                tcx.type_of(param.def_id).subst_identity(),
-            )
-            .into()
-        }
-    });
-    let bound_vars = tcx.mk_bound_variable_kinds(&bound_vars);
-    let impl_ty_substs = tcx.mk_substs(&substs);
-    let container_id = impl_ty.container_id(tcx);
-
-    let rebased_substs = impl_ty_substs.rebase_onto(tcx, container_id, impl_trait_ref.substs);
-    let impl_ty_value = tcx.type_of(impl_ty.def_id).subst_identity();
-
-    let param_env = tcx.param_env(impl_ty.def_id);
-
+        smallvec::SmallVec::with_capacity(tcx.generics_of(impl_ty.def_id).params.len());
+    // Extend the impl's identity substs with late-bound GAT vars
+    let normalize_impl_ty_substs = ty::InternalSubsts::identity_for_item(tcx, container_id)
+        .extend_to(tcx, impl_ty.def_id, |param, _| match param.kind {
+            GenericParamDefKind::Type { .. } => {
+                let kind = ty::BoundTyKind::Param(param.def_id, param.name);
+                let bound_var = ty::BoundVariableKind::Ty(kind);
+                bound_vars.push(bound_var);
+                tcx.mk_bound(
+                    ty::INNERMOST,
+                    ty::BoundTy { var: ty::BoundVar::from_usize(bound_vars.len() - 1), kind },
+                )
+                .into()
+            }
+            GenericParamDefKind::Lifetime => {
+                let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
+                let bound_var = ty::BoundVariableKind::Region(kind);
+                bound_vars.push(bound_var);
+                tcx.mk_re_late_bound(
+                    ty::INNERMOST,
+                    ty::BoundRegion { var: ty::BoundVar::from_usize(bound_vars.len() - 1), kind },
+                )
+                .into()
+            }
+            GenericParamDefKind::Const { .. } => {
+                let bound_var = ty::BoundVariableKind::Const;
+                bound_vars.push(bound_var);
+                tcx.mk_const(
+                    ty::ConstKind::Bound(
+                        ty::INNERMOST,
+                        ty::BoundVar::from_usize(bound_vars.len() - 1),
+                    ),
+                    tcx.type_of(param.def_id)
+                        .no_bound_vars()
+                        .expect("const parameter types cannot be generic"),
+                )
+                .into()
+            }
+        });
     // When checking something like
     //
     // trait X { type Y: PartialEq<<Self as X>::Y> }
@@ -1971,9 +1964,13 @@ pub(super) fn check_type_bounds<'tcx>(
     // we want <T as X>::Y to normalize to S. This is valid because we are
     // checking the default value specifically here. Add this equality to the
     // ParamEnv for normalization specifically.
+    let normalize_impl_ty = tcx.type_of(impl_ty.def_id).subst(tcx, normalize_impl_ty_substs);
+    let rebased_substs =
+        normalize_impl_ty_substs.rebase_onto(tcx, container_id, impl_trait_ref.substs);
+    let bound_vars = tcx.mk_bound_variable_kinds(&bound_vars);
     let normalize_param_env = {
         let mut predicates = param_env.caller_bounds().iter().collect::<Vec<_>>();
-        match impl_ty_value.kind() {
+        match normalize_impl_ty.kind() {
             ty::Alias(ty::Projection, proj)
                 if proj.def_id == trait_ty.def_id && proj.substs == rebased_substs =>
             {
@@ -1987,7 +1984,7 @@ pub(super) fn check_type_bounds<'tcx>(
                 ty::Binder::bind_with_vars(
                     ty::ProjectionPredicate {
                         projection_ty: tcx.mk_alias_ty(trait_ty.def_id, rebased_substs),
-                        term: impl_ty_value.into(),
+                        term: normalize_impl_ty.into(),
                     },
                     bound_vars,
                 )
diff --git a/tests/ui/generic-associated-types/gat-bounds-normalize-pred.rs b/tests/ui/generic-associated-types/gat-bounds-normalize-pred.rs
new file mode 100644
index 00000000000..b43f982283b
--- /dev/null
+++ b/tests/ui/generic-associated-types/gat-bounds-normalize-pred.rs
@@ -0,0 +1,17 @@
+// check-pass
+
+trait Foo {
+    type Assoc<T>: PartialEq<Self::Assoc<i32>>;
+}
+
+impl Foo for () {
+    type Assoc<T> = Wrapper<T>;
+}
+
+struct Wrapper<T>(T);
+
+impl<T> PartialEq<Wrapper<i32>> for Wrapper<T> {
+    fn eq(&self, _other: &Wrapper<i32>) -> bool { true }
+}
+
+fn main() {}