Auto merge of #117131 - compiler-errors:projection-oops, r=lcnr

Add all RPITITs when augmenting param-env with GAT bounds in `check_type_bounds`

When checking that associated type definitions actually satisfy their associated type bounds in `check_type_bounds`, we construct a "`normalize_param_env`" which adds a projection predicate that allows us to assume that we can project the GAT to the definition we're checking. For example, in:

```rust
type Foo {
  type Bar: Display = i32;
}
```

We would add `<Self as Foo>::Bar = i32` as a projection predicate when checking that `i32: Display` holds.

That `normalize_param_env` was, for some reason, only being used to normalize the predicate before it was registered. This is sketchy, because a nested obligation may require the GAT bound to hold, and also the projection cache is broken and doesn't differentiate projection cache keys that differ by param-envs 😿.

This `normalize_param_env` is also not sufficient when we have nested RPITITs and default trait methods, since we need to be able to assume we can normalize both the RPITIT and all of its child RPITITs to sufficiently prove all of its bounds. This is the cause of #117104, which only starts to fail for RPITITs that are nested 3 and above due to the projection-cache bug above.[^1]

## First fix

Use the `normalize_param_env` everywhere in `check_type_bounds`. This is reflected in a test I've constructed that fixes a GAT-only failure.

## Second fix

For RPITITs, install projection predicates for each RPITIT in the same function in `check_type_bounds`. This fixes #117104.

not sure who to request, so...
r? `@lcnr` hehe feel free to reassign :3

[^1]: The projection cache bug specifically occurs because we try normalizing the `assumed_wf_types` with the non-normalization param-env. This causes us to insert a projection cache entry that keeps the outermost RPITIT rigid, and it trivially satisifes all its own bounds. Super sketchy![^2]

[^2]: I haven't actually gone and fixed the projection cache bug because it's only marginally related, but I could, and it should no longer be triggered here.

6 files changed, 35 insertions, 20 deletions

diff --git a/tests/ui/generic-associated-types/assume-gat-normalization-for-nested-goals.rs b/tests/ui/generic-associated-types/assume-gat-normalization-for-nested-goals.rs
new file mode 100644
index 00000000000..7b168707239
--- /dev/null
+++ b/tests/ui/generic-associated-types/assume-gat-normalization-for-nested-goals.rs
@@ -0,0 +1,18 @@
+// check-pass
+
+#![feature(associated_type_defaults)]
+
+trait Foo {
+    type Bar<T>: Baz<Self> = i32;
+    // We should be able to prove that `i32: Baz<Self>` because of
+    // the impl below, which requires that `Self::Bar<()>: Eq<i32>`
+    // which is true, because we assume `for<T> Self::Bar<T> = i32`.
+}
+
+trait Baz<T: ?Sized> {}
+impl<T: Foo + ?Sized> Baz<T> for i32 where T::Bar<()>: Eq<i32> {}
+
+trait Eq<T> {}
+impl<T> Eq<T> for T {}
+
+fn main() {}
diff --git a/tests/ui/impl-trait/in-trait/nested-rpitit-bounds.rs b/tests/ui/impl-trait/in-trait/nested-rpitit-bounds.rs
new file mode 100644
index 00000000000..b97fd7d1ffe
--- /dev/null
+++ b/tests/ui/impl-trait/in-trait/nested-rpitit-bounds.rs
@@ -0,0 +1,11 @@
+// check-pass
+
+use std::ops::Deref;
+
+trait Foo {
+    fn foo() -> impl Deref<Target = impl Deref<Target = impl Sized>> {
+        &&()
+    }
+}
+
+fn main() {}
diff --git a/tests/ui/traits/new-solver/specialization-transmute.rs b/tests/ui/traits/new-solver/specialization-transmute.rs
index 7523b828321..f6b19e7adf5 100644
--- a/tests/ui/traits/new-solver/specialization-transmute.rs
+++ b/tests/ui/traits/new-solver/specialization-transmute.rs
@@ -10,7 +10,7 @@ trait Default {
 }
 
 impl<T> Default for T {
-    default type Id = T; //~ ERROR: type annotations needed
+    default type Id = T;
     // This will be fixed by #111994
     fn intu(&self) -> &Self::Id { //~ ERROR type annotations needed
         self
diff --git a/tests/ui/traits/new-solver/specialization-transmute.stderr b/tests/ui/traits/new-solver/specialization-transmute.stderr
index 18965a465b3..09b1405fefb 100644
--- a/tests/ui/traits/new-solver/specialization-transmute.stderr
+++ b/tests/ui/traits/new-solver/specialization-transmute.stderr
@@ -16,13 +16,6 @@ LL |     fn intu(&self) -> &Self::Id {
    |
    = note: cannot satisfy `<T as Default>::Id == _`
 
-error[E0282]: type annotations needed
-  --> $DIR/specialization-transmute.rs:13:23
-   |
-LL |     default type Id = T;
-   |                       ^ cannot infer type for associated type `<T as Default>::Id`
-
-error: aborting due to 2 previous errors; 1 warning emitted
+error: aborting due to previous error; 1 warning emitted
 
-Some errors have detailed explanations: E0282, E0284.
-For more information about an error, try `rustc --explain E0282`.
+For more information about this error, try `rustc --explain E0284`.
diff --git a/tests/ui/traits/new-solver/specialization-unconstrained.rs b/tests/ui/traits/new-solver/specialization-unconstrained.rs
index 7fd753109be..02150689ee5 100644
--- a/tests/ui/traits/new-solver/specialization-unconstrained.rs
+++ b/tests/ui/traits/new-solver/specialization-unconstrained.rs
@@ -11,7 +11,7 @@ trait Default {
 }
 
 impl<T> Default for T {
-   default type Id = T; //~ ERROR type annotations needed
+   default type Id = T;
 }
 
 fn test<T: Default<Id = U>, U>() {}
diff --git a/tests/ui/traits/new-solver/specialization-unconstrained.stderr b/tests/ui/traits/new-solver/specialization-unconstrained.stderr
index ed4dafa1484..910925cbaeb 100644
--- a/tests/ui/traits/new-solver/specialization-unconstrained.stderr
+++ b/tests/ui/traits/new-solver/specialization-unconstrained.stderr
@@ -20,13 +20,6 @@ note: required by a bound in `test`
 LL | fn test<T: Default<Id = U>, U>() {}
    |                    ^^^^^^ required by this bound in `test`
 
-error[E0282]: type annotations needed
-  --> $DIR/specialization-unconstrained.rs:14:22
-   |
-LL |    default type Id = T;
-   |                      ^ cannot infer type for associated type `<T as Default>::Id`
-
-error: aborting due to 2 previous errors; 1 warning emitted
+error: aborting due to previous error; 1 warning emitted
 
-Some errors have detailed explanations: E0282, E0284.
-For more information about an error, try `rustc --explain E0282`.
+For more information about this error, try `rustc --explain E0284`.