Rollup merge of #82516 - PoignardAzur:inherent-impl-ty, r=oli-obk

Add incomplete feature gate for inherent associate types.

Mentored by ``````@oli-obk``````

So far the only change is that instead of giving an automatic error, the following code compiles:

```rust
struct Foo;

impl Foo {
    type Bar = isize;
}
```

The backend work to make it actually usable isn't there yet. In particular, this:

```rust
let x : Foo::Bar;
```

will give you:

```sh
error[E0223]: ambiguous associated type
  --> /$RUSTC_DIR/src/test/ui/assoc-inherent.rs:15:13
   |
LL |     let x : Foo::Bar;
   |             ^^^^^^^^ help: use fully-qualified syntax: `<Foo as Trait>::Bar`
```

compiler/rustc_feature/src/active.rs

@@ -641,6 +641,9 @@ declare_features! (
     /// Allows `pub` on `macro_rules` items.
     (active, pub_macro_rules, "1.52.0", Some(78855), None),
 
+    /// Allows associated types in inherent impls.
+    (active, inherent_associated_types, "1.52.0", Some(8995), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
@@ -666,6 +669,7 @@ pub const INCOMPLETE_FEATURES: &[Symbol] = &[
     sym::unsized_locals,
     sym::capture_disjoint_fields,
     sym::const_generics_defaults,
+    sym::inherent_associated_types,
 ];
 
 /// Some features are not allowed to be used together at the same time, if