1b08d1a92c
Process alias-relate obligations in CoerceUnsized loop After #119106, we now emit `AliasRelate` goals when relating `?0` and `Alias<T, ..>` in the new solver. In the ad-hoc `CoerceUnsized` selection loop, we now may have `AliasRelate` goals which must be processed to constrain type variables which are mentioned in other goals. --- For example, in the included test, we try to coerce `&<ManuallyDrop<T> as Deref>::Target` to `&dyn Foo`. This requires proving: * 1 `&<ManuallyDrop<T> as Deref>::Target: CoerceUnsized<&dyn Foo>` * 2 `<ManuallyDrop<T> as Deref>::Target alias-relate ?0` * 3 `?0: Unsize<dyn Foo>` * 4 `?0: Foo` * 5 `?0: Sized` If we don't process goal (2.) before processing goal (3.), then we hit ambiguity since `?0` is never constrained, and therefore we bail out, refusing to coerce the types. After processing (2.), we know `?0 := T`, and the rest of the goals can be processed normally.
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