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Improve `DispatchFromDyn` and `CoerceUnsized` impl validation * Disallow arbitrary 1-ZST fields in `DispatchFromDyn` -- only `PhantomData`, and 1-ZSTs that mention no params (which is needed to support, e.g., the `Global` alloctor in `Box<T, U = Global>`). * Don't allow coercing between non-ZSTs to ZSTs (since the previous check wasn't actually checking the field tys were the same before checking the layout...) * Normalize the field before checking it's `PhantomData`. Fixes #135215 Fixes #135214 Fixes #135220 r? ```@BoxyUwU``` or reassign
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.