Structurally normalize types as needed in `projection_ty_core`
Introduce a `structurally_normalize` callback to `projection_ty_core`, and then use it before we match on the ty kind in `projection_ty_core`.
Previously we were only structurally normalizing the return type of the `handle_field` struct, but if we were to (e.g.) apply a deref projection to that type, then the resulting type is not guaranteed to be structurally normalized and any subsequent projections applied would ICE.
Fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/221
I'll leave a few comments inline to explain the changes.
r? lcnr
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Also fixesrust-lang/rust#141708
bace7f986c