7069a8c2b7
Trait upcasting coercion (part1) This revives the first part of earlier PR #60900 . It's not very clear to me which parts of that pr was design decisions, so i decide to cut it into pieces and land them incrementally. This allows more eyes on the details. This is the first part, it adds feature gates, adds feature gates tests, and implemented the unsize conversion part. (I hope i have dealt with the `ExistentialTraitRef` values correctly...) The next part will be implementing the pointer casting.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.