d4096e0412
Require TAITs to be mentioned in the signatures of functions that register hidden types for them r? `@lcnr` `@compiler-errors` This implements the lang team decision from [the TAIT design meeting](https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/design.20meeting.202023-05-31.20TAITs/near/362518164).
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.