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document + UI test `E0208` and make its output more user-friendly Cleans up `E0208`'s output a lot. It could actually be useful for someone learning about variance now. I also added a UI test for it in `tests/ui/error-codes/` and wrote some docs for it. r? `@GuillaumeGomez` another error code, can't be bothered to find the issue :P. Obviously there's some compiler stuff, so you'll have to hand it off. Part of https://github.com/rust-lang/rust/issues/61137.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.