sub-trait coercions

[nrc]

Where trait T: U { ... } , implictly coerce from &T to &U, etc.

See RFC 401.

Part of #18469

[steveklabnik]

another #18469

[frewsxcv]

What's the status of this?

[isabelmu]

I think this is a duplicate of #5665 (although that bug was filed before RFC 401 was adopted).

[Mark-Simulacrum]

Going to go ahead and close in favor of #5665.

[crlf0710]

Current tracking issue for this is #65991