Tracking issue for negative impls

[nikomatsakis]

Generalized negative impls were introduced in #68004. They were split out from "opt-in builtin traits" (#13231).

Work in progress

This issue was added in advance of #68004 landed so that I could reference it from within the code. It will be closed if we opt not to go this direction. A writeup of the general feature is available in this hackmd, but it will need to be turned into a proper RFC before this can truly advance.

Current plans

• Forbid conditional negative impls or negative impls for traits with more than one type parameter (forbid conditional, negative impls #79098)
• Forbid !Drop (negative impls of Drop)

Unresolved questions to be addressed through design process

• What should the WF requirements be? (Context)
• Should we permit combining default + negative impls like default impl !Trait for Type { }? (Context)