Tracking issue for negative impls

[nikomatsakis]

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

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

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)