Tracking issue for RFC #2056: Allow trivial constraints to appear in where clauses

[aturon]

This is a tracking issue for the RFC "Allow trivial constraints to appear in where clauses " (rust-lang/rfcs#2056).

Steps:

• Implement the RFC – Implement RFC 2056 trivial constraints in where clauses #48557
  • As noted in Tracking issue for RFC #2056: Allow trivial constraints to appear in where clauses  #48214 (comment), the new trait solver might solve a few issues with this feature, so this is currently blocked on the new trait solver (-Znext-solver)
• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge)

Unresolved questions:

• Should the lint error by default instead of warn?

[matthewjasper]

I'm working on this.

[nikomatsakis]

@matthewjasper ok. I've not thought hard about what it will take to support this. I had thought about doing it after making some more progress on general trait refactoring, but if it can be easily supported today seems fine.

[nikomatsakis]

Oh, I meant to add, please ping me on IRC/gitter with any questions! I can try to carve out some time to think about it as well.