Add experimental support for `for<T>` binders in limited positions

[compiler-errors]

Proposal

Modify the resolver and type system to support for<T> in where clauses and associated item bounds (not object types or return-position impl traits).

Why?

1. They should eventually also be useful for things like where clauses involving GATs (e.g. where for<T> Self::Assoc<T>: Trait).
2. This will allow us to test out ty::Bound and ty::Placeholder a bit more, even if currently they're pretty useless on their own (i.e. without implication predicates).
3. They should eventually work synergistically with implication predicates, if or when we add support for those (e.g. where for<T: Debug> Self::Assoc<T>: Debug).

Side-note: If anyone thinks this should be a compiler MCP, I'd be happy to move this over there. If this is better suited for a T-lang initiative, or needs a pre-RFC, then let me know.

Mentors or Reviewers

I've got a branch that mostly implements support for this, but it needs major cleaning up. I'm willing to do that work, but I just don't want to invest too much time to clean it up unless there's reasonable interest in support for type binders from other team members.
rust-lang/rust@master...compiler-errors:rust:non_lifetime_binders

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A types team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Types team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

cc @rust-lang/types

[jackh726]

@rustbot second

The actual PR for this will likely need lang okay, but from a types perspective, I think it's a good feature to experiment with.

[compiler-errors]

Cool, noted

[zyansheep]

Would this addition resolve rust-lang/rfcs#1481 ?

[compiler-errors]

@zyansheep, yes

[lcnr]

closing as accepted