Add experimental support for implication predicates

[compiler-errors]

Proposal

Implement implication predicates [A1, A2, ..] => B that have the the following behavior:

• When evaluating [A1, A2, ..] => B, add A1, A2, .. to the existing param-env and evaluate B under that new param-env.
• When evaluating whether some predicate B holds, and we have an implication predicate [A1, A2, ..] => B in the param-env, then that predicate may satisfy B if the nested obligations A1, A2, .. hold.

This proposal does not suggest adding surface syntax for these predicates yet, just adding them to the "middle" type layer.

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. I'd probably start over from scratch since I'd want to audit all the places I added todo!()s for these predicates though. I don't want to put up a PR for it unless the team is interested in supporting this in the trait system, though.

Process

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

• [x] 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

As stated on zulip, I don't think this really needs an MCP, since it aligns with our "vision" of the future type system in a-mir-formality.

But, for process, seconding.

[jackh726]

Closing as accepted.