Clean up early & late lifetime handling

[oli-obk]

Proposal

Opaque types and function item types are handling late bound lifetimes very hackily.

We think something along the lines of the ideas below can work, but mainly we want to try out lots of things here and land whatever sticks to the wall ("sticky" meaning "improves things")

• track late bound function parameters in generics_of
• try to make opaque types nicer by stopping to duplicae args/params all over the place
• make FnDef's args higher ranked (i.e. put them behind a binder)
• update all the HIR ty lowering logic for args to functions to properly map stuff to late bound parameters and check them

Sparse on purpose, we're figuring things out as we go

Mentors or Reviewers

• @BoxyUwU for high level design
• @oli-obk for hands-on reviews and keeping everyone in the loop

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]

Important

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.

Concerns or objections can formally be registered here by adding a comment.

@rustbot concern reason-for-concern
<description of the concern>

Concerns can be lifted with:

@rustbot resolve reason-for-concern

See documentation at https://forge.rust-lang.org

cc @rust-lang/types

[BoxyUwU]

@rustbot second