Add an `EarlyBinder` type

[jackh726]

Proposal

Chalk has no concept of Param (https://github.com/rust-lang/chalk/blob/e0ade19d139bc784384acc6736cd960c91dd55a1/chalk-ir/src/lib.rs#L579) or ReEarlyBound (https://github.com/rust-lang/chalk/blob/e0ade19d139bc784384acc6736cd960c91dd55a1/chalk-ir/src/lib.rs#L1308). Everything is just "bound" - the equivalent of rustc's late-bound. It's not completely clear yet whether to, in the long-term, move everything to the same type of binder in rustc or to add Param/ReEarlyBound in Chalk.

Either way, tracking when we have or haven't already substituted out these in rustc can be helpful.

This proposal is to add an EarlyBinder newtype à la Binder. As with Binder, the idea is that ty-related things with Param/ReEarlyBound inside will continue to be wrapped in EarlyBinder until they are substituted out.

There is a WIP implementation in rust-lang/rust#96883. As a first step, I'm just added an EarlyBinder newtype that is required to call subst. I also added a couple "transparent" bound_* wrappers around a couple queries that are often immediately substituted. Future PRs can be relatively small and self-contained, incrementally "pushing" EarlyBinder wrapping up.

Mentors or Reviewers

I should be able to lead the work here. @oli-obk and @nikomatsakis might review, though changes should mostly be mechanical.

Process

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

• File an issue describing the proposal.
• A compiler 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.
  • Compiler 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.

[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/compiler @rust-lang/compiler-contributors

[oli-obk]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted