Unstably support MIR-only rlibs

[saethlin]

This a spiritual successor to rust-lang/rust#38913 (implementation at rust-lang/rust#48373). I didn't intend to pick up that PR, I've just heard "MIR-only rlibs" often and the idea seems to solve a number of problems. The old tracking issue says to make an MCP, so here's one.

Proposal

Unlike the previous tracking issue, at this time I am not intending to make MIR-only rlibs the default behavior, or the behavior of the distributed standard library. @eddyb convinced me that even if the feature is nightly-only, it will be useful to the community.

In-progress implementation: rust-lang/rust#119017

If I ever become convinced that MIR-only rlibs are viable as the default behavior of rustc, or the default way that we distribute the standard library, I will create a new MCP.

Mentors or Reviewers

I don't think it will be particularly hard for me to find help on this when I get stuck, but if someone wants to volunteer I'll put a name here.

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.

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.

Concerns or objections to the proposal should be discussed on Zulip and formally registered here by adding a comment with the following syntax:

@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/compiler @rust-lang/compiler-contributors

[cjgillot]

This is meant to experiment, and the feature has a lot of potential benefits.
@rustbot second

[apiraino]

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