feat: Stabilize MSRV-aware resolver config

[epage]

What does this PR try to resolve?

This includes

• cargo generate-lockfile --ignore-rust-version
• cargo update --ignore-rust-version

This does not include

• edition = "2024"
• resolver = "3"

This is part of #9930

How should we test and review this PR?

Additional information

This is stacked on top of #14636. The commits for this PR start with the commit with a title that matches the PR title.

FCP

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[epage]

Note: the final touches are not in place for this PR but I assume people can check their boxes based on the whole and assuming comments on this PR will get resolved.

I propose that we stabilize the MSRV-aware resolver.

• Offer an opt-in config for preferring MSRV-compatible packages over those that are incompatible, regardless of version
• Packages without an MSRV are always considered compatible
• If no MSRV is defined, fallback to the current toolchain's version
• Small tweaks in the algorithm are allowed as changes in the algorithm won't cause changes in lockfiles
• This only applies to local lockfile generation and not to cargo install

This includes stabilizing

• cargo generate-lockfile --ignore-rust-version
• cargo update --ignore-rust-version

This does not include stabilizing

• edition = "2024"
• resolver = "3"
• Features around the resolver that are meant to support MSRV workflows (e.g. --update-rust-version flag)

Deviations from the RFC

• Instead of the MSRV being the lowest among the workspace's MSRVs, we prioritze packages based on how many MSRVs from the workspace they are compatible with
  • We only fallback to the current toolchain's version if no MSRV is set, rather than treating that as the MSRV for those packages

I had considered proposing we have this cover cargo install

• When viewed only as a config and keeping in mind that cargo install only reads from the user config (and so wouldn't be affected by the current repo's config), it sounds reasonable on the surface
• However, people setting something in their config for cargo install would affect any repo that doesn't have its own config. Really, cargo install does need its own [install.resolver] table.
• This also gets weirder when it comes to resolver = "3" as we'd need to describe that as conditional

@rfcbot fcp merge

[rfcbot]

Team member @epage has proposed to merge this. The next step is review by the rest of the tagged team members:

• [x] @Eh2406
• [ ] @Muscraft
• [ ] @arlosi
• [x] @ehuss
• [x] @epage
• [x] @joshtriplett
• [x] @weihanglo

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[bors]

:umbrella: The latest upstream changes (presumably #14636) made this pull request unmergeable. Please resolve the merge conflicts.

[rfcbot]

:bell: This is now entering its final comment period, as per the review above. :bell:

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[weihanglo]

@bors r+

The FCP has ended. To maximise the testing window I am going to merge this soon. Thank you all for your inputs!

[bors]

:pushpin: Commit 498d4df89fbf6fc93329ce296fd82638e1504699 has been approved by weihanglo

It is now in the queue for this repository.

[bors]

:hourglass: Testing commit 498d4df89fbf6fc93329ce296fd82638e1504699 with merge cf53cc54bb593b5ec3dc2be4b1702f50c36d24d5...

[bors]

:sunny: Test successful - checks-actions Approved by: weihanglo Pushing cf53cc54bb593b5ec3dc2be4b1702f50c36d24d5 to master...