Add semver check test command for checking API compatibility of stdlib

[Kobzol]

This PR adds a new test to bootstrap (not executed by default at the moment) that checks the API compatibility of the standard library using the https://github.com/obi1kenobi/cargo-semver-checks tool.

The command can be executed using x test std-semver-check.

(Note: I realized that x dist rust-docs-json executed twice in a row invalidates Cargo cache for some reason, that is a separate issue though).

The test is not yet executed on CI, I would do that in a separate PR (maybe pending a t-libs FCP or something).

The test checks that c-s-c is installed, and then uses git to lookup a parent baseline commit (in a future extension, we should make the baseline commit be configurable through config.toml). Then it downloads the corresponding rust-docs-json component of that commit from CI and extracts it. It generates the same JSON docs component from local sources, and then runs c-s-c on those two JSON files to compare their API.

Output when API stability is broken:

Checking semver compatibility of core
    Checking <unknown> v1.99.0-nightly  (d527bc9bf      2026-07-20) -> v1.99.0-dev (assume minor change)
     Checked [   1.282s] 196 checks: 196 pass, 57 skip
     Summary no semver update required
    Finished [   2.368s] <unknown>
Checking semver compatibility of alloc
    Checking <unknown> v1.99.0-nightly  (d527bc9bf      2026-07-20) -> v1.99.0-dev (assume minor change)
     Checked [   0.030s] 196 checks: 196 pass, 57 skip
     Summary no semver update required
    Finished [   0.127s] <unknown>
Checking semver compatibility of std
    Checking <unknown> v1.99.0-nightly  (d527bc9bf      2026-07-20) -> v1.99.0-dev (assume minor change)
     Checked [   0.055s] 196 checks: 195 pass, 1 fail, 0 warn, 57 skip

--- failure function_missing: pub fn removed or renamed ---

Description:
A publicly-visible function cannot be imported by its prior path. A `pub use` may have been removed, or the function itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/function_missing.ron

Failed in:
  function std::process::id, previously in file library/std/src/process.rs:2656

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [   0.243s] <unknown>

CC @obi1kenobi @Amanieu

r? @jieyouxu

[jieyouxu]

(I'll look at this during the weekend.)

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[jieyouxu]

Thanks, this seems reasonable, one test nit
@rustbot author

View changes since this review

[jieyouxu]

Remark: I think this will merge conflict with #159638, merge after that PR

[jieyouxu]

Question: could we add a step resolution snapshot test for ./x test std-semver-check?

[theemathas]

We should probably also run cargo semver checks in reverse each release too, and compare the output with the release notes, to make sure we don't accidentally stabilize something. That's probably for a future PR though.