Standard library semantic versioning breakage check

The x86_64-gnu-stdlib-semver-check job runs the cargo-semver-checks (c-s-c) tool on the standard library (core, alloc and std) in order to find potential unintended semantic versioning (semver) breakages. It does so by analyzing the rustdoc JSON output (from the rust-docs-json component) of the parent merge commit, and the current commit being merged. When it runs, one of five things can happen:

1. Everything proceeds correctly, c-s-c does not find any breakage.
2. The rustdoc JSON version was bumped recently, and c-s-c cannot handle it yet. This case will result in the test ending with a success, and printing a warning that c-s-c needs to be updated. Once c-s-c releases a version that supports the new JSON format, it should go to 1. again.
   • We currently install the latest released version of c-s-c in this job, so its version does not need to be updated manually in the rust-lang/rust repository.
3. c-s-c detects a breakage, but it is a false positive. In this case, please report the false positive to this Zulip channel, and bump the stamp file.
4. c-s-c detects a real breakage, and it helped you find unintended beakage. Yay! In this case, please consider reporting the success to this Zulip channel.
5. c-s-c detects a real breakage, but you want to land it anyway (maybe it is an edge case that was FCPed). In that case, bump the stamp file.

Bumping the stdlib semver stamp file

If you want to let CI pass on a PR where c-s-c detects breakage (whether it is real or not), you have to modify the src/bootstrap/stdlib-semver-check-stamp file. Please update the PR number in which you modify this file at the bottom of the file. This will ensure that the test will stay green, regardless of what c-s-c detects.

Running the check manually

You can manually run the semver check locally using ./x test std-semver-check --set rust.stdlib-semver-baseline=${PARENT}, where PARENT is a commit SHA against which you want to compare the in-tree stdlib. If you do not specify it, bootstrap will select the latest upstream commit that it finds in your local git history.