Upgrading the Rust Toolchain on Chrome OS

WARNING: These steps are only general in nature. Sometimes things can go wrong, especially with applying patches. This is expected to happen and best judgement should be used.

Follow these steps to upgrade the rust ebuild.

1. git mv rust-<old version>.ebuild rust-<new version>.ebuild
2. Add mirror to the RESTRICT variable.
3. Set STAGE0_DATE to date from https://github.com/rust-lang/rust/blob/<new version>/src/stage0.txt.
4. ebuild rust-<new version>.ebuild manifest
5. Remove mirror from RESTRICT variable.
6. ebuild rust-<new version>.ebuild compile
7. Update vendored library checksums.
8. ebuild rust-<new version>.ebuild compile
9. sudo ebuild rust-<new version>.ebuild merge
10. Upgrade rust package in profiles/targets/chromeos/package.provided
11. git mv virtual/rust/rust-<old version>.ebuild virtual/rust/rust-<new version>.ebuild
12. Upload change for review. CC reviewers from previous upgrade.
13. Kick off try-job cros tryjob -g <cl number> chromiumos-sdk-tryjob.

Before sending to CQ, ensure every file in the Manifest is in localmirror or gentoo mirror. First check for the file in gs://chromeos-mirror/gentoo/distfiles/ and then check in gs://chromeos-localmirror/distfiles/. If the file is not in either one, upload it using:

gsutil cp -a public-read \
    /var/cache/chromeos-cache/distfiles/host/<file from manifest> \
    gs://chromeos-localmirror/distfiles/