[somewhatabstract]

Summary
Measures Rust test coverage in CI and uploads it to Codecov, reported separately from TypeScript so neither language's numbers can hide the other's.

Stack

• Below: Port the checksum to Rust #2635 (Rust checksum port). This PR targets its branch.
• Above: none.

Issues
Closes #2627. Part of #2614 (Phase 1), which is under #2524.

Details

• .github/workflows/rust.yml: new coverage job on Ubuntu:
  • adds the llvm-tools-preview component, which only this job needs
  • installs a prebuilt cargo-llvm-cov with taiki-e/install-action, which is much faster than cargo install
  • runs cargo llvm-cov --workspace --locked --codecov --output-path codecov.json
  • uploads with flags: rust
  • each step has a comment, like the rest of the workflow
• .github/workflows/nodejs.yml: the Jest upload now sets flags: typescript.
• .codecov.yml:
  • The single project: default status is replaced by one status per flag (typescript, rust). Each keeps the previous default target of auto, which fails if coverage goes down.
  • flags maps typescript to src/ and rust to rust/, both with carryforward: true. The Rust workflow only runs when Rust code changes, so on a TypeScript-only PR the rust flag reuses its last report instead of showing as missing, and the same in reverse.
• An empty changeset, since the published package doesn't change.

Checked locally
cargo llvm-cov with the checksum code in place:

• calc_checksum.rs: 100% lines, functions and regions.
• checksync-cli/src/main.rs: 0%. That's accurate, since it's the empty fn main() {} until Phase 5.

Review notes

• I couldn't validate .codecov.yml with Codecov's validator, because this container's network proxy blocks it. The first CI run is the real test. Look for separate typescript and rust flags in the Codecov comment.
• Correction to Rust: report test coverage to Codecov #2627: it said TypeScript would keep "the current 100% target". The existing config actually used default: true, which is an auto target, not a fixed 100%. I kept that behavior. If you want a fixed target: 100% for TypeScript, or later for Rust, it's a one-line change.
• This is a CI change, so its own run is what proves it works.

---

Generated by Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: 9119459

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 0 packages

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.89%. Comparing base (5252210) to head (9119459).

Additional details and impacted files

@@                              Coverage Diff                              @@
##           claude/jolly-galileo-72zesp-rust-checksum    #2636      +/-   ##
=============================================================================
- Coverage                                     100.00%   99.89%   -0.11%     
=============================================================================
  Files                                             51       53       +2     
  Lines                                            959      988      +29     
  Branches                                         231      260      +29     
=============================================================================
+ Hits                                             959      987      +28     
- Misses                                             0        1       +1     

Flag	Coverage Δ
rust	96.55% <ø> (?)
typescript	100.00% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.
see 2 files with indirect coverage changes

---

Continue to review full report in Codecov by Harness.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5252210...9119459. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[codecov]

Bundle Report

Bundle size has no change ✅

[somewhatabstract]

LGTM