Migrate from rust-cache or sccache

Move one development or CI workflow at a time. Verify that mbx is active, then compare a representative warm build. For production release jobs, follow Production releases.

From rust-cache or actions/cache over target/

Replace the existing cache action with jdx/mr-boxington-action. Both actions can transport Cargo's target state, so using both for the same paths duplicates the restore and save work. See CI caches.

Before:

steps:
  - uses: actions/checkout@v7
  - uses: Swatinem/rust-cache@v2
  - run: cargo test --workspace

After:

steps:
  - uses: actions/checkout@v7
  - uses: jdx/mr-boxington-action@v1
  - run: mbx test --workspace

The action's default target payload carries the target/ directory of the Cargo workspace named by the action's working-directory input, plus Cargo's registry and git download caches under $CARGO_HOME (~/.cargo by default). That lets the workflow retain dependency artifacts and Cargo downloads. working-directory defaults to the job's working directory, normally the checkout root. If the workspace is in a subdirectory, set working-directory to it. Otherwise the action finds no reusable target state and saves nothing, not even the downloads.

The action saves a new entry only after a successful run for a push to the default branch; other runs restore without saving. These inputs let more runs save:

• save-on-pull-request for same-repository pull requests
• save-on-protected-branch for pushes to protected non-default branches
• save-on-workflow-dispatch for workflow_dispatch runs (enable it only in trusted workflows)

Fork pull requests and pushes to unprotected non-default branches never save. See GitHub Actions cache for details.

The objects payload (github-cache-mode: objects) leaves out Cargo's download caches. Cache those downloads separately if you need them.

From sccache

Both tools wrap rustc through RUSTC_WRAPPER, so they cannot be combined for the same build. When RUSTC_WRAPPER is already set, mbx defers to the existing wrapper and does not cache the build. To migrate, remove sccache from every place that configures it:

• RUSTC_WRAPPER or RUSTC_WORKSPACE_WRAPPER in CI environments and shell profiles
• build.rustc-wrapper or build.rustc-workspace-wrapper in Cargo configuration such as ~/.cargo/config.toml
• Workflow steps and settings that install or configure sccache, such as mozilla-actions/sccache-action and SCCACHE_GHA_ENABLED

Then run a build. If RUSTC_WRAPPER is still set in the environment, mbx prints a RUSTC_WRAPPER is already set warning. A leftover RUSTC_WORKSPACE_WRAPPER other than Clippy's clippy-driver gets a similar warning: mbx defers to it for workspace crates and does not cache those compilations. mbx does not warn about a wrapper left in Cargo configuration, and mbx doctor does not check for rustc wrappers.

Verify the migration

Run mbx doctor, then your usual build. If the store is still empty, mbx has to compile to fill it. If a restored Cargo target directory is already fresh, Cargo runs no compilations. Neither result proves that mbx reuses work across target directories. Follow Measure cache reuse for a controlled comparison, and use mbx explain --last to understand any remaining misses. To see why compilations bypassed, run the build through mbx explain, such as mbx explain build.