mr-boxington (mbx)

mbx wraps Cargo and reuses compiler work across projects and worktrees. Cargo still chooses what to build. Inspect the project's toolchain pins, build tasks, mise configuration, Cargo configuration, and existing compiler wrappers before changing its integration. Preserve the Rust version and build flags.

Choose the integration

Start with mbx --version and mbx doctor. Explicit mbx commands work without shell or editor setup and accept ordinary Cargo arguments:

mbx build --workspace
mbx test --workspace --all-features
mbx clippy --workspace --all-targets -- -D warnings
mbx +stable check --workspace

Use the project's actual toolchain rather than substituting stable for an existing pin. If mise manages Rust, its mr_boxington = true tool option (mise 2026.9.2 or newer) enables wrapping for mise exec -- cargo ..., mise tasks, activated shells, and mise shims. Retain the existing Rust version and other tool options when adding it. A project configuration might contain:

[tools]
rust = { version = "1.90.0", mr_boxington = true }
mr-boxington = "latest"

The version above is illustrative, not an instruction to change the project's Rust pin. A project may instead already use mise's Cargo wrapper, a [wrappers.cargo] entry with command = "mbx" and MBX_CARGO_SHIM_MODE = "1". Preserve a working integration rather than layering a second wrapper onto it.

Standalone mbx setup installs a user-level Cargo shim and rust-analyzer override. It can also add [wrappers.cargo] to a mise config and run mise reshim, and that config can be the project's committed mise.toml. mbx setup chooses the config as follows:

• --global and --local choose the global or current project's mise config.
• --yes accepts the recommendation: the config named by MISE_CONFIG_FILE when it is set; otherwise, in a mise-activated shell, the config that defines mr-boxington, then the nearest project mise config, then the global config.
• Without a flag, mbx setup prompts in an interactive mise-activated shell and preselects the recommended config.

When mbx setup selects no mise config, it changes only the shim and editor configuration. Whichever config it selects, it also removes the check override that older mbx releases wrote to the project's rust-analyzer.toml.

Use mbx setup when the task asks for the Cargo shim, mise's Cargo wrapper, or editor setup; explicit mbx commands do not need it. Native mise wrapping alone does not configure rust-analyzer. If the project already uses mr_boxington = true, do not pass --yes, --local, or --global, and choose Create the shim without activating it if mbx setup prompts. Selecting a mise config adds [wrappers.cargo], which takes precedence over the mr_boxington option, including mr_boxington = false.

With native wrapping in a mise-activated shell, mbx setup --status fails with mbx setup is installed but is not active in the selected mise config before it checks rust-analyzer. Run env -u MISE_SHELL mbx setup --status instead, and do not select a mise config to clear that message.

Even when mise wrapping works, the setup check in mbx doctor can warn that the standalone Cargo shim is not installed, or that the shim is current but not active and its directory should be prepended to PATH. mbx doctor does not recognize native mise wrapping. Verify the actual build route before changing PATH to clear either warning.

Verify real reuse

Use the same source, toolchain, profile, features, and cache directory in two fresh target directories. Choose paths that do not already contain build outputs:

mbx build --target-dir target/cache-demo-first
mbx build --target-dir target/cache-demo-second
mbx explain --last

Adapt both builds to the project's representative command. If the goal is transparent Cargo wrapping, run both through mise exec -- cargo build ... or the configured route instead of invoking mbx directly. Run them in your own environment: installing mbx does not prove that your Cargo processes use it.

Read the second build's cache counters:

• A hit means mbx restored a matching compilation.
• A miss means mbx looked up the key and found no result.
• not looked up means no usable input prediction was available.
• bypassed means the action ran without shared caching.

Cargo skipping up-to-date outputs is not an mbx cache hit. Report observed hits and remaining blockers; do not promise every action is cacheable. When finished, remove only the temporary target directories created for this check.

Diagnose before changing cache policy

mbx doctor --json
mbx explain --last
mbx explain build --workspace
mbx cache stats
mbx gc --dry-run

explain --last inspects a recorded build without running Cargo; explain build runs a build with diagnostic collection. Follow the reported reason for a miss or bypass. Different features, profiles, toolchains, and RUSTFLAGS can legitimately prevent reuse. Do not start by deleting caches or normal target directories; mbx clean and cache removal commands discard state.

For a suspected wrapper failure, run the same plain cargo command with and without MBX_DISABLE=1 (for example, MBX_DISABLE=1 cargo build) as a one-off diagnostic. mbx reads MBX_DISABLE only when it runs as plain cargo, so MBX_DISABLE=1 mbx build still builds through mbx. See Run Cargo without mbx for the PowerShell form. Keep MBX_DISABLE out of committed configuration and report both results. Review logs for credentials and private remote URLs before sharing them.

Use the setup guide, cache results, and configuration reference for editor integration, cache policy, scheduling, and remote-cache details. Add remote credentials or machine-wide scheduler changes only when the task calls for them.