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Taming Multi-Gigabyte Rust Targets: Global Build Caching with Mr Boxington (mbx) and Mise

Reclaim dozens of gigabytes of disk space from Rust targets. How to use Mr Boxington (mbx) and Mise with macOS APFS reflink zero-copy cloning to manage Cargo caches effortlessly.

Developing multiple Rust projects or large applications on macOS quickly consumes tens of gigabytes of disk space across individual target/ directories. With parallel testing or Git worktrees, every separate checkout compiles dependencies from scratch.

Traditional tools like sccache do not solve physical directory bloat. This article covers why I replaced sccache, and how to use Mr Boxington (mbx, Rust build cache) with Mise and macOS APFS reflinks to establish a globally shared compilation cache.

Limitations of Sccache

I previously configured sccache in ~/.cargo/config.toml:

[build]
rustc-wrapper = "/opt/homebrew/bin/sccache"
incremental = false

In practice, sccache has two limitations:

1. Conflict Between Incremental Compilation and Cache Hits

sccache cannot cache Rust’s incremental compilation state. To hit the cache, configuration must explicitly disable incremental compilation (incremental = false).

Disabling it means single-line edits cannot use rustc’s incremental optimizations, noticeably increasing rebuild times.

2. Uncontrolled Growth of Target Directories

sccache is a key-value store. Individual target/ directories still retain all generated object files and debug symbols.

It does not reclaim abandoned worktree artifacts or enforce a total disk budget. When disk space runs low, manual directory cleanup with cargo clean is still required.

Mr Boxington (mbx) Mechanics

Mr Boxington (CLI binary mbx) is a Rust build cache and target directory manager created by Mise author jdx.

Its mechanics:

1. No Background Daemon

mbx runs without a persistent background service. For each Cargo command, it starts a lightweight local cache agent within the process lifecycle and exits when the build completes. There are no background leaks or stalled sockets.

On APFS, mbx restores cache artifacts into project target/ directories using copy-on-write reflinks instead of physical copies.

Restoring compiled artifacts takes milliseconds. Different projects and Git worktrees share the same underlying storage blocks without consuming duplicate disk space.

3. Managed Targets and Automated Garbage Collection

mbx introduces a disk budget (default 20.0 GiB).

During builds, mbx tracks active workspaces. For stale worktrees or abandoned project targets, mbx gc removes old entries using an LRU policy, keeping total Rust cache usage within the configured budget.

Configuring Mise and mbx

Mise supports the mr_boxington option natively from version 2026.9 onward. A single command configures it globally:

Terminal window
mise use --global --tool-option mr_boxington=true rust mr-boxington

This command makes three changes:

  1. Installs the Rust toolchain and the mr-boxington (mbx) binary.
  2. Configures the Cargo wrapper in ~/.config/mise/config.toml:
wrappers = { cargo = { command = "mbx", env = { MBX_CARGO_SHIM_MODE = "1" } } }


[tools]
mr-boxington = "latest"
rust = { version = "latest", mr_boxington = true }
  1. Runs mbx setup --global, configuring intercepting wrappers for cargo and rust-analyzer.

Running cargo build or IDE rust-analyzer checks automatically routes compilation through the mbx central cache.

Chezmoi Configuration and Migration Details

When tracking this setup in Chezmoi, two details require attention:

1. Removing Old Sccache Wrappers

Both sccache and mbx work by intercepting rustc. They cannot run together in the same build. If RUSTC_WRAPPER or build.rustc-wrapper remains active, mbx yields and disables caching.

Track dot_cargo/config.toml in Chezmoi and remove the old rustc-wrapper:

~/.local/share/chezmoi/dot_cargo/config.toml
# Global Cargo configuration.
# Managed by chezmoi


[build]
# mr-boxington (mbx) manages shared cache through mise shims
# sccache removed to avoid RUSTC_WRAPPER conflict


[profile.dev]
debug = false

2. Ensuring Shim Precedence in ~/.zshenv

Non-interactive subshells spawned by Coding Agents only evaluate ~/.zshenv. In dot_zshenv.tmpl, prepend Mise shims and the mbx path:

Terminal window
export PATH="$HOME/.local/share/mise/shims:$HOME/Library/Application Support/mbx/bin:/opt/homebrew/bin:/opt/homebrew/sbin:$PATH"

Verification

Run the built-in diagnostic command:

Terminal window
mbx doctor

The output should show clean green checks:

  ok  cargo        cargo 1.98.1
  ok  rustc        rustc 1.98.1
  ok  cache        /Users/david/Library/Caches/mbx is writable
  ok  config       20.0 GiB budget, automatic gc enabled, managed targets enabled
  ok  reflink      cloning is supported (macOS APFS reflink)
  ok  setup        mise Cargo wrapper is active
0 failures, 0 warnings

Daily commands:

  • mbx stats: Inspect global cache usage, accumulated time saved, and deduplicated bytes across workspaces.
  • mbx tui: Open a live terminal dashboard showing crate progress and cache hit status.
  • mbx gc: Manually trigger cache cleanup against configured budgets.

References

Mr Boxington GitHub Repository

Mr Boxington Official Documentation

Mise Official Documentation

Dotfiles Part 3: Unifying Polyglot Toolchains and AI Agent Environments with Mise

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