A Rust build cache for agent worktrees reports 9.3 seconds against Cargo's 3 min 42 s

Panasenko, who built the Mitts build tool, reports that a parallel build of 12 rustls worktrees on Linux x86_64 took 3 min 42 s with plain Cargo and 9.3 s with Mitts. Cargo with a warm sccache took 3 min 59 s. The shared caches were prewarmed, the local target directories were empty, and the warm-up build is excluded. On a Mac, he reports that twelve actix-web worktrees took 5 min 22 s with Cargo, 5 min 23 s with sccache and 56 s with Mitts, with 3,408 compiler invocations for Cargo against 181 for Mitts.

He also tested the obvious shortcut. A shared target directory across concurrent builds at different commits failed his check: an injected test failure in one worktree was missed or leaked into a clean one, so the result was excluded from the scores. Mitts instead shares immutable, hashed build results and keeps each worktree's output separate.

The post is open about where Cargo wins. Plain Cargo finished a single cold build first on 71 of 100 Mac projects and 60 of 90 Linux x86_64 projects, and after a one-file edit Cargo still won 22 of 71 Mac projects. Each reading is a median of three repeats, which he says is not a confidence interval. Mitts is pre-release, supports Linux and macOS only, and has no remote or team cache.

Why it matters: If your agents each work in a worktree, repeated compilation is a cost you can measure on your own project before you pay for more machines. The numbers come from the tool's author on a corpus of 100 open-source Rust projects, and we have not rerun them.