Cargo that cleans up after itself

Every new worktree costs less

• Cargo
• This fork

Each worktree is a fresh checkout building one binary. Cargo gives every worktree its own target directory and compiles all dependencies again. The fork compiles them once and reuses them, so later worktrees only build the workspace crates.

Projects share dependencies too

What a cleanup looks like

After a build, at most once an hour, Cargo removes what no checkout has used for a week, then the least recently used units until the project fits. It prints one line when it frees something.

$ cargo build
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.89s
     Cleaned 680 stale build units, 10.1GiB freed (build.gc)

What changes

Your projects, Cargo.toml and .cargo/config.toml stay as they are.

• One build directory per project. Intermediate files go to ~/.cargo/targets/<name>-<hash>/, where the hash comes from the repository's .git directory. Every worktree lands there wherever it is created, reuses compiled dependencies, and only builds its own crates. Binaries still appear in ./target.
• Fixed directories. ./target and ~/.cargo/targets cannot be moved: --target-dir, CARGO_TARGET_DIR, build.target-dir and build.build-dir are ignored with a warning, so no flag, env var or tool can scatter build output elsewhere.
• Dependencies shared by every project. Registry and git dependencies build once in ~/.cargo/targets/shared-deps/ and are reused by any project that needs the same version, features and profile, like Go's build cache. cargo check, clippy, rust-analyzer and cargo doc use the same builds, so a check after a build only looks at your own crates.
• Your own crates shared by worktrees too. A workspace crate is keyed by the content of its files instead of the checkout it is in, so a new worktree of the same commit reuses every crate another worktree built, and switching back to a branch reuses what it built before. Editing a test or a binary rebuilds only that target. Crates with a build script, and crates that read their own path (env!("CARGO_MANIFEST_DIR")), still build per worktree. Worktrees also share rustc's incremental caches, so the first edit in a new worktree is incremental too. For this rustc records . as the compile directory (-Zremap-cwd-prefix=.; on stable through RUSTC_BOOTSTRAP=1 with no #![feature] allowed), so start a debugger from the workspace root.
• A size limit. Each project keeps to twice its largest build of the last day, and its units unused for a day are removed. Shared dependencies are kept for 7 days and keep to 50 GiB; a project no build has used for 7 days is removed whole. The units an edit replaces are removed right away, unless another worktree uses them. Older builds of a shared dependency that was rebuilt for new dependencies go after a day, and so do stale test directories in CARGO_TARGET_TMPDIR and binaries in ./target whose build unit is gone. Nothing a running build has locked is removed.
• No duplicate object files on macOS. Debuggers and backtraces read the objects inside each rlib, so the loose copies beside it are removed.
• Worktrees build at the same time. Fine-grained locking is on, so parallel agents do not queue on one lock. When two builds need the same dependency at once, one compiles it and the other reuses the result.
• Less debug info in dev builds. Line tables for your crates, none for dependencies. Backtraces keep file and line. A debug setting in your profile still wins.
• Metadata stored once on nightly. An rlib no longer carries a second copy of its .rmeta on a nightly rustc, about 30% off the shared dependencies.
• Updates itself. Once a day at most, a background check installs the latest release, so you never rerun the installer. CARGO_FORK_AUTO_UPDATE=0 turns it off.
• Cranelift on nightly. cargo build compiles your crates with Cranelift and runs the compiler front end on up to 8 threads. Tests and release builds keep LLVM. The installer adds the Cranelift component when you have a nightly toolchain.

# ~/.cargo/config.toml
[build.gc]
max-size = "auto"      # or "15 GiB", "none"
shared-max-size = "50 GiB"
max-unused = "1 day"
shared-max-unused = "7 days"
frequency = "1 hour"   # or "always", "never"
enabled = true