Skip to content

shared-cargo-target.md

Latest commit

 

History

History
78 lines (52 loc) · 3.27 KB

shared-cargo-target.md

File metadata and controls

78 lines (52 loc) · 3.27 KB
summary Share one Cargo target dir across all worktrees to skip redundant rebuilds (76s -> 5s)
read_when
setting up a new development machine
creating worktrees and noticing slow first builds
looking to reclaim disk space from multiple target/ directories

Shared Cargo Target Directory Across Worktrees

Why

Each git worktree gets its own target/ directory. Every first build in a new worktree recompiles all dependencies from scratch (~76s). With many worktrees this also eats massive disk space.

A shared target directory lets Cargo reuse compiled artifacts across all worktrees. Measured improvement: 76s -> 5s for a fresh worktree build.

Setup (per-machine, takes 30 seconds)

.cargo/config.toml is already gitignored, so this is local to your machine.

  1. Create .cargo/config.toml in the repo root with an absolute path to a shared target dir:

    [build]
target-dir = "/absolute/path/to/pika/.shared-target"

    Replace the path with wherever your pika repo lives. The path must be absolute because Cargo resolves relative paths from the working directory, not the config file.

  2. Done. All worktrees under pika/worktrees/ and pika/.worktrees/ are subdirectories of the repo root, so Cargo's parent-directory walk finds the same config automatically.

  3. Symlink target to .shared-target so recipes that hardcode target/ still work:

    rm -rf target && ln -s .shared-target target

  4. (Optional) Delete old per-worktree target/ dirs to reclaim disk space:

    rm -rf worktrees/*/target .worktrees/*/target

How it works

  • Cargo walks parent directories to find .cargo/config.toml
  • All worktrees share one target/ directory via the config
  • Cargo's incremental compilation handles the rest: deps compiled by any worktree are reused

Caveats

  • No concurrent builds: Cargo holds a lock on the target dir. If two worktrees build at the same time, one blocks until the other finishes.

  • Absolute path: Machine-specific, but since .cargo/config.toml is gitignored that's fine.

  • Branch switching thrash: If two worktrees have very different deps, Cargo may recompile more. In practice most worktrees share 99% of deps.

  • Stale target/ directory: If you previously built without the shared target config, a target/ directory with old artifacts may still exist in the repo root. The justfile's ios-gen-swift and other recipes reference target/$PROFILE/, so a stale target/ will shadow the correct .shared-target/ output, causing uniffi to generate bindings from an outdated dylib. After enabling the shared target, delete or replace the old target/ directory:

    rm -rf target && ln -s .shared-target target

    The symlink ensures recipes that hardcode target/ still find the correct artifacts.

Optional: sccache

As a complementary optimization, install sccache to cache individual rustc invocations by input hash:

brew install sccache

In ~/.cargo/config.toml (your home dir, not the repo):

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

This helps when the shared target dir has a cache miss. On its own it only cuts ~76s to ~45s, so it's a nice-to-have on top of the shared target, not a replacement.