External storage setup

The repository, cargo build artifacts, benchmark corpora, Git mirrors and profiling data live on an external SSD; toolchains and the cargo registry stay on the internal disk (ADR-0008). No machine-specific path is ever committed — everything below is per-developer, user-level configuration.

Cargo target directory

Put build artifacts on the external volume via your user-level~/.cargo/config.toml (never the repository):

# ~/.cargo/config.toml — adjust the volume name to your machine
[build]
target-dir = "/Volumes/<your-external-volume>/cargo-target"

[profile.dev]
debug = 1            # line tables only — halves target size

[profile.dev.package."*"]
opt-level = 2        # dependencies optimized once

[profile.release]
lto = "thin"
codegen-units = 4

Alternatively export CARGO_TARGET_DIR=... in your shell profile. The repository's committed Cargo.toml already applies the profile settings; duplicating them user-level is harmless.

Find your volume: ls /Volumes / diskutil info "/Volumes/<name>".

sccache is a worthwhile addition (cargo install sccache, then build.rustc-wrapper = "sccache" user-level).

exFAT caveats (T7-class drives ship exFAT)

• No hard links → cargo prints "hard linking files in the incremental compilation cache failed. copying files instead" and copies. Harmless, slightly slower.
• No extended attributes → macOS materialises xattrs as AppleDouble ._* sidecar files next to everything it touches, including inside .git, where git prints error: non-monotonic index ... ._pack-*.idx. The sidecars are junk metadata: .gitignore excludes them, make clean-sidecars deletes them, and QG-001 fails if one is ever tracked.
• Permanent fix (recommended, user decision): reformat the volume APFS in Disk Utility — this erases it — or add an APFS partition for development. Everything works correctly on exFAT with the mitigations above; APFS removes the noise and enables hard links.

What lives where

Internal disk	External SSD
macOS, toolchains, rustup	repository
cargo registry (~/.cargo/registry)	cargo target dir
IDE caches	benchmark corpora (M08), Git mirrors, profiling output