ci(rust): cache cargo build + Nix store (cachix) - #107
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The nix rust job recompiled the whole workspace (tauri/specta/webkit bindings,
hundreds of crates) every run — ~11 min — because nothing cached cargo's output.
Add actions/cache over ~/.cargo/{registry,git} + target, keyed on flake.lock (the
toolchain pin) + Cargo.lock, with restore-keys so a lock bump reuses the previous
target and recompiles only what changed. The devshell sets no CARGO_HOME /
CARGO_TARGET_DIR, so these host paths are what cargo actually uses.
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magic-nix-cache v8 only uploads to FlakeHub (not connected) — its GHA backend was retired, so it cached nothing. Use our public cachix cache (kasumi-proxy) instead: pulls the devshell closure as a substituter and pushes newly-built paths. Reads need no token (public), so fork PRs benefit; pushes use CACHIX_AUTH_TOKEN.
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Explain that nix develop / nix build substitute the toolchain + devshell closure from the public kasumi-proxy Cachix cache, and how to opt in locally (cachix use). Note the cargo build cache is CI-only.
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Add the kasumi-proxy Cachix cache (URL + public key) to flake.nix nixConfig so nix develop / nix build pull the toolchain + devshell closure from it without each contributor running `cachix use`. Honoured for trusted users / with --accept-flake-config (CI already sets it); README documents accepting the flake config and the prompt-free `cachix use` alternative. Pulls only — CI still pushes via the auth-token cachix-action.
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Summary
The nix
rustCI job took ~11–12 min because nothing cached cargo's output and the Nix-store cache step was a dead no-op. Two complementary cache layers fix that:actions/cacheover~/.cargo/{registry,git}+target/) — the dominant cost.clippy --all-targets+test+codegenrecompiled hundreds of crates (tauri, specta, webkit bindings) every run; restoringtarget/makes it incremental. Keyed onflake.lock(toolchain pin) +Cargo.lock, with restore-keys so a lock bump reuses the previoustarget.cachix/cachix-action, public cachekasumi-proxy) — replaces the retiredmagic-nix-cache(its GHA backend was shut down; it uploaded nothing). Pulls the devshell closure as a substituter and pushes newly-built paths. Reads need no token (public cache → fork PRs benefit); pushes use theCACHIX_AUTH_TOKENsecret.The devshell sets no
CARGO_HOME/CARGO_TARGET_DIR(verified), so~/.cargo+./targetare what cargo actually uses insidenix develop.Measured (on this branch, real CI runs)
≈ 5× faster, ~9 min saved per warm run. A CI-config-only change doesn't trigger the
rustjob (path filter), so the warm number was measured with a throwaway rust edit on this branch (reverted — the merged diff isci.ymlonly).Affected layer
.github/Verification
actionlint .github/workflows/ci.ymlcleanCARGO_HOME/CARGO_TARGET_DIRunset)Notes for reviewers
kasumi-proxyis public (managed signing, zstd compression). TheCACHIX_AUTH_TOKENsecret is set on the repo for pushes; reads are token-free.nix developclosure), the cargo cache caches the compiled Rust (target/).actions/cacheSHA reused fromrelease.yml;cachix/cachix-actionpinned to the v17 commit.