ci: cache Rust compilation with sccache#85
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rust-cache keys the entire target/ on Cargo.lock, so any dependency bump forces a full cold recompile of lux's large AWS-SDK + Tauri trees. sccache caches each crate content-addressed, so a lock change still hits for every unchanged crate. The desktop rust gate gets RUSTC_WRAPPER=sccache + the GitHub Actions cache backend, with rust-cache dropped to registry-only (cache-targets: false) since sccache now owns compilation; build-macos gets sccache alongside rust-cache (the fat-LTO link stays uncacheable, but per-arch dep compilation hits on lock changes). CARGO_INCREMENTAL=0 and sccache --show-stats for visibility.
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Adds sccache as the Rust compilation cache on both cargo CI jobs.
Why
Swatinem/rust-cachekeys the wholetarget/onCargo.lock, so any dependency bump invalidates it and forces a full cold recompile of lux's large dependency trees (full AWS SDK + Tauri). sccache caches each crate's compilation content-addressed (inputs + flags + rustc version), so aCargo.lockchange still hits for every crate that didn't actually change — the common PR case, and where the time goes.What
desktoprustgate (the frequent, non-LTO job, where this matters most):RUSTC_WRAPPER=sccachewith the GitHub Actions cache backend. rust-cache drops to registry-only (cache-targets: false) since sccache now owns compilation caching — keeps this job's GHA-cache footprint small.releasebuild-macos: sccache added alongside rust-cache. The final fat-LTO link isn't cacheable, but the universal build's per-arch dependency compilation hits across a lock bump.CARGO_INCREMENTAL=0(incremental output isn't cacheable) andsccache --show-statsafter each build so hit rates show up in the logs.Backend note
Using the GitHub Actions cache backend — zero infra, works immediately. Its 10 GB/repo cap with LRU eviction is the only limit; if it starts thrashing (the release
target/cache is large), the clean upgrade is sccache → S3 via the existing OIDC pattern (no cap, durable, no contention with rust-cache's GHA usage).