Three independent reasons Rust CI stayed slow despite sccache, fixed together because
they share the same measurement.
1. sccache only ever covered RUSTC. Every C/C++ dependency in the tree — aws-lc-sys,
openh264-sys2's vendored C++, the CMake-built libopus behind audiopus_sys — was
compiled from scratch on every job of every workflow. CMAKE_{C,CXX}_COMPILER_LAUNCHER
plus CC_/CXX_x86_64_unknown_linux_gnu route both build-script styles (cc-rs and
cmake-rs) through the same shared cache.
The CC_* vars are JOB-scoped in ci.yml and deb.yml, never workflow-scoped: the
arm64 cross image sets its own CC_x86_64_unknown_linux_gnu=pf-host-cc, the wrapper
that keeps ffmpeg-sys-next's host probe off the arm64 include dirs. Overwriting it
would surface as a header mismatch rather than as a CI config error.
2. Linking is cacheable by nothing, and these jobs relink the host, client, session,
cli, worker and tray on every run — twice per push for rpm (f43 + f44). The four
Linux builder images now install mold and carry a $CARGO_HOME/config.toml that uses
it for x86_64. aarch64 is deliberately left alone (cross driver, already-fast legs).
Each image asserts `mold --version` in its build, so an image can never ship the
flag without the linker: docker.yml goes red and :latest stays on the last good one.
3. THE EXPENSIVE ONE. ci.yml (debug) and deb.yml (release) named a byte-identical
target-cache key, under a comment claiming the release build reused ci.yml's
artifacts. It never could. actions/cache is first-saver-wins on an exact key and
ci.yml is the faster job, so the shared key always held a debug-only target/ — and,
worse, deb.yml could then never save its own, because the key was taken. Every
canary .deb has been a from-scratch release build for as long as both keys existed.
Same collision on the arm64 pair, and a third participant in
linux-client-screenshots.yml. Split into -debug-/-release- key families; that job
reads deb's tree via restore-keys but keeps its own exact key so it can never win
the save race and replace a full tree with its single-crate one.
Also: one scripts/ci/ensure-sccache.sh replaces ten copy-pasted bootstrap blocks that
had already drifted into two dialects (GNU tar --wildcards vs bsdtar), every Rust job
now ends with --show-stats so a cache regression is visible instead of just "CI got
slower", and deb.yml's web install joins every other CI install on --ignore-scripts.
No behaviour change to any artifact: same compilers, same flags, same outputs.
enricobuehler
f1dc6c9f94