GeezerVison

Problem

Every ubuntu e2e matrix leg cold-compiles the full ~275-crate dependency graph on every PR run (verified in run 28838256247: Cache not found for input keys on all legs, then 275 Compiling lines each, vs 4 on a cache-hit job). Two compounding causes:

1. Matrix key collision — all ~9 same-OS e2e legs share one cache key (${{ matrix.os }}-cargo-e2e-<lockhash>). Only one leg wins the cache reserve per run; the other 8 log Failed to save: Unable to reserve cache … another job may be creating this cache.
2. 10 GiB budget blowout — caching the whole target/ dir puts a single main run's footprint at ~8.2 GiB (windows-latest-cargo-* 3.96 GB, macos-latest-cargo-* 2.19 GB). Against GitHub's 10 GiB LRU budget, every PR save evicts the main-branch caches (the big e2e ones first) — the cache listing shows only the most recent main run's caches ever survive.

Fix

• Swap the five raw actions/cache blocks (clippy, test matrix, test-release, coverage, e2e) for Swatinem/[email protected] (SHA-pinned), which prunes caches to dependency artifacts (~5-10× smaller) and keys on rustc host triple + job automatically.
• Per-suite key: ${{ matrix.suite }} on the e2e job — no more reserve races.
• save-if: github.ref == 'refs/heads/main' everywhere — PR branches restore but never write, so they stop churning the budget.

Workspace crates (socket-patch-core/-cli) recompile each run by design — that ~1-2 min is the tradeoff that keeps the dependency caches small enough to survive. The docker-image jobs keep their deliberate no-cache stance (zizmor cache-poisoning audit).

Expected effect

e2e legs go from ~275-crate cold builds per leg per PR run to dependency-cached builds; total repo cache footprint drops well under the 10 GiB budget so main caches stop being evicted.

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