#488 ci: save rust caches only on the integration branches

[Anonymous]

Originally created by: Akarsh-Hegde

What

Adds save-if to all three Swatinem/rust-cache@v2 steps in ci.yml (rust, windows-portability, macos-universal):

save-if: ${{ github.ref == 'refs/heads/pre-main' || github.ref == 'refs/heads/main' }}

PR runs keep restoring — GitHub lets a PR run read caches from its base branch — they just stop writing a duplicate.

Why

Actions caches are scoped by ref. A branch that is both pushed to pre-main and has an open PR therefore stores two near-identical copies of every target/ cache. Measured 2026-07-19:

Cache	pre-main	pull/483/merge
macos-universal	2582 MB	2527 MB
windows-portability	1269 MB	1262 MB
rust (Linux)	1091 MB	851 MB

9.58 GB against a 10 GB per-repo quota, from six entries all created inside one 15-minute window. GitHub evicted LRU to stay under, and the casualty was the release job's own 2618 MB cache — written at 13:42, gone by ~16:55. Release run 29696118671 consequently compiled fully cold.

This frees ~4.9 GB (12.18 GB → ~7.3 GB).

Why not cache-targets: false

That was the other candidate and it's the worse trade. Those jobs are already the slowest in CI with a warm cache — on run 29695608454: Windows portability 8.1 min, macOS 6.2 min, Rust 2.3 min. Dropping target/ would add ~10 min to every PR to save ~20 min on an infrequent release.

Costs and limits, stated plainly

• A long-lived PR branch that drifts from its base gets progressively staler restores. Bounded — never worse than the cold build it would otherwise get.
• This makes the release cache able to survive, not guaranteed to. ~7.3 GB + 2.62 GB release ≈ 9.9 GB is still close to the line, and a sustained week of CI could age it out. The durable fixes are a scheduled warm-up run on pre-main that refreshes the release-namespace cache, or more quota. Neither is in this PR.
• Existing stale entries aren't pruned here. Two generations from 07-18 (2.35 GB combined) are still held on superseded lockfile hashes; clearing those would add real headroom.

Testing

save-if is confirmed present in action.yml at the pinned SHA (e18b497). The effect is only observable across runs — after merge, the check is that PR runs stop appearing as pull/* refs in gh cache list while still logging a restore.

[Anonymous]

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[Anonymous]

Ticket changed by: Akarsh-Hegde

• status: open --> closed