ci: trial main-owned Rust caching

[afourniernv]

What

Adds Swatinem/rust-cache to the existing Rust CI job.

Pull requests can restore the cache, but only main saves it. cache-bin: false keeps Cargo-installed binaries out. This follows the same pinned-action and main-writes pattern used by NeMo Relay's Rust CI, introduced as part of NVIDIA/NeMo-Relay#46.

Why

Starts the trial described in #669.

Recent successful runs spend a median of 3m30s in the Rust job. Python currently takes longer, so I do not expect this to shorten the full workflow yet. It should reduce repeated Rust compilation and return Rust results sooner.

This is intentionally experimental. The first main run will seed the cache, then we can compare 5-10 warm runs and inspect cache size, restore time, and Rust job duration. If the savings are not meaningful or the cache churn is too high, we should revert it.

Scope

• Only the core Rust job is cached.
• No custom cache keys or workspace-crate caching.
• Python, docs, portability, performance, publishing, and release workflows are unchanged.
• No public API, runtime dependency, package, or release behavior changes.

How tested

• go run github.com/rhysd/actionlint/cmd/actionlint@v1.7.12 .github/workflows/ci.yml
• git diff --check
• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• uv run ruff check .
• uv run mypy switchyard
• env -u OPENROUTER_API_KEY -u NVIDIA_API_KEY -u OPENAI_API_KEY -u ANTHROPIC_API_KEY uv run pytest tests/ -v -m "not integration" - 117 passed, 2 deselected

No live provider calls were made.

Notes for reviewers

The production diff is four YAML lines. A cache miss falls back to the normal Cargo build.

Summary by CodeRabbit

• Chores
  • Improved Rust CI caching to reduce unnecessary cache storage and limit cache updates to the main branch.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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• .github/workflows/ci.yml

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Walkthrough

The CI workflow adds Rust dependency caching with binary caching disabled. Cache saves are restricted to the main branch.

Changes

Rust CI caching

Layer / File(s)	Summary
Configure Rust dependency caching .github/workflows/ci.yml	The workflow uses Swatinem/rust-cache v2.9.1. Binary caching is disabled. Cache saves occur only on refs/heads/main.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: 🟡 Moderate · up to 99abb

This change speeds Rust CI through shared dependency caching, but pull-request jobs could access Cargo credentials if they are included in the main-branch cache. Confirming that credentials cannot enter the cached paths is needed before merge.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: an experimental Rust cache trial where cache ownership is restricted to the main branch.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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_{A rabbit checks the Rusty trail
Cache crumbs wait where builds prevail
Binaries stay tucked away
Main branch saves them for the day
CI hops faster through the hay}

_{Comment @coderabbitai help to get the list of available commands.}

[ayushag-nv]

@grahamking for review