Summary

• New composite action `setup-rust-toolchain-from-file` that reads the channel from `rust-toolchain.toml` and installs the toolchain, optionally adding the Swatinem rust-cache.
• Every Rust CI job otherwise repeats ~30 lines of boilerplate per job: sed-parse the channel from `rust-toolchain.toml`, call `actions-rust-lang/setup-rust-toolchain`, optionally call `Swatinem/rust-cache`. That boilerplate has no single source of truth and drifts independently across jobs. This action makes it one `uses:` line.

Inputs

• `path` (default `rust-toolchain.toml`) — path to the rust-toolchain file.
• `components` (default empty) — comma-separated rustup components passed through.
• `cache` (default `true`) — opt out for lint-only / fmt-only jobs that don't compile.
• `cache-shared-key` — `shared-key` passed to `Swatinem/rust-cache`.

Output

• `toolchain` — the channel resolved from the file (e.g. `nightly-2025-08-29`).

Failure modes

• Missing file → `::error file=::rust-toolchain file not found`.
• No `channel` line → `::error file=::no channel = "..." line found`.

Test plan

• action.yml YAML is well-formed.
• Indexed under "Setup Actions" in `.github/actions/README.md`.
• README documents usage, inputs, outputs, failure modes, and motivation with side-by-side before/after.
• First consumer will be a follow-up PR in `dataroadinc/wwkg` that deletes ~250 lines of duplicated boilerplate across 8 Rust jobs in `.github/workflows/build.yml`.

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