Rewrite action in Rust using octocrab

[williamdes]

Summary

Replaces the Node.js implementation (index.js + node_modules/) with a Rust crate distributed as a Docker container action. The input/output contract is unchanged — existing workflows using this action continue to work.

Ecosystem crates used

• octocrab — de-facto async GitHub REST client (pulls.get, pulls.merge, issues.remove_label, plus raw _patch for git/refs)
• tokio — async runtime
• serde / serde_json — JSON (de)serialisation
• regex — mirrors the JS new RegExp(...) for allowed-usernames-regex and filter-label
• anyhow + async-trait — error propagation and mockable client trait

No widely-used "actions-toolkit" crate exists, so the small @actions/core subset we need (workflow commands, input env-var decoding) is implemented inline in src/logger.rs and src/inputs.rs.

Structure

src/
  main.rs           # binary wrapper, exits 1 on failure
  action.rs         # decision flow ported 1:1 from index.js
  inputs.rs         # INPUT_* env-var parsing
  context.rs        # GITHUB_REPOSITORY / GITHUB_ACTOR / GITHUB_API_URL
  github_client.rs  # GithubClient trait + OctocrabClient
  logger.rs         # Logger trait + StdoutLogger + CaptureLogger
tests/integration.rs
Dockerfile          # multi-stage rust:1.82-alpine → alpine:3.20
action.yml          # now: using docker / image: Dockerfile

What's covered by tests (30/30 passing)

• input parsing: required/optional/default/empty/invalid numeric/invalid method
• env-var name normalisation
• context parsing including GHES GITHUB_API_URL
• workflow-command % / \r / \n escaping
• all five action outcomes: SkippedActor, SkippedClosed, SkippedLabelMissing, FastForwarded, Merged
• filter-label regex matching (matches JS new RegExp(filter_label))
• label removal failure is warning-only, never fails the action
• merge API failure propagates as error
• end-to-end wiring from an INPUT_*-style map to concrete merge / update_ref calls

Also clean under cargo clippy --all-targets -- -D warnings, cargo fmt --check, and cargo build --release.

Test plan

• CI lint-node → replaced by Rust test & lint (fmt + clippy + cargo test) and Build Docker image jobs
• Confirm cargo test passes in CI
• Confirm Docker image builds in CI
• Smoke-test the action on a real PR once merged to a release tag

https://claude.ai/code/session_01NeSewqCZN17eNFt5VKWkcf