Automatically creates PR when no_atomic_cas.rs needs to be updated

.github/workflows/ci.yml

@@ -231,17 +231,40 @@ jobs:
             --features unstable --ignore-unknown-features
 
   # When this job failed, run ci/no_atomic_cas.sh and commit result changes.
-  # TODO(taiki-e): Ideally, this should be automated using a bot that creates
-  #                PR when failed, but there is no bandwidth to implement it
-  #                right now...
   codegen:
     runs-on: ubuntu-latest
+    permissions:
+      contents: write
+      pull-requests: write
     steps:
       - uses: actions/checkout@v2
       - name: Install Rust
         run: rustup update nightly && rustup default nightly
       - run: ci/no_atomic_cas.sh
-      - run: git diff --exit-code
+      - run: git add -N . && git diff --exit-code
+        if: github.event_name != 'schedule'
+      - id: diff
+        run: |
+          git config user.name "Taiki Endo"
+          git config user.email "te316e89@gmail.com"
+          git add -N .
+          if ! git diff --exit-code; then
+              git add .
+              git commit -m "Update no_atomic_cas.rs"
+              echo "::set-output name=success::false"
+          fi
+        if: github.event_name == 'schedule'
+      - uses: peter-evans/create-pull-request@v3
+        with:
+          title: Update no_atomic_cas.rs
+          body: |
+            Auto-generated by [create-pull-request][1]
+            [Please close and immediately reopen this pull request to run CI.][2]
+
+            [1]: https://github.com/peter-evans/create-pull-request
+            [2]: https://github.com/peter-evans/create-pull-request/blob/HEAD/docs/concepts-guidelines.md#workarounds-to-trigger-further-workflow-runs
+          branch: update-no-atomic-cas-rs
+        if: github.event_name == 'schedule' && steps.diff.outputs.success == 'false'
 
   miri:
     name: cargo miri test