ci: add weekly job to update nightly rustc

.github/workflows/cron-weekly-update-nightly.yml

@@ -0,0 +1,43 @@
+name: Update Nightly rustc
+on:
+  schedule:
+    - cron: "0 0 * * 0" # runs weekly on Sunday at 00:00
+  workflow_dispatch: # allows manual triggering
+jobs:
+  format:
+    name: Update nightly rustc
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@nightly
+      - name: Update rust.yml to use latest nightly
+        run: |
+          set -x
+          # Not every night has a nightly, so extract the date from whatever
+          # version of the compiler dtolnay/rust-toolchain gives us.
+          NIGHTLY_DATE=$(rustc +nightly --verbose --version | sed -ne 's/^commit-date: //p')
+          # Edit in-place the main CI job for PRs. The only other dependencies
+          # on Nightly in CI are other cronjobs which are fine to use an unpinned
+          # nightly.
+          sed -i "s/toolchain: nightly-.*/toolchain: nightly-${NIGHTLY_DATE}/" .github/workflows/rust.yml || exit 1
+          echo "nightly_date=${NIGHTLY_DATE}" >> $GITHUB_ENV
+          # Plausibly there was no update to nightly over the course of a week,
+          # or dtolnay's script is broken, or something. In this case don't make
+          # an empty PR. Can also trigger if we manually run this action right
+          # before the automated one.
+          if git diff --exit-code > /dev/null; then
+              echo "Updated nightly. Opening PR."
+              echo "changes_made=true" >> $GITHUB_ENV
+          else
+              echo "Attempted to update nightly but the latest-nightly date did not change. Not opening any PR."
+              echo "changes_made=false" >> $GITHUB_ENV
+          fi
+      - name: Create Pull Request
+        if: env.changes_made == 'true'
+        uses: peter-evans/create-pull-request@v6
+        with:
+          author: Update Nightly Rustc Bot <bot@example.com>
+          title: Automated weekly update to rustc (to nightly-${{ env.nightly_date }})
+          body: |
+           Automated update to Github CI workflow `rust.yml` by [create-pull-request](https://github.com/peter-evans/create-pull-request) GitHub action
+          commit-message: Automated update to Github CI to rustc nightly-${{ env.nightly_date }}