run cargo update weekly

- Keep Cargo.lock dependencies current
- Presents output from `cargo update` in commit and PR
- Edit existing open PR, otherwise open a new one
- Skip if existing open PR is S-waiting-on-bors

.github/workflows/dependencies.yml

@@ -0,0 +1,139 @@
+# Automatically run `cargo update` periodically
+
+---
+name: Bump dependencies in Cargo.lock
+on:
+  schedule:
+    # Run weekly
+    - cron: '0 0 * * Sun'
+  workflow_dispatch:
+    # Needed so we can run it manually
+permissions:
+  contents: read
+defaults:
+  run:
+    shell: bash
+env:
+  # So cargo doesn't complain about unstable features
+  RUSTC_BOOTSTRAP: 1
+  PR_TITLE: Weekly `cargo update`
+  PR_MESSAGE: |
+    Automation to keep dependencies in `Cargo.lock` current.
+
+    The following is the output from `cargo update`:
+  COMMIT_MESSAGE: "cargo update \n\n"
+
+jobs:
+  not-waiting-on-bors:
+    name: skip if S-waiting-on-bors
+    runs-on: ubuntu-latest
+    steps:
+      - env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          # Fetch state and labels of PR
+          # Or exit successfully if PR does not exist
+          JSON=$(gh pr view cargo_update --repo $GITHUB_REPOSITORY --json labels,state || exit 0)
+          STATE=$(echo "$JSON" | jq -r '.state')
+          WAITING_ON_BORS=$(echo "$JSON" | jq '.labels[] | any(.name == "S-waiting-on-bors"; .)')
+
+          # Exit with error if open and S-waiting-on-bors
+          if [[ "$STATE" == "OPEN" && "$WAITING_ON_BORS" == "true" ]]; then
+            exit 1
+          fi
+
+  update:
+    name: update dependencies
+    needs: not-waiting-on-bors
+    runs-on: ubuntu-latest
+    steps:
+      - name: checkout the source code
+        uses: actions/checkout@v3
+        with:
+          submodules: recursive
+      - name: install the bootstrap toolchain
+        run: |
+          # Extract the stage0 version
+          TOOLCHAIN=$(jq -r '.compiler | {version,date} | join("-")' -- src/stage0.json)
+          # Install and set as default
+          rustup toolchain install --no-self-update --profile minimal $TOOLCHAIN
+          rustup default $TOOLCHAIN
+
+      - name: cargo update
+        # Remove first line that always just says "Updating crates.io index"
+        run: cargo update 2>&1 | sed '/crates.io index/d' | tee -a cargo_update.log
+      - name: upload Cargo.lock artifact for use in PR
+        uses: actions/upload-artifact@v3
+        with:
+          name: Cargo-lock
+          path: Cargo.lock
+          retention-days: 1
+      - name: upload cargo-update log artifact for use in PR
+        uses: actions/upload-artifact@v3
+        with:
+          name: cargo-updates
+          path: cargo_update.log
+          retention-days: 1
+
+  pr:
+    name: amend PR
+    needs: update
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+      pull-requests: write
+    steps:
+      - name: checkout the source code
+        uses: actions/checkout@v3
+
+      - name: download Cargo.lock from update job
+        uses: actions/download-artifact@v3
+        with:
+          name: Cargo-lock
+      - name: download cargo-update log from update job
+        uses: actions/download-artifact@v3
+        with:
+          name: cargo-updates
+
+      - name: craft PR body and commit message
+        run: |
+          echo "${COMMIT_MESSAGE}" > commit.txt
+          cat cargo_update.log >> commit.txt
+
+          echo "${PR_MESSAGE}" > body.md
+          echo '```txt' >> body.md
+          cat cargo_update.log >> body.md
+          echo '```' >> body.md
+
+      - name: commit
+        run: |
+          git config user.name github-actions
+          git config user.email github-actions@github.com
+          git switch --force-create cargo_update
+          git add ./Cargo.lock
+          git commit --no-verify --file=commit.txt
+
+      - name: push
+        run: git push --no-verify --force --set-upstream origin cargo_update
+
+      - name: edit existing open pull request
+        id: edit
+        # Don't fail job if we need to open new PR
+        continue-on-error: true
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          # Exit with error if PR is closed
+          STATE=$(gh pr view cargo_update --repo $GITHUB_REPOSITORY --json state --jq '.state')
+          if [[ "$STATE" != "OPEN" ]]; then
+            exit 1
+          fi
+
+          gh pr edit cargo_update --title "${PR_TITLE}" --body-file body.md --repo $GITHUB_REPOSITORY
+
+      - name: open new pull request
+        # Only run if there wasn't an existing PR
+        if: steps.edit.outcome != 'success'
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: gh pr create --title "${PR_TITLE}" --body-file body.md --repo $GITHUB_REPOSITORY