CI: check the presence of the changelog line in each pull request

Checking it only in the merge queue leads to deferred failures once the
decision to merge has been taken already.

.github/workflows/clippy_pr.yml

@@ -17,8 +17,38 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  changelog:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+      with:
+        # Unsetting this would make so that any malicious package could get our Github Token
+        persist-credentials: false
+
+    # Run
+    - name: Check Changelog
+      run: |
+        body=$(curl -H "Authorization: token ${{ secrets.GITHUB_TOKEN }}" -s "https://api.github.com/repos/rust-lang/rust-clippy/pulls/$PR_NUMBER" | \
+          python -c "import sys, json; print(json.load(sys.stdin)['body'])")
+        cl_line=$(grep "^changelog:\s*\S" <<< "$body") || {
+          echo "ERROR: pull request message must contain 'changelog: ...'. Add it " \
+               "and push (or force-push) the pull request to trigger a new check."
+          exit 1
+        }
+        output=$(sed "s/^changelog:\s*//" <<< "$cl_line")
+        if [[ "$output" = "none" ]]; then
+          echo "WARNING: changelog is 'none'"
+        else
+          echo "changelog: $output"
+        fi
+      env:
+        PYTHONIOENCODING: 'utf-8'
+        PR_NUMBER: '${{ github.event.number }}'
   base:
     # NOTE: If you modify this job, make sure you copy the changes to clippy_mq.yml
+    needs: changelog
     runs-on: ubuntu-latest
 
     steps: