Skip tests when merging an up-to-date branch

.github/workflows/ci.yml

@@ -15,7 +15,37 @@ env:
   CARGO_TERM_COLOR: always
 
 jobs:
+  maybe-expedite:
+    runs-on: ubuntu-latest
+
+    outputs:
+      value: ${{ steps.expedite.outputs.value }}
+
+    steps:
+      - name: Log github refs
+        run: |
+          echo '```' >> "$GITHUB_STEP_SUMMARY"
+          echo 'github.ref: ${{ github.ref }}' >> "$GITHUB_STEP_SUMMARY"
+          echo 'github.sha: ${{ github.sha }}' >> "$GITHUB_STEP_SUMMARY"
+          echo '```' >> "$GITHUB_STEP_SUMMARY"
+
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Check if merging an up-to-date branch
+        id: expedite
+        if: ${{ github.action == 'merge_group' }}
+        run: |
+          if git merge-base --is-ancestor origin/master ${{ github.event.pull_request.head.sha }}; then
+            echo "value=1" >> "$GITHUB_OUTPUT"
+          fi
+
   test:
+    needs: [maybe-expedite]
+
+    if: ${{ ! needs.maybe-expedite.outputs.value }}
+
     strategy:
       fail-fast: ${{ github.event_name == 'merge_group' }}
       matrix:
@@ -32,13 +62,6 @@ jobs:
       GROUP_RUNNER: target.'cfg(all())'.runner = 'group-runner'
 
     steps:
-      - name: Log github refs
-        run: |
-          echo '```' >> "$GITHUB_STEP_SUMMARY"
-          echo 'github.ref: ${{ github.ref }}' >> "$GITHUB_STEP_SUMMARY"
-          echo 'github.sha: ${{ github.sha }}' >> "$GITHUB_STEP_SUMMARY"
-          echo '```' >> "$GITHUB_STEP_SUMMARY"
-
       - uses: actions/checkout@v4
 
       - uses: actions/cache@v4
@@ -94,14 +117,14 @@ jobs:
   all-checks:
     needs: [test]
 
-    runs-on: ubuntu-latest
-
     # smoelius: From "Defining prerequisite jobs"
     # (https://docs.github.com/en/actions/using-jobs/using-jobs-in-a-workflow#defining-prerequisite-jobs):
     # > If you would like a job to run even if a job it is dependent on did not succeed, use the
     # > `always()` conditional expression in `jobs.<job_id>.if`.
     if: ${{ always() }}
 
+    runs-on: ubuntu-latest
+
     steps:
       - name: Check results
         if: ${{ contains(needs.*.result, 'failure') || contains(needs.*.result, 'cancelled') }}