Create an issue on nightly CI run failure

.github/workflows/ci.yml

@@ -43,6 +43,34 @@ jobs:
       - name: Run tests
         run: sh ci/run.sh ${{ matrix.os }}
 
+  create_issue:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Get today's date
+        id: date
+        run: |
+          echo "::set-output name=today::$(date "+%Y-%m-%d")"
+
+      - name: Create an issue
+        # the combination of these two lines seems to do the "if build_and_test failed, run this one"
+        needs: build_and_test
+        if: always() && github.event_name == 'schedule' && needs.build_and_test.result == 'failure'
+
+        uses: actions-ecosystem/action-create-issue@v1
+        with:
+          github_token: ${{ secrets.github_token }}
+          title: Nightly CI run failed
+          body: |
+            The nightly CI run on ${{ steps.date.outputs.today }} seems
+            to have failed.
+
+            - Check the logs at https://github.com/rust-lang/rust-semverver/actions/runs/${{ github.run_id }}
+            - Update to the latest `rustc-nightly` if necessary
+            - Investigate test run failures if any
+
+          labels: |
+            nightly-CI-failed
+
   # https://forge.rust-lang.org/infra/docs/bors.html#adding-a-new-repository-to-bors
   build_result:
     name: bors build finished