ci: gate coverage analysis on whether source code changed

.github/workflows/coveralls.yml

@@ -5,7 +5,20 @@ on:
 name: Code coverage with llvm-cov
 
 jobs:
+  Prepare:
+    runs-on: ubuntu-latest
+    outputs:
+      pr_changed_source: ${{ steps.classify_pr.outputs.pr_changed_source }}
+    steps:
+      - name: Checkout Crate
+        uses: actions/checkout@v4
+      - name: Determine what files the PR changes.
+        id: classify_pr
+        run: contrib/classify-pr.sh ${{ github.event.pull_request.head.sha }} ${{ github.event.pull_request.base.sha }}
+
   Coveralls:
+    needs: Prepare
+    if: needs.Prepare.outputs.pr_changed_source == 'true'
     name: Code coverage - stable toolchain
     runs-on: ubuntu-latest
     strategy:

contrib/classify-pr.sh

@@ -16,6 +16,10 @@ git fetch origin "$pr_tip":pr_tip
 
 pr_base=$(git merge-base master_tip pr_tip)
 
+echo "Using  master $master_tip"
+echo "Using  PR tip $pr_tip"
+echo "Using PR base $pr_base"
+
 # If something modifies any non-markdown file, it's considered a source code change.
 if git diff --name-only "$pr_base" "$pr_tip" | grep -qv "^.md$"; then
     echo "pr_changed_source=true" >> "$GITHUB_OUTPUT"