ci: Calculate delta coverage using diff-cover

Now each coverage report also includes delta coverage, i.e. coverage
scoped to current changes.  For PRs, it's code modified in the PR,
for branches it's code since the last push.

The delta coverage report is included as a separate report and also
a markdown summary is posted on GitHub Actions.

Author: kjarosh

.github/workflows/test_rust.yml

@@ -13,7 +13,8 @@ concurrency:
 env:
   FEATURES: lzma,jpegxr
   TEST_OPTS: --workspace --locked --no-fail-fast -j 4
-  LLVM_COV_OPTS: --branch --profile ci ${TEST_OPTS} --features ${FEATURES},imgtests
+  LLVM_COV_OPTS: --branch --profile ci
+  LLVM_COV_TEST_OPTS: ${LLVM_COV_OPTS} ${TEST_OPTS} --features ${FEATURES},imgtests
 
   LINUX_DEPENDENCIES: libasound2-dev mesa-vulkan-drivers libudev-dev
 
@@ -163,6 +164,37 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v5
+        with:
+          # We need history for delta coverage, but we also don't want to fetch
+          # the whole repo. 42 commits seem reasonable
+          fetch-depth: 42
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.13"
+
+      - name: Install diff-cover
+        run: pip install diff-cover==9.6.0
+
+      - name: Calculate base ref
+        id: base_ref
+        run: |
+          if [ "${{ github.event_name }}" = "pull_request" ] || [ "${{ github.event_name }}" = "pull_request_target" ]; then
+            # For PRs always compute delta with base ref
+            git fetch origin ${{ github.base_ref }}
+            echo "::notice::Using the PR base as base ref for delta coverage."
+            echo "value=origin/${{ github.base_ref }}" | tee -a $GITHUB_OUTPUT
+          elif git merge-base --is-ancestor ${{ github.event.before }} HEAD; then
+            # Use the before SHA if it's an ancestor of HEAD
+            echo "::notice::Using the last pushed commit as base ref for delta coverage."
+            echo "value=${{ github.event.before }}" | tee -a $GITHUB_OUTPUT
+          else
+            # If not, someone force pushed and just use the parent
+            echo "::warning::Failed to compute the base ref for delta coverage. Using parent commit."
+            parent=$(git rev-parse HEAD~1)
+            echo "value=${parent}" | tee -a $GITHUB_OUTPUT
+          fi
 
       - name: Install Rust toolchain
         # We need nightly for coverage due to:
@@ -192,15 +224,41 @@ jobs:
         working-directory: tests
         env:
           RUFFLE_TEST_OPTS: --ignore-known-failures
-        run: cargo llvm-cov --html ${{ env.LLVM_COV_OPTS }}
+        run: cargo llvm-cov --no-report ${{ env.LLVM_COV_TEST_OPTS }}
+
+      - name: Generate llvm-cov reports
+        run: |
+          mkdir -p target/llvm-cov
+          cargo llvm-cov report --html --output-dir  target/llvm-cov/ ${{ env.LLVM_COV_OPTS }}
+          cargo llvm-cov report --lcov --output-path target/llvm-cov/coverage.lcov ${{ env.LLVM_COV_OPTS }}
 
-      - name: Upload report
+      - name: Upload llvm-cov reports
         if: always()
         uses: actions/upload-artifact@v4
         with:
           name: llvm-cov
           path: target/llvm-cov
 
+      - name: Run diff-cover
+        run: |
+          mkdir -p target/diff-cover
+          diff-cover target/llvm-cov/coverage.lcov \
+            --html-report target/diff-cover/report.html \
+            --json-report target/diff-cover/report.json \
+            --markdown-report target/diff-cover/report.md \
+            --compare-branch ${{ steps.base_ref.outputs.value }}
+
+      - name: Upload diff-cover reports
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: diff-cover
+          path: target/diff-cover
+
+      - name: Upload diff-cover summary
+        if: always()
+        run: cat target/diff-cover/report.md >> $GITHUB_STEP_SUMMARY
+
   dependencies:
     needs: changes
     if: needs.changes.outputs.should_run == 'true'