Add coverage reporting via cargo-llvm-cov (#349)

* Add coverage reporting via cargo-llvm-cov

* Update .github/workflows/ci.yml

Co-authored-by: Ed Morley <501702+edmorley@users.noreply.github.com>
Signed-off-by: Josh W Lewis <josh.w.lewis@gmail.com>

* Update .github/workflows/ci.yml

Co-authored-by: Ed Morley <501702+edmorley@users.noreply.github.com>
Signed-off-by: Josh W Lewis <josh.w.lewis@gmail.com>

---------

Signed-off-by: Josh W Lewis <josh.w.lewis@gmail.com>
Co-authored-by: Ed Morley <501702+edmorley@users.noreply.github.com>

Author: joshwlewis

.github/workflows/ci.yml

@@ -40,6 +40,31 @@ jobs:
       - name: Run unit tests
         run: cargo test --locked
 
+  unit-test-coverage:
+    name: Generate test coverage report
+    runs-on: ubuntu-24.04
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          submodules: true
+      - name: Install nightly Rust toolchain
+        run: rustup install nightly
+      - name: Rust Cache
+        uses: Swatinem/rust-cache@f0deed1e0edfc6a9be95417288c0e1099b1eeec3 # v2.7.7
+      - name: Install cargo-llvm-cov
+        uses: taiki-e/install-action@62730e3d4f6bd81d824694e963e06d7153968c93 # v2.49.29
+        with:
+          tool: cargo-llvm-cov
+      - name: Run unit tests and generate coverage report
+        run: cargo +nightly llvm-cov --locked --html
+      - name: Upload HTML coverage report
+        uses: actions/upload-artifact@v4
+        with:
+          name: "llvm-cov-html-${{github.event.repository.name}}-${{github.sha}}"
+          path: "target/llvm-cov/html"
+          if-no-files-found: "error"
+
   integration-test:
     name: integration-tests ${{ matrix.builder }} / ${{ matrix.arch }}
     runs-on: ${{ matrix.arch == 'arm64' && 'pub-hk-ubuntu-24.04-arm-medium' || 'ubuntu-24.04' }}

Cargo.toml

@@ -10,6 +10,10 @@ edition = "2021"
 unreachable_pub = "warn"
 unsafe_code = "warn"
 unused_crate_dependencies = "warn"
+# Allows the usage of cfg(coverage_nightly).
+# cargo-llvm-cov enables that config when instrumenting our code, so we can enable
+# the experimental coverage_attribute feature.
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(coverage_nightly)'] }
 
 [workspace.lints.clippy]
 panic_in_result_fn = "warn"

buildpacks/go/src/main.rs

@@ -1,3 +1,8 @@
+// cargo-llvm-cov sets the coverage_nightly attribute when instrumenting our code. In that case,
+// we enable https://doc.rust-lang.org/beta/unstable-book/language-features/coverage-attribute.html
+// to be able selectively opt out of coverage for functions/lines/modules.
+#![cfg_attr(coverage_nightly, feature(coverage_attribute))]
+
 mod cfg;
 mod cmd;
 mod layers;