ci: migrate coverage to cargo-llvm-cov

.github/workflows/codecov.yml

@@ -21,7 +21,7 @@ jobs:
     name: Code Coverage
     runs-on: ubuntu-latest
     env:
-      TARPAULIN_VERSION: "0.32.8"
+      CARGO_LLVM_COV_VERSION: "0.8.5"
     steps:
       - name: Checkout repository
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
@@ -33,18 +33,25 @@ jobs:
         with:
           cache: true # toolchain/components are specified in rust-toolchain.toml
 
-      - name: Cache tarpaulin
+      - name: Install LLVM coverage tools
+        run: rustup component add llvm-tools-preview
+
+      - name: Cache cargo-llvm-cov
         uses: actions/cache@27d5ce7f107fe9357f9df03efb73ab90386fccae # v5.0.5
         with:
-          path: ~/.cargo/bin/cargo-tarpaulin
-          key: tarpaulin-${{ runner.os }}-${{ env.TARPAULIN_VERSION }}
-          restore-keys: |
-            tarpaulin-${{ runner.os }}-
+          path: ~/.cargo/bin/cargo-llvm-cov
+          key: cargo-llvm-cov-${{ runner.os }}-${{ env.CARGO_LLVM_COV_VERSION }}
 
-      - name: Install tarpaulin
+      - name: Install cargo-llvm-cov
         run: |
-          if ! command -v cargo-tarpaulin &> /dev/null; then
-            cargo install cargo-tarpaulin --locked --version "${TARPAULIN_VERSION}"
+          installed_version=""
+          if command -v cargo-llvm-cov &> /dev/null; then
+            installed_version="$(cargo llvm-cov --version)"
+            installed_version="${installed_version#cargo-llvm-cov }"
+          fi
+
+          if [ "$installed_version" != "$CARGO_LLVM_COV_VERSION" ]; then
+            cargo install cargo-llvm-cov --locked --version "${CARGO_LLVM_COV_VERSION}"
           fi
 
       - name: Install just
@@ -66,7 +73,7 @@ jobs:
           ls -la coverage/ || true
 
           if [ ! -f coverage/cobertura.xml ]; then
-            echo "::error::coverage/cobertura.xml not found. Tarpaulin failed to generate XML output."
+            echo "::error::coverage/cobertura.xml not found. cargo-llvm-cov failed to generate XML output."
             exit 2
           fi
           echo "::notice::Coverage report generated successfully: $(wc -l < coverage/cobertura.xml) lines"

README.md

@@ -304,6 +304,7 @@ just fix          # apply auto-fixes (mutating)
 just ci           # lint + tests + examples + bench compile
 ```
 
+For coverage commands and report locations, see [`docs/COVERAGE.md`](docs/COVERAGE.md).
 For the full set of developer commands, see `just --list` and `AGENTS.md`.
 
 ## 📝 Citation

docs/COVERAGE.md

@@ -0,0 +1,62 @@
+# Coverage
+
+la-stack uses `cargo-llvm-cov` for local and CI coverage. Coverage runs use
+Rust's LLVM source-based instrumentation with the same core test selection in
+both environments:
+
+```bash
+cargo llvm-cov --features exact --workspace --lib --tests
+```
+
+## Local HTML
+
+Generate the local developer report with:
+
+```bash
+just coverage
+```
+
+The HTML report is written to:
+
+```text
+target/llvm-cov/html/index.html
+```
+
+The report opens automatically after generation.
+
+## CI XML
+
+Generate the CI-compatible Cobertura report with:
+
+```bash
+just coverage-ci
+```
+
+The XML report is written to:
+
+```text
+coverage/cobertura.xml
+```
+
+The Codecov workflow installs Rust's `llvm-tools-preview` component, installs
+`cargo-llvm-cov`, caches the installed cargo binary by version, runs
+`just coverage-ci`, verifies `coverage/cobertura.xml`, uploads that file to
+Codecov, and archives the full `coverage/` directory. Local setup via
+`just setup-tools` installs the same Rust component and cargo subcommand.
+
+## Migration Notes
+
+- Keep `just coverage-ci` as the single source of truth for CI coverage
+  arguments; workflows should install tools and upload artifacts, not duplicate
+  the coverage command.
+- Use `--cobertura --output-path coverage/cobertura.xml` for services that
+  consume Cobertura XML.
+- Use `--open --output-dir target/llvm-cov` for local reports.
+- Preserve the crate's full coverage surface with `--features exact
+  --workspace --lib --tests`.
+- `cargo-llvm-cov` excludes workspace `tests/`, `examples/`, and `benches/`
+  source files from reports by default, while still allowing integration tests
+  to exercise library code. This matches the intended reporting surface here:
+  library implementation coverage, not test harness coverage.
+- Doc-test coverage remains intentionally disabled because `cargo-llvm-cov`
+  marks that path as unstable.

justfile

@@ -6,12 +6,23 @@
 # Use bash with strict error handling for all recipes
 set shell := ["bash", "-euo", "pipefail", "-c"]
 
+cargo_llvm_cov_version := "0.8.5"
+
 # Internal helpers: ensure external tooling is installed
 _ensure-actionlint:
     #!/usr/bin/env bash
     set -euo pipefail
     command -v actionlint >/dev/null || { echo "❌ 'actionlint' not found. See 'just setup' or https://github.com/rhysd/actionlint"; exit 1; }
 
+_ensure-cargo-llvm-cov:
+    #!/usr/bin/env bash
+    set -euo pipefail
+    if ! command -v cargo-llvm-cov >/dev/null; then
+        echo "❌ 'cargo-llvm-cov' not found. See 'just setup-tools' or install:"
+        echo "   cargo install --locked cargo-llvm-cov --version {{cargo_llvm_cov_version}}"
+        exit 1
+    fi
+
 _ensure-git-cliff:
     #!/usr/bin/env bash
     set -euo pipefail
@@ -172,7 +183,7 @@ ci: check bench-compile test-all examples
 # Clean build artifacts
 clean:
     cargo clean
-    rm -rf target/tarpaulin
+    rm -rf target/llvm-cov
     rm -rf coverage
 
 # Code quality and formatting
@@ -183,41 +194,29 @@ clippy:
 clippy-exact:
     cargo clippy --features exact --all-targets -- -D warnings -W clippy::pedantic
 
-# Coverage (cargo-tarpaulin)
+# Coverage (cargo-llvm-cov)
 #
-# Common tarpaulin arguments for all coverage runs
-# Note: -t 300 sets per-test timeout to 5 minutes (needed for slow CI environments)
-_coverage_base_args := '''--exclude-files 'benches/*' --exclude-files 'examples/*' \
-  --features exact \
+# Common cargo-llvm-cov arguments for all coverage runs.
+_coverage_base_args := '''--features exact \
   --workspace --lib --tests \
-  -t 300 --verbose --implicit-test-threads'''
+  --verbose'''
 
 # Coverage analysis for local development (HTML output)
-coverage:
+coverage: _ensure-cargo-llvm-cov
     #!/usr/bin/env bash
     set -euo pipefail
 
-    if ! command -v cargo-tarpaulin >/dev/null 2>&1; then
-        echo "cargo-tarpaulin not found. Install with: cargo install cargo-tarpaulin"
-        exit 1
-    fi
-
-    mkdir -p target/tarpaulin
-    cargo tarpaulin {{_coverage_base_args}} --out Html --output-dir target/tarpaulin
-    echo "Coverage report generated: target/tarpaulin/tarpaulin-report.html"
+    mkdir -p target/llvm-cov
+    cargo llvm-cov {{_coverage_base_args}} --open --output-dir target/llvm-cov
+    echo "Coverage report generated: target/llvm-cov/html/index.html"
 
 # Coverage analysis for CI (XML output for codecov/codacy)
-coverage-ci:
+coverage-ci: _ensure-cargo-llvm-cov
     #!/usr/bin/env bash
     set -euo pipefail
 
-    if ! command -v cargo-tarpaulin >/dev/null 2>&1; then
-        echo "cargo-tarpaulin not found. Install with: cargo install cargo-tarpaulin"
-        exit 1
-    fi
-
     mkdir -p coverage
-    cargo tarpaulin {{_coverage_base_args}} --out Xml --output-dir coverage
+    cargo llvm-cov {{_coverage_base_args}} --cobertura --output-path coverage/cobertura.xml
 
 # Default recipe shows available commands
 default:
@@ -429,7 +428,7 @@ setup-tools:
         echo "❌ 'rustup' not found. Install Rust via https://rustup.rs and re-run: just setup-tools"
         exit 1
     fi
-    rustup component add clippy rustfmt rust-docs rust-src
+    rustup component add clippy rustfmt rust-docs rust-src llvm-tools-preview
     echo ""
 
     echo "Ensuring cargo tools..."
@@ -454,21 +453,17 @@ setup-tools:
         echo "  ✓ typos"
     fi
 
-    if ! have cargo-tarpaulin; then
-        if [[ "$os" == "Linux" ]]; then
-            echo "  ⏳ Installing cargo-tarpaulin (cargo)..."
-            cargo install --locked cargo-tarpaulin
-        else
-            echo "  ⚠️  Skipping cargo-tarpaulin install on $os (coverage is typically Linux-only)"
-        fi
+    if ! have cargo-llvm-cov; then
+        echo "  ⏳ Installing cargo-llvm-cov {{cargo_llvm_cov_version}} (cargo)..."
+        cargo install --locked cargo-llvm-cov --version {{cargo_llvm_cov_version}}
     else
-        echo "  ✓ cargo-tarpaulin"
+        echo "  ✓ cargo-llvm-cov"
     fi
 
     echo ""
     echo "Verifying required commands are available..."
     missing=0
-    for cmd in uv jq taplo yamllint shfmt shellcheck actionlint node npx typos git-cliff; do
+    for cmd in uv jq taplo yamllint shfmt shellcheck actionlint node npx typos git-cliff cargo-llvm-cov; do
         if have "$cmd"; then
             echo "  ✓ $cmd"
         else