Rewrite the crate

.github/workflows/test.yml

@@ -41,3 +41,55 @@ jobs:
       # https://github.com/rust-lang/cargo/issues/6669
       - name: cargo test --doc
         run: cargo test --locked --all-features --doc
+  coverage:
+    # From https://github.com/jonhoo/rust-ci-conf/blob/main/.github/workflows/test.yml#L116
+    #
+    # use llvm-cov to build and collect coverage and outputs in a format that
+    # is compatible with codecov.io
+    #
+    # note that codecov as of v4 requires that CODECOV_TOKEN from
+    #
+    #   https://app.codecov.io/gh/<user or org>/<project>/settings
+    #
+    # is set in two places on your repo:
+    #
+    # - https://github.com/jonhoo/guardian/settings/secrets/actions
+    # - https://github.com/jonhoo/guardian/settings/secrets/dependabot
+    #
+    # (the former is needed for codecov uploads to work with Dependabot PRs)
+    #
+    # PRs coming from forks of your repo will not have access to the token, but
+    # for those, codecov allows uploading coverage reports without a token.
+    # it's all a little weird and inconvenient. see
+    #
+    #   https://github.com/codecov/feedback/issues/112
+    #
+    # for lots of more discussion
+    runs-on: ubuntu-latest
+    name: ubuntu / nightly / coverage
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: true
+      - name: Install nightly
+        uses: dtolnay/rust-toolchain@nightly
+        with:
+          toolchain: nightly
+          components: llvm-tools-preview
+      - name: cargo install cargo-llvm-cov
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-llvm-cov
+      - name: cargo generate-lockfile
+        if: hashFiles('Cargo.lock') == ''
+        run: cargo generate-lockfile
+      - name: cargo llvm-cov
+        run: cargo llvm-cov --locked --all-features --codecov --output-path lcov.info
+      - name: Record Rust version
+        run: echo "RUST=$(rustc --version)" >> "$GITHUB_ENV"
+      - name: Upload to codecov.io
+        uses: codecov/codecov-action@v5
+        with:
+          fail_ci_if_error: true
+          token: ${{ secrets.CODECOV_TOKEN }}
+          env_vars: OS,RUST