[docs] update test coverage section, put up an example in CI

.github/workflows/coverage.yml

@@ -0,0 +1,39 @@
+on:
+  push:
+    branches: [ main, auto, canary ]
+  pull_request:
+    branches:
+      - main
+
+name: Test coverage
+
+jobs:
+  coverage:
+    name: Collect test coverage
+    runs-on: ubuntu-latest
+    # nightly rust might break from time to time
+    continue-on-error: true
+    env:
+      RUSTFLAGS: -D warnings
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          # By default actions/checkout checks out a merge commit. Check out the PR head instead.
+          # https://github.com/actions/checkout#checkout-pull-request-head-commit-instead-of-merge-commit
+          ref: ${{ github.event.pull_request.head.sha }}
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          override: true
+          components: llvm-tools-preview
+      - uses: Swatinem/rust-cache@c5ed9ba6b7e1bb8aff90d43acd2f0af4990fa57c
+
+      - name: Install latest nextest release
+        uses: taiki-e/install-action@nextest
+      - name: Install cargo-llvm-cov
+        uses: taiki-e/install-action@cargo-llvm-cov
+
+      - name: Collect coverage data
+        run: cargo llvm-cov nextest
+        # TODO: generate HTML report and upload it
+        # (requires https://github.com/taiki-e/cargo-llvm-cov/pull/144#issuecomment-1072736931 to be addressed)

site/src/SUMMARY.md

@@ -17,10 +17,10 @@
   - [Target runners](book/target-runners.md)
   - [Other options](book/other-options.md)
   - [Environment variables](book/env-vars.md)
-  - [Coverage](book/coverage.md)
 - [Machine-readable output](book/machine-readable.md)
 - [Configuration](book/configuration.md)
 - [JUnit support](book/junit.md)
+- [Test coverage](book/test-coverage.md)
 ---
 - [Stability policy](book/stability.md)
 - [Experimental features](book/experimental-features.md)

site/src/book/pre-built-binaries.md

@@ -84,6 +84,6 @@ To install a version series or specific version, use this instead:
 
 Install pre-built binaries on other CI systems by downloading and extracting the respective archives, using the commands above as a guide. See [Release URLs](release-urls.md) for more about how to specify nextest versions and platforms.
 
-> If you've made it easy to install nextest on another CI system, feel free to [submit a pull request].
+> If you've made it easy to install nextest on another CI system, feel free to [submit a pull request] with documentation.
 
 [submit a pull request]: https://github.com/nextest-rs/nextest/pulls

site/src/book/test-coverage.md

@@ -0,0 +1,29 @@
+# Test coverage
+
+Test coverage support is provided by third-party tools that wrap around nextest.
+
+## llvm-cov
+
+[cargo-llvm-cov](https://github.com/taiki-e/cargo-llvm-cov) supports nextest. To generate llvm-cov data with nextest, run:
+
+```
+cargo install cargo-llvm-cov
+cargo llvm-cov nextest
+```
+
+### Using llvm-cov in GitHub Actions
+
+Install both nextest and llvm-cov in GitHub Actions, then run `cargo llvm-cov nextest`.
+
+```yaml
+- uses: taiki-e/install-action@cargo-llvm-cov
+- uses: taiki-e/install-action@nextest
+```
+
+## Integrating nextest into coverage tools
+
+Most coverage tools work by setting a few environment variables such as `RUSTFLAGS` or `RUSTC_WRAPPER`. Nextest runs Cargo for the build, which will read those environment variables as usual. This means that it should generally be quite straightforward to integrate nextest into other coverage tools.
+
+> If you've integrated nextest into a coverage tool, feel free to [submit a pull request] with documentation.
+
+[submit a pull request]: https://github.com/nextest-rs/nextest/pulls