Document coverage in the guide

Author: aDogCalledSpot

guide/src/SUMMARY.md

@@ -110,6 +110,7 @@
   - [Writing Asynchronous Tests](./wasm-bindgen-test/asynchronous-tests.md)
   - [Testing in Headless Browsers](./wasm-bindgen-test/browsers.md)
   - [Continuous Integration](./wasm-bindgen-test/continuous-integration.md)
+  - [Coverage (Experimental)](./wasm-bindgen-test/coverage.md)
 
 - [Contributing to `wasm-bindgen`](./contributing/index.md)
   - [Testing](./contributing/testing.md)

guide/src/wasm-bindgen-test/coverage.md

@@ -0,0 +1,45 @@
+# Generating Coverage Data
+
+You can ask the runner to generate coverage data from functions marked as `#[wasm_bindgen_test]` in the `.profraw` format.
+
+<div class="warning">
+  Coverage is still in an experimental state and may be unreliable and could experience
+  breaking changes at any time.
+</div>
+
+## Enabling the feature
+
+To enable this feature, you need to enable the `"unstable-coverage"` feature in your `wasm-bindgen-test` dependency.
+
+## Generating the data
+
+### `RUSTFLAGS` that need to be present
+
+Make sure you are using `RUSTFLAGS=-Cinstrument-coverage -Zno-profiler-runtime`.
+
+Due to the current limitation of `llvm-cov`, you can't get debug information out of a `.wasm`.
+Instead, [we can grab them from the LLVM IR][wasmcov], so consider adding `--emit=llvm-ir` as well.
+
+[wasmcov]: https://github.com/hknio/code-coverage-for-webassembly
+
+### Arguments to the test runner
+
+If you are using `wasm-pack` to run the tests, refer to the `wasm-pack` documentation.
+
+Otherwise, you can use the following environment variables when [executing the test runner][1] to control the coverage output:
+
+[1]: usage.html#appendix-using-wasm-bindgen-test-without-wasm-pack
+
+- `WASM_BINDGEN_UNSTABLE_TEST_COVERAGE` to generate a single `.profraw` in your current working directory.
+- `WASM_BINDGEN_UNSTABLE_TEST_PROFRAW_OUT` to control the file name of the profraw or the directory in which it is placed
+- `WASM_BINDGEN_UNSTABLE_TEST_PROFRAW_PREFIX` to add a custom prefix to the profraw files. This can be useful if you're running the tests automatically in succession.
+
+### Example
+
+```sh
+RUSTFLAGS="-Cinstrument-coverage -Zno-profiler-runtime --emit=llvm-ir" wasm-pack test --coverage --profraw-out cov_data/
+# Generate the debug info on the host
+clang target/wasm32-unknown-unknown/debug/deps/{your-dot-wasm-without-extension}.ll -Wno-override-module -c -o wasm.o
+llvm-profdata merge --sparse cov_data/*.profraw -o cov_data/coverage.profdata
+llvm-cov --instr-profile=cov_data/coverage.profdata wasm.o --format=html --output-dir=coverage/ --sources .
+```