Generating Coverage Data

You can ask the runner to generate coverage data from functions marked as #[wasm_bindgen_test] in the .profraw format.

Enabling the feature

To enable this feature, you need to enable cfg(wasm_bindgen_unstable_test_coverage).

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, we can't collect profiling symbols from the generated .wasm files. Instead, we can grab them from the LLVM IR with --emit=llvm-ir by using Clang. Usage of Clang or any LLVM tools must match the LLVM version used by Rust.

Arguments to the test runner

Like with Rust test coverage, you can use the LLVM_PROFILE_FILE environment variable to specify a path for the generated .profraw files.

Target features

This feature relies on the minicov crate, which provides a profiling runtime for WebAssembly. It in turn uses cc to compile the runtime to Wasm, which currently doesn't support accounting for target feature. Use e.g. CFLAGS_wasm32_unknown_unknown="-matomics -mbulk-memory" to account for that.

Example

Requires rust >= 1.87.0 and wasm-bindgen-test >= 0.3.57.

Install cargo-llvm-cov and run with:

CARGO_TARGET_WASM32_UNKNOWN_UNKNOWN_RUNNER=wasm-bindgen-test-runner \
CARGO_TARGET_WASM32_UNKNOWN_UNKNOWN_RUSTFLAGS="-Cinstrument-coverage -Zno-profiler-runtime -Clink-args=--no-gc-sections --cfg=wasm_bindgen_unstable_test_coverage" \
cargo +nightly llvm-cov test --target wasm32-unknown-unknown

Attribution

These methods have originally been pioneered by Hacken OÜ, see their guide as well.