Crate webassembly_test[][src]

Expand description


This crate implements a cargo test support for wasm32-unknown-unknown target:

$ cat src/
mod tests {
    use webassembly_test::webassembly_test;

    fn it_works() {
        assert_eq!(2 + 2, 4);

    fn it_does_not_work() {
        assert_eq!(2 + 2, 5);

    fn it_is_ignored() {
        assert_eq!(2 + 2, 5);

$ cargo test --target wasm32-unknown-unknown
     Running `webassembly-test-runner target/wasm32-unknown-unknown/debug/deps/hello_world.wasm`

running 3 tests
test hello_world::tests::it_works ... ok
test hello_world::tests::it_does_not_work ... FAILED
test hello_world::tests::it_is_ignored ... ignored

test result: FAILED. 1 passed; 1 failed; 1 ignored;

webassembly-test is independent from any particular wasm runtime or environment. In fact, it is more of a pattern rather than a library, and can be easily adopted to a particular use-case.

MSRV: 1.54.0 (beta at the time of writing).

Writing Tests

When writing tests, use #[webassembly_test] rather than the usual #[test] macro. Add .cargo/config which sets a runner for wasm32-unknown-unknown:

runner = "webassembly-test-runner"

Now, just cargo test --target wasm32-unknown-unknown will run the tests.

webassembly-test-runner is an example of a runner. You can install it with cargo install webassembly-test-runner. It uses wasmtime to run the tests in an empty environment. If the tested library needs environment-specific imports you need to write the runner yourself.


Implementing Your Own Runner

The webassembly_test macro is simple. For each test, it emits a corresponding wasm export with a name in special format. For the example, the names would be:


That is, $webassembly-test$ prefix, followed by ignore$, followed by the qualified name of the test function.

The test runner than loads a wasm modules, and calls all export which match the given format given format.


Implementation notes

webassembly_test uses concat!(module_path!(), #name) to set #[export_name] for test functions. The #[ignore] is encoded into a function name. Ideally, we’d use a custom wasm section to store the ignored attributes, but it seems impossible to correctly associate a custom section entry with full function name (proc macro doesn’t have access to the module path of the function). Sample runner runs all the tests sequentially in a singe instance, but it should be possible to run tests in parallel in several instances.

Attribute Macros