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Write WebAssembly (@wasm) tests ​

@wasm marks a test for cross-compilation to wasm32. Such tests run normally under dub test; --wasm additionally extracts them, compiles them with LDC for wasm32-unknown-unknown-wasm (-betterC, no druntime), and runs them with the first available WebAssembly-capable runtime:

console
$ dub test :test-runner -- --wasm
 ✓ attributes.selfContained.wasm [libs/test-runner/src/sparkles/test_runner/attributes.d:71]
1 @wasm tests passed

Each test is exported individually (run_test_<i>); a failed assert traps (bare wasm32 has no libc to print with), which the host reports as a per-test RuntimeError.

Toolchain requirements ​

  • LDC (--compiler/$DC must be an ldc; dmd has no wasm backend).
  • wasm-ld on PATH (nixpkgs: the lld package) — nixpkgs LDC does not bundle an internal linker.
  • A runtime: node, deno, or bun (a generated JS shim instantiates the module and reports per-test results), or wasmtime (--invoke run_test_<i> per test).

Missing pieces are reported as a skip, not a failure.

Constraints ​

Everything from @betterC extraction applies (public symbols only, templates/CTFE-able code by default, --include-import opt-ins), plus one more with a stock LDC:

WARNING

The tested module's whole import chain must be wasm-compatible. Bare wasm32 druntime headers static assert for many core.stdc.* modules, so any module (transitively) importing e.g. core.time fails to compile. Keep @wasm tests in modules with wasm-clean imports — or point --compiler at a wasm-enabled LDC build with full druntime/Phobos ported (e.g. the dlang.nix ldc-wasm toolchain), which lifts the restriction.