ci: test wasm32-wasi with wasmtime

Author: cuviper

.github/workflows/ci.yaml

@@ -65,16 +65,24 @@ jobs:
       - run: cargo test --verbose --package rayon
       - run: cargo test --verbose --package rayon-core
 
-  # wasm won't actually work without threading, but it builds
+  # wasm32-unknown-unknown builds, and even has the runtime fallback for
+  # unsupported threading, but we don't have an environment to execute in.
+  # wasm32-wasi can test the fallback by running in wasmtime.
   wasm:
     name: WebAssembly
     runs-on: ubuntu-latest
+    env:
+      CARGO_TARGET_WASM32_WASI_RUNNER: /home/runner/.wasmtime/bin/wasmtime
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@stable
         with:
-          target: wasm32-unknown-unknown
+          targets: wasm32-unknown-unknown,wasm32-wasi
       - run: cargo check --verbose --target wasm32-unknown-unknown
+      - run: cargo check --verbose --target wasm32-wasi
+      - run: curl https://wasmtime.dev/install.sh -sSf | bash
+      - run: cargo test --verbose --target wasm32-wasi --package rayon
+      - run: cargo test --verbose --target wasm32-wasi --package rayon-core
 
   fmt:
     name: Format