wasm: initial support for wasm32-wasi target (#4716)

This adds initial, unstable, support for the wasm32-wasi target. Not all of Tokio's
features are supported yet as WASI's non-blocking APIs are still limited.

Refs: #4827

Author: rjzak

.github/workflows/ci.yml

@@ -489,9 +489,23 @@ jobs:
       - name: Install cargo-wasi
         run: cargo install cargo-wasi
 
-      # TODO: Expand this when full WASI support lands.
-      #   Currently, this is a bare bones regression test
-      #   for features that work today with wasi.
+      - name: WASI test tokio full
+        run: cargo test -p tokio --target wasm32-wasi --features full
+        env:
+          CARGO_TARGET_WASM32_WASI_RUNNER: "wasmtime run --"
+          RUSTFLAGS: --cfg tokio_unstable -Dwarnings
+
+      - name: WASI test tokio-util full
+        run: cargo test -p tokio-util --target wasm32-wasi --features full
+        env:
+          CARGO_TARGET_WASM32_WASI_RUNNER: "wasmtime run --"
+          RUSTFLAGS: --cfg tokio_unstable -Dwarnings
+
+      - name: WASI test tokio-stream
+        run: cargo test -p tokio-stream --target wasm32-wasi --features time,net,io-util,sync
+        env:
+          CARGO_TARGET_WASM32_WASI_RUNNER: "wasmtime run --"
+          RUSTFLAGS: --cfg tokio_unstable -Dwarnings
 
       - name: test tests-integration --features wasi-rt
         # TODO: this should become: `cargo hack wasi test --each-feature`

tests-integration/tests/macros_main.rs

@@ -1,4 +1,8 @@
-#![cfg(all(feature = "macros", feature = "rt-multi-thread"))]
+#![cfg(all(
+    feature = "macros",
+    feature = "rt-multi-thread",
+    not(target_os = "wasi")
+))]
 
 #[tokio::main]
 async fn basic_main() -> usize {