feat(rust): async-spawn - build under no_std + alloc (#1600)

* async-spawn: build under no_std + alloc

* ci: guard the no_std contract for async + async-spawn with wasm32v1-none

* fmt: reorder imports in spawn.rs to satisfy rustfmt

Author: mattwilkinsonn

.github/workflows/main.yml

@@ -199,6 +199,8 @@ jobs:
       run: rustup target add wasm32-unknown-unknown
     - name: Install wasm32-wasip1 target
       run: rustup target add wasm32-wasip1
+    - name: Install wasm32v1-none target
+      run: rustup target add wasm32v1-none
 
     # Verify the output of the `./ci/rebuild-libwit-bindgen-cabi.sh` script is
     # up-to-date.
@@ -227,6 +229,10 @@ jobs:
     - run: cargo build --target wasm32-wasip1 -p wit-bindgen --no-default-features --features inter-task-wakeup
     - run: cargo build --target wasm32-wasip1 -p wit-bindgen --no-default-features --features futures-stream
 
+    # wasm32v1-none has no std, so a stray std::* usage fails to link here.
+    - run: cargo build --target wasm32v1-none -p wit-bindgen --no-default-features --features async
+    - run: cargo build --target wasm32v1-none -p wit-bindgen --no-default-features --features async-spawn
+
     # Verity that documentation can be generated for the rust bindings crate.
     - run: rustup update nightly --no-self-update
     - run: cargo +nightly doc -p wit-bindgen --no-deps

crates/guest-rust/Cargo.toml

@@ -27,7 +27,7 @@ all-features = true
 [dependencies]
 wit-bindgen-rust-macro = { path = "./macro", optional = true, default-features = false, version = "0.57.1" }
 bitflags = { workspace = true, optional = true }
-futures = { version = "0.3.30", optional = true }
+futures = { version = "0.3.30", optional = true, default-features = false, features = ["alloc"] }
 
 # When built as part of libstd
 core = { version = "1.0", optional = true, package = "rustc-std-workspace-core" }
@@ -40,7 +40,7 @@ realloc = []
 std = []
 async = []
 bitflags = ["dep:bitflags"]
-async-spawn = ['async', 'dep:futures', 'std']
+async-spawn = ['async', 'dep:futures']
 futures-stream = ['async', 'dep:futures']
 macro-string = ["wit-bindgen-rust-macro?/macro-string"]
 

crates/guest-rust/src/rt/async_support/spawn.rs

@@ -4,11 +4,11 @@
 #![allow(static_mut_refs)]
 
 use crate::rt::async_support::BoxFuture;
+use alloc::boxed::Box;
+use alloc::vec::Vec;
+use core::future::Future;
+use core::task::{Context, Poll};
 use futures::stream::{FuturesUnordered, StreamExt};
-use std::boxed::Box;
-use std::future::Future;
-use std::task::{Context, Poll};
-use std::vec::Vec;
 
 /// Any newly-deferred work queued by calls to the `spawn` function while
 /// polling the current task.