async functions do not compile correctly for WASM

[naskya]

I tried this code:

// src/lib.rs
#[unsafe(no_mangle)]
pub async fn foo() {}

#[unsafe(no_mangle)]
pub async fn bar(_: i32) {}

# Cargo.toml
[package]
name = "test"
version = "0.1.0"
edition = "2024"

[lib]
crate-type = ["cdylib"]

I expected to see this happen: foo returns no results and bar takes only one argument (or it fails to compile if async functions are not supported)

Instead, this happened: When compiled for wasm32-unknown-unknown, wasm32-wasip1 and wasm32-wasip2, it doesn’t hold

$ cargo +nightly build --release --target wasm32-unknown-unknown
$ wasm-tools print target/wasm32-unknown-unknown/release/test.wasm

(module $test.wasm
  (type (;0;) (func (result i32)))
  (type (;1;) (func (param i32 i32)))
  (table (;0;) 1 1 funcref)
  (memory (;0;) 16)
  (global $__stack_pointer (;0;) (mut i32) i32.const 1048576)
  (global (;1;) i32 i32.const 1048576)
  (global (;2;) i32 i32.const 1048576)
  (export "memory" (memory 0))
  (export "foo" (func $foo))
  (export "bar" (func $bar))
  (export "__data_end" (global 1))
  (export "__heap_base" (global 2))
  (func $foo (;0;) (type 0) (result i32)
    i32.const 0
  )
  (func $bar (;1;) (type 1) (param i32 i32)
    local.get 0
    i32.const 0
    i32.store8 offset=4
    local.get 0
    local.get 1
    i32.store
  )
  (@producers
    (language "Rust" "")
    (processed-by "rustc" "1.88.0-nightly (092a284ba 2025-04-13)")
  )
  (@custom "target_features" (after code) "\08+\0bbulk-memory+\0fbulk-memory-opt+\16call-indirect-overlong+\0amultivalue+\0fmutable-globals+\13nontrapping-fptoint+\0freference-types+\08sign-ext")
)

Notice the func $foo (;0;) (type 0) (result i32) and func $bar (;1;) (type 1) (param i32 i32).

Meta

rustc --version --verbose:

rustc 1.88.0-nightly (092a284ba 2025-04-13)
binary: rustc
commit-hash: 092a284ba0421695f2032c947765429fd7095796
commit-date: 2025-04-13
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.2

@rustbot label +O-wasm +O-wasi

[saethlin]

async fn is a function that returns a generator. They are fundamentally different from normal fn.

So the codegen here looks fine to me.

[bjorn3]

If you want to export a function to non-Rust code, you have to use extern "C". And if you do, rustc will give you the following warning for async functions:

warning: `extern` fn uses type `impl Future<Output = ()>`, which is not FFI-safe
 --> src/lib.rs:2:1
  |
2 | pub async extern "C" fn foo() {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
  |
  = note: opaque types have no C equivalent
  = note: `#[warn(improper_ctypes_definitions)]` on by default