I was initially going to file an issue about the reference not making it clear this is not allowed, but a quick test on the playground shows this actually compiles with a warning about impl Future
not being ffi-safe.
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async fn
has an opaque return type, so it shouldn't be callable by FFI even with extern "Rust"
. And there correctly are errors issued if you try to declare async fn
or fn -> impl Future
in an extern
block.
#[no_mangle]
async extern "Rust" fn hello_async() { // Allowed without warnings
println!("Hello world");
}
extern "Rust" {
async fn hello_async(); // functions in `extern` blocks cannot have qualifiers
fn hello_async() -> impl Future; // [E0562]: `impl Trait` is not allowed in `extern fn` return types
}
I don't think it's a bug as such that you can request a C-like calling convention for an async function. It's just not very useful.
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