wiggle::async_trait is defined as async_trait::async_trait(?Send)

async methods used by wiggle currently need to Not have the Send
constraint, so rather than make all use sites pass the argument
to the re-exported async_trait macro, define a new macro that
applies the argument.

crates/wiggle/macro/src/lib.rs

@@ -83,11 +83,12 @@ use syn::parse_macro_input;
 ///
 /// /// The above witx text contains one module called `$example`. So, we must
 /// /// implement this one method trait for our ctx type.
-/// #[wiggle::async_trait(?Send)]
+/// #[wiggle::async_trait]
 /// /// We specified in the `async_` field that `example::double_int_return_float`
 /// /// is an asynchronous method. Therefore, we use the `async_trait` proc macro
-/// /// (re-exported by wiggle from the crate of the same name) to define this
-/// /// trait, so that `double_int_return_float` can be an `async fn`.
+/// /// to define this trait, so that `double_int_return_float` can be an `async fn`.
+/// /// `wiggle::async_trait` is defined as `#[async_trait::async_trait(?Send)]` -
+/// /// in wiggle, async methods do not have the Send constaint.
 /// impl example::Example for YourCtxType {
 ///     /// The arrays module has two methods, shown here.
 ///     /// Note that the `GuestPtr` type comes from `wiggle`,
@@ -156,3 +157,13 @@ pub fn from_witx(args: TokenStream) -> TokenStream {
 
     TokenStream::from(quote! { #code #metadata })
 }
+
+#[proc_macro_attribute]
+pub fn async_trait(attr: TokenStream, item: TokenStream) -> TokenStream {
+    let _ = parse_macro_input!(attr as syn::parse::Nothing);
+    let item = proc_macro2::TokenStream::from(item);
+    TokenStream::from(quote! {
+        #[wiggle::async_trait_crate::async_trait(?Send)]
+        #item
+    })
+}