Move single-threaded annotation for callback-interface and traits

Author: jhugman

uniffi_macros/src/export/callback_interface.rs

@@ -7,7 +7,8 @@ use crate::{
     ffiops,
     fnsig::{FnKind, FnSignature, ReceiverArg},
     util::{
-        create_metadata_items, derive_ffi_traits, ident_to_string, mod_path, tagged_impl_header,
+        async_trait_annotation, create_metadata_items, derive_ffi_traits, ident_to_string,
+        mod_path, tagged_impl_header, wasm_single_threaded_annotation,
     },
 };
 use proc_macro2::{Span, TokenStream};
@@ -77,7 +78,11 @@ pub(super) fn trait_impl(
         .map(|sig| gen_method_impl(sig, &vtable_cell))
         .collect::<syn::Result<Vec<_>>>()?;
     let has_async_method = methods.iter().any(|m| m.is_async);
-    let impl_attributes = has_async_method.then(|| quote! { #[::async_trait::async_trait] });
+
+    // Conditionally apply the async_trait attribute with or without ?Send based on the target
+    let impl_attributes = has_async_method.then(async_trait_annotation);
+
+    let single_threaded_annotation = wasm_single_threaded_annotation();
 
     Ok(quote! {
         #[allow(missing_docs)]
@@ -105,6 +110,7 @@ pub(super) fn trait_impl(
             }
         }
 
+        #single_threaded_annotation
         ::uniffi::deps::static_assertions::assert_impl_all!(#trait_impl_ident: ::core::marker::Send);
 
         #impl_attributes

uniffi_macros/src/export/trait_interface.rs

@@ -13,7 +13,7 @@ use crate::{
     },
     ffiops,
     object::interface_meta_static_var,
-    util::{ident_to_string, tagged_impl_header},
+    util::{ident_to_string, tagged_impl_header, wasm_single_threaded_annotation},
 };
 
 pub(super) fn gen_trait_scaffolding(
@@ -155,12 +155,14 @@ pub(crate) fn ffi_converter(
     };
     let lower_self = ffiops::lower(quote! { ::std::sync::Arc<Self> });
     let try_lift_self = ffiops::try_lift(quote! { ::std::sync::Arc<Self> });
+    let single_threaded_annotation = wasm_single_threaded_annotation();
 
     quote! {
         // All traits must be `Sync + Send`. The generated scaffolding will fail to compile
         // if they are not, but unfortunately it fails with an unactionably obscure error message.
         // By asserting the requirement explicitly, we help Rust produce a more scrutable error message
         // and thus help the user debug why the requirement isn't being met.
+        #single_threaded_annotation
         ::uniffi::deps::static_assertions::assert_impl_all!(
             dyn #trait_ident: ::core::marker::Sync, ::core::marker::Send
         );

uniffi_macros/src/object.rs

@@ -4,7 +4,10 @@ use syn::DeriveInput;
 
 use crate::{
     ffiops,
-    util::{create_metadata_items, extract_docstring, ident_to_string, mod_path},
+    util::{
+        create_metadata_items, extract_docstring, ident_to_string, mod_path,
+        wasm_single_threaded_annotation,
+    },
     DeriveOptions,
 };
 use uniffi_meta::ObjectImpl;
@@ -96,19 +99,6 @@ pub fn expand_object(input: DeriveInput, options: DeriveOptions) -> syn::Result<
     })
 }
 
-fn wasm_single_threaded_annotation() -> TokenStream {
-    #[cfg(feature = "wasm-unstable-single-threaded")]
-    {
-        quote! {
-            #[cfg(not(target_arch = "wasm32"))]
-        }
-    }
-    #[cfg(not(feature = "wasm-unstable-single-threaded"))]
-    {
-        TokenStream::default()
-    }
-}
-
 fn interface_impl(object: &ObjectItem, options: &DeriveOptions) -> TokenStream {
     let name = object.name();
     let ident = object.ident();

uniffi_macros/src/util.rs

@@ -302,3 +302,32 @@ pub(crate) fn extract_docstring(attrs: &[Attribute]) -> syn::Result<String> {
         .collect::<syn::Result<Vec<_>>>()
         .map(|lines| lines.join("\n"))
 }
+
+pub(crate) fn wasm_single_threaded_annotation() -> TokenStream {
+    #[cfg(feature = "wasm-unstable-single-threaded")]
+    {
+        quote! {
+            #[cfg(not(target_arch = "wasm32"))]
+        }
+    }
+    #[cfg(not(feature = "wasm-unstable-single-threaded"))]
+    {
+        TokenStream::default()
+    }
+}
+
+pub(crate) fn async_trait_annotation() -> TokenStream {
+    #[cfg(feature = "wasm-unstable-single-threaded")]
+    {
+        quote! {
+            #[cfg_attr(not(target_arch = "wasm32"), ::async_trait::async_trait)]
+            #[cfg_attr(target_arch = "wasm32", ::async_trait::async_trait(?Send))]
+        }
+    }
+    #[cfg(not(feature = "wasm-unstable-single-threaded"))]
+    {
+        quote! {
+            #[::async_trait::async_trait]
+        }
+    }
+}