How to handle async-traits with external types

fixtures/ext-types/proc-macro-lib/Cargo.toml

@@ -18,6 +18,7 @@ name = "uniffi_ext_types_proc_macro_lib"
 
 [dependencies]
 anyhow = "1"
+async-trait = "0.1"
 bytes = "1.3"
 uniffi = { workspace = true }
 

fixtures/ext-types/proc-macro-lib/src/lib.rs

@@ -2,7 +2,8 @@ use custom_types::Handle;
 use ext_types_custom::{Guid, Ouid2};
 use std::sync::Arc;
 use uniffi_one::{
-    UniffiOneEnum, UniffiOneInterface, UniffiOneProcMacroType, UniffiOneTrait, UniffiOneType,
+    UniffiOneAsyncTrait, UniffiOneEnum, UniffiOneInterface, UniffiOneProcMacroType, UniffiOneTrait,
+    UniffiOneTraitError, UniffiOneType,
 };
 use url::Url;
 
@@ -157,6 +158,38 @@ fn get_uniffi_one_trait(t: Option<Arc<dyn UniffiOneTrait>>) -> Option<Arc<dyn Un
     t
 }
 
+#[derive(uniffi::Object)]
+pub struct UniffiOneTraitImpl;
+
+#[uniffi::export]
+impl UniffiOneTrait for UniffiOneTraitImpl {
+    fn hello(&self) -> String {
+        "uniffi-one-trait-impl".to_string()
+    }
+}
+
+#[derive(uniffi::Object)]
+pub struct UniffiOneAsyncTraitImpl;
+
+#[uniffi::export]
+#[async_trait::async_trait]
+impl UniffiOneAsyncTrait for UniffiOneAsyncTraitImpl {
+    async fn hello(&self) -> String {
+        "uniffi-one-trait-impl".to_string()
+    }
+
+    async fn try_hello(&self) -> Result<String, UniffiOneTraitError> {
+        Ok("uniffi-one-trait-impl".to_string())
+    }
+}
+
+#[uniffi::export]
+fn get_uniffi_one_async_trait(
+    t: Option<Arc<dyn UniffiOneAsyncTrait>>,
+) -> Option<Arc<dyn UniffiOneAsyncTrait>> {
+    t
+}
+
 // Some custom types via macros.
 // Another guid - here we use a regular struct.
 pub struct Uuid {

fixtures/ext-types/proc-macro-lib/tests/bindings/test_imported_types.py

@@ -49,6 +49,11 @@ async def test():
             self.assertEqual(await get_uniffi_one_async(), UniffiOneEnum.ONE)
             # This async function comes from the `proc-macro-lib` crate
             self.assertEqual(t1, await get_uniffi_one_type_async(t1))
+
+            t2 = UniffiOneTraitImpl()
+            self.assertEqual(await t2.hello(), "uniffi-one-trait-impl")
+            self.assertEqual(await t2.try_hello(), "uniffi-one-trait-impl")
+
         asyncio.run(test())
 
     def test_get_uniffi_one_proc_macro_type(self):

fixtures/ext-types/uniffi-one/Cargo.toml

@@ -12,6 +12,7 @@ name = "uniffi_one"
 
 [dependencies]
 anyhow = "1"
+async-trait = "0.1"
 bytes = "1.3"
 thiserror = "1"
 uniffi = { workspace = true }

fixtures/ext-types/uniffi-one/src/lib.rs

@@ -44,6 +44,25 @@ pub trait UniffiOneTrait: Send + Sync {
     fn hello(&self) -> String;
 }
 
+#[derive(Debug, thiserror::Error, uniffi::Error)]
+pub enum UniffiOneTraitError {
+    #[error("Callback failed")]
+    Error,
+}
+
+#[uniffi::export(with_foreign)]
+#[async_trait::async_trait]
+pub trait UniffiOneAsyncTrait: Send + Sync {
+    async fn hello(&self) -> String;
+    async fn try_hello(&self) -> Result<String, UniffiOneTraitError>;
+}
+
+// We *must* have this so support is generated. We should fix that and remove this. See #2423.
+#[uniffi::export]
+fn _just_to_get_trait_support() -> std::sync::Arc<dyn UniffiOneAsyncTrait> {
+    panic!()
+}
+
 // A couple of errors used as external types.
 #[derive(thiserror::Error, uniffi::Error, Debug)]
 pub enum UniffiOneError {