Add regression test for issue 281

Without #![feature(impl_trait_in_bindings)]:

    error[E0562]: `impl Trait` is not allowed in paths
        --> tests/test.rs:1680:42
         |
    1680 |         async fn method(&self) -> Result<impl AsRef<str> + Send + Sync, Self::Error> {
         |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         |
         = note: `impl Trait` is only allowed in arguments and return types of functions and methods

    error[E0562]: `impl Trait` is not allowed in the type of variable bindings
        --> tests/test.rs:1680:42
         |
    1680 |         async fn method(&self) -> Result<impl AsRef<str> + Send + Sync, Self::Error> {
         |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         |
         = note: `impl Trait` is only allowed in arguments and return types of functions and methods
         = note: see issue #63065 <rust-lang/rust#63065> for more information
         = help: add `#![feature(impl_trait_in_bindings)]` to the crate attributes to enable
         = note: this compiler was built on 2025-01-02; consider upgrading it if it is out of date

With #![feature(impl_trait_in_bindings)]:

    error[E0562]: `impl Trait` is not allowed in paths
        --> tests/test.rs:1680:42
         |
    1680 |         async fn method(&self) -> Result<impl AsRef<str> + Send + Sync, Self::Error> {
         |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         |
         = note: `impl Trait` is only allowed in arguments and return types of functions and methods

Author: dtolnay

tests/test.rs

@@ -1661,3 +1661,24 @@ pub mod issue277 {
 
     fn g(_: &mut &()) {}
 }
+
+// https://github.com/dtolnay/async-trait/issues/281
+pub mod issue281 {
+    use async_trait::async_trait;
+
+    #[async_trait]
+    pub trait Trait {
+        type Error;
+        async fn method(&self) -> Result<impl AsRef<str> + Send + Sync, Self::Error>;
+    }
+
+    pub struct T;
+
+    #[async_trait]
+    impl Trait for T {
+        type Error = ();
+        async fn method(&self) -> Result<impl AsRef<str> + Send + Sync, Self::Error> {
+            Ok("Hello World")
+        }
+    }
+}