Prepend early return to force unsize coercion

src/expand.rs

@@ -350,6 +350,9 @@ fn transform_block(sig: &mut Signature, block: &mut Block) {
             let _: () = { #(#decls)* #(#stmts)* };
         },
         ReturnType::Type(_, ret) => quote_spanned! {block.brace_token.span=>
+            if let ::core::option::Option::Some(__ret) = ::core::option::Option::None::<#ret> {
+                return __ret;
+            }
             let __ret: #ret = { #(#decls)* #(#stmts)* };
             #[allow(unreachable_code)]
             __ret

tests/test.rs

@@ -1258,3 +1258,23 @@ pub mod issue147 {
         }
     }
 }
+
+// https://github.com/dtolnay/async-trait/issues/149
+pub mod issue149 {
+    use async_trait::async_trait;
+
+    pub struct Thing;
+    pub trait Ret {}
+    impl Ret for Thing {}
+
+    pub async fn ok() -> &'static dyn Ret {
+        return &Thing;
+    }
+
+    #[async_trait]
+    pub trait Trait {
+        async fn fail() -> &'static dyn Ret {
+            return &Thing;
+        }
+    }
+}