Avoid unhelpful compiler suggestion (#1251)

axum-macros/tests/debug_handler/fail/generics.rs

@@ -1,6 +1,6 @@
 use axum_macros::debug_handler;
 
 #[debug_handler]
-async fn handler<T>() {}
+async fn handler<T>(extract: T) {}
 
 fn main() {}

axum-macros/tests/debug_handler/fail/generics.stderr

@@ -1,13 +1,5 @@
 error: `#[axum_macros::debug_handler]` doesn't support generic functions
  --> tests/debug_handler/fail/generics.rs:4:17
   |
-4 | async fn handler<T>() {}
+4 | async fn handler<T>(extract: T) {}
   |                 ^^^
-
-error[E0282]: type annotations needed
- --> tests/debug_handler/fail/generics.rs:4:10
-  |
-4 | async fn handler<T>() {}
-  | -----    ^^^^^^^ cannot infer type for type parameter `T` declared on the function `handler`
-  | |
-  | consider giving `future` a type