Add regression test for issue 277

    warning: the type of the `self` parameter does not need to be arbitrary
        --> tests/test.rs:1656:24
         |
    1656 |         async fn f(mut self: &Self) {
         |                        ^^^^^^^^^^^ help: consider to change this parameter to: `&'life0 self`
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_arbitrary_self_type
         = note: `#[warn(clippy::needless_arbitrary_self_type)]` on by default

tests/test.rs

@@ -1641,3 +1641,22 @@ pub mod issue266 {
         }
     }
 }
+
+// https://github.com/dtolnay/async-trait/issues/277
+pub mod issue277 {
+    use async_trait::async_trait;
+
+    #[async_trait]
+    pub trait Trait {
+        async fn f(&self);
+    }
+
+    #[async_trait]
+    impl Trait for () {
+        async fn f(mut self: &Self) {
+            g(&mut self);
+        }
+    }
+
+    fn g(_: &mut &()) {}
+}