Add regression test for issue 238

Currently fails:

    error: lifetime parameter `'impl0` only used once
        --> tests/test.rs:1603:20
         |
    1603 |     impl Trait for &Struct {
         |                    ^
         |                    |
         |                    this lifetime...
         |                    ...is used only here
         |
    note: the lint level is defined here
        --> tests/test.rs:1591:13
         |
    1591 |     #![deny(single_use_lifetimes)]
         |             ^^^^^^^^^^^^^^^^^^^^
    help: elide the single-use lifetime
         |
    1602 ~
    1603 ~     impl Trait for Struct {
         |

tests/test.rs

@@ -1585,3 +1585,22 @@ pub mod issue236 {
         }
     }
 }
+
+// https://github.com/dtolnay/async-trait/issues/238
+pub mod issue238 {
+    #![deny(single_use_lifetimes)]
+
+    use async_trait::async_trait;
+
+    #[async_trait]
+    pub trait Trait {
+        async fn f();
+    }
+
+    pub struct Struct;
+
+    #[async_trait]
+    impl Trait for &Struct {
+        async fn f() {}
+    }
+}