add #4546 test

Author: lengyijun

tests/ui/wrong_self_convention2.rs

@@ -87,3 +87,12 @@ mod issue3414 {
         }
     }
 }
+
+mod issue4546 {
+    use std::pin::Pin;
+
+    struct S;
+    impl S {
+        pub fn as_mut(self: Pin<&mut Self>) {}
+    }
+}