Add test for #5787

Author: Alexendoo

tests/ui/needless_lifetimes.rs

@@ -495,4 +495,21 @@ mod pr_9743_output_lifetime_checks {
     }
 }
 
+mod issue5787 {
+    use std::sync::MutexGuard;
+
+    struct Foo;
+
+    impl Foo {
+        // doesn't get linted without async
+        pub async fn wait<'a, T>(&self, guard: MutexGuard<'a, T>) -> MutexGuard<'a, T> {
+            guard
+        }
+    }
+
+    async fn foo<'a>(_x: &i32, y: &'a str) -> &'a str {
+        y
+    }
+}
+
 fn main() {}