remove method call receiver special casing in unused_async lint

tests/ui/unused_async.rs

@@ -55,6 +55,22 @@ mod issue9695 {
     }
 }
 
+mod issue13466 {
+    use std::future::Future;
+
+    struct Wrap<F>(F);
+    impl<F> From<F> for Wrap<F> {
+        fn from(f: F) -> Self {
+            Self(f)
+        }
+    }
+    fn takes_fut<F: Fn() -> Fut, Fut: Future>(_: Wrap<F>) {}
+    async fn unused_async() {}
+    fn fp() {
+        takes_fut(unused_async.into());
+    }
+}
+
 async fn foo() -> i32 {
     //~^ ERROR: unused `async` for function with no await statements
     4

tests/ui/unused_async.stderr

@@ -27,7 +27,7 @@ LL |     async fn f3() {}
    = help: consider removing the `async` from this function
 
 error: unused `async` for function with no await statements
-  --> tests/ui/unused_async.rs:58:1
+  --> tests/ui/unused_async.rs:74:1
    |
 LL | / async fn foo() -> i32 {
 LL | |
@@ -38,7 +38,7 @@ LL | | }
    = help: consider removing the `async` from this function
 
 error: unused `async` for function with no await statements
-  --> tests/ui/unused_async.rs:70:5
+  --> tests/ui/unused_async.rs:86:5
    |
 LL | /     async fn unused(&self) -> i32 {
 LL | |