Fix redundant_closure_call for single-expression async closures

Author: AaronC81

clippy_lints/src/redundant_closure_call.rs

@@ -85,7 +85,12 @@ impl EarlyLintPass for RedundantClosureCall {
                                 let mut hint =
                                     snippet_with_applicability(cx, block.span, "..", &mut app).into_owned();
                                 if r#async.is_async() {
-                                    hint = format!("async {}", hint);
+                                    if let ast::ExprKind::Block(_, _) = block.kind {
+                                        hint = format!("async {}", hint);
+                                    } else {
+                                        // `async x` is a syntax error, so it becomes `async { x }`
+                                        hint = format!("async {{ {} }}", hint);
+                                    }
                                 }
                                 diag.span_suggestion(expr.span, "try doing something like", hint, app);
                             }

tests/ui/redundant_closure_call_fixable.fixed

@@ -24,4 +24,5 @@ fn main() {
         let y = 2;
         x * y
     };
+    let d = async { something().await };
 }

tests/ui/redundant_closure_call_fixable.rs

@@ -24,4 +24,5 @@ fn main() {
         let y = 2;
         x * y
     })();
+    let d = (async || something().await)();
 }

tests/ui/redundant_closure_call_fixable.stderr

@@ -46,5 +46,11 @@ LL +         x * y
 LL ~     };
    |
 
-error: aborting due to 3 previous errors
+error: try not to call a closure in the expression where it is declared
+  --> $DIR/redundant_closure_call_fixable.rs:27:13
+   |
+LL |     let d = (async || something().await)();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try doing something like: `async { something().await }`
+
+error: aborting due to 4 previous errors