Do not add block around async closure (rust-lang#3946)

Author: topecongiro

src/closures.rs

@@ -170,6 +170,7 @@ fn rewrite_closure_expr(
     fn allow_multi_line(expr: &ast::Expr) -> bool {
         match expr.kind {
             ast::ExprKind::Match(..)
+            | ast::ExprKind::Async(..)
             | ast::ExprKind::Block(..)
             | ast::ExprKind::TryBlock(..)
             | ast::ExprKind::Loop(..)

tests/target/async_closure.rs

@@ -15,4 +15,8 @@ fn main() {
         let x = 3;
         x
     };
+
+    let f = |x| async {
+        println!("hello, world");
+    };
 }