Add test for box Struct{ .. } pattern

Author: ecstatic-morse

crates/ra_parser/src/grammar/expressions/atom.rs

@@ -416,6 +416,7 @@ pub(crate) fn match_arm_list(p: &mut Parser) {
 //         | X => (),
 //         box X => (),
 //         Some(box X) => (),
+//         Some(box Test{field: 0}) => (),
 //     };
 // }
 fn match_arm(p: &mut Parser) -> BlockLike {