macro_rules! fn_expr {
($return_type:ty : $body:expr) => {
(|| -> $return_type { $body })()
};
($body:expr) => {
(|| $body)()
}
fn main() {
fn_expr!{ o?.when(|&i| i > 0)?.when(|&i| i%2 == 0) };
//~^ ERROR cannot find value `o` in this scope