let current_column = 1;
let max_column = 7;
edge = match current_column {
    0                    => Edge::Left,
    a if a == max_column => Edge::Right,
    _                    => Edge::NotAnEdge
};

上面的代码来自Why is this match pattern unreachable when using non-literal patterns?

这里为什么能将if 写入到 match 的 列举参数中