老师们，下面这两种写法的区别是啥啊，为啥fn字段的生命周期标注是可省略的呢？

struct Struct01<'a> {
    field_a: Option<&'a ()>,
    field_b: fn(&()),
}

struct Struct02<'a, 'b> {
    field_a: Option<&'a ()>,
    field_b: fn(&'b ()),
}