I have a function that uses a closure as a parameter. My understanding is that FnMut should be used here, but it doesn’t work. However, it works when I change it to Fn. I hope someone can explain my mistake.

#![allow(unused)]

struct Data {
    a: u32,
    b: u64,
}

fn mark_label<F>(data: &mut Data, mark: F)
    where F: FnMut(&mut Data)  //if I change this line to F: Fn(&mut Data), it can work.
{
    mark(data)
}

fn main() {
    let mut data = Data {a: 1, b: 2};
    mark_label(&mut data, |x| x.a = 2);
    mark_label(&mut data, |x| x.b = 3);
}