Rust `Box` 是否掩盖了特征界限在这种情况下，为什么我可以将 `dyn FnOnce()` 分配给 `dyn Fn()`

我想要一个 struct ，Quox，它可以有 Select 地有每种类型的闭包，可以有(Fn，FnMut和FnOnce).我是这样打的:

#[derive(Default, Debug)]
pub struct Quox<Tfn: Fn(&Person) + Sized, Tfnmut: FnMut(&mut Person) + Sized, Tfnonce: FnOnce(&Person) + Sized> {
    soft_closure: Option<Tfn>,
    mut_closure : Option<Tfnmut>,
    move_closure: Option<Tfnonce>,
}

现在，当实例化它时，我通过Box来实现它，因为 rust 蚀投诉the trait Sized is not implemented for `dyn Fn()`rustcE0277.(也许有更好的方法，或者我误解了特征对象？)

pub fn main(){
    let mut qx = Quox::<Box<dyn Fn(&Person)>, Box<dyn FnMut(&mut Person)>, Box<dyn FnOnce(&Person)>> {
        move_closure: None,
        mut_closure : None,
        soft_closure: None,
    };
    ...

}

1. 真正让我困惑的是，当我创建相应类型的闭包时——为什么我可以随意地将它们赋给qx中的字段:

pub fn main(){
    let mut qx = Quox::<Box<dyn Fn(&Person)>, Box<dyn FnMut(&mut Person)>, Box<dyn FnOnce(&Person)>> {
        move_closure: None,
        mut_closure : None,
        soft_closure: None,
    };


    let greeting  = "Hi. We are moving soon!".to_string();
    let clsr_move = move |p:&Person| println!("Hi, {}. {} says: {}", p.name, p.name, greeting);
    let only_a_ref_clsr = |p:&Person| println!("Hi, {}.",p.name );

 
    // qx.move_closure = Some(Box::new(clsr_move));        // expected
    // qx.soft_closure = Some(Box::new(only_a_ref_clsr));  // expected

    qx.soft_closure = Some(Box::new(clsr_move));           // Why is this valid???
    qx.move_closure = Some(Box::new(only_a_ref_clsr));     // Ok Fn is supertrait to FnOnce

}

同样，令人困惑的是，我try 将(似乎)FnMut分配到Fn并没有出错.

2. 此外为什么现在将Box赋给中间变量会引发错误？？？

pub fn main(){
    let mut qx = Quox::<Box<dyn Fn(&Person)>, Box<dyn FnMut(&mut Person)>, Box<dyn FnOnce(&Person)>> {
        move_closure: None,
        mut_closure : None,
        soft_closure: None,
    };


    let greeting  = "Hi. We are moving soon!".to_string();
    let clsr_move = move |p:&Person| println!("Hi, {}. {} says: {}", p.name, p.name, greeting);
    let only_a_ref_clsr = |p:&Person| println!("Hi, {}.",p.name );

    let someboxmove = Some(Box::new(only_a_ref_clsr));
    qx.soft_closure = someboxmove;                         // Invalid. 

}

我得到的错误是:

let someboxmove: Option<Box<|&Person| -> ()>>
Go to Option | Box | Person | Global

mismatched types
expected enum `Option<Box<dyn for<'r> Fn(&'r traitobjects::Person)>>`
   found enum `Option<Box<[closure@src/traitobjects.rs:49:27: 49:66]>>`rustcE0308
traitobjects.rs(49, 27): the found closure
traitobjects.rs(66, 5): expected due to the type of this binding