语法Box<dyn (Fn() -> i32) + Clone>是有效的Rust语法（尽管那些括号是不必要的-, Box<dyn Fn() -> i32 + Clone>也可以）。但是，现在在dyn trait中有两个trait边界，而Rust不喜欢这样：

error[E0225]: only auto traits can be used as additional traits in a trait object
 --> src/lib.rs:2:41
  |
2 |     callback_a: Box<dyn (Fn() -> i32) + Clone>,
  |                         -------------   ^^^^^ additional non-auto trait
  |                         |
  |                         first non-auto trait
  |
  = help: consider creating a new trait with all of these as supertraits and using that trait here instead: `trait NewTrait: Fn<()> + Clone {}`
  = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>

就像这条信息所说的，你可以创造新的特质，将想要的特质作为超级特质：

trait CloneableI32Closure: Fn() -> i32 + Clone {}

struct Callbacks {
    callback_a: Box<dyn CloneableI32Closure>,
    // removed other callbacks for simplicity
}

但这也行不通：

error[E0038]: the trait `CloneableI32Closure` cannot be made into an object
 --> src/lib.rs:4:21
  |
4 |     callback_a: Box<dyn CloneableI32Closure>,
  |                     ^^^^^^^^^^^^^^^^^^^^^^^ `CloneableI32Closure` cannot be made into an object
  |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
 --> src/lib.rs:1:42
  |
1 | trait CloneableI32Closure: Fn() -> i32 + Clone {}
  |       -------------------                ^^^^^ ...because it requires `Self: Sized`
  |       |
  |       this trait cannot be made into an object...

Clone不是对象安全的，所以我们的新特征也不是。There are workarounds for this，但是，在这种情况下，使用Rc而不是Box可能更容易：

use std::rc::Rc;

# [derive(Clone)]

struct Callbacks {
    callback_a: Rc<dyn Fn() -> i32>,
    callback_b: Rc<dyn Fn() -> u64>,
    callback_c: Rc<dyn Fn() -> i8>,
}

Playground link
这将有一个轻微的开销来计算引用，但是，动态调度dyn特征的开销可能要高得多--如果性能很关键，泛型结构可能更适合。