让我们一个一个地分解它。 
 
 框 
 
 Box<T>是一个指向堆分配T的指针。 我们在这里使用它，因为trait对象只能存在于指针后面。 
 
 特质对象 
 
 在Box<Fn() + Send + 'static> ， Fn() + Send + 'static是一个特征对象类型。 将来，它将被写入Box<dyn (Fn() + Send + 'static)>以避免混淆。 
 
 内部dyn是对原始类型的限制。 只有当T: Fn() + Send + 'static时， Box<T>才能被强制转换为Box<Fn() + Send + 'static> 。 因此，尽管我们不知道原始类型，但我们可以假设它是Fn()和Send并具有'static生命周期” 。 
 
 Fn() 
 
 这是一个特性，就像Clone或Default 。 但是，它使用特殊的语法糖 。 
 
 
 
 Fn(A1, ..., An)是Fn<(A1, ..., An), Output=()>的语法糖。 
 
 
 Fn(A1, ..., An) -> R是Fn<(A1, ..., An), Output=R>的语法糖。 
 
 
 此语法糖也适用于以下特征： Fn ， FnMut ， FnOnce和FnBox 。 
 
 
 那么Fn是什么意思？ T: Fn(A1, ..., An) -> R表示x: T是可调用对象，其参数为A1, ..., An ，返回类型为R 例子包括函数指针和闭包。 
 
 发送 
 
 Send意味着可以通过线程发送此类型的值。 由于这是一个自动特征 ，因此可以将其指定为 dyn类型的第二个边界 （特征对象类型）。 
 
 'static界限 
 
 事实上， dyn类型（特征对象类型）必须只有一个生命周期界限 。 它被省略时推断。 推理规则在RFC 0192和RFC 1156中描述。 基本如下： 
 
 
 
 如果明确给出，请使用该生命周期。 
 
 
 否则，从内在特质推断。 例如， Box<Any>是Box<Any + 'static>因为Any: 'static 。 
 
 
 如果性状不具有适当的生命周期，则从外部类型推断。 例如， &'a Fn()是&'a (Fn() + 'a) 。 
 
 
 如果甚至失败了，它会回落到'static （对于函数签名）或匿名生命周期（对于函数体）。 
 
 
 结论 
 
 f: Box<Fn() + Send + 'static>是一个拥有指向可调用值的指针（原始类型未知并动态更改），例如闭包（没有参数或没有返回值），可以跨线程发送和程序本身一样长寿。 

Let's decompose it one-by-one.
 
Box
 
Box<T> is a pointer to heap-allocated T. We use it here because trait objects can only exist behind pointers.
 
Trait objects
 
In Box<Fn() + Send + 'static>, Fn() + Send + 'static is a trait object type. In future, it will be written Box<dyn (Fn() + Send + 'static)> to avoid confusion.
 
Inside dyn are restrictions to the original type. Box<T> can be coerced into Box<Fn() + Send + 'static> only when T: Fn() + Send + 'static. Therefore, although we don't know the original type, we can assume it was Fn() and Send and had 'static lifetime.
 
Fn()
 
This is a trait, just like Clone or Default. However, it uses a special syntax sugar.
 
 
 
Fn(A1, ..., An) is a syntax sugar for Fn<(A1, ..., An), Output=()>.
 
 
Fn(A1, ..., An) -> R is a syntax sugar for Fn<(A1, ..., An), Output=R>.
 
 
This syntax sugar also applies to the following traits: Fn, FnMut, FnOnce, and FnBox.
 
 
So what does Fn mean? T: Fn(A1, ..., An) -> R means x: T is a callable object with arguments A1, ..., An and return type R. Examples include function pointers and closures.
 
Send
 
Send means that values of this type can be sent across threads. Since this is an auto trait, it can be specified as the second bounds of dyn types (trait object types).
 
'static bound
 
In fact, dyn types (trait object types) must have exactly one lifetime bound. It's inferred when omitted. The inference rule is described in RFC 0192 and RFC 1156. It's basically as follows:
 
 
 
If explicitly given, use that lifetime.
 
 
Otherwise, it is inferred from the inner trait. For example, Box<Any> is Box<Any + 'static> because Any: 'static.
 
 
If the trait doesn't have an appropriate lifetime, it is inferred from the outer type. For example, &'a Fn() is &'a (Fn() + 'a).
 
 
If that even failed, it falls back to 'static (for a function signature) or an anonymous lifetime (for a function body).
 
 
Conclusion
 
f: Box<Fn() + Send + 'static> is an owned pointer to a callable value (with the original type unknown and dynamically change) such as closures (with no argument or no return value), which can be sent across threads and lives as long as the program itself.