大家已经注意到，Rust选择无需注解在运行时捕捉变量。这在正常使用时很方便，但是编写函数时，这种模糊性是不允许的。封闭的类型齐全，包括其捕获类型必须注明。捕获封闭使用被注释为以下特征之一的方式：
Fn: 通过引用以捕获 (&T)
FnMut: 通过可变引用需要捕获 (&mut T)
FnOnce: 通过值捕获 (T)
即使是注释，这些都是非常灵活： FnOnce的一个参数指定封闭可以通过T或 &mut T 或 &T在将捕获(如果此举是可能的，任何类型的借还应该是可能的)。 反向并非如此：如果该参数为Fn，那么什么都降低是允许的。因此，规则是：
此外，Rust会优先捕捉，可变通过变量基础变量限制最少的方式：
| // A function which takes a closure as an argument and calls |
| // it. The closure takes no input and returns nothing. |
| fn apply(f: F) where |
| F: FnOnce() { |
| // ^ TODO: Try changing this to `Fn` or `FnMut`. |
| f() |
| } |
| // A function which takes a closure and returns an `i32`. |
| fn apply_to_3(f: F) -> i32 where |
| // The closure takes an `i32` and returns an `i32`. |
| F: Fn(i32) -> i32 { |
| f(3) |
| } |
| fn main() { |
| let greeting = "hello"; |
| // A non-copy type. |
| let mut farewell = "goodbye".to_owned(); |
| // Capture 2 variables: `greeting` by reference and |
| // `farewell` by value. |
| let diary = || { |
| // `greeting` is by reference: requires `Fn`. |
| println!("I said {}.", greeting); |
| // Mutation forces `farewell` to be captured by |
| // mutable reference. Now requires `FnMut`. |
| farewell.push_str("!!!"); |
| println!("Then I screamed {}.", farewell); |
| println!("Now I can sleep. zzzzz"); |
| // Manually calling drop forces `farewell` to |
| // be captured by value. Now requires `FnOnce`. |
| drop(farewell); |
| }; |
| // Call the function which applies the closure. |
| apply(diary); |
| let double = |x| 2 * x; |
| println!("3 doubled: {}", apply_to_3(double)); |
| } |
