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));
}
