推断

类型推断引擎非常智能。它不仅仅在初始化时查看值表达式的类型，还会查看变量之后如何使用来推断其类型。这是一个类型推断的高级示例

fn main() {
    // Because of the annotation, the compiler knows that `elem` has type u8.
    let elem = 5u8;

    // Create an empty vector (a growable array).
    let mut vec = Vec::new();
    // At this point the compiler doesn't know the exact type of `vec`, it
    // just knows that it's a vector of something (`Vec<_>`).

    // Insert `elem` in the vector.
    vec.push(elem);
    // Aha! Now the compiler knows that `vec` is a vector of `u8`s (`Vec<u8>`)
    // TODO ^ Try commenting out the `vec.push(elem)` line

    println!("{:?}", vec);
}

不需要对变量进行类型注解，编译器很满意，程序员也很满意！