类型推断

类型推断引擎非常智能。它不仅查看初始化期间值表达式的类型，还会查看变量在初始化后的使用方式来推断其类型。以下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);
}

无需对变量进行类型标注，编译器和程序员都很开心！