Rust類型轉換-推導

類型推理引擎是非常靈巧的。 它在一個初始化期間會尋找 r-value 的類型。 它也著眼於如何使用變量，事後推斷其類型。這裡是類型推斷的高級示例：

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`)
    // TODO ^ Try commenting out the `vec.push(elem)` line

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

需要的變量冇有類型注釋。