类型转换

Rust 不提供基本数据类型之间的隐式类型转换（强制转换）。但是，可以使用 as 关键字执行显式类型转换（类型转换）。

整数类型之间的转换规则通常遵循 C 语言的约定，但在 C 语言中具有未定义行为的情况下除外。Rust 中所有整数类型之间的转换行为都有明确定义。

// Suppress all warnings from casts which overflow.
#![allow(overflowing_literals)]

fn main() {
    let decimal = 65.4321_f32;

    // Error! No implicit conversion
    let integer: u8 = decimal;
    // FIXME ^ Comment out this line

    // Explicit conversion
    let integer = decimal as u8;
    let character = integer as char;

    // Error! There are limitations in conversion rules.
    // A float cannot be directly converted to a char.
    let character = decimal as char;
    // FIXME ^ Comment out this line

    println!("Casting: {} -> {} -> {}", decimal, integer, character);

    // when casting any value to an unsigned type, T,
    // T::MAX + 1 is added or subtracted until the value
    // fits into the new type

    // 1000 already fits in a u16
    println!("1000 as a u16 is: {}", 1000 as u16);

    // 1000 - 256 - 256 - 256 = 232
    // Under the hood, the first 8 least significant bits (LSB) are kept,
    // while the rest towards the most significant bit (MSB) get truncated.
    println!("1000 as a u8 is : {}", 1000 as u8);
    // -1 + 256 = 255
    println!("  -1 as a u8 is : {}", (-1i8) as u8);

    // For positive numbers, this is the same as the modulus
    println!("1000 mod 256 is : {}", 1000 % 256);

    // When casting to a signed type, the (bitwise) result is the same as
    // first casting to the corresponding unsigned type. If the most significant
    // bit of that value is 1, then the value is negative.

    // Unless it already fits, of course.
    println!(" 128 as a i16 is: {}", 128 as i16);

    // In boundary case 128 value in 8-bit two's complement representation is -128
    println!(" 128 as a i8 is : {}", 128 as i8);

    // repeating the example above
    // 1000 as u8 -> 232
    println!("1000 as a u8 is : {}", 1000 as u8);
    // and the value of 232 in 8-bit two's complement representation is -24
    println!(" 232 as a i8 is : {}", 232 as i8);

    // Since Rust 1.45, the `as` keyword performs a *saturating cast*
    // when casting from float to int. If the floating point value exceeds
    // the upper bound or is less than the lower bound, the returned value
    // will be equal to the bound crossed.

    // 300.0 as u8 is 255
    println!(" 300.0 as u8 is : {}", 300.0_f32 as u8);
    // -100.0 as u8 is 0
    println!("-100.0 as u8 is : {}", -100.0_f32 as u8);
    // nan as u8 is 0
    println!("   nan as u8 is : {}", f32::NAN as u8);

    // This behavior incurs a small runtime cost and can be avoided
    // with unsafe methods, however the results might overflow and
    // return **unsound values**. Use these methods wisely:
    unsafe {
        // 300.0 as u8 is 44
        println!(" 300.0 as u8 is : {}", 300.0_f32.to_int_unchecked::<u8>());
        // -100.0 as u8 is 156
        println!("-100.0 as u8 is : {}", (-100.0_f32).to_int_unchecked::<u8>());
        // nan as u8 is 0
        println!("   nan as u8 is : {}", f32::NAN.to_int_unchecked::<u8>());
    }
}