这是我目前为止的实现，但是感觉很不优雅！

pub fn proto_to_decimal(input: &DecimalValueProto) -> Result<Decimal, String> {
    let mut num = 0;
    let mut power: i32 = (input.value.len() - 1)
        .try_into()
        .map_err(|_| "Failed to convert proto to decimal")?; //casting down from usize to i32 is failable

    for digit in input.value.iter() {
        let expansion: i128 = if power == 0 { expansion = *digit as i128 } else { expansion = (*digit as i128) * 10_i128.pow(power as u32) as i128 }
        num += expansion;
        power -= 1;
    }
    Ok(Decimal::from_i128_with_scale(num as i128, input.scale))
}