从标准输入读取的字符串在打印时具有额外的空白

Question

我在打印过程中遇到了这种异常行为：

use std::io;

fn main() {
    let mut number = String::new();
    io::stdin()
        .read_line(&mut number)
        .expect("Failed to read line");

    let mut number_len = number.len();
    number.truncate(number_len - 1); // removing trailing \n
    number_len = number.len();

    println!("Number entered is {} with length {}", number, number_len);

    for i in 0..(number_len - 1) {
        println!("Char at {} is {}", i, &number[i..i + 1]);
    }
}

输出

4200                                       <- my input
 with length 5 is 4200                     <- Oddly printed output
Char at 0 is 4
Char at 1 is 2
Char at 2 is 0
Char at 3 is 0

为什么会这样？

此外，number的长度是错误的...它应该是4，而即使在截断后它也会打印5。