所以我有一个函数，用来把字符串转换成数字：

fn to_number(string: &mut String) {
    string.trim().parse::<u32>().expect("Error");
}

如果我使用它，它似乎并没有改变传递给它的变量：

let mut x: String = " 12 ".into();
to_number(&mut x);
let y = x / 2; // Error here

产生的错误：

cannot multiply `{integer}` by `String`
the trait `Mul<String>` is not implemented for `{integer}`
the following other types implement trait `Mul<Rhs>`:
\<&'a f32 as Mul\<f32\>\>
\<&'a f64 as Mul\<f64\>\>
\<&'a i128 as Mul\<i128\>\>
\<&'a i16 as Mul\<i16\>\>
\<&'a i32 as Mul\<i32\>\>
\<&'a i64 as Mul\<i64\>\>
\<&'a i8 as Mul\<i8\>\>
\<&'a isize as Mul\<isize\>\>
and 49 others

我试过隐藏传递的字符串，但它仍然没有将变量更改为数字。