Rust 返回 &str 但不是 String 时，borrow 时间比预期长

当我编译这段代码时

    fn read(&mut self) -> Result<u8, &str> {
        if self.pos >= 512 {
            return Err("End of buffer".into());
        }
        let res = self.buf[self.pos];
        self.pos += 1;

        Ok(res)
    }
    

    fn read_u16(&mut self) -> Result<u16, &str> {
        let res = ((self.read()? as u16) << 8) | (self.read()? as u16);
        Ok(res)
    }

编译器抛出以下错误

error[E0499]: cannot borrow `*self` as mutable more than once at a time
  --> src/byte_packet_buffer.rs:53:51
   |
52 |     fn read_u16(&mut self) -> Result<u16, &str> {
   |                 - let's call the lifetime of this reference `'1`
53 |         let res = ((self.read()? as u16) << 8) | (self.read()? as u16);
   |                     ------------                  ^^^^^^^^^^^ second mutable borrow occurs here
   |                     |
   |                     first mutable borrow occurs here
   |                     returning this value requires that `*self` is borrowed for `'1`

For more information about this error, try `rustc --explain E0499`.
error: could not compile `dns_self` due to previous error

但是，如果我将返回值从&str修改为String，则编译时不会出错.

有没有人能解释为什么我在返回&str时会出错，但在返回String时却不会？