pub fn accept(&mut self, poll: &mut Poll, sock: Stream) -> Result<()> {
        // ...

        let conn = &mut self.connections[tok.into()];


        self.dispatch_by_type(conn.socket(), tok, poll); // 编译错误 cannot borrow `*self` as immutable because it is also borrowed as mutable
        

        let result = match conn.socket() { // 为什么这里不会报同样的错误
            Stream::Tcp(tcp) => {
                // ...
            },
            Stream::Unix(unix) => {
                // ...
            },
        };

        // ...
    }

error[E0502]: cannot borrow `*self` as immutable because it is also borrowed as mutable
   --> src/io.rs:545:9
    |
535 |         let conn = &mut self.connections[tok.into()];
    |                         ---------------- mutable borrow occurs here
...
545 |         self.dispatch_by_type(conn.socket(), tok, poll);
    |         ^^^^^^^^^^^^^^^^^^^^^^-------------^^^^^^^^^^^^
    |         |                     |
    |         |                     mutable borrow later used here
    |         immutable borrow occurs here

dispatch_by_type 定义：fn dispatch_by_type(&self, stream: &Stream, tok: Token, poll: &Poll) -> std::io::Result<()>

socket 方法定义：pub fn socket(&self) -> &Stream