Mutable and immutable borrow at the same time?


#1

I saw this code in an example for using tokio.

Blockquote
if let Some(n) = buf.as_ref()[4…].iter().position(|b| *b == b’\n’) {
// remove the serialized frame from the buffer.
let line = buf.split_to(n + 4);

        // Also remove the '\n'
        buf.split_to(1);

        // Deserialize the request ID
        let request_id = io::Cursor::new(&line[0..4]).get_u32::<BigEndian>();

        // Turn this data into a UTF string and return it in a Frame.
        return match str::from_utf8(&line.as_ref()[4..]) {
            Ok(s) => Ok(Some((request_id as RequestId, s.to_string()))),
            Err(_) => Err(io::Error::new(io::ErrorKind::Other, "invalid string")),
        }
    }

buf.as_ref() is a immutable borrow and buf.spit() is a mutable borrow, what am i missing here?


#2

The expression above returns an Option<usize> which does not depend on the lifetime of buf. In other words, buf is no longer borrowed at the end of this expression and can subsequently be borrowed mutably.


#3

Ah that makes sense, thanks.