A tricky ownership issue with fill_buf

code.rs

impl<'a,T: Buffer+'a> Buffer for ChunkBuffer<'a,T> {
    fn fill_buf<'a>(&'a mut self) -> IoResult<&'a [u8]> {
        if self.buffer.as_slice().contains_slice(self.boundary.as_slice()) {
            // Exit 1: Valid data in our local buffer.
            Ok(self.buffer.as_slice())
        } else if self.buffer.len() > 0 {
            // Exit 2: Add some more data to our local buffer so that it's
            // valid (see invariants for top_up).
            self.top_up()
        } else {
            {
                // Exit 3: Exit on error.
                let read = try!(self.input.fill_buf());
                if read.contains_slice(self.boundary.as_slice()) {
                    // Exit 4: Valid input from self.input. Yay!
                    return Ok(read);
                }
            }
            // Exit 4: Accumulate sufficient data in our local buffer (see
            // invariants for top_up).
            self.top_up()
        }
    }

gistfile1.txt

/home/emk/w/src/rust-opus/src/buffer.rs:168:13: 168:17 error: cannot borrow `*self` as mutable more than once at a time
/home/emk/w/src/rust-opus/src/buffer.rs:168             self.top_up()
                                                        ^~~~
/home/emk/w/src/rust-opus/src/buffer.rs:160:33: 160:43 note: previous borrow of `*self.input` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `*self.input` until the borrow ends
/home/emk/w/src/rust-opus/src/buffer.rs:160                 let read = try!(self.input.fill_buf());
                                                                            ^~~~~~~~~~
<std macros>:1:1: 3:2 note: in expansion of try!
/home/emk/w/src/rust-opus/src/buffer.rs:160:28: 160:56 note: expansion site
/home/emk/w/src/rust-opus/src/buffer.rs:170:6: 170:6 note: previous borrow ends here
/home/emk/w/src/rust-opus/src/buffer.rs:149     fn fill_buf<'a>(&'a mut self) -> IoResult<&'a [u8]> {
...
/home/emk/w/src/rust-opus/src/buffer.rs:170     }