error

16:22 error: cannot assign to immutable field `self.pos`
     self.pos += 1;
     ^~~~~~~~~~~~~

immutable-field.rs

pub struct BindIter<'a> {
    binds: &'a [String],
    pos: uint,
}

impl<'a> Iterator for &'a BindIter<'a> {
    type Item = &'a str;

    fn next(&mut self) -> Option<&'a str> {
        let pos = self.pos;

        if pos == self.binds.len() {
            return None;
        }

        self.pos += 1;
        Some(self.binds[pos].as_slice())
    }

    fn size_hint(&self) -> (uint, Option<uint>) {
        let ret = self.binds.len() - self.pos;
        (ret, Some(ret))
    }
}