Reduction showing challenge with a struct containing mutable and (intended) immutable fields.

build.log

ntalbott@lorica:~/play/rust-sandbox$ cargo build
   Compiling rust-sandbox v0.0.1 (file:///Users/ntalbott/play/rust-sandbox)
/Users/ntalbott/play/rust-sandbox/src/main.rs:18:17: 18:39 error: cannot assign to `self.offset` because it is borrowed
/Users/ntalbott/play/rust-sandbox/src/main.rs:18                 self.offset += c.len();
                                                                 ^~~~~~~~~~~~~~~~~~~~~~
/Users/ntalbott/play/rust-sandbox/src/main.rs:16:15: 16:19 note: borrow of `self.offset` occurs here
/Users/ntalbott/play/rust-sandbox/src/main.rs:16         match self.peek() {
                                                               ^~~~
/Users/ntalbott/play/rust-sandbox/src/main.rs:29:41: 29:49 error: cannot borrow `s.offset` as immutable because `s` is also borrowed as mutable
/Users/ntalbott/play/rust-sandbox/src/main.rs:29     println!("char: {}, offset: {}", c, s.offset);
                                                                                         ^~~~~~~~
note: in expansion of format_args!
<std macros>:2:23: 2:77 note: expansion site
<std macros>:1:1: 3:2 note: in expansion of println!
/Users/ntalbott/play/rust-sandbox/src/main.rs:29:5: 29:51 note: expansion site
/Users/ntalbott/play/rust-sandbox/src/main.rs:28:13: 28:14 note: previous borrow of `s` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `s` until the borrow ends
/Users/ntalbott/play/rust-sandbox/src/main.rs:28     let c = s.getch();
                                                             ^
/Users/ntalbott/play/rust-sandbox/src/main.rs:30:2: 30:2 note: previous borrow ends here
/Users/ntalbott/play/rust-sandbox/src/main.rs:26 fn main() {
/Users/ntalbott/play/rust-sandbox/src/main.rs:27     let mut s = Scanner { string: "hello, world".to_string(), offset: 0 };
/Users/ntalbott/play/rust-sandbox/src/main.rs:28     let c = s.getch();
/Users/ntalbott/play/rust-sandbox/src/main.rs:29     println!("char: {}, offset: {}", c, s.offset);
/Users/ntalbott/play/rust-sandbox/src/main.rs:30 }
                                                 ^
error: aborting due to 2 previous errors
Could not compile `rust-sandbox`.

main-with-cell.rs

use std::cell::Cell;

struct Scanner {
    string: String,
    offset: Cell<uint>
}

impl Scanner {
    fn peek(&self) -> Option<&str> {
        if self.offset.get() >= self.string.len() {
            None
        } else {
            Some(self.string.slice_from(self.offset.get()).slice_chars(0, 1))
        }
    }

    fn getch(&self) -> Option<&str> {
        match self.peek() {
            Some(c) => {
                self.offset.set(self.offset.get() + c.len());
                Some(c)
            },
            None => None
        }
    }
}

fn main() {
    let s = Scanner { string: "hello, world".to_string(), offset: Cell::new(0) };
    let c = s.getch();
    println!("char: {}, offset: {}", c.unwrap(), s.offset.get());
}

main.rs

struct Scanner {
    string: String,
    offset: uint
}

impl Scanner {
    fn peek(&self) -> Option<&str> {
        if self.offset >= self.string.len() {
            None
        } else {
            Some(self.string.slice_from(self.offset).slice_chars(0, 1))
        }
    }

    fn getch(&mut self) -> Option<&str> {
        match self.peek() {
            Some(c) => {
                self.offset += c.len();
                Some(c)
            },
            None => None
        }
    }
}

fn main() {
    let mut s = Scanner { string: "hello, world".to_string(), offset: 0 };
    let c = s.getch();
    println!("char: {}, offset: {}", c, s.offset);
}