main.rs

import to_str::*;

class bf_stack {
    priv {
        let mut data : ~[int];
    }

    new() {
        self.data = ~[0];
    }

    fn to_str() -> str {
        self.data.to_str()
    }
    
}
fn main() {
}

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

user@note ~/soft/mine/rust $ rustc main.rs && echo done && ./main 
main.rs:13:8: 13:17 error: illegal borrow unless pure: creating immutable alias to aliasable, mutable memory
main.rs:13         self.data.to_str()
                   ^~~~~~~~~
main.rs:13:8: 13:24 note: impure due to access to impure function
main.rs:13         self.data.to_str()
                   ^~~~~~~~~~~~~~~~
error: aborting due to previous error