gistfile1.rs

/// Expr wraps a 'proc' that computes a value and a new expression state.
pub struct Expr<T, Env> {
        /// Runs the expression and computes the new expression state.
        run: proc(&mut Env) -> (T, Expr<T, Env>)
}

pub fn run<Env, V>(env: &mut Env, expr: &mut Expr<V, Env>) -> V {
        let (v, n) = (expr.run)(env);
        expr.run = n.run;
        v
}

Compiler error:
/home/sven/Desktop/prob/src/expr/lib.rs:28:22: 28:30 error: cannot move out of dereference of & pointer
/home/sven/Desktop/prob/src/expr/lib.rs:28         let (v, n) = (expr.run)(env);
                                                                 ^~~~~~~~