foo3.rs

trait Serializer {
    fn emit_uint(v: uint);
    fn emit_rec(f: fn());
    fn emit_rec_field(f_name: &str, f_idx: uint, f: fn());
}

trait Serializable {
    fn serialize<S: Serializer>(s: S);
}

impl uint: Serializable {
    fn serialize<S: Serializer>(s: S) { s.emit_uint(self) }
}

type t = { mut x: uint };

impl t: Serializable {
    fn serialize<S: Serializer>(s: S) {
        do s.emit_rec || {
            s.emit_rec_field("x", 0, || self.x.serialize(s));
        }
    }
}

fn main() { }

gistfile1.txt

foo3.rs:20:40: 20:46 error: illegal borrow unless pure: creating immutable alias to mutable field
foo3.rs:20             s.emit_rec_field("x", 0, || self.x.serialize(s));
                                                   ^~~~~~
foo3.rs:20:40: 20:56 note: impure due to access to impure function
foo3.rs:20             s.emit_rec_field("x", 0, || self.x.serialize(s));
                                                   ^~~~~~~~~~~~~~~~