use std::ops::Deref; struct DerefTarget { target_field: bool, } impl DerefTarget { fn get(&self) -> &bool { &self.target_field } } struct Container { target: DerefTarget, container_field: bool, } impl Deref for Container { type Target = DerefTarget; fn deref(&self) -> &Self::Target { &self.target } } impl Container { fn bad_borrow(&mut self) { let first = self.get(); self.container_field = true; //~ ERROR E0506 first; } } fn main() {}