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