pub struct MyStruct<'a> {
    pub parent: Option<&'a mut MyStruct<'a>>,
}

impl<'a> MyStruct<'a> {
    pub fn new() -> Self {
        MyStruct { parent: None }
    }
}

fn main() {
    fn inner<'a>(_: &'a mut MyStruct<'a>) {}

    let mut i = MyStruct::new();
    inner(&mut i);
    assert!(i.parent.is_none());
}

error[E0502]: cannot borrow `i.parent` as immutable because it is also borrowed as mutable
  --> src/main.rs:18:13
   |
16 |         inner(&mut i);
   |               ------ mutable borrow occurs here
17 |     }
18 |     assert!(i.parent.is_none());
   |             ^^^^^^^^
   |             |
   |             immutable borrow occurs here
   |             mutable borrow later used here