The other option is to reserve the impl (basically a poor man's negative impl)
#[rustc_reservation_impl=
"a custom implementation would be permitted since &_ is fundamental, but be very unexpected, \
and Pin requires the lack of this impl for soundness"]
impl<T: ?Sized> DerefMut for &'_ T {
fn deref_mut(&&T) -> Self::Target {
panic!("not allowed")
}
}
#[rustc_reservation_impl=
"a custom implementation would be permitted since &_ is fundamental, but be very unexpected, \
and Pin requires the lack of this impl for soundness"]
impl<T: ?Sized> Clone for &'_ mut T {
fn clone(&&mut T) -> Self {
panic!("not allowed")
}
}