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Wrapper types

Currently Rust does not automatically reborrow Option<&mut T> or similar wrapper types. Conceptually, there is no reason why &mut T should be reborrowable but Option<&mut T> should not: the only difference between the two types is that one can also be null.

With the Reborrow trait, reborrowing wrapper types of exclusive references becomes possible using blanket implementations.

#![allow(unused)]
fn main() {
impl<T: Reborrow> for Option<T> { /* ... */ }
}