InteriorAssign trait { a := b }

bluelightzero:

For completeness, the current way to do this would be:

It could be defined to do that, or it could be defined to do:

if let Some(num) = &mut num { *num = 13; }

or

*num.as_mut().unwrap() = 13;

I didn't really mean to use Option as a motivating example, just a simple one.