Captured variable cannot escape FnMut closure body

[jofas]

@khimru

Second version doesn't change anything, it's only receives shared read-only reference… and these can be easily copied around.

This works thanks to reborrowing (I think):

Why can I assign a mutable reference to an immutable reference without violating the borrowing rules?

The following code (it compiles!) first declares a mutable variable s which has the type String. Then it creates r1 which is a mutable reference to s. Later on, it assigns r1 to r2 which is an immutable reference to s by coercing r1 into type &String and it even assign r2 to r3 which is another immutable reference to s. Isn't the reference scope of r1 fully covers the reference scopes of r2 and r3 since there is a p1.push_str("ghi") statement at the end, so it violates the borrowing rules? To fu…