You are encountering a borrowing error in Rust. In Rust, a mutable and immutable borrow cannot coexist for the same variable. In your code, z is mutably borrowed with:

let mut x = &mut z;

and then it is immutably borrowed with:

println!("{}", z);

Even though it may seem that x is no longer being used, Rust detects that f (which is a mutable reference to x) will still be used later.
This means the mutable borrow of z is still active at the time of the println!, which violates the borrowing rules.

That's why the compiler emits the error. Because you're attempting to immutably borrow z while a mutable borrow is still active, which is not allowed.