Challenge 1: A move Closure Printing a Vec — Possible Solution
====================================================================

use std::thread;

fn main() {
    let names = vec![String::from("Ada"), String::from("Grace")];

    let handle = thread::spawn(move || {
        println!("{:?}", names);
    });

    handle.join().unwrap();
}

WHY THIS WORKS AS AN ANSWER
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move || { println!("{:?}", names); } forces the closure to take
OWNERSHIP of names rather than merely borrowing it — per this
chapter's move-keyword explanation, this is required because the
spawned thread's actual lifetime is unpredictable from main's
perspective; it might still be running after main would otherwise have
dropped names.

WHAT WOULD GO WRONG WITHOUT move (IN GENERAL TERMS): the closure would
instead try to capture a REFERENCE to names. The compiler would refuse
to compile this, producing an error to the effect of "closure may
outlive the current function, but it borrows `names`, which is owned
by the current function" — essentially the compiler's way of saying it
cannot guarantee names will still be valid for as long as the spawned
thread might need it, since main's own scope (and therefore names'
lifetime) could end before the thread finishes. This is the exact
category of lifetime concern Course 1 Chapter 4 raised for ordinary
references, now surfacing again in the genuinely new context of a
thread whose actual runtime lifetime the compiler has no way to bound
in advance.