Challenge 1: Move Error, Then Fix With Clone — Possible Solution
====================================================================

Step 1 — the version that fails to compile:

fn main() {
    let s1 = String::from("hello");
    let s2 = s1;
    println!("{}", s1);
}

THE EXACT COMPILER ERROR (paraphrased from rustc's real message):
  error[E0382]: borrow of moved value: `s1`
   --> src/main.rs:4:20
    |
  2 |     let s1 = String::from("hello");
    |         -- move occurs because `s1` has type `String`, which does not implement the `Copy` trait
  3 |     let s2 = s1;
    |              -- value moved here
  4 |     println!("{}", s1);
    |                    ^^ value borrowed here after move

Step 2 — the fix, using .clone():

fn main() {
    let s1 = String::from("hello");
    let s2 = s1.clone();
    println!("{}", s1);
    println!("{}", s2);
}

WHY THIS WORKS AS AN ANSWER
------------------------------
The first version's let s2 = s1 MOVES ownership of the String's heap
data from s1 to s2, per this chapter's move semantics — s1 becomes
invalid immediately, so the later println! referencing it is rejected
at compile time, exactly the "borrow of moved value" error this
chapter's warn-box named as the most common early Rust mistake.

Replacing s1 with s1.clone() creates a genuine, independent DEEP COPY
of the string's heap data — s2 gets its own separate copy, s1 is
never moved at all, and both variables remain fully valid and usable
afterward, at the cost of the extra allocation/copy .clone() performs
(explicitly, visibly, exactly as this chapter described).