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easyrust

Day 15: Santa's Generic Obsession

“Bernard! Did I tell you? Trait bounds are the greatest thing since reindeer! The Display trick you pulled yesterday? Pure genius. I’m hooked.”

Bernard shuffled in, already regretting showing Santa Display. “What now?”

“The gifts won’t wrap themselves! I need a function—generic, reusable, beautiful. We slap on some bounds, and boom, wrapping perfection!”

Bernard peeked at the screen. “Santa, you’ve got mutable and immutable borrows fighting each other. And what’s with this raw pointer?”

Santa grinned. “Bounds solve everything, Bernard! I’ll slap a Display here, a Debug there—easy!”

The IDE chimed angrily. “Cannot borrow self as mutable while also borrowed as immutable.”

Bernard sighed. “Santa, that’s the compiler telling you no.”

Santa glared at the screen. “Warm up the sled. I’m giving the borrow checker a piece of my mind.”

Your mission

What you need to do is simple, Santa has written a function prepare_gift, but it doesn't work quite yet. The function must accept anything that implements the Display and Gift trait. And the Gift trait must have a method wrap that mutates the is_wrapped field to true.

Here's what you gotta do:

  • Add the new field to the structs is_wrapped: bool.
  • Finish the Gift trait definition, add a method called wrap.
  • Implement the Gift trait for KidsGift, ElvesGift, and ReindeerGift. Using the wrap method, it should set is_wrapped to true.
  • Update the prepare_gift function signature to accept any type that implements the Display and Gift traits.

Hints

If you're stuck or need a starting point, here are some hints to help you along the way!

Click here to show the hints.

  • The Gift trait should have a method wrap that takes a mutable reference of self. e.g.

    pub trait Gift {
    fn wrap(&mut self);
}

  • To make a function accept any type that implements multiple traits, you can use the + operator. e.g.

      fn prepare_gift<T: fmt::Display + Gift>(gift: &mut T) {
      // code remains the same
  }

  • Make sure the argument takes a mutable reference &mut T to be able to call the wrap method since wrap also mutates the struct.