Exercise: Wizard's Inventory

In this exercise, you will manage a wizard's inventory using what you have learned about borrowing and ownership.

• The wizard has a collection of spells. You need to implement functions to add spells to the inventory and to cast spells from them.

• Spells have a limited number of uses. When a spell has no uses left, it must be removed from the wizard's inventory.

# // Copyright 2023 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
{{#include exercise.rs:setup}}

    // TODO: Implement `add_spell` to take ownership of a spell and add it to
    // the wizard's inventory.
    fn add_spell(..., spell: ...) {
        todo!()
    }

    // TODO: Implement `cast_spell` to borrow a spell from the inventory and
    // cast it. The wizard's mana should decrease by the spell's cost and the
    // number of uses for the spell should decrease by 1.
    //
    // If the wizard doesn't have enough mana, the spell should fail.
    // If the spell has no uses left, it is removed from the inventory.
    fn cast_spell(..., name: ...) {
        todo!()
    }
}

{{#include exercise.rs:main}}

{{#include exercise.rs:tests}}

• The goal of this exercise is to practice the core concepts of ownership and borrowing, specifically the rule that you cannot mutate a collection while holding a reference to one of its elements.
• add_spell should take ownership of a Spell and move it into the Wizard's inventory.
• cast_spell is the core of the exercise. It needs to:
  1. Find the spell (by index or by reference).
  2. Check mana and decrement it.
  3. Decrement the spell's uses.
  4. Remove the spell if uses == 0.
• Borrow Checker Conflict: If students try to hold a reference to the spell (e.g., let spell = &mut self.spells[i]) and then call self.spells.remove(i) while that reference is still "alive" in the same scope, the borrow checker will complain. This is a great opportunity to show how to structure code to satisfy the borrow checker (e.g., by using indices or by ensuring the borrow ends before the mutation).