Implementing Interior Mutability with RefCell

Rust's ownership and borrowing rules are central to its safety guarantees, preventing data races at compile time. However, sometimes you need to mutate data that is immutably borrowed. This is where "interior mutability" patterns come into play. This challenge will guide you through implementing a scenario that requires interior mutability using Rust's RefCell.

Problem Description

You are tasked with building a simple counter that can be shared and modified across multiple threads, where the primary reference to the counter is held immutably. This is a classic use case for interior mutability. You will need to create a struct that holds a counter value and implement a method to increment this counter. The key challenge is to allow mutation of the counter's value even when the struct itself is borrowed immutably.

Key Requirements:

1. Counter Struct: Define a struct Counter that contains a single u32 field representing the counter's value.
2. increment Method: Implement an impl Counter block with a method fn increment(&self). This method should increase the internal counter value by one.
3. Interior Mutability: The increment method must be able to mutate the counter's value even though the method signature takes &self (an immutable reference).
4. Thread Safety (Conceptual): While this challenge doesn't strictly require full thread-safe concurrent access with locks, the solution should be adaptable to such scenarios. The primary goal is to demonstrate how interior mutability is achieved, which is a prerequisite for safe multithreaded mutation.

Expected Behavior:

When an instance of Counter is created with an initial value, calling its increment method multiple times should update the internal value correctly.

Important Edge Cases:

• Runtime Panics: Understand the conditions under which RefCell will panic due to multiple mutable borrows or a mutable borrow while an immutable borrow exists. While we won't explicitly test for panics in the examples, your implementation should be mindful of these rules.

Examples

Example 1:

Input:
let mut counter = Counter::new(0);
counter.increment();
counter.increment();

Output: The internal value of counter should be 2.

Explanation: The Counter is initialized to 0. Two calls to increment are made, each increasing the value by 1.

Example 2:

Input:
let counter = Counter::new(10);
counter.increment();

Output: The internal value of counter should be 11.

Explanation: The Counter is initialized to 10. One call to increment increases the value by 1.

Constraints

• The counter value will not exceed the maximum value of u32.
• The solution should be implemented purely in Rust.
• Focus on demonstrating the correct usage of interior mutability, not necessarily on complex multithreading synchronization primitives (though the pattern should enable them).

Notes

• Think about which standard library type in Rust provides interior mutability for single-threaded scenarios.
• Consider how this pattern can be extended to allow immutable borrows while simultaneously enabling mutable operations.
• The RefCell type enforces borrowing rules at runtime, unlike compile-time checks for &T and &mut T. This means you can get runtime panics if you violate its borrowing rules.