Implementing a Generic Box in Rust

The Box smart pointer in Rust is a fundamental tool for heap allocation. It allows you to move data onto the heap, which is crucial for recursive data structures, dynamic sizing, and managing ownership of potentially large data. This challenge will guide you through implementing your own generic Box type to deepen your understanding of Rust's ownership, borrowing, and smart pointer concepts.

Problem Description

Your task is to implement a generic smart pointer type named Box<T> that behaves similarly to Rust's standard library Box<T>. This Box<T> should allocate a value of type T on the heap and provide access to that value.

Key requirements:

• Heap Allocation: The Box<T> must store its value T on the heap.
• Ownership: The Box<T> must own the data it points to. When a Box<T> goes out of scope, the heap-allocated data must be deallocated.
• Dereferencing: You should be able to dereference the Box<T> to access the underlying value T.
• Generic: The Box<T> should be generic over any type T.
• Drop Implementation: Ensure proper memory deallocation by implementing the Drop trait.

Expected behavior:

• Creating a Box<T> from a value T should place T on the heap.
• Dereferencing a Box<T> using the * operator should return a reference to the heap-allocated value.
• When a Box<T> is dropped, the memory on the heap should be freed.

Examples

Example 1: Simple Value

Input:
let x = 5;
let boxed_x = Box::new(x); // Box::new is a constructor you'll implement

Output:
// The value 5 is now on the heap, owned by boxed_x.
// Dereferencing boxed_x will yield 5.
// When boxed_x goes out of scope, the memory for 5 is deallocated.

let y = *boxed_x; // y would be 5, and boxed_x would be moved (or its ownership transferred)

Example 2: Struct

Input:
struct Point {
    x: i32,
    y: i32,
}
let p = Point { x: 1, y: 2 };
let boxed_p = Box::new(p);

Output:
// The Point { x: 1, y: 2 } is on the heap.
// Dereferencing boxed_p allows access to its fields:
// assert_eq!(boxed_p.x, 1);
// assert_eq!(boxed_p.y, 2);

Example 3: Recursive Data Structure (Conceptual)

Input:
enum List {
    Cons(i32, Box<List>),
    Nil,
}
let list = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));

Output:
// This demonstrates why Box is useful. Without Box, the size of `List` would be infinite.
// Box allows us to have a known, fixed-size pointer on the stack for each `Cons` variant,
// pointing to the rest of the list on the heap.

Constraints

• You must implement Box<T> within a Rust module, or as a standalone struct.
• You are expected to use Rust's standard library allocation mechanisms (e.g., std::alloc). You should not manually manage memory using C-style malloc/free or similar low-level primitives if you want to stay within idiomatic Rust.
• The Box<T> should implement Deref<Target = T> for easy dereferencing.
• The Box<T> must implement Drop.

Notes

• Consider how to handle heap allocation and deallocation. The std::alloc module or the Box::into_raw and Box::from_raw functions from the standard library might be helpful, but focus on the core Box logic.
• Think about the Drop trait and how to ensure your allocated memory is freed exactly once when the Box goes out of scope.
• Implementing Deref will allow you to use the . operator to access fields of structs stored within the Box.
• This challenge focuses on the core mechanics of Box. You don't need to implement every single method of the standard library Box. Prioritize heap allocation, ownership, dereferencing, and dropping.