Implementing Custom Send and Sync Traits in Rust

Rust's safety guarantees are heavily reliant on its type system and the Send and Sync traits. These traits are fundamental for concurrent programming, indicating whether a type can be safely transferred between threads (Send) and if its references can be safely shared between threads (Sync). This challenge involves understanding and implementing custom versions of these traits to gain a deeper appreciation for Rust's concurrency model.

Problem Description

Your task is to create two marker traits, MySend and MySync, that mimic the behavior of Rust's built-in Send and Sync traits. You will then implement these custom traits for various data structures, demonstrating your understanding of when a type is safe to send or share across threads.

What needs to be achieved:

1. Define MySend Trait: Create a zero-sized marker trait named MySend.
2. Define MySync Trait: Create a zero-sized marker trait named MySync.
3. Implement MySend: Implement MySend for primitive types (e.g., i32, f64, bool), the String type, and a simple custom struct.
4. Implement MySync: Implement MySync for primitive types, the String type, and the same simple custom struct.
5. Demonstrate Usage (Optional but Recommended): Show how these traits would be used in a hypothetical scenario where you're trying to pass data between threads or share it.

Key Requirements:

• The MySend and MySync traits should not have any methods. They are purely marker traits.
• Implementations should reflect the safety rules of the actual Send and Sync traits.
• You should not use the #[derive(Send, Sync)] attribute. You will be implementing these traits manually or through manual blanket implementations where applicable.
• For types that contain other types, their MySend and MySync implementations should depend on the MySend and MySync implementations of their contained types.

Expected Behavior:

• Primitive types like i32, f64, and bool should be MySend and MySync.
• String should be MySend and MySync.
• A simple struct containing only primitive types should be MySend and MySync.
• A struct containing a std::rc::Rc (or a similar non-thread-safe pointer) should not be MySend or MySync. (You might need to define a type that wraps Rc to demonstrate this, as Rc itself doesn't implement Send or Sync).
• A struct containing a std::sync::Arc (which is thread-safe) should be MySend and MySync.

Important Edge Cases to Consider:

• Recursive types: While not strictly required for this challenge, think about how Send and Sync would behave with recursive data structures.
• Interior Mutability: Consider types that allow mutation through shared references (e.g., RefCell vs. Mutex). This is a key concept for Sync.

Examples

Example 1: Primitive Types

Let's consider an i32.

Input: i32

Output: impl MySend for i32 {} impl MySync for i32 {}

Explanation: Primitive types like i32 are inherently safe to send across threads and share references to, as they represent simple, immutable values (or values whose mutation is managed by Rust's ownership system).

Example 2: Custom Struct with Primitives

Consider a simple struct:

struct Point {
    x: i32,
    y: i32,
}

Input: Point struct

Output: impl MySend for Point {} impl MySync for Point {}

Explanation: Since Point only contains i32 fields, and i32 is MySend and MySync, the Point struct itself can be safely sent and shared. The implementations for Point would depend on the implementations of i32.

Example 3: Struct with Rc (Demonstrating Non-Send/Sync)

Let's create a wrapper around Rc to illustrate the concept.

use std::rc::Rc;

struct UnsafeWrapper<T>(Rc<T>);

// Assume we want to implement MySend/MySync for UnsafeWrapper

Input: UnsafeWrapper<i32>

Expected Output (Conceptual): impl MySend for UnsafeWrapper<i32> {} <-- This should FAIL to implement impl MySync for UnsafeWrapper<i32> {} <-- This should FAIL to implement

Explanation: std::rc::Rc is designed for single-threaded scenarios. It does not implement Send or Sync because its internal reference counting mechanism is not atomic and thus not safe to use concurrently across multiple threads. Therefore, any type that directly wraps Rc cannot safely be MySend or MySync.

Constraints

• You must define the MySend and MySync traits yourself.
• You are encouraged to use blanket implementations where appropriate (e.g., impl<T: MySend> MySend for &T {}) to reduce boilerplate, but this is not strictly mandatory for demonstrating understanding.
• Focus on impl<T> MySend for T where T: SendImpl { ... } style of reasoning for compound types, rather than directly deriving them.
• You may need to create dummy types to represent scenarios like the UnsafeWrapper in Example 3.
• No use of external crates is permitted, except for standard library features like String, Rc, and Arc.

Notes

• The core idea behind Send is that the ownership of a value can be transferred to another thread.
• The core idea behind Sync is that a shared reference (&T) to a value can be accessed by multiple threads concurrently. A type T is Sync if &T is Send.
• Think about the underlying data representation and how it interacts with concurrent access.
• You'll be effectively recreating the logic that Rust's compiler uses to determine Send and Sync for your own types.
• For demonstration purposes, you might write simple assert! statements or type annotations in a main function to "test" if your types are MySend or MySync (e.g., fn requires_send<T: MySend>(_: T) {}).