Implementing a Custom Future in Rust

Rust's Future trait provides a powerful abstraction for asynchronous computation. This challenge asks you to implement a simplified custom Future that represents the eventual result of a potentially long-running operation. Building this will deepen your understanding of asynchronous programming in Rust and the Future trait.

Problem Description

You are tasked with implementing a MyFuture struct that conforms to the Future trait. This MyFuture will represent the eventual result of a computation that takes a specified duration to complete. The MyFuture should take a Duration in its constructor, representing the time it will take to complete. The poll method should return Pending until the specified duration has elapsed, at which point it should return Ready with a hardcoded value of 42. You must use the std::time module for timing purposes.

Key Requirements:

• Implement the Future trait for MyFuture.
• The MyFuture struct should take a std::time::Duration in its constructor.
• The poll method should return Pending until the duration has elapsed.
• Upon completion (duration elapsed), the poll method should return Ready(42).
• Use std::time::Instant to track elapsed time.
• The MyFuture should not block the current thread. It should yield control back to the executor.

Expected Behavior:

When poll is called on a MyFuture, it should start a timer. If the timer hasn't expired, poll should return Pending. If the timer has expired, poll should return Ready(42). Subsequent calls to poll after the timer has expired should also return Ready(42).

Edge Cases to Consider:

• What happens if the duration is zero? The future should immediately return Ready(42).
• What happens if poll is called multiple times before the duration expires? The timer should only be started once.
• How do you ensure the future doesn't block the thread? (Hint: Use std::thread::sleep judiciously, or better yet, avoid it entirely and rely on the executor to wake the future up.)

Examples

Example 1:

Input: MyFuture(Duration::from_secs(2))
Output: (First poll) Pending
         (Second poll, 1 second later) Pending
         (Third poll, 2 seconds later) Ready(42)
         (Fourth poll, 2.1 seconds later) Ready(42)
Explanation: The future starts a timer for 2 seconds.  The first two polls return Pending. The third poll returns Ready(42) after the timer expires. Subsequent polls also return Ready(42).

Example 2:

Input: MyFuture(Duration::from_secs(0))
Output: (First poll) Ready(42)
Explanation: The duration is zero, so the future immediately returns Ready(42).

Example 3: (Edge Case)

Input: MyFuture(Duration::from_secs(5))
Output: (First poll) Pending
         (Second poll, 1 second later) Pending
         (Third poll, 2 seconds later) Pending
         (Fourth poll, 3 seconds later) Pending
         (Fifth poll, 4 seconds later) Pending
         (Sixth poll, 5 seconds later) Ready(42)
Explanation: Demonstrates multiple `Pending` states before reaching `Ready`.

Constraints

• The MyFuture struct should only store the duration and the Instant used for timing.
• The poll method should not block the current thread. Avoid using std::thread::sleep directly.
• The Ready value should always be 42.
• The duration must be a positive value or zero. Negative durations are not supported.

Notes

• You'll need to use the Future trait from std::future::Future.
• Consider using std::time::Instant to track elapsed time.
• Think about how to avoid blocking the current thread while waiting for the duration to expire. The executor will be responsible for waking up your future.
• This is a simplified example. Real-world futures often involve more complex logic and error handling.
• The poll method is the core of the Future trait. It's responsible for checking if the future is complete and returning the result if it is.