Implementing Timeouts for Asynchronous Operations in Rust

Many real-world applications involve asynchronous operations that might take an unpredictable amount of time to complete. To prevent these operations from blocking your program indefinitely and to handle network latency or slow external services gracefully, it's crucial to implement timeouts. This challenge focuses on building a mechanism to limit the execution time of an asynchronous task in Rust.

Problem Description

Your task is to implement a function that executes an asynchronous operation and returns its result, but only if it completes within a specified duration. If the operation exceeds the timeout, the function should return an error indicating that the timeout occurred.

Key Requirements:

The function should accept a duration (representing the timeout) and an asynchronous operation (a Future).
It should return a Result that either contains the successful output of the asynchronous operation or an error indicating a timeout.
The asynchronous operation should be cancellable. If the timeout occurs, the ongoing operation should be aborted cleanly.

Expected Behavior:

If the provided Future completes before the timeout duration, the function should return the Ok value produced by the Future.
If the Future does not complete within the timeout duration, the function should return an Err indicating a timeout. The Future should be dropped (and ideally cancelled if its underlying implementation supports it).

Edge Cases:

A timeout of zero duration should immediately result in a timeout error.
A very long-running Future that would eventually complete should still be timed out.
A Future that completes instantaneously should not time out.

Examples

Example 1:

use std::time::Duration;
use tokio::time::sleep;

async fn slow_operation(duration: Duration) -> String {
    sleep(duration).await;
    "Operation completed".to_string()
}

// Assume a `with_timeout` function exists that takes a duration and a future
// let result = with_timeout(Duration::from_millis(100), slow_operation(Duration::from_millis(50))).await;
// assert!(result.is_ok());
// assert_eq!(result.unwrap(), "Operation completed");

Output: Ok("Operation completed")

Explanation: The slow_operation takes 50 milliseconds to complete, which is less than the 100-millisecond timeout. Therefore, the operation completes successfully and returns its value.

Example 2:

use std::time::Duration;
use tokio::time::sleep;

async fn very_slow_operation() -> String {
    sleep(Duration::from_secs(5)).await; // This will take 5 seconds
    "Operation finished too late".to_string()
}

// Assume a `with_timeout` function exists
// let result = with_timeout(Duration::from_millis(100), very_slow_operation()).await;
// assert!(result.is_err());
// // The error type should indicate a timeout. For example, it might be a custom enum.
// // assert_eq!(result.unwrap_err(), TimeoutError::TimedOut);

Output: An error indicating a timeout (e.g., Err(TimeoutError::TimedOut)).

Explanation: The very_slow_operation is set to run for 5 seconds. However, the timeout is only 100 milliseconds. Since the operation does not complete within this window, a timeout error is returned.

Constraints

You should use the tokio runtime for asynchronous operations.
The timeout duration will be a std::time::Duration.
The asynchronous operation will be a std::future::Future<Output = T> where T is any type.
The implementation should be efficient and avoid unnecessary overhead.
The solution must handle the cancellation of the timed-out future gracefully.

Notes

This problem is a fundamental building block for robust asynchronous programming in Rust. Consider how you can leverage existing tokio utilities or combinators to achieve this. The key is to manage the competition between the Future completing and the timer expiring. Think about how you would represent the timeout error.