Implementing a then_or Combinator for Futures in Rust

The then_or combinator is a powerful tool for composing asynchronous operations in a functional style. It allows you to chain futures together, providing a fallback future to execute if the initial future fails. This challenge asks you to implement this combinator in Rust, enhancing your understanding of futures, combinators, and error handling in asynchronous programming.

Problem Description

You are tasked with implementing a then_or combinator for futures::Future. This combinator takes a future and another future as input. If the first future resolves successfully, its result is returned. If the first future fails (i.e., returns an Err), the second future is executed, and its result is returned. The second future should only be executed if the first future returns an error.

Key Requirements:

• The combinator must be generic over the input and output types of the futures.
• It must handle potential errors from the first future gracefully, executing the fallback future only when necessary.
• The implementation should be efficient, avoiding unnecessary computations.
• The combinator should return a new future that resolves to the result of either the first or the second future.

Expected Behavior:

The then_or combinator should behave as follows:

1. Await the first future.
2. If the first future resolves successfully (returns Ok(value)), return Ok(value).
3. If the first future resolves with an error (returns Err(error)), await the second future.
4. Return the result of the second future.

Edge Cases to Consider:

• What happens if the second future also returns an error? The combinator should propagate this error.
• What happens if both futures are already completed when then_or is called? The combinator should return the appropriate result immediately.
• Consider the ownership and borrowing of the futures. The combinator should not take ownership unless necessary.

Examples

Example 1:

Input:
future1: `Async<Result<i32, String>>` that resolves to `Ok(5)`
future2: `Async<Result<i32, String>>` that resolves to `Ok(10)`
then_or(future1, future2)

Output: Ok(5)
Explanation: The first future resolves successfully with 5, so the second future is not executed.

Example 2:

Input:
future1: `Async<Result<i32, String>>` that resolves to `Err("First future failed".to_string())`
future2: `Async<Result<i32, String>>` that resolves to `Ok(10)`
then_or(future1, future2)

Output: Ok(10)
Explanation: The first future fails, so the second future is executed, resolving to 10.

Example 3: (Error Propagation)

Input:
future1: `Async<Result<i32, String>>` that resolves to `Err("First future failed".to_string())`
future2: `Async<Result<i32, String>>` that resolves to `Err("Second future failed".to_string())`
then_or(future1, future2)

Output: Err("Second future failed".to_string())
Explanation: Both futures fail. The error from the second future is propagated.

Constraints

• The solution must use the futures crate.
• The solution must compile and run without panics.
• The solution should be reasonably efficient. Avoid unnecessary cloning or allocations.
• The solution should be generic over the result type R (e.g., Result<T, E>).
• The solution should not consume the input futures. It should return a new future.

Notes

• Consider using futures::future::select or similar functions to efficiently await both futures.
• Think about how to handle the ownership of the futures passed to the combinator. Borrowing is generally preferred.
• The Async type is a placeholder for any type implementing futures::Future. You can assume it's a future that resolves to a Result.
• This problem focuses on the logic of the combinator. Error handling and resource management are important, but the primary goal is to correctly chain the futures.