Rust Future Trait

Implementing the Future trait in Rust and extracting the value from the future without using async or external libraries can be a bit tricky, but it is possible. Below is a basic example to demonstrate how this can be achieved.

First, let's define a simple future type and implement the Future trait for it. We'll use the Future trait from the standard library. Since we are not using async, we'll need to manually manage the state of the future.

Here's a step-by-step implementation:

1. Define a simple future type.

2. Implement the Future trait for the type.

3. Create a function to poll the future and extract the value.

4. Provide an on_complete function to handle the completion.

Step 1: Define a Simple Future Type

We'll define a simple future type that wraps a value. For simplicity, let's use an Option to represent the state of the future.

use std::future::Future;

use std::pin::Pin;

use std::task::{Context, Poll};

struct SimpleFuture {

    value: Option<i32>,

}

impl SimpleFuture {

    fn new(value: i32) -> Self {

        SimpleFuture {

            value: Some(value),

        }

    }

}        

Step 2: Implement the Future Trait

We'll implement the Future trait for our SimpleFuture type. The poll method will return Poll::Ready when the future is complete, and Poll::Pending otherwise.

impl Future for SimpleFuture {

    type Output = i32;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'>) -> Poll<Self::Output> {

        if let Some(value) = self.get_mut().value.take() {

            Poll::Ready(value)

        } else {

            Poll::Pending

        }

    }

}        

Step 3: Create a Function to Poll the Future

We'll create a function to poll the future and extract the value. This function will keep polling the future until it is ready.

fn block_on<F: Future>(mut future: F) -> F::Output {

    use std::task::RawWaker;

    use std::task::RawWakerVTable;

    fn noop(_: *const ()) {}

    fn clone(_: *const ()) -> RawWaker {

        RawWaker::new(std::ptr::null(), &VTABLE)

    }

    static VTABLE: RawWakerVTable = RawWakerVTable::new(clone, noop, noop, noop);

    let waker = unsafe { std::task::Waker::from_raw(RawWaker::new(std::ptr::null(), &VTABLE)) };

    let mut context = Context::from_waker(&waker);

    let mut pinned_future = unsafe { Pin::new_unchecked(&mut future) };

    loop {

        match pinned_future.as_mut().poll(&mut context) {

            Poll::Ready(value) => return value,

            Poll::Pending => std::thread::yield_now(),

        }

    }

}        

Step 4: Provide an on_complete Function

Finally, we'll create an on_complete function to handle the completion of the future. This function will take a closure to be executed when the future is complete.

fn on_complete<F: Future, C: FnOnce(F::Output)>(future: F, callback: C) {

    let result = block_on(future);

    callback(result);

}        

Example Usage

Here is how you can use the above implementation:

fn main() {

    let future = SimpleFuture::new(42);

    on_complete(future, |result| {

        println!("Future completed with value: {}", result);

    });

}        

This example demonstrates how to manually implement and use futures in Rust without relying on async or external libraries. The block_on function simulates an executor by continuously polling the future until it is ready, and the on_complete function allows you to specify a callback to be executed upon completion.