How to Implement the Future Trait from Scratch

Async
2026-04-17T12:05:42+00:00
You cannot implement the `Future` trait from scratch in user code because it is sealed and requires unsafe compiler internals to define the `poll` method correctly. Instead, you must use the `async` keyword to create a future or wrap existing asynchronous logic in a library-provided future type like

How to Implement the Future Trait from Scratch

You cannot implement the Future trait from scratch in user code because it is sealed and requires unsafe compiler internals to define the poll method correctly. Instead, you must use the async keyword to create a future or wrap existing asynchronous logic in a library-provided future type like std::future::pending or tokio::task::spawn_blocking.

use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

fn main() {
    // This is the standard way to create a Future: using async blocks
    let my_future = async {
        println!("Hello from the future");
        42
    };

    // You cannot manually implement `impl Future for MyType` without unsafe code
    // and deep knowledge of the compiler's internal representation.
    // The code above creates a future that resolves to 42.
    // To actually run it, you would need a runtime like tokio or async-std.
    // For demonstration, we just show the creation.
    let _ = my_future; 
}