assert-unmoved

A type that asserts that the underlying type is not moved after being pinned and mutably accessed.

This is a rewrite of [futures-test]'s AssertUnmoved to allow use in more use cases. This also supports traits other than futures.

Many of the changes made in this project are also reflected upstream: [rust-lang/futures-rs#2148], [rust-lang/futures-rs#2208]

Usage

Add this to your Cargo.toml:

toml [dependencies] assert-unmoved = "0.1"

Compiler support: requires rustc 1.37+

Examples

An example of using [Pin::new_unchecked] incorrectly (should panic):

```rust use futuresutil::{ future::{self, Future}, task::{noopwaker, Context}, }; use assert_unmoved::AssertUnmoved; use std::pin::Pin;

let waker = noopwaker(); let mut cx = Context::fromwaker(&waker);

// First we allocate the future on the stack and poll it. let mut future = AssertUnmoved::new(future::pending::<()>()); let pinnedfuture = unsafe { Pin::newunchecked(&mut future) }; assert!(pinnedfuture.poll(&mut cx).ispending());

// Next we move it back to the heap and poll it again. This second call // should panic (as the future is moved). let mut boxedfuture = Box::new(future); let pinnedboxedfuture = unsafe { Pin::newunchecked(&mut *boxedfuture) }; let _ = pinnedboxedfuture.poll(&mut cx).ispending(); ```

An example of incorrect [StreamExt::next] implementation (should panic):

```rust use futuresutil::{ future::Future, stream::{self, Stream}, task::{noopwaker, Context, Poll}, }; use assert_unmoved::AssertUnmoved; use std::pin::Pin;

struct Next<'a, S: Stream>(&'a mut S);

impl Future for Next<'_, S> { type Output = Option;

fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
    // This is `Pin<&mut Type>` to `Pin<Field>` projection and is unsound
    // if `S` is not `Unpin` (you can move `S` after `Next` dropped).
    //
    // The correct projection is `Pin<&mut Type>` to `Pin<&mut Field>`.
    // In `Next`, it is `Pin<&mut Next<'_, S>>` to `Pin<&mut &mut S>`,
    // and it needs to add `S: Unpin` bounds to convert `Pin<&mut &mut S>`
    // to `Pin<&mut S>`.
    let stream: Pin<&mut S> = unsafe { self.map_unchecked_mut(|f| f.0) };
    stream.poll_next(cx)
}

}

let waker = noopwaker(); let mut cx = Context::fromwaker(&waker);

let mut stream = AssertUnmoved::new(stream::pending::<()>());

{ let next = Next(&mut stream); let mut pinnednext = Box::pin(next); assert!(pinnednext.asmut().poll(&mut cx).ispending()); }

// Move stream to the heap. let mut boxed_stream = Box::pin(stream);

let next = Next(&mut boxedstream); let mut pinnednext = Box::pin(next); // This should panic (as the future is moved). let _ = pinnednext.asmut().poll(&mut cx).is_pending(); ```

Optional features

• futures03 — Implements futures v0.3 traits for assert-unmoved types.
• tokio1 — Implements tokio v1 traits for assert-unmoved types.
• tokio03 — Implements tokio v0.3 traits for assert-unmoved types.
• tokio02 — Implements tokio v0.2 traits for assert-unmoved types.

Related Projects

• [pin-project]: A crate for safe and ergonomic pin-projection.
• [pin-project-lite]: A lightweight version of pin-project written with declarative macros.

License

Licensed under either of Apache License, Version 2.0 or MIT license at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.