Warn on dyn AsyncTrait + Unpin

What it does

This lint will detect usage of dyn AsyncTrait + Unpin for various different choices for AsyncTrait. It should also trigger when a + Send is added.

The suggested replacement should be Pin<Box<dyn AsyncTrait>>. This is strictly more powerful than the + Unpin version.

The lint should at least apply to these traits: Future, Stream, AsyncRead, AsyncWrite. It might even make sense to apply it to all traits.

Categories (optional)

• Kind: style

The advantage is that changing the type like this allows storing non-Unpin types in the trait object.

Drawbacks

None.

Example

use futures::stream::Stream;

fn new_stream() -> Box<dyn Stream<Item = i32> + Unpin> {
    Box::new(futures::stream::iter(vec![1, 2, 3]))
}

Could be written as:

use futures::stream::Stream;

fn new_stream() -> Pin<Box<dyn Stream<Item = i32>>> {
    Box::pin(futures::stream::iter(vec![1, 2, 3]))
}

Begin with the Rust examples in the issue and determine the lint's supported scope: at least Future, Stream, AsyncRead, and AsyncWrite, with + Send handled too. Done means detecting the shown dyn forms and suggesting Pin<Box<dyn ...>>; the issue names no source file or test, so repository navigation and test discovery are required.