TryStream and compiler “limitations”

[steffahn]

Reading

futures-rs/futures-core/src/stream.rs

Lines 174 to 182 in 3601bb7

	/// Poll this `TryStream` as if it were a `Stream`.
	///
	/// This method is a stopgap for a compiler limitation that prevents us from
	/// directly inheriting from the `Stream` trait; in the future it won't be
	/// needed.
	fn try_poll_next(
	self: Pin<&mut Self>,
	cx: &mut Context<'_>,
	) -> Poll<Option<Result<Self::Ok, Self::Error>>>;

I’m wondering: what’s those limitations?

Trying something like b7f417d seems to work without problems.

[taiki-e]

IIRC limitation is: #1776 (comment)

However, a recent compiler change (probably in 1.52) may have fixed this.

[steffahn]

Okay; even on older versions it still seems to work by means of writing

pub trait TryStream: Stream<Item = Result<<Self as TryStream>::Ok, <Self as TryStream>::Error>> {
    /// The type of successful values yielded by this future
    type Ok;

    /// The type of failures yielded by this future
    type Error;
}

instead of

pub trait TryStream: Stream<Item = Result<Self::Ok, Self::Error>> {
    /// The type of successful values yielded by this future
    type Ok;

    /// The type of failures yielded by this future
    type Error;
}

but perhaps that workaround wasn’t known by the author of the comment.

[steffahn]

Nevermind, even that more explicit version doesn’t work before 1.46; the TryStream trait may be older than that? Edit: Yes it is.

[steffahn]

…and I see a MSRV of 1.41 for futures if I’m interpreting that correctly. At least the comment should be updated to reflect that current Rust versions don’t have the limitation anymore. What’s the MSRV policy here? At least in an eventual 0.4 release that supertrait bound should probably be introduced; and removing the method requires a new semver-major version anyways.

[taiki-e]

The current MSRV for futures/futures-util is 1.41, and for some crates, such as futures-core, which provides only traits, the MSRV is 1.36.

The MSRV policy for futures/futures-util is basically "whatever popular distros support". (#2405)

In 0.4, the trait aliases will be moved to futures-util (#2207 (comment)), so it is okay to change the MSRV of futures-util 0.4 to 1.46 and remove the try_poll_next method.