Support `ConfiguredCancelableAsyncEnumerable<'T>`

[xperiandri]

When you call .WithCancellation(cancellationToken) on IAsyncEnumerable<'T> you get System.Runtime.CompilerServices. ConfiguredCancelableAsyncEnumerable<'T> which is not currently supported.

So that this code does not work

    static member ToFlatListAsync<'Source>(source: IQueryable<'Source>, [<Optional>] cancellationToken: CancellationToken) = task {
        let builder = ImmutableArray.CreateBuilder<'Source>()
        do!
            source.AsAsyncEnumerable().WithCancellation(cancellationToken)
            |> TaskSeq.iterAsync (fun x -> builder.Add(x))

        return builder.ToImmutable();
    }

[abelbraaksma]

This is actually very interesting. Reading up on the design decisions for WithCancellation, as Don Syme and myself were about creating our own solution, but it is, obviously, better to use dotnet's own methods, if they are applicable.

Just checked, this method is part of NetStandard 2.1, so I can actually add it. Another method in those extensions that is very interesting is ToBlockingEnumerable. This is a .NET 7+ extension, but I might take its code for the blocking operations I have, to ensure similar behavior.

[abelbraaksma]

Related discussion: #133

[abelbraaksma]

This turns out to be much harder to support than I thought. But one of the soon-to-be-made changes is going to be a move to static methods, so we can support adding a cancellation token in each call immediately, removing the need for calling WithCancellation, but that means threading an overload through the whole system.

To support this, we either need to overload every single method such that it accepts that type, or we have to go SRTP (and/or with an SRTP overload).

Instead, I may consider something like ofWithCancellation (ugly method name, I know), which would itself return an IAsyncEnumerable but with a collected cancellation token. This is, however, not ideal, as the token should be given to GetEnumerator(token). Hmm, I'll think a bit more about this.

The reason this is non-trivial is because ConfiguredCancelableAsyncEnumerable<T> is a struct and does not implement any useful interface. I'm also a little confused as to why MS chose to internally effectively call ConfigureAwait(true), which, from library code, is considered bad practice in general (and can lead to deadlocks).

[xperiandri]

Maybe raise an issues on .NET repo about the use of ConfiguredCancelableAsyncEnumerable<T>?

[abelbraaksma]

That likely wouldn't help. It is a struct for a reason. While it keeps a reference to the IAsyncEnumerable<_> internally, it makes the overhead of making your enumerable cancelable really small. There's no need for a newobj IL instruction.