[API Proposal]: Add `allows ref struct` to ConfiguredCancelableAsyncEnumerable

[huoyaoyuan]

Background and motivation

IAsyncEnumerable<T> was updated with allows ref struct in #102795 . However, ConfiguredCancelableAsyncEnumerable<T> was missed. This blocks usages of WithCancellation and ConfigureAwait on such interface instantiations.

API Proposal

namespace System.Threading.Tasks;

public static class TaskAsyncEnumerableExtensions
{
-    public static ConfiguredCancelableAsyncEnumerable<T> ConfigureAwait<T>(this IAsyncEnumerable<T> source, bool continueOnCapturedContext);
+    public static ConfiguredCancelableAsyncEnumerable<T> ConfigureAwait<T>(this IAsyncEnumerable<T> source, bool continueOnCapturedContext) where T : allows ref struct;
-    public static ConfiguredCancelableAsyncEnumerable<T> WithCancellation<T>(this IAsyncEnumerable<T> source, CancellationToken cancellationToken);
+    public static ConfiguredCancelableAsyncEnumerable<T> WithCancellation<T>(this IAsyncEnumerable<T> source, CancellationToken cancellationToken) where T : allows ref struct;
}

namespace System.Runtime.CompilerServices;

-public readonly struct ConfiguredCancelableAsyncEnumerable<T>;
+public readonly struct ConfiguredCancelableAsyncEnumerable<T> where T : allows ref struct;

API Usage

IAsyncEnumerable<Span<byte>> source = GetSource();

await foreach (source.ConfigureAwait(false).WithCancellation(ct))
{

}

Alternative Designs

Can ToBlockingEnumerable be also included? The implementation may need to be tweaked.

Risks

Should be low. Both ConfigureAwait and WithCancellation are about the ValueTask<bool> MoveNext() member of IAsyncEnumerable<T>, which shouldn't be bothered by ref struct for implementation.

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[bartonjs]

Video

• Should we also do ToBlockingEnumerable at the same time? No, it's implemented as an iterator, and therefore cannot employ that constraint.
• Looks good as proposed.

namespace System.Threading.Tasks
{
    public static class TaskAsyncEnumerableExtensions
    {
-       public static ConfiguredCancelableAsyncEnumerable<T> ConfigureAwait<T>(this IAsyncEnumerable<T> source, bool continueOnCapturedContext);
+       public static ConfiguredCancelableAsyncEnumerable<T> ConfigureAwait<T>(this IAsyncEnumerable<T> source, bool continueOnCapturedContext) where T : allows ref struct;
-       public static ConfiguredCancelableAsyncEnumerable<T> WithCancellation<T>(this IAsyncEnumerable<T> source, CancellationToken cancellationToken);
+       public static ConfiguredCancelableAsyncEnumerable<T> WithCancellation<T>(this IAsyncEnumerable<T> source, CancellationToken cancellationToken) where T : allows ref struct;
}

namespace System.Runtime.CompilerServices
{
-    public readonly struct ConfiguredCancelableAsyncEnumerable<T>;
+    public readonly struct ConfiguredCancelableAsyncEnumerable<T> where T : allows ref struct;
}

[stephentoub]

Implemented by #111953