It's worth noting that this isn't an "IL feature", but a new type that includes some knowledge of the type shape in the C# compiler. The existing F# package for async streams understands the new type, and it "just works" in F#.
So does C# with LINQ [1]. I guess C# language development tends to lean more on concrete syntax than libraries, which is a shame, but it is possible. It's also a shame LINQ can't be extended like computation expressions.