How do I get the new async semantics working in VS2017 RC?

Question

Quoting from Visual Studio 2017 RC Release Notes

Language Extensions and Analyzers

This release includes some proposed new langua

Answer 1

The TasklikeAttribute attribute name turns out isn't what's implemented in VS2017 RC, that's from a different version of the proposal. What's actually implemented relies on a type System.Runtime.CompilerServices.AsyncMethodBuilderAttribute, which appears to work exactly the same way.

I was not able to find this documented, but I was able to find this in the Roslyn tests, for example CodeGenAsyncTests.cs:

[AsyncMethodBuilder(typeof(ValueTaskMethodBuilder))]
struct ValueTask { }
...
namespace System.Runtime.CompilerServices { class AsyncMethodBuilderAttribute : System.Attribute { public AsyncMethodBuilderAttribute(System.Type t) { } } }