Proposal: Source Generation for Task returning methods

[shmuelie]

Summary

Right now, public async methods must return IAsnycAction, IAsnycActionWithProgress<TProgress>, IAsnycOperation<TResult>, or IAsnycOperationWithProgress<TResult, TProgress>. Instead, developers should be allowed to return Task and Task<T> and have a source generator create the "wrapping" version that returns the WinRT type.

Rationale

Usage of TPL things is much easier for developers, and allows for easier usage of async/await.

Important Notes

Should also support methods that have progress and cancelation.

Given RemoteThing.cs, the methods marked as [DebuggerNonUserCode] should be generated for the user

Open Questions

Should this be the default or should it be opt-in?