Again, I haven't had time to write this out more clearly yet, but there's an older thread with similar ideas if anyone is interested: Pre-Pre-RFC: async methods & bounding async fns - #63 by MajorBreakfast, and a nice summary of the argument here: rust-blog/2018-06-19-outer-return-type-approach.md at master · MajorBreakfast/rust-blog · GitHub