It's nonsense. Async in rust is just syntactic sugar around a function signature. You can merrily call async functions from sync rust, you just have to know what to do with the future you get back.
"you just have to know what to do" is the problem. You can call any color from any color, but for some colors it's trivial, e.g. sync function from a sync or async one, or a non-failing function from a failing or non-failing one.
I don't want to be able to call fallible function from an infallible one trivially, I want the compiler to force me to specify what exactly I'm going to do with an error if it happens. Likewise for async-from-sync: there are many ways I could call these: I can either create a single threaded executor and use it to complete the future to completion, or maybe I want to create a multithreaded executor, or maybe I expect the future complete in a single poll and never suspend and I don't even need a scheduler.
Well yes to all that. I still don't see the problem. An async function isn't really an async function, it's a sync function that returns a future. Would it be better if all that was manual? I've done quite a bit of stuff using manual async traits and it's painful and I highly value the syntax sugar that async brings. That said, I certainly don't want some executor running quietly behind the scenes doing async stuff for me without my explicit and full control. If I want to manually poll a future, that's for me to decide.
You seem to raise valid points and I don't disagree with you, however I don't see how it's relevant to the original concern regarding colored functions.
I suppose I'm struggling to understand what "colour" means in the context of Rust. It's surely just another word for signature. For some reason it's trotted out every time there's a discussion about async. I can only assume it's to do with the original use of the term for JavaScript async (which I know almost nothing about and have no opinion on), but I just cannot see its point in Rust async.
It has to do with the fact that most of the code in the project is not async but having to call async functions often propagates all the way to your main function. It's infectious and many people don't like it, myself included, that's why I'm working with Elixir and Golang where async is transparent and 99% automatic, or explicit but non-infectious, respectively.
I do love Rust and found a number of very valid uses for it but its async story leaves a lot to be desired. I don't enjoy writing it though I do enjoy the results.
