It's hard to say because the work for those features hasn't even been started; the MIR refactoring is still underway. Once that's done, RFCs for this stuff will start to appear, and we'll have a better idea of overall schedule. I would say that the current schedule is "longer than months, shorter than a few years".

I myself _really_ want higher kinded types, but at the same time, I would rather wait and do it right than rush and regret thing slater. We have a solid language as it is; the need to get these more advanced features is not particularly urgent, in my opinion. I like to think on the long scale when it comes to the language: if it's going to be around 40 or 50 years, we're in year one. There's plenty of time.

That's kind of a weird litmus test of the power of the language's type manipulation: it only applies to languages where functions take multiple arguments, it's a pretty niche use case (since the workaround in Rust is straightforward), and there's nothing particularly sophisticated about variadic generics (what you need to solve the problem). I could equally say that my litmus test of a language's generics system is whether the compiler prevents errors during template expansion.

Anyway, to answer your question: that's really easy in Rust, as long as you're on nightly. The FnMut trait (and friends) allow you to call functions with tuples as arguments. https://doc.rust-lang.org/std/ops/trait.FnMut.html