Relationship between Higher Kinded Polymorphism, type inference, and Currying

Question

On Hacker News there is an interesting exchange about the async\await RFC for Rust.

The author of the proposal withoutboats is responding to a comment about the usage of async-await construct vs a monadic implementation in Rust. He notes three problems, the first one I am having some trouble parsing:

Higher kinded polymorphism results in trivially undecidable type inferences without something like currying; the restrictions needed to support it would be arbitrary and weird given that Rust does not have currying (essentially some rules to reconstruct the restrictions of currying in type operator context).

I have a decent understanding of higher-kinded polymorphism, type inference, and currying (or so I naively assert), and I even see why currying is useful for higher-kinded polymorphism, i.e. type constructors of the form * -> * -> *, a type which takes a type and produces a third.

But what I don't understand is why currying is necessary to avoid "trivially undecideable type inferences." Can someone provide an example of a such an undecideable inference, and why currying helps?