[kccqzy]

Existential types are great! Although I must mention that existential types are basically the way to model OO-style information hiding. It's a great tool to have in any mature type system, although I do wonder how it is compiled. Trait objects are kind of easy to imagine: a heap-allocated dictionary of methods for that trait. I do wonder how heap allocation is being avoided in this case.

EDIT: I didn't read closely enough. Only a single type is allowed, so the traditional existential type construct ("trait objects") are still needed.

[kibwen]

There's intentionally no heap allocation or virtual dispatch when using `impl Trait`; if we were content with that, we wouldn't have bothered implementing it and just continued on as we were with `Box<Trait>`. :P