Existential types in Rust

Yes, that's not really an existential type, and we can tell because if we try to use it with different types it works - the compiler will just monomorphize your function again with the new type because this is just generics.

Here's a Jon Gjengset talk about all the places we can write "impl Trait" and they mean different things. Jon talks about the "Existential types" as Return Position Impl Trait, RPIT in 1.26. He literally says this is Existential Types about 3 minutes in.

https://www.youtube.com/watch?v=CWiz_RtA1Hw

The Argument Position Impl Trait APIT is really easy and so doesn't take long in the video - it's possible to watch the video distractedly and not realise Jon even mentioned APIT because it's like "Here's what this does, that was boring, it's just a generic with easy syntax", it is not an Existential.