[tialaramex]

To be fair RPIT is part of a family, but they're actually just a weird way to name a variety of features that it made sense to give similar syntax in Rust. "IT" in RPIT means "impl Trait" the syntax, but what that syntax does varies by position (the P).

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

Return Position Impl Trait is Existential Types, but Argument Position Impl Trait is just a simpler syntax for some Generics, same syntax, different meaning. This is actually one of the places where Rust is probably easier to pick up in use than to understand in theory. APIT and RPIT both "feel" right, but in type theory these are radically different features.

In that way it's like some natural languages, native English speakers don't notice a bunch of the grammar rules they learned as kids, those rules just "feel" right and not formally taught.