Or Go.

In Go, it's idiomatic to use map[T]struct{} for a HashSet. In fact, the compiler optimises struct{} so that it doesn't even allocate.

This is also how HashSet is implemented in Rust (internally its a HashMap<T, ()>, and zero sized structs are optimized out just like Go).

But by providing a set type, Rust is able to provide for users various set operations which Go requires you to write by hand. It becomes hard to argue that a set is simpler than a slice when you have to write your own methods dealing with union, disjunction, intersection, difference, subsets/supersets, etc every time.