Rust's HashSet is a HashMap mapping to "()", which is Rust's equivalent to "void".

That is true for the internal implementation details but HashSet provides a different API, it is not just a type alias. So you don't have to worry about semantics of what the values mean (like the bools in Go), and it has no size overhead.

Python, Ruby, Perl. Languages where hashmaps are a builtin.

Java as well: https://github.com/pengisgood/jdk-source-code/blob/master/sr...

EDIT : and Rust: https://docs.rs/hashbrown/latest/hashbrown/hash_set/index.ht... (this is used by the default implementation of HashSet).

Python has its own set type not based on hashmap/dict, and that's been the case for years.

The set implementation points out some of the differences, at https://github.com/python/cpython/blob/main/Objects/setobjec... :

   Unlike the dictionary implementation, the lookkey function can return
   NULL if the rich comparison returns an error.

   Use cases for sets differ considerably from dictionaries where looked-up
   keys are more likely to be present.  In contrast, sets are primarily
   about membership testing where the presence of an element is not known in
   advance.  Accordingly, the set implementation needs to optimize for both
   the found and not-found case.

Update: Looks like I had good reason to be surprised. Thanks everyone for the examples.