HashSet is a set data type that cannot contain the same values. (collection type)

Per geekforgeeks explanation differences between Hashmap and Hashset:

HashSet is an implementation of Set Interface which does not allow duplicate value. The main thing is, objects that are stored in HashSet must override equals() for check for equality, and hashCode() methods for no duplicate value are stored in our set. 

HashMap is an implementation of Map Interface, which maps a key to value. Duplicate keys are not allowed in a Map. HashMap allows null values and null keys. Both HashSet and HashMap are not synchronized.

To understand hashset in more detail, additional resource can be found here:

https://www.youtube.com/watch?v=UzxugFpv0dI 

You can find detailed further information from Rust Official documentation: https://doc.rust-lang.org/std/collections/struct.HashSet.html 

Resource:

https://www.geeksforgeeks.org/difference-between-hashmap-and-hashset/