A unit-like struct is a struct that does not have any fields. They are called unit-like because they behave similarly to the unit type (), which is a type that has no size and no data. Unit-like structs can be useful when you need to implement a trait on some type but don't have any data that you want to store in the type itself. Here is an example of a unit-like struct:
struct AlwaysEqual;
This struct has no fields, so it does not store any data. We can instantiate a variable of this type like this:
let subject = AlwaysEqual;
The `subject` variable will now refer to an instance of the `AlwaysEqual` struct. This instance does not store any data, but it can still be used to implement traits. For example, we could implement the `PartialEq` trait on the `AlwaysEqual` struct, which would allow us to compare two instances of the struct and determine whether they are equal.
Unit-like structs can be useful in a variety of situations. For example, they can be used to represent abstract concepts that do not have any associated data. They can also be used to implement traits on types that do not need to store any data.
