User-defined types

As the name says, user-defined types are types that are defined by you. These can be composed of several types. They may either be a wrapper over a primitive type or a composition of several user defined types. They come in three forms: structures, enumerations, and unions, or more commonly known as structs, enums, and unions. They allow you to easily express you data. The naming convention for user-defined types follows the CamelCase style. Structs and enums are more powerful than C's structs and enums, while unions in Rust are very close to C and are there mainly to interact with C code bases. We'll cover structs and enums in this section, while unions are covered in Chapter 7, Advanced Concepts.