Many programming languages have a 'prelude': a particular subset of the libraries that come with the language. Every program imports the prelude by default.

For example, it would be annoying to add use std::io::println; to every single program, and the vast majority of Rust programs will wish to print to standard output. Therefore, it makes sense to import it into every program.

Rust's prelude has three main parts:

  1. io::print and io::println.
  2. Core operators, such as Add, Mul, and Not.
  3. Various types and traits, such as Clone, Eq, and comm::Chan.