# Syntax Extensions

Before talking about Rust's different macro systems it is worthwhile to discuss the general mechanism they are built on: syntax extensions.

To do that, we must first discuss how Rust source is processed by the compiler, and the general mechanisms on which user-defined macros and proc-macros are built upon.

Note: This book will use the term syntax extension from now on when talking about all of rust's different macro kinds in general to reduce potential confusion with the upcoming declarative macro 2.0 proposal which uses the macro keyword.