A number of minor features of Rust are not central enough to have their own syntax, and yet are not implementable as functions. Instead, they are given names, and invoked through a consistent syntax: some_extension!(...).

Users of rustc can define new macros in two ways:

  • Macros define new syntax in a higher-level, declarative way.
  • Procedural Macros can be used to implement custom derive.

And one unstable way: compiler plugins.