Programming Rust, 3rd Edition

Chapter 22. Macros

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Matt Madden

Rust supports macros, a way to extend the language in ways that go beyond what you can do with functions alone. For example, we’ve seen the assert_eq! macro, which is handy for tests:

assert_eq!(gcd(6, 10), 2);

This could have been written as a generic function, but the assert_eq! macro does several things that functions can’t do. One is that when an assertion fails, assert_eq! prints an error message using a format string that the user can provide by passing extra arguments to the macro. Functions in Rust can’t take a variable number of arguments. Macros can, because the way they work is completely different.

Macros are a kind of shorthand. During compilation, before types are checked and long before any machine code is generated, each macro call is expanded—that is, it’s replaced with some Rust code. The preceding macro call expands to something ...