macro_rules!
Rust provides a powerful macro system that allows metaprogramming. As you've
seen in previous chapters, macros look like functions, except that their name
ends with a bang !, but instead of generating a function call, macros are
expanded into source code that gets compiled with the rest of the program.
However, unlike macros in C and other languages, Rust macros are expanded into
abstract syntax trees, rather than string preprocessing, so you don't get
unexpected precedence bugs.
Macros are created using the macro_rules! macro.
So why are macros useful?
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Don't repeat yourself. There are many cases where you may need similar functionality in multiple places but with different types. Often, writing a macro is a useful way to avoid repeating code. (More on this later)
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Domain-specific languages. Macros allow you to define special syntax for a specific purpose. (More on this later)
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Variadic interfaces. Sometimes you want to define an interface that takes a variable number of arguments. An example is
println!which could take any number of arguments, depending on the format string. (More on this later)