# Macros

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!(...)`.

There are two ways to define new macros:

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

## Macro Invocation

> Syntax
> MacroInvocation :
> SimplePath `!` DelimTokenTree

> DelimTokenTree :
> `(` TokenTree* `)`
> | `[` TokenTree* `]`
> | `{` TokenTree* `}`

> TokenTree :
> Tokenexcept delimiters | DelimTokenTree

> MacroInvocationSemi :
> SimplePath `!` `(` TokenTree* `)` `;`
> | SimplePath `!` `[` TokenTree* `]` `;`
> | SimplePath `!` `{` TokenTree* `}`

A macro invocation executes a macro at compile time and replaces the invocation with the result of the macro. Macros may be invoked in the following situations:

- Expressions and statements
- Patterns
- Types
- Items including associated items
- `macro_rules` transcribers

When used as an item or a statement, the MacroInvocationSemi form is used where a semicolon is required at the end when not using curly braces. Visibility qualifiers are never allowed before a macro invocation or `macro_rules` definition.

    # #![allow(unused_variables)]
    #fn main() {
    // Used as an expression.
    let x = vec![1,2,3];

    // Used as a statement.
    println!("Hello!");

    // Used in a pattern.
    macro_rules! pat {
        ($i:ident) => (Some($i))
    }

    if let pat!(x) = Some(1) {
        assert_eq!(x, 1);
    }

    // Used in a type.
    macro_rules! Tuple {
        { $A:ty, $B:ty } => { ($A, $B) };
    }

    type N2 = Tuple!(i32, i32);

    // Used as an item.
    # use std::cell::RefCell;
    thread_local!(static FOO: RefCell<u32> = RefCell::new(1));

    // Used as an associated item.
    macro_rules! const_maker {
        ($t:ty, $v:tt) => { const CONST: $t = $v; };
    }
    trait T {
        const_maker!{i32, 7}
    }

    // Macro calls within macros.
    macro_rules! example {
        () => { println!("Macro call in a macro!") };
    }
    // Outer macro `example` is expanded, then inner macro `println` is expanded.
    example!();
    #}