src/macros/designators.md

# Designators

The arguments of a macro are prefixed by a dollar sign `$` and type annotated
with a *designator*:

```rust,editable
macro_rules! create_function {
    // This macro takes an argument of designator `ident` and
    // creates a function named `$func_name`.
    // The `ident` designator is used for variable/function names.
    ($func_name:ident) => {
        fn $func_name() {
            // The `stringify!` macro converts an `ident` into a string.
            println!("You called {:?}()",
                     stringify!($func_name));
        }
    };
}

// Create functions named `foo` and `bar` with the above macro.
create_function!(foo);
create_function!(bar);

macro_rules! print_result {
    // This macro takes an expression of type `expr` and prints
    // it as a string along with its result.
    // The `expr` designator is used for expressions.
    ($expression:expr) => {
        // `stringify!` will convert the expression *as it is* into a string.
        println!("{:?} = {:?}",
                 stringify!($expression),
                 $expression);
    };
}

fn main() {
    foo();
    bar();

    print_result!(1u32 + 1);

    // Recall that blocks are expressions too!
    print_result!({
        let x = 1u32;

        x * x + 2 * x - 1
    });
}
```

These are some of the available designators:

* `block`
* `expr` is used for expressions
* `ident` is used for variable/function names
* `item`
* `literal` is used for literal constants
* `pat` (*pattern*)
* `path`
* `stmt` (*statement*)
* `tt` (*token tree*)
* `ty` (*type*)
* `vis` (*visibility qualifier*)

For a complete list, see the [Rust Reference].

[Rust Reference]: https://doc.rust-lang.org/reference/macros-by-example.html