Rust by Example

Designators

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

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

});

}

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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.