`macro_metavar_expr_concat`

The tracking issue for this feature is: #124225

In stable Rust, there is no way to create new identifiers by joining identifiers to literals or other identifiers without using procedural macros such as paste. #![feature(macro_metavar_expr_concat)] introduces a way to do this, using the concat metavariable expression.

This feature uses the syntax from macro_metavar_expr but is otherwise independent. It replaces the since-removed unstable feature concat_idents.

This is an experimental feature; it and its syntax will require a RFC before stabilization.

Overview

#![feature(macro_metavar_expr_concat)] provides the concat metavariable expression for creating new identifiers:

#![allow(unused)]
#![feature(macro_metavar_expr_concat)]

fn main() {
macro_rules! create_some_structs {
    ($name:ident) => {
        pub struct ${ concat(First, $name) };
        pub struct ${ concat(Second, $name) };
        pub struct ${ concat(Third, $name) };
    }
}

create_some_structs!(Thing);
}

This macro invocation expands to:

#![allow(unused)]
fn main() {
pub struct FirstThing;
pub struct SecondThing;
pub struct ThirdThing;
}

Syntax

This feature builds upon the metavariable expression syntax ${ .. } as specified in RFC 3086 (macro_metavar_expr). concat is available like ${ concat(items) }, where items is a comma separated sequence of idents and/or literals.

Examples

Create a function or method with a concatenated name

#![allow(unused)]
#![feature(macro_metavar_expr_concat)]

fn main() {
macro_rules! make_getter {
    ($name:ident, $field: ident, $ret:ty) => {
        impl $name {
            pub fn ${ concat(get_, $field) }(&self) -> &$ret {
                &self.$field
            }
        }
    }
}

pub struct Thing {
    description: String,
}

make_getter!(Thing, description, String);
}

This expands to:

#![allow(unused)]
fn main() {
pub struct Thing {
    description: String,
}

impl Thing {
    pub fn get_description(&self) -> &String {
        &self.description
    }
}
}

Create names for macro generated tests

#![allow(unused)]
#![feature(macro_metavar_expr_concat)]

fn main() {
macro_rules! test_math {
    ($integer:ident) => {
        #[test]
        fn ${ concat(test_, $integer, _, addition) } () {
            let a: $integer = 73;
            let b: $integer = 42;
            assert_eq!(a + b, 115)
        }

        #[test]
        fn ${ concat(test_, $integer, _, subtraction) } () {
            let a: $integer = 73;
            let b: $integer = 42;
            assert_eq!(a - b, 31)
        }
    }
}

test_math!(i32);
test_math!(u64);
test_math!(u128);
}

Running this returns the following output:

running 6 tests
test test_i32_subtraction ... ok
test test_i32_addition ... ok
test test_u128_addition ... ok
test test_u128_subtraction ... ok
test test_u64_addition ... ok
test test_u64_subtraction ... ok

test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

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