Macro with_builtin_macros::with_builtin

with_builtin!() { /* proc-macro */ }

Pre-processor (pattern) with hard-coded implementations for some of the built-in macros, so as to be able to chain their result with custom macros.

• Click here to see the list of supported macros

By using a pre-processor, this also enables "using" macros where it is otherwise forbidden.

For instance, the following code fails to compile:

fn concat_idents!(f, o, o) ()
{}

fn main ()
{
    foo();
}

But the following call does not:

use ::with_builtin_macros::with_builtin;

with_builtin!(let $fname = concat_idents!(f, o, o) in {
    fn $fname ()
    {}
});

fn main ()
{
    foo();
}

• Note: for this behavior, with some added neat features (such as case conversion), see the ::paste::paste! macro.

Even if we forget about allowed macro call-sites, there is also the issue of macro evaluation order leading to things like the following not working:

/// Some (proc-)macro that expects a string literal containing a hexadecimal
/// sequence of digits, that it decodes into the represented byte sequence.
use ::hex_literal::hex;

const BYTES: &[u8] = hex!(include_str!(concat!(
    env!("CARGO_MANIFEST_DIR"), "/", "data.hex",
)));

With with_builtin!, however, the following works:

/// Some (proc-)macro that expects a string literal containing a hexadecimal
/// sequence of digits, that it decodes into the represented byte sequence.
use ::hex_literal::hex;
use ::with_builtin_macros::with_builtin;

const BYTES: &[u8] = {
    with_builtin!(let $hex_str = include_str_from_root!("data.hex") in {
        hex!($hex_str)
    })
};