Macro cfg_block::cfg_block

macro_rules! cfg_block {
    (
        $(
            if #[cfg($meta:meta)] {
                $($item:item)*
            } else {
                $($item_f:item)*
            }
        )*
    ) => { ... };
    (
        $(
            #[$meta:meta] {
                $( $item:item )*
            }
        )*
    ) => { ... };
}

Allow applying inner procedural macros to a {} block, which helps a lot with platform-based const values.

Examples

Basic example handling differnt

use cfg_block::cfg_block;

cfg_block! {
    #[cfg(target_family = "unix")] {
        const PLATFORM: &str = "posix !";
        const MY_NUMBER: u8 = 5;
    }
    #[cfg(target_family = "windows")] {
        const PLATFORM: &str = "windows !";
        const MY_NUMBER: u16 = 20;
    }
    #[cfg(target_family = "wasm")] {
        const PLATFORM: &str = "web !";
        const MY_NUMBER: i32 = -5;
    }
}


// This is a proof of concept
match PLATFORM {
    "posix !" => assert_eq!(MY_NUMBER as i32, 5),
    "windows !" => assert_eq!(MY_NUMBER as i32, 20),
    "web !" => assert_eq!(MY_NUMBER as i32, -5),
    &_ => panic!(),
}

Or, with the if/else syntax (only works for cfg macros):

use cfg_block::cfg_block;

cfg_block! {
    if #[cfg(mips)] {
        const STR_A: &str = "where did you get this processor";
        const STR_B: &str = "mips just makes a good passing doctest";
    } else {
        const STR_A: &str = "good!";
        const STR_B: &str = "better!";
    }
}

assert_eq!(STR_A, "good!");
assert_eq!(STR_B, "better!");

Currently, if/else statements need to be in a separate cfg_block! {} from options that do not use if/else.

It is also possible to put anything that rust considers an item inside the block. Note that this does not include let bindings.

use cfg_block::cfg_block;

cfg_block! {
    if #[cfg(mips)] {
        const STR_A: &str = "where did you get this processor";
    } else {
        const STR_A: &str = "good!";
    }

    if #[cfg(not(mips))] {
        assert_eq!(STR_A, "good!");
    } else {}
}