Multiple consts within single `#[cfg()]`

help
1

Hello all,

What is the preferred way to define multiple const values based on a cfg macro? I'm looking for sort of an analog to the C #ifdef/#endif, something like the below:

#[cfg(target_family = "unix")] {
    const PLAT: Plat = Plat::Posix;
    const ENV_PATH_SEP: u8 =  0x3a;   // ":" on Posix
    const SOME_ARR: [u8; 3] = [4u8, 5, 2, 1];
}

#[cfg(target_family = "windows")] {
    const PLAT: Plat = Plat::Windows;
    const ENV_PATH_SEP: u8 = 0x3b; // ";" on Windows
    const SOME_ARR: [u8; 6] = [4u8, 5, 2, 1, 1, 1];
}

Currently the best option I have found is to either repeat the #[cfg] items (messy), or use

const PLAT: Plat = if cfg!(..) {...} else {...}

which doesn't work for arrays, since type with length must be specified before the if block.

I found Tracking issue for stmt_expr_attributes: Add attributes to expressions, etc. · Issue #15701 · rust-lang/rust · GitHub and the (I think) corresponding #![feature(stmt_expr_attributes)], but that seems to only allow the syntax within functions.

What is the current best way to accomplish this?

2

In Tokio, we define macros as seen here. Then you use them like here.

4 Likes
3

Thanks for the blazing fast response, that's perfect!

4

I don't know what the best practice is, but modules are another alternative.

3 Likes
5

Great suggestion, that's super clean. Thanks

6

I took Alice's idea and created a macro that allows specifying the macro within the block, possibly useful to others

macro_rules! cfg_block {
    ($( #[$meta:meta] {$($item:item)*} )*) => {
        $($(
            #[$meta]
            $item
        )*)*
    }
}

cfg_block! {
    // Unix config
    #[cfg(not(target_family = "windows"))] {
        const ENV_PATH_SEP: &str = ":";
        const SOME_ARR: [u8; 4] = [4u8, 5, 2, 1];
    }
    // Windows config
    #[cfg(target_family = "windows")] {
        const ENV_PATH_SEP: &str = ";";
        const SOME_ARR: [u8; 2] = [4u8, 5];
    }
}
7

Just to simplify things,, I created a simple crate in case anyone would like to quickly use the macro: cfg_block - Rust

Also added an if/else syntax that works with #[cfg()] items.

8

There's a crate for that: https://crates.io/crates/cfg-if

4 Likes
9

Wow, that's the 5th most downloaded on crates.io.... And I spent two hours trying to replicate it. Should have known I was reinventing the wheel, thanks for the tip

10

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