set compile-time configuration variable in rust

tl;dr how to use "compile-time set variables" in Rust like it's done in the C preprocessor?

In C, the preprocessor macro language allows setting preprocessor variables that can be used in later preprocessing.

Here is a contrived example in C that checks a magic list of variables once then saves that result for later use during the preprocessor stage.

#if defined(_MSC_EXTENSIONS) || defined(__IOS__)
    #define IMPORT_MAGIC_LIB 1
#else
    #define IMPORT_MAGIC_LIB 0
#endif

// ... later ...

#if IMPORT_MAGIC_LIB == 1
#include "magic_lib.h"
#else
#include "plain_lib.h"
#endif

I want to do something similar in Rust (made-up pseudo-code; will not compile)

#[if(any(target_os = "windows", target_os = "ios"))]
#set IMPORT_MAGIC_LIB : bool = true;
#[else]
#set IMPORT_MAGIC_LIB : bool = false;
#[endif]

// ... later ...

#[if(IMPORT_MAGIC_LIB)]
use ::magic_lib as my_lib;
#[else]
use ::plain_lib as my_lib;
#[endif]

What I'd like to do in Rust is check some special "magic" list just once (any(target_os = "windows", target_os = "ios")) and re-use that check elsewhere during the macro processing compile stage. I want avoid repeating the list of target_os in multiple places.

Unfortunately, Rust has no good solution for that. There are, however, two options:

1. You can create a macro, with which you wrap items and it appends the #[cfg]. tokio does that, for example:

macro_rules! cfg_magic_lib {
    { $($item:item)* } => {
        $(
            #[cfg(any(target_os = "windows", target_os = "ios"))]
            $item
        )*
    }
}
macro_rules! cfg_not_magic_lib {
    { $($item:item)* } => {
        $(
            #[cfg(not(any(target_os = "windows", target_os = "ios")))]
            $item
        )*
    }
}

cfg_magic_lib! {
    use ::magic_lib as my_lib;
}
cfg_not_magic_lib! {
    use ::plain_lib as my_lib;
}

2. You can use a build script, that will inspect the set cfgs and set a new cfg according to them:

fn main() {
    let target_os = std::env::var("CARGO_CFG_TARGET_OS").unwrap();
    if target_os == "windows" || target_os == "ios" {
        println!("cargo:rustc-cfg=magic_lib");
    }
}

Then in the library:

#[cfg(magic_lib)]
use ::magic_lib as my_lib;
#[cfg(not(magic_lib))]
use ::plain_lib as my_lib;

You can also use the cfg-if crate.