I've been doing some work on gfx and surfman lately which, due to the nature of providing cross-platform APIs with platform specific backends, can have lots of #[cfg] checks. You would wish that these could always be simple and apparent such as #[cfg(windows)] or #[cfg(feature = "x11")], but often t…

Thanks so much to all of you here, @sfackler , @Yandros , and @ratmice . With pieces of all of your input I was finally able to finish a cfg_aliases! macro_rules! macro that accomplishes what I needed! https://crates.io/crates/cfg_aliases After @ratmice brought up the target_cfg! macro from tectonic …

I agree that this is a desirable feature. FWIW, wih enough macromancy one can achieve this: make_cfg_alias! { macro let (cfg_my_combination, cfg_not_my_combination) = cfg_and_cfg_not!( all( feature = "a", not(feature = "b"), …

[image] sfackler: You can make a build script that emits should_have_x11 cfgs in the right situations: https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-cfg Ooh, that's a good idea. Definitely a simple solution. [image] Yandros: FWIW, wih enough macromancy one can achie…

Ah, the macro I used could still use some sophistication, because it doesn't allow you to have aliases reference aliases.

@Yandros I just realized what you were doing with the crazy @with_dollar! step in that macro. That is some awesome macro trickery. Anyway, I'm really trying to find out if there is a way to make my simple cfg_aliases macro in the example above more sophisticated so that it can expand aliases that a…

OK, breaking it down further it looks like it is impossible to get a macro to expand in either #[cfg()] attribute such as #[cfg(my_macro!())] or inside of the cfg!() macro such as cfg!(my_macro!()). This won't work: macro_rules! breakme { ($config:meta, $($tokens:literal)*) => { if cfg!…

@zicklag a workaround is to define macros that do: macro_rules! identity! {( $($input:tt)* ) => ( $($input)* )} macro_rules! my_macro {( @pipe_to $out:ident ! <input> ) => ( $out! { <expansion> } ); ( <input> ) => ( my_macro! { @pipe_to identity! <in…

Thanks, I'll try it out and see if it helps. :slight_smile:

Well, it looks like I'm still having trouble with the cfg! macro, I think I'll just to a proc macro with proc macro hack, which compiles real quick so I don't mind it much. My question now is how you are supposed to return errors from a proc macro? Proc macro functions don't return Results so is it…

Maybe i'm missing something, but why wouldn't cfg_attr work?

Any Such Thing as `#[cfg]` Aliases?

sfackler March 29, 2020, 2:23am 2

You can make a build script that emits should_have_x11 cfgs in the right situations: Build Scripts - The Cargo Book

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Any Such Thing as `#[cfg]` Aliases?

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