RFC: #[cfg(accessible(..) / version(..))]

[Centril]

🖼️ Rendered

⏭ Tracking issue (#[cfg(version(..))]

⏭ Tracking issue (#[cfg(accessible(..))]

📝 Summary

Permit users to #[cfg(..)] on whether:

• they have a certain minimum Rust version (#[cfg(version(1.27))]).
• a certain external path is accessible (#[cfg(accessible(::std::mem::ManuallyDrop))]).

💖 Thanks

To @eddyb, @rpjohnst, @kennytm, @aturon, and @rkruppe for discussing the feature with me.
To @Mark-Simulacrum, @MajorBreakfast, @alexreg, @alercah, and @joshtriplett for reviewing the draft version of this RFC.

[Centril]

cc @MajorBreakfast re. use case for futures. :)

[ogoffart]

Another point is that things that are gated sill need to use valid rust grammar.

Let's assume that the version 1.42 adds support for throwing function, and using .if:

#[cfg(version(1.42))]
fn foo(x : bool) -> u32 throw(SomeError) {
    x.if { something() } else { 0 }
}

This won't compile in current rust, even after this rfc is implemented, because this is not valid grammar, and rustc still check the grammar of items even if they are going to be removed by attribute macro or cfg attribute.

I suppose it would not make sense to relax the parsing rules of items that are disabled by #[cfg]

So one must wrap the item inside a macro such as cfg-if!{...} but which would not expand the items for the disabled configuration. Something like this should work: #[cfg($cfg)] identity!{ $($body)* }.

Btw, I've made a crate with a macro if_rust_version with macro_rules that allows conditional like that depending on the rust version: https://github.com/ogoffart/if_rust_version

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

The RFC will be merged soon.

[Centril]

🎉 Huzzah! This RFC has been merged! 🎉

Tracking issues:

• Tracking issue for RFC 2523, #[cfg(version(..))] rust#64796
• Tracking issue for RFC 2523, #[cfg(accessible(::path::to::thing))] rust#64797