Move cfg_if macro from libstd to libcore

[dlrobertson]

Summary

If possible I would like to move the cfg_if macro from src/libstd/macros.rs to src/libcore/macros.rs.

Motivation

The cfg_if macro could potentially increase the readability of some cfgs in no_std code. In particular it could dramatically improve the readability of some code in libcore e.g. ffi::VaList (why I'm interested in moving it 😄). I don't see anything in the macro that would make it invalid in libcore.

[alexcrichton]

Unfortunately we don't have a great way to export a macro from libcore to use in libstd that isn't part of the public API. We aren't quite ready yet I think to make this part of the public API but duplicating the definition should be fine as its a pretty simple macro

[dlrobertson]

@alexcrichton 👍 Thanks for the info. I'll hold off on duplicating it in libstd until I know I need it then.

[djc]

@alexcrichton can you elaborate on why we don't want to make cfg_if public? It came up recently in the left-pad index dicussions, seems like it would be good to expose this publicly in std and core.

[alexcrichton]

More discussion about this is at #59443 as well. I do not personally have an opinion on whether or not this should be exported from core or not. I don't personally have the motivation to push for that stabilization and/or inclusion myself.

[Mark-Simulacrum]

I am going to close this issue as it's been somewhat obsoleted by the fact that libstd depends on cfg-if (the crate) now, and libcore can always duplicate if necessary. I imagine we might even be able to get libcore to depend on cfg-if (since it's a macro-only crate I believe), but that's likely harder :)