RFC: conditional attributes

[huonw]

Currently the only way to have attributes that apply conditionally is to duplicate the whole item (e.g. #[link()] extern metadata that change per platform, or wish to have dynamic/static linking toggled externally). It'd be nice to have be able to avoid this.

Proposal: add cfg_attr(predicate, attribute) that only applies attribute if predicate is satisfied. e.g.

#[cfg_attr(windows, link(name="mylib_win"))]
#[cfg_attr(not(windows), link(name="mylib"))]
extern {
     fn mylib_some_func();
     // ...
}

currently has to be

#[cfg(windows)]
#[link(name="mylib_win")]
extern {
     fn mylib_some_func();
     // ...
}

#[cfg(not(windows))]
#[link(name="mylib")]
extern {
     fn mylib_some_func();
     // ...
}

[sfackler]

I assume that #[cfg_attr(foo, bar)] would be replaced with either #[bar] or nothing in the cfg pass?

[huonw]

Something like that.

[alexcrichton]

One snag I can imagine is that sometimes predicates are more complicated than just cfg(windows). There's an issue on a more general cfg language which I think would fix it, but this does seem like a nice solution!

[huonw]

That issue seems to be #2119.

[huonw]

Needs to become a proper RFC...