cfg meta variables

[alexcrichton]

It's quite often that a cfg directive in an attribute becomes fairy complicated when support for more platforms are added. Even worse, a cfg block is then often duplicated among many locations to configure off different items.

The thread_local/mod.rs file is a good example of the amount of duplication.

It would be nice to have something like the following

#![cfg_shortcut(foo, any(super, long, clause))]

#[cfg(foo)] ...
#[cfg(not(foo))] ...

Here the clause any(super, long, clause) would be shortened to the identifier foo which could then be used elsewhere. In general it would just be nice to make these long cfg directives much shorter and less error prone to copy-paste.

[blaenk]

Yeah this would be nice. This and bounds aliases from #804 should give us a more complete aliasing toolkit.

[lambda-fairy]

If we expand macros within attributes, then this feature would drop out as a special case.

For example:

macro_rules! foo {
    () => (any(super, long, clause))
}

#[cfg(foo!())] ...
#[cfg(not(foo!()))] ...

Users wouldn't need to learn special syntax either -- just apply their existing macro knowledge.