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.