Feature attributes are only allowed on the nightly release channel. Stable or beta compilers will not comply.

Erroneous code example:

#![feature(non_ascii_idents)] // error: `#![feature]` may not be used on the
                              //        stable release channel

If you need the feature, make sure to use a nightly release of the compiler (but be warned that the feature may be removed or altered in the future).