Feature attributes are only allowed on the nightly release channel. Stable or beta compilers will not comply.
Example of erroneous code (on a stable compiler):
#![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).