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):

```ignore (depends on release channel)
#![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).
