The `feature` attribute was badly formed.
Erroneous code example:
```compile_fail,E0556
#![feature(foo_bar_baz, foo(bar), foo = "baz", foo)] // error!
#![feature] // error!
#![feature = "foo"] // error!
```
The `feature` attribute only accept a "feature flag" and can only be used on
nightly. Example:
```ignore (only works in nightly)
#![feature(flag)]
```