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# Error code E0556

The `feature` attribute was badly formed.

Erroneous code example:

    #![allow(unused)]
    #![feature(foo_bar_baz, foo(bar), foo = "baz", foo)] // error!
    #![feature] // error!
    #![feature = "foo"] // error!
    fn main() {
    }

The `feature` attribute only accept a “feature flag” and can only be used on nightly. Example:

    #![feature(flag)]