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)]