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+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)]
+```