rustc::lint::builtin::UNUSED_FEATURES

pub static UNUSED_FEATURES: &'static Lint = &::rustc::lint::Lint{name: "UNUSED_FEATURES",
                     default_level: ::rustc::lint::Warn,
                     desc:
                         "unused or unknown features found in crate-level #[feature] directives",}

Unstable (rustc_private)