rustc::lint::builtin::UNUSED_EXTERN_CRATES

pub static UNUSED_EXTERN_CRATES: &'static Lint = &::rustc::lint::Lint{name: "UNUSED_EXTERN_CRATES",
                     default_level: ::rustc::lint::Allow,
                     desc: "extern crates that are never used",}

Unstable (rustc_private)