rustc::lint::builtin::PRIVATE_IN_PUBLIC

pub static PRIVATE_IN_PUBLIC: &'static Lint = &::rustc::lint::Lint{name: "PRIVATE_IN_PUBLIC",
                     default_level: ::rustc::lint::Warn,
                     desc:
                         "detect private items in public interfaces not caught by the old implementation",}

Unstable (rustc_private)