message: Remove unnecessary feature gates
  One wasn't necessary anymore at all. Others are only necessary when jit mode is enabled.
diff: -#![feature(rustc_private, decl_macro, never_type, hash_drain_filter, vec_into_raw_parts, once_cell)]
  +#![feature(rustc_private, decl_macro)]
  +#![cfg_attr(feature = "jit", feature(never_type, vec_into_raw_parts, once_cell))]
  #![warn(rust_2018_idioms)]
  #![warn(unused_lifetimes)]
  #![warn(unreachable_pub)]
  