Add warn(unreachable_pub) to rustc_traits.

compiler/rustc_traits/src/codegen.rs

@@ -21,7 +21,7 @@ use tracing::debug;
 /// obligations *could be* resolved if we wanted to.
 ///
 /// This also expects that `trait_ref` is fully normalized.
-pub fn codegen_select_candidate<'tcx>(
+pub(crate) fn codegen_select_candidate<'tcx>(
     tcx: TyCtxt<'tcx>,
     (param_env, trait_ref): (ty::ParamEnv<'tcx>, ty::TraitRef<'tcx>),
 ) -> Result<&'tcx ImplSource<'tcx, ()>, CodegenObligationError> {