delay error for enabling unstable lib features

If #![feature] is used outside the nightly channel for only lib
features, the check will be delayed to the stability pass after
parsing. This is done so that appropriate help messages can be shown if
the #![feature] has been used needlessly

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -702,10 +702,16 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
 }
 
 fn maybe_stage_features(sess: &Session, krate: &ast::Crate) {
+    // checks if `#![feature]` has been used to enable any lang feature
+    // does not check the same for lib features unless there's at least one
+    // declared lang feature
     use rustc_errors::Applicability;
 
     if !sess.opts.unstable_features.is_nightly_build() {
         let lang_features = &sess.features_untracked().declared_lang_features;
+        if lang_features.len() == 0 {
+            return;
+        }
         for attr in krate.attrs.iter().filter(|attr| attr.has_name(sym::feature)) {
             let mut err = struct_span_err!(
                 sess.parse_sess.span_diagnostic,

compiler/rustc_passes/src/stability.rs

@@ -929,6 +929,16 @@ pub fn check_unused_or_stable_features(tcx: TyCtxt<'_>) {
     let declared_lib_features = &tcx.features().declared_lib_features;
     let mut remaining_lib_features = FxHashMap::default();
     for (feature, span) in declared_lib_features {
+        if !tcx.sess.opts.unstable_features.is_nightly_build() {
+            struct_span_err!(
+                tcx.sess,
+                *span,
+                E0554,
+                "`#![feature]` may not be used on the {} release channel",
+                env!("CFG_RELEASE_CHANNEL")
+            )
+            .emit();
+        }
         if remaining_lib_features.contains_key(&feature) {
             // Warn if the user enables a lib feature multiple times.
             duplicate_feature_err(tcx.sess, *span, *feature);