Extract common logic for iterating over features

Two places doing the same thing is enough to motivate me to extract this to a method :)

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -622,11 +622,7 @@ fn maybe_stage_features(sess: &Session, features: &Features, krate: &ast::Crate)
 }
 
 fn check_incompatible_features(sess: &Session, features: &Features) {
-    let enabled_lang_features =
-        features.enabled_lang_features().iter().map(|feat| (feat.gate_name, feat.attr_sp));
-    let enabled_lib_features =
-        features.enabled_lib_features().iter().map(|feat| (feat.gate_name, feat.attr_sp));
-    let enabled_features = enabled_lang_features.chain(enabled_lib_features);
+    let enabled_features = features.enabled_features_iter_stable_order();
 
     for (f1, f2) in rustc_feature::INCOMPATIBLE_FEATURES
         .iter()

compiler/rustc_feature/src/unstable.rs

@@ -93,6 +93,16 @@ impl Features {
         &self.enabled_features
     }
 
+    /// Returns a iterator of enabled features in stable order.
+    pub fn enabled_features_iter_stable_order(
+        &self,
+    ) -> impl Iterator<Item = (Symbol, Span)> + Clone {
+        self.enabled_lang_features
+            .iter()
+            .map(|feat| (feat.gate_name, feat.attr_sp))
+            .chain(self.enabled_lib_features.iter().map(|feat| (feat.gate_name, feat.attr_sp)))
+    }
+
     /// Is the given feature enabled (via `#[feature(...)]`)?
     pub fn enabled(&self, feature: Symbol) -> bool {
         self.enabled_features.contains(&feature)

compiler/rustc_lint/src/builtin.rs

@@ -2331,13 +2331,9 @@ declare_lint_pass!(
 impl EarlyLintPass for IncompleteInternalFeatures {
     fn check_crate(&mut self, cx: &EarlyContext<'_>, _: &ast::Crate) {
         let features = cx.builder.features();
-        let lang_features =
-            features.enabled_lang_features().iter().map(|feat| (feat.gate_name, feat.attr_sp));
-        let lib_features =
-            features.enabled_lib_features().iter().map(|feat| (feat.gate_name, feat.attr_sp));
 
-        lang_features
-            .chain(lib_features)
+        features
+            .enabled_features_iter_stable_order()
             .filter(|(name, _)| features.incomplete(*name) || features.internal(*name))
             .for_each(|(name, span)| {
                 if features.incomplete(name) {