Correctly check never_type feature gating

Author: GuillaumeGomez

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -59,14 +59,15 @@ macro_rules! gate_legacy {
 }
 
 pub fn check_attribute(attr: &ast::Attribute, sess: &Session, features: &Features) {
-    PostExpansionVisitor { sess, features }.visit_attribute(attr)
+    PostExpansionVisitor { sess, features, is_in_generics: false }.visit_attribute(attr)
 }
 
 struct PostExpansionVisitor<'a> {
     sess: &'a Session,
 
     // `sess` contains a `Features`, but this might not be that one.
     features: &'a Features,
+    is_in_generics: bool,
 }
 
 impl<'a> PostExpansionVisitor<'a> {
@@ -349,17 +350,20 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 _ => {}
             }
         }
+        let prev = std::mem::replace(&mut self.is_in_generics, true);
         visit::walk_generics(self, g);
+        self.is_in_generics = prev;
     }
 
     fn visit_fn_ret_ty(&mut self, ret_ty: &'a ast::FnRetTy) {
-        if let ast::FnRetTy::Ty(output_ty) = ret_ty {
-            if let ast::TyKind::Never = output_ty.kind {
-                // Do nothing.
-            } else {
-                self.visit_ty(output_ty)
-            }
+        if let ast::FnRetTy::Ty(output_ty) = ret_ty
+            && let ast::TyKind::Never = output_ty.kind
+            && !self.is_in_generics
+        {
+            // Do nothing.
+            return;
         }
+        visit::walk_fn_ret_ty(self, ret_ty);
     }
 
     fn visit_expr(&mut self, e: &'a ast::Expr) {
@@ -489,7 +493,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     maybe_stage_features(sess, features, krate);
     check_incompatible_features(sess, features);
-    let mut visitor = PostExpansionVisitor { sess, features };
+    let mut visitor = PostExpansionVisitor { sess, features, is_in_generics: false };
 
     let spans = sess.parse_sess.gated_spans.spans.borrow();
     macro_rules! gate_all {