Use the scope stack instead of the decl state stack for context (#3460)

toolchain/check/context.h

@@ -123,6 +123,18 @@ class Context {
     return current_scope().scope_id;
   }
 
+  // Returns true if currently at file scope.
+  auto at_file_scope() const -> bool { return scope_stack_.size() == 1; }
+
+  // Returns the instruction kind associated with the current scope, if any.
+  auto current_scope_kind() const -> std::optional<SemIR::InstKind> {
+    auto current_scope_inst_id = current_scope().scope_inst_id;
+    if (!current_scope_inst_id.is_valid()) {
+      return std::nullopt;
+    }
+    return sem_ir_->insts().Get(current_scope_inst_id).kind();
+  }
+
   // Returns the current scope, if it is of the specified kind. Otherwise,
   // returns nullopt.
   template <typename InstT>

toolchain/check/decl_state.h

@@ -84,13 +84,6 @@ class DeclStateStack {
   // declaration currently being processed.
   auto innermost() -> DeclState& { return stack_.back(); }
 
-  // Gets the state for the declaration containing the innermost declaration.
-  // Requires that the innermost declaration is not `FileScope`.
-  auto containing() const -> const DeclState& {
-    CARBON_CHECK(stack_.size() >= 2);
-    return stack_[stack_.size() - 2];
-  }
-
   // Exits a declaration of kind `k`.
   auto Pop(DeclState::DeclKind k) -> void {
     CARBON_CHECK(stack_.back().kind == k);

toolchain/check/handle_function.cpp

@@ -17,23 +17,22 @@ static auto DiagnoseModifiers(Context& context) -> KeywordModifierSet {
                            KeywordModifierSet::Interface,
                        decl_kind);
   // Rules for abstract, virtual, and impl, which are only allowed in classes.
-  auto containing_kind = context.decl_state_stack().containing().kind;
-  if (containing_kind != DeclState::Class) {
-    ForbidModifiersOnDecl(context, KeywordModifierSet::Method, decl_kind,
-                          " outside of a class");
-  } else {
-    auto containing_decl_modifiers =
-        context.decl_state_stack().containing().modifier_set;
-    if (!(containing_decl_modifiers & KeywordModifierSet::Class)) {
+  if (auto class_decl = context.GetCurrentScopeAs<SemIR::ClassDecl>()) {
+    auto inheritance_kind =
+        context.classes().Get(class_decl->class_id).inheritance_kind;
+    if (inheritance_kind == SemIR::Class::Final) {
       ForbidModifiersOnDecl(context, KeywordModifierSet::Virtual, decl_kind,
                             " in a non-abstract non-base `class` definition",
-                            context.decl_state_stack().containing().first_node);
+                            class_decl->parse_node);
     }
-    if (!(containing_decl_modifiers & KeywordModifierSet::Abstract)) {
+    if (inheritance_kind != SemIR::Class::Abstract) {
       ForbidModifiersOnDecl(context, KeywordModifierSet::Abstract, decl_kind,
                             " in a non-abstract `class` definition",
-                            context.decl_state_stack().containing().first_node);
+                            class_decl->parse_node);
     }
+  } else {
+    ForbidModifiersOnDecl(context, KeywordModifierSet::Method, decl_kind,
+                          " outside of a class");
   }
   RequireDefaultFinalOnlyInInterfaces(context, decl_kind);
 

toolchain/check/modifiers.cpp

@@ -64,34 +64,33 @@ auto ForbidModifiersOnDecl(Context& context, KeywordModifierSet forbidden,
 
 auto CheckAccessModifiersOnDecl(Context& context, Lex::TokenKind decl_kind)
     -> void {
-  switch (context.decl_state_stack().containing().kind) {
-    case DeclState::FileScope:
-      ForbidModifiersOnDecl(
-          context, KeywordModifierSet::Protected, decl_kind,
-          " at file scope, `protected` is only allowed on class members");
-      break;
-
-    case DeclState::Class:
-      // Both `private` and `protected` allowed in a class definition.
-      break;
-
-    default:
-      // Otherwise neither `private` nor `protected` allowed.
-      ForbidModifiersOnDecl(context, KeywordModifierSet::Protected, decl_kind,
-                            ", `protected` is only allowed on class members");
-      ForbidModifiersOnDecl(
-          context, KeywordModifierSet::Private, decl_kind,
-          ", `private` is only allowed on class members and at file scope");
-      break;
+  if (context.at_file_scope()) {
+    ForbidModifiersOnDecl(
+        context, KeywordModifierSet::Protected, decl_kind,
+        " at file scope, `protected` is only allowed on class members");
+    return;
   }
+
+  if (auto kind = context.current_scope_kind()) {
+    if (*kind == SemIR::ClassDecl::Kind) {
+      // Both `private` and `protected` allowed in a class definition.
+      return;
+    }
+  }
+
+  // Otherwise neither `private` nor `protected` allowed.
+  ForbidModifiersOnDecl(context, KeywordModifierSet::Protected, decl_kind,
+                        ", `protected` is only allowed on class members");
+  ForbidModifiersOnDecl(
+      context, KeywordModifierSet::Private, decl_kind,
+      ", `private` is only allowed on class members and at file scope");
 }
 
 auto RequireDefaultFinalOnlyInInterfaces(Context& context,
                                          Lex::TokenKind decl_kind) -> void {
-  if (context.decl_state_stack().containing().kind != DeclState::Interface) {
-    ForbidModifiersOnDecl(context, KeywordModifierSet::Interface, decl_kind,
-                          " outside of an interface");
-  }
+  // TODO: Skip this if *context.current_scope_kind() == SemIR::InterfaceDecl
+  ForbidModifiersOnDecl(context, KeywordModifierSet::Interface, decl_kind,
+                        " outside of an interface");
 }
 
 }  // namespace Carbon::Check