Refactor BuildFunctionDecl (#5098)

Trying to break apart the function on reasonable boundaries because it's
big. Changes behavior of class functions with virtual modifiers to
remove the modifier when diagnosing, which is more consistent with other
modifier diagnostics.

Author: jonmeow

toolchain/check/decl_name_stack.cpp

@@ -18,7 +18,7 @@
 
 namespace Carbon::Check {
 
-auto DeclNameStack::NameContext::prev_inst_id() -> SemIR::InstId {
+auto DeclNameStack::NameContext::prev_inst_id() const -> SemIR::InstId {
   switch (state) {
     case NameContext::State::Error:
       // The name is malformed and a diagnostic has already been emitted.

toolchain/check/decl_name_stack.h

@@ -118,7 +118,7 @@ class DeclNameStack {
 
     // Returns any name collision found, or `None`. Requires a non-poisoned
     // value.
-    auto prev_inst_id() -> SemIR::InstId;
+    auto prev_inst_id() const -> SemIR::InstId;
 
     // Returns the name_id for a new instruction. This is `None` when the name
     // resolved.

toolchain/check/handle_function.cpp

@@ -15,13 +15,15 @@
 #include "toolchain/check/import_ref.h"
 #include "toolchain/check/inst.h"
 #include "toolchain/check/interface.h"
+#include "toolchain/check/keyword_modifier_set.h"
 #include "toolchain/check/literal.h"
 #include "toolchain/check/merge.h"
 #include "toolchain/check/modifiers.h"
 #include "toolchain/check/name_component.h"
 #include "toolchain/check/name_lookup.h"
 #include "toolchain/check/type.h"
 #include "toolchain/check/type_completion.h"
+#include "toolchain/parse/node_ids.h"
 #include "toolchain/sem_ir/builtin_function_kind.h"
 #include "toolchain/sem_ir/entry_point.h"
 #include "toolchain/sem_ir/function.h"
@@ -75,11 +77,33 @@ auto HandleParseNode(Context& context, Parse::ReturnTypeId node_id) -> bool {
   return true;
 }
 
-static auto DiagnoseModifiers(Context& context, DeclIntroducerState& introducer,
+// Returns the ID of the self parameter pattern, or None.
+// TODO: Do this during initial traversal of implicit params.
+static auto FindSelfPattern(Context& context,
+                            SemIR::InstBlockId implicit_param_patterns_id)
+    -> SemIR::InstId {
+  auto implicit_param_patterns =
+      context.inst_blocks().GetOrEmpty(implicit_param_patterns_id);
+  if (const auto* i = llvm::find_if(implicit_param_patterns,
+                                    [&](auto implicit_param_id) {
+                                      return SemIR::IsSelfPattern(
+                                          context.sem_ir(), implicit_param_id);
+                                    });
+      i != implicit_param_patterns.end()) {
+    return *i;
+  }
+  return SemIR::InstId::None;
+}
+
+// Diagnoses issues with the modifiers, removing modifiers that shouldn't be
+// present.
+static auto DiagnoseModifiers(Context& context,
+                              Parse::AnyFunctionDeclId node_id,
+                              DeclIntroducerState& introducer,
                               bool is_definition,
                               SemIR::InstId parent_scope_inst_id,
-                              std::optional<SemIR::Inst> parent_scope_inst)
-    -> void {
+                              std::optional<SemIR::Inst> parent_scope_inst,
+                              SemIR::InstId self_param_id) -> void {
   CheckAccessModifiersOnDecl(context, introducer, parent_scope_inst);
   LimitModifiersOnDecl(context, introducer,
                        KeywordModifierSet::Access | KeywordModifierSet::Extern |
@@ -90,6 +114,33 @@ static auto DiagnoseModifiers(Context& context, DeclIntroducerState& introducer,
   CheckMethodModifiersOnFunction(context, introducer, parent_scope_inst_id,
                                  parent_scope_inst);
   RequireDefaultFinalOnlyInInterfaces(context, introducer, parent_scope_inst);
+
+  if (introducer.modifier_set.HasAnyOf(KeywordModifierSet::Interface)) {
+    // TODO: Once we are saving the modifiers for a function, add check that
+    // the function may only be defined if it is marked `default` or `final`.
+    context.TODO(introducer.modifier_node_id(ModifierOrder::Decl),
+                 "interface modifier");
+  }
+
+  if (!self_param_id.has_value() &&
+      introducer.modifier_set.HasAnyOf(KeywordModifierSet::Method)) {
+    CARBON_DIAGNOSTIC(VirtualWithoutSelf, Error, "virtual class function");
+    context.emitter().Build(node_id, VirtualWithoutSelf).Emit();
+    // TODO: Remove the incorrect modifier.
+    // introducer.modifier_set.Remove(KeywordModifierSet::Method);
+  }
+}
+
+// Returns the virtual-family modifier as an enum.
+static auto GetVirtualModifier(const KeywordModifierSet& modifier_set)
+    -> SemIR::Function::VirtualModifier {
+  return modifier_set.ToEnum<SemIR::Function::VirtualModifier>()
+      .Case(KeywordModifierSet::Virtual,
+            SemIR::Function::VirtualModifier::Virtual)
+      .Case(KeywordModifierSet::Abstract,
+            SemIR::Function::VirtualModifier::Abstract)
+      .Case(KeywordModifierSet::Impl, SemIR::Function::VirtualModifier::Impl)
+      .Default(SemIR::Function::VirtualModifier::None);
 }
 
 // Tries to merge new_function into prev_function_id. Since new_function won't
@@ -140,11 +191,17 @@ static auto MergeFunctionRedecl(Context& context,
 
 // Check whether this is a redeclaration, merging if needed.
 static auto TryMergeRedecl(Context& context, Parse::AnyFunctionDeclId node_id,
-                           SemIR::NameId name_id, SemIR::InstId prev_id,
-                           SemIR::LocId name_loc_id,
+                           const DeclNameStack::NameContext& name_context,
                            SemIR::FunctionDecl& function_decl,
                            SemIR::Function& function_info, bool is_definition)
     -> void {
+  if (name_context.state == DeclNameStack::NameContext::State::Poisoned) {
+    DiagnosePoisonedName(context, name_context.name_id_for_new_inst(),
+                         name_context.poisoning_loc_id, name_context.loc_id);
+    return;
+  }
+
+  auto prev_id = name_context.prev_inst_id();
   if (!prev_id.has_value()) {
     return;
   }
@@ -185,7 +242,8 @@ static auto TryMergeRedecl(Context& context, Parse::AnyFunctionDeclId node_id,
   }
 
   if (!prev_function_id.has_value()) {
-    DiagnoseDuplicateName(context, name_id, name_loc_id, prev_id);
+    DiagnoseDuplicateName(context, name_context.name_id, name_context.loc_id,
+                          prev_id);
     return;
   }
 
@@ -197,6 +255,89 @@ static auto TryMergeRedecl(Context& context, Parse::AnyFunctionDeclId node_id,
   }
 }
 
+// Adds the declaration to name lookup when appropriate.
+static auto MaybeAddToNameLookup(
+    Context& context, const DeclNameStack::NameContext& name_context,
+    const KeywordModifierSet& modifier_set,
+    const std::optional<SemIR::Inst>& parent_scope_inst, SemIR::InstId decl_id)
+    -> void {
+  if (name_context.state == DeclNameStack::NameContext::State::Poisoned ||
+      name_context.prev_inst_id().has_value()) {
+    return;
+  }
+
+  // At interface scope, a function declaration introduces an associated
+  // function.
+  auto lookup_result_id = decl_id;
+  if (parent_scope_inst && !name_context.has_qualifiers) {
+    if (auto interface_scope =
+            parent_scope_inst->TryAs<SemIR::InterfaceDecl>()) {
+      lookup_result_id = BuildAssociatedEntity(
+          context, interface_scope->interface_id, decl_id);
+    }
+  }
+
+  context.decl_name_stack().AddName(name_context, lookup_result_id,
+                                    modifier_set.GetAccessKind());
+}
+
+// If the function is the entry point, do corresponding validation.
+static auto ValidateForEntryPoint(Context& context,
+                                  Parse::AnyFunctionDeclId node_id,
+                                  SemIR::FunctionId function_id,
+                                  const SemIR::Function& function_info)
+    -> void {
+  if (!SemIR::IsEntryPoint(context.sem_ir(), function_id)) {
+    return;
+  }
+
+  auto return_type_id = function_info.GetDeclaredReturnType(context.sem_ir());
+  // TODO: Update this once valid signatures for the entry point are decided.
+  if (function_info.implicit_param_patterns_id.has_value() ||
+      !function_info.param_patterns_id.has_value() ||
+      !context.inst_blocks().Get(function_info.param_patterns_id).empty() ||
+      (return_type_id.has_value() &&
+       return_type_id != GetTupleType(context, {}) &&
+       // TODO: Decide on valid return types for `Main.Run`. Perhaps we should
+       // have an interface for this.
+       return_type_id != MakeIntType(context, node_id, SemIR::IntKind::Signed,
+                                     context.ints().Add(32)))) {
+    CARBON_DIAGNOSTIC(InvalidMainRunSignature, Error,
+                      "invalid signature for `Main.Run` function; expected "
+                      "`fn ()` or `fn () -> i32`");
+    context.emitter().Emit(node_id, InvalidMainRunSignature);
+  }
+}
+
+// Requests a vtable be created when processing a virtual function.
+static auto RequestVtableIfVirtual(
+    Context& context, Parse::AnyFunctionDeclId node_id,
+    SemIR::Function::VirtualModifier virtual_modifier,
+    const std::optional<SemIR::Inst>& parent_scope_inst, SemIR::InstId decl_id)
+    -> void {
+  // In order to request a vtable, the function must be virtual, and in a class
+  // scope.
+  if (virtual_modifier == SemIR::Function::VirtualModifier::None ||
+      !parent_scope_inst) {
+    return;
+  }
+  auto class_decl = parent_scope_inst->TryAs<SemIR::ClassDecl>();
+  if (!class_decl) {
+    return;
+  }
+
+  auto& class_info = context.classes().Get(class_decl->class_id);
+  if (virtual_modifier == SemIR::Function::VirtualModifier::Impl &&
+      !class_info.base_id.has_value()) {
+    CARBON_DIAGNOSTIC(ImplWithoutBase, Error, "impl without base class");
+    context.emitter().Build(node_id, ImplWithoutBase).Emit();
+  }
+  // TODO: If this is an `impl` function, check there's a matching base
+  // function that's impl or virtual.
+  class_info.is_dynamic = true;
+  context.vtable_stack().AddInstId(decl_id);
+}
+
 // Build a FunctionDecl describing the signature of a function. This
 // handles the common logic shared by function declaration syntax and function
 // definition syntax.
@@ -216,6 +357,8 @@ static auto BuildFunctionDecl(Context& context,
     context.TODO(node_id, "function with positional parameters");
     name.param_patterns_id = SemIR::InstBlockId::Empty;
   }
+  auto self_param_id =
+      FindSelfPattern(context, name.implicit_param_patterns_id);
 
   auto name_context = context.decl_name_stack().FinishName(name);
   context.node_stack()
@@ -226,89 +369,34 @@ static auto BuildFunctionDecl(Context& context,
       context.name_scopes().GetInstIfValid(name_context.parent_scope_id);
   auto introducer =
       context.decl_introducer_state_stack().Pop<Lex::TokenKind::Fn>();
-  DiagnoseModifiers(context, introducer, is_definition, parent_scope_inst_id,
-                    parent_scope_inst);
+  DiagnoseModifiers(context, node_id, introducer, is_definition,
+                    parent_scope_inst_id, parent_scope_inst, self_param_id);
   bool is_extern = introducer.modifier_set.HasAnyOf(KeywordModifierSet::Extern);
-  auto virtual_modifier =
-      introducer.modifier_set.ToEnum<SemIR::Function::VirtualModifier>()
-          .Case(KeywordModifierSet::Virtual,
-                SemIR::Function::VirtualModifier::Virtual)
-          .Case(KeywordModifierSet::Abstract,
-                SemIR::Function::VirtualModifier::Abstract)
-          .Case(KeywordModifierSet::Impl,
-                SemIR::Function::VirtualModifier::Impl)
-          .Default(SemIR::Function::VirtualModifier::None);
-  SemIR::Class* virtual_class_info = nullptr;
-  if (virtual_modifier != SemIR::Function::VirtualModifier::None &&
-      parent_scope_inst) {
-    if (auto class_decl = parent_scope_inst->TryAs<SemIR::ClassDecl>()) {
-      virtual_class_info = &context.classes().Get(class_decl->class_id);
-      if (virtual_modifier == SemIR::Function::VirtualModifier::Impl &&
-          !virtual_class_info->base_id.has_value()) {
-        CARBON_DIAGNOSTIC(ImplWithoutBase, Error, "impl without base class");
-        context.emitter().Build(node_id, ImplWithoutBase).Emit();
-      }
-      // TODO: If this is an `impl` function, check there's a matching base
-      // function that's impl or virtual.
-      virtual_class_info->is_dynamic = true;
-    }
-  }
-  if (introducer.modifier_set.HasAnyOf(KeywordModifierSet::Interface)) {
-    // TODO: Once we are saving the modifiers for a function, add check that
-    // the function may only be defined if it is marked `default` or `final`.
-    context.TODO(introducer.modifier_node_id(ModifierOrder::Decl),
-                 "interface modifier");
-  }
+  auto virtual_modifier = GetVirtualModifier(introducer.modifier_set);
 
   // Add the function declaration.
-  auto decl_block_id = context.inst_block_stack().Pop();
-  auto function_decl = SemIR::FunctionDecl{
-      SemIR::TypeId::None, SemIR::FunctionId::None, decl_block_id};
+  SemIR::FunctionDecl function_decl = {SemIR::TypeId::None,
+                                       SemIR::FunctionId::None,
+                                       context.inst_block_stack().Pop()};
   auto decl_id =
       AddPlaceholderInst(context, SemIR::LocIdAndInst(node_id, function_decl));
+  RequestVtableIfVirtual(context, node_id, virtual_modifier, parent_scope_inst,
+                         decl_id);
 
-  // Find self parameter pattern.
-  // TODO: Do this during initial traversal of implicit params.
-  auto self_param_id = SemIR::InstId::None;
-  auto implicit_param_patterns =
-      context.inst_blocks().GetOrEmpty(name.implicit_param_patterns_id);
-  if (const auto* i = llvm::find_if(implicit_param_patterns,
-                                    [&](auto implicit_param_id) {
-                                      return SemIR::IsSelfPattern(
-                                          context.sem_ir(), implicit_param_id);
-                                    });
-      i != implicit_param_patterns.end()) {
-    self_param_id = *i;
-  }
-
-  if (virtual_modifier != SemIR::Function::VirtualModifier::None &&
-      !self_param_id.has_value()) {
-    CARBON_DIAGNOSTIC(VirtualWithoutSelf, Error, "virtual class function");
-    context.emitter().Build(node_id, VirtualWithoutSelf).Emit();
-  }
   // Build the function entity. This will be merged into an existing function if
   // there is one, or otherwise added to the function store.
   auto function_info =
-      SemIR::Function{{name_context.MakeEntityWithParamsBase(
-                          name, decl_id, is_extern, introducer.extern_library)},
+      SemIR::Function{name_context.MakeEntityWithParamsBase(
+                          name, decl_id, is_extern, introducer.extern_library),
                       {.return_slot_pattern_id = name.return_slot_pattern_id,
                        .virtual_modifier = virtual_modifier,
                        .self_param_id = self_param_id}};
   if (is_definition) {
     function_info.definition_id = decl_id;
   }
-  if (virtual_class_info) {
-    context.vtable_stack().AddInstId(decl_id);
-  }
 
-  if (name_context.state == DeclNameStack::NameContext::State::Poisoned) {
-    DiagnosePoisonedName(context, name_context.name_id_for_new_inst(),
-                         name_context.poisoning_loc_id, name_context.loc_id);
-  } else {
-    TryMergeRedecl(context, node_id, name_context.name_id,
-                   name_context.prev_inst_id(), name_context.loc_id,
-                   function_decl, function_info, is_definition);
-  }
+  TryMergeRedecl(context, node_id, name_context, function_decl, function_info,
+                 is_definition);
 
   // Create a new function if this isn't a valid redeclaration.
   if (!function_decl.function_id.has_value()) {
@@ -338,43 +426,12 @@ static auto BuildFunctionDecl(Context& context,
     context.emitter().Emit(TokenOnly(node_id), DefinedAbstractFunction);
   }
 
-  // Check if we need to add this to name lookup, now that the function decl is
-  // done.
-  if (name_context.state != DeclNameStack::NameContext::State::Poisoned &&
-      !name_context.prev_inst_id().has_value()) {
-    // At interface scope, a function declaration introduces an associated
-    // function.
-    auto lookup_result_id = decl_id;
-    if (parent_scope_inst && !name_context.has_qualifiers) {
-      if (auto interface_scope =
-              parent_scope_inst->TryAs<SemIR::InterfaceDecl>()) {
-        lookup_result_id = BuildAssociatedEntity(
-            context, interface_scope->interface_id, decl_id);
-      }
-    }
+  // Add to name lookup if needed, now that the decl is built.
+  MaybeAddToNameLookup(context, name_context, introducer.modifier_set,
+                       parent_scope_inst, decl_id);
 
-    context.decl_name_stack().AddName(name_context, lookup_result_id,
-                                      introducer.modifier_set.GetAccessKind());
-  }
-
-  if (SemIR::IsEntryPoint(context.sem_ir(), function_decl.function_id)) {
-    auto return_type_id = function_info.GetDeclaredReturnType(context.sem_ir());
-    // TODO: Update this once valid signatures for the entry point are decided.
-    if (function_info.implicit_param_patterns_id.has_value() ||
-        !function_info.param_patterns_id.has_value() ||
-        !context.inst_blocks().Get(function_info.param_patterns_id).empty() ||
-        (return_type_id.has_value() &&
-         return_type_id != GetTupleType(context, {}) &&
-         // TODO: Decide on valid return types for `Main.Run`. Perhaps we should
-         // have an interface for this.
-         return_type_id != MakeIntType(context, node_id, SemIR::IntKind::Signed,
-                                       context.ints().Add(32)))) {
-      CARBON_DIAGNOSTIC(InvalidMainRunSignature, Error,
-                        "invalid signature for `Main.Run` function; expected "
-                        "`fn ()` or `fn () -> i32`");
-      context.emitter().Emit(node_id, InvalidMainRunSignature);
-    }
-  }
+  ValidateForEntryPoint(context, node_id, function_decl.function_id,
+                        function_info);
 
   if (!is_definition && context.sem_ir().is_impl() && !is_extern) {
     context.definitions_required().push_back(decl_id);