Roundtrip (export/reimport) class declarations (#7182)

Roundtrip (export/reimport) class declarations

The remapping was previously implemented using name_scopes, which aren't
created for class declarations, only definitions - causing the reimport
to import a fresh copy of the type that mismatched with the original (as
seen in the test baseline).

By changing the mapping to use the reverse part of the clang_decls
mapping this should generalize better (& we probably should further
migrate to that mapping). Though it did trip over some issue with
exactly which instruction is used as the key in the clang_decls map -
this change moves towards standardizing on the first decl id of the
class as its map key.

toolchain/check/cpp/export.cpp

@@ -116,8 +116,7 @@ auto ExportNameScopeToCpp(Context& context, SemIR::LocId loc_id,
 }
 
 auto ExportClassToCpp(Context& context, SemIR::LocId loc_id,
-                      SemIR::InstId class_inst_id, SemIR::ClassType class_type)
-    -> clang::TagDecl* {
+                      SemIR::ClassType class_type) -> clang::TagDecl* {
   // TODO: A lot of logic in this function is shared with ExportNameScopeToCpp.
   // This should be refactored.
 
@@ -131,9 +130,11 @@ auto ExportClassToCpp(Context& context, SemIR::LocId loc_id,
   // If this class was produced by importing a C++ declaration or has
   // already been exported to C++, return the corresponding Clang declaration.
   // That could either be a CXXRecordDecl or an EnumDecl.
-  if (auto* decl_context =
-          GetClangDeclContextForScope(context, class_info.scope_id)) {
-    return cast<clang::TagDecl>(decl_context);
+  if (auto clang_decl_id =
+          context.clang_decls().Lookup(class_info.first_decl_id());
+      clang_decl_id.has_value()) {
+    return cast<clang::TagDecl>(
+        context.clang_decls().Get(clang_decl_id).key.decl);
   }
 
   auto* identifier_info = GetClangIdentifierInfo(context, class_info.name_id);
@@ -157,8 +158,8 @@ auto ExportClassToCpp(Context& context, SemIR::LocId loc_id,
 
   auto key =
       SemIR::ClangDeclKey::ForNonFunctionDecl(cast<clang::Decl>(record_decl));
-  auto clang_decl_id =
-      context.clang_decls().Add({.key = key, .inst_id = class_inst_id});
+  auto clang_decl_id = context.clang_decls().Add(
+      {.key = key, .inst_id = class_info.first_decl_id()});
   if (class_info.scope_id.has_value()) {
     // TODO: Record the Carbon class -> clang declaration mapping for incomplete
     // classes too.

toolchain/check/cpp/export.h

@@ -28,8 +28,7 @@ auto ExportNameScopeToCpp(Context& context, SemIR::LocId loc_id,
 // Otherwise, creates a new C++ class and returns it. Returns nullptr if the
 // class could not be exported and an error was diagnosed.
 auto ExportClassToCpp(Context& context, SemIR::LocId loc_id,
-                      SemIR::InstId class_inst_id, SemIR::ClassType class_type)
-    -> clang::TagDecl*;
+                      SemIR::ClassType class_type) -> clang::TagDecl*;
 
 // Export all `SemIR::FieldDecl`s in the class body as `clang::FieldDecl`s.
 auto ExportAllFieldsToCpp(Context& context, SemIR::Class& class_info) -> void;

toolchain/check/cpp/type_mapping.cpp

@@ -226,8 +226,8 @@ static auto TryMapClassType(Context& context, SemIR::TypeInstId class_inst_id,
   }
 
   // Otherwise, find the existing C++ declaration or create a new one.
-  auto* tag_decl = ExportClassToCpp(context, SemIR::LocId(class_inst_id),
-                                    class_inst_id, class_type);
+  auto* tag_decl =
+      ExportClassToCpp(context, SemIR::LocId(class_inst_id), class_type);
   if (!tag_decl) {
     return clang::QualType();
   }