Mangle class declarations #6617 (#6648)

In some situations we need to mangle class declarations, which then are
not NameScopes (can't scope anything inside a declaration) - so make the
mangler able to cope with that situation by mangling the name of the
class directly rather than relying on generic NameScope mangling to
handle the class case.

toolchain/lower/mangler.cpp

@@ -83,11 +83,15 @@ auto Mangler::MangleInverseQualifiedNameScope(llvm::raw_ostream& os,
         auto self_inst = insts().Get(self_const_inst_id);
         CARBON_KIND_SWITCH(self_inst) {
           case CARBON_KIND(SemIR::ClassType class_type): {
-            auto next_name_scope_id =
-                sem_ir().classes().Get(class_type.class_id).scope_id;
-            names_to_render.push_back({.name_scope_id = next_name_scope_id,
-                                       .specific_id = class_type.specific_id,
-                                       .prefix = '\0'});
+            const auto& class_info =
+                sem_ir().classes().Get(class_type.class_id);
+
+            names_to_render.push_back(
+                {.name_scope_id = class_info.parent_scope_id,
+                 .specific_id = class_type.specific_id,
+                 .prefix = '.'});
+
+            MangleUnqualifiedClass(os, class_info, class_type.specific_id);
             break;
           }
           case SemIR::AutoType::Kind:
@@ -128,8 +132,8 @@ auto Mangler::MangleInverseQualifiedNameScope(llvm::raw_ostream& os,
         continue;
       }
       case CARBON_KIND(SemIR::ClassDecl class_decl): {
-        MangleNameId(os, sem_ir().classes().Get(class_decl.class_id).name_id);
-        MangleSpecificId(os, specific_id);
+        MangleUnqualifiedClass(os, sem_ir().classes().Get(class_decl.class_id),
+                               specific_id);
         break;
       }
       case CARBON_KIND(SemIR::InterfaceDecl interface_decl): {
@@ -264,4 +268,10 @@ auto Mangler::MangleVTable(const SemIR::Class& class_info,
   return os.TakeStr();
 }
 
+auto Mangler::MangleUnqualifiedClass(llvm::raw_ostream& os,
+                                     const SemIR::Class& class_info,
+                                     SemIR::SpecificId specific_id) -> void {
+  MangleNameId(os, class_info.name_id);
+  MangleSpecificId(os, specific_id);
+}
 }  // namespace Carbon::Lower

toolchain/lower/mangler.h

@@ -58,6 +58,11 @@ class Mangler {
                                        SemIR::NameScopeId name_scope_id)
       -> void;
 
+  // Mangle the unqualified name of the specified `Class`.
+  auto MangleUnqualifiedClass(llvm::raw_ostream& os,
+                              const SemIR::Class& class_info,
+                              SemIR::SpecificId specific_id) -> void;
+
   // Generates a mangled name using Clang mangling for imported C++ functions.
   auto MangleCppClang(const clang::NamedDecl* decl) -> std::string;
 

toolchain/lower/testdata/interface/mangle_declared_class.carbon

@@ -0,0 +1,42 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/convert.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/lower/testdata/interface/mangle_declared_class.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/lower/testdata/interface/mangle_declared_class.carbon
+
+interface I {
+  fn F();
+}
+class C;
+impl C as I {
+  fn F() {}
+}
+
+// CHECK:STDOUT: ; ModuleID = 'mangle_declared_class.carbon'
+// CHECK:STDOUT: source_filename = "mangle_declared_class.carbon"
+// CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: nounwind
+// CHECK:STDOUT: define void @"_CF.C.Main:I.Main"() #0 !dbg !4 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   ret void, !dbg !7
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: attributes #0 = { nounwind }
+// CHECK:STDOUT:
+// CHECK:STDOUT: !llvm.module.flags = !{!0, !1}
+// CHECK:STDOUT: !llvm.dbg.cu = !{!2}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !0 = !{i32 7, !"Dwarf Version", i32 5}
+// CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
+// CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
+// CHECK:STDOUT: !3 = !DIFile(filename: "mangle_declared_class.carbon", directory: "")
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.C.Main:I.Main", scope: null, file: !3, line: 18, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
+// CHECK:STDOUT: !6 = !{null}
+// CHECK:STDOUT: !7 = !DILocation(line: 18, column: 3, scope: !4)