Support calling simple Carbon functions from C++ (#6967)

For now, only functions with no parameters and a `()` return type are
supported.

toolchain/check/cpp/generate_ast.cpp

@@ -389,6 +389,48 @@ auto CarbonExternalASTSource::MapInstIdToClangDecl(
           *ast_context_, clang::TagTypeKind::Class, &decl_context,
           clang::SourceLocation(), clang::SourceLocation(), identifier_info);
     }
+    case SemIR::StructValue::Kind: {
+      auto callee = GetCallee(context_->sem_ir(), target_constant);
+      auto* callee_function = std::get_if<SemIR::CalleeFunction>(&callee);
+      if (!callee_function) {
+        return nullptr;
+      }
+      const SemIR::Function& function =
+          context_->functions().Get(callee_function->function_id);
+      auto* identifier_info =
+          GetClangIdentifierInfo(*context_, function.name_id);
+
+      if (function.call_param_ranges.explicit_size() != 0) {
+        context_->TODO(target_inst_id,
+                       "unsupported: C++ calling a Carbon function with "
+                       "parameters");
+        return nullptr;
+      }
+
+      if (function.return_type_inst_id != SemIR::TypeInstId::None) {
+        context_->TODO(target_inst_id,
+                       "unsupported: C++ calling a Carbon function with "
+                       "return type other than `()`");
+        return nullptr;
+      }
+
+      // TODO: support non-empty parameter lists.
+      llvm::SmallVector<clang::QualType> cpp_param_types;
+
+      // TODO: support non-void return types.
+      auto cpp_return_type = ast_context_->VoidTy;
+
+      auto cpp_function_type = ast_context_->getFunctionType(
+          cpp_return_type, cpp_param_types,
+          clang::FunctionProtoType::ExtProtoInfo());
+
+      return clang::FunctionDecl::Create(
+          *ast_context_, &decl_context,
+          /*StartLoc=*/clang::SourceLocation(),
+          /*NLoc=*/clang::SourceLocation(),
+          clang::DeclarationName(identifier_info), cpp_function_type,
+          /*TInfo=*/nullptr, clang::SC_Extern);
+    }
     default:
       return nullptr;
   }

toolchain/check/testdata/interop/cpp/reverse/function.carbon

@@ -0,0 +1,69 @@
+// 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/int.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/reverse/function.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/reverse/function.carbon
+
+// --- other.carbon
+
+package Other;
+fn F1() {}
+fn F2() -> i32 { return 0; }
+fn F3(_: i32) {}
+
+// --- function.carbon
+
+library "[[@TEST_NAME]]";
+
+import Other;
+import Cpp inline '''
+void G() {
+  Carbon::Other::F1();
+}
+''';
+
+// --- fail_todo_non_void.carbon
+
+library "[[@TEST_NAME]]";
+
+// CHECK:STDERR: fail_todo_non_void.carbon:[[@LINE+5]]:1: in import [InImport]
+// CHECK:STDERR: other.carbon:4:1: error: semantics TODO: `unsupported: C++ calling a Carbon function with return type other than `()`` [SemanticsTodo]
+// CHECK:STDERR: fn F2() -> i32 { return 0; }
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~
+// CHECK:STDERR:
+import Other;
+import Cpp inline '''
+void G() {
+  // CHECK:STDERR: fail_todo_non_void.carbon:[[@LINE+4]]:18: error: no member named 'F2' in namespace 'Carbon::Other' [CppInteropParseError]
+  // CHECK:STDERR:    16 |   Carbon::Other::F2();
+  // CHECK:STDERR:       |                  ^~
+  // CHECK:STDERR:
+  Carbon::Other::F2();
+}
+''';
+
+// --- fail_todo_args.carbon
+
+library "[[@TEST_NAME]]";
+
+// CHECK:STDERR: fail_todo_args.carbon:[[@LINE+5]]:1: in import [InImport]
+// CHECK:STDERR: other.carbon:5:1: error: semantics TODO: `unsupported: C++ calling a Carbon function with parameters` [SemanticsTodo]
+// CHECK:STDERR: fn F3(_: i32) {}
+// CHECK:STDERR: ^~~~~~~~~~~~~~~
+// CHECK:STDERR:
+import Other;
+import Cpp inline '''
+void G() {
+  // CHECK:STDERR: fail_todo_args.carbon:[[@LINE+4]]:18: error: no member named 'F3' in namespace 'Carbon::Other' [CppInteropParseError]
+  // CHECK:STDERR:    16 |   Carbon::Other::F3();
+  // CHECK:STDERR:       |                  ^~
+  // CHECK:STDERR:
+  Carbon::Other::F3();
+}
+''';

toolchain/lower/testdata/interop/cpp/reverse/function.carbon

@@ -0,0 +1,84 @@
+// 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/none.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/lower/testdata/interop/cpp/reverse/function.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/lower/testdata/interop/cpp/reverse/function.carbon
+
+// --- other.carbon
+package Other;
+fn F() {}
+// --- function.carbon
+
+library "[[@TEST_NAME]]";
+
+import Other;
+import Cpp inline '''
+void G() {
+  // TODO: this is not actually calling the Carbon function yet, because
+  // it's not using the right mangled name.
+  Carbon::Other::F();
+}
+''';
+
+// CHECK:STDOUT: ; ModuleID = 'other.carbon'
+// CHECK:STDOUT: source_filename = "other.carbon"
+// CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: nounwind
+// CHECK:STDOUT: define void @_CF.Other() #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: "other.carbon", directory: "")
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Other", scope: null, file: !3, line: 2, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
+// CHECK:STDOUT: !6 = !{null}
+// CHECK:STDOUT: !7 = !DILocation(line: 2, column: 1, scope: !4)
+// CHECK:STDOUT: ; ModuleID = 'function.carbon'
+// CHECK:STDOUT: source_filename = "function.carbon"
+// CHECK:STDOUT: target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+// CHECK:STDOUT: target triple = "x86_64-unknown-linux-gnu"
+// CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: mustprogress uwtable
+// CHECK:STDOUT: define dso_local void @_Z1Gv() #0 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   call void @_ZN6Carbon5Other1FEv()
+// CHECK:STDOUT:   ret void
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: declare void @_ZN6Carbon5Other1FEv() #1
+// CHECK:STDOUT:
+// CHECK:STDOUT: declare void @_CF.Other()
+// CHECK:STDOUT:
+// CHECK:STDOUT: attributes #0 = { mustprogress uwtable "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
+// CHECK:STDOUT: attributes #1 = { "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
+// CHECK:STDOUT:
+// CHECK:STDOUT: !llvm.module.flags = !{!0, !1, !2, !3, !4}
+// CHECK:STDOUT: !llvm.dbg.cu = !{!5}
+// CHECK:STDOUT: !llvm.errno.tbaa = !{!7}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !0 = !{i32 7, !"Dwarf Version", i32 5}
+// CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
+// CHECK:STDOUT: !2 = !{i32 8, !"PIC Level", i32 2}
+// CHECK:STDOUT: !3 = !{i32 7, !"PIE Level", i32 2}
+// CHECK:STDOUT: !4 = !{i32 7, !"uwtable", i32 2}
+// CHECK:STDOUT: !5 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !6, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
+// CHECK:STDOUT: !6 = !DIFile(filename: "function.carbon", directory: "")
+// CHECK:STDOUT: !7 = !{!8, !8, i64 0}
+// CHECK:STDOUT: !8 = !{!"int", !9, i64 0}
+// CHECK:STDOUT: !9 = !{!"omnipotent char", !10, i64 0}
+// CHECK:STDOUT: !10 = !{!"Simple C++ TBAA"}