Restrict the Cpp package name (#4618)

We're looking at using this for imports, and it gets awkward if we allow
users to declare entries.

toolchain/check/check.cpp

@@ -545,11 +545,12 @@ static auto GetImportKey(UnitInfo& unit_info, IdentifierId file_package_id,
   return {package_name, library_name};
 }
 
-static constexpr llvm::StringLiteral ExplicitMainName = "Main";
+static constexpr llvm::StringLiteral CppPackageName = "Cpp";
+static constexpr llvm::StringLiteral MainPackageName = "Main";
 
 static auto RenderImportKey(ImportKey import_key) -> std::string {
   if (import_key.first.empty()) {
-    import_key.first = ExplicitMainName;
+    import_key.first = MainPackageName;
   }
   if (import_key.second.empty()) {
     return import_key.first.str();
@@ -573,7 +574,7 @@ static auto TrackImport(Map<ImportKey, UnitInfo*>& api_map,
 
   // True if the import has `Main` as the package name, even if it comes from
   // the file's packaging (diagnostics may differentiate).
-  bool is_explicit_main = import_key.first == ExplicitMainName;
+  bool is_explicit_main = import_key.first == MainPackageName;
 
   // Explicit imports need more validation than implicit ones. We try to do
   // these in an order of imports that should be removed, followed by imports
@@ -688,6 +689,11 @@ static auto TrackImport(Map<ImportKey, UnitInfo*>& api_map,
   } else {
     // The imported api is missing.
     package_imports.has_load_error = true;
+    if (!explicit_import_map && import_key.first == CppPackageName) {
+      // Don't diagnose the implicit import in `impl package Cpp`, because we'll
+      // have diagnosed the use of `Cpp` in the declaration.
+      return;
+    }
     CARBON_DIAGNOSTIC(LibraryApiNotFound, Error,
                       "corresponding API for '{0}' not found", std::string);
     CARBON_DIAGNOSTIC(ImportNotFound, Error, "imported API '{0}' not found",
@@ -714,9 +720,9 @@ static auto BuildApiMapAndDiagnosePackaging(
                                 // Construct a boring key for Main//default.
                                 : ImportKey{"", ""};
 
-    // Diagnose explicit `Main` uses before they become marked as possible
+    // Diagnose restricted package names before they become marked as possible
     // APIs.
-    if (import_key.first == ExplicitMainName) {
+    if (import_key.first == MainPackageName) {
       CARBON_DIAGNOSTIC(ExplicitMainPackage, Error,
                         "`Main//default` must omit `package` declaration");
       CARBON_DIAGNOSTIC(
@@ -726,6 +732,11 @@ static auto BuildApiMapAndDiagnosePackaging(
                              import_key.second.empty() ? ExplicitMainPackage
                                                        : ExplicitMainLibrary);
       continue;
+    } else if (import_key.first == CppPackageName) {
+      CARBON_DIAGNOSTIC(CppPackageDeclaration, Error,
+                        "`Cpp` cannot be used by a `package` declaration");
+      unit_info.emitter.Emit(packaging->names.node_id, CppPackageDeclaration);
+      continue;
     }
 
     bool is_impl = packaging && packaging->is_impl;

toolchain/check/testdata/packages/fail_package_main.carbon

@@ -1,106 +0,0 @@
-// 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
-//
-// AUTOUPDATE
-// TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/packages/fail_package_main.carbon
-// TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/packages/fail_package_main.carbon
-
-// --- fail_main.carbon
-
-// CHECK:STDERR: fail_main.carbon:[[@LINE+4]]:1: error: `Main//default` must omit `package` declaration [ExplicitMainPackage]
-// CHECK:STDERR: package Main;
-// CHECK:STDERR: ^~~~~~~
-// CHECK:STDERR:
-package Main;
-
-// --- fail_main_impl.carbon
-
-// CHECK:STDERR: fail_main_impl.carbon:[[@LINE+4]]:6: error: `Main//default` must omit `package` declaration [ExplicitMainPackage]
-// CHECK:STDERR: impl package Main;
-// CHECK:STDERR:      ^~~~~~~
-// CHECK:STDERR:
-impl package Main;
-
-// --- fail_raw_main.carbon
-
-// `Main` isn't a keyword, so this fails the same way.
-// CHECK:STDERR: fail_raw_main.carbon:[[@LINE+4]]:1: error: `Main//default` must omit `package` declaration [ExplicitMainPackage]
-// CHECK:STDERR: package r#Main;
-// CHECK:STDERR: ^~~~~~~
-// CHECK:STDERR:
-package r#Main;
-
-// --- fail_main_lib.carbon
-
-// CHECK:STDERR: fail_main_lib.carbon:[[@LINE+3]]:1: error: use `library` declaration in `Main` package libraries [ExplicitMainLibrary]
-// CHECK:STDERR: package Main library "main_lib";
-// CHECK:STDERR: ^~~~~~~
-package Main library "[[@TEST_NAME]]";
-
-// CHECK:STDOUT: --- fail_main.carbon
-// CHECK:STDOUT:
-// CHECK:STDOUT: imports {
-// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [template] {
-// CHECK:STDOUT:     import Core//prelude
-// CHECK:STDOUT:     import Core//prelude/...
-// CHECK:STDOUT:   }
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: file {
-// CHECK:STDOUT:   package: <namespace> = namespace [template] {
-// CHECK:STDOUT:     .Core = imports.%Core
-// CHECK:STDOUT:   }
-// CHECK:STDOUT:   %Core.import = import Core
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: --- fail_main_impl.carbon
-// CHECK:STDOUT:
-// CHECK:STDOUT: imports {
-// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [template] {
-// CHECK:STDOUT:     import Core//prelude
-// CHECK:STDOUT:     import Core//prelude/...
-// CHECK:STDOUT:   }
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: file {
-// CHECK:STDOUT:   package: <namespace> = namespace [template] {
-// CHECK:STDOUT:     .Core = imports.%Core
-// CHECK:STDOUT:   }
-// CHECK:STDOUT:   %Core.import = import Core
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: --- fail_raw_main.carbon
-// CHECK:STDOUT:
-// CHECK:STDOUT: imports {
-// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [template] {
-// CHECK:STDOUT:     import Core//prelude
-// CHECK:STDOUT:     import Core//prelude/...
-// CHECK:STDOUT:   }
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: file {
-// CHECK:STDOUT:   package: <namespace> = namespace [template] {
-// CHECK:STDOUT:     .Core = imports.%Core
-// CHECK:STDOUT:   }
-// CHECK:STDOUT:   %Core.import = import Core
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: --- fail_main_lib.carbon
-// CHECK:STDOUT:
-// CHECK:STDOUT: imports {
-// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [template] {
-// CHECK:STDOUT:     import Core//prelude
-// CHECK:STDOUT:     import Core//prelude/...
-// CHECK:STDOUT:   }
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: file {
-// CHECK:STDOUT:   package: <namespace> = namespace [template] {
-// CHECK:STDOUT:     .Core = imports.%Core
-// CHECK:STDOUT:   }
-// CHECK:STDOUT:   %Core.import = import Core
-// CHECK:STDOUT: }
-// CHECK:STDOUT:

toolchain/check/testdata/packages/no_prelude/restricted_package_names.carbon

@@ -0,0 +1,148 @@
+// 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
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/packages/no_prelude/restricted_package_names.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/packages/no_prelude/restricted_package_names.carbon
+
+// --- lowercase_main.carbon
+
+// The restriction is case-sensitive.
+package main;
+
+// --- fail_main.carbon
+
+// CHECK:STDERR: fail_main.carbon:[[@LINE+4]]:1: error: `Main//default` must omit `package` declaration [ExplicitMainPackage]
+// CHECK:STDERR: package Main;
+// CHECK:STDERR: ^~~~~~~
+// CHECK:STDERR:
+package Main;
+
+// --- fail_main_impl.carbon
+
+// CHECK:STDERR: fail_main_impl.carbon:[[@LINE+4]]:6: error: `Main//default` must omit `package` declaration [ExplicitMainPackage]
+// CHECK:STDERR: impl package Main;
+// CHECK:STDERR:      ^~~~~~~
+// CHECK:STDERR:
+impl package Main;
+
+// --- fail_raw_main.carbon
+
+// `Main` isn't a keyword, so this fails the same way.
+// CHECK:STDERR: fail_raw_main.carbon:[[@LINE+4]]:1: error: `Main//default` must omit `package` declaration [ExplicitMainPackage]
+// CHECK:STDERR: package r#Main;
+// CHECK:STDERR: ^~~~~~~
+// CHECK:STDERR:
+package r#Main;
+
+// --- fail_main_lib.carbon
+
+// CHECK:STDERR: fail_main_lib.carbon:[[@LINE+4]]:1: error: use `library` declaration in `Main` package libraries [ExplicitMainLibrary]
+// CHECK:STDERR: package Main library "main_lib";
+// CHECK:STDERR: ^~~~~~~
+// CHECK:STDERR:
+package Main library "[[@TEST_NAME]]";
+
+// --- lowercase_cpp.carbon
+
+// The restriction is case-sensitive.
+package cpp;
+
+// --- fail_cpp.carbon
+
+// CHECK:STDERR: fail_cpp.carbon:[[@LINE+4]]:1: error: `Cpp` cannot be used by a `package` declaration [CppPackageDeclaration]
+// CHECK:STDERR: package Cpp;
+// CHECK:STDERR: ^~~~~~~
+// CHECK:STDERR:
+package Cpp;
+
+// --- fail_cpp.impl.carbon
+
+// CHECK:STDERR: fail_cpp.impl.carbon:[[@LINE+4]]:6: error: `Cpp` cannot be used by a `package` declaration [CppPackageDeclaration]
+// CHECK:STDERR: impl package Cpp;
+// CHECK:STDERR:      ^~~~~~~
+// CHECK:STDERR:
+impl package Cpp;
+
+// --- fail_raw_cpp.carbon
+
+// `cpp` isn't a keyword, so this fails the same way.
+// CHECK:STDERR: fail_raw_cpp.carbon:[[@LINE+4]]:1: error: `Cpp` cannot be used by a `package` declaration [CppPackageDeclaration]
+// CHECK:STDERR: package r#Cpp;
+// CHECK:STDERR: ^~~~~~~
+// CHECK:STDERR:
+package r#Cpp;
+
+// --- fail_cpp_lib.carbon
+
+// CHECK:STDERR: fail_cpp_lib.carbon:[[@LINE+3]]:1: error: `Cpp` cannot be used by a `package` declaration [CppPackageDeclaration]
+// CHECK:STDERR: package Cpp library "cpp_lib";
+// CHECK:STDERR: ^~~~~~~
+package Cpp library "[[@TEST_NAME]]";
+
+// CHECK:STDOUT: --- lowercase_main.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_main.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_main_impl.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_raw_main.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_main_lib.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- lowercase_cpp.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_cpp.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_cpp.impl.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     has_error
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %default.import = import <invalid>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_raw_cpp.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_cpp_lib.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:

toolchain/diagnostics/diagnostic_kind.def

@@ -152,6 +152,7 @@ CARBON_DIAGNOSTIC_KIND(ImportNotFound)
 CARBON_DIAGNOSTIC_KIND(ImportCycleDetected)
 CARBON_DIAGNOSTIC_KIND(ExplicitMainPackage)
 CARBON_DIAGNOSTIC_KIND(ExplicitMainLibrary)
+CARBON_DIAGNOSTIC_KIND(CppPackageDeclaration)
 CARBON_DIAGNOSTIC_KIND(ImportMainPackage)
 CARBON_DIAGNOSTIC_KIND(ImportMainDefaultLibrary)
 CARBON_DIAGNOSTIC_KIND(ImportCurrentPackageByName)