Add coverage testing for parse node kinds. (#4436)

This refactors the diagnostic kind coverage check into something that
also works for node kinds. Then, since this points out a few node kinds
that aren't having their parse verified, I'm adding minor tests for
those.

toolchain/diagnostics/coverage_test.cpp

@@ -0,0 +1,73 @@
+// 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 <gtest/gtest.h>
+
+#include "absl/flags/flag.h"
+#include "toolchain/diagnostics/diagnostic_kind.h"
+#include "toolchain/testing/coverage_helper.h"
+
+ABSL_FLAG(std::string, testdata_manifest, "",
+          "A path to a file containing repo-relative names of test files.");
+
+namespace Carbon {
+namespace {
+
+constexpr DiagnosticKind DiagnosticKinds[] = {
+#define CARBON_DIAGNOSTIC_KIND(Name) DiagnosticKind::Name,
+#include "toolchain/diagnostics/diagnostic_kind.def"
+};
+
+constexpr DiagnosticKind UntestedDiagnosticKinds[] = {
+    // These exist only for unit tests.
+    DiagnosticKind::TestDiagnostic,
+    DiagnosticKind::TestDiagnosticNote,
+
+    // These diagnose filesystem issues that are hard to unit test.
+    DiagnosticKind::ErrorReadingFile,
+    DiagnosticKind::ErrorStattingFile,
+    DiagnosticKind::FileTooLarge,
+
+    // Int literals are currently limited to i32. Once that's fixed, this
+    // should be tested.
+    DiagnosticKind::ArrayBoundTooLarge,
+
+    // This isn't feasible to test with a normal testcase, but is tested in
+    // lex/tokenized_buffer_test.cpp.
+    DiagnosticKind::TooManyTokens,
+
+    // TODO: Should look closer at these, but adding tests is a high risk of
+    // loss in merge conflicts due to the amount of tests being changed right
+    // now.
+    DiagnosticKind::ExternLibraryInImporter,
+    DiagnosticKind::ExternLibraryOnDefinition,
+    DiagnosticKind::HexadecimalEscapeMissingDigits,
+    DiagnosticKind::ImplOfUndefinedInterface,
+    DiagnosticKind::IncompleteTypeInFunctionParam,
+    DiagnosticKind::InvalidDigit,
+    DiagnosticKind::InvalidDigitSeparator,
+    DiagnosticKind::InvalidHorizontalWhitespaceInString,
+    DiagnosticKind::MismatchedIndentInString,
+    DiagnosticKind::ModifierPrivateNotAllowed,
+    DiagnosticKind::MultiLineStringWithDoubleQuotes,
+    DiagnosticKind::NameAmbiguousDueToExtend,
+    DiagnosticKind::TooManyDigits,
+    DiagnosticKind::UnaryOperatorRequiresWhitespace,
+    DiagnosticKind::UnicodeEscapeSurrogate,
+    DiagnosticKind::UnicodeEscapeTooLarge,
+    DiagnosticKind::UnknownBaseSpecifier,
+    DiagnosticKind::UnsupportedCRLineEnding,
+    DiagnosticKind::UnsupportedLFCRLineEnding,
+};
+
+// Looks for diagnostic kinds that aren't covered by a file_test.
+TEST(Coverage, DiagnosticKind) {
+  Testing::TestKindCoverage(absl::GetFlag(FLAGS_testdata_manifest),
+                            R"(^ *// CHECK:STDERR: .*\.carbon:.* \[(\w+)\]$)",
+                            llvm::ArrayRef(DiagnosticKinds),
+                            llvm::ArrayRef(UntestedDiagnosticKinds));
+}
+
+}  // namespace
+}  // namespace Carbon