When running FileTests, verify that autoupdate wouldn't make changes. (#3232)

Trying to more proactively catch when autoupdate is missed. Most of the
execution time of these tests should be in running the program under
test, not processing output, so this should have marginal overhead in
order to produce a useful reminder.

testing/file_test/file_test_base.cpp

@@ -80,7 +80,19 @@ auto FileTestBase::TestBody() -> void {
       << "Tests should be prefixed with `fail_` if and only if running them "
          "is expected to fail.";
 
-  // Check results.
+  // Check results. Include a reminder of the autoupdate command for any
+  // stdout/stderr differences.
+  std::string update_message;
+  if (context.autoupdate_line_number) {
+    update_message = llvm::formatv(
+        "If these differences are expected, try the autoupdater:\n"
+        "\tbazel run {0} -- --autoupdate --file_tests={1}",
+        target, test_name_);
+  } else {
+    update_message =
+        "If these differences are expected, content must be updated manually.";
+  }
+  SCOPED_TRACE(update_message);
   if (context.check_subset) {
     EXPECT_THAT(SplitOutput(context.stdout),
                 IsSupersetOf(context.expected_stdout));
@@ -93,19 +105,17 @@ auto FileTestBase::TestBody() -> void {
     EXPECT_THAT(SplitOutput(context.stderr),
                 ElementsAreArray(context.expected_stderr));
   }
+
+  // If there are no other test failures, check if autoupdate would make
+  // changes. We don't do this when there _are_ failures because the
+  // SCOPED_TRACE already contains the autoupdate reminder.
+  if (!HasFailure() && RunAutoupdater(context, /*dry_run=*/true)) {
+    ADD_FAILURE() << "Autoupdate would make changes to the file content.";
+  }
 }
 
-auto FileTestBase::Autoupdate() -> ErrorOr<bool> {
-  // Add a crash trace entry mentioning which file we're updating.
-  llvm::PrettyStackTraceFormat stack_trace_entry("performing autoupdate for %s",
-                                                 test_name_);
-
-  TestContext context;
-  auto run_result = ProcessTestFileAndRun(context);
-  if (!run_result.ok()) {
-    return ErrorBuilder() << "Error updating " << test_name_ << ": "
-                          << run_result.error();
-  }
+auto FileTestBase::RunAutoupdater(const TestContext& context, bool dry_run)
+    -> bool {
   if (!context.autoupdate_line_number) {
     return false;
   }
@@ -132,7 +142,21 @@ auto FileTestBase::Autoupdate() -> ErrorOr<bool> {
              GetDefaultFileRE(expected_filenames),
              GetLineNumberReplacements(expected_filenames),
              [&](std::string& line) { DoExtraCheckReplacements(line); })
-      .Run();
+      .Run(dry_run);
+}
+
+auto FileTestBase::Autoupdate() -> ErrorOr<bool> {
+  // Add a crash trace entry mentioning which file we're updating.
+  llvm::PrettyStackTraceFormat stack_trace_entry("performing autoupdate for %s",
+                                                 test_name_);
+
+  TestContext context;
+  auto run_result = ProcessTestFileAndRun(context);
+  if (!run_result.ok()) {
+    return ErrorBuilder() << "Error updating " << test_name_ << ": "
+                          << run_result.error();
+  }
+  return RunAutoupdater(context, /*dry_run=*/false);
 }
 
 auto FileTestBase::GetLineNumberReplacements(