Reland "[vm/nnbd] Auto-detect null-safety from entry point in kernel compilers"

This is a reland of fd19c8cc25
No changes were made to the original CL.

The following fixes landed separately:
* Flutter tests which use --enable-experiment=non-nullable are
  disabled in https://github.com/flutter/flutter/pull/57235.
* Auto-detection of null-safety mode from dill file in gen_snapshot is
  implemented in https://dart-review.googlesource.com/c/sdk/+/148152.

Original change's description:
> [vm/nnbd] Auto-detect null-safety from entry point in kernel compilers
>
> If --null-safety flag is not specified, tools should infer null safety
> mode from a legacy status of an entry point script.
> This change adds such auto-detection to kernel compilers (front-end
> server and gen_kernel).
>
> Issue: https://github.com/dart-lang/sdk/issues/41206
> Change-Id: If78491d08390c28b98e86857ec7d61520433dee9
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/148024
> Reviewed-by: Ryan Macnak <rmacnak@google.com>
> Reviewed-by: Alexander Aprelev <aam@google.com>
> Commit-Queue: Alexander Markov <alexmarkov@google.com>

Change-Id: Ibc185caa6975087a35c89ff69faf3f6c11fcf14c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/148201
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

pkg/frontend_server/lib/frontend_server.dart

@@ -152,7 +152,7 @@ ArgParser argParser = ArgParser(allowTrailingOptions: true)
   ..addFlag('null-safety',
       help:
           'Respect the nullability of types at runtime in casts and instance checks.',
-      defaultsTo: false)
+      defaultsTo: null)
   ..addMultiOption('enable-experiment',
       help: 'Comma separated list of experimental features, eg set-literals.',
       hide: true)
@@ -385,7 +385,8 @@ class FrontendCompiler implements CompilerInterface {
       ..experimentalFlags = parseExperimentalFlags(
           parseExperimentalArguments(options['enable-experiment']),
           onError: (msg) => errors.add(msg))
-      ..nnbdMode = options['null-safety'] ? NnbdMode.Strong : NnbdMode.Weak
+      ..nnbdMode =
+          (options['null-safety'] == true) ? NnbdMode.Strong : NnbdMode.Weak
       ..onDiagnostic = (DiagnosticMessage message) {
         bool printMessage;
         switch (message.severity) {
@@ -446,6 +447,11 @@ class FrontendCompiler implements CompilerInterface {
       }
     }
 
+    if (options['null-safety'] == null &&
+        compilerOptions.experimentalFlags[ExperimentalFlag.nonNullable]) {
+      await autoDetectNullSafetyMode(_mainSource, compilerOptions);
+    }
+
     compilerOptions.bytecode = options['gen-bytecode'];
     final BytecodeOptions bytecodeOptions = BytecodeOptions(
       enableAsserts: options['enable-asserts'],
@@ -459,7 +465,7 @@ class FrontendCompiler implements CompilerInterface {
     compilerOptions.target = createFrontEndTarget(
       options['target'],
       trackWidgetCreation: options['track-widget-creation'],
-      nullSafety: options['null-safety'],
+      nullSafety: compilerOptions.nnbdMode == NnbdMode.Strong,
     );
     if (compilerOptions.target == null) {
       print('Failed to create front-end target ${options['target']}.');

pkg/vm/bin/kernel_service.dart

@@ -31,7 +31,6 @@ import 'package:build_integration/file_system/multi_root.dart';
 import 'package:front_end/src/api_prototype/experimental_flags.dart';
 import 'package:front_end/src/api_prototype/front_end.dart' as fe
     show CompilerResult;
-import 'package:front_end/src/api_prototype/language_version.dart';
 import 'package:front_end/src/api_prototype/memory_file_system.dart';
 import 'package:front_end/src/api_unstable/vm.dart';
 import 'package:kernel/binary/ast_to_binary.dart';
@@ -46,7 +45,10 @@ import 'package:vm/bytecode/gen_bytecode.dart'
 import 'package:vm/bytecode/options.dart' show BytecodeOptions;
 import 'package:vm/incremental_compiler.dart';
 import 'package:vm/kernel_front_end.dart'
-    show createLoadedLibrariesSet, runWithFrontEndCompilerContext;
+    show
+        autoDetectNullSafetyMode,
+        createLoadedLibrariesSet,
+        runWithFrontEndCompilerContext;
 import 'package:vm/http_filesystem.dart';
 import 'package:vm/target/vm.dart' show VmTarget;
 import 'package:front_end/src/api_prototype/compiler_options.dart'
@@ -90,12 +92,6 @@ const int kNullSafetyOptionUnspecified = 0;
 const int kNullSafetyOptionWeak = 1;
 const int kNullSafetyOptionStrong = 2;
 
-Future<void> autoDetectNullSafetyMode(
-    Uri script, CompilerOptions options) async {
-  var isLegacy = await uriUsesLegacyLanguageVersion(script, options);
-  options.nnbdMode = isLegacy ? NnbdMode.Weak : NnbdMode.Strong;
-}
-
 abstract class Compiler {
   final int isolateId;
   final FileSystem fileSystem;

pkg/vm/lib/kernel_front_end.dart

@@ -15,6 +15,9 @@ import 'package:build_integration/file_system/multi_root.dart'
 
 import 'package:crypto/crypto.dart';
 
+import 'package:front_end/src/api_prototype/language_version.dart'
+    show uriUsesLegacyLanguageVersion;
+
 import 'package:front_end/src/api_unstable/vm.dart'
     show
         CompilerContext,
@@ -22,6 +25,7 @@ import 'package:front_end/src/api_unstable/vm.dart'
         CompilerResult,
         DiagnosticMessage,
         DiagnosticMessageHandler,
+        ExperimentalFlag,
         FileSystem,
         FileSystemEntity,
         FileSystemException,
@@ -110,7 +114,7 @@ void declareCompilerOptions(ArgParser args) {
   args.addFlag('null-safety',
       help:
           'Respect the nullability of types at runtime in casts and instance checks.',
-      defaultsTo: false);
+      defaultsTo: null);
   args.addFlag('split-output-by-packages',
       help:
           'Split resulting kernel file into multiple files (one per package).',
@@ -200,12 +204,6 @@ Future<int> runCompiler(ArgResults options, String usage) async {
     aot: aot,
   )..parseCommandLineFlags(options['bytecode-options']);
 
-  final target = createFrontEndTarget(targetName, nullSafety: nullSafety);
-  if (target == null) {
-    print('Failed to create front-end target $targetName.');
-    return badUsageExitCode;
-  }
-
   final fileSystem =
       createFrontEndFileSystem(fileSystemScheme, fileSystemRoots);
 
@@ -227,19 +225,30 @@ Future<int> runCompiler(ArgResults options, String usage) async {
 
   final CompilerOptions compilerOptions = new CompilerOptions()
     ..sdkSummary = platformKernelUri
-    ..target = target
     ..fileSystem = fileSystem
     ..additionalDills = additionalDills
     ..packagesFileUri = packagesUri
     ..experimentalFlags = parseExperimentalFlags(
         parseExperimentalArguments(experimentalFlags),
         onError: print)
-    ..nnbdMode = nullSafety ? NnbdMode.Strong : NnbdMode.Weak
+    ..nnbdMode = (nullSafety == true) ? NnbdMode.Strong : NnbdMode.Weak
     ..onDiagnostic = (DiagnosticMessage m) {
       errorDetector(m);
     }
     ..embedSourceText = embedSources;
 
+  if (nullSafety == null &&
+      compilerOptions.experimentalFlags[ExperimentalFlag.nonNullable]) {
+    await autoDetectNullSafetyMode(mainUri, compilerOptions);
+  }
+
+  compilerOptions.target = createFrontEndTarget(targetName,
+      nullSafety: compilerOptions.nnbdMode == NnbdMode.Strong);
+  if (compilerOptions.target == null) {
+    print('Failed to create front-end target $targetName.');
+    return badUsageExitCode;
+  }
+
   final results = await compileToKernel(mainUri, compilerOptions,
       includePlatform: additionalDills.isNotEmpty,
       aot: aot,
@@ -558,6 +567,13 @@ bool parseCommandLineDefines(
   return true;
 }
 
+/// Detect null safety mode from an entry point and set [options.nnbdMode].
+Future<void> autoDetectNullSafetyMode(
+    Uri script, CompilerOptions options) async {
+  var isLegacy = await uriUsesLegacyLanguageVersion(script, options);
+  options.nnbdMode = isLegacy ? NnbdMode.Weak : NnbdMode.Strong;
+}
+
 /// Create front-end target with given name.
 Target createFrontEndTarget(String targetName,
     {bool trackWidgetCreation = false, bool nullSafety = false}) {

pkg/vm/test/snapshot/instruction_sizes_test.dart

@@ -8,7 +8,6 @@ import 'package:path/path.dart' as path;
 import 'package:test/test.dart';
 
 import 'package:vm/snapshot/instruction_sizes.dart' as instruction_sizes;
-import 'package:vm/snapshot/name.dart';
 
 void main() async {
   if (!Platform.executable.contains('dart-sdk')) {

runtime/tests/vm/dart/null_safety_autodetection_in_kernel_compiler_test.dart

@@ -0,0 +1,44 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Tests auto-detection of null safety mode in gen_kernel tool.
+
+import 'dart:io' show File, Platform;
+
+import 'package:path/path.dart' as path;
+import 'snapshot_test_helper.dart';
+
+compileAndRunTest(String comment, String expectedOutput) async {
+  await withTempDir((String temp) async {
+    final testScriptPath = path.join(temp, 'test.dart');
+    File(testScriptPath).writeAsStringSync('''
+      // $comment
+
+      void main() {
+        try {
+          null as int;
+          print('weak mode');
+        } on TypeError {
+          print('strong mode');
+        }
+      }
+    ''');
+
+    final testDillPath = path.join(temp, 'test.dill');
+    await runGenKernel('BUILD DILL FILE', [
+      '--enable-experiment=non-nullable',
+      '--output=$testDillPath',
+      testScriptPath,
+    ]);
+
+    final result = await runBinary(
+        'RUN TEST FROM DILL', Platform.executable, [testDillPath]);
+    expectOutput(expectedOutput, result);
+  });
+}
+
+main() async {
+  await compileAndRunTest('', 'strong mode');
+  await compileAndRunTest('@dart=2.7', 'weak mode');
+}

runtime/tests/vm/vm.status

@@ -17,6 +17,7 @@ dart/data_uri_import_test/none: SkipByDesign
 dart/emit_aot_size_info_flag_test: Pass, Slow # Spawns several subprocesses
 dart/minimal_kernel_bytecode_test: Pass, Slow # Spawns several subprocesses
 dart/minimal_kernel_test: Pass, Slow # Spawns several subprocesses
+dart/null_safety_autodetection_in_kernel_compiler_test: Pass, Slow # Spawns several subprocesses
 dart/slow_path_shared_stub_test: Pass, Slow # Uses --shared-slow-path-triggers-gc flag.
 dart/snapshot_version_test: Skip # This test is a Dart1 test (script snapshot)
 dart/stack_overflow_shared_test: Pass, Slow # Uses --shared-slow-path-triggers-gc flag.
@@ -52,6 +53,7 @@ dart/appjit*: SkipByDesign # Test needs to a particular opt-counter value
 dart/kernel_determinism_test: SkipSlow
 dart/minimal_kernel_bytecode_test: SkipSlow # gen_kernel is too slow with optimization_counter_threshold
 dart/minimal_kernel_test: SkipSlow # gen_kernel is too slow with optimization_counter_threshold
+dart/null_safety_autodetection_in_kernel_compiler_test: SkipSlow # gen_kernel is too slow with optimization_counter_threshold
 
 [ $builder_tag == tsan ]
 dart/appjit_cha_deopt_test: SkipSlow
@@ -79,6 +81,7 @@ cc/VerifyImplicit_Crash: Crash # Negative tests of VerifiedMemory should crash i
 dart/appjit_cha_deopt_test: Pass, Slow # Quite slow in debug mode, uses --optimization-counter-threshold=100
 dart/minimal_kernel_bytecode_test: SkipSlow # gen_kernel is too slow in debug mode
 dart/minimal_kernel_test: SkipSlow # gen_kernel is too slow in debug mode
+dart/null_safety_autodetection_in_kernel_compiler_test: SkipSlow # gen_kernel is too slow in debug mode
 dart/spawn_shutdown_test: Pass, Slow # VM Shutdown test, It can take some time for all the isolates to shutdown in a Debug build.
 
 [ $mode == product ]
@@ -141,6 +144,7 @@ dart/bytecode_and_ast_mix_test: SkipByDesign # Test needs to run from source
 dart/kernel_determinism_test: SkipByDesign # Test needs to run from source
 dart/minimal_kernel_bytecode_test: SkipByDesign # Test needs to run from source
 dart/minimal_kernel_test: SkipByDesign # Test needs to run from source
+dart/null_safety_autodetection_in_kernel_compiler_test: SkipByDesign # Test needs to run from source
 dart/snapshot_depfile_test: SkipByDesign # Test needs to run from source
 
 [ $compiler == dartkp && ($runtime == dart_precompiled || $runtime == vm) ]
@@ -202,6 +206,7 @@ dart/appjit*: SkipSlow # DFE too slow
 dart/data_uri_spawn_test: Skip # Please triage.
 dart/minimal_kernel_bytecode_test: SkipSlow # gen_kernel is too slow on simulated architectures
 dart/minimal_kernel_test: SkipSlow # gen_kernel is too slow on simulated architectures
+dart/null_safety_autodetection_in_kernel_compiler_test: SkipSlow # gen_kernel is too slow on simulated architectures
 dart/snapshot_version_test: RuntimeError # Please triage.
 
 [ $hot_reload || $hot_reload_rollback ]
@@ -302,6 +307,7 @@ dart/disassemble_determinism_test: SkipSlow # Runs expensive fibonacci(32) compu
 dart/issue_31959_31960_test: SkipSlow
 dart/minimal_kernel_bytecode_test: SkipSlow # gen_kernel is too slow in hot reload testing mode
 dart/minimal_kernel_test: SkipSlow # gen_kernel is too slow in hot reload testing mode
+dart/null_safety_autodetection_in_kernel_compiler_test: SkipSlow # gen_kernel is too slow in hot reload testing mode
 dart/print_flow_graph_determinism_test: SkipSlow
 dart/slow_path_shared_stub_test: SkipSlow # Too slow with --shared-slow-path-triggers-gc flag and not relevant outside precompiled.
 dart/spawn_infinite_loop_test: Skip # We can shutdown an isolate before it reloads.