[flutter_tools] warn when main library is relocated (#62930)

packages/flutter_tools/lib/src/run_hot.dart

@@ -376,10 +376,19 @@ class HotRunner extends ResidentRunner {
       asyncScanning: hotRunnerConfig.asyncScanning,
       packageConfig: flutterDevices[0].devFS.lastPackageConfig,
     );
+    final File entrypointFile = globals.fs.file(mainPath);
+    if (!entrypointFile.existsSync()) {
+      globals.printError(
+        'The entrypoint file (i.e. the file with main()) ${entrypointFile.path} '
+        'cannot be found. Moving or renaming this file will prevent changes to '
+        'its contents from being discovered during hot reload/restart until '
+        'flutter is restarted or the file is restored.'
+      );
+    }
     final UpdateFSReport results = UpdateFSReport(success: true);
     for (final FlutterDevice device in flutterDevices) {
       results.incorporateResults(await device.updateDevFS(
-        mainUri: globals.fs.file(mainPath).absolute.uri,
+        mainUri: entrypointFile.absolute.uri,
         target: target,
         bundle: assetBundle,
         firstBuildTime: firstBuildTime,

packages/flutter_tools/test/general.shard/resident_runner_test.dart

@@ -609,6 +609,67 @@ void main() {
     Usage: () => MockUsage(),
   }));
 
+  testUsingContext('ResidentRunner reports error with missing entrypoint file', () => testbed.run(() async {
+    fakeVmServiceHost = FakeVmServiceHost(requests: <VmServiceExpectation>[
+      listViews,
+      listViews,
+      listViews,
+      listViews,
+      FakeVmServiceRequest(
+        method: 'getIsolate',
+        args: <String, Object>{
+          'isolateId': '1',
+        },
+        jsonResponse: fakeUnpausedIsolate.toJson(),
+      ),
+      FakeVmServiceRequest(
+        method: 'ext.flutter.reassemble',
+        args: <String, Object>{
+          'isolateId': fakeUnpausedIsolate.id,
+        },
+      ),
+    ]);
+    when(mockDevice.sdkNameAndVersion).thenAnswer((Invocation invocation) async {
+      return 'Example';
+    });
+    when(mockDevice.targetPlatform).thenAnswer((Invocation invocation) async {
+      return TargetPlatform.android_arm;
+    });
+    when(mockDevice.isLocalEmulator).thenAnswer((Invocation invocation) async {
+      return false;
+    });
+    final Completer<DebugConnectionInfo> onConnectionInfo = Completer<DebugConnectionInfo>.sync();
+    final Completer<void> onAppStart = Completer<void>.sync();
+    unawaited(residentRunner.attach(
+      appStartedCompleter: onAppStart,
+      connectionInfoCompleter: onConnectionInfo,
+    ));
+    await onAppStart.future;
+    when(mockFlutterDevice.updateDevFS(
+      mainUri: anyNamed('mainUri'),
+      target: anyNamed('target'),
+      bundle: anyNamed('bundle'),
+      firstBuildTime: anyNamed('firstBuildTime'),
+      bundleFirstUpload: anyNamed('bundleFirstUpload'),
+      bundleDirty: anyNamed('bundleDirty'),
+      fullRestart: anyNamed('fullRestart'),
+      projectRootPath: anyNamed('projectRootPath'),
+      pathToReload: anyNamed('pathToReload'),
+      invalidatedFiles: anyNamed('invalidatedFiles'),
+      dillOutputPath: anyNamed('dillOutputPath'),
+      packageConfig: anyNamed('packageConfig'),
+    )).thenAnswer((Invocation invocation) async {
+      return UpdateFSReport(success: true);
+    });
+
+    final OperationResult result = await residentRunner.restart(fullRestart: false);
+
+    expect(globals.fs.file(globals.fs.path.join('lib', 'main.dart')), isNot(exists));
+    expect(testLogger.errorText, contains('The entrypoint file (i.e. the file with main())'));
+    expect(result.fatal, false);
+    expect(result.code, 0);
+  }));
+
 
   testUsingContext('ResidentRunner can send target platform to analytics from hot reload', () => testbed.run(() async {
     fakeVmServiceHost = FakeVmServiceHost(requests: <VmServiceExpectation>[