Add a validator that checks that the lib directory is not empty.

utils/pub/validator/lib.dart

Index: utils/pub/validator/lib.dart
diff --git a/utils/pub/validator/lib.dart b/utils/pub/validator/lib.dart
new file mode 100644
index 0000000000000000000000000000000000000000..64f44c6365f5694c8468c8b73d513a0a171a7416
--- /dev/null
+++ b/utils/pub/validator/lib.dart
@@ -0,0 +1,43 @@
+// Copyright (c) 2012, 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.
+
+library lib_validator;
+
+import 'dart:io';
+
+import '../entrypoint.dart';
+import '../io.dart';
+import '../system_cache.dart';
+import '../utils.dart';
+import '../validator.dart';
+
+// TODO(nweiz): When issue 7196 is fixed, complain about non-Dart files in lib.
+/// A validator that checks that libraries in "lib/" (and not "lib/src/") exist
+/// and are well-formed.
+class LibValidator extends Validator {
+  LibValidator(Entrypoint entrypoint)
+    : super(entrypoint);
+
+  Future validate() {
+    var libDir = join(entrypoint.root.dir, "lib");
+    return dirExists(libDir).chain((libDirExists) {
+      if (!libDirExists) {
+        errors.add('Your package must have a "lib/" directory so users have '

[Bob Nystrom, 2012/12/12 00:11:51]

In pub install, it's only a warning if you depend on a package with no lib
directory. Can it be a warning here too?

[nweiz, 2012/12/12 00:53:34]

We decided to make this an error because there's no utility at all in publishing
a package that contains no resources and is un-importable, other than squatting
on a package name, which I don't think we want to encourage.

I thought the warning from depending on a package without a lib directory was
just a holdover from when we supported both package layouts. There's no reason
that shouldn't be an error, either.

+            'something to import.');
+        return new Future.immediate(null);
+      }
+
+      return listDir(libDir).transform((files) {
+        files = files.map((file) => relativeTo(file, libDir));
+        if (files.isEmpty) {
+          errors.add('The "lib/" directory may not be empty so users have '
+              'something to import');
+        } else if (files.length == 1 && files.first == "src") {
+          errors.add('The "lib/" directory must contain something other than '
+              '"src/" so users have something to import');
+        }
+      });
+    });
+  }
+}