Backport Ref<T> to 4.0 (#114)

* Backport Ref<T> to 4.0

* Find the best hhvm that still passes travis

* Remove failing hhvm versions

Only hhvm 4.0 passes, since the others assume that `<?hh` means `// strict` instead of `// partial`

.travis.yml

@@ -2,7 +2,8 @@ sudo: required
 language: generic
 services: docker
 env:
- - HHVM_VERSION=latest
+ - HHVM_VERSION=4.0-latest
+
 install:
  - docker pull hhvm/hhvm:$HHVM_VERSION
 script:

src/Ref.php

@@ -0,0 +1,28 @@
+<?hh // strict
+/*
+ *  Copyright (c) 2004-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the MIT license found in the
+ *  LICENSE file in the root directory of this source tree.
+ *
+ */
+
+namespace HH\Lib;
+
+/** Wrapper class for getting object (byref) semantics for a value type.
+ *
+ * This is especially useful for mutating values outside of a lambda's scope.
+ *
+ * In general, it's preferable to refactor to use return values or `inout`
+ * parameters instead of using this class - however, a `Ref` of a Hack array
+ * is generally preferable to a Hack collection - e.g. prefer `Ref<vec<T>>`
+ * over `Vector<T>`.
+ *
+ * `C\reduce()` and `C\reduce_with_key()` can also be used in some situations
+ * to avoid this class.
+ */
+final class Ref<T> {
+  <<__RxShallow>>
+  public function __construct(public T $value) {}
+}