[core] Move mbgl::Pass and mbgl::PassRefPtr to a separate header

CMakeLists.txt

@@ -266,6 +266,7 @@ add_library(
     ${PROJECT_SOURCE_DIR}/include/mbgl/util/logging.hpp
     ${PROJECT_SOURCE_DIR}/include/mbgl/util/noncopyable.hpp
     ${PROJECT_SOURCE_DIR}/include/mbgl/util/optional.hpp
+    ${PROJECT_SOURCE_DIR}/include/mbgl/util/pass_types.hpp
     ${PROJECT_SOURCE_DIR}/include/mbgl/util/platform.hpp
     ${PROJECT_SOURCE_DIR}/include/mbgl/util/premultiply.hpp
     ${PROJECT_SOURCE_DIR}/include/mbgl/util/projection.hpp

include/mbgl/actor/scheduler.hpp

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <mbgl/util/pass_types.hpp>
+
 #include <mapbox/weak.hpp>
 
 #include <functional>
@@ -9,23 +11,6 @@ namespace mbgl {
 
 class Mailbox;
 
-// Using this type as a return type enforces the client to retain the returned object.
-// TODO:  Move to a separate file if/when other clients for this aux API turn up.
-template <typename T>
-class Pass {
-public:
-    Pass(T&& obj_) : obj(std::forward<T>(obj_)) {}
-    Pass(Pass&&) = default;
-    Pass(const Pass&) = delete;
-    operator T() && { return std::move(obj); }
-
-private:
-    T obj;
-};
-
-template <typename T>
-using PassRefPtr = Pass<std::shared_ptr<T>>;
-
 /*
     A `Scheduler` is responsible for coordinating the processing of messages by
     one or more actors via their mailboxes. It's an abstract interface. Currently,

include/mbgl/util/pass_types.hpp

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <memory>
+
+namespace mbgl {
+
+// Using this type as a return type enforces the client to retain the returned object.
+template <typename T>
+class Pass {
+public:
+    Pass(T&& obj_) : obj(std::forward<T>(obj_)) {}
+    Pass(Pass&&) noexcept = default;
+    Pass(const Pass&) = delete;
+    Pass& operator=(const Pass&) = delete;
+    operator T() && { return std::move(obj); }
+
+private:
+    T obj;
+};
+
+template <typename T>
+using PassRefPtr = Pass<std::shared_ptr<T>>;
+
+} // namespace mbgl