Add move only range test

Signed-off-by: Nahuel Espinosa <nespinosa@ekumenlabs.com>

beluga/include/beluga/actions/assign.hpp

@@ -84,6 +84,12 @@ struct assign_fn {
 /**
  * This can be appended to any view closure to evaluate it eagerly and assign the result
  * to a given range, effectively converting any view into an action.
+ *
+ * The assignment of elements into the container may involve copy which can be less efficient than
+ * move because lvalue references are produced during the indirection call. Users can opt-in to use
+ * `ranges::views::move` to adapt the range in order for their elements to always produce an rvalue
+ * reference during the indirection call which implies move. In this case, make sure that the input
+ * views don't require accessing the elements in the adapted range more than once.
  */
 inline constexpr detail::assign_fn assign;
 

beluga/test/beluga/actions/test_assign.cpp

@@ -22,9 +22,12 @@
 #include <range/v3/action/drop.hpp>
 #include <range/v3/action/remove.hpp>
 #include <range/v3/algorithm/equal.hpp>
+#include <range/v3/view/move.hpp>
 #include <range/v3/view/reverse.hpp>
 #include <range/v3/view/transform.hpp>
 
+#include <memory>
+
 namespace {
 
 TEST(AssignAction, ViewToAction) {
@@ -33,6 +36,16 @@ TEST(AssignAction, ViewToAction) {
   ASSERT_TRUE(ranges::equal(input, std::vector{3, 2, 1}));
 }
 
+TEST(AssignAction, MoveOnlyRange) {
+  auto input = std::vector<std::unique_ptr<int>>{};
+  input.emplace_back(std::make_unique<int>(1));
+  input.emplace_back(std::make_unique<int>(2));
+  input.emplace_back(std::make_unique<int>(3));
+  input |= ranges::views::move | ranges::views::reverse | beluga::actions::assign;
+  ASSERT_EQ(*input.front(), 3);
+  ASSERT_EQ(*input.back(), 1);
+}
+
 TEST(AssignAction, ViewToActionComposition) {
   auto reverse_and_assign = ranges::views::reverse | beluga::actions::assign;
   auto input = std::vector{1, 2, 3};