Fix scatter output size for structs.

1. Output struct size must match target column, not source.
2. Test case for #1.
3. Also, move scatter_struct_tests and scatter_list_tests
   from .cu to .cpp, for faster compile.

cpp/include/cudf/detail/scatter.cuh

@@ -317,7 +317,7 @@ struct column_scatterer_impl<struct_view> {
 
     // Need to put the result column in a vector to call `gather_bitmask`
     std::vector<std::unique_ptr<column>> result;
-    result.emplace_back(cudf::make_structs_column(source.size(),
+    result.emplace_back(cudf::make_structs_column(target.size(),
                                                   std::move(output_struct_members),
                                                   0,
                                                   rmm::device_buffer{0, stream, mr},

cpp/tests/CMakeLists.txt

@@ -206,8 +206,8 @@ ConfigureTest(COPYING_TEST
     copying/pack_tests.cu
     copying/sample_tests.cpp
     copying/scatter_tests.cpp
-    copying/scatter_list_tests.cu
-    copying/scatter_struct_tests.cu
+    copying/scatter_list_tests.cpp
+    copying/scatter_struct_tests.cpp
     copying/segmented_gather_list_tests.cpp
     copying/shift_tests.cpp
     copying/slice_tests.cpp

cpp/tests/copying/scatter_list_tests.cu → cpp/tests/copying/scatter_list_tests.cpp

@@ -18,8 +18,6 @@
 
 #include <cudf/column/column_view.hpp>
 #include <cudf/copying.hpp>
-#include <cudf/detail/iterator.cuh>
-#include <cudf/detail/scatter.cuh>
 #include <cudf/lists/lists_column_view.hpp>
 #include <cudf/table/table.hpp>
 #include <cudf/table/table_view.hpp>

cpp/tests/copying/scatter_struct_tests.cu → cpp/tests/copying/scatter_struct_tests.cpp

@@ -291,3 +291,24 @@ TYPED_TEST(TypedStructScatterTest, ScatterStructOfListsTest)
   auto const scatter_map = int32s_col{-3, -2, -1, 5, 4, 3, 2}.release();
   test_scatter(structs_src, structs_tgt, structs_expected, scatter_map);
 }
+
+TYPED_TEST(TypedStructScatterTest, ScatterSourceSmallerThanTarget)
+{
+  using namespace cudf;
+  using namespace cudf::test;
+
+  using fixed_width     = fixed_width_column_wrapper<TypeParam, int32_t>;
+  using structs_col     = structs_column_wrapper;
+  using scatter_map_col = fixed_width_column_wrapper<offset_type, int32_t>;
+
+  auto source_child   = fixed_width{22, 55};
+  auto target_child   = fixed_width{0, 1, 2, 3, 4, 5, 6};
+  auto expected_child = fixed_width{0, 1, 22, 3, 4, 55, 6};
+
+  auto const source      = structs_col{{source_child}}.release();
+  auto const target      = structs_col{{target_child}}.release();
+  auto const scatter_map = scatter_map_col{2, 5}.release();
+  auto const expected    = structs_col{{expected_child}}.release();
+
+  test_scatter(source, target, expected, scatter_map);
+}