Remove obsolete cudf::strings::replace_nulls (#7965)

The `cudf::strings::replace_nulls()` is a public API that was replaced by `cudf::replace_nulls()`. 
The strings one should not be used since the base one handles any column type.

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Conor Hoekstra (https://github.com/codereport)
  - Nghia Truong (https://github.com/ttnghia)
  - Christopher Harris (https://github.com/cwharris)
  - Mike Wilson (https://github.com/hyperbolic2346)

URL: #7965

cpp/include/cudf/strings/detail/replace.hpp

@@ -78,10 +78,22 @@ std::unique_ptr<column> replace(
   rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource());
 
 /**
- * @copydoc cudf::strings::replace(strings_column_view const&, string_scalar const&,
- * rmm::mr::device_memory_resource*)
+ * @brief Replaces any null string entries with the given string.
  *
- * @param[in] stream CUDA stream used for device memory operations and kernel launches.
+ * This returns a strings column with no null entries.
+ *
+ * @code{.pseudo}
+ * Example:
+ * s = ["hello", nullptr, "goodbye"]
+ * r = replace_nulls(s,"**")
+ * r is now ["hello", "**", "goodbye"]
+ * @endcode
+ *
+ * @param strings Strings column for this operation.
+ * @param repl Replacement string for null entries. Default is empty string.
+ * @param stream CUDA stream used for device memory operations and kernel launches.
+ * @param mr Device memory resource used to allocate the returned column's device memory.
+ * @return New strings column.
  */
 std::unique_ptr<column> replace_nulls(
   strings_column_view const& strings,

cpp/include/cudf/strings/replace.hpp

@@ -151,28 +151,6 @@ std::unique_ptr<column> replace(
   strings_column_view const& repls,
   rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource());
 
-/**
- * @brief Replaces any null string entries with the given string.
- *
- * This returns a strings column with no null entries.
- *
- * @code{.pseudo}
- * Example:
- * s = ["hello", nullptr, "goodbye"]
- * r = replace_nulls(s,"**")
- * r is now ["hello", "**", "goodbye"]
- * @endcode
- *
- * @param strings Strings column for this operation.
- * @param repl Replacement string for null entries. Default is empty string.
- * @param mr Device memory resource used to allocate the returned column's device memory.
- * @return New strings column.
- */
-std::unique_ptr<column> replace_nulls(
-  strings_column_view const& strings,
-  string_scalar const& repl           = string_scalar(""),
-  rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource());
-
 /** @} */  // end of doxygen group
 }  // namespace strings
 }  // namespace cudf

cpp/src/strings/replace/replace.cu

@@ -871,13 +871,5 @@ std::unique_ptr<column> replace(strings_column_view const& strings,
   return detail::replace(strings, targets, repls, rmm::cuda_stream_default, mr);
 }
 
-std::unique_ptr<column> replace_nulls(strings_column_view const& strings,
-                                      string_scalar const& repl,
-                                      rmm::mr::device_memory_resource* mr)
-{
-  CUDF_FUNC_RANGE();
-  return detail::replace_nulls(strings, repl, rmm::cuda_stream_default, mr);
-}
-
 }  // namespace strings
 }  // namespace cudf

cpp/tests/strings/replace_tests.cpp

@@ -337,30 +337,6 @@ TEST_F(StringsReplaceTest, ReplaceMulti)
   }
 }
 
-TEST_F(StringsReplaceTest, ReplaceNulls)
-{
-  std::vector<const char*> h_strings{"Héllo", "thesé", nullptr, "ARE THE", "tést strings", ""};
-
-  cudf::test::strings_column_wrapper strings(
-    h_strings.begin(),
-    h_strings.end(),
-    thrust::make_transform_iterator(h_strings.begin(), [](auto str) { return str != nullptr; }));
-  auto strings_view = cudf::strings_column_view(strings);
-
-  {
-    auto results = cudf::strings::replace_nulls(strings_view, cudf::string_scalar("___"));
-    std::vector<const char*> h_expected{"Héllo", "thesé", "___", "ARE THE", "tést strings", ""};
-    cudf::test::strings_column_wrapper expected(h_expected.begin(), h_expected.end());
-    CUDF_TEST_EXPECT_COLUMNS_EQUAL(*results, expected);
-  }
-  {
-    auto results = cudf::strings::replace_nulls(strings_view);
-    std::vector<const char*> h_expected{"Héllo", "thesé", "", "ARE THE", "tést strings", ""};
-    cudf::test::strings_column_wrapper expected(h_expected.begin(), h_expected.end());
-    CUDF_TEST_EXPECT_COLUMNS_EQUAL(*results, expected);
-  }
-}
-
 TEST_F(StringsReplaceTest, EmptyStringsColumn)
 {
   cudf::column_view zero_size_strings_column(