From 5b84ab3b6852787f24f48534e7060603c051d5bf Mon Sep 17 00:00:00 2001
From: Ben Deane <ben@elbeno.com>
Date: Wed, 29 Oct 2025 09:33:02 -0600
Subject: [PATCH] :sparkles: Rename a field in a message

Problem:
- It is sometimes useful to rename a field in a message. Since fields are
  identified by name, this cannot be done with the existing `extend` mechanism.

Solution:
- Introduce `rename_field<Msg, OldName, NewName>` for this purpose.
---
 include/msg/field.hpp                    |  5 ++++
 include/msg/message.hpp                  | 34 ++++++++++++++++++++++++
 test/msg/fail/CMakeLists.txt             |  2 ++
 test/msg/fail/rename_field_duplicate.cpp | 15 +++++++++++
 test/msg/fail/rename_field_not_found.cpp | 13 +++++++++
 test/msg/message.cpp                     |  9 +++++++
 6 files changed, 78 insertions(+)
 create mode 100644 test/msg/fail/rename_field_duplicate.cpp
 create mode 100644 test/msg/fail/rename_field_not_found.cpp

diff --git a/include/msg/field.hpp b/include/msg/field.hpp
index ecc59947..d8c7011d 100644
--- a/include/msg/field.hpp
+++ b/include/msg/field.hpp
@@ -535,6 +535,11 @@ class field_t : public field_spec_t<Name, T, detail::field_size<Ats...>>,
     using with_new_type =
         field_t<Name, U, detail::with_default<U>, match::always_t, Ats...>;
 
+    // ======================================================================
+    // rename field
+    template <stdx::ct_string NewName>
+    using with_new_name = field_t<NewName, T, Default, M, Ats...>;
+
     // ======================================================================
     // shift a field
     template <auto N, typename Unit = bit_unit>
diff --git a/include/msg/message.hpp b/include/msg/message.hpp
index 0859b588..5e59b757 100644
--- a/include/msg/message.hpp
+++ b/include/msg/message.hpp
@@ -385,6 +385,7 @@ struct message {
     using num_fields_t = std::integral_constant<std::size_t, sizeof...(Fields)>;
     template <std::size_t I> using nth_field_t = stdx::nth_t<I, Fields...>;
 
+    using name_t = stdx::cts_t<Name>;
     using env_t = Env;
     using access_t = msg_access<Name, Fields...>;
     using default_storage_t = typename access_t::default_storage_t;
@@ -789,4 +790,37 @@ struct field_locator<Name, Env, Fields...> {
 
 template <stdx::ct_string Name, typename... Ts>
 using relaxed_message = typename detail::field_locator<Name, Ts...>::msg_type;
+
+namespace detail {
+template <typename Msg> struct replace_fields_q {
+    template <typename... Fs>
+    using fn = ::msg::message<Msg::name_t::value, typename Msg::env_t, Fs...>;
+};
+
+template <typename Msg, stdx::ct_string OldName, stdx::ct_string NewName>
+struct with_renamed_field {
+    using split_fields = boost::mp11::mp_partition_q<typename Msg::fields_t,
+                                                     matching_name<OldName>>;
+
+    using untouched_fs = boost::mp11::mp_second<split_fields>;
+    static_assert(boost::mp11::mp_count_if_q<untouched_fs,
+                                             matching_name<NewName>>::value ==
+                      0,
+                  "rename_field: New field name already exists in message");
+
+    using replaced_fs = boost::mp11::mp_first<split_fields>;
+    static_assert(not boost::mp11::mp_empty<replaced_fs>::value,
+                  "rename_field: Old field name not found in message");
+
+    using new_f = typename boost::mp11::mp_first<
+        replaced_fs>::template with_new_name<NewName>;
+
+    using new_fields = boost::mp11::mp_push_back<untouched_fs, new_f>;
+    using msg = boost::mp11::mp_apply_q<replace_fields_q<Msg>, new_fields>;
+};
+} // namespace detail
+
+template <typename Msg, stdx::ct_string OldName, stdx::ct_string NewName>
+using rename_field =
+    typename detail::with_renamed_field<Msg, OldName, NewName>::msg;
 } // namespace msg
diff --git a/test/msg/fail/CMakeLists.txt b/test/msg/fail/CMakeLists.txt
index 0af5cc91..9a1627dd 100644
--- a/test/msg/fail/CMakeLists.txt
+++ b/test/msg/fail/CMakeLists.txt
@@ -21,4 +21,6 @@ add_compile_fail_test(message_set_nonexistent_field.cpp LIBRARIES warnings
                       cib_msg)
 add_compile_fail_test(message_uninitialized_field.cpp LIBRARIES warnings
                       cib_msg)
+add_compile_fail_test(rename_field_duplicate.cpp LIBRARIES warnings cib_msg)
+add_compile_fail_test(rename_field_not_found.cpp LIBRARIES warnings cib_msg)
 add_compile_fail_test(view_upsize.cpp LIBRARIES warnings cib_msg)
diff --git a/test/msg/fail/rename_field_duplicate.cpp b/test/msg/fail/rename_field_duplicate.cpp
new file mode 100644
index 00000000..fe31c8a3
--- /dev/null
+++ b/test/msg/fail/rename_field_duplicate.cpp
@@ -0,0 +1,15 @@
+#include <msg/message.hpp>
+
+// EXPECT: New field name already exists in message
+namespace {
+using namespace msg;
+
+using test_field1 =
+    field<"f1", std::uint32_t>::located<at{0_dw, 31_msb, 24_lsb}>;
+using test_field2 =
+    field<"f2", std::uint32_t>::located<at{0_dw, 23_msb, 16_lsb}>;
+
+using msg_defn = message<"test_msg", test_field1, test_field2>;
+} // namespace
+
+auto main() -> int { using M = rename_field<msg_defn, "f1", "f2">; }
diff --git a/test/msg/fail/rename_field_not_found.cpp b/test/msg/fail/rename_field_not_found.cpp
new file mode 100644
index 00000000..ed7b6d79
--- /dev/null
+++ b/test/msg/fail/rename_field_not_found.cpp
@@ -0,0 +1,13 @@
+#include <msg/message.hpp>
+
+// EXPECT: Old field name not found in message
+namespace {
+using namespace msg;
+
+using test_field1 =
+    field<"f1", std::uint32_t>::located<at{0_dw, 31_msb, 24_lsb}>;
+
+using msg_defn = message<"test_msg", test_field1>;
+} // namespace
+
+auto main() -> int { using M = rename_field<msg_defn, "f", "F">; }
diff --git a/test/msg/message.cpp b/test/msg/message.cpp
index 75a4a863..35c2192c 100644
--- a/test/msg/message.cpp
+++ b/test/msg/message.cpp
@@ -505,6 +505,15 @@ TEST_CASE("extend message with retyped field", "[message]") {
     STATIC_REQUIRE(std::is_same_v<defn, expected_defn>);
 }
 
+TEST_CASE("rename field in message", "[message]") {
+    using base_defn = message<"msg_base", id_field, field1>;
+    using defn = rename_field<base_defn, "f1", "f1_new">;
+    using expected_f =
+        field<"f1_new", std::uint32_t>::located<at{0_dw, 15_msb, 0_lsb}>;
+    using expected_defn = message<"msg_base", id_field, expected_f>;
+    STATIC_REQUIRE(std::is_same_v<defn, expected_defn>);
+}
+
 TEST_CASE("message equivalence (owning)", "[message]") {
     test_msg m1{"f1"_field = 0xba11, "f2"_field = 0x42, "f3"_field = 0xd00d};
     test_msg m2{"f1"_field = 0xba11, "f2"_field = 0x42, "f3"_field = 0xd00d};