✅ Add message equivalence matcher for Catch2

Problem:
There is no example way to test message equivalence.

Solution:
Provide a test that implements a Catch2 matcher. A similar concept exists for GoogleTest.

Author: elbeno

include/msg/message.hpp

@@ -399,11 +399,12 @@ template <stdx::ct_string Name, typename... Fields> struct message {
         [[nodiscard]] constexpr auto data() const { return storage; }
 
         [[nodiscard]] constexpr auto as_owning() { return owner_t{*this}; }
+
+        using const_view_t =
+            view_t<stdx::span<std::add_const_t<typename Span::value_type>,
+                              stdx::ct_capacity_v<Span>>>;
         [[nodiscard]] constexpr auto as_const_view() const {
-            using cv_t =
-                view_t<stdx::span<std::add_const_t<typename Span::value_type>,
-                                  stdx::ct_capacity_v<Span>>>;
-            return cv_t{*this};
+            return const_view_t{*this};
         }
 
       private:
@@ -500,6 +501,10 @@ template <stdx::ct_string Name, typename... Fields> struct message {
         [[nodiscard]] constexpr auto as_mutable_view() LIFETIMEBOUND {
             return view_t{*this};
         }
+
+        using const_view_t =
+            view_t<stdx::span<std::add_const_t<typename Storage::value_type>,
+                              stdx::ct_capacity_v<Storage>>>;
         [[nodiscard]] constexpr auto as_const_view() const LIFETIMEBOUND {
             return view_t{*this};
         }

test/msg/message.cpp

@@ -2,6 +2,7 @@
 #include <msg/message.hpp>
 
 #include <catch2/catch_test_macros.hpp>
+#include <catch2/matchers/catch_matchers_templated.hpp>
 
 #include <algorithm>
 #include <array>
@@ -500,3 +501,40 @@ TEST_CASE("message equivalence (views)", "[message]") {
     auto cv2 = other.as_const_view();
     CHECK(not equivalent(cv1, cv2));
 }
+
+namespace {
+template <typename View>
+struct MsgEquivMatcher : Catch::Matchers::MatcherGenericBase {
+    MsgEquivMatcher(View v) : view{v} {}
+
+    template <typename OtherMsg> bool match(OtherMsg const &other) const {
+        return msg::equivalent(view, other);
+    }
+
+    std::string describe() const override {
+        return "Equivalent: " + Catch::rangeToString(view.data());
+    }
+
+  private:
+    View view;
+};
+
+template <typename Msg>
+auto MsgEquiv(Msg const &msg) -> MsgEquivMatcher<typename Msg::const_view_t> {
+    return {msg.as_const_view()};
+}
+} // namespace
+
+TEST_CASE("message equivalence matcher", "[message]") {
+    owning<msg_defn> m{"f1"_field = 0xba11, "f2"_field = 0x42,
+                       "f3"_field = 0xd00d};
+    auto cv1 = m.as_const_view();
+    auto mv1 = m.as_mutable_view();
+
+    CHECK_THAT(cv1, MsgEquiv(m));
+    CHECK_THAT(mv1, MsgEquiv(m));
+
+    owning<msg_defn> m2{"f1"_field = 0xba12, "f2"_field = 0x42,
+                        "f3"_field = 0xd00d};
+    CHECK_THAT(m2, not MsgEquiv(m));
+}