Avoid allocating temporary basic_json for cbor and msgpack object keys (#5328)

* avoid allocating temporary basic_json for CBOR and MessagePack object keys

Signed-off-by: alexprabhat99 <alexpbara@gmail.com>

* add size() to the custom object key test type

UBJSON and BJData access object keys through size() and c_str()
directly, so the key type now provides both and the comment says why.

Signed-off-by: alexprabhat99 <alexpbara@gmail.com>

* address review: drop key size()/c_str(), test keys below the depth limit

Nothing in the library calls size() or c_str() on an object key, so the
test key type only keeps data(), which JSON_DIAGNOSTICS needs.

The CBOR and MessagePack custom key tests now also nest objects deeper
than detail::recursion_depth_limit(), so keys written by
write_cbor_iterative and write_msgpack_iterative are covered as well.

Signed-off-by: alexprabhat99 <alexpbara@gmail.com>

---------

Signed-off-by: alexprabhat99 <alexpbara@gmail.com>

tests/src/unit-cbor.cpp

@@ -28,6 +28,7 @@ using nlohmann::json;
 #include "make_test_data_available.hpp"
 #include "round_trip_corpus.hpp"
 #include "test_utils.hpp"
+#include "custom_object_key_type.hpp"
 #include "sax_countdown.hpp"
 using utils::SaxCountdown;
 
@@ -3357,3 +3358,55 @@ TEST_CASE("CBOR large strings and binaries (chunked reader)")
         }
     }
 }
+
+TEST_CASE("CBOR supports custom object key types")
+{
+    using custom_json = custom_object_key_test::json;
+    using custom_key = custom_object_key_test::key;
+
+    custom_json::object_t object;
+    object.emplace(custom_key{"short"}, 1);
+    object.emplace(
+              custom_key{"a key longer than twenty-three characters"},
+              2);
+
+    const custom_json value(std::move(object));
+    const auto encoded = custom_json::to_cbor(value);
+
+    CHECK(nlohmann::json::from_cbor(encoded) == nlohmann::json
+    {
+        {"short", 1},
+        {"a key longer than twenty-three characters", 2}
+    });
+}
+
+TEST_CASE("CBOR supports custom object key types nested deeper than the recursion depth limit")
+{
+    // below detail::recursion_depth_limit(), keys are written by
+    // write_cbor_iterative instead of write_cbor
+    using custom_json = custom_object_key_test::json;
+    using custom_key = custom_object_key_test::key;
+
+    const std::size_t depth = nlohmann::detail::recursion_depth_limit() + 10;
+
+    custom_json value = 1;
+    nlohmann::json expected = 1;
+    for (std::size_t i = 0; i < depth; ++i)
+    {
+        // alternate short keys with ones long enough to need a length byte
+        const std::string name = (i % 2 == 0) ? "k" + std::to_string(i)
+                                 : "a key longer than thirty-one characters " + std::to_string(i);
+
+        custom_json::object_t object;
+        object.emplace(custom_key{name}, std::move(value));
+        value = custom_json(std::move(object));
+
+        nlohmann::json::object_t expected_object;
+        expected_object.emplace(name, std::move(expected));
+        expected = nlohmann::json(std::move(expected_object));
+    }
+
+    const auto encoded = custom_json::to_cbor(value);
+    CHECK(encoded == nlohmann::json::to_cbor(expected));
+    CHECK(nlohmann::json::from_cbor(encoded) == expected);
+}