Decoder: optimize decoding of optional and variant for scalar types

Currently, we always emplace an optional or variant in order to pass the
value they contain forward, as a reference. This is required for complex
nested types. However, if we are dealing with a scalar type, we can read
its value into the optional or variant in-place.

Closes #84

src/mpp/Dec.hpp

@@ -1172,6 +1172,20 @@ bool jump_read_key(BUF& buf, T... t)
 	return jump_find_key<PAIRS, FAMILY, PATH>(val, IS, buf, t...);
 }
 
+template<compact::Family FAMILY, size_t SUBRULE,
+	 class DST, class BUF, class = void>
+struct is_object_readable_by_value : std::false_type {};
+
+template<compact::Family FAMILY, size_t SUBRULE,
+	 class DST, class BUF>
+struct is_object_readable_by_value<FAMILY, SUBRULE, DST, BUF,
+    typename std::enable_if_t<rule_by_family_t<FAMILY>::is_readable_by_value &&
+	    std::is_assignable_v<DST, decltype(read_value<FAMILY, SUBRULE>(std::declval<BUF>()))>, bool>::type> : std::true_type { };
+
+template<compact::Family FAMILY, size_t SUBRULE,
+	class DST, class BUF>
+constexpr bool is_object_readable_by_value_v = is_object_readable_by_value<FAMILY, SUBRULE, DST, BUF>::value;
+
 template <compact::Family FAMILY, size_t SUBRULE,
 	  class PATH, class BUF, class... T>
 bool jump_read_optional(BUF& buf, T... t)
@@ -1184,10 +1198,15 @@ bool jump_read_optional(BUF& buf, T... t)
 		dst.reset();
 		return decode_next<PATH>(buf, t...);
 	} else {
-		if (!dst.has_value())
-			dst.emplace();
-		using NEXT_PATH = path_push_t<PATH, PIT_OPTIONAL>;
-		return decode_impl<NEXT_PATH>(buf, t...);
+		if constexpr (is_object_readable_by_value_v<FAMILY, SUBRULE, dst_t, BUF>) {
+			dst = read_value<FAMILY, SUBRULE>(buf);
+			return decode_next<PATH>(buf, t...);
+		} else {
+			if (!dst.has_value())
+				dst.emplace();
+			using NEXT_PATH = path_push_t<PATH, PIT_OPTIONAL>;
+			return decode_impl<NEXT_PATH>(buf, t...);
+		}
 	}
 }
 
@@ -1219,7 +1238,12 @@ bool jump_read_variant(BUF& buf, T... t)
 	auto&& dst = unwrap(path_resolve(PATH{}, t...));
 	using dst_t = std::remove_reference_t<decltype(dst)>;
 	static_assert(tnt::is_variant_v<dst_t>);
-	return jump_read_variant_impl<0, FAMILY, SUBRULE, PATH>(buf, t...);
+	if constexpr (is_object_readable_by_value<FAMILY, SUBRULE, dst_t, BUF>()) {
+		dst = read_value<FAMILY, SUBRULE>(buf);
+		return decode_next<PATH>(buf, t...);
+	} else {
+		return jump_read_variant_impl<0, FAMILY, SUBRULE, PATH>(buf, t...);
+	}
 }
 
 template <compact::Family FAMILY, size_t SUBRULE,

src/mpp/Rules.hpp

@@ -133,6 +133,8 @@ struct BaseRule {
 	static constexpr uint32_t children_multiplier =
 		FAMILY == compact::MP_ARR ? 1 :
 		FAMILY == compact::MP_MAP ? 2 : 0;
+	// The encoded object can be read by value.
+	static constexpr bool is_readable_by_value = !has_data && !has_ext && !has_children;
 	// The rule has simplex form.
 	static constexpr bool has_simplex = FAMILY == compact::MP_NIL ||
 					    FAMILY == compact::MP_IGNR ||