add constexpr to SPDLOG_LEVEL_NAMES declaration

for #1791

include/spdlog/common-inl.h

@@ -9,11 +9,11 @@
 
 namespace spdlog {
 namespace level {
-static string_view_t level_string_views[] SPDLOG_LEVEL_NAMES;
+static FMT_CONSTEXPR string_view_t level_string_views[] SPDLOG_LEVEL_NAMES;
 
 static const char *short_level_names[] SPDLOG_SHORT_LEVEL_NAMES;
 
-SPDLOG_INLINE string_view_t &to_string_view(spdlog::level::level_enum l) SPDLOG_NOEXCEPT
+SPDLOG_INLINE const string_view_t &to_string_view(spdlog::level::level_enum l) SPDLOG_NOEXCEPT
 {
     return level_string_views[l];
 }

include/spdlog/common.h

@@ -163,7 +163,7 @@ enum level_enum
     }
 #endif
 
-SPDLOG_API string_view_t &to_string_view(spdlog::level::level_enum l) SPDLOG_NOEXCEPT;
+SPDLOG_API const string_view_t &to_string_view(spdlog::level::level_enum l) SPDLOG_NOEXCEPT;
 SPDLOG_API const char *to_short_c_str(spdlog::level::level_enum l) SPDLOG_NOEXCEPT;
 SPDLOG_API spdlog::level::level_enum from_str(const std::string &name) SPDLOG_NOEXCEPT;
 

include/spdlog/pattern_formatter-inl.h

@@ -127,7 +127,7 @@ public:
 
     void format(const details::log_msg &msg, const std::tm &, memory_buf_t &dest) override
     {
-        string_view_t &level_name = level::to_string_view(msg.level);
+        const string_view_t &level_name = level::to_string_view(msg.level);
         ScopedPadder p(level_name.size(), padinfo_, dest);
         fmt_helper::append_string_view(level_name, dest);
     }