m_option: add M_OPT_PRIVATE

Used to mark private options that shouldn't be exposed to the users.

Author: kasper93

options/m_config_frontend.c

@@ -400,7 +400,9 @@ static int handle_set_opt_flags(struct m_config *config,
         return M_OPT_INVALID;
 
     // Check if this option isn't forbidden in the current mode
-    if ((flags & M_SETOPT_FROM_CONFIG_FILE) && (optflags & M_OPT_NOCFG)) {
+    if ((flags & M_SETOPT_FROM_CONFIG_FILE) &&
+        (optflags & (M_OPT_NOCFG | M_OPT_PRIVATE)))
+    {
         MP_ERR(config, "The %s option can't be used in a config file.\n",
                co->name);
         return M_OPT_INVALID;
@@ -841,6 +843,8 @@ void m_config_print_option_list(const struct m_config *config, const char *name)
     for (int i = 0; i < config->num_opts; i++) {
         struct m_config_option *co = &sorted[i];
         const struct m_option *opt = co->opt;
+        if (opt->flags & M_OPT_PRIVATE)
+            continue;
         if (strcmp(name, "*") != 0 && !strstr(co->name, name))
             continue;
         MP_INFO(config, " %s%-30s", prefix, co->name);

options/m_option.h

@@ -488,7 +488,11 @@ char *format_file_size(int64_t size);
 // Like M_OPT_TYPE_OPTIONAL_PARAM.
 #define M_OPT_OPTIONAL_PARAM    (UINT64_C(1) << 54)
 
-static_assert(!(UPDATE_OPTS_MASK & M_OPT_OPTIONAL_PARAM), "");
+// Option is private: forbidden in config files and hidden from --help listing.
+// Can still be used as a property by builtin scripts.
+#define M_OPT_PRIVATE           (UINT64_C(1) << 53)
+
+static_assert(!(UPDATE_OPTS_MASK & M_OPT_PRIVATE), "");
 
 // These flags are used to describe special parser capabilities or behavior.