refactor(options): make immutable and hidden options distinct

Problem: Currently, the `immutable` property of options can be applied for options that are hidden and options whose value simply can't be changed. Which is problematic when attempting to convert an option like `'maxcombine'` into an immutable option, because trying to `:set` an immutable option currently gives an error, which is only desired behavior for hidden options, not options that are actually immutable.

Solution: Separate the `immutable` property into two distinct `hidden` and `immutable` properties. Change all options with the `immutable` property to use the `hidden` property instead. Also add `p_mco` as an `immutable` option, as its value cannot be changed, and the underlying variable is not used anywhere.

src/nvim/generators/gen_options.lua

@@ -164,14 +164,19 @@ local function dump_option(i, o)
   if o.enable_if then
     w(get_cond(o.enable_if))
   end
+
+  -- Options cannot be both hidden and immutable.
+  assert(not o.hidden or not o.immutable)
+
   if o.varname then
     w('    .var=&' .. o.varname)
-  -- Immutable options can directly point to the default value.
-  elseif o.immutable then
+  -- Hidden and immutable options can directly point to the default value.
+  elseif o.hidden or o.immutable then
     w(('    .var=&options[%u].def_val'):format(i - 1))
   elseif #o.scope == 1 and o.scope[1] == 'window' then
     w('    .var=VAR_WIN')
   end
+  w('    .hidden=' .. (o.hidden and 'true' or 'false'))
   w('    .immutable=' .. (o.immutable and 'true' or 'false'))
   if #o.scope == 1 and o.scope[1] == 'global' then
     w('    .indir=PV_NONE')

src/nvim/option.c

@@ -2898,8 +2898,6 @@ static const char *validate_num_option(OptIndex opt_idx, void *varp, OptInt *new
     } else if (value > p_wiw) {
       return e_winwidth;
     }
-  } else if (varp == &p_mco) {
-    *newval = MAX_MCO;
   } else if (varp == &p_titlelen) {
     if (value < 0) {
       return e_positive;
@@ -3483,11 +3481,11 @@ static const char *did_set_option(OptIndex opt_idx, void *varp, OptVal old_value
     .os_win = curwin
   };
 
-  if (direct) {
-    // Don't do any extra processing if setting directly.
+  if (direct || opt->immutable) {
+    // Don't do any extra processing if setting directly or if option is immutable.
   }
-  // Disallow changing immutable options.
-  else if (opt->immutable && !optval_equal(old_value, new_value)) {
+  // Disallow changing hidden options.
+  else if (opt->hidden && !optval_equal(old_value, new_value)) {
     errmsg = e_unsupportedoption;
   }
   // Disallow changing some options from secure mode.
@@ -3511,8 +3509,9 @@ static const char *did_set_option(OptIndex opt_idx, void *varp, OptVal old_value
     restore_chartab = did_set_cb_args.os_restore_chartab;
   }
 
-  // If an error is detected, restore the previous value and don't do any further processing.
-  if (errmsg != NULL) {
+  // If option is immutable or if an error is detected, restore the previous value and don't do any
+  // further processing.
+  if (opt->immutable || errmsg != NULL) {
     set_option_varp(opt_idx, varp, old_value, true);
     // When resetting some values, need to act on it.
     if (restore_chartab) {
@@ -4227,7 +4226,7 @@ static int optval_default(OptIndex opt_idx, void *varp)
   vimoption_T *opt = &options[opt_idx];
 
   // Hidden or immutable options always use their default value.
-  if (varp == NULL || opt->immutable) {
+  if (varp == NULL || opt->hidden || opt->immutable) {
     return true;
   }
 

src/nvim/option.h

@@ -49,7 +49,9 @@ typedef struct {
                             ///< buffer-local option: global value
   idopt_T indir;            ///< global option: PV_NONE;
                             ///< local option: indirect option index
-  bool immutable;           ///< option value cannot be changed from the default value.
+  bool hidden;              ///< option is hidden and cannot be set.
+  bool immutable;           ///< option value can be set but any attempt to change the option is
+                            ///  ignored.
 
   /// callback function to invoke after an option is modified to validate and
   /// apply the new value.

src/nvim/option_vars.h

@@ -129,6 +129,8 @@
 #define DFLT_FO_VIM     "tcqj"
 #define FO_ALL          "tcro/q2vlb1mMBn,aw]jp"   // for do_set()
 
+#define MAX_MCO  6  // fixed value for 'maxcombine'
+
 // characters for the p_cpo option:
 #define CPO_ALTREAD     'a'     // ":read" sets alternate file name
 #define CPO_ALTWRITE    'A'     // ":write" sets alternate file name
@@ -549,8 +551,6 @@ EXTERN char *p_mef;             ///< 'makeef'
 EXTERN char *p_mp;              ///< 'makeprg'
 EXTERN char *p_mps;             ///< 'matchpairs'
 EXTERN OptInt p_mat;            ///< 'matchtime'
-EXTERN OptInt p_mco;            ///< 'maxcombine'
-#define MAX_MCO  6  // fixed value for 'maxcombine'
 EXTERN OptInt p_mfd;            ///< 'maxfuncdepth'
 EXTERN OptInt p_mmd;            ///< 'maxmapdepth'
 EXTERN OptInt p_mmp;            ///< 'maxmempattern'

src/nvim/options.lua

@@ -7,6 +7,7 @@
 --- @field varname? string
 --- @field pv_name? string
 --- @field type 'boolean'|'number'|'string'
+--- @field hidden? boolean
 --- @field immutable? boolean
 --- @field list? 'comma'|'onecomma'|'commacolon'|'onecommacolon'|'flags'|'flagscomma'
 --- @field scope vim.option_scope[]
@@ -1340,7 +1341,7 @@ return {
       scope = { 'global' },
       short_desc = N_('No description'),
       type = 'boolean',
-      immutable = true,
+      hidden = true,
     },
     {
       abbreviation = 'cpt',
@@ -2299,7 +2300,7 @@ return {
       scope = { 'global' },
       short_desc = N_('No description'),
       type = 'boolean',
-      immutable = true,
+      hidden = true,
     },
     {
       abbreviation = 'emo',
@@ -3888,7 +3889,7 @@ return {
       scope = { 'global' },
       short_desc = N_('No description'),
       type = 'boolean',
-      immutable = true,
+      hidden = true,
     },
     {
       abbreviation = 'hkp',
@@ -3897,7 +3898,7 @@ return {
       scope = { 'global' },
       short_desc = N_('No description'),
       type = 'boolean',
-      immutable = true,
+      hidden = true,
     },
     {
       abbreviation = 'hls',
@@ -4295,7 +4296,7 @@ return {
       scope = { 'global' },
       short_desc = N_('No description'),
       type = 'boolean',
-      immutable = true,
+      hidden = true,
     },
     {
       abbreviation = 'isf',
@@ -5141,12 +5142,12 @@ return {
     },
     {
       abbreviation = 'mco',
-      defaults = { if_true = 6 },
+      defaults = { if_true = imacros('MAX_MCO') },
       full_name = 'maxcombine',
       scope = { 'global' },
       short_desc = N_('maximum nr of combining characters displayed'),
       type = 'number',
-      varname = 'p_mco',
+      immutable = true,
     },
     {
       abbreviation = 'mfd',
@@ -6047,7 +6048,7 @@ return {
       scope = { 'global' },
       short_desc = N_('enable prompt in Ex mode'),
       type = 'boolean',
-      immutable = true,
+      hidden = true,
     },
     {
       abbreviation = 'pb',
@@ -6304,7 +6305,7 @@ return {
       scope = { 'global' },
       short_desc = N_('No description'),
       type = 'boolean',
-      immutable = true,
+      hidden = true,
     },
     {
       defaults = { if_true = 2 },
@@ -8843,7 +8844,7 @@ return {
       scope = { 'global' },
       short_desc = N_('No description'),
       type = 'boolean',
-      immutable = true,
+      hidden = true,
     },
     {
       abbreviation = 'tw',
@@ -9088,7 +9089,7 @@ return {
       scope = { 'global' },
       short_desc = N_('No description'),
       type = 'boolean',
-      immutable = true,
+      hidden = true,
     },
     {
       abbreviation = 'udir',