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.

1 files changed, 8 insertions, 9 deletions

diff --git a/src/nvim/option.c b/src/nvim/option.c
index 73a5af5520..e4f32ebec4 100644
--- a/src/nvim/option.c
+++ b/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;
   }