feat(options)!: disallow setting hidden options #28400

Problem:
There are three different ways of marking an option as hidden, `enable_if
= false`, `hidden = true` and `immutable = true`. These also have different
behaviors. Options hidden with `enable_if = false` can't have their value
fetched using Vim script or the API, but options hidden with `hidden = true` or
`immutable = true` can. On the other hand, options with `hidden = true` do not
error when trying to set their value, but options with `immutable = true` do.

Solution:
Remove `enable_if = false`, remove the `hidden` property for options, and use
`immutable = true` to mark an option as hidden instead. Also make hidden option
variable pointers always point to the default value, which allows fetching the
value of every hidden option using Vim script and the API. This does also mean
that trying to set a hidden option will now give an error instead of just being
ignored.

1 files changed, 2 insertions, 3 deletions

diff --git a/src/nvim/api/options.c b/src/nvim/api/options.c
index 1a0edd551e..96866d80ba 100644
--- a/src/nvim/api/options.c
+++ b/src/nvim/api/options.c
@@ -80,7 +80,7 @@ static int validate_option_value_args(Dict(option) *opts, char *name, OptIndex *
   *opt_idxp = find_option(name);
   int flags = get_option_attrs(*opt_idxp);
   if (flags == 0) {
-    // hidden or unknown option
+    // unknown option
     api_set_error(err, kErrorTypeValidation, "Unknown option '%s'", name);
   } else if (*req_scope == kOptReqBuf || *req_scope == kOptReqWin) {
     // if 'buf' or 'win' is passed, make sure the option supports it
@@ -175,7 +175,6 @@ Object nvim_get_option_value(String name, Dict(option) *opts, Error *err)
   }
 
   OptVal value = get_option_value_for(opt_idx, scope, req_scope, from, err);
-  bool hidden = is_option_hidden(opt_idx);
 
   if (ftbuf != NULL) {
     // restore curwin/curbuf and a few other things
@@ -189,7 +188,7 @@ Object nvim_get_option_value(String name, Dict(option) *opts, Error *err)
     goto err;
   }
 
-  VALIDATE_S(!hidden && value.type != kOptValTypeNil, "option", name.data, {
+  VALIDATE_S(value.type != kOptValTypeNil, "option", name.data, {
     goto err;
   });