fix(options): better handling of empty values

Problem:

Whether an option is allowed to be empty isn't well defined and
isn't properly checked.

Solution:

- For non-list string options, explicitly check the option value
  if it is empty.
- Annotate non-list string options that can accept an empty value.
  - Adjust command completion to ignore the empty value.
- Render values in Lua meta files

2 files changed, 22 insertions, 11 deletions

diff --git a/src/nvim/options.lua b/src/nvim/options.lua
index 15a4e8ddc2..2425dcb93e 100644
--- a/src/nvim/options.lua
+++ b/src/nvim/options.lua
@@ -7,7 +7,7 @@
 --- @field alias? string|string[]
 --- @field short_desc? string|fun(): string
 --- @field varname? string
---- @field type vim.option_type|vim.option_type[]
+--- @field type vim.option_type
 --- @field immutable? boolean
 --- @field list? 'comma'|'onecomma'|'commacolon'|'onecommacolon'|'flags'|'flagscomma'
 --- @field scope vim.option_scope[]
@@ -834,7 +834,7 @@ return {
       abbreviation = 'bh',
       cb = 'did_set_bufhidden',
       defaults = { if_true = '' },
-      values = { 'hide', 'unload', 'delete', 'wipe' },
+      values = { '', 'hide', 'unload', 'delete', 'wipe' },
       desc = [=[
         This option specifies what happens when a buffer is no longer
         displayed in a window:
@@ -888,11 +888,12 @@ return {
       cb = 'did_set_buftype',
       defaults = { if_true = '' },
       values = {
+        '',
+        'acwrite',
+        'help',
         'nofile',
         'nowrite',
         'quickfix',
-        'help',
-        'acwrite',
         'terminal',
         'prompt',
       },
@@ -1554,7 +1555,7 @@ return {
       abbreviation = 'csl',
       cb = 'did_set_completeslash',
       defaults = { if_true = '' },
-      values = { 'slash', 'backslash' },
+      values = { '', 'slash', 'backslash' },
       desc = [=[
         		only modifiable in MS-Windows
         When this option is set it overrules 'shellslash' for completion:
@@ -2017,8 +2018,10 @@ return {
         "msg" and "throw" are useful for debugging 'foldexpr', 'formatexpr' or
         'indentexpr'.
       ]=],
+      -- TODO(lewis6991): bug, values currently cannot be combined
       expand_cb = 'expand_set_debug',
       full_name = 'debug',
+      list = 'comma',
       scope = { 'global' },
       short_desc = N_('to "msg" to see all error messages'),
       type = 'string',
@@ -4299,7 +4302,7 @@ return {
       abbreviation = 'icm',
       cb = 'did_set_inccommand',
       defaults = { if_true = 'nosplit' },
-      values = { 'nosplit', 'split' },
+      values = { 'nosplit', 'split', '' },
       desc = [=[
         When nonempty, shows the effects of |:substitute|, |:smagic|,
         |:snomagic| and user commands with the |:command-preview| flag as you
@@ -5735,7 +5738,7 @@ return {
       abbreviation = 'mousem',
       cb = 'did_set_mousemodel',
       defaults = { if_true = 'popup_setpos' },
-      values = { 'extend', 'popup', 'popup_setpos', 'mac' },
+      values = { 'extend', 'popup', 'popup_setpos' },
       desc = [=[
         Sets the model to use for the mouse.  The name mostly specifies what
         the right mouse button is used for:
diff --git a/src/nvim/optionstr.c b/src/nvim/optionstr.c
index 9b7b50ae04..eac9ea02e0 100644
--- a/src/nvim/optionstr.c
+++ b/src/nvim/optionstr.c
@@ -395,7 +395,9 @@ static int expand_set_opt_string(optexpand_T *args, const char **values, size_t
   }
 
   for (const char **val = values; *val != NULL; val++) {
-    if (include_orig_val && *option_val != NUL) {
+    if (**val == NUL) {
+      continue;  // Ignore empty
+    } else if (include_orig_val && *option_val != NUL) {
       if (strcmp(*val, option_val) == 0) {
         continue;
       }
@@ -1091,7 +1093,7 @@ int expand_set_cursorlineopt(optexpand_T *args, int *numMatches, char ***matches
 /// The 'debug' option is changed.
 const char *did_set_debug(optset_T *args FUNC_ATTR_UNUSED)
 {
-  return did_set_opt_strings(p_debug, opt_debug_values, false);
+  return did_set_opt_strings(p_debug, opt_debug_values, true);
 }
 
 int expand_set_debug(optexpand_T *args, int *numMatches, char ***matches)
@@ -2545,7 +2547,7 @@ int expand_set_winhighlight(optexpand_T *args, int *numMatches, char ***matches)
 /// @param list  when true: accept a list of values
 ///
 /// @return  OK for correct value, FAIL otherwise. Empty is always OK.
-static int check_opt_strings(char *val, const char **values, int list)
+static int check_opt_strings(char *val, const char **values, bool list)
 {
   return opt_strings_flags(val, values, NULL, list);
 }
@@ -2562,7 +2564,10 @@ static int opt_strings_flags(const char *val, const char **values, unsigned *fla
 {
   unsigned new_flags = 0;
 
-  while (*val) {
+  // If not list and val is empty, then force one iteration of the while loop
+  bool iter_one = (*val == NUL) && !list;
+
+  while (*val || iter_one) {
     for (unsigned i = 0;; i++) {
       if (values[i] == NULL) {          // val not found in values[]
         return FAIL;
@@ -2577,6 +2582,9 @@ static int opt_strings_flags(const char *val, const char **values, unsigned *fla
         break;                  // check next item in val list
       }
     }
+    if (iter_one) {
+      break;
+    }
   }
   if (flagp != NULL) {
     *flagp = new_flags;