Merge pull request #25456 from zeertzjq/vim-9.0.1958

vim-patch:9.0.{1958,1960,1961,1968}: string option completion

18 files changed, 1178 insertions, 164 deletions

diff --git a/src/nvim/autocmd.c b/src/nvim/autocmd.c
index 657760914f..a40f7d8c26 100644
--- a/src/nvim/autocmd.c
+++ b/src/nvim/autocmd.c
@@ -2213,6 +2213,13 @@ char *expand_get_event_name(expand_T *xp, int idx)
   return event_names[idx - next_augroup_id].name;
 }
 
+/// Function given to ExpandGeneric() to obtain the list of event names. Don't
+/// include groups.
+char *get_event_name_no_group(expand_T *xp FUNC_ATTR_UNUSED, int idx)
+{
+  return event_names[idx].name;
+}
+
 /// Check whether given autocommand is supported
 ///
 /// @param[in]  event  Event to check.
diff --git a/src/nvim/cmdexpand.c b/src/nvim/cmdexpand.c
index d733ffe6ab..893deadd13 100644
--- a/src/nvim/cmdexpand.c
+++ b/src/nvim/cmdexpand.c
@@ -69,9 +69,6 @@
 #include "nvim/vim.h"
 #include "nvim/window.h"
 
-/// Type used by ExpandGeneric()
-typedef char *(*CompleteListItemGetter)(expand_T *, int);
-
 /// Type used by call_user_expand_func
 typedef void *(*user_expand_func_T)(const char *, int, typval_T *);
 
@@ -107,6 +104,8 @@ static bool cmdline_fuzzy_completion_supported(const expand_T *const xp)
          && xp->xp_context != EXPAND_HELP
          && xp->xp_context != EXPAND_LUA
          && xp->xp_context != EXPAND_OLD_SETTING
+         && xp->xp_context != EXPAND_STRING_SETTING
+         && xp->xp_context != EXPAND_SETTING_SUBTRACT
          && xp->xp_context != EXPAND_OWNSYNTAX
          && xp->xp_context != EXPAND_PACKADD
          && xp->xp_context != EXPAND_RUNTIME
@@ -2599,7 +2598,7 @@ static int ExpandOther(char *pat, expand_T *xp, regmatch_T *rmp, char ***matches
     { EXPAND_MENUNAMES, get_menu_names, false, true },
     { EXPAND_SYNTAX, get_syntax_name, true, true },
     { EXPAND_SYNTIME, get_syntime_arg, true, true },
-    { EXPAND_HIGHLIGHT, (ExpandFunc)get_highlight_name, true, false },
+    { EXPAND_HIGHLIGHT, get_highlight_name, true, false },
     { EXPAND_EVENTS, expand_get_event_name, true, false },
     { EXPAND_AUGROUP, expand_get_augroup_name, true, false },
     { EXPAND_SIGN, get_sign_name, true, true },
@@ -2694,8 +2693,7 @@ static int ExpandFromContext(expand_T *xp, char *pat, char ***matches, int *numM
     return OK;
   }
   if (xp->xp_context == EXPAND_OLD_SETTING) {
-    ExpandOldSetting(numMatches, matches);
-    return OK;
+    return ExpandOldSetting(numMatches, matches);
   }
   if (xp->xp_context == EXPAND_BUFFERS) {
     return ExpandBufnames(pat, numMatches, matches, options);
@@ -2765,6 +2763,10 @@ static int ExpandFromContext(expand_T *xp, char *pat, char ***matches, int *numM
   if (xp->xp_context == EXPAND_SETTINGS
       || xp->xp_context == EXPAND_BOOL_SETTINGS) {
     ret = ExpandSettings(xp, &regmatch, pat, numMatches, matches, fuzzy);
+  } else if (xp->xp_context == EXPAND_STRING_SETTING) {
+    ret = ExpandStringSetting(xp, &regmatch, numMatches, matches);
+  } else if (xp->xp_context == EXPAND_SETTING_SUBTRACT) {
+    ret = ExpandSettingSubtract(xp, &regmatch, numMatches, matches);
   } else if (xp->xp_context == EXPAND_MAPPINGS) {
     ret = ExpandMappings(pat, &regmatch, numMatches, matches);
   } else if (xp->xp_context == EXPAND_USER_DEFINED) {
@@ -2788,9 +2790,8 @@ static int ExpandFromContext(expand_T *xp, char *pat, char ***matches, int *numM
 /// program.  Matching strings are copied into an array, which is returned.
 ///
 /// @param func  returns a string from the list
-static void ExpandGeneric(const char *const pat, expand_T *xp, regmatch_T *regmatch,
-                          char ***matches, int *numMatches, CompleteListItemGetter func,
-                          int escaped)
+void ExpandGeneric(const char *const pat, expand_T *xp, regmatch_T *regmatch, char ***matches,
+                   int *numMatches, CompleteListItemGetter func, bool escaped)
 {
   const bool fuzzy = cmdline_fuzzy_complete(pat);
   *matches = NULL;
@@ -2863,6 +2864,7 @@ static void ExpandGeneric(const char *const pat, expand_T *xp, regmatch_T *regma
   // in the specified order.
   const bool sort_matches = !fuzzy
                             && xp->xp_context != EXPAND_MENUNAMES
+                            && xp->xp_context != EXPAND_STRING_SETTING
                             && xp->xp_context != EXPAND_MENUS
                             && xp->xp_context != EXPAND_SCRIPTNAMES;
 
@@ -3221,8 +3223,7 @@ void globpath(char *path, char *file, garray_T *ga, int expand_options, bool dir
 
       char **p;
       int num_p = 0;
-      (void)ExpandFromContext(&xpc, buf, &p, &num_p,
-                              WILD_SILENT | expand_options);
+      (void)ExpandFromContext(&xpc, buf, &p, &num_p, WILD_SILENT | expand_options);
       if (num_p > 0) {
         ExpandEscape(&xpc, buf, num_p, p, WILD_SILENT | expand_options);
 
diff --git a/src/nvim/cmdexpand.h b/src/nvim/cmdexpand.h
index 32c23c5d66..f37c693a22 100644
--- a/src/nvim/cmdexpand.h
+++ b/src/nvim/cmdexpand.h
@@ -41,6 +41,9 @@ enum {
   BUF_DIFF_FILTER           = 0x2000,
 };
 
+/// Type used by ExpandGeneric()
+typedef char *(*CompleteListItemGetter)(expand_T *, int);
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "cmdexpand.h.generated.h"
 #endif
diff --git a/src/nvim/diff.c b/src/nvim/diff.c
index 3ab1da76f4..cb76cf17fc 100644
--- a/src/nvim/diff.c
+++ b/src/nvim/diff.c
@@ -2467,6 +2467,7 @@ int diffopt_changed(void)
 
   char *p = p_dip;
   while (*p != NUL) {
+    // Note: Keep this in sync with p_dip_values
     if (strncmp(p, "filler", 6) == 0) {
       p += 6;
       diff_flags_new |= DIFF_FILLER;
@@ -2513,6 +2514,7 @@ int diffopt_changed(void)
       p += 8;
       diff_flags_new |= DIFF_INTERNAL;
     } else if (strncmp(p, "algorithm:", 10) == 0) {
+      // Note: Keep this in sync with p_dip_algorithm_values.
       p += 10;
       if (strncmp(p, "myers", 5) == 0) {
         p += 5;
diff --git a/src/nvim/ex_getln.c b/src/nvim/ex_getln.c
index 6fa607d569..2d1633f312 100644
--- a/src/nvim/ex_getln.c
+++ b/src/nvim/ex_getln.c
@@ -2766,6 +2766,7 @@ int check_opt_wim(void)
   }
 
   for (char *p = p_wim; *p; p++) {
+    // Note: Keep this in sync with p_wim_values.
     for (i = 0; ASCII_ISALPHA(p[i]); i++) {}
     if (p[i] != NUL && p[i] != ',' && p[i] != ':') {
       return FAIL;
diff --git a/src/nvim/generators/gen_options.lua b/src/nvim/generators/gen_options.lua
index 0932a1357f..05def71caa 100644
--- a/src/nvim/generators/gen_options.lua
+++ b/src/nvim/generators/gen_options.lua
@@ -35,6 +35,8 @@ local redraw_flags={
 local list_flags={
   comma='P_COMMA',
   onecomma='P_ONECOMMA',
+  commacolon='P_COMMA|P_COLON',
+  onecommacolon='P_ONECOMMA|P_COLON',
   flags='P_FLAGLIST',
   flagscomma='P_COMMA|P_FLAGLIST',
 }
@@ -166,6 +168,9 @@ local function dump_option(i, o)
   if o.cb then
     w('    .opt_did_set_cb=' .. o.cb)
   end
+  if o.expand_cb then
+    w('    .opt_expand_cb=' .. o.expand_cb)
+  end
   if o.enable_if then
     w('#else')
     w('    .var=NULL')
diff --git a/src/nvim/highlight_group.c b/src/nvim/highlight_group.c
index 080dc79e0e..d2cdcb2516 100644
--- a/src/nvim/highlight_group.c
+++ b/src/nvim/highlight_group.c
@@ -2283,10 +2283,10 @@ static void highlight_list_two(int cnt, int attr)
 }
 
 /// Function given to ExpandGeneric() to obtain the list of group names.
-const char *get_highlight_name(expand_T *const xp, int idx)
+char *get_highlight_name(expand_T *const xp, int idx)
   FUNC_ATTR_WARN_UNUSED_RESULT
 {
-  return get_highlight_name_ext(xp, idx, true);
+  return (char *)get_highlight_name_ext(xp, idx, true);
 }
 
 /// Obtain a highlight group name.
diff --git a/src/nvim/indent.c b/src/nvim/indent.c
index b7bc23cda5..55235e454c 100644
--- a/src/nvim/indent.c
+++ b/src/nvim/indent.c
@@ -760,6 +760,7 @@ bool briopt_check(win_T *wp)
 
   char *p = wp->w_p_briopt;
   while (*p != NUL) {
+    // Note: Keep this in sync with p_briopt_values
     if (strncmp(p, "shift:", 6) == 0
         && ((p[6] == '-' && ascii_isdigit(p[7])) || ascii_isdigit(p[6]))) {
       p += 6;
diff --git a/src/nvim/mbyte.c b/src/nvim/mbyte.c
index 7b7c822b3b..124855fd08 100644
--- a/src/nvim/mbyte.c
+++ b/src/nvim/mbyte.c
@@ -2830,3 +2830,14 @@ void f_charclass(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   }
   rettv->vval.v_number = mb_get_class(argvars[0].vval.v_string);
 }
+
+/// Function given to ExpandGeneric() to obtain the possible arguments of the
+/// encoding options.
+char *get_encoding_name(expand_T *xp FUNC_ATTR_UNUSED, int idx)
+{
+  if (idx >= (int)ARRAY_SIZE(enc_canon_table)) {
+    return NULL;
+  }
+
+  return (char *)enc_canon_table[idx].name;
+}
diff --git a/src/nvim/option.c b/src/nvim/option.c
index eef5e66aeb..f771760822 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -136,13 +136,6 @@ static char *p_vsts_nopaste;
 
 #define OPTION_COUNT ARRAY_SIZE(options)
 
-typedef enum {
-  OP_NONE = 0,
-  OP_ADDING,      ///< "opt+=arg"
-  OP_PREPENDING,  ///< "opt^=arg"
-  OP_REMOVING,    ///< "opt-=arg"
-} set_op_T;
-
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "option.c.generated.h"
 #endif
@@ -888,7 +881,7 @@ static char *stropt_copy_value(char *origval, char **argp, set_op_T op,
   // For MS-Windows backslashes before normal file name characters
   // are not removed, and keep backslash at start, for "\\machine\path",
   // but do remove it for "\\\\machine\\path".
-  // The reverse is found in ExpandOldSetting().
+  // The reverse is found in escape_option_str_cmdline().
   while (*arg != NUL && !ascii_iswhite(*arg)) {
     if (*arg == '\\' && arg[1] != NUL
 #ifdef BACKSLASH_IN_FILENAME
@@ -1168,7 +1161,7 @@ static void do_set_option_string(int opt_idx, int opt_flags, char **argp, int ne
   // be triggered that can cause havoc.
   *errmsg = did_set_string_option(curbuf, curwin, opt_idx, (char **)varp, oldval,
                                   errbuf, errbuflen,
-                                  opt_flags, value_checked);
+                                  opt_flags, op, value_checked);
 
   secure = secure_saved;
 
@@ -5305,8 +5298,10 @@ void set_imsearch_global(buf_T *buf)
 }
 
 static int expand_option_idx = -1;
+static int expand_option_start_col = 0;
 static char expand_option_name[5] = { 't', '_', NUL, NUL, NUL };
 static int expand_option_flags = 0;
+static bool expand_option_append = false;
 
 /// @param opt_flags  OPT_GLOBAL and/or OPT_LOCAL
 void set_context_in_set_cmd(expand_T *xp, char *arg, int opt_flags)
@@ -5405,7 +5400,15 @@ void set_context_in_set_cmd(expand_T *xp, char *arg, int opt_flags)
     }
   }
   // handle "-=" and "+="
+  expand_option_append = false;
+  bool expand_option_subtract = false;
   if ((nextchar == '-' || nextchar == '+' || nextchar == '^') && p[1] == '=') {
+    if (nextchar == '-') {
+      expand_option_subtract = true;
+    }
+    if (nextchar == '+' || nextchar == '^') {
+      expand_option_append = true;
+    }
     p++;
     nextchar = '=';
   }
@@ -5414,28 +5417,51 @@ void set_context_in_set_cmd(expand_T *xp, char *arg, int opt_flags)
     xp->xp_context = EXPAND_UNSUCCESSFUL;
     return;
   }
-  if (p[1] == NUL) {
-    xp->xp_context = EXPAND_OLD_SETTING;
-    if (is_term_option) {
-      expand_option_idx = -1;
-    } else {
-      expand_option_idx = opt_idx;
-    }
-    xp->xp_pattern = p + 1;
-    return;
-  }
-  xp->xp_context = EXPAND_NOTHING;
-  if (is_term_option || (flags & P_NUM)) {
-    return;
+
+  // Below are for handling expanding a specific option's value after the '=' or ':'
+
+  if (is_term_option) {
+    expand_option_idx = -1;
+  } else {
+    expand_option_idx = opt_idx;
   }
 
   xp->xp_pattern = p + 1;
+  expand_option_start_col = (int)(p + 1 - xp->xp_line);
 
+  // Certain options currently have special case handling to reuse the
+  // expansion logic with other commands.
   if (options[opt_idx].var == &p_syn) {
     xp->xp_context = EXPAND_OWNSYNTAX;
     return;
   }
+  if (options[opt_idx].var == &p_ft) {
+    xp->xp_context = EXPAND_FILETYPE;
+    return;
+  }
 
+  // Now pick. If the option has a custom expander, use that. Otherwise, just
+  // fill with the existing option value.
+  if (expand_option_subtract) {
+    xp->xp_context = EXPAND_SETTING_SUBTRACT;
+    return;
+  } else if (expand_option_idx >= 0
+             && options[expand_option_idx].opt_expand_cb != NULL) {
+    xp->xp_context = EXPAND_STRING_SETTING;
+  } else if (*xp->xp_pattern == NUL) {
+    xp->xp_context = EXPAND_OLD_SETTING;
+    return;
+  } else {
+    xp->xp_context = EXPAND_NOTHING;
+  }
+
+  if (is_term_option || (flags & P_NUM)) {
+    return;
+  }
+
+  // Only string options below
+
+  // Options that have P_EXPAND are considered to all use file/dir expansion.
   if (flags & P_EXPAND) {
     p = options[opt_idx].var;
     if (p == (char *)&p_bdir
@@ -5451,8 +5477,6 @@ void set_context_in_set_cmd(expand_T *xp, char *arg, int opt_flags)
       } else {
         xp->xp_backslash = XP_BS_ONE;
       }
-    } else if (p == (char *)&p_ft) {
-      xp->xp_context = EXPAND_FILETYPE;
     } else {
       xp->xp_context = EXPAND_FILES;
       // for 'tags' need three backslashes for a space
@@ -5464,27 +5488,45 @@ void set_context_in_set_cmd(expand_T *xp, char *arg, int opt_flags)
     }
   }
 
-  // For an option that is a list of file names, find the start of the
-  // last file name.
-  for (p = arg + strlen(arg) - 1; p > xp->xp_pattern; p--) {
-    // count number of backslashes before ' ' or ','
-    if (*p == ' ' || *p == ',') {
-      char *s = p;
-      while (s > xp->xp_pattern && *(s - 1) == '\\') {
-        s--;
-      }
-      if ((*p == ' ' && (xp->xp_backslash == XP_BS_THREE && (p - s) < 3))
-          || (*p == ',' && (flags & P_COMMA) && ((p - s) & 1) == 0)) {
-        xp->xp_pattern = p + 1;
-        break;
+  // For an option that is a list of file names, or comma/colon-separated
+  // values, split it by the delimiter and find the start of the current
+  // pattern, while accounting for backslash-escaped space/commas/colons.
+  // Triple-backslashed escaped file names (e.g. 'path') can also be
+  // delimited by space.
+  if ((flags & P_EXPAND) || (flags & P_COMMA) || (flags & P_COLON)) {
+    for (p = arg + strlen(arg) - 1; p > xp->xp_pattern; p--) {
+      // count number of backslashes before ' ' or ','
+      if (*p == ' ' || *p == ',' || (*p == ':' && (flags & P_COLON))) {
+        char *s = p;
+        while (s > xp->xp_pattern && *(s - 1) == '\\') {
+          s--;
+        }
+        if ((*p == ' ' && (xp->xp_backslash == XP_BS_THREE && (p - s) < 3))
+            || (*p == ',' && (flags & P_COMMA) && ((p - s) % 1) == 0)
+            || (*p == ':' && (flags & P_COLON))) {
+          xp->xp_pattern = p + 1;
+          break;
+        }
       }
     }
+  }
 
-    // for 'spellsuggest' start at "file:"
-    if (options[opt_idx].var == &p_sps
-        && strncmp(p, "file:", 5) == 0) {
-      xp->xp_pattern = p + 5;
-      break;
+  // An option that is a list of single-character flags should always start
+  // at the end as we don't complete words.
+  if (flags & P_FLAGLIST) {
+    xp->xp_pattern = arg + strlen(arg);
+  }
+
+  // Some options can either be using file/dir expansions, or custom value
+  // expansion depending on what the user typed. Unfortunately we have to
+  // manually handle it here to make sure we have the correct xp_context set.
+  // for 'spellsuggest' start at "file:"
+  if (options[opt_idx].var == &p_sps) {
+    if (strncmp(xp->xp_pattern, "file:", 5) == 0) {
+      xp->xp_pattern += 5;
+      return;
+    } else if (options[expand_option_idx].opt_expand_cb != NULL) {
+      xp->xp_context = EXPAND_STRING_SETTING;
     }
   }
 }
@@ -5609,7 +5651,33 @@ int ExpandSettings(expand_T *xp, regmatch_T *regmatch, char *fuzzystr, int *numM
   return OK;
 }
 
-void ExpandOldSetting(int *numMatches, char ***matches)
+/// Escape an option value that can be used on the command-line with :set.
+/// Caller needs to free the returned string, unless NULL is returned.
+static char *escape_option_str_cmdline(char *var)
+{
+  // A backslash is required before some characters.  This is the reverse of
+  // what happens in do_set().
+  char *buf = vim_strsave_escaped(var, escape_chars);
+
+#ifdef BACKSLASH_IN_FILENAME
+  // For MS-Windows et al. we don't double backslashes at the start and
+  // before a file name character.
+  // The reverse is found at stropt_copy_value().
+  for (var = buf; *var != NUL; MB_PTR_ADV(var)) {
+    if (var[0] == '\\' && var[1] == '\\'
+        && expand_option_idx >= 0
+        && (options[expand_option_idx].flags & P_EXPAND)
+        && vim_isfilec((uint8_t)var[2])
+        && (var[2] != '\\' || (var == buf && var[4] != '\\'))) {
+      STRMOVE(var, var + 1);
+    }
+  }
+#endif
+  return buf;
+}
+
+/// Expansion handler for :set= when we just want to fill in with the existing value.
+int ExpandOldSetting(int *numMatches, char ***matches)
 {
   char *var = NULL;
 
@@ -5629,26 +5697,149 @@ void ExpandOldSetting(int *numMatches, char ***matches)
     var = "";
   }
 
-  // A backslash is required before some characters.  This is the reverse of
-  // what happens in do_set().
-  char *buf = vim_strsave_escaped(var, escape_chars);
+  char *buf = escape_option_str_cmdline(var);
 
-#ifdef BACKSLASH_IN_FILENAME
-  // For MS-Windows et al. we don't double backslashes at the start and
-  // before a file name character.
-  for (var = buf; *var != NUL; MB_PTR_ADV(var)) {
-    if (var[0] == '\\' && var[1] == '\\'
-        && expand_option_idx >= 0
-        && (options[expand_option_idx].flags & P_EXPAND)
-        && vim_isfilec((uint8_t)var[2])
-        && (var[2] != '\\' || (var == buf && var[4] != '\\'))) {
-      STRMOVE(var, var + 1);
+  (*matches)[0] = buf;
+  *numMatches = 1;
+  return OK;
+}
+
+/// Expansion handler for :set=/:set+= when the option has a custom expansion handler.
+int ExpandStringSetting(expand_T *xp, regmatch_T *regmatch, int *numMatches, char ***matches)
+{
+  if (expand_option_idx < 0
+      || options[expand_option_idx].opt_expand_cb == NULL) {
+    // Not supposed to reach this. This function is only for options with
+    // custom expansion callbacks.
+    return FAIL;
+  }
+
+  optexpand_T args = {
+    .oe_varp = get_varp_scope(&options[expand_option_idx], expand_option_flags),
+    .oe_append = expand_option_append,
+    .oe_regmatch = regmatch,
+    .oe_xp = xp,
+    .oe_set_arg = xp->xp_line + expand_option_start_col,
+  };
+  args.oe_include_orig_val = !expand_option_append && (*args.oe_set_arg == NUL);
+
+  // Retrieve the existing value, but escape it as a reverse of setting it.
+  // We technically only need to do this when oe_append or
+  // oe_include_orig_val is true.
+  option_value2string(&options[expand_option_idx], expand_option_flags);
+  char *var = NameBuff;
+  char *buf = escape_option_str_cmdline(var);
+  args.oe_opt_value = buf;
+
+  int num_ret = options[expand_option_idx].opt_expand_cb(&args, numMatches, matches);
+
+  xfree(buf);
+  return num_ret;
+}
+
+/// Expansion handler for :set-=
+int ExpandSettingSubtract(expand_T *xp, regmatch_T *regmatch, int *numMatches, char ***matches)
+{
+  if (expand_option_idx < 0) {
+    // term option
+    return ExpandOldSetting(numMatches, matches);
+  }
+
+  char *option_val = *(char **)get_option_varp_scope_from(expand_option_idx,
+                                                          expand_option_flags,
+                                                          curbuf, curwin);
+
+  uint32_t option_flags = options[expand_option_idx].flags;
+
+  if (option_flags & P_NUM) {
+    return ExpandOldSetting(numMatches, matches);
+  } else if (option_flags & P_COMMA) {
+    // Split the option by comma, then present each option to the user if
+    // it matches the pattern.
+    // This condition needs to go first, because 'whichwrap' has both
+    // P_COMMA and P_FLAGLIST.
+
+    if (*option_val == NUL) {
+      return FAIL;
     }
+
+    // Make a copy as we need to inject null characters destructively.
+    char *option_copy = xstrdup(option_val);
+    char *next_val = option_copy;
+
+    garray_T ga;
+    ga_init(&ga, sizeof(char *), 10);
+
+    do {
+      char *item = next_val;
+      char *comma = vim_strchr(next_val, ',');
+      while (comma != NULL && comma != next_val && *(comma - 1) == '\\') {
+        // "\," is interpreted as a literal comma rather than option
+        // separator when reading options in copy_option_part(). Skip
+        // it.
+        comma = vim_strchr(comma + 1, ',');
+      }
+      if (comma != NULL) {
+        *comma = NUL;  // null-terminate this value, required by later functions
+        next_val = comma + 1;
+      } else {
+        next_val = NULL;
+      }
+
+      if (*item == NUL) {
+        // empty value, don't add to list
+        continue;
+      }
+
+      if (!vim_regexec(regmatch, item, (colnr_T)0)) {
+        continue;
+      }
+
+      char *buf = escape_option_str_cmdline(item);
+      GA_APPEND(char *, &ga, buf);
+    } while (next_val != NULL);
+
+    xfree(option_copy);
+
+    *matches = ga.ga_data;
+    *numMatches = ga.ga_len;
+    return OK;
+  } else if (option_flags & P_FLAGLIST) {
+    // Only present the flags that are set on the option as the other flags
+    // are not meaningful to do set-= on.
+
+    if (*xp->xp_pattern != NUL) {
+      // Don't suggest anything if cmdline is non-empty. Vim's set-=
+      // behavior requires consecutive strings and it's usually
+      // unintuitive to users if ther try to subtract multiple flags at
+      // once.
+      return FAIL;
+    }
+
+    size_t num_flags = strlen(option_val);
+    if (num_flags == 0) {
+      return FAIL;
+    }
+
+    *matches = xmalloc(sizeof(char *) * (num_flags + 1));
+
+    int count = 0;
+
+    (*matches)[count++] = xstrdup(option_val);
+
+    if (num_flags > 1) {
+      // If more than one flags, split the flags up and expose each
+      // character as individual choice.
+      for (char *flag = option_val; *flag != NUL; flag++) {
+        (*matches)[count++] = xstrnsave(flag, 1);
+      }
+    }
+
+    *numMatches = count;
+    return OK;
   }
-#endif
 
-  *matches[0] = buf;
-  *numMatches = 1;
+  return ExpandOldSetting(numMatches, matches);
 }
 
 /// Get the value for the numeric or string option///opp in a nice format into
@@ -5772,6 +5963,7 @@ int fill_culopt_flags(char *val, win_T *wp)
     p = val;
   }
   while (*p != NUL) {
+    // Note: Keep this in sync with p_culopt_values.
     if (strncmp(p, "line", 4) == 0) {
       p += 4;
       culopt_flags_new |= CULOPT_LINE;
diff --git a/src/nvim/option.h b/src/nvim/option.h
index 9e3bf25bc3..8417d152e7 100644
--- a/src/nvim/option.h
+++ b/src/nvim/option.h
@@ -7,6 +7,7 @@
 #include "nvim/eval/typval_defs.h"
 #include "nvim/ex_cmds_defs.h"
 #include "nvim/option_defs.h"
+#include "nvim/search.h"
 
 /// The options that are local to a window or buffer have "indir" set to one of
 /// these values.  Special values:
@@ -45,7 +46,15 @@ typedef struct vimoption {
                      ///< local option: indirect option index
                      ///< callback function to invoke after an option is modified to validate and
                      ///< apply the new value.
+
+  /// callback function to invoke after an option is modified to validate and
+  /// apply the new value.
   opt_did_set_cb_T opt_did_set_cb;
+
+  /// callback function to invoke when expanding possible values on the
+  /// cmdline. Only useful for string options.
+  opt_expand_cb_T opt_expand_cb;
+
   void *def_val;     ///< default values for variable (neovim!!)
   LastSet last_set;  ///< script in which the option was last set
 } vimoption_T;
diff --git a/src/nvim/option_defs.h b/src/nvim/option_defs.h
index f078f6073c..dd637aacf5 100644
--- a/src/nvim/option_defs.h
+++ b/src/nvim/option_defs.h
@@ -3,6 +3,7 @@
 
 #include "nvim/api/private/defs.h"
 #include "nvim/eval/typval_defs.h"
+#include "nvim/regexp_defs.h"
 #include "nvim/types.h"
 
 /// Option value type
@@ -25,6 +26,14 @@ typedef struct {
   } data;
 } OptVal;
 
+/// :set operator types
+typedef enum {
+  OP_NONE = 0,
+  OP_ADDING,      ///< "opt+=arg"
+  OP_PREPENDING,  ///< "opt^=arg"
+  OP_REMOVING,    ///< "opt-=arg"
+} set_op_T;
+
 /// Argument for the callback function (opt_did_set_cb_T) invoked after an
 /// option value is modified.
 typedef struct {
@@ -33,6 +42,7 @@ typedef struct {
   void *os_varp;
   int os_idx;
   int os_flags;
+  set_op_T os_op;
 
   /// old value of the option (can be a string, number or a boolean)
   union {
@@ -80,4 +90,40 @@ typedef struct {
 /// Otherwise returns an error message.
 typedef const char *(*opt_did_set_cb_T)(optset_T *args);
 
+/// Argument for the callback function (opt_expand_cb_T) invoked after a string
+/// option value is expanded for cmdline completion.
+typedef struct {
+  /// Pointer to the option variable. It's always a string.
+  char *oe_varp;
+  /// The original option value, escaped.
+  char *oe_opt_value;
+
+  /// true if using set+= instead of set=
+  bool oe_append;
+  /// true if we would like to add the original option value as the first choice.
+  bool oe_include_orig_val;
+
+  /// Regex from the cmdline, for matching potential options against.
+  regmatch_T *oe_regmatch;
+  /// The expansion context.
+  expand_T *oe_xp;
+
+  /// The full argument passed to :set. For example, if the user inputs
+  /// ":set dip=icase,algorithm:my<Tab>", oe_xp->xp_pattern will only have
+  /// "my", but oe_set_arg will contain the whole "icase,algorithm:my".
+  char *oe_set_arg;
+} optexpand_T;
+
+/// Type for the callback function that is invoked when expanding possible
+/// string option values during cmdline completion.
+///
+/// Strings in returned matches will be managed and freed by caller.
+///
+/// Returns OK if the expansion succeeded (numMatches and matches have to be
+/// set). Otherwise returns FAIL.
+///
+/// Note: If returned FAIL or *numMatches is 0, *matches will NOT be freed by
+/// caller.
+typedef int (*opt_expand_cb_T)(optexpand_T *args, int *numMatches, char ***matches);
+
 #endif  // NVIM_OPTION_DEFS_H
diff --git a/src/nvim/option_vars.h b/src/nvim/option_vars.h
index d8bbce21b3..86e7f4cee8 100644
--- a/src/nvim/option_vars.h
+++ b/src/nvim/option_vars.h
@@ -16,6 +16,7 @@
                                     ///< the same.
 #define P_EXPAND       0x10U        ///< environment expansion.  NOTE: P_EXPAND can
                                     ///< never be used for local or hidden options
+#define P_NO_DEF_EXP   0x20U        ///< do not expand default value
 #define P_NODEFAULT    0x40U        ///< don't set to default value
 #define P_DEF_ALLOCED  0x80U        ///< default value is in allocated memory, must
                                     ///< use free() when assigning new value
@@ -51,8 +52,10 @@
 #define P_RWINONLY     0x10000000U  ///< only redraw current window
 #define P_MLE          0x20000000U  ///< under control of 'modelineexpr'
 #define P_FUNC         0x40000000U  ///< accept a function reference or a lambda
-
-#define P_NO_DEF_EXP   0x80000000U  ///< Do not expand default value.
+#define P_COLON        0x80000000U  ///< values use colons to create sublists
+// Warning: Currently we have used all 32 bits for option flags, and adding more
+//          flags will overflow it. Adding another flag will need to change how
+//          it's stored first.
 
 #define HIGHLIGHT_INIT \
   "8:SpecialKey,~:EndOfBuffer,z:TermCursor,Z:TermCursorNC,@:NonText,d:Directory,e:ErrorMsg," \
@@ -183,7 +186,7 @@
 #define CPO_VI          "aAbBcCdDeEfFiIJKlLmMnoOpPqrRsStuvWxXyZ$!%+>;_"
 
 // characters for p_ww option:
-#define WW_ALL          "bshl<>[],~"
+#define WW_ALL          "bshl<>[]~"
 
 // characters for p_mouse option:
 #define MOUSE_NORMAL    'n'             // use mouse in Normal mode
@@ -757,9 +760,9 @@ extern char *p_vfile;           ///< 'verbosefile'
 EXTERN int p_warn;              ///< 'warn'
 EXTERN char *p_wop;             ///< 'wildoptions'
 EXTERN unsigned wop_flags;
-#define WOP_TAGFILE             0x01
-#define WOP_PUM                 0x02
-#define WOP_FUZZY               0x04
+#define WOP_FUZZY               0x01
+#define WOP_TAGFILE             0x02
+#define WOP_PUM                 0x04
 EXTERN OptInt p_window;         ///< 'window'
 EXTERN char *p_wak;             ///< 'winaltkeys'
 EXTERN char *p_wig;             ///< 'wildignore'
diff --git a/src/nvim/options.lua b/src/nvim/options.lua
index cd1d760836..bae9b56ef4 100644
--- a/src/nvim/options.lua
+++ b/src/nvim/options.lua
@@ -6,7 +6,7 @@
 --- @field varname? string
 --- @field pv_name? string
 --- @field type 'bool'|'number'|'string'
---- @field list? 'comma'|'onecomma'|'flags'|'flagscomma'
+--- @field list? 'comma'|'onecomma'|'commacolon'|'onecommacolon'|'flags'|'flagscomma'
 --- @field scope vim.option_scope[]
 --- @field deny_duplicates? boolean
 --- @field enable_if? string|false
@@ -25,6 +25,7 @@
 --- @field alloced? true
 --- @field redraw? vim.option_redraw[]
 --- @field cb? string
+--- @field expand_cb? string
 --- @field tags? string[]
 
 --- @class vim.option_defaults
@@ -192,6 +193,7 @@ return {
         set to one of CJK locales.  See Unicode Standard Annex #11
         (https://www.unicode.org/reports/tr11).
       ]=],
+      expand_cb = 'expand_set_ambiwidth',
       full_name = 'ambiwidth',
       redraw = { 'all_windows', 'ui_option' },
       scope = { 'global' },
@@ -331,6 +333,7 @@ return {
         option, you must load syntax.vim again to see the result.  This can be
         done with ":syntax on".
       ]=],
+      expand_cb = 'expand_set_background',
       full_name = 'background',
       scope = { 'global' },
       short_desc = N_('"dark" or "light", used for highlight colors'),
@@ -357,6 +360,7 @@ return {
         When the value is empty, Vi compatible backspacing is used, none of