fix(options): ":set foo?" for Lua callbacks #41100

Problem:
`:set foo=<tab>` and `:set foo?` for func/expr options set to a Lua
function, always displays "v:lua".

The existing "<Lua N: file:line>" form was avoided because the ref id
N changes on every read of the same option.

Solution:
- Don't attempt to tab-complete Lua functions.
- Show "<Lua file:line>" (or "<Lua>" if source file is unknown).

Example:

    :set operatorfunc?
      operatorfunc=<Lua ~/.config/nvim/init.lua:42>

src/nvim/eval/typval.c

@@ -1898,7 +1898,7 @@ void callback_copy(Callback *dest, Callback *src)
 char *callback_to_string(Callback *cb, Arena *arena)
 {
   if (cb->type == kCallbackLua) {
-    return nlua_funcref_str(cb->data.luaref, arena);
+    return nlua_funcref_str(cb->data.luaref, arena, true);
   }
 
   const size_t msglen = 100;

src/nvim/eval/vars.c

@@ -3303,7 +3303,7 @@ typval_T opt_to_tv(Object value, bool numbool)
   case kObjectTypeLuaRef:
     // Lua callback option (e.g. 'operatorfunc'): show a human-readable hint (same as `:map` does).
     rettv.v_type = VAR_STRING;
-    rettv.vval.v_string = nlua_funcref_str(value.data.luaref, NULL);
+    rettv.vval.v_string = nlua_funcref_str(value.data.luaref, NULL, true);
     break;
   default:
     abort();  // Should never happen.

src/nvim/lua/executor.c

@@ -2453,7 +2453,7 @@ int nlua_do_ucmd(ucmd_T *cmd, exarg_T *eap, bool preview)
 /// String representation of a Lua function reference
 ///
 /// @return Allocated string
-char *nlua_funcref_str(LuaRef ref, Arena *arena)
+char *nlua_funcref_str(LuaRef ref, Arena *arena, bool show_ref)
 {
   lua_State *const lstate = global_lstate;
 
@@ -2469,13 +2469,14 @@ char *nlua_funcref_str(LuaRef ref, Arena *arena)
   lua_Debug ar;
   if (lua_getinfo(lstate, ">S", &ar) && *ar.source == '@' && ar.linedefined >= 0) {
     char *src = home_replace_save(NULL, ar.source + 1);
-    String str = arena_printf(arena, "<Lua %d: %s:%d>", ref, src, ar.linedefined);
+    String str = show_ref ? arena_printf(arena, "<Lua %d: %s:%d>", ref, src, ar.linedefined)
+                          : arena_printf(arena, "<Lua %s:%d>", src, ar.linedefined);
     xfree(src);
     return str.data;
   }
 
 plain: {}
-  return arena_printf(arena, "<Lua %d>", ref).data;
+  return show_ref ? arena_printf(arena, "<Lua %d>", ref).data : arena_printf(arena, "<Lua>").data;
 }
 
 /// Execute the vim._core.defaults module to set up default mappings and autocommands

src/nvim/mapping.c

@@ -255,7 +255,7 @@ static void showmap(mapblock_T *mp, bool local)
   // Use false below if we only want things like <Up> to show up as such on
   // the rhs, and not M-x etc, true gets both -- webb
   if (mp->m_luaref != LUA_NOREF) {
-    char *str = nlua_funcref_str(mp->m_luaref, NULL);
+    char *str = nlua_funcref_str(mp->m_luaref, NULL, true);
     msg_puts_hl(str, HLF_8, false);
     xfree(str);
   } else if (mp->m_str[0] == NUL) {
@@ -2206,7 +2206,7 @@ static void get_maparg(typval_T *argvars, typval_T *rettv, int exact)
         rettv->vval.v_string = str2special_save(rhs, false, false);
       }
     } else if (rhs_lua != LUA_NOREF) {
-      rettv->vval.v_string = nlua_funcref_str(mp->m_luaref, NULL);
+      rettv->vval.v_string = nlua_funcref_str(mp->m_luaref, NULL, true);
     }
   } else {
     // Return a dictionary.

src/nvim/msgpack_rpc/packer.c

@@ -212,7 +212,7 @@ void mpack_object_inner(Object *current, Object *container, size_t container_idx
       // see nlua_pop_Object.
 
       // Human-readable hint string (`:map`-style "<Lua …>" string).
-      char *repr = nlua_funcref_str(current->data.luaref, NULL);
+      char *repr = nlua_funcref_str(current->data.luaref, NULL, true);
       api_free_luaref(current->data.luaref);
       current->data.luaref = LUA_NOREF;
       mpack_str(cstr_as_string(repr), packer);

src/nvim/option.c

@@ -3448,8 +3448,8 @@ static ObjectType optval_type(Object o)
   }
 }
 
-/// True for a "callback option" (varp stores `Callback`): func option (kOptFlagFunc, e.g.
-/// 'operatorfunc') or expr option (kOptFlagExpr, e.g. 'foldexpr').
+/// True for a "callback option" (varp stores `Callback`): func option (e.g. 'operatorfunc') or expr
+/// option (e.g. 'foldexpr').
 static bool is_callback_option(OptIndex opt_idx)
 {
   return (options[opt_idx].flags & (kOptFlagFunc | kOptFlagExpr)) != 0;
@@ -3472,7 +3472,7 @@ Object optval_own(OptIndex opt_idx, Object value)
 static Object optval_snapshot(Object value)
 {
   if (value.type == kObjectTypeLuaRef) {
-    return CSTR_AS_OBJ(nlua_funcref_str(value.data.luaref, NULL));
+    return CSTR_AS_OBJ(nlua_funcref_str(value.data.luaref, NULL, true));
   }
   return copy_object(value, NULL);
 }
@@ -3486,14 +3486,14 @@ static Object optval_snapshot_varp(OptIndex opt_idx, void *varp)
   return snapshot;
 }
 
-/// Converts a callback option's stored Callback to an option value Object.
-/// Always OWNED (a copied name/expr string, or a fresh LuaRef): free with optval_free().
+/// Converts a callback option's stored Callback to an optval Object.
+/// Always OWNED (a copied name/expr string, or a new LuaRef): free with optval_free().
 ///
-/// - Lua callback => `LuaRef`
-/// - named funcref => name
-/// - partial => name
-/// - expr option => expression string
-/// - an unset callback => empty string
+/// - Lua callback   => `LuaRef`
+/// - named funcref  => name
+/// - partial        => name
+/// - expr option    => expression string
+/// - unset callback => empty string
 static Object opt_from_callback(Callback *cb)
 {
   switch (cb->type) {
@@ -3520,10 +3520,10 @@ static Object opt_from_callback(Callback *cb)
 /// Builds a callback option's Callback from an option value Object. Borrows `value` (copies its
 /// string/ref; the caller retains ownership).
 ///
-/// - `LuaRef` ("func" or "expr" option)    => Lua callback
-/// - func option with `is_expr=false`      => string parsed as a function name or lambda
-/// - expr option with non-empty string     => expression string (`<SID>`/`s:` resolves to `<SNR>`)
-/// - anything else (empty string, `Unset`) => `CALLBACK_NONE`
+/// - `LuaRef` ("func"/"expr" options)   => Lua callback
+/// - func option with `is_expr=false`   => string parsed as a function name or lambda
+/// - expr option with non-empty string  => expression string (`<SID>`/`s:` resolves to `<SNR>`)
+/// - other (empty string, `Unset`)      => `CALLBACK_NONE`
 static Callback opt_to_callback(Object value, bool is_expr)
 {
   Callback cb = CALLBACK_NONE;
@@ -3653,7 +3653,7 @@ static char *optval_to_cstr(Object o, bool quote)
     return buf;
   }
   case kObjectTypeLuaRef:
-    return xstrdup("v:lua");  // A Lua callback has no name; show a stable handle.
+    return nlua_funcref_str(o.data.luaref, NULL, false);  // show_ref=false: ref changes every time.
   default:
     abort();  // Should not happen.
   }
@@ -6467,7 +6467,7 @@ int ExpandOldSetting(int *numMatches, char ***matches)
   char *var = NULL;
 
   *numMatches = 0;
-  *matches = xmalloc(sizeof(char *));
+  *matches = NULL;
 
   // For a terminal key code expand_option_idx is kOptInvalid.
   if (expand_option_idx == kOptInvalid) {
@@ -6475,6 +6475,14 @@ int ExpandOldSetting(int *numMatches, char ***matches)
   }
 
   if (expand_option_idx != kOptInvalid) {
+    Object o = opt_from_varp(expand_option_idx,
+                             get_varp_scope(&options[expand_option_idx], expand_option_flags));
+    bool is_luafn = o.type == kObjectTypeLuaRef;
+    optval_free_read(expand_option_idx, o);
+    if (is_luafn) {
+      // Special case: ":set x=<tab>" cannot meaningfully complete a Lua function.
+      return OK;
+    }
     // Put string of option value in NameBuff.
     optval_fmt(expand_option_idx, expand_option_flags);
     var = NameBuff;
@@ -6484,6 +6492,7 @@ int ExpandOldSetting(int *numMatches, char ***matches)
 
   char *buf = escape_option_str_cmdline(var);
 
+  *matches = xmalloc(sizeof(char *));
   (*matches)[0] = buf;
   *numMatches = 1;
   return OK;

src/nvim/usercmd.c

@@ -613,7 +613,7 @@ static void uc_list(char *name, size_t name_len)
       msg_outtrans(IObuff, 0, false);
 
       if (cmd->uc_luaref != LUA_NOREF) {
-        char *fn = nlua_funcref_str(cmd->uc_luaref, NULL);
+        char *fn = nlua_funcref_str(cmd->uc_luaref, NULL, true);
         msg_puts_hl(fn, HLF_8, false);
         xfree(fn);
         // put the description on a new line

test/functional/options/options_spec.lua

@@ -218,7 +218,7 @@ describe('callback (func/expr) options', function()
     eq('GoodFunc', eval('&operatorfunc'))
   end)
 
-  it('with ":set all"', function()
+  it(':set', function()
     command('set operatorfunc=SomeFunc')
     command('set quickfixtextfunc={\\ d\\ ->\\ []\\ }')
     exec_lua(function()
@@ -227,6 +227,20 @@ describe('callback (func/expr) options', function()
     end)
     command('set all')
     n.assert_alive()
+
+    -- ":set opt?" shows funcref/lambda name, or "<Lua …>".
+    local function set_q(opt)
+      return n.api.nvim_exec2(('set %s?'):format(opt), { output = true }).output
+    end
+    eq('  operatorfunc=SomeFunc', set_q('operatorfunc'))
+    matches("^  quickfixtextfunc=function%('<lambda>%d+'%)$", set_q('quickfixtextfunc'))
+    matches('^  omnifunc=<Lua .+:%d+>$', set_q('omnifunc'))
+    matches('^  foldexpr=<Lua .+:%d+>$', set_q('foldexpr'))
+    eq(set_q('omnifunc'), set_q('omnifunc')) -- Stable: no per-read ref id.
+
+    -- ":set opt=<Tab>" completes a funcref value, but not a Lua function.
+    eq({ 'SomeFunc' }, n.fn.getcompletion('set operatorfunc=', 'cmdline'))
+    eq({}, n.fn.getcompletion('set omnifunc=', 'cmdline'))
   end)
 
   it("expr option ('foldexpr') accepts string, reads back string", function()