feat(api): implement nvim_{add,del}_user_command

Add support for adding and removing custom user commands with the Nvim
API.

17 files changed, 740 insertions, 99 deletions

diff --git a/runtime/doc/api.txt b/runtime/doc/api.txt
index 8fb6290e50..13b7c2f61a 100644
--- a/runtime/doc/api.txt
+++ b/runtime/doc/api.txt
@@ -626,6 +626,56 @@ nvim__stats()                                                  *nvim__stats()*
                 Return: ~
                     Map of various internal stats.
 
+                                                     *nvim_add_user_command()*
+nvim_add_user_command({name}, {command}, {*opts})
+                Create a new user command |user-commands|
+
+                {name} is the name of the new command. The name must begin
+                with an uppercase letter.
+
+                {command} is the replacement text or Lua function to execute.
+
+                Example: >
+                   :call nvim_add_user_command('SayHello', 'echo "Hello world!"', {})
+                   :SayHello
+                   Hello world!
+<
+
+                Parameters: ~
+                    {name}     Name of the new user command. Must begin with
+                               an uppercase letter.
+                    {command}  Replacement command to execute when this user
+                               command is executed. When called from Lua, the
+                               command can also be a Lua function. The
+                               function is called with a single table argument
+                               that contains the following keys:
+                               • args: (string) The args passed to the
+                                 command, if any |<args>|
+                               • bang: (boolean) "true" if the command was
+                                 executed with a ! modifier |<bang>|
+                               • line1: (number) The starting line of the
+                                 command range |<line1>|
+                               • line2: (number) The final line of the command
+                                 range |<line2>|
+                               • range: (number) The number of items in the
+                                 command range: 0, 1, or 2 |<range>|
+                               • count: (number) Any count supplied |<count>|
+                               • reg: (string) The optional register, if
+                                 specified |<reg>|
+                               • mods: (string) Command modifiers, if any
+                                 |<mods>|
+                    {opts}     Optional command attributes. See
+                               |command-attributes| for more details. To use
+                               boolean attributes (such as |:command-bang| or
+                               |:command-bar|) set the value to "true". When
+                               using a Lua function for {command} you can also
+                               provide a "desc" key that will be displayed
+                               when listing commands. In addition to the
+                               string options listed in |:command-complete|,
+                               the "complete" key also accepts a Lua function
+                               which works like the "customlist" completion
+                               mode |:command-complete-customlist|.
+
 nvim_call_atomic({calls})                                 *nvim_call_atomic()*
                 Calls many API methods atomically.
 
@@ -714,6 +764,12 @@ nvim_del_mark({name})                                        *nvim_del_mark()*
                     |nvim_buf_del_mark()|
                     |nvim_get_mark()|
 
+nvim_del_user_command({name})                        *nvim_del_user_command()*
+                Delete a user-defined command.
+
+                Parameters: ~
+                    {name}  Name of the command to delete.
+
 nvim_del_var({name})                                          *nvim_del_var()*
                 Removes a global (g:) variable.
 
@@ -1790,6 +1846,16 @@ nvim__buf_redraw_range({buffer}, {first}, {last})
 nvim__buf_stats({buffer})                                  *nvim__buf_stats()*
                 TODO: Documentation
 
+                                                 *nvim_buf_add_user_command()*
+nvim_buf_add_user_command({buffer}, {name}, {command}, {*opts})
+                Create a new user command |user-commands| in the given buffer.
+
+                Parameters: ~
+                    {buffer}  Buffer handle, or 0 for current buffer.
+
+                See also: ~
+                    nvim_add_user_command
+
 nvim_buf_attach({buffer}, {send_buffer}, {opts})           *nvim_buf_attach()*
                 Activates buffer-update events on a channel, or as Lua
                 callbacks.
@@ -1925,6 +1991,18 @@ nvim_buf_del_mark({buffer}, {name})                      *nvim_buf_del_mark()*
                     |nvim_buf_set_mark()|
                     |nvim_del_mark()|
 
+                                                 *nvim_buf_del_user_command()*
+nvim_buf_del_user_command({buffer}, {name})
+                Delete a buffer-local user-defined command.
+
+                Only commands created with |:command-buffer| or
+                |nvim_buf_add_user_command()| can be deleted with this
+                function.
+
+                Parameters: ~
+                    {buffer}  Buffer handle, or 0 for current buffer.
+                    {name}    Name of the command to delete.
+
 nvim_buf_del_var({buffer}, {name})                        *nvim_buf_del_var()*
                 Removes a buffer-scoped (b:) variable
 
diff --git a/runtime/doc/map.txt b/runtime/doc/map.txt
index 238ef39bd3..f6d9e45d64 100644
--- a/runtime/doc/map.txt
+++ b/runtime/doc/map.txt
@@ -1247,8 +1247,8 @@ See |:verbose-cmd| for more information.
 
 
 Command attributes ~
-
-User-defined commands are treated by Vim just like any other Ex commands.  They
+							*command-attributes*
+User-defined commands are treated by Nvim just like any other Ex commands.  They
 can have arguments, or have a range specified.  Arguments are subject to
 completion as filenames, buffers, etc.  Exactly how this works depends upon the
 command's attributes, which are specified when the command is defined.
diff --git a/src/nvim/api/buffer.c b/src/nvim/api/buffer.c
index ce9c4e27ad..bacb991c4b 100644
--- a/src/nvim/api/buffer.c
+++ b/src/nvim/api/buffer.c
@@ -1273,6 +1273,63 @@ Object nvim_buf_call(Buffer buffer, LuaRef fun, Error *err)
   return res;
 }
 
+/// Create a new user command |user-commands| in the given buffer.
+///
+/// @param  buffer  Buffer handle, or 0 for current buffer.
+/// @param[out] err Error details, if any.
+/// @see nvim_add_user_command
+void nvim_buf_add_user_command(Buffer buffer, String name, Object command,
+                               Dict(user_command) *opts, Error *err)
+  FUNC_API_SINCE(9)
+{
+  buf_T *target_buf = find_buffer_by_handle(buffer, err);
+  if (ERROR_SET(err)) {
+    return;
+  }
+
+  buf_T *save_curbuf = curbuf;
+  curbuf = target_buf;
+  add_user_command(name, command, opts, UC_BUFFER, err);
+  curbuf = save_curbuf;
+}
+
+/// Delete a buffer-local user-defined command.
+///
+/// Only commands created with |:command-buffer| or
+/// |nvim_buf_add_user_command()| can be deleted with this function.
+///
+/// @param  buffer  Buffer handle, or 0 for current buffer.
+/// @param  name    Name of the command to delete.
+/// @param[out] err Error details, if any.
+void nvim_buf_del_user_command(Buffer buffer, String name, Error *err)
+  FUNC_API_SINCE(9)
+{
+  garray_T *gap;
+  if (buffer == -1) {
+    gap = &ucmds;
+  } else {
+    buf_T *buf = find_buffer_by_handle(buffer, err);
+    gap = &buf->b_ucmds;
+  }
+
+  for (int i = 0; i < gap->ga_len; i++) {
+    ucmd_T *cmd = USER_CMD_GA(gap, i);
+    if (!STRCMP(name.data, cmd->uc_name)) {
+      free_ucmd(cmd);
+
+      gap->ga_len -= 1;
+
+      if (i < gap->ga_len) {
+        memmove(cmd, cmd + 1, (size_t)(gap->ga_len - i) * sizeof(ucmd_T));
+      }
+
+      return;
+    }
+  }
+
+  api_set_error(err, kErrorTypeException, "No such user-defined command: %s", name.data);
+}
+
 Dictionary nvim__buf_stats(Buffer buffer, Error *err)
 {
   Dictionary rv = ARRAY_DICT_INIT;
diff --git a/src/nvim/api/keysets.lua b/src/nvim/api/keysets.lua
index f3e7f2f1dc..e47dec9eb7 100644
--- a/src/nvim/api/keysets.lua
+++ b/src/nvim/api/keysets.lua
@@ -33,6 +33,18 @@ return {
   get_commands = {
     "builtin";
   };
+  user_command = {
+    "addr";
+    "bang";
+    "bar";
+    "complete";
+    "count";
+    "desc";
+    "force";
+    "nargs";
+    "range";
+    "register";
+  };
   float_config = {
     "row";
     "col";
diff --git a/src/nvim/api/private/helpers.c b/src/nvim/api/private/helpers.c
index 9b407eab8b..962fce6952 100644
--- a/src/nvim/api/private/helpers.c
+++ b/src/nvim/api/private/helpers.c
@@ -961,6 +961,10 @@ Object copy_object(Object obj)
 
   case kObjectTypeDictionary:
     return DICTIONARY_OBJ(copy_dictionary(obj.data.dictionary));
+
+  case kObjectTypeLuaRef:
+    return LUAREF_OBJ(api_new_luaref(obj.data.luaref));
+
   default:
     abort();
   }
@@ -1342,3 +1346,184 @@ const char *get_default_stl_hl(win_T *wp)
     return "StatusLineNC";
   }
 }
+
+void add_user_command(String name, Object command, Dict(user_command) *opts, int flags, Error *err)
+{
+  uint32_t argt = 0;
+  long def = -1;
+  cmd_addr_T addr_type_arg = ADDR_NONE;
+  int compl = EXPAND_NOTHING;
+  char *compl_arg = NULL;
+  char *rep = NULL;
+  LuaRef luaref = LUA_NOREF;
+  LuaRef compl_luaref = LUA_NOREF;
+
+  if (HAS_KEY(opts->range) && HAS_KEY(opts->count)) {
+    api_set_error(err, kErrorTypeValidation, "'range' and 'count' are mutually exclusive");
+    goto err;
+  }
+
+  if (opts->nargs.type == kObjectTypeInteger) {
+    switch (opts->nargs.data.integer) {
+    case 0:
+      // Default value, nothing to do
+      break;
+    case 1:
+      argt |= EX_EXTRA | EX_NOSPC | EX_NEEDARG;
+      break;
+    default:
+      api_set_error(err, kErrorTypeValidation, "Invalid value for 'nargs'");
+      goto err;
+    }
+  } else if (opts->nargs.type == kObjectTypeString) {
+    if (opts->nargs.data.string.size > 1) {
+      api_set_error(err, kErrorTypeValidation, "Invalid value for 'nargs'");
+      goto err;
+    }
+
+    switch (opts->nargs.data.string.data[0]) {
+    case '*':
+      argt |= EX_EXTRA;
+      break;
+    case '?':
+      argt |= EX_EXTRA | EX_NOSPC;
+      break;
+    case '+':
+      argt |= EX_EXTRA | EX_NEEDARG;
+      break;
+    default:
+      api_set_error(err, kErrorTypeValidation, "Invalid value for 'nargs'");
+      goto err;
+    }
+  } else if (HAS_KEY(opts->nargs)) {
+    api_set_error(err, kErrorTypeValidation, "Invalid value for 'nargs'");
+    goto err;
+  }
+
+  if (HAS_KEY(opts->complete) && !argt) {
+    api_set_error(err, kErrorTypeValidation, "'complete' used without 'nargs'");
+    goto err;
+  }
+
+  if (opts->range.type == kObjectTypeBoolean) {
+    if (opts->range.data.boolean) {
+      argt |= EX_RANGE;
+      addr_type_arg = ADDR_LINES;
+    }
+  } else if (opts->range.type == kObjectTypeString) {
+    if (opts->range.data.string.data[0] == '%' && opts->range.data.string.size == 1) {
+      argt |= EX_RANGE | EX_DFLALL;
+      addr_type_arg = ADDR_LINES;
+    } else {
+      api_set_error(err, kErrorTypeValidation, "Invalid value for 'range'");
+      goto err;
+    }
+  } else if (opts->range.type == kObjectTypeInteger) {
+    argt |= EX_RANGE | EX_ZEROR;
+    def = opts->range.data.integer;
+    addr_type_arg = ADDR_LINES;
+  } else if (HAS_KEY(opts->range)) {
+    api_set_error(err, kErrorTypeValidation, "Invalid value for 'range'");
+    goto err;
+  }
+
+  if (opts->count.type == kObjectTypeBoolean) {
+    if (opts->count.data.boolean) {
+      argt |= EX_COUNT | EX_ZEROR | EX_RANGE;
+      addr_type_arg = ADDR_OTHER;
+      def = 0;
+    }
+  } else if (opts->count.type == kObjectTypeInteger) {
+    argt |= EX_COUNT | EX_ZEROR | EX_RANGE;
+    addr_type_arg = ADDR_OTHER;
+    def = opts->count.data.integer;
+  } else if (HAS_KEY(opts->count)) {
+    api_set_error(err, kErrorTypeValidation, "Invalid value for 'count'");
+    goto err;
+  }
+
+  if (opts->addr.type == kObjectTypeString) {
+    if (parse_addr_type_arg((char_u *)opts->addr.data.string.data, (int)opts->addr.data.string.size,
+                            &addr_type_arg) != OK) {
+      api_set_error(err, kErrorTypeValidation, "Invalid value for 'addr'");
+      goto err;
+    }
+
+    if (addr_type_arg != ADDR_LINES) {
+      argt |= EX_ZEROR;
+    }
+  } else if (HAS_KEY(opts->addr)) {
+    api_set_error(err, kErrorTypeValidation, "Invalid value for 'addr'");
+    goto err;
+  }
+
+  if (api_object_to_bool(opts->bang, "bang", false, err)) {
+    argt |= EX_BANG;
+  } else if (ERROR_SET(err)) {
+    goto err;
+  }
+
+  if (api_object_to_bool(opts->bar, "bar", false, err)) {
+    argt |= EX_TRLBAR;
+  } else if (ERROR_SET(err)) {
+    goto err;
+  }
+
+
+  if (api_object_to_bool(opts->register_, "register", false, err)) {
+    argt |= EX_REGSTR;
+  } else if (ERROR_SET(err)) {
+    goto err;
+  }
+
+  bool force = api_object_to_bool(opts->force, "force", false, err);
+  if (ERROR_SET(err)) {
+    goto err;
+  }
+
+  if (opts->complete.type == kObjectTypeLuaRef) {
+    compl = EXPAND_USER_LUA;
+    compl_luaref = api_new_luaref(opts->complete.data.luaref);
+  } else if (opts->complete.type == kObjectTypeString) {
+    if (parse_compl_arg((char_u *)opts->complete.data.string.data,
+                        (int)opts->complete.data.string.size, &compl, &argt,
+                        (char_u **)&compl_arg) != OK) {
+      api_set_error(err, kErrorTypeValidation, "Invalid value for 'complete'");
+      goto err;
+    }
+  } else if (HAS_KEY(opts->complete)) {
+    api_set_error(err, kErrorTypeValidation, "Invalid value for 'complete'");
+    goto err;
+  }
+
+  switch (command.type) {
+  case kObjectTypeLuaRef:
+    luaref = api_new_luaref(command.data.luaref);
+    if (opts->desc.type == kObjectTypeString) {
+      rep = opts->desc.data.string.data;
+    } else {
+      snprintf((char *)IObuff, IOSIZE, "<Lua function %d>", luaref);
+      rep = (char *)IObuff;
+    }
+    break;
+  case kObjectTypeString:
+    rep = command.data.string.data;
+    break;
+  default:
+    api_set_error(err, kErrorTypeValidation, "'command' must be a string or Lua function");
+    goto err;
+  }
+
+  if (uc_add_command((char_u *)name.data, name.size, (char_u *)rep, argt, def, flags,
+                     compl, (char_u *)compl_arg, compl_luaref, addr_type_arg, luaref,
+                     force) != OK) {
+    api_set_error(err, kErrorTypeException, "Failed to create user command");
+    goto err;
+  }
+
+  return;
+
+err:
+  NLUA_CLEAR_REF(luaref);
+  NLUA_CLEAR_REF(compl_luaref);
+}
diff --git a/src/nvim/api/vim.c b/src/nvim/api/vim.c
index 4a0222234d..ac979d0064 100644
--- a/src/nvim/api/vim.c
+++ b/src/nvim/api/vim.c
@@ -2363,3 +2363,52 @@ Dictionary nvim_eval_statusline(String str, Dict(eval_statusline) *opts, Error *
 
   return result;
 }
+
+/// Create a new user command |user-commands|
+///
+/// {name} is the name of the new command. The name must begin with an uppercase letter.
+///
+/// {command} is the replacement text or Lua function to execute.
+///
+/// Example:
+/// <pre>
+///    :call nvim_add_user_command('SayHello', 'echo "Hello world!"', {})
+///    :SayHello
+///    Hello world!
+/// </pre>
+///
+/// @param  name    Name of the new user command. Must begin with an uppercase letter.
+/// @param  command Replacement command to execute when this user command is executed. When called
+///                 from Lua, the command can also be a Lua function. The function is called with a
+///                 single table argument that contains the following keys:
+///                 - args: (string) The args passed to the command, if any |<args>|
+///                 - bang: (boolean) "true" if the command was executed with a ! modifier |<bang>|
+///                 - line1: (number) The starting line of the command range |<line1>|
+///                 - line2: (number) The final line of the command range |<line2>|
+///                 - range: (number) The number of items in the command range: 0, 1, or 2 |<range>|
+///                 - count: (number) Any count supplied |<count>|
+///                 - reg: (string) The optional register, if specified |<reg>|
+///                 - mods: (string) Command modifiers, if any |<mods>|
+/// @param  opts    Optional command attributes. See |command-attributes| for more details. To use
+///                 boolean attributes (such as |:command-bang| or |:command-bar|) set the value to
+///                 "true". When using a Lua function for {command} you can also provide a "desc"
+///                 key that will be displayed when listing commands. In addition to the string
+///                 options listed in |:command-complete|, the "complete" key also accepts a Lua
+///                 function which works like the "customlist" completion mode
+///                 |:command-complete-customlist|.
+/// @param[out] err Error details, if any.
+void nvim_add_user_command(String name, Object command, Dict(user_command) *opts, Error *err)
+  FUNC_API_SINCE(9)
+{
+  add_user_command(name, command, opts, 0, err);
+}
+
+/// Delete a user-defined command.
+///
+/// @param  name    Name of the command to delete.
+/// @param[out] err Error details, if any.
+void nvim_del_user_command(String name, Error *err)
+  FUNC_API_SINCE(9)
+{
+  nvim_buf_del_user_command(-1, name, err);
+}
diff --git a/src/nvim/ex_cmds_defs.h b/src/nvim/ex_cmds_defs.h
index ea899b660b..e5eab61f9e 100644
--- a/src/nvim/ex_cmds_defs.h
+++ b/src/nvim/ex_cmds_defs.h
@@ -192,6 +192,7 @@ struct expand {
   int xp_context;               // type of expansion
   size_t xp_pattern_len;        // bytes in xp_pattern before cursor
   char_u *xp_arg;               // completion function
+  LuaRef xp_luaref;             // Ref to Lua completion function
   sctx_T xp_script_ctx;         // SCTX for completion function
   int xp_backslash;             // one of the XP_BS_ values
 #ifndef BACKSLASH_IN_FILENAME
diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index 230cbd4b60..7bf4a4a65a 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -81,23 +81,7 @@
 static int quitmore = 0;
 static bool ex_pressedreturn = false;
 
-typedef struct ucmd {
-  char_u *uc_name;         // The command name
-  uint32_t uc_argt;             // The argument type
-  char_u *uc_rep;          // The command's replacement string
-  long uc_def;                  // The default value for a range/count
-  int uc_compl;                 // completion type
-  cmd_addr_T uc_addr_type;      // The command's address type
-  sctx_T uc_script_ctx;         // SCTX where the command was defined
-  char_u *uc_compl_arg;    // completion argument if any
-} ucmd_T;
-
-#define UC_BUFFER       1       // -buffer: local to current buffer
-
-static garray_T ucmds = { 0, 0, sizeof(ucmd_T), 4, NULL };
-
-#define USER_CMD(i) (&((ucmd_T *)(ucmds.ga_data))[i])
-#define USER_CMD_GA(gap, i) (&((ucmd_T *)((gap)->ga_data))[i])
+garray_T ucmds = { 0, 0, sizeof(ucmd_T), 4, NULL };
 
 // Whether a command index indicates a user command.
 #define IS_USER_CMDIDX(idx) ((int)(idx) < 0)
@@ -2761,6 +2745,7 @@ static char_u *find_ucmd(exarg_T *eap, char_u *p, int *full, expand_T *xp, int *
             *complp = uc->uc_compl;
           }
           if (xp != NULL) {
+            xp->xp_luaref = uc->uc_compl_luaref;
             xp->xp_arg = uc->uc_compl_arg;
             xp->xp_script_ctx = uc->uc_script_ctx;
             xp->xp_script_ctx.sc_lnum += sourcing_lnum;
@@ -5171,8 +5156,9 @@ char_u *get_command_name(expand_T *xp, int idx)
   return cmdnames[idx].cmd_name;
 }
 
-static int uc_add_command(char_u *name, size_t name_len, char_u *rep, uint32_t argt, long def,
-                          int flags, int compl, char_u *compl_arg, cmd_addr_T addr_type, bool force)
+int uc_add_command(char_u *name, size_t name_len, char_u *rep, uint32_t argt, long def, int flags,
+                   int compl, char_u *compl_arg, LuaRef compl_luaref, cmd_addr_T addr_type,
+                   LuaRef luaref, bool force)
   FUNC_ATTR_NONNULL_ARG(1, 3)
 {
   ucmd_T *cmd = NULL;
@@ -5226,6 +5212,8 @@ static int uc_add_command(char_u *name, size_t name_len, char_u *rep, uint32_t a
 
       XFREE_CLEAR(cmd->uc_rep);
       XFREE_CLEAR(cmd->uc_compl_arg);
+      NLUA_CLEAR_REF(cmd->uc_luaref);
+      NLUA_CLEAR_REF(cmd->uc_compl_luaref);
       break;
     }
 
@@ -5256,13 +5244,17 @@ static int uc_add_command(char_u *name, size_t name_len, char_u *rep, uint32_t a
   cmd->uc_script_ctx = current_sctx;
   cmd->uc_script_ctx.sc_lnum += sourcing_lnum;
   cmd->uc_compl_arg = compl_arg;
+  cmd->uc_compl_luaref = compl_luaref;
   cmd->uc_addr_type = addr_type;
+  cmd->uc_luaref = luaref;
 
   return OK;
 
 fail:
   xfree(rep_buf);
   xfree(compl_arg);
+  NLUA_CLEAR_REF(luaref);
+  NLUA_CLEAR_REF(compl_luaref);
   return FAIL;
 }
 
@@ -5301,6 +5293,7 @@ static const char *command_complete[] =
   [EXPAND_CSCOPE] = "cscope",
   [EXPAND_USER_DEFINED] = "custom",
   [EXPAND_USER_LIST] = "customlist",
+  [EXPAND_USER_LUA] = "<Lua function>",
   [EXPAND_DIFF_BUFFERS] = "diff_buffer",
   [EXPAND_DIRECTORIES] = "dir",
   [EXPAND_ENV_VARS] = "environment",
@@ -5702,8 +5695,8 @@ static void ex_command(exarg_T *eap)
   } else if (compl > 0 && (argt & EX_EXTRA) == 0) {
     emsg(_(e_complete_used_without_nargs));
   } else {
-    uc_add_command(name, end - name, p, argt, def, flags, compl, compl_arg,
-                   addr_type_arg, eap->forceit);
+    uc_add_command(name, end - name, p, argt, def, flags, compl, compl_arg, LUA_NOREF,
+                   addr_type_arg, LUA_NOREF, eap->forceit);
   }
 }
 
@@ -5717,11 +5710,13 @@ void ex_comclear(exarg_T *eap)
   uc_clear(&curbuf->b_ucmds);
 }
 
-static void free_ucmd(ucmd_T *cmd)
+void free_ucmd(ucmd_T *cmd)
 {
   xfree(cmd->uc_name);
   xfree(cmd->uc_rep);
   xfree(cmd->uc_compl_arg);
+  NLUA_CLEAR_REF(cmd->uc_compl_luaref);
+  NLUA_CLEAR_REF(cmd->uc_luaref);
 }
 
 /*
@@ -5759,9 +5754,7 @@ static void ex_delcommand(exarg_T *eap)
     return;
   }
 
-  xfree(cmd->uc_name);
-  xfree(cmd->uc_rep);
-  xfree(cmd->uc_compl_arg);
+  free_ucmd(cmd);
 
   --gap->ga_len;
 
@@ -5843,7 +5836,7 @@ static char_u *uc_split_args(char_u *arg, size_t *lenp)
   return buf;
 }
 
-static size_t add_cmd_modifier(char_u *buf, char *mod_str, bool *multi_mods)
+static size_t add_cmd_modifier(char *buf, char *mod_str, bool *multi_mods)
 {
   size_t result = STRLEN(mod_str);
   if (*multi_mods) {
@@ -6044,70 +6037,8 @@ static size_t uc_check_code(char_u *code, size_t len, char_u *buf, ucmd_T *cmd,
       *buf = '\0';
     }
 
-    bool multi_mods = false;
-
-    // :aboveleft and :leftabove
-    if (cmdmod.split & WSP_ABOVE) {
-      result += add_cmd_modifier(buf, "aboveleft", &multi_mods);
-    }
-    // :belowright and :rightbelow
-    if (cmdmod.split & WSP_BELOW) {
-      result += add_cmd_modifier(buf, "belowright", &multi_mods);
-    }
-    // :botright
-    if (cmdmod.split & WSP_BOT) {
-      result += add_cmd_modifier(buf, "botright", &multi_mods);
-    }
-
-    typedef struct {
-      bool *set;
-      char *name;
-    } mod_entry_T;
-    static mod_entry_T mod_entries[] = {
-      { &cmdmod.browse, "browse" },
-      { &cmdmod.confirm, "confirm" },
-      { &cmdmod.hide, "hide" },
-      { &cmdmod.keepalt, "keepalt" },
-      { &cmdmod.keepjumps, "keepjumps" },
-      { &cmdmod.keepmarks, "keepmarks" },
-      { &cmdmod.keeppatterns, "keeppatterns" },
-      { &cmdmod.lockmarks, "lockmarks" },
-      { &cmdmod.noswapfile, "noswapfile" }
-    };
-    // the modifiers that are simple flags
-    for (size_t i = 0; i < ARRAY_SIZE(mod_entries); i++) {
-      if (*mod_entries[i].set) {
-        result += add_cmd_modifier(buf, mod_entries[i].name, &multi_mods);
-      }
-    }
-
-    // TODO(vim): How to support :noautocmd?
-    // TODO(vim): How to support :sandbox?
+    result += uc_mods((char *)buf);
 
-    // :silent
-    if (msg_silent > 0) {
-      result += add_cmd_modifier(buf, emsg_silent > 0 ? "silent!" : "silent",
-                                 &multi_mods);
-    }
-    // :tab
-    if (cmdmod.tab > 0) {
-      result += add_cmd_modifier(buf, "tab", &multi_mods);
-    }
-    // :topleft
-    if (cmdmod.split & WSP_TOP) {
-      result += add_cmd_modifier(buf, "topleft", &multi_mods);
-    }
-
-    // TODO(vim): How to support :unsilent?
-
-    // :verbose
-    if (p_verbose > 0) {
-      result += add_cmd_modifier(buf, "verbose", &multi_mods);
-    }
-    // :vertical
-    if (cmdmod.split & WSP_VERT) {
-      result += add_cmd_modifier(buf, "vertical", &multi_mods);
-    }
     if (quote && buf != NULL) {
       buf += result - 2;
       *buf = '"';
@@ -6152,6 +6083,76 @@ static size_t uc_check_code(char_u *code, size_t len, char_u *buf, ucmd_T *cmd,
   return result;
 }
 
+size_t uc_mods(char *buf)
+{
+  size_t result = 0;
+  bool multi_mods = false;
+
+  // :aboveleft and :leftabove
+  if (cmdmod.split & WSP_ABOVE) {
+    result += add_cmd_modifier(buf, "aboveleft", &multi_mods);
+  }
+  // :belowright and :rightbelow
+  if (cmdmod.split & WSP_BELOW) {
+    result += add_cmd_modifier(buf, "belowright", &multi_mods);
+  }
+  // :botright
+  if (cmdmod.split & WSP_BOT) {
+    result += add_cmd_modifier(buf, "botright", &multi_mods);
+  }
+
+  typedef struct {
+    bool *set;
+    char *name;
+  } mod_entry_T;
+  static mod_entry_T mod_entries[] = {
+    { &cmdmod.browse, "browse" },
+    { &cmdmod.confirm, "confirm" },
+    { &cmdmod.hide, "hide" },
+    { &cmdmod.keepalt, "keepalt" },
+    { &cmdmod.keepjumps, "keepjumps" },
+    { &cmdmod.keepmarks, "keepmarks" },
+    { &cmdmod.keeppatterns, "keeppatterns" },
+    { &cmdmod.lockmarks, "lockmarks" },
+    { &cmdmod.noswapfile, "noswapfile" }
+  };
+  // the modifiers that are simple flags
+  for (size_t i = 0; i < ARRAY_SIZE(mod_entries); i++) {
+    if (*mod_entries[i].set) {
+      result += add_cmd_modifier(buf, mod_entries[i].name, &multi_mods);
+    }
+  }
+
+  // TODO(vim): How to support :noautocmd?
+  // TODO(vim): How to support :sandbox?
+
+  // :silent
+  if (msg_silent > 0) {
+    result += add_cmd_modifier(buf, emsg_silent > 0 ? "silent!" : "silent", &multi_mods);
+  }
+  // :tab
+  if (cmdmod.tab > 0) {
+    result += add_cmd_modifier(buf, "tab", &multi_mods);
+  }
+  // :topleft
+  if (cmdmod.split & WSP_TOP) {
+    result += add_cmd_modifier(buf, "topleft", &multi_mods);
+  }
+
+  // TODO(vim): How to support :unsilent?
+
+  // :verbose
+  if (p_verbose > 0) {
+    result += add_cmd_modifier(buf, "verbose", &multi_mods);
+  }
+  // :vertical
+  if (cmdmod.split & WSP_VERT) {
+    result += add_cmd_modifier(buf, "vertical", &multi_mods);
+  }
+
+  return result;
+}
+
 static void do_ucmd(exarg_T *eap)
 {
   char_u *buf;
@@ -6174,6 +6175,11 @@ static void do_ucmd(exarg_T *eap)
     cmd = USER_CMD_GA(&curbuf->b_ucmds, eap->useridx);
   }
 
+  if (cmd->uc_luaref > 0) {
+    nlua_do_ucmd(cmd, eap);
+    return;
+  }
+
   /*
    * Replace <> in the command by the arguments.
    * First round: "buf" is NULL, compute length, allocate "buf".
diff --git a/src/nvim/ex_docmd.h b/src/nvim/ex_docmd.h
index a302d4a3c5..abf6ec347b 100644
--- a/src/nvim/ex_docmd.h
+++ b/src/nvim/ex_docmd.h
@@ -32,6 +32,26 @@ typedef struct {
   tasave_T tabuf;
 } save_state_T;
 
+typedef struct ucmd {
+  char_u *uc_name;              // The command name
+  uint32_t uc_argt;             // The argument type
+  char_u *uc_rep;               // The command's replacement string
+  long uc_def;                  // The default value for a range/count
+  int uc_compl;                 // completion type
+  cmd_addr_T uc_addr_type;      // The command's address type
+  sctx_T uc_script_ctx;         // SCTX where the command was defined
+  char_u *uc_compl_arg;         // completion argument if any
+  LuaRef uc_compl_luaref;       // Reference to Lua completion function
+  LuaRef uc_luaref;             // Reference to Lua function
+} ucmd_T;
+
+#define UC_BUFFER       1       // -buffer: local to current buffer
+
+extern garray_T ucmds;
+
+#define USER_CMD(i) (&((ucmd_T *)(ucmds.ga_data))[i])
+#define USER_CMD_GA(gap, i) (&((ucmd_T *)((gap)->ga_data))[i])
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "ex_docmd.h.generated.h"
 #endif
diff --git a/src/nvim/ex_getln.c b/src/nvim/ex_getln.c
index ba2238ace2..67fd5a4efc 100644
--- a/src/nvim/ex_getln.c
+++ b/src/nvim/ex_getln.c
@@ -4983,6 +4983,9 @@ static int ExpandFromContext(expand_T *xp, char_u *pat, int *num_file, char_u **
   if (xp->xp_context == EXPAND_USER_LIST) {
     return ExpandUserList(xp, num_file, file);
   }
+  if (xp->xp_context == EXPAND_USER_LUA) {
+    return ExpandUserLua(xp, num_file, file);
+  }
   if (xp->xp_context == EXPAND_PACKADD) {
     return ExpandPackAddDir(pat, num_file, file);
   }
@@ -5411,6 +5414,35 @@ static int ExpandUserList(expand_T *xp, int *num_file, char_u ***file)
   return OK;
 }
 
+static int ExpandUserLua(expand_T *xp, int *num_file, char_u ***file)
+{
+  typval_T rettv;
+  nlua_call_user_expand_func(xp, &rettv);
+  if (rettv.v_type != VAR_LIST) {
+    tv_clear(&rettv);
+    return FAIL;
+  }
+
+  list_T *const retlist = rettv.vval.v_list;
+
+  garray_T ga;
+  ga_init(&ga, (int)sizeof(char *), 3);
+  // Loop over the items in the list.
+  TV_LIST_ITER_CONST(retlist, li, {
+    if (TV_LIST_ITEM_TV(li)->v_type != VAR_STRING
+        || TV_LIST_ITEM_TV(li)->vval.v_string == NULL) {
+      continue;  // Skip non-string items and empty strings.
+    }
+
+    GA_APPEND(char *, &ga, xstrdup((const char *)TV_LIST_ITEM_TV(li)->vval.v_string));
+  });
+  tv_list_unref(retlist);
+
+  *file = ga.ga_data;
+  *num_file = ga.ga_len;
+  return OK;
+}
+
 /// Expand color scheme, compiler or filetype names.
 /// Search from 'runtimepath':
 ///   'runtimepath'/{dirnames}/{pat}.vim
diff --git a/src/nvim/generators/gen_api_dispatch.lua b/src/nvim/generators/gen_api_dispatch.lua
index 21f8c3855e..c6dd25154b 100644
--- a/src/nvim/generators/gen_api_dispatch.lua
+++ b/src/nvim/generators/gen_api_dispatch.lua
@@ -441,8 +441,8 @@ local function process_function(fn)
     local cparam = string.format('arg%u', j)
     local param_type = real_type(param[1])
     local lc_param_type = real_type(param[1]):lower()
-    local extra = ((param_type == "Object" or param_type == "Dictionary") and "false, ") or ""
-    if param[1] == "DictionaryOf(LuaRef)" then
+    local extra = param_type == "Dictionary" and "false, " or ""
+    if param[1] == "Object" or param[1] == "DictionaryOf(LuaRef)" then
       extra = "true, "
     end
     local errshift = 0
diff --git a/src/nvim/generators/gen_keysets.lua b/src/nvim/generators/gen_keysets.lua
index 63ef202fe1..01d8c1d357 100644
--- a/src/nvim/generators/gen_keysets.lua
+++ b/src/nvim/generators/gen_keysets.lua
@@ -26,6 +26,17 @@ local defspipe = io.open(defs_file, 'wb')
 
 local keysets = require'api.keysets'
 
+local keywords = {
+  register = true,
+}
+
+local function sanitize(key)
+  if keywords[key] then
+    return key .. "_"
+  end
+  return key
+end
+
 for name, keys in pairs(keysets) do
   local neworder, hashfun = hashy.hashy_hash(name, keys, function (idx)
     return name.."_table["..idx.."].str"
@@ -33,7 +44,7 @@ for name, keys in pairs(keysets) do
 
   defspipe:write("typedef struct {\n")
   for _, key in ipairs(neworder) do
-    defspipe:write("  Object "..key..";\n")
+    defspipe:write("  Object "..sanitize(key)..";\n")
   end
   defspipe:write("} KeyDict_"..name..";\n\n")
 
@@ -41,7 +52,7 @@ for name, keys in pairs(keysets) do
 
   funcspipe:write("KeySetLink "..name.."_table[] = {\n")
   for _, key in ipairs(neworder) do
-    funcspipe:write('  {"'..key..'", offsetof(KeyDict_'..name..", "..key..")},\n")
+    funcspipe:write('  {"'..key..'", offsetof(KeyDict_'..name..", "..sanitize(key)..")},\n")
   end
     funcspipe:write('  {NULL, 0},\n')
   funcspipe:write("};\n\n")
diff --git a/src/nvim/lua/executor.c b/src/nvim/lua/executor.c
index 107ff22913..7d43d21d53 100644
--- a/src/nvim/lua/executor.c
+++ b/src/nvim/lua/executor.c
@@ -18,6 +18,7 @@
 #include "nvim/event/loop.h"
 #include "nvim/event/time.h"
 #include "nvim/ex_cmds2.h"
+#include "nvim/ex_docmd.h"
 #include "nvim/ex_getln.h"
 #include "nvim/extmark.h"
 #include "nvim/func_attr.h"
@@ -914,6 +915,24 @@ void nlua_typval_call(const char *str, size_t len, typval_T *const args, int arg
   }
 }
 
+void nlua_call_user_expand_func(expand_T *xp, typval_T *ret_tv)
+  FUNC_ATTR_NONNULL_ALL
+{
+  lua_State *const lstate = global_lstate;
+
+  nlua_pushref(lstate, xp->xp_luaref);
+  lua_pushstring(lstate, (char *)xp->xp_pattern);
+  lua_pushstring(lstate, (char *)xp->xp_line);
+  lua_pushinteger(lstate, xp->xp_col);
+
+  if (nlua_pcall(lstate, 3, 1)) {
+    nlua_error(lstate, _("E5108: Error executing Lua function: %.*s"));
+    return;
+  }
+
+  nlua_pop_typval(lstate, ret_tv);
+}
+
 static void nlua_typval_exec(const char *lcmd, size_t lcmd_len, const char *name,
                              typval_T *const args, int argcount, bool special, typval_T *ret_tv)
 {
@@ -1432,3 +1451,48 @@ void nlua_execute_on_key(int c)
 #endif
 }
 
+void nlua_do_ucmd(ucmd_T *cmd, exarg_T *eap)
+{
+  lua_State *const lstate = global_lstate;
+
+  nlua_pushref(lstate, cmd->uc_luaref);
+
+  lua_newtable(lstate);
+  lua_pushboolean(lstate, eap->forceit == 1);
+  lua_setfield(lstate, -2, "bang");
+
+  lua_pushinteger(lstate, eap->line1);
+  lua_setfield(lstate, -2, "line1");
+
+  lua_pushinteger(lstate, eap->line2);
+  lua_setfield(lstate, -2, "line2");
+
+  lua_pushstring(lstate, (const char *)eap->arg);
+  lua_setfield(lstate, -2, "args");
+
+  lua_pushstring(lstate, (const char *)&eap->regname);
+  lua_setfield(lstate, -2, "reg");
+
+  lua_pushinteger(lstate, eap->addr_count);
+  lua_setfield(lstate, -2, "range");
+
+  if (eap->addr_count > 0) {
+    lua_pushinteger(lstate, eap->line2);
+  } else {
+    lua_pushinteger(lstate, cmd->uc_def);
+  }
+  lua_setfield(lstate, -2, "count");
+
+  // The size of this buffer is chosen empirically to be large enough to hold
+  // every possible modifier (with room to spare). If the list of possible
+  // modifiers grows this may need to be updated.
+  char buf[200] = { 0 };
+  (void)uc_mods(buf);
+  lua_pushstring(lstate, buf);
+  lua_setfield(lstate, -2, "mods");
+
+  if (nlua_pcall(lstate, 1, 0)) {
+    nlua_error(lstate, _("Error executing Lua callback: %.*s"));
+  }
+}
+
diff --git a/src/nvim/lua/executor.h b/src/nvim/lua/executor.h
index a1f66bd02b..bf78f7ec5e 100644
--- a/src/nvim/lua/executor.h
+++ b/src/nvim/lua/executor.h
@@ -7,6 +7,7 @@
 #include "nvim/api/private/defs.h"
 #include "nvim/eval/typval.h"
 #include "nvim/ex_cmds_defs.h"
+#include "nvim/ex_docmd.h"
 #include "nvim/func_attr.h"
 #include "nvim/lua/converter.h"
 
diff --git a/src/nvim/vim.h b/src/nvim/vim.h
index 2f8ddd1e88..6e0e9922a6 100644
--- a/src/nvim/vim.h
+++ b/src/nvim/vim.h
@@ -143,6 +143,7 @@ enum {
   EXPAND_COMPILER,
   EXPAND_USER_DEFINED,
   EXPAND_USER_LIST,
+  EXPAND_USER_LUA,
   EXPAND_SHELLCMD,
   EXPAND_CSCOPE,
   EXPAND_SIGN,
diff --git a/test/functional/api/command_spec.lua b/test/functional/api/command_spec.lua
index 6f929ad1ca..6c2c136edc 100644
--- a/test/functional/api/command_spec.lua
+++ b/test/functional/api/command_spec.lua
@@ -6,8 +6,14 @@ local command = helpers.command
 local curbufmeths = helpers.curbufmeths
 local eq = helpers.eq
 local meths = helpers.meths
+local bufmeths = helpers.bufmeths
+local matches = helpers.matches
 local source = helpers.source
 local pcall_err = helpers.pcall_err
+local exec_lua = helpers.exec_lua
+local assert_alive = helpers.assert_alive
+local feed = helpers.feed
+local funcs = helpers.funcs
 
 describe('nvim_get_commands', function()
   local cmd_dict  = { addr=NIL, bang=false, bar=false, complete=NIL, complete_arg=NIL, count=NIL, definition='echo "Hello World"', name='Hello', nargs='1', range=NIL, register=false, script_id=0, }
@@ -78,3 +84,118 @@ describe('nvim_get_commands', function()
     eq({Cmd2=cmd2, Cmd3=cmd3, Cmd4=cmd4, Finger=cmd1, TestCmd=cmd0}, meths.get_commands({builtin=false}))
   end)
 end)
+
+describe('nvim_add_user_command', function()
+  before_each(clear)
+
+  it('works with strings', function()
+    meths.add_user_command('SomeCommand', 'let g:command_fired = <args>', {nargs = 1})
+    meths.command('SomeCommand 42')
+    eq(42, meths.eval('g:command_fired'))
+  end)
+
+  it('works with Lua functions', function()
+    exec_lua [[
+      result = {}
+      vim.api.nvim_add_user_command('CommandWithLuaCallback', function(opts)
+        result = opts
+      end, {
+        nargs = "*",
+        bang = true,
+        count = 2,
+      })
+    ]]
+
+    eq({
+      args = "hello",
+      bang = false,
+      line1 = 1,
+      line2 = 1,
+      mods = "",
+      range = 0,
+      count = 2,
+      reg = "",
+    }, exec_lua [[
+      vim.api.nvim_command('CommandWithLuaCallback hello')
+      return result
+    ]])
+
+    eq({
+      args = "",
+      bang = true,
+      line1 = 10,
+      line2 = 10,
+      mods = "botright",
+      range = 1,
+      count = 10,
+      reg = "",
+    }, exec_lua [[
+      vim.api.nvim_command('botright 10CommandWithLuaCallback!')
+      return result
+    ]])
+
+    eq({
+      args = "",
+      bang = false,
+      line1 = 1,
+      line2 = 42,
+      mods = "",
+      range = 1,
+      count = 42,
+      reg = "",
+    }, exec_lua [[
+      vim.api.nvim_command('CommandWithLuaCallback 42')
+      return result
+    ]])
+  end)
+
+  it('can define buffer-local commands', function()
+    local bufnr = meths.create_buf(false, false)
+    bufmeths.add_user_command(bufnr, "Hello", "", {})
+    matches("Not an editor command: Hello", pcall_err(meths.command, "Hello"))
+    meths.set_current_buf(bufnr)
+    meths.command("Hello")
+    assert_alive()
+  end)
+
+  it('can use a Lua complete function', function()
+    exec_lua [[
+      vim.api.nvim_add_user_command('Test', '', {
+        nargs = "*",
+        complete = function(arg, cmdline, pos)
+          local options = {"aaa", "bbb", "ccc"}
+          local t = {}
+          for _, v in ipairs(options) do
+            if string.find(v, "^" .. arg) then
+              table.insert(t, v)
+            end
+          end
+          return t
+        end,
+      })
+    ]]
+
+    feed(':Test a<Tab>')
+    eq('Test aaa', funcs.getcmdline())
+    feed('<C-U>Test b<Tab>')
+    eq('Test bbb', funcs.getcmdline())
+  end)
+end)
+
+describe('nvim_del_user_command', function()
+  before_each(clear)
+
+  it('can delete global commands', function()
+    meths.add_user_command('Hello', 'echo "Hi"', {})
+    meths.command('Hello')
+    meths.del_user_command('Hello')
+    matches("Not an editor command: Hello", pcall_err(meths.command, "Hello"))
+  end)
+
+  it('can delete buffer-local commands', function()
+    bufmeths.add_user_command(0, 'Hello', 'echo "Hi"', {})
+    meths.command('Hello')
+    bufmeths.del_user_command(0, 'Hello')
+    matches("Not an editor command: Hello", pcall_err(meths.command, "Hello"))
+  end)
+end)
diff --git a/test/functional/lua/api_spec.lua b/test/functional/lua/api_spec.lua
index 81e00bba6d..f6038e23fe 100644
--- a/test/functional/lua/api_spec.lua
+++ b/test/functional/lua/api_spec.lua
@@ -8,6 +8,7 @@ local clear = helpers.clear
 local eval = helpers.eval
 local NIL = helpers.NIL
 local eq = helpers.eq
+local exec_lua = helpers.exec_lua
 
 before_each(clear)
 
@@ -111,6 +112,12 @@ describe('luaeval(vim.api.…)', function()
     eq(7, eval([[type(luaeval('vim.api.nvim__id(nil)'))]]))
 
     eq({foo=1, bar={42, {{baz=true}, 5}}}, funcs.luaeval('vim.api.nvim__id({foo=1, bar={42, {{baz=true}, 5}}})'))
+
+    eq(true, funcs.luaeval('vim.api.nvim__id(vim.api.nvim__id)(true)'))
+    eq(42, exec_lua [[
+      local f = vim.api.nvim__id({42, vim.api.nvim__id})
+      return f[2](f[1])
+    ]])
   end)
 
   it('correctly converts container objects with type_idx to API objects', function()
@@ -159,12 +166,8 @@ describe('luaeval(vim.api.…)', function()
 
   it('errors out correctly when working with API', function()
     -- Conversion errors
-    eq('Vim(call):E5108: Error executing lua [string "luaeval()"]:1: Cannot convert given lua type',
-       remove_trace(exc_exec([[call luaeval("vim.api.nvim__id(vim.api.nvim__id)")]])))
     eq('Vim(call):E5108: Error executing lua [string "luaeval()"]:1: Cannot convert given lua table',
        remove_trace(exc_exec([[call luaeval("vim.api.nvim__id({1, foo=42})")]])))
-    eq('Vim(call):E5108: Error executing lua [string "luaeval()"]:1: Cannot convert given lua type',
-       remove_trace(exc_exec([[call luaeval("vim.api.nvim__id({42, vim.api.nvim__id})")]])))
     -- Errors in number of arguments
     eq('Vim(call):E5108: Error executing lua [string "luaeval()"]:1: Expected 1 argument',
        remove_trace(exc_exec([[call luaeval("vim.api.nvim__id()")]])))