Make it possible to inject tab via cmdutils.Value

doc/contributing.asciidoc

@@ -483,6 +483,7 @@ The following arguments are supported for `@cmdutils.argument`:
 - `value`: Tell qutebrowser to fill the argument with special values:
    - `value=cmdutils.Value.count`: The `count` given by the user to the command.
    - `value=cmdutils.Value.win_id`: The window ID of the current window.
+   - `value=cmdutils.Value.tab`: The tab object which is currently focused.
 - `completion`: A completion function (see `qutebrowser.completions.models.*`)
   to use when completing arguments for the given command.
 - `choices`: The allowed string choices for the argument.

qutebrowser/commands/command.py

@@ -193,7 +193,7 @@ class Command:
                 raise TypeError("{}: handler has count parameter "
                                 "without default!".format(self.name))
             return True
-        elif arg_info.value == usertypes.CommandValue.win_id:
+        elif isinstance(arg_info.value, usertypes.CommandValue):
             return True
         else:
             raise TypeError("{}: Invalid value={!r} for argument '{}'!"
@@ -441,6 +441,11 @@ class Command:
                 self._add_special_arg(value=win_id, param=param,
                                       args=args, kwargs=kwargs)
                 continue
+            elif arg_info.value == usertypes.CommandValue.tab:
+                tab = self._get_objreg(win_id=win_id, name='tab', scope='tab')
+                self._add_special_arg(value=tab, param=param,
+                                      args=args, kwargs=kwargs)
+                continue
             elif arg_info.value is None:
                 pass
             else:

qutebrowser/utils/usertypes.py

@@ -262,6 +262,7 @@ class CommandValue(enum.Enum):
 
     count = 1
     win_id = 2
+    tab = 3
 
 
 class Question(QObject):