wibox.widget.base: add __tostring method to widgets

Fixes https://github.com/awesomeWM/awesome/issues/337.
Closes https://github.com/awesomeWM/awesome/pull/341.

lib/gears/object.lua

@@ -112,6 +112,21 @@ function object.mt:__call(...)
     return new(...)
 end
 
+--- Helper function to get the module name out of `debug.getinfo`.
+-- @usage
+--  local mt = {}
+--  mt.__tostring = function(o)
+--      return require("gears.object").modulename(2)
+--  end
+--  return setmetatable(ret, mt)
+--
+-- @tparam[opt=2] integer level Level for `debug.getinfo(level, "S")`.
+--   Typically 2 or 3.
+-- @treturn string The module name, e.g. "wibox.widget.background".
+function object.modulename(level)
+    return debug.getinfo(level, "S").source:gsub(".*/lib/", ""):gsub("/", "."):gsub("%.lua", "")
+end
+
 return setmetatable(object, object.mt)
 
 -- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80

lib/wibox/widget/base.lua

@@ -64,7 +64,9 @@ end
 -- that the needed signals are added and mouse input handling is set up.
 -- @param proxy If this is set, the returned widget will be a proxy for this
 --              widget. It will be equivalent to this widget.
-function base.make_widget(proxy)
+-- @tparam[opt] string widget_name Name of the widget.  If not set, it will be
+--   set automatically via `gears.object.modulename`.
+function base.make_widget(proxy, widget_name)
     local ret = object()
 
     -- This signal is used by layouts to find out when they have to update.
@@ -101,7 +103,13 @@ function base.make_widget(proxy)
         ret._fit_geometry_cache = setmetatable({}, { __mode = 'v' })
     end)
 
-    return ret
+    -- Add __tostring method to metatable.
+    ret.widget_name = widget_name or object.modulename(3)
+    local mt = {}
+    mt.__tostring = function(o)
+        return ret.widget_name
+    end
+    return setmetatable(ret, mt)
 end
 
 --- Generate an empty widget which takes no space and displays nothing