optimise **kwargs copying away when it's not being used

Cython/Compiler/Nodes.py

@@ -3380,7 +3380,7 @@ class DefNodeWrapper(FuncDefNode):
             code.putln("}")
 
         if self.starstar_arg:
-            if self.star_arg:
+            if self.star_arg or not self.starstar_arg.entry.cf_used:
                 kwarg_check = "unlikely(%s)" % Naming.kwds_cname
             else:
                 kwarg_check = "%s" % Naming.kwds_cname
@@ -3394,9 +3394,8 @@ class DefNodeWrapper(FuncDefNode):
                 kwarg_check, Naming.kwds_cname, self.name,
                 bool(self.starstar_arg), self.error_value()))
 
-        if self.starstar_arg:
-            allow_null = all(ref.node.allow_null for ref in self.starstar_arg.entry.cf_references)
-            if allow_null:
+        if self.starstar_arg and self.starstar_arg.entry.cf_used:
+            if all(ref.node.allow_null for ref in self.starstar_arg.entry.cf_references):
                 code.putln("if (%s) {" % kwarg_check)
                 code.putln("%s = PyDict_Copy(%s); if (unlikely(!%s)) return %s;" % (
                         self.starstar_arg.entry.cname,
@@ -3405,12 +3404,11 @@ class DefNodeWrapper(FuncDefNode):
                         self.error_value()))
                 code.put_gotref(self.starstar_arg.entry.cname)
                 code.putln("} else {")
-                code.putln("%s = NULL;" % (
-                    self.starstar_arg.entry.cname,))
+                code.putln("%s = NULL;" % (self.starstar_arg.entry.cname,))
                 code.putln("}")
                 self.starstar_arg.entry.xdecref_cleanup = 1
             else:
-                code.putln("%s = (%s) ? PyDict_Copy(%s) : PyDict_New();" % (
+                code.put("%s = (%s) ? PyDict_Copy(%s) : PyDict_New(); " % (
                         self.starstar_arg.entry.cname,
                         Naming.kwds_cname,
                         Naming.kwds_cname))

tests/run/kwargs_passthrough.pyx

@@ -19,6 +19,48 @@ def wrap_passthrough(f):
     return wrapper
 
 
+@cython.test_fail_if_path_exists('//KeywordArgsNode')
+def unused(*args, **kwargs):
+    """
+    >>> unused()
+    ()
+    >>> unused(1, 2)
+    (1, 2)
+    """
+    return args
+
+
+@cython.test_fail_if_path_exists('//KeywordArgsNode')
+def used_in_closure(**kwargs):
+    """
+    >>> used_in_closure()
+    >>> d = {}
+    >>> used_in_closure(**d)
+    >>> d  # must not be modified
+    {}
+    """
+    def func():
+        kwargs['test'] = 1
+    return func()
+
+
+@cython.test_fail_if_path_exists('//KeywordArgsNode')
+def modify_in_closure(**kwargs):
+    """
+    >>> func = modify_in_closure()
+    >>> func()
+
+    >>> d = {}
+    >>> func = modify_in_closure(**d)
+    >>> func()
+    >>> d  # must not be modified
+    {}
+    """
+    def func():
+        kwargs['test'] = 1
+    return func
+
+
 @cython.test_assert_path_exists('//KeywordArgsNode')
 def wrap_passthrough_more(f):
     """