@ivan-shrimp Hi, thank you for contributing. Is this still a draft or done?

This PR is complete as is. Feel free to merge this first.

I've marked this as draft because I'd like to fix the **-unpacking case as well; however I haven't got the time to fix that recently. The crux of that problem lies in two parts:

• We currently treat {**x} the same as dict.update:

  RustPython/vm/src/frame.rs

  Lines 754 to 762 in 2a41072

  	bytecode::Instruction::DictUpdate => {
  	let other = self.pop_value();
  	let dict = self
  	.top_value()
  	.downcast_ref::<PyDict>()
  	.expect("exact dict expected");
  	dict.merge_object(other, vm)?;
  	Ok(None)
  	}

  This means we evaluate {**[[1, 2]]} to {1: 2} when we should emit an error instead.

• We currently assume that the x in f(**x) inherits from and behaves exactly like a dict:

  RustPython/vm/src/frame.rs

  Lines 1414 to 1437 in 2a41072

  	fn execute_build_map_for_call(&mut self, vm: &VirtualMachine, size: u32) -> FrameResult {
  	let size = size as usize;
  	let map_obj = vm.ctx.new_dict();
  	for obj in self.pop_multiple(size) {
  	// Take all key-value pairs from the dict:
  	let dict: PyDictRef = obj.downcast().map_err(|obj| {
  	vm.new_type_error(format!("'{}' object is not a mapping", obj.class().name()))
  	})?;
  	for (key, value) in dict {
  	if map_obj.contains_key(&*key, vm) {
  	let key_repr = &key.repr(vm)?;
  	let msg = format!(
  	"got multiple values for keyword argument {}",
  	key_repr.as_str()
  	);
  	return Err(vm.new_type_error(msg));
  	}
  	map_obj.set_item(&*key, value, vm)?;
  	}
  	}
  	self.push_value(map_obj.into());
  	Ok(None)
  	}

  This means custom mappings and OrderedDict (which inherits from dict but maintains a separate internal ordering) will not work. A corresponding test case is here:

  RustPython/Lib/test/test_call.py

  Lines 16 to 29 in 2a41072

  	# TODO: RUSTPYTHON
  	@unittest.expectedFailure
  	def test_kwargs_order(self):
  	# bpo-34320: **kwargs should preserve order of passed OrderedDict
  	od = collections.OrderedDict([('a', 1), ('b', 2)])
  	od.move_to_end('a')
  	expected = list(od.items())
  	def fn(**kw):
  	return kw
  	res = fn(**od)
  	self.assertIsInstance(res, dict)
  	self.assertEqual(list(res.items()), expected)

In both cases, we should instead perform some "update using mapping" operation. Also, since the unpacking process is externally observable (evaluation order; and for calls, how non-str keys are rejected, and how duplicate keys are rejected), there is almost no alternative to effectively copying from CPython.

Feel free to take this up; I won't have the time in the very near future, and for myself it's easy to work around this.