Implement null weight trader (#2957)

xcm/src/v0/traits.rs

@@ -25,6 +25,7 @@ use super::{MultiLocation, Xcm};
 pub enum Error {
 	Undefined,
 	Overflow,
+	/// The operation is intentionally unsupported.
 	Unimplemented,
 	UnhandledXcmVersion,
 	UnhandledXcmMessage,

xcm/xcm-executor/src/traits/weight.rs

@@ -61,3 +61,10 @@ pub trait WeightTrader: Sized {
 	/// Default implementation refunds nothing.
 	fn refund_weight(&mut self, _weight: Weight) -> MultiAsset { MultiAsset::None }
 }
+
+impl WeightTrader for () {
+	fn new() -> Self { () }
+	fn buy_weight(&mut self, _: Weight, _: Assets) -> Result<Assets, Error> {
+		Err(Error::Unimplemented)
+	}
+}