From ce457a91e1530df227086081597869409977ddc7 Mon Sep 17 00:00:00 2001
From: Gavin Wood <gavin@parity.io>
Date: Thu, 29 Apr 2021 15:35:07 +0200
Subject: [PATCH] Implement null weight trader (#2957)

---
 polkadot/xcm/src/v0/traits.rs                  | 1 +
 polkadot/xcm/xcm-executor/src/traits/weight.rs | 7 +++++++
 2 files changed, 8 insertions(+)

diff --git a/polkadot/xcm/src/v0/traits.rs b/polkadot/xcm/src/v0/traits.rs
index 8734bb6c7f..f0df7db03e 100644
--- a/polkadot/xcm/src/v0/traits.rs
+++ b/polkadot/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,
diff --git a/polkadot/xcm/xcm-executor/src/traits/weight.rs b/polkadot/xcm/xcm-executor/src/traits/weight.rs
index 1d6d5eeec4..5fd8963090 100644
--- a/polkadot/xcm/xcm-executor/src/traits/weight.rs
+++ b/polkadot/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)
+	}
+}