Different XCM builders, default one requires fee payment (#2253)

Adding on top of the new builder pattern for creating XCM programs, I'm
adding some more APIs:

```rust
let paying_fees: Xcm<()> = Xcm::builder() // Only allow paying for fees
  .withdraw_asset() // First instruction has to load the holding register
  .buy_execution() // Second instruction has to be `buy_execution`
  .build();

let paying_fees_invalid: Xcm<()> = Xcm::builder()
  .withdraw_asset()
  .build(); // Invalid, need to pay for fees

let not_paying_fees: Xcm<()> = Xcm::builder_unpaid()
  .unpaid_execution() // Needed
  .withdraw_asset()
  .deposit_asset()
  .build();

let all_goes: Xcm<()> = Xcm::builder_unsafe() // You can do anything
  .withdraw_asset()
  .deposit_asset()
  .build();
```

The invalid bits are because the methods don't even exist on the types
that you'd want to call them on.

---------

Co-authored-by: command-bot <>

polkadot/xcm/procedural/tests/builder_pattern.rs

@@ -0,0 +1,81 @@
+// Copyright (C) Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Polkadot is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Test the methods generated by the Builder derive macro.
+//! Tests directly on the actual Xcm struct and Instruction enum.
+
+use xcm::latest::prelude::*;
+
+#[test]
+fn builder_pattern_works() {
+	let asset: MultiAsset = (Here, 100u128).into();
+	let beneficiary: MultiLocation = AccountId32 { id: [0u8; 32], network: None }.into();
+	let message: Xcm<()> = Xcm::builder()
+		.receive_teleported_asset(asset.clone().into())
+		.buy_execution(asset.clone(), Unlimited)
+		.deposit_asset(asset.clone().into(), beneficiary)
+		.build();
+	assert_eq!(
+		message,
+		Xcm(vec![
+			ReceiveTeleportedAsset(asset.clone().into()),
+			BuyExecution { fees: asset.clone(), weight_limit: Unlimited },
+			DepositAsset { assets: asset.into(), beneficiary },
+		])
+	);
+}
+
+#[test]
+fn default_builder_requires_buy_execution() {
+	let asset: MultiAsset = (Here, 100u128).into();
+	let beneficiary: MultiLocation = AccountId32 { id: [0u8; 32], network: None }.into();
+	// This is invalid, since it doesn't pay for fees.
+	// This is enforced by the runtime, because the build() method doesn't exist
+	// on the resulting type.
+	// let message: Xcm<()> = Xcm::builder()
+	//     .withdraw_asset(asset.clone().into())
+	//     .deposit_asset(asset.into(), beneficiary)
+	//     .build();
+
+	// To be able to do that, we need to use the explicitly unpaid variant
+	let message: Xcm<()> = Xcm::builder_unpaid()
+		.unpaid_execution(Unlimited, None)
+		.withdraw_asset(asset.clone().into())
+		.deposit_asset(asset.clone().into(), beneficiary)
+		.build(); // This works
+	assert_eq!(
+		message,
+		Xcm(vec![
+			UnpaidExecution { weight_limit: Unlimited, check_origin: None },
+			WithdrawAsset(asset.clone().into()),
+			DepositAsset { assets: asset.clone().into(), beneficiary },
+		])
+	);
+
+	// The other option doesn't have any limits whatsoever, so it should
+	// only be used when you really know what you're doing.
+	let message: Xcm<()> = Xcm::builder_unsafe()
+		.withdraw_asset(asset.clone().into())
+		.deposit_asset(asset.clone().into(), beneficiary)
+		.build();
+	assert_eq!(
+		message,
+		Xcm(vec![
+			WithdrawAsset(asset.clone().into()),
+			DepositAsset { assets: asset.clone().into(), beneficiary },
+		])
+	);
+}