allow clear_origin in safe xcm builder (paritytech#4777)

Fixes paritytech#3770 

Added `clear_origin` as an allowed command after commands that load the
holdings register, in the safe xcm builder.

Checklist
- [x] My PR includes a detailed description as outlined in the
"Description" section above
- [x] My PR follows the [labeling
requirements](https://github.com/paritytech/polkadot-sdk/blob/master/docs/contributor/CONTRIBUTING.md#Process)
of this project (at minimum one label for T required)
- [x] I have made corresponding changes to the documentation (if
applicable)
- [x] I have added tests that prove my fix is effective or that my
feature works (if applicable)

---------

Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: gupnik <mail.guptanikhil@gmail.com>

polkadot/xcm/procedural/src/builder_pattern.rs

@@ -233,6 +233,32 @@ fn generate_builder_impl(name: &Ident, data_enum: &DataEnum) -> Result<TokenStre
 		}
 	};
 
+	// Some operations are allowed after the holding register is loaded
+	let allowed_after_load_holding_methods: Vec<TokenStream2> = data_enum
+		.variants
+		.iter()
+		.filter(|variant| variant.ident == "ClearOrigin")
+		.map(|variant| {
+			let variant_name = &variant.ident;
+			let method_name_string = &variant_name.to_string().to_snake_case();
+			let method_name = syn::Ident::new(method_name_string, variant_name.span());
+			let docs = get_doc_comments(variant);
+			let method = match &variant.fields {
+				Fields::Unit => {
+					quote! {
+						#(#docs)*
+						pub fn #method_name(mut self) -> XcmBuilder<Call, LoadedHolding> {
+							self.instructions.push(#name::<Call>::#variant_name);
+							self
+						}
+					}
+				},
+				_ => return Err(Error::new_spanned(variant, "ClearOrigin should have no fields")),
+			};
+			Ok(method)
+		})
+		.collect::<std::result::Result<Vec<_>, _>>()?;
+
 	// Then we require fees to be paid
 	let buy_execution_method = data_enum
 		.variants
@@ -276,6 +302,7 @@ fn generate_builder_impl(name: &Ident, data_enum: &DataEnum) -> Result<TokenStre
 
 	let second_impl = quote! {
 		impl<Call> XcmBuilder<Call, LoadedHolding> {
+			#(#allowed_after_load_holding_methods)*
 			#buy_execution_method
 		}
 	};

polkadot/xcm/procedural/tests/builder_pattern.rs

@@ -79,3 +79,24 @@ fn default_builder_requires_buy_execution() {
 		])
 	);
 }
+
+#[test]
+fn default_builder_allows_clear_origin_before_buy_execution() {
+	let asset: Asset = (Here, 100u128).into();
+	let beneficiary: Location = [0u8; 32].into();
+	let message: Xcm<()> = Xcm::builder()
+		.receive_teleported_asset(asset.clone())
+		.clear_origin()
+		.buy_execution(asset.clone(), Unlimited)
+		.deposit_asset(asset.clone(), beneficiary.clone())
+		.build();
+	assert_eq!(
+		message,
+		Xcm(vec![
+			ReceiveTeleportedAsset(asset.clone().into()),
+			ClearOrigin,
+			BuyExecution { fees: asset.clone(), weight_limit: Unlimited },
+			DepositAsset { assets: asset.into(), beneficiary },
+		])
+	);
+}

prdoc/pr_4777.prdoc

@@ -0,0 +1,27 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: XCM builder pattern allows clear_origin before buy_execution.
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Added clear_origin as an allowed command after commands that load the holdings register, in the safe xcm builder.
+      Previously, although it's logically allowed, an XCM could not be built like this:
+      ```rust
+      let xcm = Xcm::builder()
+          .withdraw_asset((Parent, 100u128))
+          .clear_origin()
+          .buy_execution((Parent, 1u128))
+          .deposit_asset(All, [0u8; 32])
+          .build();
+      ```
+      You had to use the unsafe_builder.
+      Now, it's allowed using the default builder.
+
+crates: 
+- name: "xcm-procedural"
+  bump: minor
+- name: "staging-xcm"
+  bump: minor
+