XCM builder pattern

polkadot/xcm/procedural/src/builder_pattern.rs

@@ -0,0 +1,95 @@
+use proc_macro::TokenStream;
+use proc_macro2::TokenStream as TokenStream2;
+use syn::{parse_macro_input, DeriveInput, Data, Error, Fields, Meta, Expr, Lit};
+use quote::{quote, format_ident};
+use inflector::Inflector;
+
+pub fn derive(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+	let builder_impl = match &input.data {
+		Data::Enum(data_enum) => generate_methods_for_enum(input.ident, data_enum),
+		_ => return Error::new_spanned(&input, "Expected the `Instruction` enum")
+			.to_compile_error().into(),
+	};
+	let output = quote! {
+		pub struct XcmBuilder<Call>(Vec<Instruction<Call>>);
+		impl<Call> Xcm<Call> {
+			pub fn builder() -> XcmBuilder<Call> {
+				XcmBuilder::<Call>(Vec::new())
+			}
+		}
+		#builder_impl
+	};
+	output.into()
+}
+
+fn generate_methods_for_enum(name: syn::Ident, data_enum: &syn::DataEnum) -> TokenStream2 {
+	let methods = data_enum.variants.iter().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: Vec<_> = variant.attrs.iter().filter_map(|attr| {
+			if attr.path().is_ident("doc") {
+				match &attr.meta {
+					Meta::NameValue(pair) => match &pair.value {
+						Expr::Lit(inner) => match &inner.lit {
+							Lit::Str(string_literal) => Some(string_literal.value()),
+							_ => None,
+						},
+						_ => None,
+					},
+					_ => None,
+				}
+			} else {
+				None
+			}
+		}).map(|doc| syn::parse_str::<TokenStream2>(&format!("/// {}", doc)).unwrap()).collect();
+		let method = match &variant.fields {
+			Fields::Unit => {
+				quote! {
+					pub fn #method_name(mut self) -> Self {
+						self.0.push(#name::<Call>::#variant_name);
+						self
+					}
+				}
+			},
+			Fields::Unnamed(fields) => {
+				let arg_names: Vec<_> = fields.unnamed.iter().enumerate()
+					.map(|(index, _)| format_ident!("arg{}", index))
+					.collect();
+				let arg_types: Vec<_> = fields.unnamed.iter().map(|field| &field.ty)
+					.collect();
+				quote! {
+					pub fn #method_name(mut self, #(#arg_names: #arg_types),*) -> Self {
+						self.0.push(#name::<Call>::#variant_name(#(#arg_names),*));
+						self
+					}
+				}
+			},
+			Fields::Named(fields) => {
+	            let arg_names: Vec<_> = fields.named.iter().map(|field| &field.ident).collect();
+                let arg_types: Vec<_> = fields.named.iter().map(|field| &field.ty).collect();
+				quote! {
+					pub fn #method_name(mut self, #(#arg_names: #arg_types),*) -> Self {
+						self.0.push(#name::<Call>::#variant_name { #(#arg_names),* });
+						self
+					}
+				}
+			},
+		};
+		quote! {
+			#(#docs)*
+			#method
+		}
+	});
+	let output = quote! {
+		impl<Call> XcmBuilder<Call> {
+			#(#methods)*
+
+			pub fn build(self) -> Xcm<Call> {
+				Xcm(self.0)
+			}
+		}
+	};
+	output
+}

polkadot/xcm/procedural/src/lib.rs

@@ -21,6 +21,7 @@ use proc_macro::TokenStream;
 mod v2;
 mod v3;
 mod weight_info;
+mod builder_pattern;
 
 #[proc_macro]
 pub fn impl_conversion_functions_for_multilocation_v2(input: TokenStream) -> TokenStream {
@@ -47,3 +48,15 @@ pub fn impl_conversion_functions_for_junctions_v3(input: TokenStream) -> TokenSt
 		.unwrap_or_else(syn::Error::into_compile_error)
 		.into()
 }
+
+/// This is called on the `Instruction` enum, not on the `Xcm` struct,
+/// and allows for the following syntax for building XCMs:
+/// let message = Xcm::builder()
+/// 	.withdraw_asset(assets)
+/// 	.buy_execution(fees, weight_limit)
+/// 	.deposit_asset(assets, beneficiary)
+/// 	.build();
+#[proc_macro_derive(Builder)]
+pub fn derive_builder(input: TokenStream) -> TokenStream {
+	builder_pattern::derive(input)
+}

polkadot/xcm/src/v3/mod.rs

@@ -401,7 +401,7 @@ impl XcmContext {
 ///
 /// This is the inner XCM format and is version-sensitive. Messages are typically passed using the
 /// outer XCM format, known as `VersionedXcm`.
-#[derive(Derivative, Encode, Decode, TypeInfo, xcm_procedural::XcmWeightInfoTrait)]
+#[derive(Derivative, Encode, Decode, TypeInfo, xcm_procedural::XcmWeightInfoTrait, xcm_procedural::Builder)]
 #[derivative(Clone(bound = ""), Eq(bound = ""), PartialEq(bound = ""), Debug(bound = ""))]
 #[codec(encode_bound())]
 #[codec(decode_bound())]

polkadot/xcm/xcm-simulator/example/src/lib.rs

@@ -649,4 +649,20 @@ mod tests {
 			);
 		});
 	}
+
+	#[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()
+			.withdraw_asset(asset.clone().into())
+			.buy_execution(asset.clone(), Unlimited)
+			.deposit_asset(asset.clone().into(), beneficiary)
+			.build();
+		assert_eq!(message, Xcm(vec![
+			WithdrawAsset(asset.clone().into()),
+			BuyExecution { fees: asset.clone(), weight_limit: Unlimited },
+			DepositAsset { assets: asset.into(), beneficiary },
+		]));
+	}
 }