Use derive macro to generate XcmWeightInfo trait (#4618)

* Use derive macro to generate XcmWeightInfo trait

* Move the Weight type alias out of proc macro

polkadot/xcm/procedural/Cargo.toml

@@ -11,3 +11,4 @@ proc-macro = true
 proc-macro2 = "1.0.36"
 quote = "1.0.10"
 syn = "1.0.83"
+Inflector = "0.11.4"

polkadot/xcm/procedural/src/lib.rs

@@ -20,6 +20,7 @@ use proc_macro::TokenStream;
 
 mod v0;
 mod v1;
+mod weight_info;
 
 #[proc_macro]
 pub fn impl_conversion_functions_for_multilocation_v0(input: TokenStream) -> TokenStream {
@@ -34,3 +35,8 @@ pub fn impl_conversion_functions_for_multilocation_v1(input: TokenStream) -> Tok
 		.unwrap_or_else(syn::Error::into_compile_error)
 		.into()
 }
+
+#[proc_macro_derive(XcmWeightInfoTrait)]
+pub fn derive_xcm_weight_info(item: TokenStream) -> TokenStream {
+	weight_info::derive(item)
+}

polkadot/xcm/procedural/src/weight_info.rs

@@ -0,0 +1,67 @@
+// Copyright 2021 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/>.
+
+use inflector::Inflector;
+use quote::format_ident;
+
+pub fn derive(item: proc_macro::TokenStream) -> proc_macro::TokenStream {
+	let input: syn::DeriveInput = match syn::parse(item) {
+		Ok(input) => input,
+		Err(e) => return e.into_compile_error().into(),
+	};
+
+	let syn::DeriveInput { generics, data, .. } = input;
+
+	match data {
+		syn::Data::Enum(syn::DataEnum { variants, .. }) => {
+			let methods = variants.into_iter().map(|syn::Variant { ident, fields, .. }| {
+				let snake_cased_ident = format_ident!("{}", ident.to_string().to_snake_case());
+				let ref_fields =
+					fields.into_iter().enumerate().map(|(idx, syn::Field { ident, ty, .. })| {
+						let field_name = ident.unwrap_or_else(|| format_ident!("_{}", idx));
+						let field_ty = match ty {
+							syn::Type::Reference(r) => {
+								// If the type is already a reference, do nothing
+								quote::quote!(#r)
+							},
+							t => {
+								// Otherwise, make it a reference
+								quote::quote!(&#t)
+							},
+						};
+
+						quote::quote!(#field_name: #field_ty,)
+					});
+				quote::quote!(fn #snake_cased_ident( #(#ref_fields)* ) -> Weight;)
+			});
+
+			let res = quote::quote! {
+				pub trait XcmWeightInfo #generics {
+					#(#methods)*
+				}
+			};
+			res.into()
+		},
+		syn::Data::Struct(syn::DataStruct { struct_token, .. }) => {
+			let msg = "structs are not supported by 'derive(XcmWeightInfo)'";
+			syn::Error::new(struct_token.span, msg).into_compile_error().into()
+		},
+		syn::Data::Union(syn::DataUnion { union_token, .. }) => {
+			let msg = "unions are not supported by 'derive(XcmWeightInfo)'";
+			syn::Error::new(union_token.span, msg).into_compile_error().into()
+		},
+	}
+}