Add new CompareFields trait and derive

eth2/utils/compare_fields_derive/src/lib.rs

@@ -0,0 +1,58 @@
+#![recursion_limit = "256"]
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{parse_macro_input, DeriveInput};
+
+#[proc_macro_derive(CompareFields)]
+pub fn compare_fields_derive(input: TokenStream) -> TokenStream {
+    let item = parse_macro_input!(input as DeriveInput);
+
+    let name = &item.ident;
+    let (impl_generics, ty_generics, where_clause) = &item.generics.split_for_impl();
+
+    let struct_data = match &item.data {
+        syn::Data::Struct(s) => s,
+        _ => panic!("compare_fields_derive only supports structs."),
+    };
+
+    let mut idents_a = vec![];
+    let mut field_names = vec![];
+
+    for field in struct_data.fields.iter() {
+        let ident = match &field.ident {
+            Some(ref ident) => ident,
+            _ => panic!("compare_fields_derive only supports named struct fields."),
+        };
+
+        field_names.push(format!("{:}", ident));
+        idents_a.push(ident);
+    }
+
+    let idents_b = idents_a.clone();
+    let idents_c = idents_a.clone();
+    let idents_d = idents_a.clone();
+
+    let output = quote! {
+        impl #impl_generics compare_fields::CompareFields for #name #ty_generics #where_clause {
+            fn compare_fields(&self, b: &Self) -> Vec<compare_fields::FieldComparison> {
+                let mut comparisons = vec![];
+
+                #(
+                    comparisons.push(
+                        compare_fields::FieldComparison {
+                            equal: self.#idents_a == b.#idents_b,
+                            field_name: #field_names.to_string(),
+                            a: format!("{:?}", self.#idents_c),
+                            b: format!("{:?}", b.#idents_d),
+                        }
+                    );
+                )*
+
+                comparisons
+            }
+        }
+    };
+    output.into()
+}