added test_random_derive implementation (lighthouse-246)

eth2/utils/test_random_derive/src/lib.rs

@@ -2,14 +2,42 @@ extern crate proc_macro;
 
 use crate::proc_macro::TokenStream;
 use quote::quote;
-use syn;
-use syn::DeriveInput;
+use syn::{parse_macro_input, DeriveInput};
 
 #[proc_macro_derive(TestRandom)]
 pub fn test_random_derive(input: TokenStream) -> TokenStream {
-    let ast = syn::parse(input).unwrap();
+    let ast = parse_macro_input!(input as DeriveInput);
+    let name = &ast.ident;
 
-    impl_test_random(&ast)
+    let struct_data = match &ast.data {
+        syn::Data::Struct(s) => s,
+        _ => panic!("test_random_derive only supports structs."),
+    };
+
+    let field_names = get_named_field_idents_and_types(&struct_data);
+
+    let output = quote! {
+        impl<T: RngCore> TestRandom<T> for #name {
+            fn random_for_test(rng: &mut T) -> Self {
+               Self {
+                    #(
+                        #field_names: <_>::random_for_test(rng)
+                    )*
+               }
+            }
+        }
+    };
+
+    output.into()
 }
 
-fn impl_test_random(ast: &DeriveInput) -> TokenStream {}
+fn get_named_field_idents_and_types(struct_data: &syn::DataStruct) -> Vec<(&syn::Ident)> {
+    struct_data
+        .fields
+        .iter()
+        .map(|f| match &f.ident {
+            Some(ref ident) => ident,
+            _ => panic!("test_random_derive only supports named struct fields."),
+        })
+        .collect()
+}