Add ssz_derive crate.

It appears to be fully functional at this stage.

eth2/utils/ssz_derive/src/lib.rs

@@ -0,0 +1,80 @@
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{parse_macro_input, DeriveInput};
+
+fn get_named_field_idents<'a>(struct_data: &'a syn::DataStruct) -> Vec<&'a syn::Ident> {
+    struct_data.fields.iter().map(|f| {
+        match &f.ident {
+            Some(ref ident) => ident,
+            _ => panic!("ssz_derive only supports named struct fields.")
+        }
+    }).collect()
+}
+
+#[proc_macro_derive(Encode)]
+pub fn ssz_encode_derive(input: TokenStream) -> TokenStream {
+    let item = parse_macro_input!(input as DeriveInput);
+
+    let name = &item.ident;
+
+    let struct_data = match &item.data {
+        syn::Data::Struct(s) => s,
+        _ => panic!("ssz_derive only supports structs.")
+    };
+
+    let field_idents = get_named_field_idents(&struct_data);
+
+    let output = quote! {
+        impl Encodable for #name {
+            fn ssz_append(&self, s: &mut SszStream) {
+                #(
+                    s.append(&self.#field_idents);
+                )*
+            }
+        }
+    };
+    output.into()
+}
+
+#[proc_macro_derive(Decode)]
+pub fn ssz_decode_derive(input: TokenStream) -> TokenStream {
+    let item = parse_macro_input!(input as DeriveInput);
+
+    let name = &item.ident;
+
+    let struct_data = match &item.data {
+        syn::Data::Struct(s) => s,
+        _ => panic!("ssz_derive only supports structs.")
+    };
+
+    let field_idents = get_named_field_idents(&struct_data);
+
+    // Using a var in an iteration always consumes the var, therefore we must make a `fields_a` and
+    // a `fields_b` in order to perform two loops.
+    //
+    // https://github.com/dtolnay/quote/issues/8
+    let field_idents_a = &field_idents;
+    let field_idents_b = &field_idents;
+
+    let output = quote! {
+        impl Decodable for #name {
+            fn ssz_decode(bytes: &[u8], i: usize) -> Result<(Self, usize), DecodeError> {
+                #(
+                    let (#field_idents_a, i) = <_>::ssz_decode(bytes, i)?;
+                )*
+
+                Ok((
+                    Self {
+                        #(
+                            #field_idents_b,
+                        )*
+                    },
+                    i
+                ))
+            }
+        }
+    };
+    output.into()
+}