Add ssz_derive crate.

It appears to be fully functional at this stage.

Cargo.toml

@@ -12,6 +12,7 @@ members = [
 	"eth2/utils/int_to_bytes",
 	"eth2/utils/slot_clock",
 	"eth2/utils/ssz",
+	"eth2/utils/ssz_derive",
 	"eth2/utils/swap_or_not_shuffle",
 	"eth2/utils/fisher_yates_shuffle",
 	"beacon_node",

eth2/types/Cargo.toml

@@ -18,6 +18,7 @@ serde_derive = "1.0"
 serde_json = "1.0"
 slog = "^2.2.3"
 ssz = { path = "../utils/ssz" }
+ssz_derive = { path = "../utils/ssz_derive" }
 swap_or_not_shuffle = { path = "../utils/swap_or_not_shuffle" }
 
 [dev-dependencies]

eth2/utils/ssz_derive/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "ssz_derive"
+version = "0.1.0"
+authors = ["Paul Hauner <paul@paulhauner.com>"]
+edition = "2018"
+description = "Procedural derive macros for SSZ encoding and decoding."
+
+[lib]
+proc-macro = true
+
+[dependencies]
+syn = "0.15"
+quote = "0.6"
+ssz = { path = "../ssz" }

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()
+}