Add macros for easy SSZ impl via From trait

eth2/utils/ssz/src/lib.rs

@@ -18,3 +18,90 @@ where
 {
     val.as_ssz_bytes()
 }
+
+#[macro_export]
+macro_rules! impl_encode_via_from {
+    ($impl_type: ty, $from_type: ty) => {
+        impl Encodable for $impl_type {
+            fn is_ssz_fixed_len() -> bool {
+                <$from_type as Encodable>::is_ssz_fixed_len()
+            }
+
+            fn ssz_fixed_len() -> usize {
+                <$from_type as Encodable>::ssz_fixed_len()
+            }
+
+            fn ssz_append(&self, buf: &mut Vec<u8>) {
+                let conv: $from_type = self.clone().into();
+
+                conv.ssz_append(buf)
+            }
+        }
+    };
+}
+
+#[macro_export]
+macro_rules! impl_decode_via_from {
+    ($impl_type: ty, $from_type: tt) => {
+        impl Decodable for $impl_type {
+            fn is_ssz_fixed_len() -> bool {
+                <$from_type as Decodable>::is_ssz_fixed_len()
+            }
+
+            fn ssz_fixed_len() -> usize {
+                <$from_type as Decodable>::ssz_fixed_len()
+            }
+
+            fn from_ssz_bytes(bytes: &[u8]) -> Result<Self, ssz::DecodeError> {
+                $from_type::from_ssz_bytes(bytes).and_then(|dec| Ok(dec.into()))
+            }
+        }
+    };
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate as ssz;
+
+    #[derive(PartialEq, Debug, Clone, Copy)]
+    struct Wrapper(u64);
+
+    impl From<u64> for Wrapper {
+        fn from(x: u64) -> Wrapper {
+            Wrapper(x)
+        }
+    }
+
+    impl From<Wrapper> for u64 {
+        fn from(x: Wrapper) -> u64 {
+            x.0
+        }
+    }
+
+    impl_encode_via_from!(Wrapper, u64);
+    impl_decode_via_from!(Wrapper, u64);
+
+    #[test]
+    fn impl_encode_via_from() {
+        let check_encode = |a: u64, b: Wrapper| assert_eq!(a.as_ssz_bytes(), b.as_ssz_bytes());
+
+        check_encode(0, Wrapper(0));
+        check_encode(1, Wrapper(1));
+        check_encode(42, Wrapper(42));
+    }
+
+    #[test]
+    fn impl_decode_via_from() {
+        let check_decode = |bytes: Vec<u8>| {
+            let a = u64::from_ssz_bytes(&bytes).unwrap();
+            let b = Wrapper::from_ssz_bytes(&bytes).unwrap();
+
+            assert_eq!(a, b.into())
+        };
+
+        check_decode(vec![0, 0, 0, 0, 0, 0, 0, 0]);
+        check_decode(vec![1, 0, 0, 0, 0, 0, 0, 0]);
+        check_decode(vec![1, 0, 0, 0, 2, 0, 0, 0]);
+    }
+}