[metadata] add 0x prefix to hex string if not present (#522)

* Add 0x prefix if missing

* Fmt

crates/metadata/src/layout/mod.rs

@@ -17,7 +17,10 @@ mod tests;
 
 use crate::{
     serde_hex,
-    utils::serialize_as_byte_str,
+    utils::{
+        deserialize_from_byte_str,
+        serialize_as_byte_str,
+    },
 };
 use derive_more::From;
 use ink_prelude::collections::btree_map::BTreeMap;
@@ -265,13 +268,13 @@ pub struct HashingStrategy {
     /// An optional prefix to the computed hash.
     #[serde(
         serialize_with = "serialize_as_byte_str",
-        deserialize_with = "serde_hex::deserialize"
+        deserialize_with = "deserialize_from_byte_str"
     )]
     prefix: Vec<u8>,
     /// An optional postfix to the computed hash.
     #[serde(
         serialize_with = "serialize_as_byte_str",
-        deserialize_with = "serde_hex::deserialize"
+        deserialize_with = "deserialize_from_byte_str"
     )]
     postfix: Vec<u8>,
 }

crates/metadata/src/utils.rs

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use crate::serde_hex;
+
 /// Serializes the given bytes as byte string.
 pub fn serialize_as_byte_str<S>(bytes: &[u8], serializer: S) -> Result<S::Ok, S::Error>
 where
@@ -21,5 +23,36 @@ where
         // Return empty string without prepended `0x`.
         return serializer.serialize_str("")
     }
-    crate::serde_hex::serialize(bytes, serializer)
+    serde_hex::serialize(bytes, serializer)
+}
+
+/// Deserializes the given hex string with optional `0x` prefix.
+pub fn deserialize_from_byte_str<'de, D>(deserializer: D) -> Result<Vec<u8>, D::Error>
+where
+    D: serde::Deserializer<'de>,
+{
+    struct Visitor;
+
+    impl<'b> serde::de::Visitor<'b> for Visitor {
+        type Value = Vec<u8>;
+
+        fn expecting(&self, formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
+            write!(formatter, "hex string with optional 0x prefix")
+        }
+
+        fn visit_str<E: serde::de::Error>(self, v: &str) -> Result<Self::Value, E> {
+            let result = if v.starts_with("0x") {
+                serde_hex::from_hex(v)
+            } else {
+                serde_hex::from_hex(&format!("0x{}", v))
+            };
+            result.map_err(E::custom)
+        }
+
+        fn visit_string<E: serde::de::Error>(self, v: String) -> Result<Self::Value, E> {
+            self.visit_str(&v)
+        }
+    }
+
+    deserializer.deserialize_str(Visitor)
 }