Optimize hexadecimal conversions in serde_utils to speed up JSON seri…

…alization of byte collections

Slow serialization to JSON was claimed to be the problem that led to 2e92e00.
BlobSidecar serialization is now around 15.4 times faster.
BlobSidecar deserialization is now around 12 times faster.
SignedBeaconBlock serialization is now up to 1.5 times faster.
SignedBeaconBlock deserialization is now up to 1.1 times faster.

faster-hex might be faster than const-hex in some cases, but faster-hex does not provide the API we require.
See https://crates.io/crates/const-hex/1.14.0 for benchmark results comparing various hexadecimal conversion crates.

Cargo.toml

@@ -300,6 +300,7 @@ bytesize = { version = '1', features = ['serde'] }
 cached = '0.53'
 chrono = '0.4'
 clap = { version = '4', features = ['derive'] }
+const-hex = '1.14'
 const_format = '0.2'
 constant_time_eq = '0.3'
 conv = '0.3'
@@ -339,7 +340,6 @@ hash_hasher = '2'
 hashlink = '0.9'
 hex = { version = '0.4', features = ['serde'] }
 hex-literal = '0.4'
-hex_fmt = '0.3'
 hmac = '0.12'
 http = '1'
 http-body-util = '0.1'

serde_utils/Cargo.toml

@@ -7,9 +7,9 @@ authors = ["Grandine <[email protected]>"]
 workspace = true
 
 [dependencies]
+const-hex = { workspace = true }
 generic-array = { workspace = true }
 hex = { workspace = true }
-hex_fmt = { workspace = true }
 itertools = { workspace = true }
 num-traits = { workspace = true }
 serde = { workspace = true }

serde_utils/src/prefixed_hex_or_bytes_array.rs

@@ -29,7 +29,7 @@ pub fn deserialize<'de, D: Deserializer<'de>, const N: usize>(
             let digits = shared::strip_hex_prefix(string)?;
 
             let mut bytes = [0; N];
-            hex::decode_to_slice(digits, &mut bytes).map_err(E::custom)?;
+            const_hex::decode_to_slice(digits, &mut bytes).map_err(E::custom)?;
 
             Ok(bytes)
         }

serde_utils/src/prefixed_hex_or_bytes_cow.rs

@@ -34,7 +34,7 @@ pub fn deserialize<'de, D: Deserializer<'de>>(deserializer: D) -> Result<Cow<'de
 
         fn visit_str<E: Error>(self, string: &str) -> Result<Self::Value, E> {
             let digits = shared::strip_hex_prefix(string)?;
-            let bytes = hex::decode(digits).map_err(E::custom)?;
+            let bytes = const_hex::decode(digits).map_err(E::custom)?;
             Ok(Cow::Owned(bytes))
         }
     }

serde_utils/src/prefixed_hex_or_bytes_generic_array.rs

@@ -43,7 +43,7 @@ pub fn deserialize<'de, D: Deserializer<'de>, N: ArrayLength<u8>>(
             let digits = shared::strip_hex_prefix(string)?;
 
             let mut bytes = GenericArray::default();
-            hex::decode_to_slice(digits, &mut bytes).map_err(E::custom)?;
+            const_hex::decode_to_slice(digits, &mut bytes).map_err(E::custom)?;
 
             Ok(bytes)
         }

serde_utils/src/prefixed_hex_or_bytes_slice.rs

@@ -1,9 +1,8 @@
-use hex_fmt::HexFmt;
 use serde::Serializer;
 
 pub fn serialize<S: Serializer>(bytes: impl AsRef<[u8]>, serializer: S) -> Result<S::Ok, S::Error> {
     if serializer.is_human_readable() {
-        serializer.collect_str(&format_args!("0x{}", HexFmt(bytes)))
+        serializer.serialize_str(const_hex::encode_prefixed(bytes).as_str())
     } else {
         serializer.serialize_bytes(bytes.as_ref())
     }