How to handle optional fields with serde

Use the #[serde(default)] attribute on a struct field to provide a default value when the field is missing from the input.

use serde::{Deserialize, Serialize};

#[derive(Serialize, Deserialize)]
struct Config {
    name: String,
    #[serde(default)]
    port: u16,
}

When deserializing, if port is absent, it defaults to 0 (the Default implementation for u16). For custom defaults, implement Default for the type or use #[serde(default = "my_default_func")].

Handling optional fields with serde lets your program manage missing data gracefully by filling in a standard placeholder value instead of crashing. It is like a form that automatically selects a default option if the user leaves a field blank. You use this when parsing configuration files or API responses where some fields are optional.