Rust Serde Custom Field Deserializer

use std::fmt;

use serde::de::{self, Deserializer, SeqAccess, Visitor};
use serde::{Deserialize, Serialize};

fn deserialize_origins<'de, D>(deserializer: D) -> Result<Vec<String>, D::Error>
where
    D: Deserializer<'de>,
{
    struct OriginsVisitor;

    impl<'de> Visitor<'de> for OriginsVisitor {
        type Value = Vec<String>;

        fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
            formatter.write_str("a space-separated string or an array of strings")
        }

        fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
        where
            E: de::Error,
        {
            Ok(value.split_whitespace().map(str::to_owned).collect())
        }

        fn visit_seq<A>(self, mut sequence: A) -> Result<Self::Value, A::Error>
        where
            A: SeqAccess<'de>,
        {
            let mut origins = Vec::new();
            while let Some(origin) = sequence.next_element::<String>()? {
                origins.push(origin);
            }
            Ok(origins)
        }
    }

    deserializer.deserialize_any(OriginsVisitor)
}

fn default_replicas() -> u8 {
    2
}

#[derive(Debug, Deserialize, Serialize)]
#[serde(deny_unknown_fields, rename_all = "kebab-case")]
struct ServiceConfig {
    name: String,
    #[serde(default = "default_replicas")]
    replicas: u8,
    #[serde(default, deserialize_with = "deserialize_origins")]
    allowed_origins: Vec<String>,
}

fn main() {
    let expanded = r#"{
        "name": "checkout-api",
        "replicas": 4,
        "allowed-origins": "https://shop.example https://admin.example"
    }"#;
    let config: ServiceConfig = serde_json::from_str(expanded).unwrap();
    println!("{config:?}");
    assert_eq!(config.allowed_origins.len(), 2);

    let array_form = r#"{"name":"worker","allowed-origins":["https://a.example"]}"#;
    let config: ServiceConfig = serde_json::from_str(array_form).unwrap();
    println!("default replicas: {}", config.replicas);
    assert_eq!(config.replicas, 2);

    let unknown = r#"{"name":"api","replica":3}"#;
    let error = serde_json::from_str::<ServiceConfig>(unknown).unwrap_err();
    println!("rejected unknown field: {error}");
}