🧱 Lenient Deserialize

A Rust crate for fault-tolerant deserialization using Serde. Automatically wraps fields to ensure invalid values fall back to sensible defaults — instead of breaking the entire deserialization process.

This is useful when handling user inputs, external APIs, or anything where input reliability is uncertain.

✨ Features

• Lenient<T>: wraps any type to gracefully fallback to T::default() on deserialization failure.
• Optional<T>: alias for Lenient<Option<T>>.
• #[derive(LenientDeserialize)]: a procedural macro to generate fault-tolerant wrappers for entire structs.
• Support for field-level #[lenient] and #[optional] attributes.
• Optional error logging via tracing.
• Ergonomic access with Deref and DerefMut on Lenient<T>.

📦 Crate Structure

This workspace contains two crates:

lenient/
├── lenient/            # Main library (Lenient wrapper, re-exports macro)
├── lenient_derive/     # Procedural macro crate (LenientDeserialize)

🚀 Quick Start

1. Add Dependencies

In your Cargo.toml:

[dependencies]
serde = { version = "1.0", features = ["derive"] }
serde_json = "1.0"
tracing = "0.1"
lenient = { path = "./lenient" }

2. Example: Lenient Field Wrapping

use lenient::LenientDeserialize;
use serde::Deserialize;

#[derive(Debug, Default)]
struct Age(pub u32);
impl<'de> Deserialize<'de> for Age {
    fn deserialize<D>(de: D) -> Result<Self, D::Error>
    where D: serde::Deserializer<'de> {
        let val = u32::deserialize(de)?;
        Ok(Age(val))
    }
}

#[derive(Debug, Default)]
struct Score(pub u8);
impl<'de> Deserialize<'de> for Score {
    fn deserialize<D>(de: D) -> Result<Self, D::Error>
    where D: serde::Deserializer<'de> {
        let val = u8::deserialize(de)?;
        Ok(Score(val))
    }
}

#[derive(Debug, Default, LenientDeserialize)]
struct UserProfile {
    #[lenient]
    age: Age,
    #[optional]
    score: Score,
    #[lenient]
    nickname: String,
}

3. Example Input Handling with Deref

use serde_json::json;

let input = json!({ "age": "oops", "score": 90, "nickname": "Nina" });
let profile: UserProfile = serde_json::from_value(input).unwrap();

let age = profile.age.0;
let score = profile.score.0;
let nickname = &profile.nickname;

println!("User: {nickname}, Age: {age}, Score: {score}");

🧪 Tests

Run Unit Tests for lenient

cargo test -p lenient

Sample Test

#[test]
fn test_lenient_invalid() {
    let json = r#"{ "age": "invalid", "score": 55, "nickname": "test" }"#;

    let result: UserProfile = serde_json::from_str(json).unwrap();
    assert_eq!(result.age.0, 0); // fallback to default
    assert_eq!(result.nickname, "test");
}

🔖 License

MIT