## Cargo.toml
```toml
[package]
name = "json_parser"
version = "0.1.0"
edition = "2021"

[dependencies]
serde = { version = "1.0", features = ["derive"] }
serde_json = "1.0"

```

## src/lib.rs
```rust
use serde::{Deserialize, Serialize};

#[derive(Debug, Deserialize)]
struct User {
    name: String,
    age: u32,
}

fn solution(json_string: &str) -> Option<User> {
    match serde_json::from_str::<User>(json_string) {
        Ok(user) => Some(user),
        Err(_) => None,
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_parse_valid_json() {
        let json = r#"{"name": "John Doe", "age": 30}"#;
        assert_eq!(solution(json), Some(User { name: "John Doe".to_string(), age: 30 }));
    }

    #[test]
    fn test_parse_invalid_json() {
        let json = r#"{"name": "John Doe"}"#; // Missing "age" field
        assert_eq!(solution(json), None);
    }
}
```

## Build
```bash
cargo build --tests
```

## Test
```bash
cargo test
```