Structural Typing for Rust

Define structs with optional fields, tracked at the type level. Inspired by TypeScript, RDF, and this talk.

Installation

cargo add structural-typing

Example

use structural_typing::{structural, presence::Present};

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

fn main() {
    // Build with some fields
    let user = User::empty()
        .name("Alice".to_owned())
        .email("alice@example.com".to_owned());

    // This compiles - greet() requires name field
    println!("{}", user.greet());

    let user = user.age(30);
    // Now all fields present
}

// Methods that require specific fields
// Note: #[structural] generates a snake_case module (User → user)
impl<F: user::Fields<name = Present>> User<F> {
    fn greet(&self) -> String {
        format!("Hello, {}!", self.name)
    }
}

See examples/ for comprehensive usage including merge, split, serde integration, and more.

Features

Type-level field tracking (Present, Optional, Absent)
Compile-time enforcement of field requirements
Builder pattern with .field() that infers presence from value type
Type-safe merge and split
Automatic serde support
Zero runtime overhead

Status

Experimental. API subject to change.

License