Crate structural_typing 

Structural Typing for Rust

Define a struct once and use it with different field combinations, tracked at compile time.

Installation

cargo add structural-typing

If using derives on #[structural] structs, also add:

cargo add derive-where

Quick Example

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

#[structural]
struct User { id: u32, name: String, email: String }

type Create = select!(user: name, email);

fn create_user(data: User<Create>) -> User {
    data.id(generate_id())
}

fn update_user<F: user::Fields<id = Present>>(data: User<F>) {
    if let Some(name) = data.get_name() {
        update_in_db(data.id, name);
    }
}

Core Concepts

• Field States: Present, Optional, Absent
• Builder API: .field(value) infers presence from type
• Type Selection: select! macro or type aliases
• Bounded Impls: Require specific fields via trait bounds
• Merge and Split: Combine partial structs or extract field subsets

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

Constraints

• Named structs only (not tuple structs or enums)
• No generic parameters
• At least one field required

Modules

access

Runtime field access trait for checking presence and getting values. Runtime field access for structural types.

presence

Type-level presence markers and traits for field state tracking. Type-level markers for field presence states.

split

Traits for splitting structs into selected fields and remainder. Split structs into selected fields and remainder.

Macros

select

Construct a FieldSet by selecting fields from a module.

Attribute Macros

structural

Generates type-safe structural typing support for a struct.