Crate structural_typing 

Structural Typing for Rust

Track which struct fields are present/absent at the type level.

Quick Example

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

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

// Build incrementally - type tracks which fields are set
let user = User::empty().name("Alice".to_owned());

// Methods can require specific fields
impl<F: user::Fields<name = Present>> User<F> {
    fn greet(&self) -> String {
        format!("Hello, {}!", self.name)
    }
}

user.greet(); // ✓ Compiles - name is Present
// User::empty().greet(); // ✗ Compile error - name is Absent

// Use select! to create field sets
type NameAndEmail = select!(user: name, email);

Core Concepts

• Field States: Present (has value), Optional (has Option<T>), Absent (no value)
• Builder API: .field(value) infers presence from type (T → Present, Option → Optional, PhantomData<T> → Absent)
• Type Selection: select!(module: field1, ?field2) macro or type aliases like with::name::Present<with::email::Optional>
• Bounded Impls: Methods requiring specific fields via trait bounds

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

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.