Attribute Macro structural 

#[structural]

Generates type-safe structural typing support for a struct.

This macro transforms a struct to track which fields are present/absent at the type level, enabling compile-time enforcement of field requirements.

Note: If you use #[derive(...)] on your struct, you must add derive-where to your dependencies:

cargo add derive-where

Generated Code

For a struct MyStruct, this macro generates:

• A my_struct module containing:
  • Fields trait - for type-level field requirements
  • FieldSet<...> - zero-cost marker type tracking field presence
  • Type aliases in with:: module for field presence combinations
• Builder methods: .field(value) that infers presence from value type
• Getter methods: .get_field(), .get_field_mut()
• .merge() - combine two partial structs
• .split() / .try_split() - split into selected fields and remainder

Field States

• Present: Field has a value of type T
• Optional: Field has type Option<T>
• Absent: Field is PhantomData<T> (zero cost)

Example

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

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

// Build incrementally
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

Restrictions

• Only named structs supported (not tuple structs or enums)
• No generic parameters allowed
• Struct must have at least one field