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 with Fields trait, FieldSet<...> type, and with:: aliases
• Builder methods: .field(value)
• .merge(), .extract(), .try_extract() operations

Supports user-defined generic type parameters and nested structural types (see examples/nested.rs).

Field States

• Present: Value of type T
• Optional: Option<T>
• Absent: PhantomData<T>

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

Serde Support

Deserialization uses a helper struct where all fields are Option<T> with #[serde(default)], then converts via TryFrom. Present fields error if missing, Optional fields become Some/None, Absent fields are ignored.

Limitations:

• Present fields cannot use the field type’s Default impl during deserialization
• Optional fields cannot distinguish between missing and explicit null in JSON
• Absent fields silently ignore data if present in input
• All field types must implement Deserialize even if marked Absent

Custom deserializers: If using #[serde(deserialize_with)], the function must produce Option<T>.

Incompatible: #[serde(default)], #[serde(skip)], #[serde(skip_deserializing)], #[serde(flatten)]. Compatible: rename, alias, rename_all, deserialize_with (with Option<T> output).

Restrictions

• Named structs only
• At least one field