Macro select 

macro_rules! select {
    ($($module:ident)::+ : .. $spread:ty) => { ... };
    ($($module:ident)::+ :) => { ... };
    ($($module:ident)::+ : $field:ident, .. $spread:ty) => { ... };
    ($($module:ident)::+ : $field:ident ?, .. $spread:ty) => { ... };
    ($($module:ident)::+ : $field:ident -, .. $spread:ty) => { ... };
    ($($module:ident)::+ : $field:ident < $presence:ty >, .. $spread:ty) => { ... };
    ($($module:ident)::+ : $field:ident) => { ... };
    ($($module:ident)::+ : $field:ident,) => { ... };
    ($($module:ident)::+ : $field:ident ?) => { ... };
    ($($module:ident)::+ : $field:ident ?,) => { ... };
    ($($module:ident)::+ : $field:ident -) => { ... };
    ($($module:ident)::+ : $field:ident -,) => { ... };
    ($($module:ident)::+ : $field:ident < $presence:ty >) => { ... };
    ($($module:ident)::+ : $field:ident < $presence:ty >,) => { ... };
    ($($module:ident)::+ : $field:ident ?, $($rest:tt)+) => { ... };
    ($($module:ident)::+ : $field:ident -, $($rest:tt)+) => { ... };
    ($($module:ident)::+ : $field:ident < $presence:ty >, $($rest:tt)+) => { ... };
    ($($module:ident)::+ : $field:ident, $($rest:tt)+) => { ... };
}

Construct a FieldSet by selecting fields from a module.

Presence States

Suffix	Presence	Field Type
(none)	Present	T
?	Optional	Option<T>
-	Absent	PhantomData<T>

Access is always user.field. In generic code use .get() from the Access trait.

Syntax Reference

Pattern	Description
select!(mod: field)	field is Present (required)
select!(mod: field?)	field is Optional
select!(mod: field-)	field is Absent
select!(mod: field<P>)	field has custom presence type P
select!(mod: all)	all fields Present
select!(mod: all?)	all fields Optional
select!(mod: all-)	all fields Absent
select!(mod: f1, f2?)	multiple fields (f1=Present, f2=Optional)
select!(mod: f, ..F)	spread: f=Present, rest inherited from F
select!(mod: f?, ..F)	spread: f=Optional, rest inherited from F
select!(mod: f, all-)	f=Present, all others Absent

Examples

// Concrete types for API boundaries
type Create = select!(user: name, email);      // Both required
type Update = select!(user: name?, email?);    // Both optional

// Generic: add id field to any existing fields
type WithId<F: user::Fields> = select!(user: id, ..F);

// Function using concrete type
fn create(user: User<Create>) -> User<WithId<Create>> { ... }