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#![cfg(feature = "frunk")]

use frunk::{from_generic, generic::Generic, hlist::h_cons, into_generic, HCons, HNil};
use std::ops::Add;

/// Constructs a structure from another structure and a field.
///
/// Let `X` be a structure of `field1, ..., fieldN`, `Y` be `field1, ...,
/// fieldN, fieldN+1`. Both `X` and `Y` implement [`Generic`]. Then `Y::up(x,
/// fieldN+1)` constructs `Y` from all the fields of `x: X` plus `Y`'s
/// `fieldN+1`.
///
/// [`Generic`]: https://docs.rs/frunk/latest/frunk/generic/trait.Generic.html
pub trait UpState: Sized {
    fn up<Src, F>(src: Src, field: F) -> Self
    where
        Src: Generic,
        Self: Generic<Repr = <<Src as Generic>::Repr as Add<HCons<F, HNil>>>::Output>,
        <Src as Generic>::Repr: Add<HCons<F, HNil>>,
    {
        from_generic(into_generic(src) + h_cons(field, HNil))
    }
}

impl<Dst> UpState for Dst {}