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// This file is part of ICU4X. For terms of use, please see the file
// called LICENSE at the top level of the ICU4X source tree
//! Types to enable polymorphic carts.
use crate::CloneableCart;
use core::ops::Deref;
use stable_deref_trait::StableDeref;
/// A cart that can be one type or the other. Enables ergonomic polymorphic carts.
///
/// `EitherCart` enables yokes originating from different data sources and therefore
/// having different cart types to be merged into the same yoke type, but still being
/// able to recover the original cart type if necessary.
///
/// All relevant Cart traits are implemented for `EitherCart`, and carts can be
/// safely wrapped in an `EitherCart`.
///
/// Also see [`Yoke::erase_box_cart()`](crate::Yoke::erase_box_cart).
///
/// # Examples
///
/// ```
/// use std::borrow::Cow;
/// use std::rc::Rc;
/// use yoke::either::EitherCart;
/// use yoke::Yoke;
///
/// let y1: Yoke<&'static str, Rc<str>> =
/// Yoke::attach_to_zero_copy_cart("reference counted hello world".into());
///
/// let y2: Yoke<&'static str, &str> = Yoke::attach_to_zero_copy_cart("borrowed hello world");
///
/// type CombinedYoke<'a> = Yoke<&'static str, EitherCart<Rc<str>, &'a str>>;
///
/// // Both yokes can be combined into a single yoke type despite different carts
/// let y3: CombinedYoke = y1.wrap_cart_in_either_a();
/// let y4: CombinedYoke = y2.wrap_cart_in_either_b();
///
/// assert_eq!(*y3.get(), "reference counted hello world");
/// assert_eq!(*y4.get(), "borrowed hello world");
///
/// // The resulting yoke is cloneable if both cart types implement CloneableCart
/// let y5 = y4.clone();
/// assert_eq!(*y5.get(), "borrowed hello world");
/// ```
#[derive(Clone, PartialEq, Eq, Debug)]
#[allow(clippy::exhaustive_enums)] // stable
pub enum EitherCart<C0, C1> {
A(C0),
B(C1),
}
impl<C0, C1, T> Deref for EitherCart<C0, C1>
where
C0: Deref<Target = T>,
C1: Deref<Target = T>,
{
type Target = T;
fn deref(&self) -> &T {
use EitherCart::*;
match self {
A(a) => a.deref(),
B(b) => b.deref(),
}
}
}
// Safe because both sub-types implement the trait.
unsafe impl<C0, C1, T> StableDeref for EitherCart<C0, C1>
where
C0: StableDeref,
C1: StableDeref,
C0: Deref<Target = T>,
C1: Deref<Target = T>,
{
}
// Safe because both sub-types implement the trait.
unsafe impl<C0, C1> CloneableCart for EitherCart<C0, C1>
where
C0: CloneableCart,
C1: CloneableCart,
{
}