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/// Encodes the rules for what types can be flattened into other types
/// via the `#[cynic(flatten)]` attribute.
///
/// This trait is sealed and can't be implemented by users of cynic.
pub trait FlattensInto<T>: private::Sealed<T> {}

impl<T> FlattensInto<Vec<T>> for Vec<Option<T>> {}
impl<T> FlattensInto<Vec<T>> for Option<Vec<T>> {}
impl<T> FlattensInto<Option<Vec<T>>> for Option<Vec<Option<T>>> {}
impl<T> FlattensInto<Vec<T>> for Option<Vec<Option<T>>> {}

mod private {
    pub trait Sealed<T> {}

    impl<T> Sealed<Vec<T>> for Vec<Option<T>> {}
    impl<T> Sealed<Vec<T>> for Option<Vec<T>> {}
    impl<T> Sealed<Option<Vec<T>>> for Option<Vec<Option<T>>> {}
    impl<T> Sealed<Vec<T>> for Option<Vec<Option<T>>> {}
}