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use std::cell::Cell;
use std::marker::PhantomData;

// Super nice trick taken from the rlua library.
// Can be used to pin a lifetime so that all functions which use
// that lifetime can only us that single lifetime and not one which
// is variant over that lifetime
pub type Invariant<'a> = PhantomData<Cell<&'a ()>>;

/// The marker trait which requires [`Send`] when `"parallel"` feature is used
#[cfg(not(feature = "parallel"))]
pub trait ParallelSend {}

#[cfg(feature = "parallel")]
pub trait ParallelSend: Send {}

#[cfg(not(feature = "parallel"))]
impl<T> ParallelSend for T {}

#[cfg(feature = "parallel")]
impl<T: Send> ParallelSend for T {}