//! Type definitions for putting shared ownership and synchronized mutation behind the `threading` feature toggle.//!//! That way, single-threaded applications will not have to use thread-safe primitives, and simply do not specify the 'threading' feature. #[cfg(feature = "parallel")]mod _impl {    use std::sync::Arc;     /// A thread-safe cell which can be written to only once.    ///    /// Note: We use `once_cell` here because `std::sync::OnceLock::get_or_try_init()` is not yet stable.    /// Once it's stabilized, we can switch to `std::sync::OnceLock`.    #[cfg(feature = "once_cell")]    pub type OnceCell<T> = once_cell::sync::OnceCell<T>;    /// A reference counted pointer type for shared ownership.    pub type OwnShared<T> = Arc<T>;    /// A synchronization primitive which can start read-only and transition to support mutation.    pub type MutableOnDemand<T> = parking_lot::RwLock<T>;    /// A synchronization primitive which provides read-write access right away.    pub type Mutable<T> = parking_lot::Mutex<T>;    /// A guarded reference suitable for safekeeping in a struct.    pub type RefGuard<'a, T> = parking_lot::RwLockReadGuard<'a, T>;    /// A mapped reference created from a `RefGuard`    pub type MappedRefGuard<'a, U> = parking_lot::MappedRwLockReadGuard<'a, U>;     /// Get a shared reference through a [`MutableOnDemand`] for read-only access.    pub fn get_ref<T>(v: &MutableOnDemand<T>) -> RefGuard<'_, T> {        v.read()    }     /// Get a mutable reference through a [`MutableOnDemand`] for read-write access.    pub fn get_mut<T>(v: &MutableOnDemand<T>) -> parking_lot::RwLockWriteGuard<'_, T> {        v.write()    }     /// Get a mutable reference to the underlying data, with semantics similar to [Arc::make_mut()].    pub fn make_mut<T: Clone>(this: &mut OwnShared<T>) -> &mut T {        OwnShared::make_mut(this)    }     /// Get a mutable reference through a [`Mutable`] for read-write access.    pub fn lock<T>(v: &Mutable<T>) -> parking_lot::MutexGuard<'_, T> {        v.lock()    }     /// Downgrade a handle previously obtained with [`get_mut()`] to drop mutation support.    pub fn downgrade_mut_to_ref<'a, T>(        v: parking_lot::RwLockWriteGuard<'a, T>,        _orig: &'a MutableOnDemand<T>,    ) -> RefGuard<'a, T> {        parking_lot::RwLockWriteGuard::downgrade(v)    }     /// Map a read guard into a sub-type it contains.    pub fn map_ref<T, U: ?Sized>(v: RefGuard<'_, T>, f: impl FnOnce(&T) -> &U) -> MappedRefGuard<'_, U> {        parking_lot::RwLockReadGuard::map(v, f)    }} #[cfg(not(feature = "parallel"))]mod _impl {    use std::{        cell::{Ref, RefCell, RefMut},        rc::Rc,    };     /// A thread-safe cell which can be written to only once.    ///    /// Note: We use `once_cell` here because `std::cell::OnceCell::get_or_try_init()` is not yet stable.    /// Once it's stabilized, we can switch to `std::cell::OnceCell`.    #[cfg(feature = "once_cell")]    pub type OnceCell<T> = once_cell::unsync::OnceCell<T>;    /// A reference counted pointer type for shared ownership.    pub type OwnShared<T> = Rc<T>;    /// A synchronization primitive which can start read-only and transition to support mutation.    pub type MutableOnDemand<T> = RefCell<T>;    /// A synchronization primitive which provides read-write access right away.    pub type Mutable<T> = RefCell<T>;    /// A guarded reference suitable for safekeeping in a struct.    pub type RefGuard<'a, T> = Ref<'a, T>;    /// A mapped reference created from a RefGuard    pub type MappedRefGuard<'a, U> = Ref<'a, U>;     /// Get a shared reference through a [`MutableOnDemand`] for read-only access.    pub fn get_mut<T>(v: &RefCell<T>) -> RefMut<'_, T> {        v.borrow_mut()    }     /// Get a mutable reference to the underlying data, with semantics similar to [Rc::make_mut()].    pub fn make_mut<T: Clone>(this: &mut OwnShared<T>) -> &mut T {        OwnShared::make_mut(this)    }     /// Get a mutable reference through a [`Mutable`] for read-write access.    pub fn lock<T>(v: &Mutable<T>) -> RefMut<'_, T> {        v.borrow_mut()    }     /// Get a mutable reference through a [`MutableOnDemand`] for read-write access.    pub fn get_ref<T>(v: &RefCell<T>) -> RefGuard<'_, T> {        v.borrow()    }     /// Downgrade a handle previously obtained with [`upgrade_ref_to_mut()`] to drop mutation support.    pub fn downgrade_mut_to_ref<'a, T>(v: RefMut<'a, T>, orig: &'a RefCell<T>) -> RefGuard<'a, T> {        drop(v);        orig.borrow()    }     /// Map a read guard into a sub-type it contains.    pub fn map_ref<T, U: ?Sized>(v: RefGuard<'_, T>, f: impl FnOnce(&T) -> &U) -> MappedRefGuard<'_, U> {        Ref::map(v, f)    }} pub use _impl::*;