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#[cfg(feature = "std")]
pub(crate) use once_cell::sync::Lazy;
#[cfg(not(feature = "std"))]
pub(crate) use self::spin::Lazy;
#[cfg(not(feature = "std"))]
mod spin {
//! This is the `once_cell::sync::Lazy` type, but modified to use our
//! `spin::Once` type rather than `OnceCell`. This is used to replace
//! `once_cell::sync::Lazy` on `no-std` builds.
use crate::spin::Once;
use core::{cell::Cell, fmt, ops::Deref};
/// Re-implementation of `once_cell::sync::Lazy` on top of `spin::Once`
/// rather than `OnceCell`.
///
/// This is used when the standard library is disabled.
pub(crate) struct Lazy<T, F = fn() -> T> {
cell: Once<T>,
init: Cell<Option<F>>,
}
impl<T: fmt::Debug, F> fmt::Debug for Lazy<T, F> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Lazy")
.field("cell", &self.cell)
.field("init", &"..")
.finish()
}
}
// We never create a `&F` from a `&Lazy<T, F>` so it is fine to not impl
// `Sync` for `F`. We do create a `&mut Option<F>` in `force`, but this is
// properly synchronized, so it only happens once so it also does not
// contribute to this impl.
unsafe impl<T, F: Send> Sync for Lazy<T, F> where Once<T>: Sync {}
// auto-derived `Send` impl is OK.
impl<T, F> Lazy<T, F> {
/// Creates a new lazy value with the given initializing function.
pub(crate) const fn new(init: F) -> Lazy<T, F> {
Lazy {
cell: Once::new(),
init: Cell::new(Some(init)),
}
}
}
impl<T, F: FnOnce() -> T> Lazy<T, F> {
/// Forces the evaluation of this lazy value and returns a reference to
/// the result.
///
/// This is equivalent to the `Deref` impl, but is explicit.
pub(crate) fn force(this: &Lazy<T, F>) -> &T {
this.cell.call_once(|| match this.init.take() {
Some(f) => f(),
None => panic!("Lazy instance has previously been poisoned"),
})
}
}
impl<T, F: FnOnce() -> T> Deref for Lazy<T, F> {
type Target = T;
fn deref(&self) -> &T {
Lazy::force(self)
}
}
impl<T: Default> Default for Lazy<T> {
/// Creates a new lazy value using `Default` as the initializing function.
fn default() -> Lazy<T> {
Lazy::new(T::default)
}
}
}