1#[cfg(feature = "std")]
2pub(crate) use once_cell::sync::Lazy;
3
4#[cfg(not(feature = "std"))]
5pub(crate) use self::spin::Lazy;
6
7#[cfg(not(feature = "std"))]
8mod spin {
9 //! This is the `once_cell::sync::Lazy` type, but modified to use our
10 //! `spin::Once` type rather than `OnceCell`. This is used to replace
11 //! `once_cell::sync::Lazy` on `no-std` builds.
12 use crate::spin::Once;
13 use core::{cell::Cell, fmt, ops::Deref};
14
15 /// Re-implementation of `once_cell::sync::Lazy` on top of `spin::Once`
16 /// rather than `OnceCell`.
17 ///
18 /// This is used when the standard library is disabled.
19 pub(crate) struct Lazy<T, F = fn() -> T> {
20 cell: Once<T>,
21 init: Cell<Option<F>>,
22 }
23
24 impl<T: fmt::Debug, F> fmt::Debug for Lazy<T, F> {
25 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
26 f.debug_struct("Lazy")
27 .field("cell", &self.cell)
28 .field("init", &"..")
29 .finish()
30 }
31 }
32
33 // We never create a `&F` from a `&Lazy<T, F>` so it is fine to not impl
34 // `Sync` for `F`. We do create a `&mut Option<F>` in `force`, but this is
35 // properly synchronized, so it only happens once so it also does not
36 // contribute to this impl.
37 unsafe impl<T, F: Send> Sync for Lazy<T, F> where Once<T>: Sync {}
38 // auto-derived `Send` impl is OK.
39
40 impl<T, F> Lazy<T, F> {
41 /// Creates a new lazy value with the given initializing function.
42 pub(crate) const fn new(init: F) -> Lazy<T, F> {
43 Lazy {
44 cell: Once::new(),
45 init: Cell::new(Some(init)),
46 }
47 }
48 }
49
50 impl<T, F: FnOnce() -> T> Lazy<T, F> {
51 /// Forces the evaluation of this lazy value and returns a reference to
52 /// the result.
53 ///
54 /// This is equivalent to the `Deref` impl, but is explicit.
55 pub(crate) fn force(this: &Lazy<T, F>) -> &T {
56 this.cell.call_once(|| match this.init.take() {
57 Some(f) => f(),
58 None => panic!("Lazy instance has previously been poisoned"),
59 })
60 }
61 }
62
63 impl<T, F: FnOnce() -> T> Deref for Lazy<T, F> {
64 type Target = T;
65 fn deref(&self) -> &T {
66 Lazy::force(self)
67 }
68 }
69
70 impl<T: Default> Default for Lazy<T> {
71 /// Creates a new lazy value using `Default` as the initializing function.
72 fn default() -> Lazy<T> {
73 Lazy::new(T::default)
74 }
75 }
76}
77