tracing_core/
lazy.rs

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}
tracing_core/lazy.rs

tracing_core/
lazy.rs
