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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-17 12:20:39 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-17 12:20:39 +0000 |
commit | 1376c5a617be5c25655d0d7cb63e3beaa5a6e026 (patch) | |
tree | 3bb8d61aee02bc7a15eab3f36e3b921afc2075d0 /library/core/src/cell/lazy.rs | |
parent | Releasing progress-linux version 1.69.0+dfsg1-1~progress7.99u1. (diff) | |
download | rustc-1376c5a617be5c25655d0d7cb63e3beaa5a6e026.tar.xz rustc-1376c5a617be5c25655d0d7cb63e3beaa5a6e026.zip |
Merging upstream version 1.70.0+dfsg1.
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'library/core/src/cell/lazy.rs')
-rw-r--r-- | library/core/src/cell/lazy.rs | 106 |
1 files changed, 86 insertions, 20 deletions
diff --git a/library/core/src/cell/lazy.rs b/library/core/src/cell/lazy.rs index 65d12c25c..44adcfa1a 100644 --- a/library/core/src/cell/lazy.rs +++ b/library/core/src/cell/lazy.rs @@ -1,6 +1,13 @@ -use crate::cell::{Cell, OnceCell}; -use crate::fmt; use crate::ops::Deref; +use crate::{fmt, mem}; + +use super::UnsafeCell; + +enum State<T, F> { + Uninit(F), + Init(T), + Poisoned, +} /// A value which is initialized on the first access. /// @@ -11,7 +18,7 @@ use crate::ops::Deref; /// # Examples /// /// ``` -/// #![feature(once_cell)] +/// #![feature(lazy_cell)] /// /// use std::cell::LazyCell; /// @@ -29,10 +36,9 @@ use crate::ops::Deref; /// // 92 /// // 92 /// ``` -#[unstable(feature = "once_cell", issue = "74465")] +#[unstable(feature = "lazy_cell", issue = "109736")] pub struct LazyCell<T, F = fn() -> T> { - cell: OnceCell<T>, - init: Cell<Option<F>>, + state: UnsafeCell<State<T, F>>, } impl<T, F: FnOnce() -> T> LazyCell<T, F> { @@ -41,7 +47,7 @@ impl<T, F: FnOnce() -> T> LazyCell<T, F> { /// # Examples /// /// ``` - /// #![feature(once_cell)] + /// #![feature(lazy_cell)] /// /// use std::cell::LazyCell; /// @@ -52,9 +58,9 @@ impl<T, F: FnOnce() -> T> LazyCell<T, F> { /// assert_eq!(&*lazy, "HELLO, WORLD!"); /// ``` #[inline] - #[unstable(feature = "once_cell", issue = "74465")] - pub const fn new(init: F) -> LazyCell<T, F> { - LazyCell { cell: OnceCell::new(), init: Cell::new(Some(init)) } + #[unstable(feature = "lazy_cell", issue = "109736")] + pub const fn new(f: F) -> LazyCell<T, F> { + LazyCell { state: UnsafeCell::new(State::Uninit(f)) } } /// Forces the evaluation of this lazy value and returns a reference to @@ -65,7 +71,7 @@ impl<T, F: FnOnce() -> T> LazyCell<T, F> { /// # Examples /// /// ``` - /// #![feature(once_cell)] + /// #![feature(lazy_cell)] /// /// use std::cell::LazyCell; /// @@ -75,16 +81,71 @@ impl<T, F: FnOnce() -> T> LazyCell<T, F> { /// assert_eq!(&*lazy, &92); /// ``` #[inline] - #[unstable(feature = "once_cell", issue = "74465")] + #[unstable(feature = "lazy_cell", issue = "109736")] pub fn force(this: &LazyCell<T, F>) -> &T { - this.cell.get_or_init(|| match this.init.take() { - Some(f) => f(), - None => panic!("`Lazy` instance has previously been poisoned"), - }) + // SAFETY: + // This invalidates any mutable references to the data. The resulting + // reference lives either until the end of the borrow of `this` (in the + // initialized case) or is invalidated in `really_init` (in the + // uninitialized case; `really_init` will create and return a fresh reference). + let state = unsafe { &*this.state.get() }; + match state { + State::Init(data) => data, + // SAFETY: The state is uninitialized. + State::Uninit(_) => unsafe { LazyCell::really_init(this) }, + State::Poisoned => panic!("LazyCell has previously been poisoned"), + } + } + + /// # Safety + /// May only be called when the state is `Uninit`. + #[cold] + unsafe fn really_init(this: &LazyCell<T, F>) -> &T { + // SAFETY: + // This function is only called when the state is uninitialized, + // so no references to `state` can exist except for the reference + // in `force`, which is invalidated here and not accessed again. + let state = unsafe { &mut *this.state.get() }; + // Temporarily mark the state as poisoned. This prevents reentrant + // accesses and correctly poisons the cell if the closure panicked. + let State::Uninit(f) = mem::replace(state, State::Poisoned) else { unreachable!() }; + + let data = f(); + + // SAFETY: + // If the closure accessed the cell through something like a reentrant + // mutex, but caught the panic resulting from the state being poisoned, + // the mutable borrow for `state` will be invalidated, so we need to + // go through the `UnsafeCell` pointer here. The state can only be + // poisoned at this point, so using `write` to skip the destructor + // of `State` should help the optimizer. + unsafe { this.state.get().write(State::Init(data)) }; + + // SAFETY: + // The previous references were invalidated by the `write` call above, + // so do a new shared borrow of the state instead. + let state = unsafe { &*this.state.get() }; + let State::Init(data) = state else { unreachable!() }; + data + } +} + +impl<T, F> LazyCell<T, F> { + #[inline] + fn get(&self) -> Option<&T> { + // SAFETY: + // This is sound for the same reason as in `force`: once the state is + // initialized, it will not be mutably accessed again, so this reference + // will stay valid for the duration of the borrow to `self`. + let state = unsafe { &*self.state.get() }; + match state { + State::Init(data) => Some(data), + _ => None, + } } } -#[unstable(feature = "once_cell", issue = "74465")] +#[unstable(feature = "lazy_cell", issue = "109736")] impl<T, F: FnOnce() -> T> Deref for LazyCell<T, F> { type Target = T; #[inline] @@ -93,7 +154,7 @@ impl<T, F: FnOnce() -> T> Deref for LazyCell<T, F> { } } -#[unstable(feature = "once_cell", issue = "74465")] +#[unstable(feature = "lazy_cell", issue = "109736")] impl<T: Default> Default for LazyCell<T> { /// Creates a new lazy value using `Default` as the initializing function. #[inline] @@ -102,9 +163,14 @@ impl<T: Default> Default for LazyCell<T> { } } -#[unstable(feature = "once_cell", issue = "74465")] +#[unstable(feature = "lazy_cell", issue = "109736")] impl<T: fmt::Debug, F> fmt::Debug for LazyCell<T, F> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { - f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish() + let mut d = f.debug_tuple("LazyCell"); + match self.get() { + Some(data) => d.field(data), + None => d.field(&format_args!("<uninit>")), + }; + d.finish() } } |