Implement ACP 429: add `Lazy{Cell,Lock}::get[_mut]` and `force_mut`

In the implementation of `force_mut`, I chose performance over safety.
For `LazyLock` this isn't really a choice; the code has to be unsafe.
But for `LazyCell`, we can have a full-safe implementation, but it will
be a bit less performant, so I went with the unsafe approach.

3 files changed, 27 insertions, 0 deletions

diff --git a/library/std/src/sys/sync/once/futex.rs b/library/std/src/sys/sync/once/futex.rs
index 2c8a054282b..25588a4217b 100644
--- a/library/std/src/sys/sync/once/futex.rs
+++ b/library/std/src/sys/sync/once/futex.rs
@@ -91,6 +91,15 @@ impl Once {
         }
     }
 
+    #[inline]
+    pub(crate) fn set_state(&mut self, new_state: ExclusiveState) {
+        *self.state_and_queued.get_mut() = match new_state {
+            ExclusiveState::Incomplete => INCOMPLETE,
+            ExclusiveState::Poisoned => POISONED,
+            ExclusiveState::Complete => COMPLETE,
+        };
+    }
+
     #[cold]
     #[track_caller]
     pub fn wait(&self, ignore_poisoning: bool) {
diff --git a/library/std/src/sys/sync/once/no_threads.rs b/library/std/src/sys/sync/once/no_threads.rs
index 12c1d9f5a6c..cdcffe790f5 100644
--- a/library/std/src/sys/sync/once/no_threads.rs
+++ b/library/std/src/sys/sync/once/no_threads.rs
@@ -55,6 +55,15 @@ impl Once {
         }
     }
 
+    #[inline]
+    pub(crate) fn set_state(&mut self, new_state: ExclusiveState) {
+        self.state.set(match new_state {
+            ExclusiveState::Incomplete => State::Incomplete,
+            ExclusiveState::Poisoned => State::Poisoned,
+            ExclusiveState::Complete => State::Complete,
+        });
+    }
+
     #[cold]
     #[track_caller]
     pub fn wait(&self, _ignore_poisoning: bool) {
diff --git a/library/std/src/sys/sync/once/queue.rs b/library/std/src/sys/sync/once/queue.rs
index 86f72c82008..17abaf0bf26 100644
--- a/library/std/src/sys/sync/once/queue.rs
+++ b/library/std/src/sys/sync/once/queue.rs
@@ -140,6 +140,15 @@ impl Once {
         }
     }
 
+    #[inline]
+    pub(crate) fn set_state(&mut self, new_state: ExclusiveState) {
+        *self.state_and_queue.get_mut() = match new_state {
+            ExclusiveState::Incomplete => ptr::without_provenance_mut(INCOMPLETE),
+            ExclusiveState::Poisoned => ptr::without_provenance_mut(POISONED),
+            ExclusiveState::Complete => ptr::without_provenance_mut(COMPLETE),
+        };
+    }
+
     #[cold]
     #[track_caller]
     pub fn wait(&self, ignore_poisoning: bool) {