modify LazyLock poison panic message

Fixes an issue where if the underlying `Once` panics because it is
poisoned, the panic displays the wrong message.

Also consolidates all of the `Once` panic messages in one place so that
we don't have to edit the message in 3 different files if we want to
change it.

Signed-off-by: Connor Tsui <[email protected]>

Author: connortsui20

library/std/src/sync/lazy_lock.rs

@@ -244,21 +244,30 @@ impl<T, F: FnOnce() -> T> LazyLock<T, F> {
     #[inline]
     #[stable(feature = "lazy_cell", since = "1.80.0")]
     pub fn force(this: &LazyLock<T, F>) -> &T {
-        this.once.call_once(|| {
+        let mut is_poisoned = false;
+
+        this.once.call_once_force(|state| {
+            if state.is_poisoned() {
+                is_poisoned = true;
+            }
+
             // SAFETY: `call_once` only runs this closure once, ever.
             let data = unsafe { &mut *this.data.get() };
             let f = unsafe { ManuallyDrop::take(&mut data.f) };
             let value = f();
             data.value = ManuallyDrop::new(value);
         });
 
+        if is_poisoned {
+            panic_poisoned()
+        }
+
         // SAFETY:
         // There are four possible scenarios:
         // * the closure was called and initialized `value`.
         // * the closure was called and panicked, so this point is never reached.
         // * the closure was not called, but a previous call initialized `value`.
-        // * the closure was not called because the Once is poisoned, so this point
-        //   is never reached.
+        // * the closure was not called because the Once is poisoned, which we handled above.
         // So `value` has definitely been initialized and will not be modified again.
         unsafe { &*(*this.data.get()).value }
     }

library/std/src/sys/sync/once/futex.rs

@@ -112,7 +112,7 @@ impl Once {
                 COMPLETE => return,
                 POISONED if !ignore_poisoning => {
                     // Panic to propagate the poison.
-                    panic!("Once instance has previously been poisoned");
+                    panic!("{}", super::ONCE_POISON_PANIC_MSG);
                 }
                 _ => {
                     // Set the QUEUED bit if it has not already been set.
@@ -147,7 +147,7 @@ impl Once {
                 COMPLETE => return,
                 POISONED if !ignore_poisoning => {
                     // Panic to propagate the poison.
-                    panic!("Once instance has previously been poisoned");
+                    panic!("{}", super::ONCE_POISON_PANIC_MSG);
                 }
                 INCOMPLETE | POISONED => {
                     // Try to register the current thread as the one running.

library/std/src/sys/sync/once/mod.rs

@@ -7,6 +7,11 @@
 // This also gives us the opportunity to optimize the implementation a bit which
 // should help the fast path on call sites.
 
+/// The panic message used when a `Once` is accessed after being poisoned.
+///
+/// This message is checked by `LazyLock` to detect when a `Once` has been poisoned.
+pub(crate) const ONCE_POISON_PANIC_MSG: &str = "Once instance has previously been poisoned";
+
 cfg_select! {
     any(
         all(target_os = "windows", not(target_vendor="win7")),

library/std/src/sys/sync/once/no_threads.rs

@@ -76,7 +76,7 @@ impl Once {
         match state {
             State::Poisoned if !ignore_poisoning => {
                 // Panic to propagate the poison.
-                panic!("Once instance has previously been poisoned");
+                panic!("{}", super::ONCE_POISON_PANIC_MSG);
             }
             State::Incomplete | State::Poisoned => {
                 self.state.set(State::Running);

library/std/src/sys/sync/once/queue.rs

@@ -159,7 +159,7 @@ impl Once {
                 COMPLETE => return,
                 POISONED if !ignore_poisoning => {
                     // Panic to propagate the poison.
-                    panic!("Once instance has previously been poisoned");
+                    panic!("{}", super::ONCE_POISON_PANIC_MSG);
                 }
                 _ => {
                     current = wait(&self.state_and_queue, current, !ignore_poisoning);
@@ -189,7 +189,7 @@ impl Once {
                 COMPLETE => break,
                 POISONED if !ignore_poisoning => {
                     // Panic to propagate the poison.
-                    panic!("Once instance has previously been poisoned");
+                    panic!("{}", super::ONCE_POISON_PANIC_MSG);
                 }
                 POISONED | INCOMPLETE => {
                     // Try to register this thread as the one RUNNING.

library/std/tests/sync/lazy_lock.rs

@@ -165,3 +165,32 @@ fn lazy_force_mut() {
     p.clear();
     LazyLock::force_mut(&mut lazy);
 }
+
+/// Verifies that when a `LazyLock` is poisoned, it panics with the correct error message ("LazyLock
+/// instance has previously been poisoned") instead of the underlying `Once` error message.
+#[test]
+#[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
+#[should_panic(expected = "LazyLock instance has previously been poisoned")]
+fn lazy_lock_poison_message() {
+    let lazy: LazyLock<String> = LazyLock::new(|| panic!("initialization failed"));
+
+    // First access will panic during initialization.
+    let _ = panic::catch_unwind(|| {
+        let _ = &*lazy;
+    });
+
+    // Second access should panic with the poisoned message.
+    let _ = &*lazy;
+}
+
+// Verifies that when the initialization closure panics with a custom message, that message is
+// preserved and not overridden by `LazyLock`.
+#[test]
+#[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
+#[should_panic(expected = "custom panic message from closure")]
+fn lazy_lock_preserves_closure_panic_message() {
+    let lazy: LazyLock<String> = LazyLock::new(|| panic!("custom panic message from closure"));
+
+    // This should panic with the original message from the closure.
+    let _ = &*lazy;
+}