Fix LazyLock poison panic message

library/std/src/sync/lazy_lock.rs

@@ -2,8 +2,9 @@ use super::poison::once::ExclusiveState;
 use crate::cell::UnsafeCell;
 use crate::mem::ManuallyDrop;
 use crate::ops::{Deref, DerefMut};
-use crate::panic::{RefUnwindSafe, UnwindSafe};
+use crate::panic::{self, AssertUnwindSafe, RefUnwindSafe, UnwindSafe};
 use crate::sync::Once;
+use crate::sys::sync::once::ONCE_POISON_PANIC_MSG;
 use crate::{fmt, ptr};
 
 // We use the state of a Once as discriminant value. Upon creation, the state is
@@ -244,21 +245,38 @@ 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(|| {
-            // 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);
-        });
+        let call_once_closure = || {
+            this.once.call_once(|| {
+                // 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 the internal `Once` is poisoned, it will panic with "Once instance has previously been
+        // poisoned", but we want to panic with "LazyLock instance ..." instead.
+        let result = panic::catch_unwind(AssertUnwindSafe(call_once_closure));
+
+        if let Err(payload) = result {
+            // Check if this is the `Once` poison panic.
+            if let Some(s) = payload.downcast_ref::<String>()
+                && s == ONCE_POISON_PANIC_MSG
+            {
+                panic_poisoned();
+            } else {
+                // This panic came from the closure `f`, so we don't need to change the message.
+                panic::resume_unwind(payload);
+            }
+        }
 
         // 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 called and panicked, which we handled above.
         // * 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/mod.rs

@@ -1,6 +1,6 @@
 mod condvar;
 mod mutex;
-mod once;
+pub(crate) mod once; // For `ONCE_POISON_PANIC_MSG`.
 mod once_box;
 mod rwlock;
 mod thread_parking;

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_if::cfg_if! {
     if #[cfg(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;
+}