[Aditya-9-6]

Motivation

In SemaphorePermit::merge and OwnedSemaphorePermit::merge, permit counts are added together:

self.permits += other.permits;

Because permits was stored internally as u32, merging permits whose sum exceeds u32::MAX (~4.29 billion) causes an integer overflow. In debug builds this panics (attempt to add with overflow), while in release builds it silently wraps, causing the semaphore to permanently lose permits when the permit is dropped.

As noted in #8395, a semaphore can legitimately hold billions of permits (since Semaphore::MAX_PERMITS is usize::MAX >> 3), making this reachable. Furthermore, Semaphore::new(permits: usize) and num_permits(&self) -> usize already operate in usize, and split and drop convert to/from usize.

Solution

Per maintainer guidance on #8395:

1. Changed the internal permits field in both SemaphorePermit and OwnedSemaphorePermit from u32 to usize.
2. Updated merge in both types to use checked_add with .expect("number of permits overflowed") to prevent silent overflow.
3. Updated split in both types to eliminate the unnecessary u32 intermediate conversion.
4. Simplified num_permits(&self) -> usize and Drop to work directly with usize.
5. Preserved exact public API signatures and backward compatibility.
6. Added targeted unit tests in tests/sync_semaphore.rs and tests/sync_semaphore_owned.rs verifying merging permits beyond u32::MAX on 64-bit platforms.

Fixes #8395

[Darksonn]

Thanks.