Author of the original WTF::ParkingLot here (what rust’s parking_lot is based on).

I’m surprised that this only compared to std on one platform (Linux).

The main benefit of parking lot is that it makes locks very small, which then encourages the use of fine grained locking. For example, in JavaScriptCore (ParkingLot’s first customer), we stuff a 2-bit lock into every object header - so if there is ever a need to do some locking for internal VM reasons on any object we can do that without increasing the size of the object

> The main benefit of parking lot is that it makes locks very small, which then encourages the use of fine grained locking. For example, in JavaScriptCore (ParkingLot’s first customer), we stuff a 2-bit lock into every object header - so if there is ever a need to do some locking for internal VM reasons on any object we can do that without increasing the size of the object

IMHO that's a very cool feature which is essentially wasted when using it as a `Mutex<InnerBlah>` because the mutex's size will get rounded up to the alignment of `InnerBlah`. And even when not doing that, afaict `parking_lot` doesn't expose a way to use the remaining six bits in `parking_lot::RawMutex`. I think the new std mutexes made the right choice to use a different design.

> I’m surprised that this only compared to std on one platform (Linux).

Can't speak for the author, but I suspect a lot of people really only care about performance under Linux. I write software that I often develop from a Mac but almost entirely deploy on Linux. (But speaking of Macs: std::mutex doesn't yet use futexes on macOS. Might happen soon. https://github.com/rust-lang/rust/pull/122408)

How can a parking_lot lock be less than 1 byte? does this uses unsafe?

Rust in general doesn't support bit-level objects unless you cast things to [u8] and do some shifts and masking manually (that is, like C), which of course is wildly unsafe for data structures with safety invariants