> This means SRW locks on Windows, and futex-based locks on Linux, some BSDs, and Wasm.

Note that the SRW Locks are gone, except if you're on a very old Windows. So today the Rust built-in std mutex for your platform is almost certainly basically a futex though if it is on Windows it is not called a futex and from some angles is better - the same core ideas of the futex apply, we only ask the OS to do any work when we're contended, there is no OS limited resource (other than memory) and our uncontended operations are as fast as they could ever be.

SRW Locks were problematic because they're bulkier than a futex (though mostly when contended) and they have a subtle bug and for a long time it was unclear when Microsoft would get around to fixing that which isn't a huge plus sign for an important intrinsic used in all the high performance software on a $$$ commercial OS...

Mara's work (which you linked) is probably more work, and more important, but it's not actually the most recent large reworking of Rust's Mutex implementation.