Auto merge of #77380 - fusion-engineering-forks:unbox-the-mutex, r=dtolnay

Unbox mutexes and condvars on some platforms

Both mutexes and condition variables contained a Box containing the actual os-specific object. This was done because moving these objects may cause undefined behaviour on some platforms.

However, this is not needed on Windows[1], Wasm[2], cloudabi[2], and 'unsupported'[3], were the box was only needlessly making them less efficient.

This change gets rid of the box on those platforms.

On those platforms, `Condvar` can no longer verify it is only used with one `Mutex`, as mutexes no longer have a stable address. This was addressed and considered acceptable in #76932.

[1]\: https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-initializesrwlock
[2]\: These are just a single atomic integer together with futex wait/wake calls/instructions.
[3]\: The `unsupported` platform doesn't support multiple threads at all.

library/std/src/sys/unix/condvar.rs

@@ -6,6 +6,8 @@ pub struct Condvar {
     inner: UnsafeCell<libc::pthread_cond_t>,
 }
 
+pub type MovableCondvar = Box<Condvar>;
+
 unsafe impl Send for Condvar {}
 unsafe impl Sync for Condvar {}
 

library/std/src/sys/unix/mutex.rs

@@ -5,6 +5,8 @@ pub struct Mutex {
     inner: UnsafeCell<libc::pthread_mutex_t>,
 }
 
+pub type MovableMutex = Box<Mutex>;
+
 #[inline]
 pub unsafe fn raw(m: &Mutex) -> *mut libc::pthread_mutex_t {
     m.inner.get()