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.

2 files changed, 7 insertions, 0 deletions

diff --git a/library/std/src/sys/windows/condvar.rs b/library/std/src/sys/windows/condvar.rs
index 8f7f6854cc2..44547a5c51a 100644
--- a/library/std/src/sys/windows/condvar.rs
+++ b/library/std/src/sys/windows/condvar.rs
@@ -8,6 +8,8 @@ pub struct Condvar {
     inner: UnsafeCell<c::CONDITION_VARIABLE>,
 }
 
+pub type MovableCondvar = Condvar;
+
 unsafe impl Send for Condvar {}
 unsafe impl Sync for Condvar {}
 
diff --git a/library/std/src/sys/windows/mutex.rs b/library/std/src/sys/windows/mutex.rs
index e2aaca59fe2..fa51b006c34 100644
--- a/library/std/src/sys/windows/mutex.rs
+++ b/library/std/src/sys/windows/mutex.rs
@@ -29,6 +29,11 @@ pub struct Mutex {
     lock: AtomicUsize,
 }
 
+// Windows SRW Locks are movable (while not borrowed).
+// ReentrantMutexes (in Inner) are not, but those are stored indirectly through
+// a Box, so do not move when the Mutex it self is moved.
+pub type MovableMutex = Mutex;
+
 unsafe impl Send for Mutex {}
 unsafe impl Sync for Mutex {}