docs: Documented Send and Sync requirements for Mutex + MutexGuard

Author: ranger-ross

library/std/src/sync/poison/mutex.rs

@@ -181,10 +181,24 @@ pub struct Mutex<T: ?Sized> {
     data: UnsafeCell<T>,
 }
 
-// these are the only places where `T: Send` matters; all other
-// functionality works fine on a single thread.
+/// Mutex is a container that wraps `T`, so it's necessary for `T` to be `Send`
+/// to safely send `Mutex` to another thread. This ensures that the protected
+/// data can be accessed safely from multiple threads without causing data races
+/// or other unsafe behavior.
+///
+/// [`Mutex<T>`] provides mutable access to `T` to one thread at a time. However, it's essential
+/// for `T` to be `Send` because it's not safe for non-`Send` structures to be accessed in
+/// this manner. For instance, consider [`Rc`], a non-atomic reference counted smart pointer,
+/// which is not `Send`. With `Rc`, we can have multiple copies pointing to the same heap
+/// allocation with a non-atomic reference count. If we were to use `Mutex<Rc<_>>`, it would
+/// only protect one instance of `Rc` from shared access, leaving other copies vulnerable
+/// to potential data races.
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: ?Sized + Send> Send for Mutex<T> {}
+
+/// [`Mutex`] can be `Sync` even if its inner type `T` is not `Sync` itself.
+/// This is because [`Mutex`] provides a safe interface for accessing `T` through
+/// locking mechanisms, ensuring that only one thread can access `T` at a time.
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}
 
@@ -211,8 +225,18 @@ pub struct MutexGuard<'a, T: ?Sized + 'a> {
     poison: poison::Guard,
 }
 
+/// A [`MutexGuard`] is not `Send` to maximize platform portablity.
+/// On platforms that use POSIX thread (commonly referred to as pthreads) there is a requirement to
+/// release mutex locks on the same thread they were acquired.
+/// For this reason, [`MutexGuard`] must not implement `Send` to prevent it being dropped from
+/// another thread.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> !Send for MutexGuard<'_, T> {}
+
+/// A [`MutexGuard`] can be `Sync` even though it is not `Send` because ownership is not transfer to
+/// a new thread. Because ownership always stays on the original thread `Sync`
+/// is safe to implement for [`MutexGuard`].
+/// See the `!Send` implementation on [`MutexGuard`] for more details.
 #[stable(feature = "mutexguard", since = "1.19.0")]
 unsafe impl<T: ?Sized + Sync> Sync for MutexGuard<'_, T> {}