error[E0277]: `std::cell::Cell<i32>` cannot be shared between threads safely
  --> $DIR/mutexguard-sync.rs:11:5
   |
LL | fn test_sync<T: Sync>(_t: T) {}
   | ---------------------------- required by `test_sync`
...
LL |     test_sync(guard);
   |     ^^^^^^^^^ `std::cell::Cell<i32>` cannot be shared between threads safely
   |
   = help: the trait `std::marker::Sync` is not implemented for `std::cell::Cell<i32>`
   = note: required because of the requirements on the impl of `std::marker::Sync` for `std::sync::MutexGuard<'_, std::cell::Cell<i32>>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.