What does () mean as an argument in a function where a parameter of type T is expected?

Question

I am new to Rust and I was reading the Dining Philosophers' tutorial when I found this:

Mutex::new(())

I don't know what the argument inside new means. I read the documentation for Mutex and I still have no idea what it means. I would appreciate an explanation about what is happening under the hood.

Answer 1

() is the empty tuple, also called the unit type -- a tuple with no member types. It is also the only valid value of said type. It has a size of zero (note that it is still Sized, just with a size of 0), making it nonexistent at runtime. This has several useful effects, one of which is being used here.

Here, () is used to create a Mutex with no owned data -- it's just an unlockable and lockable mutex. If we explicitly write out the type inference with the turbofish operator ::<>, we could also write:

Mutex::<()>::new( () )

That is, we're creating a new Mutex that contains a () with the initial value ().