Rust: 聊聊AtomicPtr＜()＞和 *const ()

在Bytes库在github源码（https://docs.rs/bytes/1.1.0/src/bytes/bytes.rs.html#94-100）有关Bytes的定义中，

pub struct Bytes {
 	ptr: *const u8,
 	 len: usize, // inlined "trait object"
   	data: AtomicPtr<()>, 
  	 vtable: &'static Vtable,
   }

其中的data字段中有(),一般地，()表示一个空元组。但在这里，是不是这个意思？
我们看到，data可以用这个来赋值：

AtomicPtr::new(ptr::null_mut())

那ptr::null_mut()又是啥？查看https://doc.rust-lang.org/stable/std/ptr/fn.null_mut.html，可以看到：

pub const fn null_mut() -> *mut T {
    from_raw_parts_mut(invalid_mut(0), ())
}
//再查invaild_mut
pub const fn invalid_mut(addr: usize) -> *mut T {
    // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
    // We use transmute rather than a cast so tools like Miri can tell that this
    // is *not* the same as from_exposed_addr.
    // SAFETY: every valid integer is also a valid pointer (as long as you don't dereference that
    // pointer).
    unsafe { mem::transmute(addr) }
}

表示：Creates a null mutable raw pointer.类似，创建一个不变的raw pointer有，

pub const fn null() -> *const T
where
    T: Thin + ?Sized,

进而，查看https://doc.rust-lang.org/src/core/sync/atomic.rs.html#175，可以了解AtomicPtr的定义，

pub struct AtomicPtr {
    p: UnsafeCell<*mut T>,
}

也就是可以了解，Bytes中的data字段是一个用AtomicPtr封装起来的一个函数的可变raw pointer.

当然，上面的data还可以这样赋值：

// slice是Box<[u8]>
//let ptr = Box::into_raw(slice) as *mut u8;
//let data = ptr as usize
data: AtomicPtr::new(data as *mut _),

另外，*const ()也自然可以联想到，它可能是表示指向一个固定的函数的raw pointer.

fn foo(i32) -> i32 {
    666666
}
// Crucially, we `as`-cast to a raw pointer before `transmute`ing to a function pointer.
// This avoids an integer-to-pointer `transmute`, which can be problematic.
// Transmuting between raw pointers and function pointers (i.e., two pointer types) is fine.
let pointer = foo as *const ();
let function = unsafe {
    std::mem::transmute::<*const (), fn(i32) -> i32>(pointer)
};
assert_eq!(function(42), 666666);