drop(x);

  x;

The main difference to C's free is that you simply cannot double-free a value (without using unsafe). That's because a value is only dropped when it falls out of scope -- and since it values only have one owner (and references must not outlive values) there cannot be any dangling references after it's dropped.

The documentation for std::mem::drop is explaining that dropping a value is a property of the language and scoping rules -- thus there is nothing for the function to do other than take a value and let it go out of scope.