What is this “[0 … 255] =” syntax in C?

... is an extension provided by GCC

https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html#Designated-Inits

To initialize a range of elements to the same value, write [first ...
last] = value. This is a GNU extension. For example,

 int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };

&& is another extension

https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html#Labels-as-Values

You can get the address of a label defined in the current function (or
a containing function) with the unary operator &&. The value has
type void *. This value is a constant and can be used wherever a
constant of that type is valid. For example:

 void *ptr;
 /* ... */
 ptr = &&foo;