xiaosuo@gentux test $ cat unamed.c
struct test {
        struct {
                int a;
        };
};

int main()
{
        struct test t1;
        t1.a = 0; /* it is OK */

        struct test t2 = {.a = 0}; /* error */

        return 0;
}
xiaosuo@gentux test $ gcc unamed.c
unamed.c: In function 'main':
unamed.c:12: error: unknown field 'a' specified in initializer