Warning(s):

source_file.c:17:16: warning: unused variable 'j' [-Wunused-variable]
    struct foo j = foo_make(i);
               ^
source_file.c:17:29: warning: variable 'i' is uninitialized when used here [-Wuninitialized]
    struct foo j = foo_make(i);
                            ^
source_file.c:11:22: note: expanded from macro 'foo_make'
        struct foo: (x),                             \
                     ^
source_file.c:16:5: note: variable 'i' is declared here
    struct foo i;
    ^
2 warnings generated.