gistfile1.txt

const Module = struct {
    sp: i32,
    memory: Memory,
};

const Memory = struct {
    ram: []i32,
};  

pub fn main() void {
 var module = Module { .sp = 0, .memory = null};
}

$ zig build-exe struct.zig
/tmp/struct.zig:13:43: error: expected type 'Memory', found '(null)'
 var module = Module { .sp = 0, .memory = null};
                                          ^