> Structs in Crystal are are always passed by copy, so modifying them can be problematic. For example, my_struct.x = 7 is fine but array_of_structs[2].x = 5 will not work. To work around this, copy the whole struct, modify it, then write it back. Better yet, avoid the need to modify structs (work with them like with immutable objects).

I can see that becoming problematic for simulations (lots of particles, for example) and games especially. At some point you're going to want pass-by-reference and in-place mutations for better performance (ideally hidden behind some form of encapsulation of course). Is there no way around this?