`readonly` struct, `in` modifier and defensive copies

Hi people, I just encountered the

in

in

modifier for structs.
As far as I'm concerned the

in

in

modifier can negatively affect performance if the struct I'm passing is NOT

readonly

readonly

, because it might create defensive copies - however, how can I avoid defensive copies without marking my struct

readonly

readonly

? Is there a rule of thumb when defensive copies are created?
Thanks in advance!