Certain FIDL types can be made optional with no change to the wire shape of their containing message with the addition of the :optional constraint. Further, the table layout is always optional, while the struct layout never is. To make a struct optional, it must be wrapped in a box<T>, thereby changing the wire shape of its containing message.

Base type	Optional version	Does optionality change the wire layout?
struct {...}	box<struct {...}>	Yes
table {...}	table {...}	No
union {...}	union {...}:optional	No
vector<T>	vector<T>:optional	No
string	string:optional	No
zx.Handle	zx.Handle:optional	No
client_end:P	client_end:<P, optional>	No
server_end:P	server_end:<P, optional>	No

All other types (bits, enum, array<T, N>, and the primitive types) cannot be made optional.

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