Feature request: Support key on character arrays in structs

[dbechrd]

In the same vein as https://github.com/google/flatbuffers/issues/4195, it would be nice if it were possible to easily use fixed-length character buffers in structs alongside the automatic sorting and LookupByKey code gen provided by (key).

In other words, it would be cool if this schema were valid:

struct UserAccount {
    username:[uint8:32] (key);
    password:[uint8:32];
}

Currently, it throws the error: error: 'username' field must be string or scalar type

I'm working around this by using this schema instead:

table UserAccount {
    username:string (key);
    password:string;
}

along with the CreateString overload that takes an explicit length (e.g. fbb.CreateString("dbechrd", 32)) in order to pre-allocate enough storage to allow me to mutate the username later, but then it becomes very tedious to copy UserAccounts around in C++ since they're now tables instead of structs, and there doesn't seem to be any straightforward way to get the data() and size() of a table (it's probably possible via reflection, but that's not ideal).

Dunno if this is feasible; just a thought.

[dbaileychess]

I would be supportive of this. A fixed-size array is a scalar IMO, and anything else you can place in a struct is technically a scalar type as well. So other structs could also be keys as well.

[sunwen18]

I created a PR #7623, this one only enables using an array of scalar type as key, will have follow up PRs for using struct and array of struct as key.

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