Make the function generic and return result(T, err), where T is the generic typed supplied by the caller (turbo fish syntax). Not sure if it will compile though.
Use whatever serve uses under the hood. They obviously have some way that allows them to return an arbitrary type. Alternatively implement it yourself by creating an enum that can be either string, int or bool. Will require matching by the caller after the function returns.
I know in e.g. java you regularly do if x.instanceof(y), but rust lends itself really badly to this type of programming.