Meta variant structs and enums

Meta variants are an optional feature, useful for scenarios where you'd want nested enums at the top-level. structs will be created for all combinations of meta_variants and variants, names in the format {BaseName}{MetaVariantName}{VariantName}. Additionally, enums will be created for each meta_variant named {BaseName}{MetaVariantName}.

For example:

#![allow(unused)]
fn main() {
#[superstruct(meta_variants(Baz, Qux), variants(Foo, Bar))]
struct MyStruct {
    name: String,
    #[superstruct(only(Foo))]
    location: u16,
    #[superstruct(meta_only(Baz))]
    score: u64,
    #[superstruct(only(Bar), meta_only(Qux))]
    id: usize,
}
}

Here the BaseName is MyStruct and there are two variants in the meta-enum called Baz and Qux.

The generated enums are:

#![allow(unused)]
fn main() {
enum MyStruct {
    Baz(MyStructBaz),
    Qux(MyStructQux),
}

enum MyStructBaz {
    Foo(MyStructBazFoo),
    Bar(MyStructBazBar),
}

enum MyStructQux {
    Foo(MyStructQuxFoo),
    Bar(MyStructQuxBar),
}
}

The generated variant structs are:

#![allow(unused)]
fn main() {
struct MyStructBazFoo {
    name: String,
    location: u16,
    score: u64,
}

struct MyStructBazBar {
    name: String,
    score: u64,
}

struct MyStructQuxFoo {
    name: String,
    location: u16,
}

struct MyStructQuxBar {
    name: String,
    id: usize,
}
}

Note how the only attribute still applies, and a new meta_only attribute can be used to control the presence of fields in each meta variant.

For more information see Struct attributes.