Anonymous enums would be so good

One of the best features of Rust is the algebraic type system, of which the most outstanding feature is the enum not as a restricted integer but as a sum type. But there is more: enum variants can be effectively treated as anonymous structs, refactoring what would be something like this:

struct A;
struct B(usize, usize);
struct C {
  foo: isize,
  bar: String,
}

enum Composed {
  A(A),
  B(B),
  C(C),
}

in this beauty:

enum Composed {
  A,
  B(usize, usize),
  C{
    foo: isize,
    bar: String,
  },
}

This makes writing types containing multiple possible values easy, the best of the strongly typed languages I tried. And if you find the need to separate a variant, the first option is always available.

Lately, I struggle with the opposite problem: types containing some common data. The pattern I find myself writing often is:

struct Foo {
  common: T // Some common data...
  content: FooContent
}
enum FooContent {
  // alternative data
}

The problem I have with this is that FooContent is often pure boilerplate. Really rarely do you manipulate it outside of Foo, and in a lot of libraries you can find the same pattern. The only alternative I found is to rewrite common to be part of every variant but is verbose and needs a lot of boilerplate for extraction. So, why can't we have:

struct Foo {
  common: T // Some common data...
  content: enum {
    // alternative data
  }
}

It's the part I miss about C anonymous unions and struct, and it would make for great type definitions (Error types in particular). It just seems the natural counterpart of enum struct variants. The only problems I see are how to type Foo.content and references to it, and how to match against it. For the first one, I would say that anonymous types exist (see closures) while for the second there is the need for some new syntax, something like Foo { common: _, contend: ContentVariant }.

Am I missing something subtle (or obvious) that makes this more difficult than it seems? Or there is some alternative type of definition that makes it unnecessary?