Test #[serde(flatten)] support

[webmaster128]

Closes #20

[ethanfrey]

Nice test to show this works.

But, it would be good to ensure it works for the particular case we would use in CosmWasm... enums flattening their variants (which are other enums).

I hope this works, but may bring up some unreachable!() instruction it would be good to flag

[ethanfrey]

This is a great test case. But it would be good to have one or two for the specific version we would use in CosmWasm.

struct ExecMsg1 {
  something: String,
}

struct ExecMsg2 {
  other: u64,
}

/// one example to test
enum ExecMsg {
  #[serde(flatten)]
  One(ExecMsg1),
  #[serde(flatten)]
  Two(ExecMsg2),
}

/// another example to test
enum ExecMsg {
  #[serde(flatten)]
  One(ExecMsg1),
  // important to leave top level along with a flattened variant
  SetAge { age: u32 },
  // also interesting to see if we can use this format with them (unflattened) 
  Custom(CustomData),
}

You can make it a bit nicer than that, but combining a cw20 ExecMsg and my contracts custom one is a clear case.

[webmaster128]

Yeah, right. I can easily make the flatten test for enums instead of structs. This is the part we need for CosmWasm.

[webmaster128]

Turns out flatten is not for enums (see feature request).

So this PR would close the ticket and if we hit new issues, we can create more refined ones.

[ethanfrey]

Great link.
Yes, we need untagged for that, which is another issue #43

This PR closes the linked issue well

[ethanfrey]

lgtm (comments already addressed)