Could `impl_deserialize_for_internally_tagged_enum!` handle a default case?

[aesteve]

Hello and thanks a lot for your work.

Would it be possible (and would it make sense) for impl_deserialize_for_internally_tagged_enum to handle a default case?

Like "for every other value of this tag, just deserialise to this enum variant.

Not sure how common this is but I'm facing this exact pattern where I'd like to handle a few variants that I know are specific, and all the rest can just be fall into a generic struct.

I hope it's clear, sorry if it's already possible and I missed it. Happy to update the docs to highlight it in this case.

Thanks a lot.

[Mingun]

It is not clear for me what exactly you need. Perhaps this is already could be implemented using existing serde API. Could you provide an example of your Rust struct and XML that you want to map?

[aesteve]

Sorry for the confusion, let me try to explain this a bit better.

<Root>
  <SomeElement type="Specific">
      <MoreSpecific>a</MoreSpecific>
      <Common>123</Common>
  </SomeElement>
  <SomeElement type="AnotherType">
     <Common>MaybeNotAnInt</Common>
  </SomeElement>
  <SomeElement type="YetAnotherType">
     <Common>AnotherString</Common>
  </SomeElement>
</Root>

I could map to the following enum:

enum Elements {
  Specific({Common: u32, MoreSpecific: String}),
  TheRest({Common: String})
}

As of now, I deserialize to:

struct CommonStructure {
  r#type: String,
  Common: String,
  MoreSpecific: Option<String>
}

and then match on r#type to create the proper value.

I'd really like to use impl_deserialize_for_internally_tagged_enum but that means (unless I'm missing something) I have to list every possible value for type= which I can't know for sure.

The idea is that I'd like "everything that's not type=Specific to be handled as TheRest, no matter @type.

I'm sorry I can't explain it better, but basically getting the equivalent of the _ => ... pattern in a match expression.

[Mingun]

Ah, I didn't immediately realize it was about macro impl_deserialize_for_internally_tagged_enum! from quick-xml. Probably this could be implemented. You should replace default arm (_) here

quick-xml/src/serde_helpers.rs

Lines 181 to 186 in 1be35e1

	match tag.as_ref() {
	$(
	$variant_tag => Ok(deserialize_variant!( de, $enum, $($variant)+ )),
	)*
	_ => Err(A::Error::unknown_field(&tag, &[$($variant_tag),+])),
	}

with Ok(deserialize_variant!(...)) with the name of default variant. Feel free to submit a PR!

[aesteve]

Thanks for the detailed answer.

Unfortunately I have never written a single macro, so it's probably gonna take a while for me to find the syntax to add an optional argument (so that it's backward compatible), and how to deal with this in the body macro.

I'll try to, at some point, but can't promise I'll manage to 😅

Thanks again.

[Mingun]

Fixed in #634