PrefixedAttName lost in subtree when serializing

[valkum]

Another (possibly) bug I encountered while trying to implement more epp specs (launch-1.0 which requires rfc7848 see https://datatracker.ietf.org/doc/html/rfc7848#page-11 for example).

If you have types in one namespace that reference types in another namespace and you want to use prefixes, the serializer will assume the wrong default namespace for all elements below the first prefixed namespace.

Repro:

use instant_xml::{FromXml, ToXml};

const NS1: &str = "urn:ns1";
const NS2: &str = "urn:ns2";

#[derive(FromXml, ToXml, PartialEq, Debug)]
#[xml(ns(NS1, ns2 = NS2))]
struct Root {
    a: A,
    // This uses a field from a different namespace
    b: B,
}

#[derive(FromXml, ToXml, PartialEq, Debug)]
#[xml(ns(NS1))]
struct A {
    value: String,
}

// These are structs in the NS2 namespace
#[derive(FromXml, ToXml, PartialEq, Debug)]
#[xml(ns(NS2))]
struct B {
    value: String,
    c: C,
}

#[derive(FromXml, ToXml, PartialEq, Debug)]
#[xml(ns(NS2))]
struct C {
    value: String,
}

fn main() {
    let root = Root {
        a: A {
            value: "Value A".to_string(),
        },
        b: B {
            value: "Value B".to_string(),
            c: C {
                value: "Value C".to_string(),
            },
        },
    };

    let xml_string = instant_xml::to_string(&root).unwrap();
    println!("Serialized XML:\n{}", xml_string);
}

Serializes to

<Root xmlns="urn:ns1" xmlns:ns2="urn:ns2">
  <A>
    <value>Value A</value>
  </A>
  <ns2:B>
    <value>Value B</value>
    <C>
      <value>Value C</value>
    </C>
  </ns2:B>
</Root>

But should serialize to either:

<Root xmlns="urn:ns1" xmlns:ns2="urn:ns2">
  <A>
    <value>Value A</value>
  </A>
  <ns2:B>
    <ns2:value>Value B</ns2:value>
    <ns2:C>
      <ns2:value>Value C</ns2:value>
    </ns2:C>
  </ns2:B>
</Root>

Note that you can remove the prefix ns from Root, to get a <B xmlns="urn:ns2"> which will emit the correct code with default namspacing rules.

From the XML name spec:

The scope of a namespace declaration declaring a prefix extends from the beginning of the start-tag in which it appears to the end of the corresponding end-tag, excluding the scope of any inner declarations with the same NSAttName part. In the case of an empty tag, the scope is the tag itself.
Thus the ns2 prefix is missing on C and the respective values inside <ns2:B> and </ns2:B>

[djc]

I think it should serialize to this:

<Root xmlns="urn:ns1" xmlns:ns2="urn:ns2">
  <A>
    <value>Value A</value>
  </A>
  <B xmlns="urn:ns2">
    <value>Value B</value>
    <C>
      <value>Value C</value>
    </C>
  </B>
</Root>

That seems like a bug that shouldn't be too hard to fix?

[valkum]

While this should be equivalent, most examples in EPP specs use prefixed names. In the meantime, I switched to using default namespaces, which at least emit valid namespaced code.

Being able to emit xml using proper prefixed names might be a win, as most examples in the EPP space are using prefixed names, which makes it easier to compare output against examples.

This also would make emitted xml with intertwined namespaces hard to follow, I guess.
For example:

<Root xmlns="urn:ns1" xmlns:ns2="urn:ns2">
  <A>
    <value>Value A</value>
  </A>
  <ns2:B>
    <ns2:value>Value B</ns2:value>
    <ns2:C>
      <ns2:value>Value C</ns2:value>
    </ns2:C>
    <A>
      <value>Value A</value>
    </A>
  </ns2:B>
</Root>

Using only default namespaces would cause this to be:

<Root xmlns="urn:ns1">
  <A>
    <value>Value A</value>
  </A>
  <B xmlns="urn:ns2">
    <value>Value B</value>
    <C>
      <value>Value C</value>
    </C>
    <A xmlns="urn:ns2">
      <value>Value A</value>
    </A>
  </B>
</Root>

In my eyes this is harder to follow.

[djc]

I don't think I'm motivated right now to work on "cosmetics", like following a particular style -- I'm focused on correctness (making sure serialization and deserialization are correct) and expressiveness (can useful XML schemas be expressed in Rust types), so suggest keeping issues focused on one issue at a time rather than intermixing different concerns.