Breaking change to deserialization in 0.27.0 - Err value: Custom("missing field ...")

[spazm]

Unexpected breaking change to deserialization behavior. (AKA a bug was fixed 😄 )

Updating to current v0.27.1 caused my deserializer library to fail with a Custom("missing field '...').
Note: the error message uses backticks (`) rather than single quotes ('), but those are hard to write in markdown. This error did not make it clear how to find the problem. Updating from v0.23.0 to v0.26.0 was transparent. Updating to v0.27.1 caused errors.

After a considerable amount of digging, I found that this was an intentional change. I don't know where I should have looked to find information about the change and how to migrate my code. The documentation for quick_xml::de was updated with the new syntax, but it doesn't show that the syntax changed in this version.

The change:
Deserialization used to treat attributes and elements equivalently when de-serializing. This was changed in v0.27.0, as part of the serializer rewrite in #490 . Specifically commit 8b50c64

The description of #490 contains this "Notable Change" notice

Change how attributes is defined. Now attribute should have a name started with @ character. Without that fields serialized as elements and can be deserialized only from elements. Deserialization from element or from attribute into one field is not possible anymore. It's a weird wish anyway

Prior to 0.27, these two documents could both be deserialized into the same struct definition:

<a b="1"><c>2</c></a>

<a><b>1</b><c>2</c></a>

#[derive(Debug, Deserialize, PartialEq)]
struct A {
  b: String,
  c: String,
}

With the change to v0.27 those two documents are distinct and deserialize into different structures. Attributes now have a @ prefix and will need to a serde(rename) to convert to a valid type name. The two example documents now require slightly different struct definitions:

// <a b="1"><c>2</c></a>

#[derive(Debug, Deserialize, PartialEq)]
struct A {
  #[serde(rename = "@b")]
  b: String,
  c: String,
}

// <a><b>1</b><c>2</c></a>

#[derive(Debug, Deserialize, PartialEq)]
struct A {
  b: String,
  c: String,
}

[spazm]

I hope this helps you, random future github user, if you find this issue while searching for the meaning a Custom("missing field ...") error in code that worked prior to updating.

Hey maintainers, thanks for your work on this module!!
And especially for bringing it back from stalled status with new collaborators, woo!!

[spazm]

I am excited that this new nomenclature will let me describe which struct fields should be serialized as attributes vs elements.

I did enjoy the robust flexibility of parsing either form with the same struct code. XML that doesn't have an associated DTD can easily get munged between attr and elems during read-edit-export processes.

[Mingun]

Well, the changelog mentions this change, but maybe not highlights it very well as it should. In any case, I glad that you found the root cause and shared your experience :)

[spazm]

👍

[istellino-chub]

@spazm @Mingun is there a way to make that all attributes works as in the past version using #serde(rename.. or somehow? because when using quick-xml version 22.0 attributes flow normally and now with version 0.27 no

[Mingun]

You should name your attributes starting with @ character, using #[serde(rename = "@...")] or analogues. The @ convention works also for tag and content names in enums.