Fix serde 1.0.181 compatibility

[Mingun]

This PR reworks our mapping of enums to XML. Actually, it is more correct, because examples in our documentation was not checked and not worked.

Summary of changes

Rules:

• In $value field the representation is always the same as top-level representation;
• In $text field the representation is always the same as in normal field, but surrounding tags with field name are removed;
• In normal field the representation is always contains a tag with field name

In $text fields enums represented as xs:simpleType

In tables below the following type is used when I say about normal, $text or $value fields:

struct Container {
  field: Enum,

  #[serde(rename = "$text")]
  text: Enum,

  #[serde(rename = "$value")]
  value: Enum,
}

Normal enum variant

Old

Before this PR the representation was:

Kind	Top-level, in normal and $value fields	In $text field
Unit	<Unit/>	(empty)
Newtype	<Newtype>42</Newtype>	42
Tuple	<Tuple>42</Tuple><Tuple>answer</Tuple>	42 answer
Struct	<Struct><q>42</q><a>answer</a></Struct>	Err(Unsupported)

New

In this PR the representation changed to:

Kind	Top-level and in $value field	In normal field	In $text field
Unit	<Unit/>	<field>Unit</field>	Unit
Newtype	<Newtype>42</Newtype>	Err(Unsupported)	Err(Unsupported)
Tuple	<Tuple>42</Tuple><Tuple>answer</Tuple>	Err(Unsupported)	Err(Unsupported)
Struct	<Struct><q>42</q><a>answer</a></Struct>	Err(Unsupported)	Err(Unsupported)

$text enum variant

<field> in top-level serialization is the name of top-level element, serialization is impossible if it is not defined in the serializer.

Old

Before this PR the representation was:

Kind	In normal field	In $text and $value fields
Unit	<field/>	(empty)
Newtype	<field>42</field>	42
Tuple	<field>42 answer</field>	42 answer
Struct	Err(Unsupported)	Err(Unsupported)

New

In this PR the representation changed to:

Kind	Top-level and in $value field	In normal field	In $text field
Unit	(empty)	<field/>	(empty)
Newtype	42	Err(Unsupported)1	Err(Unsupported)2
Tuple	42 answer	Err(Unsupported)3	Err(Unsupported)4
Struct	Err(Unsupported)	Err(Unsupported)	Err(Unsupported)

Closes #630

I think, this will be the last merged PR for this release cycle, because it seems that no one will be reviewed to the end of week. I plan to release 0.31.0 on weekend.

Footnotes

1. If this serialize as <field>42</field> then it will be ambiguity during deserialization, because it clash with Unit representation in normal field ↩

2. If this serialize as 42 then it will be ambiguity during deserialization, because it clash with Unit representation in $text field ↩

3. If this serialize as <field>42 answer</field> then it will be ambiguity during deserialization, because it clash with Unit representation in normal field ↩

4. If this serialize as 42 answer then it will be ambiguity during deserialization, because it clash with Unit representation in $text field ↩

[dralley]

I don't have time to review this this week. Merge away

[Mingun]

@dralley , heh, I not quite experienced in English slang. "Merge away" -- is this "ok, you can merge this, I anyway does not known serde's much" or "please don't merge, I want to do a review when I'll have time" 😅?

[dralley]

It means you can merge it freely without needing my say so

You can merge it, yes.