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xso: re-implement #[xml(lang)] on top of #[xml(attribute)]

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This gives us all the goodies of `default`, `type_` and `codec` without
having to duplicate lots of code (and I think the `match`-iness of the
new macro code is still within limits).

However, we still keep them as separate `#[xml(..)]` attributes, because
their semantics are very different and it is sensible to make them stand
out.

skip-changelog, because `#[xml(lang)]` was introduced in this version.
This commit is contained in:
Jonas Schäfer 2025-04-27 10:32:17 +02:00
commit 67f3ffaced
6 changed files with 198 additions and 138 deletions
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33
xso/src/from_xml_doc.md
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@ -684,17 +684,38 @@ assert_eq!(foo, Foo {
The `lang` meta allows to access the (potentially inherited) logical
`xml:lang` value as defined in
[XML 1.0 § 2.12](https://www.w3.org/TR/REC-xml/#sec-lang-tag).
[XML 1.0 § 2.12](https://www.w3.org/TR/REC-xml/#sec-lang-tag). For `FromXml`,
the field's type must implement [`FromXmlText`] and for `AsXml`, the field's
type must implement [`AsOptionalXmlText`].
This meta supports no arguments and can only be used on fields of type
`Option<String>`.
| Key | Value type | Description |
| --- | --- | --- |
| `default` | *flag* | If present, an absent attribute will substitute the default value instead of raising an error. |
| `type_` | *type* | Optional explicit type specification. Only allowed within `#[xml(extract(fields(..)))]`. |
| `codec` | optional *expression* | [`TextCodec`] implementation which is used to encode or decode the field. |
Unlike `#[xml(attribute = "xml:lang")]`, the `#[xml(lang)]` meta takes
inheritance into account.
Unlike `#[xml(attribute = "xml:lang")]`, using `#[xml(lang)]` takes
the inheritance of the `xml:lang` attribute into account.
**Note:** Using this meta is not roundtrip-safe. `rxml` will always emit its
**Note:** Using this meta is not roundtrip-safe. `xso` will always emit its
value on serialisation, even if it was inherited during deserialisation.
If `default` is specified and there is no `xml:lang` specified at the point of
the element, the value is generated using [`core::default::Default`],
requiring the field type to implement the `Default` trait for a `FromXml`
derivation. `default` has no influence on `AsXml`. If `default` is not
specified, an error is raised if `xml:lang` has not been set on the element
or any of its ancestors.
Note that no error is generated (by `xso`) for `xml:lang` values of `""`.
If `type_` is specified and the `lang` meta is used within an
`#[xml(extract(fields(..)))]` meta, the specified type is used instead of the
field type on which the `extract` is declared.
If `codec` is given, the given `codec` value must implement
[`TextCodec<T>`][`TextCodec`] where `T` is the type of the field.
#### Example
```rust