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 was a bit tricky to build, because it is possible to have an
indirection through a `static` there. Thanks to Rust's extensive
const-fn capabilities, though, it's in fact possible to cover all cases.
We still do two different checks to improve user experience. If we can,
from within the proc macro, determine that two fields refer to the same
XML attribute (because their namespace/name values use the same Rust
tokens), then we reject the fields with a clear error message pointing
at both fields.
In the other case, when there's e.g. `#[xml(lang)]` and
`#[xml(attribute(namespace = rxml::XMLNS_XML, name = "lang"))]`, the
macro cannot be sure that XMLNS_XML is in fact the XML namespace. For
that case, we generate code which is evaluated at compile time (and
has no runtime impact) which panics if the namespace and name of two
attribute-matching fields is the same.
The error message will be less clear (because it contains extra,
unchangeable wording like "evaluation of constant value failed" and "the
evaluated program panicked at", which may be a bit confusing) than the
message generated by the macros themselves, but it's a price we have to
pay unfortunately.
Note that this check may seem cosmetic and purely for better user
experience, but it is in fact needed to avoid generating not-well-formed
and/or not-namespace-well-formed XML: As `AsXml` generates `xso::Item`,
where each attribute is emitted separated (and not aggregated in a
map structure), a naive (and efficient) implementation of a writer might
not double-check that no duplicate attributes are generated.
This allows exactly one arbitrary payload in any element, and is handled
after every other element with a more specific matcher has been parsed.
Both the #[xml(element(n = 1))] meta and its shortcut #[xml(element)]
are allowed and treated the exact same way.
This decreases indentation levels on the various implemenations, it
groups the implementations together physically, and (spoiler alert!)
we'll actually need the dyn Field trait object-ness (much) later on.
