The previous wording was a bit ominous in places ("Because of the
unfortunate situation as described in `FromXmlText`"). This should be
clearer and provides hopefully clearer instructions.
The `xso::text::Base64` struct remains as a shorthand (because frankly,
I find the const names in the base64 crate very unwieldly), but you can
now use any of the base64 engines as codec.
This text codec was previously implemented only for Option<String>, this
extends it to all types implementing those two traits, such as numbers
or JIDs.
This allows stateful or configurable codecs without having to express
all configuration in the type name itself. For example, we could have a
Base64 type with configurable Base64 engines without having to duplicate
the Base64 type itself.
(Note that the different engines in the Base64 crate are values, not
types.)
This introduces support for `Cow<'_, [u8]>` (which is not needed
currently, but still good to have) and generalizes the `Option<T>`
implementation so that it doesn't have to be copied for every other type
supported to be Base64'd (we may add support for `bytes::Bytes` at some
point, for instance).
Now it's not limited to a single feature gate per type (or even just
feature gates: it should now also be possible to add constraints
based on OS, for example) anymore.
This codec converts from a colon-separated case-insensitive hexadecimal
string into a Vec of bytes, and back to a lowercase colon-separated
hexadecimal string. Each byte must be separated by exactly one colon.
Text codecs allow to customize the conversion of data from/to XML,
in particular in two scenarios:
1. When the type for which the behaviour is to be defined comes from a
foreign crate, preventing the implementation of
FromXmlText/IntoXmlText.
2. When there is not one obvious, or more than one sensible, way to
convert a value to XML text and back.
The traits have undergone a couple iterations and this is what we end up
with. The core issue which makes this entire thing ugly is the
Orphan Rule, preventing some trait implementations relating to types
which haven't been defined in this crate.
In an ideal world, we would implement FromXmlText and IntoXmlText for
all types implementing FromStr and/or fmt::Display.
This comes with two severe issues:
1. Downstream crates cannot chose to have different
parsing/serialisation behaviour for "normal" text vs. xml.
2. We ourselves cannot define a behaviour for `Option<T>`. `Option<T>`
does not implement `FromStr` (nor `Display`), but the standard
library *could* do that at some point, and thus Rust doesn't let us
implement e.g. `FromXmlText for Option<T> where T: FromXmlText`,
if we also implement it on `T: FromStr`.
The second one hurts particularly once we get to optional attributes:
For these, we need to "detect" that the type is in fact `Option<T>`,
because we then need to invoke `FromXmlText` on `T` instead of
`Option<T>`. Unfortunately, we cannot do that: macros operate on token
streams and we have no type information available.
We can of course match on the name `Option`, but that breaks down when
users re-import `Option` under a different name. Even just enumerating
all the possible correct ways of using `Option` from the standard
library (there are more than three) would be a nuisance at best.
Hence, we need *another* trait or at least a specialized implementation
of `FromXmlText for Option<T>`, and we cannot do that if we blanket-impl
`FromXmlText` on `T: FromStr`.
That makes the traits what they are, and introduces the requirement that
we know about any upstream crate which anyone might want to parse from
or to XML. This sucks a lot, but that's the state of the world. We are
late to the party, and we cannot expect everyone to do the same they
have done for `serde` (many crates have a `feature = "serde"` which then
provides Serialize/Deserialize trait impls for their types).
