xso: add traits for XML text <-> value conversion

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).

xso/Cargo.toml

@@ -14,6 +14,14 @@ rxml = { version = "0.11.0", default-features = false }
 minidom = { version = "^0.15" }
 xso_proc = { version = "0.0.2", optional = true }
 
+# optional dependencies to provide text conversion to/from types from these crates
+# NOTE: because we don't have public/private dependencies yet and cargo
+# defaults to picking the highest matching version by default, the only
+# sensible thing we can do here is to depend on the least version of the most
+# recent semver of each crate.
+jid = { version = "^0.10", optional = true }
+uuid = { version = "^1", optional = true }
+
 [features]
 macros = [ "dep:xso_proc" ]
 minidom = [ "xso_proc/minidom"]

xso/src/lib.rs

@@ -1,3 +1,4 @@
+#![cfg_attr(docsrs, feature(doc_cfg))]
 #![forbid(unsafe_code)]
 #![warn(missing_docs)]
 /*!
@@ -22,6 +23,7 @@ use of this library in parsing XML streams like specified in RFC 6120.
 pub mod error;
 #[cfg(feature = "minidom")]
 pub mod minidom_compat;
+mod text;
 
 #[doc(hidden)]
 pub mod exports {
@@ -30,6 +32,8 @@ pub mod exports {
     pub use rxml;
 }
 
+use std::borrow::Cow;
+
 #[doc = include_str!("from_xml_doc.md")]
 #[doc(inline)]
 #[cfg(feature = "macros")]
@@ -130,6 +134,126 @@ pub trait FromXml {
     ) -> Result<Self::Builder, self::error::FromEventsError>;
 }
 
+/// Trait allowing to convert XML text to a value.
+///
+/// This trait is similar to [`std::str::FromStr`], however, due to
+/// restrictions imposed by the orphan rule, a separate trait is needed.
+/// Implementations for many standard library types are available. In
+/// addition, the following feature flags can enable more implementations:
+///
+/// - `jid`: `jid::Jid`, `jid::BareJid`, `jid::FullJid`
+/// - `uuid`: `uuid::Uuid`
+///
+/// Because of this unfortunate situation, we are **extremely liberal** with
+/// accepting optional dependencies for this purpose. You are very welcome to
+/// make merge requests against this crate adding support for parsing
+/// third-party crates.
+pub trait FromXmlText: Sized {
+    /// Convert the given XML text to a value.
+    fn from_xml_text(data: String) -> Result<Self, self::error::Error>;
+}
+
+impl FromXmlText for String {
+    fn from_xml_text(data: String) -> Result<Self, self::error::Error> {
+        Ok(data)
+    }
+}
+
+impl<T: FromXmlText, B: ToOwned<Owned = T>> FromXmlText for Cow<'_, B> {
+    fn from_xml_text(data: String) -> Result<Self, self::error::Error> {
+        Ok(Cow::Owned(T::from_xml_text(data)?))
+    }
+}
+
+impl<T: FromXmlText> FromXmlText for Option<T> {
+    fn from_xml_text(data: String) -> Result<Self, self::error::Error> {
+        Ok(Some(T::from_xml_text(data)?))
+    }
+}
+
+impl<T: FromXmlText> FromXmlText for Box<T> {
+    fn from_xml_text(data: String) -> Result<Self, self::error::Error> {
+        Ok(Box::new(T::from_xml_text(data)?))
+    }
+}
+
+/// Trait to convert a value to an XML text string.
+///
+/// This trait is implemented for many standard library types implementing
+/// [`std::fmt::Display`]. In addition, the following feature flags can enable
+/// more implementations:
+///
+/// - `jid`: `jid::Jid`, `jid::BareJid`, `jid::FullJid`
+/// - `uuid`: `uuid::Uuid`
+///
+/// Because of the unfortunate situation as described in [`FromXmlText`], we
+/// are **extremely liberal** with accepting optional dependencies for this
+/// purpose. You are very welcome to make merge requests against this crate
+/// adding support for parsing third-party crates.
+pub trait IntoXmlText: Sized {
+    /// Convert the value to an XML string in a context where an absent value
+    /// cannot be represented.
+    fn into_xml_text(self) -> Result<String, self::error::Error>;
+
+    /// Convert the value to an XML string in a context where an absent value
+    /// can be represented.
+    ///
+    /// The provided implementation will always return the result of
+    /// [`Self::into_xml_text`] wrapped into `Some(.)`. By re-implementing
+    /// this method, implementors can customize the behaviour for certain
+    /// values.
+    fn into_optional_xml_text(self) -> Result<Option<String>, self::error::Error> {
+        Ok(Some(self.into_xml_text()?))
+    }
+}
+
+impl IntoXmlText for String {
+    fn into_xml_text(self) -> Result<String, self::error::Error> {
+        Ok(self)
+    }
+}
+
+impl<T: IntoXmlText> IntoXmlText for Box<T> {
+    fn into_xml_text(self) -> Result<String, self::error::Error> {
+        T::into_xml_text(*self)
+    }
+}
+
+impl<T: IntoXmlText, B: ToOwned<Owned = T>> IntoXmlText for Cow<'_, B> {
+    fn into_xml_text(self) -> Result<String, self::error::Error> {
+        T::into_xml_text(self.into_owned())
+    }
+}
+
+/// Specialized variant of [`IntoXmlText`].
+///
+/// Do **not** implement this unless you cannot implement [`IntoXmlText`]:
+/// implementing [`IntoXmlText`] is more versatile and an
+/// [`IntoOptionalXmlText`] implementation is automatically provided.
+///
+/// If you need to customize the behaviour of the [`IntoOptionalXmlText`]
+/// blanket implementation, implement a custom
+/// [`IntoXmlText::into_optional_xml_text`] instead.
+pub trait IntoOptionalXmlText {
+    /// Convert the value to an XML string in a context where an absent value
+    /// can be represented.
+    fn into_optional_xml_text(self) -> Result<Option<String>, self::error::Error>;
+}
+
+impl<T: IntoXmlText> IntoOptionalXmlText for T {
+    fn into_optional_xml_text(self) -> Result<Option<String>, self::error::Error> {
+        <Self as IntoXmlText>::into_optional_xml_text(self)
+    }
+}
+
+impl<T: IntoOptionalXmlText> IntoOptionalXmlText for Option<T> {
+    fn into_optional_xml_text(self) -> Result<Option<String>, self::error::Error> {
+        self.map(T::into_optional_xml_text)
+            .transpose()
+            .map(Option::flatten)
+    }
+}
+
 /// Attempt to transform a type implementing [`IntoXml`] into another
 /// type which implements [`FromXml`].
 pub fn transform<T: FromXml, F: IntoXml>(from: F) -> Result<T, self::error::Error> {

xso/src/text.rs

@@ -0,0 +1,105 @@
+// Copyright (c) 2024 Jonas Schäfer <jonas@zombofant.net>
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+//! Module containing implementations for conversions to/from XML text.
+
+use crate::{error::Error, FromXmlText, IntoXmlText};
+
+#[cfg(feature = "jid")]
+use jid;
+#[cfg(feature = "uuid")]
+use uuid;
+
+macro_rules! convert_via_fromstr_and_display {
+    ($($(#[cfg(feature = $feature:literal)])?$t:ty,)+) => {
+        $(
+            $(
+                #[cfg(feature = $feature)]
+                #[cfg_attr(docsrs, doc(cfg(feature = $feature)))]
+            )?
+            impl FromXmlText for $t {
+                fn from_xml_text(s: String) -> Result<Self, Error> {
+                    s.parse().map_err(Error::text_parse_error)
+                }
+            }
+
+            $(
+                #[cfg(feature = $feature)]
+                #[cfg_attr(docsrs, doc(cfg(feature = $feature)))]
+            )?
+            impl IntoXmlText for $t {
+                fn into_xml_text(self) -> Result<String, Error> {
+                    Ok(self.to_string())
+                }
+            }
+        )+
+    }
+}
+
+/// This provides an implementation compliant with xsd::bool.
+impl FromXmlText for bool {
+    fn from_xml_text(s: String) -> Result<Self, Error> {
+        match s.as_str() {
+            "1" => "true",
+            "0" => "false",
+            other => other,
+        }
+        .parse()
+        .map_err(Error::text_parse_error)
+    }
+}
+
+/// This provides an implementation compliant with xsd::bool.
+impl IntoXmlText for bool {
+    fn into_xml_text(self) -> Result<String, Error> {
+        Ok(self.to_string())
+    }
+}
+
+convert_via_fromstr_and_display! {
+    u8,
+    u16,
+    u32,
+    u64,
+    u128,
+    usize,
+    i8,
+    i16,
+    i32,
+    i64,
+    i128,
+    isize,
+    f32,
+    f64,
+    std::net::IpAddr,
+    std::net::Ipv4Addr,
+    std::net::Ipv6Addr,
+    std::net::SocketAddr,
+    std::net::SocketAddrV4,
+    std::net::SocketAddrV6,
+    std::num::NonZeroU8,
+    std::num::NonZeroU16,
+    std::num::NonZeroU32,
+    std::num::NonZeroU64,
+    std::num::NonZeroU128,
+    std::num::NonZeroUsize,
+    std::num::NonZeroI8,
+    std::num::NonZeroI16,
+    std::num::NonZeroI32,
+    std::num::NonZeroI64,
+    std::num::NonZeroI128,
+    std::num::NonZeroIsize,
+
+    #[cfg(feature = "uuid")]
+    uuid::Uuid,
+
+    #[cfg(feature = "jid")]
+    jid::Jid,
+    #[cfg(feature = "jid")]
+    jid::FullJid,
+    #[cfg(feature = "jid")]
+    jid::BareJid,
+}