xso: add type-erased and dyn-compatible variant of AsXml

That way, we don't need to know the specific type of iterator or even
iteree anymore. This can turn out useful when working with `Box<dyn _>`,
that is, in contexts where we don't know the (possible or actual) types
at compile time.

xso/ChangeLog

@@ -53,6 +53,7 @@ Version NEXT:
       - `xso::convert_via_fromstr_and_display`, a declarative macro to provide
         `AsXmlText` and `FromXmlText` implementations based on the standard
         library's `Display` and `FromStr` traits.
+      - `AsXmlDyn`, a dyn-compatible variant of `AsXml` (!573).
     * Changes
       - Generated AsXml iterator and FromXml builder types are now
         doc(hidden), to not clutter hand-written documentation with auto

xso/src/asxml.rs

@@ -127,6 +127,28 @@ impl<T: AsXml> fmt::Display for PrintRawXml<'_, T> {
     }
 }
 
+/// Dyn-compatible version of [`AsXml`].
+///
+/// This trait is automatically implemented for all types which implement
+/// `AsXml`.
+pub trait AsXmlDyn {
+    /// Return an iterator which emits the contents of the struct or enum as
+    /// serialisable [`Item`] items.
+    fn as_xml_dyn_iter<'x>(
+        &'x self,
+    ) -> Result<Box<dyn Iterator<Item = Result<Item<'x>, Error>> + 'x>, Error>;
+}
+
+impl<T: AsXml> AsXmlDyn for T {
+    /// Return an iterator which emits the contents of the struct or enum as
+    /// serialisable [`Item`] items by calling [`AsXml::as_xml_dyn_iter`].
+    fn as_xml_dyn_iter<'x>(
+        &'x self,
+    ) -> Result<Box<dyn Iterator<Item = Result<Item<'x>, Error>> + 'x>, Error> {
+        <T as AsXml>::as_xml_dyn_iter(self)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

xso/src/lib.rs

@@ -100,7 +100,7 @@ pub mod exports {
     }
 }
 
-use alloc::{borrow::Cow, string::String, vec::Vec};
+use alloc::{borrow::Cow, boxed::Box, string::String, vec::Vec};
 
 #[doc(inline)]
 pub use fromxml::Context;
@@ -150,6 +150,22 @@ pub trait AsXml {
     /// Return an iterator which emits the contents of the struct or enum as
     /// serialisable [`Item`] items.
     fn as_xml_iter(&self) -> Result<Self::ItemIter<'_>, self::error::Error>;
+
+    /// Return the same iterator as [`as_xml_iter`][`Self::as_xml_iter`], but
+    /// boxed to erase the concrete iterator type.
+    ///
+    /// The provided implementation uses a simple cast. In most cases, it does
+    /// not make sense to override the implementation. The only exception is
+    /// if [`Self::ItemIter`] is already a boxed type, in which case
+    /// overriding this method can avoid double-boxing the iterator.
+    fn as_xml_dyn_iter<'x>(
+        &'x self,
+    ) -> Result<
+        Box<dyn Iterator<Item = Result<Item<'x>, self::error::Error>> + 'x>,
+        self::error::Error,
+    > {
+        self.as_xml_iter().map(|x| Box::new(x) as Box<_>)
+    }
 }
 
 /// Trait for a temporary object allowing to construct a struct from