xso-proc: introduce trait for StructDef

Can you see it coming?

xso-proc/src/common.rs

@@ -0,0 +1,57 @@
+// 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/.
+
+//! Definitions common to both enums and structs
+
+use proc_macro2::TokenStream;
+use syn::*;
+
+/// Parts necessary to construct a `::xso::FromXml` implementation.
+pub(crate) struct FromXmlParts {
+    /// Additional items necessary for the implementation.
+    pub(crate) defs: TokenStream,
+
+    /// The body of the `::xso::FromXml::from_xml` function.
+    pub(crate) from_events_body: TokenStream,
+
+    /// The name of the type which is the `::xso::FromXml::Builder`.
+    pub(crate) builder_ty_ident: Ident,
+}
+
+/// Parts necessary to construct a `::xso::AsXml` implementation.
+pub(crate) struct AsXmlParts {
+    /// Additional items necessary for the implementation.
+    pub(crate) defs: TokenStream,
+
+    /// The body of the `::xso::AsXml::as_xml_iter` function.
+    pub(crate) as_xml_iter_body: TokenStream,
+
+    /// The type which is the `::xso::AsXml::ItemIter`.
+    pub(crate) item_iter_ty: Type,
+
+    /// The lifetime name used in `item_iter_ty`.
+    pub(crate) item_iter_ty_lifetime: Lifetime,
+}
+
+/// Trait describing the definition of the XML (de-)serialisation for an item
+/// (enum or struct).
+pub(crate) trait ItemDef {
+    /// Construct the parts necessary for the caller to build an
+    /// `xso::FromXml` implementation for the item.
+    fn make_from_events_builder(
+        &self,
+        vis: &Visibility,
+        name_ident: &Ident,
+        attrs_ident: &Ident,
+    ) -> Result<FromXmlParts>;
+
+    /// Construct the parts necessary for the caller to build an `xso::AsXml`
+    /// implementation for the item.
+    fn make_as_xml_iter(&self, vis: &Visibility) -> Result<AsXmlParts>;
+
+    /// Return true iff the user requested debug output.
+    fn debug(&self) -> bool;
+}