xso-proc: refactor field implementations into separate files

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.

xso-proc/src/field/text.rs

@@ -0,0 +1,89 @@
+// 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/.
+
+//! This module concerns the processing of text content.
+//!
+//! In particular, it provides the `#[xml(text)]` implementation.
+
+use proc_macro2::Span;
+use quote::quote;
+use syn::*;
+
+use crate::error_message::ParentRef;
+use crate::scope::{AsItemsScope, FromEventsScope};
+use crate::types::{
+    as_xml_text_fn, from_xml_text_fn, string_ty, text_codec_decode_fn, text_codec_encode_fn,
+};
+
+use super::{Field, FieldBuilderPart, FieldIteratorPart, FieldTempInit};
+
+/// The field maps to the character data of the element.
+pub(super) struct TextField {
+    /// Optional codec to use
+    pub(super) codec: Option<Expr>,
+}
+
+impl Field for TextField {
+    fn make_builder_part(
+        &self,
+        scope: &FromEventsScope,
+        _container_name: &ParentRef,
+        member: &Member,
+        ty: &Type,
+    ) -> Result<FieldBuilderPart> {
+        let FromEventsScope { ref text, .. } = scope;
+        let field_access = scope.access_field(member);
+        let finalize = match self.codec {
+            Some(ref codec) => {
+                let decode = text_codec_decode_fn(ty.clone());
+                quote! {
+                    #decode(&#codec, #field_access)?
+                }
+            }
+            None => {
+                let from_xml_text = from_xml_text_fn(ty.clone());
+                quote! { #from_xml_text(#field_access)? }
+            }
+        };
+
+        Ok(FieldBuilderPart::Text {
+            value: FieldTempInit {
+                init: quote! { ::std::string::String::new() },
+                ty: string_ty(Span::call_site()),
+            },
+            collect: quote! {
+                #field_access.push_str(#text.as_str());
+            },
+            finalize,
+        })
+    }
+
+    fn make_iterator_part(
+        &self,
+        _scope: &AsItemsScope,
+        _container_name: &ParentRef,
+        bound_name: &Ident,
+        _member: &Member,
+        ty: &Type,
+    ) -> Result<FieldIteratorPart> {
+        let generator = match self.codec {
+            Some(ref codec) => {
+                let encode = text_codec_encode_fn(ty.clone());
+                quote! { #encode(&#codec, #bound_name)? }
+            }
+            None => {
+                let as_xml_text = as_xml_text_fn(ty.clone());
+                quote! { ::core::option::Option::Some(#as_xml_text(#bound_name)?) }
+            }
+        };
+
+        Ok(FieldIteratorPart::Text { generator })
+    }
+
+    fn captures_text(&self) -> bool {
+        true
+    }
+}