xso-proc: add support for parsing attributes into Strings

This is bare-bones and is missing many features which we intend to add
in future commits, such as parsing from attributes whose names differ
from the field names and parsing into non-String types.

xso-proc/src/scope.rs

@@ -0,0 +1,73 @@
+// 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/.
+
+//! Identifiers used within generated code.
+
+use proc_macro2::Span;
+use syn::*;
+
+/// Container struct for various identifiers used throughout the parser code.
+///
+/// This struct is passed around from the [`crate::compound::Compound`]
+/// downward to the code generators in order to ensure that everyone is on the
+/// same page about which identifiers are used for what.
+///
+/// The recommended usage is to bind the names which are needed into the local
+/// scope like this:
+///
+/// ```text
+/// # let scope = FromEventsScope::new();
+/// let FromEventsScope {
+///     ref attrs,
+///     ..
+/// } = scope;
+/// ```
+pub(crate) struct FromEventsScope {
+    /// Accesses the `AttrMap` from code in
+    /// [`crate::field::FieldBuilderPart::Init`].
+    pub(crate) attrs: Ident,
+}
+
+impl FromEventsScope {
+    /// Create a fresh scope with all necessary identifiers.
+    pub(crate) fn new() -> Self {
+        // Sadly, `Ident::new` is not `const`, so we have to create even the
+        // well-known identifiers from scratch all the time.
+        Self {
+            attrs: Ident::new("attrs", Span::call_site()),
+        }
+    }
+}
+
+/// Container struct for various identifiers used throughout the generator
+/// code.
+///
+/// This struct is passed around from the [`crate::compound::Compound`]
+/// downward to the code generators in order to ensure that everyone is on the
+/// same page about which identifiers are used for what.
+///
+/// See [`FromEventsScope`] for recommendations on the usage.
+pub(crate) struct IntoEventsScope {
+    /// Accesses the `AttrMap` from code in
+    /// [`crate::field::FieldIteratorPart::Header`].
+    pub(crate) attrs: Ident,
+}
+
+impl IntoEventsScope {
+    /// Create a fresh scope with all necessary identifiers.
+    pub(crate) fn new() -> Self {
+        Self {
+            attrs: Ident::new("attrs", Span::call_site()),
+        }
+    }
+}
+
+pub(crate) fn mangle_member(member: &Member) -> Ident {
+    match member {
+        Member::Named(member) => quote::format_ident!("f{}", member),
+        Member::Unnamed(member) => quote::format_ident!("f_u{}", member.index),
+    }
+}