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/error_message.rs

@@ -0,0 +1,81 @@
+// 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/.
+
+//! Infrastructure for contextual error messages
+
+use std::fmt;
+
+use syn::*;
+
+/// Reference to a compound field's parent
+///
+/// This reference can be converted to a hopefully-useful human-readable
+/// string via [`std::fmt::Display`].
+#[derive(Clone, Debug)]
+pub(super) enum ParentRef {
+    /// The parent is addressable by a path, e.g. a struct type or enum
+    /// variant.
+    Named(Path),
+}
+
+impl From<Path> for ParentRef {
+    fn from(other: Path) -> Self {
+        Self::Named(other)
+    }
+}
+
+impl From<&Path> for ParentRef {
+    fn from(other: &Path) -> Self {
+        Self::Named(other.clone())
+    }
+}
+
+impl fmt::Display for ParentRef {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Self::Named(name) => {
+                let mut first = true;
+                for segment in name.segments.iter() {
+                    if !first || name.leading_colon.is_some() {
+                        write!(f, "::")?;
+                    }
+                    first = false;
+                    write!(f, "{}", segment.ident)?;
+                }
+                write!(f, " element")
+            }
+        }
+    }
+}
+
+/// Ephemeral struct to create a nice human-readable representation of
+/// [`syn::Member`].
+///
+/// It implements [`std::fmt::Display`] for that purpose and is otherwise of
+/// little use.
+#[repr(transparent)]
+struct FieldName<'x>(&'x Member);
+
+impl fmt::Display for FieldName<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self.0 {
+            Member::Named(v) => write!(f, "field '{}'", v),
+            Member::Unnamed(v) => write!(f, "unnamed field {}", v.index),
+        }
+    }
+}
+
+/// Create a string error message for a missing attribute.
+///
+/// `parent_name` should point at the compound which is being parsed and
+/// `field` should be the field to which the attribute belongs.
+pub(super) fn on_missing_attribute(parent_name: &ParentRef, field: &Member) -> String {
+    format!(
+        "Required attribute {} on {} missing.",
+        FieldName(&field),
+        parent_name
+    )
+}