One record answers another, from a list

Three small things that together make a board work: a link on every
listed record to a page about it (ResourceSpec.link, {id} and {field}
in the address); a state mail whose `to` is a field of the record this
one belongs to ({notice.email}), with that record's fields in the
text, and a lint that the form carries the record field it reaches
through; and value: "{user.email}" filled from the session and hidden,
so someone the site knows is not asked who they are.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GT37Z1Xtfd9pUuQtMTg6Yt

src/content/validate.rs

@@ -135,6 +135,27 @@ pub fn validate_questions(
         }
     }
 
+    // A mail to the record this one belongs to goes through a field of
+    // the form: `{notice.email}` needs a `type: record` field called
+    // `notice` on every form that records into the bucket, or the mail
+    // has nowhere to go.
+    for (bucket, schema) in aggregates {
+        for (state, spec) in &schema.mail_for_state {
+            let Some(parent) = crate::mail::parent_of(&spec.to) else { continue };
+            for question in questions.values() {
+                for alternative in question.alternatives.iter().filter(|a| a.record_as.as_deref() == Some(bucket.as_str())) {
+                    let has = alternative.features.iter().flat_map(|f| &f.requirements).any(|r| r.name == parent && r.kind == "record" && r.of.is_some());
+                    if !has {
+                        anyhow::bail!(
+                            "question {:?} alternative {:?}: records into {bucket:?}, whose state {state:?} mails {:?}, so it must carry a `type: record` field called {parent:?} with `of`",
+                            question.id, alternative.name, spec.to
+                        );
+                    }
+                }
+            }
+        }
+    }
+
     // A state that grants a group makes an account for the person the
     // record is about, so the record has to know who they are. Same
     // shape as an invite's own rule, one step further back: the form