check: warn on a mail placeholder every form marks optional

Found the same way: 'your brief for {organization}, {org_contact}' with
the contact optional reads ', .' for most people.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/mail.rs

@@ -13,8 +13,9 @@
 //!   the account is made and its sign-in link reaches nobody;
 //! - **fail** an invite mail without its `{link}`, or with a
 //!   placeholder the invite cannot fill;
-//! - **warn** a placeholder only some of the bucket's forms collect: it
-//!   is blank in the mails about the others;
+//! - **warn** a placeholder only some of the bucket's forms collect, or
+//!   one every form marks optional: it is blank in the mails about the
+//!   others, or about anyone who skipped it;
 //! - **warn** a dialogue that does not close: the person is mailed on
 //!   the way in and not told how their case ended;
 //! - **warn** a mail that tells a person their case is where they may
@@ -158,6 +159,15 @@ pub fn check(questions: &HashMap<String, Question>, aggregates: &HashMap<String,
                 let having: Vec<&String> = bucket_forms.iter().filter(|(_, f)| f.contains(&key)).map(|(n, _)| n).collect();
                 if having.is_empty() {
                     fail(format!("{at}: {{{key}}} is not a field any form recording into {bucket:?} collects, nor one portal fills ({}) - it would be sent blank", CASE_VARS.join(", ")));
+                } else if questions
+                    .values()
+                    .flat_map(|q| &q.alternatives)
+                    .filter(|a| a.record_as.as_deref() == Some(bucket.as_str()))
+                    .flat_map(|a| a.features.iter().flat_map(|f| &f.requirements))
+                    .filter(|r| r.name == key)
+                    .all(|r| r.optional)
+                {
+                    warns.push(format!("{at}: {{{key}}} is optional on every form - blank for anyone who skipped it"));
                 } else if having.len() < bucket_forms.len() {
                     let missing: Vec<&String> = bucket_forms.iter().filter(|(_, f)| !f.contains(&key)).map(|(n, _)| n).collect();
                     warns.push(format!("{at}: {{{key}}} is blank for records sent from {}", missing.iter().map(|n| format!("{n:?}")).collect::<Vec<_>>().join(", ")));
@@ -289,6 +299,15 @@ mod tests {
         assert!(f.iter().any(|x| x.text.contains("no {link}")));
     }
 
+    #[test]
+    fn an_optional_answer_in_a_mail_is_found() {
+        let agg = AGG.replace("Got it, {name}", "Got it, {nick}");
+        let form = FORM.replace("          - { name: email, type: email }\n", "          - { name: email, type: email }\n          - { name: nick, optional: true }\n");
+        let (q, a) = repo(&agg, &[("/", &form), ("/decide/trial", DECIDE)]);
+        let f = check(&q, &a, &site("mail: { from: x@y.no }\n"));
+        assert!(f.iter().any(|x| x.level == Level::Warn && x.text.contains("{nick}") && x.text.contains("optional")), "{f:?}");
+    }
+
     #[test]
     fn a_long_answer_in_a_subject_line_is_found() {
         let agg = AGG.replace("Got it, {name}", "About {brief}");