feat: cooked up something fierce

src/model/bank.rs

@@ -403,6 +403,15 @@ fn validate_item(
         if seen.contains(&o.id.as_str()) {
             issues.push(format!("option {pos}: duplicate option id `{}`", o.id));
         }
+        if let Some(retirement) = &o.retired {
+            if retirement.reason.trim().is_empty() {
+                issues.push(format!(
+                    "option {pos}: retired without a reason. The reason is the finding — what \
+                     the option did or failed to do — and it is the only part of a retirement \
+                     that is worth anything later."
+                ));
+            }
+        }
         seen.push(&o.id);
 
         let credit = o.credit();
@@ -442,14 +451,27 @@ fn validate_item(
     }
 
     // --- key ---
+    // Counted over the pool that can still be drawn: a retired option is a
+    // record, not an offer.
+    let (live_keys, live_distractors) = it.pool();
     let keys = it.key_indices();
     match it.format {
         Format::SingleBestAnswer => {
-            if keys.len() != 1 {
-                issues.push(format!(
-                    "single_best_answer needs exactly one keyed option, has {}",
-                    keys.len()
-                ));
+            // Several defensible keys is a pool, not a bug — it is what lets you
+            // test whether "fourth" or "last of the four" is doing the work.
+            // Exactly one of them reaches a student, and that is the
+            // placement's business: see
+            // [`crate::catalog::Catalog::validate_record`].
+            if live_keys.is_empty() {
+                issues.push("single_best_answer needs at least one keyed option".into());
+            }
+            if live_distractors.is_empty() {
+                let retired = it.options.iter().any(|o| o.retired.is_some());
+                issues.push(if retired {
+                    "every distractor is retired, so nothing can be drawn against the key".into()
+                } else {
+                    "has no option that is not keyed correct, so it asks nothing".to_string()
+                });
             }
         }
         Format::MultipleResponse => {
@@ -780,20 +802,43 @@ mod tests {
     options:
       - { id: A, text: a, correct: true }
       - { id: B, text: b, correct: true }
+  - id: q-a-003
+    status: draft
+    level: 1
+    format: single_best_answer
+    stem: s
+    options:
+      - { id: o-key-one, text: a, correct: true }
+      - { id: o-key-two, text: b, correct: true }
+      - { id: o-wrong-one, text: c }
+      - { id: o-wrong-two, text: d }
 "#,
         );
         let issues = b.validate(None);
+        // No key at all is still a bank problem: nothing can be drawn from it.
         assert!(
             issues
                 .iter()
-                .any(|i| i.contains("exactly one keyed option"))
+                .any(|i| i.starts_with("q-a-001") && i.contains("at least one keyed option")),
+            "{issues:?}"
         );
-        assert_eq!(
+        // Keying every option is still a bank problem, for the older reason: a
+        // question with nothing to choose against asks nothing.
+        assert!(
             issues
                 .iter()
-                .filter(|i| i.contains("exactly one keyed option"))
-                .count(),
-            2
+                .any(|i| i.starts_with("q-a-002") && i.contains("asks nothing")),
+            "{issues:?}"
+        );
+        // Two defensible keys alongside real distractors is not a problem. It
+        // is the pool doing its job — it is what lets you test whether the
+        // wording of the key is what students are answering. Exactly one of
+        // them reaches a student, and that is checked against the placement
+        // that administers it, in `Catalog::validate_placement`.
+        assert!(
+            !issues.iter().any(|i| i.starts_with("q-a-003")
+                && (i.contains("keyed option") || i.contains("asks nothing"))),
+            "{issues:?}"
         );
     }