test: assert mermaid/math parse errors surface a placeholder reason

Author: wilfreddenton

src/math.rs

@@ -206,6 +206,19 @@ mod tests {
         assert!(cache.get(&key).is_some());
     }
 
+    #[test]
+    fn parse_error_surfaces_reason() {
+        // An unknown control sequence yields a message shown in the placeholder.
+        let j = job("\\unknowncmd{x}", true);
+        let key = key_for(&j);
+        let cache = ImageCache::new();
+        render_math_blocking(&[(key.clone(), j)], &cache);
+        match cache.get(&key) {
+            Some(ImageState::Failed(Some(msg))) => assert!(!msg.is_empty()),
+            _ => panic!("expected Failed(Some(reason))"),
+        }
+    }
+
     #[test]
     fn key_stable_and_distinguishes_display() {
         assert_eq!(key_for(&job("x", true)), key_for(&job("x", true)));

src/mermaid.rs

@@ -132,6 +132,22 @@ mod tests {
         assert!(cache.get(&key).is_some());
     }
 
+    #[test]
+    fn parse_error_surfaces_reason() {
+        // A parseable header with an unsupported construct yields a message (not just a
+        // generic failure), so the placeholder can show the author what's wrong.
+        let src = "classDiagram\n  class A {\n    <<interface>>\n  }";
+        let key = key_for(src);
+        let cache = ImageCache::new();
+        render_mermaid_blocking(&[(key.clone(), src.to_string())], &cache);
+        match cache.get(&key) {
+            Some(ImageState::Failed(Some(msg))) => {
+                assert!(!msg.is_empty(), "error message should be non-empty");
+            }
+            _ => panic!("expected Failed(Some(reason))"),
+        }
+    }
+
     #[test]
     fn key_is_stable_per_source() {
         assert_eq!(key_for("graph LR; A-->B"), key_for("graph LR; A-->B"));