Fix formatting of YAML merge key error messages

src/metadata/error.rs

@@ -347,12 +347,13 @@ impl std::fmt::Display for YamlMergeKeyError {
         let yaml = to_yaml(&self.value);
 
         if saphyr::YamlEmitter::new(&mut output).dump(&yaml).is_ok() {
+            // The emitter starts the dumped YAML with ---\n, so strip that.
             write!(
                 f,
                 "invalid YAML merge key value at line {} column {}: {}",
                 self.value.span.start.line(),
                 self.value.span.start.col(),
-                output
+                output.get(4..).unwrap_or_default()
             )
         } else {
             write!(

src/metadata/yaml/merge.rs

@@ -72,3 +72,49 @@ fn merge_hashes(
     }
     hash1
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    mod process_merge_keys {
+        use crate::metadata::parse;
+
+        use super::*;
+
+        #[test]
+        fn should_error_if_merge_key_value_has_a_single_value_that_is_not_a_hash() {
+            let yaml = parse(
+                "
+- &anchor1 test
+- <<: *anchor1
+  value: test-value-2",
+            );
+
+            let error_message = process_merge_keys(yaml).unwrap_err().to_string();
+
+            assert_eq!(
+                "invalid YAML merge key value at line 3 column 6: test",
+                error_message
+            );
+        }
+
+        #[test]
+        fn should_error_if_merge_key_value_is_an_array_of_non_hash_values() {
+            let yaml = parse(
+                "
+- &anchor1 {key: test-key}
+- &anchor2 test
+- <<: [*anchor1, *anchor2]
+  value: test-value-2",
+            );
+
+            let error_message = process_merge_keys(yaml).unwrap_err().to_string();
+
+            assert_eq!(
+                "invalid YAML merge key value at line 4 column 17: test",
+                error_message
+            );
+        }
+    }
+}