marked-yaml: Check that we never reach the expecting() function in Sp…

…anned<T>

Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>

marked-yaml/src/spanned_serde.rs

@@ -140,8 +140,8 @@ where
         {
             type Value = Spanned<T>;
 
-            fn expecting(&self, formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
-                formatter.write_str("a MarkedNode of some kind")
+            fn expecting(&self, _formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
+                unreachable!()
             }
 
             fn visit_map<V>(self, mut visitor: V) -> Result<Self::Value, V::Error>
@@ -222,6 +222,36 @@ where
     }
 }
 
+#[cfg(test)]
+mod spanned_tests {
+    use super::Spanned;
+    use serde::{forward_to_deserialize_any, Deserialize, Deserializer};
+
+    #[test]
+    #[should_panic]
+    fn spanned_always_map() {
+        struct NotSpanned;
+        impl<'de> Deserializer<'de> for NotSpanned {
+            type Error = super::Error;
+
+            fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+            where
+                V: serde::de::Visitor<'de>,
+            {
+                visitor.visit_bool(false)
+            }
+
+            forward_to_deserialize_any! [
+                bool i8 i16 i32 i64 u8 u16 u32 u64 f32 f64 char str string bytes byte_buf
+                unit unit_struct newtype_struct seq tuple tuple_struct
+                map identifier ignored_any option struct enum
+            ];
+        }
+        type T = Spanned<bool>;
+        let _ = T::deserialize(NotSpanned);
+    }
+}
+
 // -------------------------------------------------------------------------------
 
 /// Errors which can come from deserialisation