Improve error reporting for missing values

instant-xml-macros/src/de.rs

@@ -63,7 +63,7 @@ fn deserialize_scalar_enum(
 
                 let value = match deserializer.take_str() {
                     #variants
-                    _ => return Err(Error::UnexpectedValue),
+                    _ => return Err(Error::UnexpectedValue("enum variant not found")),
                 };
 
                 *into = Some(value);

instant-xml/src/impls.rs

@@ -25,7 +25,7 @@ impl<'xml, T: FromStr> FromXml<'xml> for FromXmlStr<T> {
                 *into = Some(FromXmlStr(value));
                 Ok(())
             }
-            Err(_) => Err(Error::UnexpectedValue),
+            Err(_) => Err(Error::UnexpectedValue("unable to parse value")),
         }
     }
 
@@ -205,7 +205,9 @@ impl<'xml> FromXml<'xml> for &'xml str {
                 *into = Some(s);
                 Ok(())
             }
-            Some(Cow::Owned(_)) => Err(Error::UnexpectedValue),
+            Some(Cow::Owned(_)) => Err(Error::UnexpectedValue(
+                "string with escape characters cannot be deserialized as &str",
+            )),
             None => Err(Error::MissingValue(&Kind::Scalar)),
         }
     }

instant-xml/src/lib.rs

@@ -60,7 +60,7 @@ pub fn from_str<'xml, T: FromXml<'xml>>(input: &'xml str) -> Result<T, Error> {
     };
 
     if id != expected {
-        return Err(Error::UnexpectedValue);
+        return Err(Error::UnexpectedValue("unexpected root"));
     }
 
     let mut value = None;
@@ -99,7 +99,7 @@ pub enum Error {
     #[error("unexpected end of stream")]
     UnexpectedEndOfStream,
     #[error("unexpected value")]
-    UnexpectedValue,
+    UnexpectedValue(&'static str),
     #[error("unexpected tag")]
     UnexpectedTag,
     #[error("missing tag")]

instant-xml/tests/de-ns.rs

@@ -39,7 +39,7 @@ fn default_namespaces() {
         from_str(
             "<NestedDe xmlns=\"WRONG\" xmlns:bar=\"BAZ\"><bar:flag>true</bar:flag></NestedDe>"
         ),
-        Err::<NestedDe, _>(Error::UnexpectedValue)
+        Err::<NestedDe, _>(Error::UnexpectedValue("unexpected root"))
     );
 
     // Correct child namespace

instant-xml/tests/entities.rs

@@ -30,7 +30,7 @@ fn escape_back() {
         from_str(
             "<StructSpecialEntities xmlns=\"URI\"><string>&lt;&gt;&amp;&quot;&apos;adsad&quot;</string><str>str&amp;</str></StructSpecialEntities>"
         ),
-        Err::<StructSpecialEntities, _>(Error::UnexpectedValue)
+        Err::<StructSpecialEntities, _>(Error::UnexpectedValue("string with escape characters cannot be deserialized as &str"))
     );
 
     // Borrowed