feat: Improve error from try_deserialize

Author: barskern

src/error.rs

@@ -289,6 +289,10 @@ impl de::Error for ConfigError {
     fn custom<T: fmt::Display>(msg: T) -> Self {
         Self::Message(msg.to_string())
     }
+
+    fn missing_field(field: &'static str) -> Self {
+        Self::NotFound(field.into())
+    }
 }
 
 impl ser::Error for ConfigError {

tests/testsuite/errors.rs

@@ -158,7 +158,7 @@ fn test_get_missing_field() {
     let res = c.get::<InnerSettings>("inner");
     assert_data_eq!(
         res.unwrap_err().to_string(),
-        str!["missing field `value2` for key `inner`"]
+        str![[r#"missing configuration field "value2" for key `inner`"#]]
     );
 }
 
@@ -184,7 +184,7 @@ fn test_get_missing_field_file() {
     let res = c.get::<InnerSettings>("inner");
     assert_data_eq!(
         res.unwrap_err().to_string(),
-        str!["missing field `value2` for key `inner`"]
+        str![[r#"missing configuration field "value2" for key `inner`"#]]
     );
 }
 
@@ -468,6 +468,6 @@ fn test_deserialize_missing_field_file() {
     let res = c.try_deserialize::<Settings>();
     assert_data_eq!(
         res.unwrap_err().to_string(),
-        str!["missing field `value2` for key `inner`"]
+        str![[r#"missing configuration field "inner.value2""#]]
     );
 }