Fix #580: deserialize newtypes using Visitor::visit_newtype_struct in…

…stead of visit_seq

The problem was in `SeqItemDeserializer::deserialize_newtype_struct`. Previously it was
the same as `SeqItemDeserializer::deserialize_seq`, but deserializing sequences in this
deserializer assumes, that those sequences are `xs:list`s, because deserializer itself
represents a list element.

In the original bug report an `UnexpectedEof` is returning. The error was changed in the
pre-previous commit (3d5ed69) and in this it is fixed.

The following tests checks that all's OK:

- Deserializer::deserialize_newtype_struct is reached by:
  - serde-de:
    - newtype::excess_attribute
    - newtype::simple
  - serde-se:
    - with_root::newtype
    - without_root::newtype

- MapValueDeserializer::deserialize_newtype_struct is reached by:
  - serde-issues:
    - issue343

- SeqItemDeserializer::deserialize_newtype_struct is reached by:
  - serde-issues:
    - issue580

Changelog.md

@@ -22,6 +22,7 @@ MSRV bumped to 1.56! Crate now uses Rust 2021 edition.
 
 - [#660]: Fixed incorrect deserialization of `xs:list`s from empty tags (`<tag/>`
   or `<tag></tag>`). Previously an `DeError::UnexpectedEof")` was returned in that case
+- [#580]: Fixed incorrect deserialization of vectors of newtypes from sequences of tags.
 
 ### Misc Changes
 
@@ -35,6 +36,7 @@ MSRV bumped to 1.56! Crate now uses Rust 2021 edition.
   (and newly added `ElementWriter::write_inner_content_async` of course).
 
 [#545]: https://github.com/tafia/quick-xml/pull/545
+[#580]: https://github.com/tafia/quick-xml/issues/580
 [#619]: https://github.com/tafia/quick-xml/issues/619
 [#635]: https://github.com/tafia/quick-xml/pull/635
 [#643]: https://github.com/tafia/quick-xml/pull/643

src/de/map.rs

@@ -550,6 +550,19 @@ where
         deserialize_option!(self.map.de, self, visitor)
     }
 
+    /// Forwards deserialization of the inner type. Always calls [`Visitor::visit_newtype_struct`]
+    /// with the same deserializer.
+    fn deserialize_newtype_struct<V>(
+        self,
+        _name: &'static str,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        visitor.visit_newtype_struct(self)
+    }
+
     /// Deserializes each `<tag>` in
     /// ```xml
     /// <any-tag>
@@ -869,6 +882,20 @@ where
         deserialize_option!(self.map.de, self, visitor)
     }
 
+    /// Forwards deserialization of the inner type. Always calls [`Visitor::visit_newtype_struct`]
+    /// with the [`SimpleTypeDeserializer`].
+    fn deserialize_newtype_struct<V>(
+        mut self,
+        _name: &'static str,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        let text = self.read_string()?;
+        visitor.visit_newtype_struct(SimpleTypeDeserializer::from_text(text))
+    }
+
     /// This method deserializes a sequence inside of element that itself is a
     /// sequence element:
     ///

src/de/mod.rs

@@ -1904,18 +1904,6 @@ macro_rules! deserialize_primitives {
             self.deserialize_unit(visitor)
         }
 
-        /// Representation of the newtypes the same as one-element [tuple](#method.deserialize_tuple).
-        fn deserialize_newtype_struct<V>(
-            self,
-            _name: &'static str,
-            visitor: V,
-        ) -> Result<V::Value, DeError>
-        where
-            V: Visitor<'de>,
-        {
-            self.deserialize_tuple(1, visitor)
-        }
-
         /// Representation of tuples the same as [sequences](#method.deserialize_seq).
         fn deserialize_tuple<V>(self, _len: usize, visitor: V) -> Result<V::Value, DeError>
         where
@@ -2839,6 +2827,19 @@ where
         }
     }
 
+    /// Forwards deserialization of the inner type. Always calls [`Visitor::visit_newtype_struct`]
+    /// with the same deserializer.
+    fn deserialize_newtype_struct<V>(
+        self,
+        _name: &'static str,
+        visitor: V,
+    ) -> Result<V::Value, DeError>
+    where
+        V: Visitor<'de>,
+    {
+        visitor.visit_newtype_struct(self)
+    }
+
     fn deserialize_enum<V>(
         self,
         _name: &'static str,