Ensure, that map keys serialized as valid xml names

Changelog.md

@@ -14,11 +14,17 @@
 
 ### Bug Fixes
 
+- [#468]: Ensure, that serialization of map keys always produces valid XML names.
+  In particular, that means that maps with numeric and numeric-like keys (for
+  example, `"42"`) no longer can be serialized because [XML name] cannot start
+  from a digit
+
 ### Misc Changes
 
 - [#468]: Content of `DeError::Unsupported` changed from `&'static str` to `Cow<'static, str>`
 
 [#468]: https://github.com/tafia/quick-xml/pull/468
+[XML name]: https://www.w3.org/TR/REC-xml/#NT-Name
 
 ## 0.24.0 -- 2022-08-28
 

src/errors.rs

@@ -188,6 +188,15 @@ pub mod serialize {
         /// An attempt to deserialize to a type, that is not supported by the XML
         /// store at current position, for example, attempt to deserialize `struct`
         /// from attribute or attempt to deserialize binary data.
+        ///
+        /// Serialized type cannot be represented in an XML due to violation of the
+        /// XML rules in the final XML document. For example, attempt to serialize
+        /// a `HashMap<{integer}, ...>` would cause this error because [XML name]
+        /// cannot start from a digit or a hyphen (minus sign). The same result
+        /// would occur if map key is a complex type that serialized not as
+        /// a primitive type (i.e. string, char, bool, unit struct or unit variant).
+        ///
+        /// [XML name]: https://www.w3.org/TR/REC-xml/#sec-common-syn
         Unsupported(Cow<'static, str>),
         /// Too many events were skipped while deserializing a sequence, event limit
         /// exceeded. The limit was provided as an argument
@@ -294,4 +303,11 @@ pub mod serialize {
             Self::InvalidFloat(e)
         }
     }
+
+    impl From<fmt::Error> for DeError {
+        #[inline]
+        fn from(e: fmt::Error) -> Self {
+            Self::Custom(e.to_string())
+        }
+    }
 }