Don't require both attribute key and value to be of same type

The key value isn't escaped, so we can use AsRef to allow either value.

Therefore it is unnecessary to convert both parts of the tuple to the
same type in order to produce an Attribute

src/events/attributes.rs

@@ -291,42 +291,48 @@ impl<'a> std::fmt::Debug for Attribute<'a> {
     }
 }
 
-impl<'a> From<(&'a [u8], &'a [u8])> for Attribute<'a> {
-    /// Creates new attribute from raw bytes.
-    /// Does not apply any transformation to both key and value.
+impl<'a, A> From<(&'a A, &'a str)> for Attribute<'a>
+where
+    A: AsRef<[u8]> + ?Sized,
+{
+    /// Creates new attribute from text representation.
+    /// Key is stored as-is, but the value will be escaped.
     ///
     /// # Examples
     ///
     /// ```
     /// use quick_xml::events::attributes::Attribute;
     ///
-    /// let features = Attribute::from(("features".as_bytes(), "Bells &amp; whistles".as_bytes()));
+    /// let features = Attribute::from(("features", "Bells & whistles"));
     /// assert_eq!(features.value, "Bells &amp; whistles".as_bytes());
     /// ```
-    fn from(val: (&'a [u8], &'a [u8])) -> Attribute<'a> {
+    fn from(val: (&'a A, &'a str)) -> Attribute<'a> {
         Attribute {
-            key: val.0,
-            value: Cow::from(val.1),
+            key: val.0.as_ref(),
+            value: escape(val.1.as_bytes()),
         }
     }
 }
 
-impl<'a> From<(&'a str, &'a str)> for Attribute<'a> {
-    /// Creates new attribute from text representation.
-    /// Key is stored as-is, but the value will be escaped.
+impl<'a, A> From<(&'a A, &'a [u8])> for Attribute<'a>
+where
+    A: AsRef<[u8]> + ?Sized,
+{
+    /// Creates new attribute from raw bytes.
+    /// Does not apply any transformation to both key and value.
     ///
     /// # Examples
     ///
     /// ```
     /// use quick_xml::events::attributes::Attribute;
     ///
-    /// let features = Attribute::from(("features", "Bells & whistles"));
+    /// let features = Attribute::from(("features".as_bytes(), "Bells &amp; whistles".as_bytes()));
     /// assert_eq!(features.value, "Bells &amp; whistles".as_bytes());
     /// ```
-    fn from(val: (&'a str, &'a str)) -> Attribute<'a> {
+    fn from(val: (&'a A, &'a [u8])) -> Attribute<'a> {
         Attribute {
-            key: val.0.as_bytes(),
-            value: escape(val.1.as_bytes()),
+            key: val.0.as_ref(),
+            value: Cow::from(val.1),
         }
     }
 }

src/writer.rs

@@ -1,7 +1,7 @@
 //! A module to handle `Writer`
 
 use crate::errors::{Error, Result};
-use crate::events::{attributes::Attribute, BytesEnd, BytesStart, BytesText, Event};
+use crate::events::{attributes::Attribute, BytesStart, BytesText, Event};
 use std::io::Write;
 
 /// XML writer.