Add reserved namespace bindings.

This adds xml and xmlns namespace bindings. These are defined at
https://www.w3.org/TR/xml-names11/#xmlReserved.

Changelog.md

@@ -12,10 +12,15 @@
 
 ### New Features
 
+- [#545]: Resolve well-known namespaces (`xml` and `xmlns`) to their appropriate URIs.
+  Also, enforce namespace constraints related to these well-known namespaces.
+
 ### Bug Fixes
 
 ### Misc Changes
 
+[#545]: https://github.com/tafia/quick-xml/pull/545
+
 
 ## 0.30.0 -- 2023-07-23
 

src/errors.rs

@@ -47,6 +47,22 @@ pub enum Error {
     EscapeError(EscapeError),
     /// Specified namespace prefix is unknown, cannot resolve namespace for it
     UnknownPrefix(Vec<u8>),
+    /// Error for when a reserved namespace is set incorrectly.
+    ///
+    /// This error returned in following cases:
+    /// - the XML document attempts to bind `xml` prefix to something other than
+    ///   `http://www.w3.org/XML/1998/namespace`
+    /// - the XML document attempts to bind `xmlns` prefix
+    /// - the XML document attempts to bind some prefix (except `xml`) to
+    ///   `http://www.w3.org/XML/1998/namespace`
+    /// - the XML document attempts to bind some prefix to
+    ///   `http://www.w3.org/2000/xmlns/`
+    InvalidPrefixBind {
+        /// The prefix that is tried to be bound
+        prefix: Vec<u8>,
+        /// Namespace to which prefix tried to be bound
+        namespace: Vec<u8>,
+    },
 }
 
 impl From<IoError> for Error {
@@ -121,6 +137,13 @@ impl fmt::Display for Error {
                 write_byte_string(f, prefix)?;
                 f.write_str("'")
             }
+            Error::InvalidPrefixBind { prefix, namespace } => {
+                f.write_str("The namespace prefix '")?;
+                write_byte_string(f, prefix)?;
+                f.write_str("' cannot be bound to '")?;
+                write_byte_string(f, namespace)?;
+                f.write_str("'")
+            }
         }
     }
 }