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

@@ -12,6 +12,11 @@
 
 ### New Features
 
+- [#541]: Deserialize specially named `$text` enum variant in [externally tagged]
+  enums from textual content
+- [#545]: Resolve well-known namespaces (xml and xmlns) to their approrpriate URIs.
+  Also, enforce namespace constraints related to these well-known namespaces.
+
 ### Bug Fixes
 
 ### Misc Changes

src/errors.rs

@@ -9,6 +9,16 @@ use std::str::Utf8Error;
 use std::string::FromUtf8Error;
 use std::sync::Arc;
 
+#[derive(Clone, Debug)]
+pub struct ReservedNamespacePrefixError {
+    pub prefix: String,
+}
+
+#[derive(Clone, Debug)]
+pub struct ReservedNamespaceNameError {
+    pub name: String,
+}
+
 /// The error type used by this crate.
 #[derive(Clone, Debug)]
 pub enum Error {
@@ -45,6 +55,10 @@ pub enum Error {
     InvalidAttr(AttrError),
     /// Escape error
     EscapeError(EscapeError),
+    /// Error for when a reserved namespace is explicity set incorrectly.
+    ReservedNamespacePrefixError(ReservedNamespacePrefixError),
+    /// Error for when a reserved namespace name is explicitly set incorrectly.
+    ReservedNamespaceNameError(ReservedNamespaceNameError),
     /// Specified namespace prefix is unknown, cannot resolve namespace for it
     UnknownPrefix(Vec<u8>),
 }
@@ -120,7 +134,13 @@ impl fmt::Display for Error {
                 f.write_str("Unknown namespace prefix '")?;
                 write_byte_string(f, prefix)?;
                 f.write_str("'")
-            }
+            },
+            Error::ReservedNamespacePrefixError(e) => write!(
+                f, "The namespace prefix `{}` is invalid.", e.prefix
+            ),
+            Error::ReservedNamespaceNameError(e) => write!(
+                f, "The namespace name `{}` is invalid.", e.name
+            ),
         }
     }
 }