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 +12,8 @@
 
 ### New Features
 
+- [#545]: resolve well-known namespaces to their approrpriate URIs
+
 ### Bug Fixes
 
 ### Misc Changes

src/name.rs

@@ -399,6 +399,24 @@ pub(crate) struct NamespaceResolver {
     nesting_level: i32,
 }
 
+/// These constants define the reserved namespaces for the xml standard.
+///
+/// The prefix `xml` is by definition bound to the namespace name
+/// `http://www.w3.org/XML/1998/namespace`. It may, but need not, be declared, and must not be
+/// undeclared or bound to any other namespace name. Other prefixes must not be bound to this
+/// namespace name, and it must not be declared as the default namespace.
+///
+/// The prefix `xmlns` is used only to declare namespace bindings and is by definition bound
+/// to the namespace name http://www.w3.org/2000/xmlns/. It must not be declared or
+/// undeclared. Other prefixes must not be bound to this namespace name, and it must not be
+///  declared as the default namespace. Element names must not have the prefix xmlns.
+///
+/// [reserved namespaces]: https://www.w3.org/TR/xml-names11/#xmlReserved
+static WELL_KNOWN_NAMESPACES: &'static [[&str; 2]] = &[
+    ["xml", "http://www.w3.org/XML/1998/namespace"],
+    ["xmlns", "http://www.w3.org/2000/xmlns/"],
+];
+
 impl NamespaceResolver {
     /// Begins a new scope and add to it all [namespace bindings] that found in
     /// the specified start element.
@@ -542,7 +560,13 @@ impl NamespaceResolver {
     #[inline]
     fn maybe_unknown(prefix: Option<Prefix>) -> ResolveResult<'static> {
         match prefix {
-            Some(p) => ResolveResult::Unknown(p.into_inner().to_vec()),
+            Some(p) => WELL_KNOWN_NAMESPACES
+                .iter()
+                .find(|x| p == Prefix(x[0].as_bytes()))
+                .map_or_else(
+                    || ResolveResult::Unknown(p.into_inner().to_vec()),
+                    |[_, uri]| ResolveResult::Bound(Namespace(uri.as_bytes())),
+                ),
             None => ResolveResult::Unbound,
         }
     }
@@ -787,6 +811,63 @@ mod namespaces {
         }
     }
 
+    mod builtin_prefixes {
+        use super::*;
+        use pretty_assertions::assert_eq;
+
+        #[test]
+        fn undeclared_reserved_prefix_xml() {
+            let prefix_name = "xml";
+            let namespace_uri = "http://www.w3.org/XML/1998/namespace";
+
+            let prefix = Prefix(prefix_name.as_bytes());
+            let namespace = Namespace(namespace_uri.as_bytes());
+
+            let resolver = NamespaceResolver::default();
+            let tag = b"random";
+
+            let name_buf = [prefix.into_inner(), tag].join(&b":"[..]);
+            let name = QName(&name_buf);
+
+            assert_eq!(
+                resolver.resolve(name, b"", true),
+                (Bound(namespace), LocalName(tag))
+            );
+
+            assert_eq!(
+                resolver.resolve(name.clone(), b"", false),
+                (Bound(namespace), LocalName(tag))
+            );
+            assert_eq!(resolver.find(name.clone(), b""), Bound(namespace));
+        }
+
+        #[test]
+        fn undeclared_reserved_prefix_xmlns() {
+            let prefix_name = "xmlns";
+            let namespace_uri = "http://www.w3.org/2000/xmlns/";
+
+            let prefix = Prefix(prefix_name.as_bytes());
+            let namespace = Namespace(namespace_uri.as_bytes());
+
+            let resolver = NamespaceResolver::default();
+            let tag = b"random";
+
+            let name_buf = [prefix.into_inner(), tag].join(&b":"[..]);
+            let name = QName(&name_buf);
+
+            assert_eq!(
+                resolver.resolve(name, b"", true),
+                (Bound(namespace), LocalName(tag))
+            );
+
+            assert_eq!(
+                resolver.resolve(name.clone(), b"", false),
+                (Bound(namespace), LocalName(tag))
+            );
+            assert_eq!(resolver.find(name.clone(), b""), Bound(namespace));
+        }
+    }
+
     #[test]
     fn undeclared_prefix() {
         let name = QName(b"unknown:prefix");