Use XML file headers as language detection hints

Fixes #630

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 Added support for Objective-C and VHDL.
 
+Files starting with `<?xml` are now parsed as XML.
+
 ### Display
 
 The default display width for tabs has changed to 4.

src/parse/guess_language.rs

@@ -416,6 +416,10 @@ fn looks_like_objc(path: &Path, src: &str) -> bool {
     false
 }
 
+fn looks_like_xml(src: &str) -> bool {
+    src.starts_with("<?xml")
+}
+
 pub(crate) fn guess(
     path: &Path,
     src: &str,
@@ -458,6 +462,10 @@ pub(crate) fn guess(
         return Some(lang);
     }
 
+    if looks_like_xml(src) {
+        return Some(Language::Xml);
+    }
+
     None
 }
 
@@ -651,6 +659,12 @@ mod tests {
         assert_eq!(guess(path, "# -*-python-*-", &[]), Some(Python));
     }
 
+    #[test]
+    fn test_guess_by_xml_header() {
+        let path = Path::new("foo");
+        assert_eq!(guess(path, "<?xml version=\"1.0\" encoding=\"utf-8\"?>", &[]), Some(Xml));
+    }
+
     #[test]
     fn test_guess_unknown() {
         let path = Path::new("jfkdlsjfkdsljfkdsljf");