Handle null host with leading empty path fragment

url/src/parser.rs

@@ -1281,6 +1281,16 @@ impl<'a> Parser<'a> {
             self.serialization.push_str(path.trim_start_matches('/'));
         }
 
+        // This prevents web+demo:/.//not-a-host/ or web+demo:/path/..//not-a-host/,
+        // when parsed and then serialized, from ending up as web+demo://not-a-host/
+        // (they end up as web+demo:/.//not-a-host/).
+        if !*has_host && self.serialization[path_start..].starts_with("//") {
+            // If url’s host is null, url does not have an opaque path,
+            // url’s path’s size is greater than 1, and url’s path[0] is the empty string,
+            // then append U+002F (/) followed by U+002E (.) to output.
+            self.serialization.insert_str(path_start, "/.");
+        }
+
         input
     }
 

url/tests/unit.rs

@@ -954,6 +954,16 @@ fn no_panic() {
     url::quirks::set_hostname(&mut url, "//eom/datcom/\\\\t\\://eom/data.cs").unwrap();
 }
 
+#[test]
+fn test_null_host_with_leading_empty_path_fragment() {
+    // since Note in item 3 of URL serializing in the URL Standard
+    // https://url.spec.whatwg.org/#url-serializing
+    let url = Url::parse("m:/.//\\").unwrap();
+    let encoded = url.as_str();
+    let reparsed = Url::parse(encoded).unwrap();
+    assert_eq!(reparsed, url);
+}
+
 #[test]
 fn pop_if_empty_in_bounds() {
     let mut url = Url::parse("m://").unwrap();