Prevent panic when calling `url::Url::set_host` with `None`.

Fixes https://github.com/servo/rust-url/issues/243.

src/lib.rs

@@ -998,12 +998,18 @@ impl Url {
         }
 
         if let Some(host) = host {
+            if host == "" && SchemeType::from(self.scheme()).is_special() {
+                return Err(ParseError::EmptyHost);
+            }
             self.set_host_internal(try!(Host::parse(host)), None)
         } else if self.has_host() {
+            if SchemeType::from(self.scheme()).is_special() {
+                return Err(ParseError::EmptyHost)
+            }
             debug_assert!(self.byte_at(self.scheme_end) == b':');
             debug_assert!(self.byte_at(self.path_start) == b'/');
             let new_path_start = self.scheme_end + 1;
-            self.serialization.drain(self.path_start as usize..new_path_start as usize);
+            self.serialization.drain(new_path_start as usize..self.path_start as usize);
             let offset = self.path_start - new_path_start;
             self.path_start = new_path_start;
             self.username_end = new_path_start;

tests/unit.rs

@@ -317,3 +317,19 @@ fn append_empty_segment_then_mutate() {
     url.assert_invariants();
     assert_eq!(url.to_string(), "http://localhost:6767/foo/bar?a=b");
 }
+
+#[test]
+/// https://github.com/servo/rust-url/issues/243
+fn test_set_host() {
+    let mut url = Url::parse("https://example.net/hello").unwrap();
+    url.set_host(Some("foo.com")).unwrap();
+    assert_eq!(url.as_str(), "https://foo.com/hello");
+    assert!(url.set_host(None).is_err());
+    assert_eq!(url.as_str(), "https://foo.com/hello");
+    assert!(url.set_host(Some("")).is_err());
+    assert_eq!(url.as_str(), "https://foo.com/hello");
+
+    let mut url = Url::parse("foobar://example.net/hello").unwrap();
+    url.set_host(None).unwrap();
+    assert_eq!(url.as_str(), "foobar:/hello");
+}