Use the conversion traits from js.

components/util/str.rs

@@ -4,15 +4,23 @@
 
 use app_units::Au;
 use cssparser::{self, Color, RGBA};
+use js::conversions::{FromJSValConvertible, ToJSValConvertible, latin1_to_string};
+use js::jsapi::{JSContext, JSString, HandleValue, MutableHandleValue};
+use js::jsapi::{JS_GetTwoByteStringCharsAndLength, JS_StringHasLatin1Chars};
+use js::rust::ToString;
 use libc::c_char;
 use num_lib::ToPrimitive;
+use opts;
 use std::ascii::AsciiExt;
 use std::borrow::ToOwned;
+use std::char;
 use std::convert::AsRef;
 use std::ffi::CStr;
 use std::fmt;
 use std::iter::{Filter, Peekable};
 use std::ops::{Deref, DerefMut};
+use std::ptr;
+use std::slice;
 use std::str::{CharIndices, FromStr, Split, from_utf8};
 
 #[derive(Clone, PartialOrd, Ord, PartialEq, Eq, Deserialize, Serialize, Hash, Debug)]
@@ -71,6 +79,82 @@ impl Into<Vec<u8>> for DOMString {
     }
 }
 
+// https://heycam.github.io/webidl/#es-DOMString
+impl ToJSValConvertible for DOMString {
+    unsafe fn to_jsval(&self, cx: *mut JSContext, rval: MutableHandleValue) {
+        (**self).to_jsval(cx, rval);
+    }
+}
+
+/// Behavior for stringification of `JSVal`s.
+#[derive(PartialEq)]
+pub enum StringificationBehavior {
+    /// Convert `null` to the string `"null"`.
+    Default,
+    /// Convert `null` to the empty string.
+    Empty,
+}
+
+/// Convert the given `JSString` to a `DOMString`. Fails if the string does not
+/// contain valid UTF-16.
+pub unsafe fn jsstring_to_str(cx: *mut JSContext, s: *mut JSString) -> DOMString {
+    let latin1 = JS_StringHasLatin1Chars(s);
+    DOMString(if latin1 {
+        latin1_to_string(cx, s)
+    } else {
+        let mut length = 0;
+        let chars = JS_GetTwoByteStringCharsAndLength(cx, ptr::null(), s, &mut length);
+        assert!(!chars.is_null());
+        let potentially_ill_formed_utf16 = slice::from_raw_parts(chars, length as usize);
+        let mut s = String::with_capacity(length as usize);
+        for item in char::decode_utf16(potentially_ill_formed_utf16.iter().cloned()) {
+            match item {
+                Ok(c) => s.push(c),
+                Err(_) => {
+                    // FIXME: Add more info like document URL in the message?
+                    macro_rules! message {
+                        () => {
+                            "Found an unpaired surrogate in a DOM string. \
+                             If you see this in real web content, \
+                             please comment on https://github.com/servo/servo/issues/6564"
+                        }
+                    }
+                    if opts::get().replace_surrogates {
+                        error!(message!());
+                        s.push('\u{FFFD}');
+                    } else {
+                        panic!(concat!(message!(), " Use `-Z replace-surrogates` \
+                            on the command line to make this non-fatal."));
+                    }
+                }
+            }
+        }
+        s
+    })
+}
+
+// https://heycam.github.io/webidl/#es-DOMString
+impl FromJSValConvertible for DOMString {
+    type Config = StringificationBehavior;
+    unsafe fn from_jsval(cx: *mut JSContext,
+                         value: HandleValue,
+                         null_behavior: StringificationBehavior)
+                         -> Result<DOMString, ()> {
+        if null_behavior == StringificationBehavior::Empty &&
+           value.get().is_null() {
+            Ok(DOMString::new())
+        } else {
+            let jsstr = ToString(cx, value);
+            if jsstr.is_null() {
+                debug!("ToString failed");
+                Err(())
+            } else {
+                Ok(jsstring_to_str(cx, jsstr))
+            }
+        }
+    }
+}
+
 pub type StaticCharVec = &'static [char];
 pub type StaticStringVec = &'static [&'static str];