Auto merge of #14653 - upsuper:bug1321176, r=Manishearth

stylo: Fix assertion for unresolvable url

<!-- Please describe your changes on the following line: -->
This is the Servo part of [bug 1321176](https://bugzilla.mozilla.org/show_bug.cgi?id=1321176), which has been reviewed by @emilio, @Manishearth, and @heycam.

r? @Manishearth

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/14653)
<!-- Reviewable:end -->

components/script/dom/element.rs

@@ -397,7 +397,7 @@ impl LayoutElementHelpers for LayoutJS<Element> {
                 PropertyDeclaration::BackgroundImage(DeclaredValue::Value(
                     background_image::SpecifiedValue(vec![
                         background_image::single_value::SpecifiedValue(Some(
-                            specified::Image::for_cascade(Some(url.into()), specified::url::UrlExtraData { })
+                            specified::Image::for_cascade(url.into(), specified::url::UrlExtraData { })
                         ))
                     ])))));
         }

components/style/gecko/conversions.rs

@@ -133,7 +133,9 @@ impl nsStyleImage {
                 self.set_gradient(gradient)
             },
             Image::Url(ref url) if with_url => {
-                let (ptr, len) = url.as_slice_components();
+                let (ptr, len) = match url.as_slice_components() {
+                    Ok(value) | Err(value) => value
+                };
                 let extra_data = url.extra_data();
                 unsafe {
                     Gecko_SetUrlImageValue(self,

components/style/properties/gecko.mako.rs

@@ -1152,7 +1152,10 @@ fn static_assert() {
             Either::Second(_none) => debug_assert!(self.gecko.mBinding.mRawPtr.is_null()),
             Either::First(ref url) => {
                 let extra_data = url.extra_data();
-                let (ptr, len) = url.as_slice_components();
+                let (ptr, len) = match url.as_slice_components() {
+                    Ok(value) => value,
+                    Err(_) => (ptr::null(), 0),
+                };
                 unsafe {
                     Gecko_SetMozBinding(&mut self.gecko,
                                         ptr,
@@ -1737,7 +1740,9 @@ fn static_assert() {
                 }
             }
             Either::First(ref url) => {
-                let (ptr, len) = url.as_slice_components();
+                let (ptr, len) = match url.as_slice_components() {
+                    Ok(value) | Err(value) => value
+                };
                 let extra_data = url.extra_data();
                 unsafe {
                     Gecko_SetListStyleImage(&mut self.gecko,
@@ -2433,7 +2438,9 @@ clip-path
         for i in 0..v.images.len() {
             let image = &v.images[i];
             let extra_data = image.url.extra_data();
-            let (ptr, len) = image.url.as_slice_components();
+            let (ptr, len) = match image.url.as_slice_components() {
+                Ok(value) | Err(value) => value,
+            };
             unsafe {
                 Gecko_SetCursorImage(&mut self.gecko.mCursorImages[i],
                                      ptr, len as u32,

components/style/values/specified/image.rs

@@ -45,7 +45,7 @@ impl Image {
 
     /// Creates an already specified image value from an already resolved URL
     /// for insertion in the cascade.
-    pub fn for_cascade(url: Option<ServoUrl>, extra_data: UrlExtraData) -> Self {
+    pub fn for_cascade(url: ServoUrl, extra_data: UrlExtraData) -> Self {
         Image::Url(SpecifiedUrl::for_cascade(url, extra_data))
     }
 }

components/style/values/specified/url.rs

@@ -13,7 +13,6 @@ use parser::ParserContextExtraData;
 use servo_url::ServoUrl;
 use std::borrow::Cow;
 use std::fmt::{self, Write};
-use std::ptr;
 use std::sync::Arc;
 use style_traits::ToCss;
 use values::NoViewportPercentage;
@@ -121,19 +120,24 @@ impl SpecifiedUrl {
     }
 
     /// Little helper for Gecko's ffi.
-    pub fn as_slice_components(&self) -> (*const u8, usize) {
+    #[cfg(feature = "gecko")]
+    pub fn as_slice_components(&self) -> Result<(*const u8, usize), (*const u8, usize)> {
         match self.resolved {
-            Some(ref url) => (url.as_str().as_ptr(), url.as_str().len()),
-            None => (ptr::null(), 0),
+            Some(ref url) => Ok((url.as_str().as_ptr(), url.as_str().len())),
+            None => {
+                let url = self.original.as_ref()
+                    .expect("We should always have either the original or the resolved value");
+                Err((url.as_str().as_ptr(), url.as_str().len()))
+            }
         }
     }
 
     /// Creates an already specified url value from an already resolved URL
     /// for insertion in the cascade.
-    pub fn for_cascade(url: Option<ServoUrl>, extra_data: UrlExtraData) -> Self {
+    pub fn for_cascade(url: ServoUrl, extra_data: UrlExtraData) -> Self {
         SpecifiedUrl {
             original: None,
-            resolved: url,
+            resolved: Some(url),
             extra_data: extra_data,
         }
     }