Don't resolve URLs at parse time for Stylo.

It's a bit unfortunate the use separate implementations of SpecifiedUrl for Servo
and Gecko, but they're different enough at this point that I don't think it really
makes sense to try to share everything. Splitting them out has some nice
simplifications as well.

I recognize that there's still some potential correctness issues for Servo using
the resolved URI in various places where the original URI may be the right thing,
but I've got too much on my plate to look into that for now.

MozReview-Commit-ID: BeDu93TQ4Ow

components/style/servo/url.rs

@@ -0,0 +1,127 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! Common handling for the specified value CSS url() values.
+
+use cssparser::CssStringWriter;
+use parser::ParserContext;
+use servo_url::ServoUrl;
+use std::borrow::Cow;
+use std::fmt::{self, Write};
+use std::sync::Arc;
+use style_traits::ToCss;
+
+/// A specified url() value for servo.
+///
+/// Servo eagerly resolves SpecifiedUrls, which it can then take advantage of
+/// when computing values. In contrast, Gecko uses a different URL backend, so
+/// eagerly resolving with rust-url would be duplicated work.
+///
+/// However, this approach is still not necessarily optimal: See
+/// https://bugzilla.mozilla.org/show_bug.cgi?id=1347435#c6
+#[derive(Clone, Debug, HeapSizeOf, Serialize, Deserialize)]
+pub struct SpecifiedUrl {
+    /// The original URI. This might be optional since we may insert computed
+    /// values of images into the cascade directly, and we don't bother to
+    /// convert their serialization.
+    ///
+    /// Refcounted since cloning this should be cheap and data: uris can be
+    /// really large.
+    original: Option<Arc<String>>,
+
+    /// The resolved value for the url, if valid.
+    resolved: Option<ServoUrl>,
+}
+
+impl SpecifiedUrl {
+    /// Try to parse a URL from a string value that is a valid CSS token for a
+    /// URL. Never fails - the API is only fallible to be compatible with the
+    /// gecko version.
+    pub fn parse_from_string<'a>(url: Cow<'a, str>,
+                                 context: &ParserContext)
+                                 -> Result<Self, ()> {
+        let serialization = Arc::new(url.into_owned());
+        let resolved = context.base_url.join(&serialization).ok();
+        Ok(SpecifiedUrl {
+            original: Some(serialization),
+            resolved: resolved,
+        })
+    }
+
+    /// Returns true if the URL is definitely invalid. For Servo URLs, we can
+    /// use its |resolved| status.
+    pub fn is_invalid(&self) -> bool {
+        self.resolved.is_none()
+    }
+
+    /// Returns true if this URL looks like a fragment.
+    /// See https://drafts.csswg.org/css-values/#local-urls
+    ///
+    /// Since Servo currently stores resolved URLs, this is hard to implement. We
+    /// either need to change servo to lazily resolve (like Gecko), or note this
+    /// information in the tokenizer.
+    pub fn is_fragment(&self) -> bool {
+        error!("Can't determine whether the url is a fragment.");
+        false
+    }
+
+    /// Returns the resolved url if it was valid.
+    pub fn url(&self) -> Option<&ServoUrl> {
+        self.resolved.as_ref()
+    }
+
+    /// Return the resolved url as string, or the empty string if it's invalid.
+    ///
+    /// TODO(emilio): Should we return the original one if needed?
+    pub fn as_str(&self) -> &str {
+        match self.resolved {
+            Some(ref url) => url.as_str(),
+            None => "",
+        }
+    }
+
+    /// Creates an already specified url value from an already resolved URL
+    /// for insertion in the cascade.
+    pub fn for_cascade(url: ServoUrl) -> Self {
+        SpecifiedUrl {
+            original: None,
+            resolved: Some(url),
+        }
+    }
+
+    /// Gets a new url from a string for unit tests.
+    pub fn new_for_testing(url: &str) -> Self {
+        SpecifiedUrl {
+            original: Some(Arc::new(url.into())),
+            resolved: ServoUrl::parse(url).ok(),
+        }
+    }
+}
+
+impl PartialEq for SpecifiedUrl {
+    fn eq(&self, other: &Self) -> bool {
+        // TODO(emilio): maybe we care about equality of the specified values if
+        // present? Seems not.
+        self.resolved == other.resolved
+    }
+}
+
+impl ToCss for SpecifiedUrl {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+        try!(dest.write_str("url(\""));
+        let string = match self.original {
+            Some(ref original) => &**original,
+            None => match self.resolved {
+                Some(ref url) => url.as_str(),
+                // This can only happen if the url wasn't specified by the
+                // user *and* it's an invalid url that has been transformed
+                // back to specified value via the "uncompute" functionality.
+                None => "about:invalid",
+            }
+        };
+
+        try!(CssStringWriter::new(dest).write_str(string));
+        dest.write_str("\")")
+    }
+}