Auto merge of #15077 - emilio:stylo-medialist, r=heycam,Manishearth,upsuper

Media query parsing and evaluation in stylo using nsMediaFeatures

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Reviewed in the bug.

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components/style/build_gecko.rs

@@ -99,7 +99,7 @@ mod bindings {
 
     fn add_include(name: &str) -> String {
         let mut added_paths = ADDED_PATHS.lock().unwrap();
-        let file = search_include(name).unwrap();
+        let file = search_include(name).expect("Include not found!");
         let result = String::from(file.to_str().unwrap());
         add_headers_recursively(file, &mut *added_paths);
         result
@@ -235,6 +235,8 @@ mod bindings {
             .include(add_include("mozilla/ServoElementSnapshot.h"))
             .include(add_include("mozilla/dom/Element.h"))
             .include(add_include("mozilla/ServoBindings.h"))
+            .include(add_include("nsMediaFeatures.h"))
+            .include(add_include("nsMediaList.h"))
             // FIXME(emilio): Incrementally remove these "pub use"s. Probably
             // mozilla::css and mozilla::dom are easier.
             .raw_line("pub use self::root::*;")
@@ -296,6 +298,14 @@ mod bindings {
             "nsChangeHint",
             "nsCSSKeyword",
             "nsCSSPropertyID",
+            "nsCSSProps",
+
+            // FIXME(emilio): These three can go away once
+            // https://github.com/servo/rust-bindgen/pull/397 has landed.
+            "nsStyleStructID",
+            "nsStyleAnimType",
+            "UseCounter",
+
             "nsCSSRect",
             "nsCSSRect_heap",
             "nsCSSShadowArray",
@@ -316,6 +326,10 @@ mod bindings {
             "nsMainThreadPtrHandle",
             "nsMainThreadPtrHolder",
             "nsMargin",
+            "nsMediaExpression",
+            "nsMediaFeature",
+            "nsMediaFeatures",
+            "nsMediaList",
             "nsRect",
             "nsRestyleHint",
             "nsresult",
@@ -376,6 +390,7 @@ mod bindings {
             "mozilla::DefaultDelete",
         ];
         let opaque_types = [
+            "std::pair__PCCP",
             "std::namespace::atomic___base", "std::atomic__My_base",
             "nsAString_internal_char_traits",
             "nsAString_internal_incompatible_char_type",
@@ -401,7 +416,6 @@ mod bindings {
                                         // for clang.
             "nsPIDOMWindow",  // <- Takes the vtable from a template parameter, and we can't
                               //    generate it conditionally.
-            "RawGeckoPresContext", // Just passing it through.
             "JS::Rooted",
             "mozilla::Maybe",
             "gfxSize",  // <- union { struct { T width; T height; }; T components[2] };
@@ -483,6 +497,7 @@ mod bindings {
             "RawGeckoAnimationValueList",
             "RawServoAnimationValue",
             "RawGeckoPresContext",
+            "RawGeckoPresContextOwned",
             "ThreadSafeURIHolder",
             "ThreadSafePrincipalHolder",
             "CSSPseudoClassType",
@@ -503,6 +518,7 @@ mod bindings {
             "nsCursorImage",
             "nsFont",
             "nsIAtom",
+            "nsMediaFeature",
             "nsRestyleHint",
             "nsStyleBackground",
             "nsStyleBorder",

components/style/gecko/data.rs

@@ -7,11 +7,10 @@
 use animation::Animation;
 use atomic_refcell::{AtomicRef, AtomicRefCell, AtomicRefMut};
 use dom::OpaqueNode;
-use euclid::size::TypedSize2D;
-use gecko_bindings::bindings::RawGeckoPresContextBorrowed;
 use gecko_bindings::bindings::RawServoStyleSet;
+use gecko_bindings::structs::RawGeckoPresContextOwned;
 use gecko_bindings::sugar::ownership::{HasBoxFFI, HasFFI, HasSimpleFFI};
-use media_queries::{Device, MediaType};
+use media_queries::Device;
 use num_cpus;
 use parking_lot::RwLock;
 use properties::ComputedValues;
@@ -21,7 +20,6 @@ use std::collections::HashMap;
 use std::env;
 use std::sync::Arc;
 use std::sync::mpsc::{Receiver, Sender, channel};
-use style_traits::ViewportPx;
 use stylesheets::Stylesheet;
 use stylist::Stylist;
 
@@ -37,6 +35,9 @@ pub struct PerDocumentStyleDataImpl {
     /// Whether the stylesheets list above has changed since the last restyle.
     pub stylesheets_changed: bool,
 
+    /// Whether the device has changed since the last restyle.
+    pub device_changed: bool,
+
     // FIXME(bholley): Hook these up to something.
     /// Unused. Will go away when we actually implement transitions and
     /// animations properly.
@@ -57,9 +58,6 @@ pub struct PerDocumentStyleDataImpl {
 
     /// The number of threads of the work queue.
     pub num_threads: usize,
-
-    /// Default computed values for this document.
-    pub default_computed_values: Arc<ComputedValues>
 }
 
 /// The data itself is an `AtomicRefCell`, which guarantees the proper semantics
@@ -78,15 +76,8 @@ lazy_static! {
 
 impl PerDocumentStyleData {
     /// Create a dummy `PerDocumentStyleData`.
-    pub fn new(pres_context: RawGeckoPresContextBorrowed) -> Self {
-        // FIXME(bholley): Real window size.
-        let window_size: TypedSize2D<f32, ViewportPx> = TypedSize2D::new(800.0, 600.0);
-        let default_computed_values = ComputedValues::default_values(pres_context);
-
-        // FIXME(bz): We're going to need to either update the computed values
-        // in the Stylist's Device or give the Stylist a new Device when our
-        // default_computed_values changes.
-        let device = Device::new(MediaType::Screen, window_size, &default_computed_values);
+    pub fn new(pres_context: RawGeckoPresContextOwned) -> Self {
+        let device = Device::new(pres_context);
 
         let (new_anims_sender, new_anims_receiver) = channel();
 
@@ -94,6 +85,7 @@ impl PerDocumentStyleData {
             stylist: Arc::new(Stylist::new(device)),
             stylesheets: vec![],
             stylesheets_changed: true,
+            device_changed: true,
             new_animations_sender: new_anims_sender,
             new_animations_receiver: new_anims_receiver,
             running_animations: Arc::new(RwLock::new(HashMap::new())),
@@ -106,7 +98,6 @@ impl PerDocumentStyleData {
                 rayon::ThreadPool::new(configuration).ok()
             },
             num_threads: *NUM_THREADS,
-            default_computed_values: default_computed_values,
         }))
     }
 
@@ -124,15 +115,30 @@ impl PerDocumentStyleData {
 impl PerDocumentStyleDataImpl {
     /// Recreate the style data if the stylesheets have changed.
     pub fn flush_stylesheets(&mut self) {
-        // The stylist wants to be flushed if either the stylesheets change or the
-        // device dimensions change. When we add support for media queries, we'll
-        // need to detect the latter case and trigger a flush as well.
+        let mut stylist = if self.device_changed || self.stylesheets_changed {
+            Some(Arc::get_mut(&mut self.stylist).unwrap())
+        } else {
+            None
+        };
+
+        if self.device_changed {
+            Arc::get_mut(&mut stylist.as_mut().unwrap().device).unwrap().reset();
+            self.device_changed = false;
+            // Force a stylesheet flush if the device has changed.
+            self.stylesheets_changed = true;
+        }
+
         if self.stylesheets_changed {
-            let _ = Arc::get_mut(&mut self.stylist).unwrap()
-                                                   .update(&self.stylesheets, None, true);
+            let _ = stylist.unwrap().update(&self.stylesheets, None, true);
             self.stylesheets_changed = false;
         }
     }
+
+    /// Get the default computed values for this document.
+    pub fn default_computed_values(&self) -> &Arc<ComputedValues> {
+        debug_assert!(!self.device_changed, "A device flush was pending");
+        self.stylist.device.default_values_arc()
+    }
 }
 
 unsafe impl HasFFI for PerDocumentStyleData {

components/style/gecko/media_queries.rs

@@ -0,0 +1,566 @@
+/* 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/. */
+
+//! Gecko's media-query device and expression representation.
+
+use app_units::Au;
+use cssparser::{CssStringWriter, Parser, Token};
+use euclid::Size2D;
+use gecko_bindings::bindings;
+use gecko_bindings::structs::{nsCSSValue, nsCSSUnit, nsStringBuffer, nsresult};
+use gecko_bindings::structs::{nsMediaExpression_Range, nsMediaFeature};
+use gecko_bindings::structs::{nsMediaFeature_ValueType, nsMediaFeature_RangeType, nsMediaFeature_RequirementFlags};
+use gecko_bindings::structs::RawGeckoPresContextOwned;
+use media_queries::MediaType;
+use properties::ComputedValues;
+use std::ascii::AsciiExt;
+use std::fmt::{self, Write};
+use std::sync::Arc;
+use string_cache::Atom;
+use style_traits::ToCss;
+use style_traits::viewport::ViewportConstraints;
+use values::{CSSFloat, specified};
+use values::computed::{self, ToComputedValue};
+
+/// The `Device` in Gecko wraps a pres context, has a default values computed,
+/// and contains all the viewport rule state.
+pub struct Device {
+    /// NB: The pres context lifetime is tied to the styleset, who owns the
+    /// stylist, and thus the `Device`, so having a raw pres context pointer
+    /// here is fine.
+    pres_context: RawGeckoPresContextOwned,
+    default_values: Arc<ComputedValues>,
+    viewport_override: Option<ViewportConstraints>,
+}
+
+impl Device {
+    /// Trivially constructs a new `Device`.
+    pub fn new(pres_context: RawGeckoPresContextOwned) -> Self {
+        assert!(!pres_context.is_null());
+        Device {
+            pres_context: pres_context,
+            default_values: ComputedValues::default_values(unsafe { &*pres_context }),
+            viewport_override: None,
+        }
+    }
+
+    /// Tells the device that a new viewport rule has been found, and stores the
+    /// relevant viewport constraints.
+    pub fn account_for_viewport_rule(&mut self,
+                                     constraints: &ViewportConstraints) {
+        self.viewport_override = Some(constraints.clone());
+    }
+
+    /// Returns the default computed values as a reference, in order to match
+    /// Servo.
+    pub fn default_values(&self) -> &ComputedValues {
+        &*self.default_values
+    }
+
+    /// Returns the default computed values as an `Arc`, in order to avoid
+    /// clones.
+    pub fn default_values_arc(&self) -> &Arc<ComputedValues> {
+        &self.default_values
+    }
+
+    /// Recreates all the temporary state that the `Device` stores.
+    ///
+    /// This includes the viewport override from `@viewport` rules, and also the
+    /// default computed values.
+    pub fn reset(&mut self) {
+        // NB: A following stylesheet flush will populate this if appropriate.
+        self.viewport_override = None;
+        self.default_values = ComputedValues::default_values(unsafe { &*self.pres_context });
+    }
+
+    /// Returns the current media type of the device.
+    pub fn media_type(&self) -> MediaType {
+        unsafe {
+            // FIXME(emilio): Gecko allows emulating random media with
+            // mIsEmulatingMedia / mMediaEmulated . Refactor both sides so that
+            // is supported (probably just making MediaType an Atom).
+            if (*self.pres_context).mMedium == atom!("screen").as_ptr() {
+                MediaType::Screen
+            } else {
+                debug_assert!((*self.pres_context).mMedium == atom!("print").as_ptr());
+                MediaType::Print
+            }
+        }
+    }
+
+    /// Returns the current viewport size in app units.
+    pub fn au_viewport_size(&self) -> Size2D<Au> {
+        self.viewport_override.as_ref().map(|v| {
+            Size2D::new(Au::from_f32_px(v.size.width),
+                        Au::from_f32_px(v.size.height))
+        }).unwrap_or_else(|| {
+            // TODO(emilio): Grab from pres context.
+            Size2D::new(Au::from_f32_px(1024.0),
+                        Au::from_f32_px(768.0))
+        })
+    }
+}
+
+unsafe impl Sync for Device {}
+unsafe impl Send for Device {}
+
+/// A expression for gecko contains a reference to the media feature, the value
+/// the media query contained, and the range to evaluate.
+#[derive(Debug, Clone)]
+pub struct Expression {
+    feature: &'static nsMediaFeature,
+    value: Option<MediaExpressionValue>,
+    range: nsMediaExpression_Range
+}
+
+impl ToCss for Expression {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+        where W: fmt::Write,
+    {
+        dest.write_str("(")?;
+        match self.range {
+            nsMediaExpression_Range::eMin => dest.write_str("min-")?,
+            nsMediaExpression_Range::eMax => dest.write_str("max-")?,
+            nsMediaExpression_Range::eEqual => {},
+        }
+
+        // NB: CssStringWriter not needed, feature names are under control.
+        write!(dest, "{}", Atom::from(unsafe { *self.feature.mName }))?;
+
+        if let Some(ref val) = self.value {
+            dest.write_str(": ")?;
+            val.to_css(dest)?;
+        }
+
+        dest.write_str(")")
+    }
+}
+
+/// A resolution.
+#[derive(Debug, Clone)]
+pub enum Resolution {
+    /// Dots per inch.
+    Dpi(CSSFloat),
+    /// Dots per pixel.
+    Dppx(CSSFloat),
+    /// Dots per centimeter.
+    Dpcm(CSSFloat),
+}
+
+impl Resolution {
+    fn to_dpi(&self) -> CSSFloat {
+        match *self {
+            Resolution::Dpi(f) => f,
+            Resolution::Dppx(f) => f * 96.0,
+            Resolution::Dpcm(f) => f * 2.54,
+        }
+    }
+
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
+        let (value, unit) = match try!(input.next()) {
+            Token::Dimension(value, unit) => {
+                (value.value, unit)
+            },
+            _ => return Err(()),
+        };
+
+        Ok(match_ignore_ascii_case! { unit,
+            "dpi" => Resolution::Dpi(value),
+            "dppx" => Resolution::Dppx(value),
+            "dpcm" => Resolution::Dpcm(value),
+            _ => return Err(())
+        })
+    }
+}
+
+impl ToCss for Resolution {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+        where W: fmt::Write,
+    {
+        match *self {
+            Resolution::Dpi(v) => write!(dest, "{}dpi", v),
+            Resolution::Dppx(v) => write!(dest, "{}dppx", v),
+            Resolution::Dpcm(v) => write!(dest, "{}dpcm", v),
+        }
+    }
+}
+
+unsafe fn string_from_ns_string_buffer(buffer: *const nsStringBuffer) -> String {
+    use std::slice;
+    debug_assert!(!buffer.is_null());
+    let data = buffer.offset(1) as *const u16;
+    let mut length = 0;
+    let mut iter = data;
+    while *iter != 0 {
+        length += 1;
+        iter = iter.offset(1);
+    }
+    String::from_utf16_lossy(slice::from_raw_parts(data, length))
+}
+
+/// A value found or expected in a media expression.
+#[derive(Debug, Clone)]
+pub enum MediaExpressionValue {
+    /// A length.
+    Length(specified::Length),
+    /// A (non-negative) integer.
+    Integer(u32),
+    /// A floating point value.
+    Float(CSSFloat),
+    /// A boolean value, specified as an integer (i.e., either 0 or 1).
+    BoolInteger(bool),
+    /// Two integers separated by '/', with optional whitespace on either side
+    /// of the '/'.
+    IntRatio(u32, u32),
+    /// A resolution.
+    Resolution(Resolution),
+    /// An enumerated value, defined by the variant keyword table in the
+    /// feature's `mData` member.
+    Enumerated(u32),
+    /// An identifier.
+    ///
+    /// TODO(emilio): Maybe atomize?
+    Ident(String),
+}
+
+impl MediaExpressionValue {
+    fn from_css_value(for_expr: &Expression, css_value: &nsCSSValue) -> Option<Self> {
+        // NB: If there's a null value, that means that we don't support the
+        // feature.
+        if css_value.mUnit == nsCSSUnit::eCSSUnit_Null {
+            return None;
+        }
+
+        match for_expr.feature.mValueType {
+            nsMediaFeature_ValueType::eLength => {
+                debug_assert!(css_value.mUnit == nsCSSUnit::eCSSUnit_Pixel);
+                let pixels = css_value.float_unchecked();
+                Some(MediaExpressionValue::Length(
+                        specified::Length::Absolute(Au::from_f32_px(pixels))))
+            }
+            nsMediaFeature_ValueType::eInteger => {
+                let i = css_value.integer_unchecked();
+                debug_assert!(i >= 0);
+                Some(MediaExpressionValue::Integer(i as u32))
+            }
+            nsMediaFeature_ValueType::eFloat => {
+                debug_assert!(css_value.mUnit == nsCSSUnit::eCSSUnit_Number);
+                Some(MediaExpressionValue::Float(css_value.float_unchecked()))
+            }
+            nsMediaFeature_ValueType::eBoolInteger => {
+                debug_assert!(css_value.mUnit == nsCSSUnit::eCSSUnit_Integer);
+                let i = css_value.integer_unchecked();
+                debug_assert!(i == 0 || i == 1);
+                Some(MediaExpressionValue::BoolInteger(i == 1))
+            }
+            nsMediaFeature_ValueType::eResolution => {
+                debug_assert!(css_value.mUnit == nsCSSUnit::eCSSUnit_Inch);
+                Some(MediaExpressionValue::Resolution(Resolution::Dpi(css_value.float_unchecked())))
+            }
+            nsMediaFeature_ValueType::eEnumerated => {
+                debug_assert!(css_value.mUnit == nsCSSUnit::eCSSUnit_Enumerated);
+                let value = css_value.integer_unchecked();
+                debug_assert!(value >= 0);
+                Some(MediaExpressionValue::Enumerated(value as u32))
+            }
+            nsMediaFeature_ValueType::eIdent => {
+                debug_assert!(css_value.mUnit == nsCSSUnit::eCSSUnit_Ident);
+                let string = unsafe {
+                    string_from_ns_string_buffer(*css_value.mValue.mString.as_ref())
+                };
+                Some(MediaExpressionValue::Ident(string))
+            }
+            nsMediaFeature_ValueType::eIntRatio => {
+                let array = unsafe { css_value.array_unchecked() };
+                debug_assert_eq!(array.len(), 2);
+                let first = array[0].integer_unchecked();
+                let second = array[1].integer_unchecked();
+
+                debug_assert!(first >= 0 && second >= 0);
+                Some(MediaExpressionValue::IntRatio(first as u32, second as u32))
+            }
+        }
+    }
+}
+
+impl ToCss for MediaExpressionValue {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+        where W: fmt::Write,
+    {
+        match *self {
+            MediaExpressionValue::Length(ref l) => l.to_css(dest),
+            MediaExpressionValue::Integer(v) => write!(dest, "{}", v),
+            MediaExpressionValue::Float(v) => write!(dest, "{}", v),
+            MediaExpressionValue::BoolInteger(v) => {
+                dest.write_str(if v { "1" } else { "0" })
+            },
+            MediaExpressionValue::IntRatio(a, b) => {
+                write!(dest, "{}/{}", a, b)
+            },
+            MediaExpressionValue::Resolution(ref r) => r.to_css(dest),
+            MediaExpressionValue::Ident(ref ident) => {
+                CssStringWriter::new(dest).write_str(ident)
+            }
+            MediaExpressionValue::Enumerated(..) => {
+                // TODO(emilio): Use the CSS keyword table.
+                unimplemented!()
+            }
+        }
+    }
+}
+
+fn starts_with_ignore_ascii_case(string: &str, prefix: &str) -> bool {
+    string.len() > prefix.len() &&
+      string[0..prefix.len()].eq_ignore_ascii_case(prefix)
+}
+
+fn find_feature<F>(mut f: F) -> Option<&'static nsMediaFeature>
+    where F: FnMut(&'static nsMediaFeature) -> bool,
+{
+    // FIXME(emilio): With build-time bindgen, we would be able to use
+    // structs::nsMediaFeatures_features. That would unfortunately break MSVC
+    // builds, or require one bindings file per platform.
+    //
+    // I'm not into any of those, so meanwhile let's use a FFI function.
+    unsafe {
+        let mut features = bindings::Gecko_GetMediaFeatures();
+        while !(*features).mName.is_null() {
+            if f(&*features) {
+                return Some(&*features);
+            }
+            features = features.offset(1);
+        }
+    }
+
+    None
+}
+
+impl Expression {
+    /// Trivially construct a new expression.
+    fn new(feature: &'static nsMediaFeature,
+           value: Option<MediaExpressionValue>,
+           range: nsMediaExpression_Range) -> Self {
+        Expression {
+            feature: feature,
+            value: value,
+            range: range,
+        }
+    }
+
+    /// Parse a media expression of the form:
+    ///
+    /// ```
+    /// (media-feature: media-value)
+    /// ```
+    pub fn parse(input: &mut Parser) -> Result<Self, ()> {
+        try!(input.expect_parenthesis_block());
+        input.parse_nested_block(|input| {
+            let ident = try!(input.expect_ident());
+
+            let mut flags = 0;
+            let mut feature_name = &*ident;
+
+            // TODO(emilio): this is under a pref in Gecko.
+            if starts_with_ignore_ascii_case(feature_name, "-webkit-") {
+                feature_name = &feature_name[8..];
+                flags |= nsMediaFeature_RequirementFlags::eHasWebkitPrefix as u8;
+            }
+
+            let range = if starts_with_ignore_ascii_case(feature_name, "min-") {
+                feature_name = &feature_name[4..];
+                nsMediaExpression_Range::eMin
+            } else if starts_with_ignore_ascii_case(feature_name, "max-") {
+                feature_name = &feature_name[4..];
+                nsMediaExpression_Range::eMax
+            } else {
+                nsMediaExpression_Range::eEqual
+            };
+
+            let atom = Atom::from(feature_name);
+            let feature =
+                match find_feature(|f| atom.as_ptr() == unsafe { *f.mName }) {
+                    Some(f) => f,
+                    None => return Err(()),
+                };
+
+            if (feature.mReqFlags & !flags) != 0 {
+                return Err(());
+            }
+
+            if range != nsMediaExpression_Range::eEqual &&
+                feature.mRangeType != nsMediaFeature_RangeType::eMinMaxAllowed {
+                return Err(());
+            }
+
+            // If there's no colon, this is a media query of the form
+            // '(<feature>)', that is, there's no value specified.
+            //
+            // FIXME(emilio): We need to check for range operators too here when
+            // we support them, see:
+            //
+            // https://drafts.csswg.org/mediaqueries/#mq-ranges
+            if input.try(|i| i.expect_colon()).is_err() {
+                return Ok(Expression::new(feature, None, range));
+            }
+
+            let value = match feature.mValueType {
+                nsMediaFeature_ValueType::eLength => {
+                    MediaExpressionValue::Length(
+                        specified::Length::parse_non_negative(input)?)
+                },
+                nsMediaFeature_ValueType::eInteger => {
+                    let i = input.expect_integer()?;
+                    if i < 0 {
+                        return Err(())
+                    }
+                    MediaExpressionValue::Integer(i as u32)
+                }
+                nsMediaFeature_ValueType::eBoolInteger => {
+                    let i = input.expect_integer()?;
+                    if i < 0 || i > 1 {
+                        return Err(())
+                    }
+                    MediaExpressionValue::BoolInteger(i == 1)
+                }
+                nsMediaFeature_ValueType::eFloat => {
+                    MediaExpressionValue::Float(input.expect_number()?)
+                }
+                nsMediaFeature_ValueType::eIntRatio => {
+                    let a = input.expect_integer()?;
+                    if a <= 0 {
+                        return Err(())
+                    }
+
+                    input.expect_delim('/')?;
+
+                    let b = input.expect_integer()?;
+                    if b <= 0 {
+                        return Err(())
+                    }
+                    MediaExpressionValue::IntRatio(a as u32, b as u32)
+                }
+                nsMediaFeature_ValueType::eResolution => {
+                    MediaExpressionValue::Resolution(Resolution::parse(input)?)
+                }
+                nsMediaFeature_ValueType::eEnumerated => {
+                    // TODO(emilio): Use Gecko's CSS keyword table to parse
+                    // this.
+                    return Err(())
+                }
+                nsMediaFeature_ValueType::eIdent => {
+                    MediaExpressionValue::Ident(input.expect_ident()?.into_owned())
+                }
+            };
+
+            Ok(Expression::new(feature, Some(value), range))
+        })
+    }
+
+    /// Returns whether this media query evaluates to true for the given device.
+    pub fn matches(&self, device: &Device) -> bool {
+        let mut css_value = nsCSSValue::null();
+        let result = unsafe {
+            (self.feature.mGetter.unwrap())(device.pres_context,
+                                            self.feature,
+                                            &mut css_value)
+        };
+
+        if result != nsresult::NS_OK {
+            // FIXME(emilio): This doesn't seem possible from reading gecko
+            // code, probably we should just clean up that function and return
+            // void.
+            error!("Media feature getter errored: {:?}, {:?}",
+                   result, Atom::from(unsafe { *self.feature.mName }));
+            return false;
+        }
+
+        let value = match MediaExpressionValue::from_css_value(self, &css_value) {
+            Some(v) => v,
+            None => return false,
+        };
+
+        self.evaluate_against(device, &value)
+    }
+
+    fn evaluate_against(&self,
+                        device: &Device,
+                        actual_value: &MediaExpressionValue)
+                        -> bool {
+        use self::MediaExpressionValue::*;
+        use std::cmp::Ordering;
+
+        debug_assert!(self.range == nsMediaExpression_Range::eEqual ||
+                      self.feature.mRangeType == nsMediaFeature_RangeType::eMinMaxAllowed,
+                      "Whoops, wrong range");
+
+        let default_values = device.default_values();
+
+        // http://dev.w3.org/csswg/mediaqueries3/#units
+        // em units are relative to the initial font-size.
+        let context = computed::Context {
+            is_root_element: false,
+            viewport_size: device.au_viewport_size(),
+            inherited_style: default_values,
+            // This cloning business is kind of dumb.... It's because Context
+            // insists on having an actual ComputedValues inside itself.
+            style: default_values.clone(),
+            font_metrics_provider: None
+        };
+
+        let required_value = match self.value {
+            Some(ref v) => v,
+            None => {
+                // If there's no value, always match unless it's a zero length
+                // or a zero integer or boolean.
+                return match *actual_value {
+                    BoolInteger(v) => v,
+                    Integer(v) => v != 0,
+                    Length(ref l) => l.to_computed_value(&context) != Au(0),
+                    _ => true,
+                }
+            }
+        };
+
+        // FIXME(emilio): Handle the possible floating point errors?
+        let cmp = match (required_value, actual_value) {
+            (&Length(ref one), &Length(ref other)) => {
+                one.to_computed_value(&context)
+                    .cmp(&other.to_computed_value(&context))
+            }
+            (&Integer(one), &Integer(ref other)) => one.cmp(other),
+            (&BoolInteger(one), &BoolInteger(ref other)) => one.cmp(other),
+            (&Float(one), &Float(ref other)) => one.partial_cmp(other).unwrap(),
+            (&IntRatio(one_num, one_den), &IntRatio(other_num, other_den)) => {
+                (one_num * other_den).partial_cmp(&(other_num * one_den)).unwrap()
+            }
+            (&Resolution(ref one), &Resolution(ref other)) => {
+                let actual_dpi = unsafe {
+                    if (*device.pres_context).mOverrideDPPX > 0.0 {
+                        self::Resolution::Dppx((*device.pres_context).mOverrideDPPX)
+                            .to_dpi()
+                    } else {
+                        other.to_dpi()
+                    }
+                };
+
+                one.to_dpi().partial_cmp(&actual_dpi).unwrap()
+            }
+            (&Ident(ref one), &Ident(ref other)) => {
+                debug_assert!(self.feature.mRangeType != nsMediaFeature_RangeType::eMinMaxAllowed);
+                return one == other;
+            }
+            (&Enumerated(..), &Enumerated(..)) => {
+                // TODO(emilio)
+                unimplemented!();
+            }
+            _ => unreachable!(),
+        };
+
+        cmp == Ordering::Equal || match self.range {
+            nsMediaExpression_Range::eMin => cmp == Ordering::Less,
+            nsMediaExpression_Range::eEqual => false,
+            nsMediaExpression_Range::eMax => cmp == Ordering::Greater,
+        }
+    }
+}

components/style/gecko/mod.rs

@@ -6,12 +6,7 @@
 
 pub mod conversions;
 pub mod data;
-
-// TODO(emilio): Implement Gecko media query parsing and evaluation using
-// nsMediaFeatures.
-#[path = "../servo/media_queries.rs"]
 pub mod media_queries;
-
 pub mod restyle_damage;
 pub mod selector_parser;
 pub mod snapshot;

components/style/gecko_bindings/bindings.rs

@@ -9,6 +9,7 @@ use gecko_bindings::structs::RawGeckoNode;
 use gecko_bindings::structs::RawGeckoAnimationValueList;
 use gecko_bindings::structs::RawServoAnimationValue;
 use gecko_bindings::structs::RawGeckoPresContext;
+use gecko_bindings::structs::RawGeckoPresContextOwned;
 use gecko_bindings::structs::ThreadSafeURIHolder;
 use gecko_bindings::structs::ThreadSafePrincipalHolder;
 use gecko_bindings::structs::CSSPseudoClassType;
@@ -29,6 +30,7 @@ use gecko_bindings::structs::nsChangeHint;
 use gecko_bindings::structs::nsCursorImage;
 use gecko_bindings::structs::nsFont;
 use gecko_bindings::structs::nsIAtom;
+use gecko_bindings::structs::nsMediaFeature;
 use gecko_bindings::structs::nsRestyleHint;
 use gecko_bindings::structs::nsStyleBackground;
 unsafe impl Send for nsStyleBackground {}
@@ -748,6 +750,9 @@ extern "C" {
     pub fn Gecko_CSSValue_GetArrayItem(css_value: nsCSSValueBorrowedMut,
                                        index: i32) -> nsCSSValueBorrowedMut;
 }
+extern "C" {
+    pub fn Gecko_CSSValue_Drop(css_value: nsCSSValueBorrowedMut);
+}
 extern "C" {
     pub fn Gecko_AddRefCSSValueSharedListArbitraryThread(aPtr:
                                                              *mut nsCSSValueSharedList);
@@ -759,6 +764,9 @@ extern "C" {
 extern "C" {
     pub fn Gecko_PropertyId_IsPrefEnabled(id: nsCSSPropertyID) -> bool;
 }
+extern "C" {
+    pub fn Gecko_GetMediaFeatures() -> *const nsMediaFeature;
+}
 extern "C" {
     pub fn Gecko_Construct_Default_nsStyleFont(ptr: *mut nsStyleFont,
                                                pres_context:
@@ -1114,14 +1122,11 @@ extern "C" {
      -> ServoCssRulesStrong;
 }
 extern "C" {
-    pub fn Servo_StyleSet_Init(pres_context: RawGeckoPresContextBorrowed)
+    pub fn Servo_StyleSet_Init(pres_context: RawGeckoPresContextOwned)
      -> RawServoStyleSetOwned;
 }
 extern "C" {
-    pub fn Servo_StyleSet_RecomputeDefaultStyles(set:
-                                                     RawServoStyleSetBorrowed,
-                                                 pres_context:
-                                                     RawGeckoPresContextBorrowed);
+    pub fn Servo_StyleSet_RebuildData(set: RawServoStyleSetBorrowed);
 }
 extern "C" {
     pub fn Servo_StyleSet_AppendStyleSheet(set: RawServoStyleSetBorrowed,

components/style/gecko_bindings/sugar/mod.rs

@@ -6,6 +6,7 @@
 
 mod ns_com_ptr;
 mod ns_css_shadow_array;
+mod ns_css_value;
 mod ns_style_auto_array;
 pub mod ns_style_coord;
 mod ns_t_array;

components/style/gecko_bindings/sugar/ns_css_value.rs

@@ -0,0 +1,78 @@
+/* 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/. */
+
+//! Little helpers for `nsCSSValue`.
+
+use gecko_bindings::bindings::Gecko_CSSValue_Drop;
+use gecko_bindings::structs::{nsCSSValue, nsCSSUnit, nsCSSValue_Array};
+use std::mem;
+use std::ops::Index;
+use std::slice;
+
+impl nsCSSValue {
+    /// Create a CSSValue with null unit, useful to be used as a return value.
+    #[inline]
+    pub fn null() -> Self {
+        unsafe { mem::zeroed() }
+    }
+
+    /// Returns this nsCSSValue value as an integer, unchecked in release
+    /// builds.
+    pub fn integer_unchecked(&self) -> i32 {
+        debug_assert!(self.mUnit == nsCSSUnit::eCSSUnit_Integer ||
+                      self.mUnit == nsCSSUnit::eCSSUnit_Enumerated ||
+                      self.mUnit == nsCSSUnit::eCSSUnit_EnumColor);
+        unsafe { *self.mValue.mInt.as_ref() }
+    }
+
+    /// Returns this nsCSSValue value as a floating point value, unchecked in
+    /// release builds.
+    pub fn float_unchecked(&self) -> f32 {
+        debug_assert!(nsCSSUnit::eCSSUnit_Number as u32 <= self.mUnit as u32);
+        unsafe { *self.mValue.mFloat.as_ref() }
+    }
+
+    /// Returns this nsCSSValue as a nsCSSValue::Array, unchecked in release
+    /// builds.
+    pub unsafe fn array_unchecked(&self) -> &nsCSSValue_Array {
+        debug_assert!(nsCSSUnit::eCSSUnit_Array as u32 <= self.mUnit as u32 &&
+                      self.mUnit as u32 <= nsCSSUnit::eCSSUnit_Calc_Divided as u32);
+        let array = *self.mValue.mArray.as_ref();
+        debug_assert!(!array.is_null());
+        &*array
+    }
+}
+
+impl Drop for nsCSSValue {
+    fn drop(&mut self) {
+        unsafe { Gecko_CSSValue_Drop(self) };
+    }
+}
+
+impl nsCSSValue_Array {
+    /// Return the length of this `nsCSSShadowArray`
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.mCount
+    }
+
+    #[inline]
+    fn buffer(&self) -> *const nsCSSValue {
+        self.mArray.as_ptr()
+    }
+
+    /// Get the array as a slice of nsCSSValues.
+    #[inline]
+    pub fn as_slice(&self) -> &[nsCSSValue] {
+        unsafe { slice::from_raw_parts(self.buffer(), self.len()) }
+    }
+}
+
+impl Index<usize> for nsCSSValue_Array {
+    type Output = nsCSSValue;
+    #[inline]
+    fn index(&self, i: usize) -> &nsCSSValue {
+        &self.as_slice()[i]
+    }
+}

components/style/media_queries.rs

@@ -19,7 +19,7 @@ pub use servo::media_queries::{Device, Expression};
 pub use gecko::media_queries::{Device, Expression};
 
 /// A type that encapsulates a media query list.
-#[derive(Debug, PartialEq)]
+#[derive(Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct MediaList {
     /// The list of media queries.
@@ -66,7 +66,7 @@ impl ToCss for Qualifier {
 /// A [media query][mq].
 ///
 /// [mq]: https://drafts.csswg.org/mediaqueries/
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct MediaQuery {
     /// The qualifier for this query.

components/style/servo/media_queries.rs

@@ -10,8 +10,6 @@ use euclid::{Size2D, TypedSize2D};
 use media_queries::MediaType;
 use properties::ComputedValues;
 use std::fmt;
-#[cfg(feature = "gecko")]
-use std::sync::Arc;
 use style_traits::{ToCss, ViewportPx};
 use style_traits::viewport::ViewportConstraints;
 use values::computed::{self, ToComputedValue};
@@ -21,24 +19,16 @@ use values::specified;
 /// is displayed in.
 ///
 /// This is the struct against which media queries are evaluated.
-#[derive(Debug)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Debug, HeapSizeOf)]
 pub struct Device {
     /// The current media type used by de device.
     media_type: MediaType,
     /// The current viewport size, in viewport pixels.
     viewport_size: TypedSize2D<f32, ViewportPx>,
-    /// A set of default computed values for this document.
-    ///
-    /// This allows handling zoom correctly, among other things. Gecko-only for
-    /// now, see #14773.
-    #[cfg(feature = "gecko")]
-    default_values: Arc<ComputedValues>,
 }
 
 impl Device {
     /// Trivially construct a new `Device`.
-    #[cfg(feature = "servo")]
     pub fn new(media_type: MediaType,
                viewport_size: TypedSize2D<f32, ViewportPx>)
                -> Device {
@@ -48,30 +38,11 @@ impl Device {
         }
     }
 
-    /// Trivially construct a new `Device`.
-    #[cfg(feature = "gecko")]
-    pub fn new(media_type:
-               MediaType, viewport_size: TypedSize2D<f32, ViewportPx>,
-               default_values: &Arc<ComputedValues>) -> Device {
-        Device {
-            media_type: media_type,
-            viewport_size: viewport_size,
-            default_values: default_values.clone(),
-        }
-    }
-
     /// Return the default computed values for this device.
-    #[cfg(feature = "servo")]
     pub fn default_values(&self) -> &ComputedValues {
         ComputedValues::initial_values()
     }
 
-    /// Return the default computed values for this device.
-    #[cfg(feature = "gecko")]
-    pub fn default_values(&self) -> &ComputedValues {
-        &*self.default_values
-    }
-
     /// Returns the viewport size of the current device in app units, needed,
     /// among other things, to resolve viewport units.
     #[inline]

components/style/stylist.rs

@@ -49,6 +49,20 @@ pub use ::fnv::FnvHashMap;
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct Stylist {
     /// Device that the stylist is currently evaluating against.
+    ///
+    /// This field deserves a bigger comment due to the different use that Gecko
+    /// and Servo give to it (that we should eventually unify).
+    ///
+    /// With Gecko, the device is never changed. Gecko manually tracks whether
+    /// the device data should be reconstructed, and "resets" the state of the
+    /// device.
+    ///
+    /// On Servo, on the other hand, the device is a really cheap representation
+    /// that is recreated each time some constraint changes and calling
+    /// `set_device`.
+    ///
+    /// In both cases, the device is actually _owned_ by the Stylist, and it's
+    /// only an `Arc` so we can implement `add_stylesheet` more idiomatically.
     pub device: Arc<Device>,
 
     /// Viewport constraints based on the current device.
@@ -146,6 +160,18 @@ impl Stylist {
             return false;
         }
 
+        let cascaded_rule = ViewportRule {
+            declarations: viewport::Cascade::from_stylesheets(doc_stylesheets, &self.device).finish(),
+        };
+
+        self.viewport_constraints =
+            ViewportConstraints::maybe_new(&self.device, &cascaded_rule);
+
+        if let Some(ref constraints) = self.viewport_constraints {
+            Arc::get_mut(&mut self.device).unwrap()
+                .account_for_viewport_rule(constraints);
+        }
+
         self.element_map = PerPseudoElementSelectorMap::new();
         self.pseudos_map = Default::default();
         self.animations = Default::default();
@@ -394,6 +420,13 @@ impl Stylist {
     ///
     /// Probably worth to make the stylist own a single `Device`, and have a
     /// `update_device` function?
+    ///
+    /// feature = "servo" because gecko only has one device, and manually tracks
+    /// when the device is dirty.
+    ///
+    /// FIXME(emilio): The semantics of the device for Servo and Gecko are
+    /// different enough we may want to unify them.
+    #[cfg(feature = "servo")]
     pub fn set_device(&mut self, mut device: Device, stylesheets: &[Arc<Stylesheet>]) {
         let cascaded_rule = ViewportRule {
             declarations: viewport::Cascade::from_stylesheets(stylesheets, &device).finish(),