style: Use OwnedSlice for will-change.

We could use ArcSlice if wanted I guess, your call. Though will change is not
supposed to be used very frequently.

Differential Revision: https://phabricator.services.mozilla.com/D30548

components/style/properties/gecko.mako.rs

@@ -2513,7 +2513,7 @@ fn static_assert() {
                           transition-timing-function transition-property
                           transform-style
                           rotate scroll-snap-points-x scroll-snap-points-y
-                          scroll-snap-coordinate -moz-binding will-change
+                          scroll-snap-coordinate -moz-binding
                           offset-path shape-outside
                           translate scale -webkit-line-clamp""" %>
 <%self:impl_trait style_struct_name="Box" skip_longhands="${skip_box_longhands}">
@@ -2829,66 +2829,6 @@ fn static_assert() {
     ${impl_individual_transform('translate', 'Translate', 'mSpecifiedTranslate')}
     ${impl_individual_transform('scale', 'Scale', 'mSpecifiedScale')}
 
-    pub fn set_will_change(&mut self, v: longhands::will_change::computed_value::T) {
-        use crate::gecko_bindings::bindings::{Gecko_AppendWillChange, Gecko_ClearWillChange};
-        use crate::values::specified::box_::{WillChangeBits, WillChange};
-
-        match v {
-            WillChange::AnimateableFeatures { features, bits } => {
-                unsafe {
-                    Gecko_ClearWillChange(&mut *self.gecko, features.len());
-                }
-
-                for feature in features.iter() {
-                    unsafe {
-                        Gecko_AppendWillChange(&mut *self.gecko, feature.0.as_ptr())
-                    }
-                }
-
-                self.gecko.mWillChangeBitField = bits;
-            },
-            WillChange::Auto => {
-                unsafe {
-                    Gecko_ClearWillChange(&mut *self.gecko, 0);
-                }
-                self.gecko.mWillChangeBitField = WillChangeBits::empty();
-            },
-        };
-    }
-
-    pub fn copy_will_change_from(&mut self, other: &Self) {
-        use crate::gecko_bindings::bindings::Gecko_CopyWillChangeFrom;
-
-        self.gecko.mWillChangeBitField = other.gecko.mWillChangeBitField;
-        unsafe {
-            Gecko_CopyWillChangeFrom(&mut *self.gecko, &*other.gecko);
-        }
-    }
-
-    pub fn reset_will_change(&mut self, other: &Self) {
-        self.copy_will_change_from(other)
-    }
-
-    pub fn clone_will_change(&self) -> longhands::will_change::computed_value::T {
-        use crate::values::CustomIdent;
-        use crate::values::specified::box_::WillChange;
-
-        if self.gecko.mWillChange.len() == 0 {
-            return WillChange::Auto
-        }
-
-        let custom_idents: Vec<CustomIdent> = self.gecko.mWillChange.iter().map(|gecko_atom| {
-            unsafe {
-                CustomIdent(Atom::from_raw(gecko_atom.mRawPtr))
-            }
-        }).collect();
-
-        WillChange::AnimateableFeatures {
-            features: custom_idents.into_boxed_slice(),
-            bits: self.gecko.mWillChangeBitField,
-        }
-    }
-
     <% impl_shape_source("shape_outside", "mShapeOutside") %>
 
     pub fn set_offset_path(&mut self, v: longhands::offset_path::computed_value::T) {

components/style/values/specified/box.rs

@@ -642,6 +642,7 @@ pub enum OverflowClipBox {
 #[derive(
     Clone,
     Debug,
+    Default,
     MallocSizeOf,
     PartialEq,
     SpecifiedValueInfo,
@@ -650,38 +651,38 @@ pub enum OverflowClipBox {
     ToResolvedValue,
     ToShmem,
 )]
-/// Provides a rendering hint to the user agent,
-/// stating what kinds of changes the author expects
-/// to perform on the element
+#[css(comma)]
+#[repr(C)]
+/// Provides a rendering hint to the user agent, stating what kinds of changes
+/// the author expects to perform on the element.
+///
+/// `auto` is represented by an empty `features` list.
 ///
 /// <https://drafts.csswg.org/css-will-change/#will-change>
-pub enum WillChange {
-    /// Expresses no particular intent
-    Auto,
-    /// <custom-ident>
-    #[css(comma)]
-    AnimateableFeatures {
+pub struct WillChange {
     /// The features that are supposed to change.
-        #[css(iterable)]
-        features: Box<[CustomIdent]>,
+    ///
+    /// TODO(emilio): Consider using ArcSlice since we just clone them from the
+    /// specified value? That'd save an allocation, which could be worth it.
+    #[css(iterable, if_empty = "auto")]
+    features: crate::OwnedSlice<CustomIdent>,
     /// A bitfield with the kind of change that the value will create, based
     /// on the above field.
     #[css(skip)]
     bits: WillChangeBits,
-    },
 }
 
 impl WillChange {
     #[inline]
     /// Get default value of `will-change` as `auto`
-    pub fn auto() -> WillChange {
-        WillChange::Auto
+    pub fn auto() -> Self {
+        Self::default()
     }
 }
 
 bitflags! {
     /// The change bits that we care about.
-    #[derive(MallocSizeOf, SpecifiedValueInfo, ToComputedValue, ToResolvedValue, ToShmem)]
+    #[derive(Default, MallocSizeOf, SpecifiedValueInfo, ToComputedValue, ToResolvedValue, ToShmem)]
     #[repr(C)]
     pub struct WillChangeBits: u8 {
         /// Whether the stacking context will change.
@@ -746,7 +747,7 @@ impl Parse for WillChange {
             .try(|input| input.expect_ident_matching("auto"))
             .is_ok()
         {
-            return Ok(WillChange::Auto);
+            return Ok(Self::default());
         }
 
         let mut bits = WillChangeBits::empty();
@@ -767,8 +768,8 @@ impl Parse for WillChange {
             Ok(ident)
         })?;
 
-        Ok(WillChange::AnimateableFeatures {
-            features: custom_idents.into_boxed_slice(),
+        Ok(Self {
+            features: custom_idents.into(),
             bits,
         })
     }