style: Add bindings for ArcSlice and ThinArc, and use them to reduce copies of SVG path data.

As I said over bug 1549593, the eventual goal is to use ArcSlice in all
inherited properties. But this seemed like a good first candidate that doesn't
require me to move around a lot more code, since we were already using cbindgen
for the path commands.

Differential Revision: https://phabricator.services.mozilla.com/D30134

components/style/values/specified/svg_path.rs

@@ -29,16 +29,15 @@ use style_traits::{CssWriter, ParseError, StyleParseErrorKind, ToCss};
     ToResolvedValue,
     ToShmem,
 )]
-pub struct SVGPathData(Box<[PathCommand]>);
+#[repr(C)]
+pub struct SVGPathData(
+    // TODO(emilio): Should probably measure this somehow only from the
+    // specified values.
+    #[ignore_malloc_size_of = "Arc"]
+    pub crate::ArcSlice<PathCommand>
+);
 
 impl SVGPathData {
-    /// Return SVGPathData by a slice of PathCommand.
-    #[inline]
-    pub fn new(cmd: Box<[PathCommand]>) -> Self {
-        debug_assert!(!cmd.is_empty());
-        SVGPathData(cmd)
-    }
-
     /// Get the array of PathCommand.
     #[inline]
     pub fn commands(&self) -> &[PathCommand] {
@@ -46,9 +45,9 @@ impl SVGPathData {
         &self.0
     }
 
-    /// Create a normalized copy of this path by converting each relative command to an absolute
-    /// command.
-    fn normalize(&self) -> Self {
+    /// Create a normalized copy of this path by converting each relative
+    /// command to an absolute command.
+    fn normalize(&self) -> Box<[PathCommand]> {
         let mut state = PathTraversalState {
             subpath_start: CoordPair::new(0.0, 0.0),
             pos: CoordPair::new(0.0, 0.0),
@@ -58,7 +57,7 @@ impl SVGPathData {
             .iter()
             .map(|seg| seg.normalize(&mut state))
             .collect::<Vec<_>>();
-        SVGPathData(result.into_boxed_slice())
+        result.into_boxed_slice()
     }
 }
 
@@ -71,7 +70,7 @@ impl ToCss for SVGPathData {
         dest.write_char('"')?;
         {
             let mut writer = SequenceWriter::new(dest, " ");
-            for command in self.0.iter() {
+            for command in self.commands() {
                 writer.item(command)?;
             }
         }
@@ -104,7 +103,7 @@ impl Parse for SVGPathData {
             }
         }
 
-        Ok(SVGPathData::new(path_parser.path.into_boxed_slice()))
+        Ok(SVGPathData(crate::ArcSlice::from_iter(path_parser.path.into_iter())))
     }
 }
 
@@ -114,14 +113,17 @@ impl Animate for SVGPathData {
             return Err(());
         }
 
+        // FIXME(emilio): This allocates three copies of the path, that's not
+        // great! Specially, once we're normalized once, we don't need to
+        // re-normalize again.
         let result = self
             .normalize()
-            .0
             .iter()
-            .zip(other.normalize().0.iter())
+            .zip(other.normalize().iter())
             .map(|(a, b)| a.animate(&b, procedure))
             .collect::<Result<Vec<_>, _>>()?;
-        Ok(SVGPathData::new(result.into_boxed_slice()))
+
+        Ok(SVGPathData(crate::ArcSlice::from_iter(result.into_iter())))
     }
 }
 
@@ -131,9 +133,8 @@ impl ComputeSquaredDistance for SVGPathData {
             return Err(());
         }
         self.normalize()
-            .0
             .iter()
-            .zip(other.normalize().0.iter())
+            .zip(other.normalize().iter())
             .map(|(this, other)| this.compute_squared_distance(&other))
             .sum()
     }