Optimize monotonic conversion a little bit

geometry/src/clip.rs

@@ -286,7 +286,7 @@ trait ContourClipper where Self::Edge: TEdge {
             }
         }
 
-        let input = mem::replace(self.contour_mut(), Contour::new());
+        let input = self.contour_mut().take();
         for mut segment in input.iter() {
             // Easy cases.
             match edge.trivially_test_segment(&segment) {

geometry/src/outline.rs

@@ -178,6 +178,17 @@ impl Contour {
         Contour { points: vec![], flags: vec![], bounds: Rect::zero() }
     }
 
+    // Replaces this contour with a new one, with arrays preallocated to match `self`.
+    #[inline]
+    pub(crate) fn take(&mut self) -> Contour {
+        let length = self.len() as usize;
+        mem::replace(self, Contour {
+            points: Vec::with_capacity(length),
+            flags: Vec::with_capacity(length),
+            bounds: Rect::zero(),
+        })
+    }
+
     #[inline]
     pub fn iter(&self) -> ContourIter {
         ContourIter { contour: self, index: 1 }
@@ -345,7 +356,8 @@ impl Contour {
 
     #[inline]
     pub fn make_monotonic(&mut self) {
-        let contour = mem::replace(self, Contour::new());
+        // TODO(pcwalton): Make monotonic in place?
+        let contour = self.take();
         for segment in MonotonicConversionIter::new(contour.iter()) {
             self.push_segment(segment);
         }