--- a/layout/style/nsStyleStruct.cpp
+++ b/layout/style/nsStyleStruct.cpp
@@ -988,22 +988,31 @@ StyleBasicShape::GetShapeTypeName() cons
       return eCSSKeyword_inset;
   }
   MOZ_ASSERT_UNREACHABLE("unexpected type");
   return eCSSKeyword_UNKNOWN;
 }
 
 // --------------------
 // StyleShapeSource
+StyleShapeSource::StyleShapeSource()
+  : mBasicShape()
+{
+}
 
 StyleShapeSource::StyleShapeSource(const StyleShapeSource& aSource)
 {
   DoCopy(aSource);
 }
 
+StyleShapeSource::~StyleShapeSource()
+{
+  DoDestroy();
+}
+
 StyleShapeSource&
 StyleShapeSource::operator=(const StyleShapeSource& aOther)
 {
   if (this != &aOther) {
     DoCopy(aOther);
   }
 
   return *this;
@@ -1035,28 +1044,31 @@ StyleShapeSource::operator==(const Style
   MOZ_ASSERT_UNREACHABLE("Unexpected shape source type!");
   return true;
 }
 
 void
 StyleShapeSource::SetURL(css::URLValue* aValue)
 {
   MOZ_ASSERT(aValue);
-  if (!mShapeImage) {
-    mShapeImage = MakeUnique<nsStyleImage>();
+  if (mType != StyleShapeSourceType::Image &&
+      mType != StyleShapeSourceType::URL) {
+    DoDestroy();
+    new (&mShapeImage) UniquePtr<nsStyleImage>(new nsStyleImage());
   }
   mShapeImage->SetURLValue(do_AddRef(aValue));
   mType = StyleShapeSourceType::URL;
 }
 
 void
 StyleShapeSource::SetShapeImage(UniquePtr<nsStyleImage> aShapeImage)
 {
   MOZ_ASSERT(aShapeImage);
-  mShapeImage = std::move(aShapeImage);
+  DoDestroy();
+  new (&mShapeImage) UniquePtr<nsStyleImage>(std::move(aShapeImage));
   mType = StyleShapeSourceType::Image;
 }
 
 imgIRequest*
 StyleShapeSource::GetShapeImageData() const
 {
   if (mType != StyleShapeSourceType::Image) {
     return nullptr;
@@ -1067,24 +1079,26 @@ StyleShapeSource::GetShapeImageData() co
   return mShapeImage->GetImageData();
 }
 
 void
 StyleShapeSource::SetBasicShape(UniquePtr<StyleBasicShape> aBasicShape,
                                 StyleGeometryBox aReferenceBox)
 {
   MOZ_ASSERT(aBasicShape);
-  mBasicShape = std::move(aBasicShape);
+  DoDestroy();
+  new (&mBasicShape) UniquePtr<StyleBasicShape>(std::move(aBasicShape));
   mReferenceBox = aReferenceBox;
   mType = StyleShapeSourceType::Shape;
 }
 
 void
 StyleShapeSource::SetReferenceBox(StyleGeometryBox aReferenceBox)
 {
+  DoDestroy();
   mReferenceBox = aReferenceBox;
   mType = StyleShapeSourceType::Box;
 }
 
 void
 StyleShapeSource::DoCopy(const StyleShapeSource& aOther)
 {
   switch (aOther.mType) {
@@ -1107,16 +1121,35 @@ StyleShapeSource::DoCopy(const StyleShap
       break;
 
     case StyleShapeSourceType::Box:
       SetReferenceBox(aOther.GetReferenceBox());
       break;
   }
 }
 
+void
+StyleShapeSource::DoDestroy()
+{
+  switch (mType) {
+    case StyleShapeSourceType::Shape:
+      mBasicShape.~UniquePtr<StyleBasicShape>();
+      break;
+    case StyleShapeSourceType::Image:
+    case StyleShapeSourceType::URL:
+      mShapeImage.~UniquePtr<nsStyleImage>();
+      break;
+    case StyleShapeSourceType::None:
+    case StyleShapeSourceType::Box:
+      // Not a union type, so do nothing.
+      break;
+  }
+  mType = StyleShapeSourceType::None;
+}
+
 // --------------------
 // nsStyleFilter
 //
 nsStyleFilter::nsStyleFilter()
   : mType(NS_STYLE_FILTER_NONE)
   , mDropShadow(nullptr)
 {
   MOZ_COUNT_CTOR(nsStyleFilter);

--- a/layout/style/nsStyleStruct.h
+++ b/layout/style/nsStyleStruct.h
@@ -1968,23 +1968,21 @@ private:
   // position of center for ellipse or circle
   mozilla::Position mPosition;
   // corner radii for inset (0 if not set)
   nsStyleCorners mRadius;
 };
 
 struct StyleShapeSource final
 {
-  StyleShapeSource() = default;
+  StyleShapeSource();
 
   StyleShapeSource(const StyleShapeSource& aSource);
 
-  ~StyleShapeSource()
-  {
-  }
+  ~StyleShapeSource();
 
   StyleShapeSource& operator=(const StyleShapeSource& aOther);
 
   bool operator==(const StyleShapeSource& aOther) const;
 
   bool operator!=(const StyleShapeSource& aOther) const
   {
     return !(*this == aOther);
@@ -2036,19 +2034,24 @@ struct StyleShapeSource final
   }
 
   void SetReferenceBox(StyleGeometryBox aReferenceBox);
 
 private:
   void* operator new(size_t) = delete;
 
   void DoCopy(const StyleShapeSource& aOther);
-
-  mozilla::UniquePtr<StyleBasicShape> mBasicShape;
-  mozilla::UniquePtr<nsStyleImage> mShapeImage;
+  void DoDestroy();
+
+  union {
+    mozilla::UniquePtr<StyleBasicShape> mBasicShape;
+    mozilla::UniquePtr<nsStyleImage> mShapeImage;
+    // TODO: Bug 1429298, implement SVG Path function.
+    // TODO: Bug 1480665, implement ray() function.
+  };
   StyleShapeSourceType mType = StyleShapeSourceType::None;
   StyleGeometryBox mReferenceBox = StyleGeometryBox::NoBox;
 };
 
 } // namespace mozilla
 
 struct MOZ_NEEDS_MEMMOVABLE_MEMBERS nsStyleDisplay
 {

--- a/servo/components/style/gecko/conversions.rs
+++ b/servo/components/style/gecko/conversions.rs
@@ -654,17 +654,17 @@ pub mod basic_shape {
         ) -> Option<ShapeSource<BasicShape, ReferenceBox, ImageOrUrl>>
         where
             ReferenceBox: From<StyleGeometryBox>,
         {
             match self.mType {
                 StyleShapeSourceType::None => Some(ShapeSource::None),
                 StyleShapeSourceType::Box => Some(ShapeSource::Box(self.mReferenceBox.into())),
                 StyleShapeSourceType::Shape => {
-                    let other_shape = unsafe { &*self.mBasicShape.mPtr };
+                    let other_shape = unsafe { &*self.__bindgen_anon_1.mBasicShape.as_ref().mPtr };
                     let shape = other_shape.into();
                     let reference_box = if self.mReferenceBox == StyleGeometryBox::NoBox {
                         None
                     } else {
                         Some(self.mReferenceBox.into())
                     };
                     Some(ShapeSource::Shape(shape, reference_box))
                 },
@@ -672,17 +672,17 @@ pub mod basic_shape {
             }
         }
     }
 
     impl<'a> From<&'a StyleShapeSource> for ClippingShape {
         fn from(other: &'a StyleShapeSource) -> Self {
             match other.mType {
                 StyleShapeSourceType::URL => unsafe {
-                    let shape_image = &*other.mShapeImage.mPtr;
+                    let shape_image = &*other.__bindgen_anon_1.mShapeImage.as_ref().mPtr;
                     let other_url = RefPtr::new(*shape_image.__bindgen_anon_1.mURLValue.as_ref());
                     let url = ComputedUrl::from_url_value(other_url);
                     ShapeSource::ImageOrUrl(url)
                 },
                 StyleShapeSourceType::Image => {
                     unreachable!("ClippingShape doesn't support Image!");
                 },
                 _ => other
@@ -694,17 +694,17 @@ pub mod basic_shape {
 
     impl<'a> From<&'a StyleShapeSource> for FloatAreaShape {
         fn from(other: &'a StyleShapeSource) -> Self {
             match other.mType {
                 StyleShapeSourceType::URL => {
                     unreachable!("FloatAreaShape doesn't support URL!");
                 },
                 StyleShapeSourceType::Image => unsafe {
-                    let shape_image = &*other.mShapeImage.mPtr;
+                    let shape_image = &*other.__bindgen_anon_1.mShapeImage.as_ref().mPtr;
                     let image = shape_image.into_image().expect("Cannot convert to Image");
                     ShapeSource::ImageOrUrl(image)
                 },
                 _ => other
                     .into_shape_source()
                     .expect("Couldn't convert to StyleSource!"),
             }
         }

--- a/servo/components/style/properties/gecko.mako.rs
+++ b/servo/components/style/properties/gecko.mako.rs
@@ -4955,17 +4955,17 @@ fn static_assert() {
                 unsafe {
                     bindings::Gecko_StyleShapeSource_SetURLValue(${ident}, url.0.url_value.get())
                 }
             }
             % elif ident == "shape_outside":
             ShapeSource::ImageOrUrl(image) => {
                 unsafe {
                     bindings::Gecko_NewShapeImage(${ident});
-                    let style_image = &mut *${ident}.mShapeImage.mPtr;
+                    let style_image = &mut *${ident}.__bindgen_anon_1.mShapeImage.as_mut().mPtr;
                     style_image.set(image);
                 }
             }
             % else:
                <% raise Exception("Unknown property: %s" % ident) %>
             }
             % endif
             ShapeSource::None => {} // don't change the type
@@ -4975,17 +4975,17 @@ fn static_assert() {
             }
             ShapeSource::Shape(servo_shape, maybe_box) => {
                 fn init_shape(${ident}: &mut StyleShapeSource, basic_shape_type: StyleBasicShapeType)
                               -> &mut StyleBasicShape {
                     unsafe {
                         // Create StyleBasicShape in StyleShapeSource. mReferenceBox and mType
                         // will be set manually later.
                         Gecko_NewBasicShape(${ident}, basic_shape_type);
-                        &mut *${ident}.mBasicShape.mPtr
+                        &mut *${ident}.__bindgen_anon_1.mBasicShape.as_mut().mPtr
                     }
                 }
                 match servo_shape {
                     BasicShape::Inset(inset) => {
                         let shape = init_shape(${ident}, StyleBasicShapeType::Inset);
                         unsafe { shape.mCoordinates.set_len(4) };
 
                         // set_len() can't call constructors, so the coordinates