canvas: Remove Backend trait (#38262)

After #38214 `Backend` trait is only used as DrawTarget builder and as
type holder. By moving types and DrawTarget creation into
`GenericDrawTarget` trait, we can completely remove `Backend` trait.

Testing: Just refactor, but code is covered by WPT tests.

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

components/canvas/backend.rs

@@ -12,28 +12,25 @@ use webrender_api::ImageDescriptor;
 
 use crate::canvas_data::{Filter, TextRun};
 
-pub(crate) trait Backend: Clone + Sized {
-    type DrawTarget: GenericDrawTarget<Self>;
-    type SourceSurface;
-
-    fn create_drawtarget(&self, size: Size2D<u64>) -> Self::DrawTarget;
-}
-
 // This defines required methods for a DrawTarget (currently only implemented for raqote).  The
 // prototypes are derived from the now-removed Azure backend's methods.
-pub(crate) trait GenericDrawTarget<B: Backend> {
+pub(crate) trait GenericDrawTarget {
+    type SourceSurface;
+
+    fn new(size: Size2D<u32>) -> Self;
+    fn create_similar_draw_target(&self, size: &Size2D<i32>) -> Self;
+
     fn clear_rect(&mut self, rect: &Rect<f32>, transform: Transform2D<f32>);
     fn copy_surface(
         &mut self,
-        surface: B::SourceSurface,
+        surface: Self::SourceSurface,
         source: Rect<i32>,
         destination: Point2D<i32>,
     );
-    fn create_similar_draw_target(&self, size: &Size2D<i32>) -> Self;
-    fn create_source_surface_from_data(&self, data: Snapshot) -> Option<B::SourceSurface>;
+    fn create_source_surface_from_data(&self, data: Snapshot) -> Option<Self::SourceSurface>;
     fn draw_surface(
         &mut self,
-        surface: B::SourceSurface,
+        surface: Self::SourceSurface,
         dest: Rect<f64>,
         source: Rect<f64>,
         filter: Filter,
@@ -42,7 +39,7 @@ pub(crate) trait GenericDrawTarget<B: Backend> {
     );
     fn draw_surface_with_shadow(
         &self,
-        surface: B::SourceSurface,
+        surface: Self::SourceSurface,
         dest: &Point2D<f32>,
         shadow_options: ShadowOptions,
         composition_options: CompositionOptions,
@@ -89,7 +86,7 @@ pub(crate) trait GenericDrawTarget<B: Backend> {
         composition_options: CompositionOptions,
         transform: Transform2D<f32>,
     );
-    fn surface(&self) -> B::SourceSurface;
+    fn surface(&self) -> Self::SourceSurface;
     fn image_descriptor_and_serializable_data(&self) -> (ImageDescriptor, SerializableImageData);
     fn snapshot(&self) -> Snapshot;
 }

components/canvas/canvas_data.rs

@@ -20,7 +20,7 @@ use range::Range;
 use unicode_script::Script;
 use webrender_api::ImageKey;
 
-use crate::backend::{Backend, GenericDrawTarget as _};
+use crate::backend::GenericDrawTarget;
 
 // Asserts on WR texture cache update for zero sized image with raw data.
 // https://github.com/servo/webrender/blob/main/webrender/src/texture_cache.rs#L1475
@@ -102,28 +102,25 @@ pub(crate) enum Filter {
     Nearest,
 }
 
-pub(crate) struct CanvasData<B: Backend> {
-    backend: B,
-    drawtarget: B::DrawTarget,
+pub(crate) struct CanvasData<DrawTarget: GenericDrawTarget> {
+    drawtarget: DrawTarget,
     compositor_api: CrossProcessCompositorApi,
     image_key: ImageKey,
     font_context: Arc<FontContext>,
 }
 
-impl<B: Backend> CanvasData<B> {
+impl<DrawTarget: GenericDrawTarget> CanvasData<DrawTarget> {
     pub(crate) fn new(
         size: Size2D<u64>,
         compositor_api: CrossProcessCompositorApi,
         font_context: Arc<FontContext>,
-        backend: B,
-    ) -> CanvasData<B> {
+    ) -> CanvasData<DrawTarget> {
         let size = size.max(MIN_WR_IMAGE_SIZE);
-        let draw_target = backend.create_drawtarget(size);
+        let draw_target = DrawTarget::new(size.cast());
         let image_key = compositor_api.generate_image_key_blocking().unwrap();
         let (descriptor, data) = draw_target.image_descriptor_and_serializable_data();
         compositor_api.add_image(image_key, descriptor, data);
         CanvasData {
-            backend,
             drawtarget: draw_target,
             compositor_api,
             image_key,
@@ -155,8 +152,8 @@ impl<B: Backend> CanvasData<B> {
             snapshot
         };
 
-        let writer = |draw_target: &mut B::DrawTarget, transform| {
-            write_image::<B>(
+        let writer = |draw_target: &mut DrawTarget, transform| {
+            write_image::<DrawTarget>(
                 draw_target,
                 snapshot,
                 dest_rect,
@@ -496,7 +493,7 @@ impl<B: Backend> CanvasData<B> {
                 shadow_options,
                 composition_options,
                 transform,
-                |new_draw_target: &mut B::DrawTarget, transform| {
+                |new_draw_target, transform| {
                     new_draw_target.fill_rect(rect, style, composition_options, transform);
                 },
             );
@@ -538,7 +535,7 @@ impl<B: Backend> CanvasData<B> {
                 shadow_options,
                 composition_options,
                 transform,
-                |new_draw_target: &mut B::DrawTarget, transform| {
+                |new_draw_target, transform| {
                     new_draw_target.stroke_rect(
                         rect,
                         style,
@@ -629,9 +626,7 @@ impl<B: Backend> CanvasData<B> {
             .max(MIN_WR_IMAGE_SIZE);
 
         // Step 1. Clear canvas's bitmap to transparent black.
-        self.drawtarget = self
-            .backend
-            .create_drawtarget(Size2D::new(size.width, size.height));
+        self.drawtarget = DrawTarget::new(Size2D::new(size.width, size.height).cast());
 
         self.update_image_rendering();
     }
@@ -658,7 +653,7 @@ impl<B: Backend> CanvasData<B> {
         );
     }
 
-    fn create_draw_target_for_shadow(&self, source_rect: &Rect<f32>) -> B::DrawTarget {
+    fn create_draw_target_for_shadow(&self, source_rect: &Rect<f32>) -> DrawTarget {
         self.drawtarget.create_similar_draw_target(&Size2D::new(
             source_rect.size.width as i32,
             source_rect.size.height as i32,
@@ -673,7 +668,7 @@ impl<B: Backend> CanvasData<B> {
         transform: Transform2D<f32>,
         draw_shadow_source: F,
     ) where
-        F: FnOnce(&mut B::DrawTarget, Transform2D<f32>),
+        F: FnOnce(&mut DrawTarget, Transform2D<f32>),
     {
         let shadow_src_rect = transform.outer_transformed_rect(rect);
         let mut new_draw_target = self.create_draw_target_for_shadow(&shadow_src_rect);
@@ -755,7 +750,7 @@ impl<B: Backend> CanvasData<B> {
     }
 }
 
-impl<B: Backend> Drop for CanvasData<B> {
+impl<D: GenericDrawTarget> Drop for CanvasData<D> {
     fn drop(&mut self) {
         self.compositor_api.delete_image(self.image_key);
     }
@@ -771,8 +766,8 @@ const IDEOGRAPHIC_BASELINE_DEFAULT: f32 = 0.5;
 /// dest_rect: Area of the destination target where the pixels will be copied
 /// smoothing_enabled: It determines if smoothing is applied to the image result
 /// premultiply: Determines whenever the image data should be premultiplied or not
-fn write_image<B: Backend>(
-    draw_target: &mut B::DrawTarget,
+fn write_image<DrawTarget: GenericDrawTarget>(
+    draw_target: &mut DrawTarget,
     snapshot: Snapshot,
     dest_rect: Rect<f64>,
     smoothing_enabled: bool,

components/canvas/canvas_paint_thread.rs

@@ -23,7 +23,6 @@ use pixels::Snapshot;
 use webrender_api::ImageKey;
 
 use crate::canvas_data::*;
-use crate::raqote_backend::RaqoteBackend;
 
 pub struct CanvasPaintThread {
     canvases: HashMap<CanvasId, Canvas>,
@@ -120,12 +119,8 @@ impl CanvasPaintThread {
         let canvas_id = self.next_canvas_id;
         self.next_canvas_id.0 += 1;
 
-        let canvas_data = CanvasData::new(
-            size,
-            self.compositor_api.clone(),
-            self.font_context.clone(),
-            RaqoteBackend,
-        );
+        let canvas_data =
+            CanvasData::new(size, self.compositor_api.clone(), self.font_context.clone());
         let image_key = canvas_data.image_key();
         self.canvases.insert(canvas_id, Canvas::Raqote(canvas_data));
 
@@ -301,7 +296,7 @@ impl CanvasPaintThread {
 }
 
 enum Canvas {
-    Raqote(CanvasData<RaqoteBackend>),
+    Raqote(CanvasData<raqote::DrawTarget>),
 }
 
 impl Canvas {

components/canvas/raqote_backend.rs

@@ -19,7 +19,7 @@ use raqote::{DrawOptions, PathBuilder, StrokeStyle};
 use style::color::AbsoluteColor;
 use webrender_api::{ImageDescriptor, ImageDescriptorFlags, ImageFormat};
 
-use crate::backend::{Backend, GenericDrawTarget};
+use crate::backend::GenericDrawTarget;
 use crate::canvas_data::{Filter, TextRun};
 
 thread_local! {
@@ -30,18 +30,6 @@ thread_local! {
     static SHARED_FONT_CACHE: RefCell<HashMap<FontIdentifier, Font>> = RefCell::default();
 }
 
-#[derive(Clone, Default)]
-pub(crate) struct RaqoteBackend;
-
-impl Backend for RaqoteBackend {
-    type DrawTarget = raqote::DrawTarget;
-    type SourceSurface = Vec<u8>; // TODO: See if we can avoid the alloc (probably?)
-
-    fn create_drawtarget(&self, size: Size2D<u64>) -> Self::DrawTarget {
-        raqote::DrawTarget::new(size.width as i32, size.height as i32)
-    }
-}
-
 #[derive(Clone)]
 pub enum Pattern {
     // argb
@@ -199,9 +187,15 @@ fn create_gradient_stops(gradient_stops: Vec<CanvasGradientStop>) -> Vec<raqote:
     stops
 }
 
-impl GenericDrawTarget<RaqoteBackend> for raqote::DrawTarget {
+impl GenericDrawTarget for raqote::DrawTarget {
+    type SourceSurface = Vec<u8>; // TODO: See if we can avoid the alloc (probably?)
+
+    fn new(size: Size2D<u32>) -> Self {
+        raqote::DrawTarget::new(size.width as i32, size.height as i32)
+    }
+
     fn clear_rect(&mut self, rect: &Rect<f32>, transform: Transform2D<f32>) {
-        <Self as GenericDrawTarget<RaqoteBackend>>::fill_rect(
+        <Self as GenericDrawTarget>::fill_rect(
             self,
             rect,
             FillOrStrokeStyle::Color(AbsoluteColor::TRANSPARENT_BLACK),
@@ -215,7 +209,7 @@ impl GenericDrawTarget<RaqoteBackend> for raqote::DrawTarget {
     #[allow(unsafe_code)]
     fn copy_surface(
         &mut self,
-        surface: <RaqoteBackend as Backend>::SourceSurface,
+        surface: Self::SourceSurface,
         source: Rect<i32>,
         destination: Point2D<i32>,
     ) {
@@ -226,16 +220,10 @@ impl GenericDrawTarget<RaqoteBackend> for raqote::DrawTarget {
         raqote::DrawTarget::copy_surface(self, &dt, source.to_box2d(), destination);
     }
 
-    fn create_similar_draw_target(
-        &self,
-        size: &Size2D<i32>,
-    ) -> <RaqoteBackend as Backend>::DrawTarget {
+    fn create_similar_draw_target(&self, size: &Size2D<i32>) -> Self {
         raqote::DrawTarget::new(size.width, size.height)
     }
-    fn create_source_surface_from_data(
-        &self,
-        data: Snapshot,
-    ) -> Option<<RaqoteBackend as Backend>::SourceSurface> {
+    fn create_source_surface_from_data(&self, data: Snapshot) -> Option<Self::SourceSurface> {
         Some(
             data.to_vec(
                 Some(SnapshotAlphaMode::Transparent {
@@ -249,7 +237,7 @@ impl GenericDrawTarget<RaqoteBackend> for raqote::DrawTarget {
     #[allow(unsafe_code)]
     fn draw_surface(
         &mut self,
-        surface: <RaqoteBackend as Backend>::SourceSurface,
+        surface: Self::SourceSurface,
         dest: Rect<f64>,
         src: Rect<f64>,
         filter: Filter,
@@ -297,7 +285,7 @@ impl GenericDrawTarget<RaqoteBackend> for raqote::DrawTarget {
     }
     fn draw_surface_with_shadow(
         &self,
-        _surface: <RaqoteBackend as Backend>::SourceSurface,
+        _surface: Self::SourceSurface,
         _dest: &Point2D<f32>,
         _shadow_options: ShadowOptions,
         _composition_options: CompositionOptions,
@@ -398,13 +386,7 @@ impl GenericDrawTarget<RaqoteBackend> for raqote::DrawTarget {
             rect.size.height,
         );
 
-        <Self as GenericDrawTarget<RaqoteBackend>>::fill(
-            self,
-            &pb,
-            style,
-            composition_options,
-            transform,
-        );
+        <Self as GenericDrawTarget>::fill(self, &pb, style, composition_options, transform);
     }
     fn get_size(&self) -> Size2D<i32> {
         Size2D::new(self.width(), self.height())
@@ -419,7 +401,7 @@ impl GenericDrawTarget<RaqoteBackend> for raqote::DrawTarget {
     fn push_clip_rect(&mut self, rect: &Rect<i32>) {
         self.push_clip_rect(rect.to_box2d());
     }
-    fn surface(&self) -> <RaqoteBackend as Backend>::SourceSurface {
+    fn surface(&self) -> Self::SourceSurface {
         self.get_data_u8().to_vec()
     }
     fn stroke(