Auto merge of #17150 - asajeffrey:script-paint-worklets-plumbing, r=jdm

Implemented the plumbing for paint worklets

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This PR implements the plumbing for paint worklets:

* Adding CSS values for paint worklets.
* Implementing a skeleton for the `PaintWorkletGlobalScope` webidl.
* Implementing an executor for paint worklet tasks, and passing it from script to layout.
* Building the display list items for paint worklet images.

This PR does not implement registering or calling paint worklets in JS.

Before it merges, this PR needs a reftest added for basic paint worklet functionality.

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] There are tests for these changes

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components/style/values/generics/image.rs

@@ -27,6 +27,10 @@ pub enum Image<Gradient, ImageRect> {
     Rect(ImageRect),
     /// A `-moz-element(# <element-id>)`
     Element(Atom),
+    /// A paint worklet image.
+    /// https://drafts.css-houdini.org/css-paint-api/
+    #[cfg(feature = "servo")]
+    PaintWorklet(PaintWorklet),
 }
 
 /// A CSS gradient.
@@ -128,6 +132,23 @@ pub struct ColorStop<Color, LengthOrPercentage> {
     pub position: Option<LengthOrPercentage>,
 }
 
+/// Specified values for a paint worklet.
+/// https://drafts.css-houdini.org/css-paint-api/
+#[derive(Clone, Debug, PartialEq, ToComputedValue)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+pub struct PaintWorklet {
+    /// The name the worklet was registered with.
+    pub name: Atom,
+}
+
+impl ToCss for PaintWorklet {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+        dest.write_str("paint(")?;
+        serialize_identifier(&*self.name.to_string(), dest)?;
+        dest.write_str(")")
+    }
+}
+
 /// Values for `moz-image-rect`.
 ///
 /// `-moz-image-rect(<uri>, top, right, bottom, left);`
@@ -150,6 +171,8 @@ impl<G, R> fmt::Debug for Image<G, R>
             Image::Url(ref url) => url.to_css(f),
             Image::Gradient(ref grad) => grad.fmt(f),
             Image::Rect(ref rect) => rect.fmt(f),
+            #[cfg(feature = "servo")]
+            Image::PaintWorklet(ref paint_worklet) => paint_worklet.fmt(f),
             Image::Element(ref selector) => {
                 f.write_str("-moz-element(#")?;
                 serialize_identifier(&selector.to_string(), f)?;
@@ -167,6 +190,8 @@ impl<G, R> ToCss for Image<G, R>
             Image::Url(ref url) => url.to_css(dest),
             Image::Gradient(ref gradient) => gradient.to_css(dest),
             Image::Rect(ref rect) => rect.to_css(dest),
+            #[cfg(feature = "servo")]
+            Image::PaintWorklet(ref paint_worklet) => paint_worklet.to_css(dest),
             Image::Element(ref selector) => {
                 dest.write_str("-moz-element(#")?;
                 serialize_identifier(&selector.to_string(), dest)?;

components/style/values/specified/image.rs

@@ -22,6 +22,7 @@ use values::generics::image::{EndingShape as GenericEndingShape, Gradient as Gen
 use values::generics::image::{GradientItem as GenericGradientItem, GradientKind as GenericGradientKind};
 use values::generics::image::{Image as GenericImage, ImageRect as GenericImageRect};
 use values::generics::image::{LineDirection as GenericsLineDirection, ShapeExtent};
+use values::generics::image::PaintWorklet;
 use values::generics::position::Position as GenericPosition;
 use values::specified::{Angle, CSSColor, Color, Length, LengthOrPercentage};
 use values::specified::{Number, NumberOrPercentage, Percentage};
@@ -98,6 +99,12 @@ impl Parse for Image {
         if let Ok(gradient) = input.try(|i| Gradient::parse(context, i)) {
             return Ok(GenericImage::Gradient(gradient));
         }
+        #[cfg(feature = "servo")]
+        {
+            if let Ok(paint_worklet) = input.try(|i| PaintWorklet::parse(context, i)) {
+                return Ok(GenericImage::PaintWorklet(paint_worklet));
+            }
+        }
         #[cfg(feature = "gecko")]
         {
             if let Ok(mut image_rect) = input.try(|input| ImageRect::parse(context, input)) {
@@ -673,6 +680,18 @@ impl Parse for ColorStop {
     }
 }
 
+impl Parse for PaintWorklet {
+    fn parse(_context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
+        input.expect_function_matching("paint")?;
+        input.parse_nested_block(|i| {
+            let name = i.expect_ident()?;
+            Ok(PaintWorklet {
+                name: Atom::from(name),
+            })
+        })
+    }
+}
+
 impl Parse for ImageRect {
     fn parse(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
         input.try(|i| i.expect_function_matching("-moz-image-rect"))?;