Auto merge of #15465 - austinprete:caret-color, r=Wafflespeanut

Implement parsing/serialization for caret-color

<!-- Please describe your changes on the following line: -->
This pull request implements parsing and serialization for the caret-color CSS property, per issue #15309 .

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15309

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- [X] There are tests for these changes

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components/style/properties/helpers/animated_properties.mako.rs

@@ -29,11 +29,12 @@ use std::fmt;
 use style_traits::ToCss;
 use super::ComputedValues;
 use values::CSSFloat;
-use values::Either;
+use values::{Auto, Either};
 use values::computed::{Angle, LengthOrPercentageOrAuto, LengthOrPercentageOrNone};
 use values::computed::{BorderRadiusSize, ClipRect, LengthOrNone};
 use values::computed::{CalcLengthOrPercentage, Context, LengthOrPercentage};
 use values::computed::{MaxLength, MinLength};
+use values::computed::ColorOrAuto;
 use values::computed::position::{HorizontalPosition, Position, VerticalPosition};
 use values::computed::ToComputedValue;
 use values::specified::Angle as SpecifiedAngle;
@@ -1796,3 +1797,21 @@ impl Interpolate for TransformList {
     }
 }
 
+/// https://drafts.csswg.org/css-transitions-1/#animtype-color
+impl Interpolate for ColorOrAuto {
+    #[inline]
+    fn interpolate(&self, other: &Self, progress: f64) -> Result<Self, ()> {
+        match (*self, *other) {
+            (Either::First(ref this), Either::First(ref other)) => {
+                this.interpolate(&other, progress).map(Either::First)
+            },
+            (Either::Second(Auto), Either::Second(Auto)) => {
+                Ok(Either::Second(Auto))
+            },
+            _ => {
+                let interpolated = if progress < 0.5 { *self } else { *other };
+                Ok(interpolated)
+            }
+        }
+    }
+}

components/style/properties/longhand/ui.mako.rs

@@ -30,3 +30,10 @@ ${helpers.single_keyword("-moz-window-dragging", "default drag no-drag", product
                          gecko_enum_prefix="StyleWindowDragging",
                          animatable=False,
                          spec="None (Nonstandard Firefox-only property)")}
+
+${helpers.predefined_type("caret-color",
+                          "ColorOrAuto",
+                          "Either::Second(Auto)",
+                          spec="https://drafts.csswg.org/css-ui/#caret-color",
+                          animatable="True",
+                          products="none")}

components/style/values/computed/mod.rs

@@ -363,3 +363,6 @@ impl ClipRectOrAuto {
         }
     }
 }
+
+/// <color> | auto
+pub type ColorOrAuto = Either<CSSColor, Auto>;

components/style/values/specified/mod.rs

@@ -998,3 +998,6 @@ impl Parse for ClipRect {
 
 /// rect(...) | auto
 pub type ClipRectOrAuto = Either<ClipRect, Auto>;
+
+/// <color> | auto
+pub type ColorOrAuto = Either<CSSColor, Auto>;