Bug 1900530 - Allow gradient defined by a single color stop. r=layout-reviewers,emilio

Differential Revision: https://phabricator.services.mozilla.com/D233218

gfx/wr/webrender_api/src/gradient_builder.rs

@@ -127,7 +127,7 @@ impl GradientBuilder {
     /// the amount to adjust the gradient line start and stop.
     fn normalize(&mut self, extend_mode: di::ExtendMode) -> (f32, f32) {
         let stops = &mut self.stops;
-        assert!(stops.len() >= 2);
+        assert!(stops.len() >= 1);
 
         let first = *stops.first().unwrap();
         let last = *stops.last().unwrap();

layout/painting/nsCSSRenderingGradients.cpp

@@ -608,8 +608,8 @@ static nsTArray<ColorStop> ComputeColorStopsForItems(
     ComputedStyle* aComputedStyle,
     Span<const StyleGenericGradientItem<StyleColor, T>> aItems,
     CSSCoord aLineLength) {
-  MOZ_ASSERT(aItems.Length() >= 2,
-             "The parser should reject gradients with less than two stops");
+  MOZ_ASSERT(!aItems.IsEmpty(),
+             "The parser should reject gradients with no stops");
 
   nsTArray<ColorStop> stops(aItems.Length());
 

layout/painting/nsCSSRenderingGradients.h

@@ -52,12 +52,14 @@ class MOZ_STACK_CLASS ColorStopInterpolator {
 
   void CreateStops() {
     // This loop intentionally iterates extra stops at the beginning and end
-    // if extending was requested.
-    uint32_t iterStops = mStops.Length() - 1 + (mExtend ? 2 : 0);
+    // if extending was requested, or in the degenerate case where only one
+    // color stop was specified.
+    const bool extend = mExtend || mStops.Length() == 1;
+    const uint32_t iterStops = mStops.Length() - 1 + (extend ? 2 : 0);
     for (uint32_t i = 0; i < iterStops; i++) {
-      auto thisindex = mExtend ? (i == 0 ? 0 : i - 1) : i;
+      auto thisindex = extend ? (i == 0 ? 0 : i - 1) : i;
       auto nextindex =
-          mExtend && (i == iterStops - 1 || i == 0) ? thisindex : thisindex + 1;
+          extend && (i == iterStops - 1 || i == 0) ? thisindex : thisindex + 1;
       const auto& start = mStops[thisindex];
       const auto& end = mStops[nextindex];
       float startPosition = start.mPosition;
@@ -68,7 +70,7 @@ class MOZ_STACK_CLASS ColorStopInterpolator {
       //
       // See https://bugzilla.mozilla.org/show_bug.cgi?id=1885716 for more info.
       uint32_t extraStops = 0;
-      if (mExtend) {
+      if (extend) {
         // If we're extending, we just need a single new stop, which will
         // duplicate the end being extended; do not create interpolated stops
         // within in the extension area!

servo/components/style/values/specified/image.rs

@@ -786,7 +786,7 @@ impl Gradient {
     ) -> Result<LengthPercentageItemList, ParseError<'i>> {
         let items =
             generic::GradientItem::parse_comma_separated(context, input, LengthPercentage::parse)?;
-        if items.len() < 2 {
+        if items.is_empty() {
             return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
         }
         Ok(items)
@@ -966,7 +966,7 @@ impl Gradient {
             AngleOrPercentage::parse_with_unitless,
         )?;
 
-        if items.len() < 2 {
+        if items.is_empty() {
             return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
         }
 
@@ -1249,7 +1249,7 @@ impl<T> generic::GradientItem<Color, T> {
             }
         }
 
-        if !seen_stop || items.len() < 2 {
+        if !seen_stop || items.is_empty() {
             return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
         }
         Ok(items.into())