style: Improve #[derive(Parse)].

I want to do this so that I can get rid of Either<>. The reasons for getting rid
of either are multiple:

 * It doesn't generate as nice C++ code using cbindgen.
 * It isn't that nice to use either from Rust.
 * cbindgen has bugs with zero-sized types.

I started using this for ColorOrAuto and a few others, for now.

Differential Revision: https://phabricator.services.mozilla.com/D19844

components/style/values/generics/svg.rs

@@ -136,6 +136,7 @@ impl<ColorType: Parse, UrlPaintServer: Parse> Parse for SVGPaint<ColorType, UrlP
     Debug,
     MallocSizeOf,
     PartialEq,
+    Parse,
     SpecifiedValueInfo,
     ToAnimatedValue,
     ToAnimatedZero,
@@ -143,28 +144,26 @@ impl<ColorType: Parse, UrlPaintServer: Parse> Parse for SVGPaint<ColorType, UrlP
     ToCss,
 )]
 pub enum SvgLengthPercentageOrNumber<LengthPercentage, Number> {
+    /// <number>
+    ///
+    /// Note that this needs to be before, so it gets parsed before the length,
+    /// to handle `0` correctly as a number instead of a `<length>`.
+    Number(Number),
     /// <length> | <percentage>
     LengthPercentage(LengthPercentage),
-    /// <number>
-    Number(Number),
 }
 
-/// Parsing the SvgLengthPercentageOrNumber. At first, we need to parse number
-/// since prevent converting to the length.
-impl<LengthPercentageType: Parse, NumberType: Parse> Parse
-    for SvgLengthPercentageOrNumber<LengthPercentageType, NumberType>
-{
-    fn parse<'i, 't>(
-        context: &ParserContext,
-        input: &mut Parser<'i, 't>,
-    ) -> Result<Self, ParseError<'i>> {
-        if let Ok(num) = input.try(|i| NumberType::parse(context, i)) {
-            return Ok(SvgLengthPercentageOrNumber::Number(num));
-        }
+/// Whether the `context-value` value is enabled.
+#[cfg(feature = "gecko")]
+pub fn is_context_value_enabled(_: &ParserContext) -> bool {
+    use crate::gecko_bindings::structs::mozilla;
+    unsafe { mozilla::StaticPrefs_sVarCache_gfx_font_rendering_opentype_svg_enabled }
+}
 
-        let lp = LengthPercentageType::parse(context, input)?;
-        Ok(SvgLengthPercentageOrNumber::LengthPercentage(lp))
-    }
+/// Whether the `context-value` value is enabled.
+#[cfg(not(feature = "gecko"))]
+pub fn is_context_value_enabled(_: &ParserContext) -> bool {
+    false
 }
 
 /// An SVG length value supports `context-value` in addition to length.
@@ -175,6 +174,7 @@ impl<LengthPercentageType: Parse, NumberType: Parse> Parse
     Debug,
     MallocSizeOf,
     PartialEq,
+    Parse,
     SpecifiedValueInfo,
     ToAnimatedValue,
     ToAnimatedZero,
@@ -185,6 +185,7 @@ pub enum SVGLength<LengthType> {
     /// `<length> | <percentage> | <number>`
     Length(LengthType),
     /// `context-value`
+    #[parse(condition = "is_context_value_enabled")]
     ContextValue,
 }
 
@@ -216,6 +217,7 @@ pub enum SVGStrokeDashArray<LengthType> {
     Debug,
     MallocSizeOf,
     PartialEq,
+    Parse,
     SpecifiedValueInfo,
     ToAnimatedZero,
     ToComputedValue,