Auto merge of #19918 - emilio:font-stuff-is-a-mess, r=nox

style: Refactor font-feature-settings and font-variation-settings.

This fixes all known issues with serialization and parsing of these two
properties, and in particular calc handling and such:

  https://bugzilla.mozilla.org/show_bug.cgi?id=1434692
  https://bugzilla.mozilla.org/show_bug.cgi?id=1434724

Also does a fair amount of cleanup and all that, which was needed, this code
was a mess.

There are further cleanups that can be done, like renaming the
font-variation-settings animation stuff.

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components/style/values/specified/mod.rs

@@ -9,6 +9,7 @@
 use Namespace;
 use context::QuirksMode;
 use cssparser::{Parser, Token, serialize_identifier};
+use num_traits::One;
 use parser::{ParserContext, Parse};
 use self::url::SpecifiedUrl;
 #[allow(unused_imports)] use std::ascii::AsciiExt;
@@ -33,7 +34,7 @@ pub use self::background::{BackgroundRepeat, BackgroundSize};
 pub use self::border::{BorderCornerRadius, BorderImageSlice, BorderImageWidth};
 pub use self::border::{BorderImageSideWidth, BorderRadius, BorderSideWidth, BorderSpacing};
 pub use self::font::{FontSize, FontSizeAdjust, FontSynthesis, FontWeight, FontVariantAlternates};
-pub use self::font::{FontFamily, FontLanguageOverride, FontVariantSettings, FontVariantEastAsian};
+pub use self::font::{FontFamily, FontLanguageOverride, FontVariationSettings, FontVariantEastAsian};
 pub use self::font::{FontVariantLigatures, FontVariantNumeric, FontFeatureSettings};
 pub use self::font::{MozScriptLevel, MozScriptMinSize, MozScriptSizeMultiplier, XTextZoom, XLang};
 pub use self::box_::{AnimationIterationCount, AnimationName, Display, OverscrollBehavior, Contain};
@@ -386,12 +387,28 @@ impl ToComputedValue for Opacity {
 /// An specified `<integer>`, optionally coming from a `calc()` expression.
 ///
 /// <https://drafts.csswg.org/css-values/#integers>
-#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq, PartialOrd)]
+#[derive(Clone, Copy, Debug, Eq, MallocSizeOf, PartialEq, PartialOrd)]
 pub struct Integer {
     value: CSSInteger,
     was_calc: bool,
 }
 
+impl One for Integer {
+    #[inline]
+    fn one() -> Self {
+        Self::new(1)
+    }
+}
+
+// This is not great, because it loses calc-ness, but it's necessary for One.
+impl ::std::ops::Mul<Integer> for Integer {
+    type Output = Self;
+
+    fn mul(self, other: Self) -> Self {
+        Self::new(self.value * other.value)
+    }
+}
+
 impl Integer {
     /// Trivially constructs a new `Integer` value.
     pub fn new(val: CSSInteger) -> Self {
@@ -439,7 +456,7 @@ impl Integer {
     pub fn parse_with_minimum<'i, 't>(
         context: &ParserContext,
         input: &mut Parser<'i, 't>,
-        min: i32
+        min: i32,
     ) -> Result<Integer, ParseError<'i>> {
         match Integer::parse(context, input) {
             // FIXME(emilio): The spec asks us to avoid rejecting it at parse
@@ -502,10 +519,11 @@ impl ToCss for Integer {
 pub type IntegerOrAuto = Either<Integer, Auto>;
 
 impl IntegerOrAuto {
-    #[allow(missing_docs)]
-    pub fn parse_positive<'i, 't>(context: &ParserContext,
-                                  input: &mut Parser<'i, 't>)
-                                  -> Result<IntegerOrAuto, ParseError<'i>> {
+    /// Parse `auto` or a positive integer.
+    pub fn parse_positive<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<IntegerOrAuto, ParseError<'i>> {
         match IntegerOrAuto::parse(context, input) {
             Ok(Either::First(integer)) if integer.value() <= 0 => {
                 Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError))