Auto merge of #18131 - emilio:calc-serialization, r=canaltinova

style: Multiple calc serialization fixes.

This puts us in line with the spec as written, except for caveat in https://github.com/w3c/csswg-drafts/issues/1731.

Need to fix computed calc() too, I'm less confident that we won't need to change test expectations.

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components/style/values/computed/length.rs

@@ -195,11 +195,21 @@ impl From<LengthOrPercentageOrNone> for Option<CalcLengthOrPercentage> {
 
 impl ToCss for CalcLengthOrPercentage {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match (self.length, self.percentage) {
-            (l, Some(p)) if l == Au(0) => p.to_css(dest),
-            (l, Some(p)) => write!(dest, "calc({}px + {}%)", Au::to_px(l), p.0 * 100.),
-            (l, None) => write!(dest, "{}px", Au::to_px(l)),
-        }
+        use num_traits::Zero;
+
+        let (length, percentage) = match (self.length, self.percentage) {
+            (l, None) => return l.to_css(dest),
+            (l, Some(p)) if l == Au(0) => return p.to_css(dest),
+            (l, Some(p)) => (l, p),
+        };
+
+        dest.write_str("calc(")?;
+        percentage.to_css(dest)?;
+
+        dest.write_str(if length < Zero::zero() { " - " } else { " + " })?;
+        length.abs().to_css(dest)?;
+
+        dest.write_str(")")
     }
 }
 

components/style/values/computed/percentage.rs

@@ -11,7 +11,7 @@ use values::CSSFloat;
 use values::animated::ToAnimatedZero;
 
 /// A computed percentage.
-#[derive(Clone, ComputeSquaredDistance, Copy, Debug, Default, HasViewportPercentage, PartialEq)]
+#[derive(Clone, ComputeSquaredDistance, Copy, Debug, Default, HasViewportPercentage, PartialEq, PartialOrd)]
 #[cfg_attr(feature = "servo", derive(Deserialize, HeapSizeOf, Serialize))]
 pub struct Percentage(pub CSSFloat);
 
@@ -27,6 +27,12 @@ impl Percentage {
     pub fn hundred() -> Self {
         Percentage(1.)
     }
+
+    /// Returns the absolute value for this percentage.
+    #[inline]
+    pub fn abs(&self) -> Self {
+        Percentage(self.0.abs())
+    }
 }
 
 /// https://drafts.csswg.org/css-transitions/#animtype-percentage