Auto merge of #14049 - Wafflespeanut:lon, r=SimonSapin

Add a generic type for sharing some CSS types

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This would be useful for types such as `T(pub Option<U>)` or any type that's a mixture of two types (like `LengthOrFoo` for example). Well, it's a bit ugly, especially because we have to address the types as `Either::First` and `Either::Second` everywhere, but I don't have a brighter idea 😄

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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- [x] These changes do not require tests because it's a refactor

r? @SimonSapin (cc @Manishearth @emilio)

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components/style/properties/helpers.mako.rs

@@ -178,7 +178,7 @@
             use parser::{ParserContext, ParserContextExtraData};
             use properties::{CSSWideKeyword, DeclaredValue, Shorthand};
         % endif
-        #[allow(unused_imports)]
+        use values::{Auto, Either, None_, Normal};
         use cascade_info::CascadeInfo;
         use error_reporting::ParseErrorReporter;
         use parser::Parse;

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

@@ -22,6 +22,7 @@ use std::cmp;
 use std::fmt;
 use style_traits::ToCss;
 use super::ComputedValues;
+use values::Either;
 use values::computed::{Angle, LengthOrPercentageOrAuto, LengthOrPercentageOrNone};
 use values::computed::{BorderRadiusSize, LengthOrNone};
 use values::computed::{CalcLengthOrPercentage, LengthOrPercentage};
@@ -682,8 +683,8 @@ impl Interpolate for BoxShadow {
 impl Interpolate for LengthOrNone {
     fn interpolate(&self, other: &Self, progress: f64) -> Result<Self, ()> {
         match (*self, *other) {
-            (LengthOrNone::Length(ref len), LengthOrNone::Length(ref other)) =>
-                len.interpolate(&other, progress).map(LengthOrNone::Length),
+            (Either::First(ref length), Either::First(ref other)) =>
+                length.interpolate(&other, progress).map(Either::First),
             _ => Err(()),
         }
     }

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

@@ -1395,7 +1395,7 @@ ${helpers.single_keyword("transform-style",
 
 ${helpers.predefined_type("perspective",
                           "LengthOrNone",
-                          "computed::LengthOrNone::None",
+                          "Either::Second(None_)",
                           products="servo",
                           animatable=True)}
 

components/style/properties/properties.mako.rs

@@ -30,6 +30,7 @@ use logical_geometry::WritingMode;
 use parser::{ParserContext, ParserContextExtraData};
 use style_traits::ToCss;
 use stylesheets::Origin;
+#[cfg(feature = "servo")] use values::Either;
 use values::{HasViewportPercentage, computed};
 use cascade_info::CascadeInfo;
 use rule_tree::StrongRuleNode;
@@ -1249,7 +1250,7 @@ impl ComputedValues {
             if effects.transform.0.is_some() {
                 return transform_style::T::flat;
             }
-            if effects.perspective != computed::LengthOrNone::None {
+            if let Either::First(ref _length) = effects.perspective {
                 return transform_style::T::flat;
             }
         }