Auto merge of #16780 - canaltinova:shorthand-aliases, r=emilio

Add comments to shorthand alias logic that explains the interesting bahavior

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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 they are just comments and variable name change.

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components/style/properties/declaration_block.rs

@@ -482,7 +482,7 @@ impl ToCss for PropertyDeclarationBlock {
                     // Substeps 7 and 8
                     // We need to check the shorthand whether it's an alias property or not.
                     // If it's an alias property, it should be serialized like its longhand.
-                    if shorthand.flags().contains(ALIAS_PROPERTY) {
+                    if shorthand.flags().contains(SHORTHAND_ALIAS_PROPERTY) {
                         append_serialization::<_, Cloned<slice::Iter< _>>, _>(
                              dest,
                              &property,

components/style/properties/properties.mako.rs

@@ -484,8 +484,8 @@ bitflags! {
         /// This property has values that can establish a containing block for
         /// absolutely positioned elements.
         const ABSPOS_CB = 1 << 2,
-        /// This property(shorthand) is an alias of another property.
-        const ALIAS_PROPERTY = 1 << 3,
+        /// This shorthand property is an alias of another property.
+        const SHORTHAND_ALIAS_PROPERTY = 1 << 3,
     }
 }
 
@@ -1263,7 +1263,10 @@ impl ToCss for PropertyDeclaration {
             PropertyDeclaration::WithVariables(_, ref with_variables) => {
                 // https://drafts.csswg.org/css-variables/#variables-in-shorthands
                 match with_variables.from_shorthand {
-                    Some(shorthand) if shorthand.flags().contains(ALIAS_PROPERTY) =>
+                    // Normally, we shouldn't be printing variables here if they came from
+                    // shorthands. But we should allow properties that came from shorthand
+                    // aliases. That also matches with the Gecko behavior.
+                    Some(shorthand) if shorthand.flags().contains(SHORTHAND_ALIAS_PROPERTY) =>
                         dest.write_str(&*with_variables.css)?,
                     None => dest.write_str(&*with_variables.css)?,
                     _ => {},
@@ -1345,7 +1348,11 @@ impl PropertyDeclaration {
                         Some(&*with_variables.css)
                     } else { None }
                 } else {
-                    if shorthand.flags().contains(ALIAS_PROPERTY) {
+                    // Normally, longhand property that doesn't come from a shorthand
+                    // should return None here. But we return Some to longhands if they
+                    // came from a shorthand alias. Because for example, we should be able to
+                    // get -moz-transform's value from transform.
+                    if shorthand.flags().contains(SHORTHAND_ALIAS_PROPERTY) {
                         return Some(&*with_variables.css);
                     }
                     None

components/style/properties/shorthand/box.mako.rs

@@ -327,7 +327,7 @@ macro_rules! try_parse_one {
 
 <%helpers:shorthand name="-moz-transform" products="gecko"
                     sub_properties="transform"
-                    flags="ALIAS_PROPERTY"
+                    flags="SHORTHAND_ALIAS_PROPERTY"
                     spec="Non-standard: https://developer.mozilla.org/en-US/docs/Web/CSS/transform">
     use properties::longhands::transform;