Auto merge of #17630 - canaltinova:grid-shorthand, r=wafflespeanut

stylo: Properly serialize grid shorthand

It was failing to serialize correctly when grid was `<'grid-template'>` or some different edge cases.

---
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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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components/style/properties/longhand/position.mako.rs

@@ -307,9 +307,7 @@ ${helpers.predefined_type("object-position",
         #[derive(PartialEq, Clone, Eq, Copy, Debug)]
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
         pub struct T {
-            // This needs to be an Option because we need an indicator to determine whether this property is
+            pub autoflow: AutoFlow,
-            // parsed or not in `grid` shorthand. Otherwise we can't properly serialize it.
-            pub autoflow: Option<AutoFlow>,
             pub dense: bool,
         }
     }
@@ -319,7 +317,7 @@ ${helpers.predefined_type("object-position",
 
     impl ToCss for computed_value::T {
         fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-            dest.write_str(match self.autoflow.unwrap_or(computed_value::AutoFlow::Row) {
+            dest.write_str(match self.autoflow {
                 computed_value::AutoFlow::Column => "column",
                 computed_value::AutoFlow::Row => "row"
             })?;
@@ -332,9 +330,8 @@ ${helpers.predefined_type("object-position",
     #[inline]
     pub fn get_initial_value() -> computed_value::T {
         computed_value::T {
-            // None should resolve to `computed_value::AutoFlow::Row` in layout.
+            autoflow: computed_value::AutoFlow::Row,
-            autoflow: None,
+            dense: false,
-            dense: false
         }
     }
 
@@ -370,7 +367,7 @@ ${helpers.predefined_type("object-position",
 
         if value.is_some() || dense {
             Ok(computed_value::T {
-                autoflow: value,
+                autoflow: value.unwrap_or(AutoFlow::Row),
                 dense: dense,
             })
         } else {
@@ -385,12 +382,12 @@ ${helpers.predefined_type("object-position",
             use self::computed_value::AutoFlow;
 
             SpecifiedValue {
-                autoflow: Some(
+                autoflow:
                     if bits & structs::NS_STYLE_GRID_AUTO_FLOW_ROW as u8 != 0 {
                         AutoFlow::Row
                     } else {
                         AutoFlow::Column
-                    }),
+                    },
                 dense:
                     bits & structs::NS_STYLE_GRID_AUTO_FLOW_DENSE as u8 != 0,
             }
@@ -403,7 +400,7 @@ ${helpers.predefined_type("object-position",
             use gecko_bindings::structs;
             use self::computed_value::AutoFlow;
 
-            let mut result: u8 = match v.autoflow.unwrap_or(AutoFlow::Row) {
+            let mut result: u8 = match v.autoflow {
                 AutoFlow::Row => structs::NS_STYLE_GRID_AUTO_FLOW_ROW as u8,
                 AutoFlow::Column => structs::NS_STYLE_GRID_AUTO_FLOW_COLUMN as u8,
             };

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

@@ -460,7 +460,7 @@
 
             auto_flow.map(|flow| {
                 SpecifiedAutoFlow {
-                    autoflow: Some(flow),
+                    autoflow: flow,
                     dense: dense,
                 }
             }).ok_or(StyleParseError::UnspecifiedError.into())
@@ -473,10 +473,10 @@
         } else if let Ok(rows) = input.try(|i| GridTemplateComponent::parse(context, i)) {
             temp_rows = rows;
             input.expect_delim('/')?;
-            flow = parse_auto_flow(input, true)?;
+            flow = parse_auto_flow(input, false)?;
             auto_cols = grid_auto_columns::parse(context, input).unwrap_or_default();
         } else {
-            flow = parse_auto_flow(input, false)?;
+            flow = parse_auto_flow(input, true)?;
             auto_rows = input.try(|i| grid_auto_rows::parse(context, i)).unwrap_or_default();
             input.expect_delim('/')?;
             temp_cols = GridTemplateComponent::parse(context, input)?;
@@ -495,16 +495,30 @@
         })
     }
 
+    /// Returns true if every sub property value of `grid` shorthand is initial.
+    impl<'a> LonghandsToSerialize<'a> {
+        fn is_initial(&self) -> bool {
+            *self.grid_template_rows == GridTemplateComponent::None &&
+            *self.grid_template_columns == GridTemplateComponent::None &&
+            *self.grid_template_areas == Either::Second(None_) &&
+            *self.grid_auto_rows == TrackSize::default() &&
+            *self.grid_auto_columns == TrackSize::default() &&
+            *self.grid_auto_flow == grid_auto_flow::get_initial_value()
+        }
+    }
+
     impl<'a> ToCss for LonghandsToSerialize<'a> {
         fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-            if self.grid_auto_flow.autoflow.is_none() {
+            if *self.grid_template_areas != Either::Second(None_) ||
+               (*self.grid_template_rows != GridTemplateComponent::None &&
+                   *self.grid_template_columns != GridTemplateComponent::None) ||
+               self.is_initial() {
                 return super::grid_template::serialize_grid_template(self.grid_template_rows,
                                                                      self.grid_template_columns,
                                                                      self.grid_template_areas, dest);
             }
 
-            // This unwrap is safe because we checked the None part in previous if condition.
+            if self.grid_auto_flow.autoflow == AutoFlow::Column {
-            if self.grid_auto_flow.autoflow.unwrap() == AutoFlow::Row {
                 self.grid_template_rows.to_css(dest)?;
                 dest.write_str(" / auto-flow")?;
                 if self.grid_auto_flow.dense {