Auto merge of #16322 - tamamu:place-shorthand, r=upsuper

Implement alignment shorthand properties

<!-- Please describe your changes on the following line: -->
I implemented the shorthand properties, but it may includes some bugs.

Currently, `mach test-unit -p style` caught some errors (see #15954). I already tried `mach build-geckolib`, though there is no improvement. Please check my code.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #15954

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- [x] There are tests for these changes

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

@@ -137,3 +137,98 @@
   }
 
 </%helpers:shorthand>
+
+<%helpers:shorthand name="place-content" sub_properties="align-content justify-content"
+                    spec="https://drafts.csswg.org/css-align/#propdef-place-content"
+                    products="gecko" disable_when_testing="True">
+    use properties::longhands::align_content;
+    use properties::longhands::justify_content;
+
+    pub fn parse_value(context: &ParserContext, input: &mut Parser) -> Result<Longhands, ()> {
+        let align = align_content::parse(context, input)?;
+        let justify = input.try(|input| justify_content::parse(context, input))
+                           .unwrap_or(justify_content::SpecifiedValue::from(align));
+
+        Ok(Longhands {
+            align_content: align,
+            justify_content: justify,
+        })
+    }
+
+    impl<'a> ToCss for LonghandsToSerialize<'a> {
+        fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+            if self.align_content == self.justify_content {
+                self.align_content.to_css(dest)
+            } else {
+                self.justify_content.to_css(dest)?;
+                dest.write_str(" ")?;
+                self.justify_content.to_css(dest)
+            }
+        }
+    }
+</%helpers:shorthand>
+
+<%helpers:shorthand name="place-self" sub_properties="align-self justify-self"
+                    spec="https://drafts.csswg.org/css-align/#place-self-property"
+                    products="gecko" disable_when_testing="True">
+    use values::specified::align::AlignJustifySelf;
+    use parser::Parse;
+
+    pub fn parse_value(context: &ParserContext, input: &mut Parser) -> Result<Longhands, ()> {
+        let align = AlignJustifySelf::parse(context, input)?;
+        let justify = input.try(|input| AlignJustifySelf::parse(context, input)).unwrap_or(align.clone());
+
+        Ok(Longhands {
+            align_self: align,
+            justify_self: justify,
+        })
+    }
+
+    impl<'a> ToCss for LonghandsToSerialize<'a> {
+        fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+            if self.align_self == self.justify_self {
+                self.align_self.to_css(dest)
+            } else {
+                self.align_self.to_css(dest)?;
+                dest.write_str(" ")?;
+                self.justify_self.to_css(dest)
+            }
+        }
+    }
+</%helpers:shorthand>
+
+<%helpers:shorthand name="place-items" sub_properties="align-items justify-items"
+                    spec="https://drafts.csswg.org/css-align/#place-items-property"
+                    products="gecko" disable_when_testing="True">
+    use values::specified::align::{AlignItems, JustifyItems};
+    use parser::Parse;
+
+    impl From<AlignItems> for JustifyItems {
+        fn from(align: AlignItems) -> JustifyItems {
+            JustifyItems(align.0)
+        }
+    }
+
+    pub fn parse_value(context: &ParserContext, input: &mut Parser) -> Result<Longhands, ()> {
+        let align = AlignItems::parse(context, input)?;
+        let justify = input.try(|input| JustifyItems::parse(context, input))
+                           .unwrap_or(JustifyItems::from(align));
+
+        Ok(Longhands {
+            align_items: align,
+            justify_items: justify,
+        })
+    }
+
+    impl<'a> ToCss for LonghandsToSerialize<'a> {
+        fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+            if self.align_items.0 == self.justify_items.0 {
+                self.align_items.to_css(dest)
+            } else {
+                self.align_items.to_css(dest)?;
+                dest.write_str(" ")?;
+                self.justify_items.to_css(dest)
+            }
+        }
+    }
+</%helpers:shorthand>