style: Part 2: Support fit-content() in style

Support fit-content for preferred size, min size, and max size. This
patch only implement the style system. For layout part, we will do that
in the following patches.

Differential Revision: https://phabricator.services.mozilla.com/D107161

components/style/values/computed/length.rs

@@ -188,7 +188,8 @@ impl Size {
             GenericSize::MinContent |
             GenericSize::MaxContent |
             GenericSize::MozFitContent |
-            GenericSize::MozAvailable => false
+            GenericSize::MozAvailable |
+            GenericSize::FitContentFunction(_) => false
         }
     }
 }

components/style/values/generics/length.rs

@@ -161,6 +161,9 @@ pub enum GenericSize<LengthPercent> {
     #[cfg(feature = "gecko")]
     #[animation(error)]
     MozAvailable,
+    #[animation(error)]
+    #[css(function = "fit-content")]
+    FitContentFunction(LengthPercent)
 }
 
 pub use self::GenericSize as Size;
@@ -215,6 +218,9 @@ pub enum GenericMaxSize<LengthPercent> {
     #[cfg(feature = "gecko")]
     #[animation(error)]
     MozAvailable,
+    #[animation(error)]
+    #[css(function = "fit-content")]
+    FitContentFunction(LengthPercent),
 }
 
 pub use self::GenericMaxSize as MaxSize;

components/style/values/specified/length.rs

@@ -1247,6 +1247,30 @@ macro_rules! parse_size_non_length {
     }};
 }
 
+#[cfg(feature = "gecko")]
+fn is_fit_content_function_enabled() -> bool {
+    static_prefs::pref!("layout.css.fit-content-function.enabled")
+}
+#[cfg(feature = "servo")]
+fn is_fit_content_function_enabled() -> bool {
+    false
+}
+
+macro_rules! parse_fit_content_function {
+    ($size:ident, $input:expr, $context:expr, $allow_quirks:expr) => {
+        if is_fit_content_function_enabled() {
+            if let Ok(length) = $input.try_parse(|input| {
+                input.expect_function_matching("fit-content")?;
+                input.parse_nested_block(|i| {
+                    NonNegativeLengthPercentage::parse_quirky($context, i, $allow_quirks)
+                })
+            }) {
+                return Ok($size::FitContentFunction(length));
+            }
+        }
+    };
+}
+
 impl Size {
     /// Parses, with quirks.
     pub fn parse_quirky<'i, 't>(
@@ -1255,6 +1279,7 @@ impl Size {
         allow_quirks: AllowQuirks,
     ) -> Result<Self, ParseError<'i>> {
         parse_size_non_length!(Size, input, "auto" => Auto);
+        parse_fit_content_function!(Size, input, context, allow_quirks);
 
         let length = NonNegativeLengthPercentage::parse_quirky(context, input, allow_quirks)?;
         Ok(GenericSize::LengthPercentage(length))
@@ -1287,6 +1312,7 @@ impl MaxSize {
         allow_quirks: AllowQuirks,
     ) -> Result<Self, ParseError<'i>> {
         parse_size_non_length!(MaxSize, input, "none" => None);
+        parse_fit_content_function!(MaxSize, input, context, allow_quirks);
 
         let length = NonNegativeLengthPercentage::parse_quirky(context, input, allow_quirks)?;
         Ok(GenericMaxSize::LengthPercentage(length))