style: Make PropertyId::parse less of a footgun.

Bug: 1466095
Reviewed-by: xidorn
MozReview-Commit-ID: 2BmtSDPmHj9

components/style/properties/properties.mako.rs

@@ -1595,7 +1595,7 @@ impl PropertyId {
     /// Returns a given property from the string `s`.
     ///
     /// Returns Err(()) for unknown non-custom properties.
-    pub fn parse(property_name: &str) -> Result<Self, ()> {
+    fn parse_unchecked(property_name: &str) -> Result<Self, ()> {
         // FIXME(https://github.com/rust-lang/rust/issues/33156): remove this
         // enum and use PropertyId when stable Rust allows destructors in
         // statics.
@@ -1639,13 +1639,39 @@ impl PropertyId {
                 PropertyId::ShorthandAlias(id, alias)
             },
             None => {
-                return ::custom_properties::parse_name(property_name).map(|name| {
-                    PropertyId::Custom(::custom_properties::Name::from(name))
-                })
+                let name = ::custom_properties::parse_name(property_name)?;
+                PropertyId::Custom(::custom_properties::Name::from(name))
             },
         })
     }
 
+    /// Parses a property name, and returns an error if it's unknown or isn't
+    /// enabled for all content.
+    #[inline]
+    pub fn parse_enabled_for_all_content(name: &str) -> Result<Self, ()> {
+        let id = Self::parse_unchecked(name)?;
+
+        if !id.enabled_for_all_content() {
+            return Err(());
+        }
+
+        Ok(id)
+    }
+
+
+    /// Parses a property name, and returns an error if it's unknown or isn't
+    /// allowed in this context.
+    #[inline]
+    pub fn parse(name: &str, context: &ParserContext) -> Result<Self, ()> {
+        let id = Self::parse_unchecked(name)?;
+
+        if !id.allowed_in(context) {
+            return Err(());
+        }
+
+        Ok(id)
+    }
+
     /// Returns a property id from Gecko's nsCSSPropertyID.
     #[cfg(feature = "gecko")]
     #[allow(non_upper_case_globals)]
@@ -1715,8 +1741,7 @@ impl PropertyId {
         }
     }
 
-    /// Returns the non-custom property id for this property.
-    pub fn non_custom_id(&self) -> Option<NonCustomPropertyId> {
+    fn non_custom_id(&self) -> Option<NonCustomPropertyId> {
         Some(match *self {
             PropertyId::Custom(_) => return None,
             PropertyId::Shorthand(shorthand_id) => shorthand_id.into(),
@@ -1751,8 +1776,7 @@ impl PropertyId {
         id.enabled_for_all_content()
     }
 
-    /// Returns whether the property is allowed in a given context.
-    pub fn allowed_in(&self, context: &ParserContext) -> bool {
+    fn allowed_in(&self, context: &ParserContext) -> bool {
         let id = match self.non_custom_id() {
             // Custom properties are allowed everywhere
             None => return true,
@@ -1974,12 +1998,7 @@ impl PropertyDeclaration {
         input: &mut Parser<'i, 't>,
     ) -> Result<(), ParseError<'i>> {
         assert!(declarations.is_empty());
-
-        if !id.allowed_in(context) {
-            return Err(input.new_custom_error(
-                StyleParseErrorKind::UnknownProperty(name)
-            ));
-        }
+        debug_assert!(id.allowed_in(context), "{:?}", id);
 
         let start = input.state();
         match id {