style: Allow CSS wide-keywords in custom property fallback.

Differential Revision: https://phabricator.services.mozilla.com/D28349

components/style/properties/properties.mako.rs

@@ -1085,6 +1085,8 @@ impl LonghandId {
         }
     }
 
+    // TODO(emilio): Should we use a function table like CASCADE_PROPERTY does
+    // to avoid blowing up code-size here?
     fn parse_value<'i, 't>(
         &self,
         context: &ParserContext,
@@ -1532,12 +1534,28 @@ impl UnparsedValue {
         quirks_mode: QuirksMode,
         environment: &::custom_properties::CssEnvironment,
     ) -> PropertyDeclaration {
-        crate::custom_properties::substitute(
+        let invalid_at_computed_value_time = || {
+            let keyword = if longhand_id.inherited() {
+                CSSWideKeyword::Inherit
+            } else {
+                CSSWideKeyword::Initial
+            };
+            PropertyDeclaration::CSSWideKeyword(WideKeywordDeclaration {
+                id: longhand_id,
+                keyword,
+            })
+        };
+
+        let css = match crate::custom_properties::substitute(
             &self.css,
             self.first_token_type,
             custom_properties,
             environment,
-        ).ok().and_then(|css| {
+        ) {
+            Ok(css) => css,
+            Err(..) => return invalid_at_computed_value_time(),
+        };
+
         // As of this writing, only the base URL is used for property
         // values.
         //
@@ -1559,12 +1577,21 @@ impl UnparsedValue {
         );
 
         let mut input = ParserInput::new(&css);
-            Parser::new(&mut input).parse_entirely(|input| {
+        let mut input = Parser::new(&mut input);
+        input.skip_whitespace();  // Unnecessary for correctness, but may help try() rewind less.
+        if let Ok(keyword) = input.try(CSSWideKeyword::parse) {
+            return PropertyDeclaration::CSSWideKeyword(WideKeywordDeclaration {
+                id: longhand_id,
+                keyword,
+            });
+        }
+
+        let declaration = input.parse_entirely(|input| {
             match self.from_shorthand {
                 None => longhand_id.parse_value(&context, input),
                 Some(ShorthandId::All) => {
-                        // No need to parse the 'all' shorthand as anything other than a CSS-wide
+                    // No need to parse the 'all' shorthand as anything other
-                        // keyword, after variable substitution.
+                    // than a CSS-wide keyword, after variable substitution.
                     Err(input.new_custom_error(SelectorParseErrorKind::UnexpectedIdent("all".into())))
                 }
                 % for shorthand in data.shorthands_except_all():
@@ -1585,21 +1612,12 @@ impl UnparsedValue {
                 }
                 % endfor
             }
-            })
+        });
-            .ok()
+
-        })
+        match declaration {
-        .unwrap_or_else(|| {
+            Ok(decl) => decl,
-            // Invalid at computed-value time.
+            Err(..) => invalid_at_computed_value_time(),
-            let keyword = if longhand_id.inherited() {
+        }
-                CSSWideKeyword::Inherit
-            } else {
-                CSSWideKeyword::Initial
-            };
-            PropertyDeclaration::CSSWideKeyword(WideKeywordDeclaration {
-                id: longhand_id,
-                keyword,
-            })
-        })
     }
 }