servo: Merge #20586 - style: Respect the -moz- gradient pref again (from emilio:moz-gradient-pref); r=xidorn

This hopefully allows us to unship them eventually.

Bug: 1451874
Reviewed-by: xidorn
MozReview-Commit-ID: 2MDRZLmLMM0
Source-Repo: https://github.com/servo/servo
Source-Revision: 954ac8157f1acac6454ced09bfc948a0bcce75e2

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : d599106eac67425f2e8a2efd245a26fedea223e5

servo/components/style/values/specified/image.rs

@@ -227,9 +227,21 @@ impl Parse for Gradient {
 
         let (shape, repeating, mut compat_mode) = match result {
             Some(result) => result,
-            None => return Err(input.new_custom_error(StyleParseErrorKind::UnexpectedFunction(func.clone()))),
+            None => return Err(input.new_custom_error(StyleParseErrorKind::UnexpectedFunction(func))),
         };
 
+        #[cfg(feature = "gecko")]
+        {
+            use gecko_bindings::structs;
+            if compat_mode == CompatMode::Moz &&
+                !unsafe { structs::StaticPrefs_sVarCache_layout_css_prefixes_gradients }
+            {
+                return Err(input.new_custom_error(
+                    StyleParseErrorKind::UnexpectedFunction(func)
+                ));
+            }
+        }
+
         let (kind, items) = input.parse_nested_block(|i| {
             let shape = match shape {
                 Shape::Linear => GradientKind::parse_linear(context, i, &mut compat_mode)?,