Fix Stylo tests to pass on both Stable and Nightly Rust.

This is on top of https://github.com/servo/servo/pull/19285.

Rust Nightly has new enum memory layout optimizations:
https://github.com/rust-lang/rust/pull/45225

components/style/properties/build.py

@@ -32,7 +32,8 @@ def main():
 
     properties = data.PropertiesData(product=product)
     template = os.path.join(BASE, "properties.mako.rs")
-    rust = render(template, product=product, data=properties, __file__=template)
+    rust = render(template, product=product, data=properties, __file__=template,
+                  RUSTC_HAS_PR45225=os.environ.get("RUSTC_HAS_PR45225"))
     if output == "style-crate":
         write(os.environ["OUT_DIR"], "properties.rs", rust)
         if product == "gecko":

components/style/properties/data.py

@@ -140,7 +140,7 @@ class Keyword(object):
 def arg_to_bool(arg):
     if isinstance(arg, bool):
         return arg
-    assert arg in ["True", "False"]
+    assert arg in ["True", "False"], "Unexpected value for boolean arguement: " + repr(arg)
     return arg == "True"
 
 

components/style/properties/longhand/inherited_svg.mako.rs

@@ -66,6 +66,7 @@ ${helpers.predefined_type(
     "stroke-width", "SVGWidth",
     "::values::computed::NonNegativeLength::new(1.).into()",
     products="gecko",
+    boxed=not RUSTC_HAS_PR45225,
     animation_value_type="::values::computed::SVGWidth",
     spec="https://www.w3.org/TR/SVG2/painting.html#StrokeWidth")}
 
@@ -100,6 +101,7 @@ ${helpers.predefined_type(
     "stroke-dashoffset", "SVGLength",
     "Au(0).into()",
     products="gecko",
+    boxed=not RUSTC_HAS_PR45225,
     animation_value_type="ComputedValue",
     spec="https://www.w3.org/TR/SVG2/painting.html#StrokeDashing")}
 

components/style/properties/longhand/pointing.mako.rs

@@ -177,6 +177,7 @@ ${helpers.predefined_type(
     "Either::Second(Auto)",
     spec="https://drafts.csswg.org/css-ui/#caret-color",
     animation_value_type="Either<AnimatedColor, Auto>",
+    boxed=not RUSTC_HAS_PR45225,
     ignored_when_colors_disabled=True,
     products="gecko",
 )}