Auto merge of #14789 - Manishearth:supports, r=SimonSapin

Support @supports

fixes #14786

cc @heycam @upsuper
r? @SimonSapin

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/14789)
<!-- Reviewable:end -->

tests/unit/style/parsing/mod.rs

@@ -39,6 +39,25 @@ macro_rules! assert_roundtrip_with_context {
     }
 }
 
+macro_rules! assert_roundtrip {
+    ($fun:expr, $string:expr) => {
+        assert_roundtrip!($fun, $string, $string);
+    };
+    ($fun:expr,$input:expr, $output:expr) => {
+        let mut parser = Parser::new($input);
+        let parsed = $fun(&mut parser)
+                     .expect(&format!("Failed to parse {}", $input));
+        let serialized = ToCss::to_css_string(&parsed);
+        assert_eq!(serialized, $output);
+
+        let mut parser = Parser::new(&serialized);
+        let re_parsed = $fun(&mut parser)
+                        .expect(&format!("Failed to parse serialization {}", $input));
+        let re_serialized = ToCss::to_css_string(&re_parsed);
+        assert_eq!(serialized, re_serialized);
+    }
+}
+
 macro_rules! parse_longhand {
     ($name:ident, $s:expr) => {{
         let url = ::servo_url::ServoUrl::parse("http://localhost").unwrap();
@@ -58,3 +77,4 @@ mod inherited_text;
 mod mask;
 mod position;
 mod selectors;
+mod supports;

tests/unit/style/parsing/supports.rs

@@ -0,0 +1,14 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use cssparser::Parser;
+use style::supports::SupportsCondition;
+use style_traits::ToCss;
+
+#[test]
+fn test_supports_condition() {
+    assert_roundtrip!(SupportsCondition::parse, "(margin: 1px)");
+    assert_roundtrip!(SupportsCondition::parse, "not (--be: to be)");
+    assert_roundtrip!(SupportsCondition::parse, "(color: blue) and future-extension(4)");
+}