servo: Merge #12487 - Add FFI glue for Gecko to implement 1-arg CSS.supports() with stylo (from heycam:supports-ffi); r=emilio

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This adds an FFI function for Gecko to call to implement the 1-arg version of `CSS.supports()`.  This will be useful for producing an automated analysis of CSS properties we lack support for in geckolib.  Corresponding Gecko bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1287382

r? @emilio

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because this is a geckolib-only change, and we don't have testing for that yet :(

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Source-Repo: https://github.com/servo/servo
Source-Revision: f88ecda3628ec2af3a4e4c7fdebeda47b4008d6f

servo/ports/geckolib/gecko_bindings/bindings.rs

@@ -288,6 +288,8 @@ extern "C" {
                                                   *mut ServoDeclarationBlock);
     pub fn Servo_ClearDeclarationBlockCachePointer(declarations:
                                                        *mut ServoDeclarationBlock);
+    pub fn Servo_CSSSupports(property: *const u8, property_length: u32,
+                             value: *const u8, value_length: u32) -> bool;
     pub fn Servo_GetComputedValues(node: *mut RawGeckoNode)
      -> *mut ServoComputedValues;
     pub fn Servo_GetComputedValuesForAnonymousBox(parentStyleOrNull:

servo/ports/geckolib/glue.rs

@@ -28,13 +28,13 @@ use style::dom::{TDocument, TElement, TNode};
 use style::error_reporting::StdoutErrorReporter;
 use style::parallel;
 use style::parser::ParserContextExtraData;
-use style::properties::{ComputedValues, PropertyDeclarationBlock};
+use style::properties::{ComputedValues, PropertyDeclarationBlock, parse_one_declaration};
 use style::selector_impl::{SelectorImplExt, PseudoElementCascadeType};
 use style::sequential;
 use style::stylesheets::Origin;
 use traversal::RecalcStyleOnly;
 use url::Url;
-use wrapper::{GeckoDocument, GeckoElement, GeckoNode, NonOpaqueStyleData};
+use wrapper::{DUMMY_BASE_URL, GeckoDocument, GeckoElement, GeckoNode, NonOpaqueStyleData};
 
 // TODO: This is ugly and should go away once we get an atom back-end.
 pub fn pseudo_element_from_atom(pseudo: *mut nsIAtom,
@@ -471,3 +471,18 @@ pub extern "C" fn Servo_ClearDeclarationBlockCachePointer(declarations: *mut Ser
     let declarations = unsafe { (declarations as *mut GeckoDeclarationBlock).as_mut().unwrap() };
     declarations.cache = ptr::null_mut();
 }
+
+#[no_mangle]
+pub extern "C" fn Servo_CSSSupports(property: *const u8, property_length: u32,
+                                    value: *const u8, value_length: u32) -> bool {
+    let property = unsafe { from_utf8_unchecked(slice::from_raw_parts(property, property_length as usize)) };
+    let value    = unsafe { from_utf8_unchecked(slice::from_raw_parts(value, value_length as usize)) };
+
+    let base_url = &*DUMMY_BASE_URL;
+    let extra_data = ParserContextExtraData::default();
+
+    match parse_one_declaration(&property, &value, &base_url, Box::new(StdoutErrorReporter), extra_data) {
+        Ok(decls) => !decls.is_empty(),
+        Err(()) => false,
+    }
+}