style: Add collect_values function to SpecifiedValueInfo trait for collecting possible values.

This is the basic structure of the stuff. Following patches will fill
the gap between Gecko and Servo on value generating, and finally hook
it into InspectorUtils.

Bug: 1434130
Reviewed-by: emilio
MozReview-Commit-ID: KNLAfFBiY6e

ports/geckolib/glue.rs

@@ -5,12 +5,13 @@
 use cssparser::{ParseErrorKind, Parser, ParserInput, SourceLocation};
 use cssparser::ToCss as ParserToCss;
 use malloc_size_of::MallocSizeOfOps;
-use nsstring::nsCString;
+use nsstring::{nsCString, nsStringRepr};
 use selectors::{NthIndexCache, SelectorList};
 use selectors::matching::{MatchingContext, MatchingMode, matches_selector};
 use servo_arc::{Arc, ArcBorrow, RawOffsetArc};
 use smallvec::SmallVec;
 use std::cell::RefCell;
+use std::collections::BTreeSet;
 use std::fmt::Write;
 use std::iter;
 use std::mem;
@@ -928,20 +929,36 @@ pub extern "C" fn Servo_ComputedValues_ExtractAnimationValue(
     }
 }
 
+macro_rules! parse_enabled_property_name {
+    ($prop_name:ident, $found:ident, $default:expr) => {{
+        let prop_name = $prop_name.as_ref().unwrap().as_str_unchecked();
+        // XXX This can be simplified once Option::filter is stable.
+        let prop_id = PropertyId::parse(prop_name).ok().and_then(|p| {
+            if p.enabled_for_all_content() {
+                Some(p)
+            } else {
+                None
+            }
+        });
+        match prop_id {
+            Some(p) => {
+                *$found = true;
+                p
+            }
+            None => {
+                *$found = false;
+                return $default;
+            }
+        }
+    }}
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn Servo_Property_IsShorthand(
     prop_name: *const nsACString,
     found: *mut bool
 ) -> bool {
-    let prop_id = PropertyId::parse(prop_name.as_ref().unwrap().as_str_unchecked());
-    let prop_id = match prop_id {
-        Ok(ref p) if p.enabled_for_all_content() => p,
-        _ => {
-            *found = false;
-            return false;
-        }
-    };
-    *found = true;
+    let prop_id = parse_enabled_property_name!(prop_name, found, false);
     prop_id.is_shorthand()
 }
 
@@ -970,16 +987,7 @@ pub unsafe extern "C" fn Servo_Property_SupportsType(
     ty: u32,
     found: *mut bool,
 ) -> bool {
-    let prop_id = PropertyId::parse(prop_name.as_ref().unwrap().as_str_unchecked());
-    let prop_id = match prop_id {
-        Ok(ref p) if p.enabled_for_all_content() => p,
-        _ => {
-            *found = false;
-            return false;
-        }
-    };
-
-    *found = true;
+    let prop_id = parse_enabled_property_name!(prop_name, found, false);
     // This should match the constants in InspectorUtils.
     // (Let's don't bother importing InspectorUtilsBinding into bindings
     // because it is not used anywhere else, and issue here would be
@@ -993,6 +1001,24 @@ pub unsafe extern "C" fn Servo_Property_SupportsType(
     prop_id.supports_type(ty)
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn Servo_Property_GetCSSValuesForProperty(
+    prop_name: *const nsACString,
+    found: *mut bool,
+    result: *mut nsTArray<nsStringRepr>,
+) {
+    let prop_id = parse_enabled_property_name!(prop_name, found, ());
+    // Use B-tree set for unique and sorted result.
+    let mut values = BTreeSet::<&'static str>::new();
+    prop_id.collect_property_completion_keywords(&mut |list| values.extend(list.iter()));
+
+    let result = result.as_mut().unwrap();
+    bindings::Gecko_ResizeTArrayForStrings(result, values.len() as u32);
+    for (src, dest) in values.iter().zip(result.iter_mut()) {
+        dest.write_str(src).unwrap();
+    }
+}
+
 #[no_mangle]
 pub extern "C" fn Servo_Property_IsAnimatable(property: nsCSSPropertyID) -> bool {
     use style::properties::animated_properties;