Bug 1918463: Fix `inset` auto-completion. r=firefox-style-system-reviewers,emilio

Prevent `anchor()`'s inner keywords from suggested, and make the suggestion of
`anchor()` only if the relevant pref is flipped.

Differential Revision: https://phabricator.services.mozilla.com/D222018

layout/inspector/tests/test_bug877690.html

@@ -135,6 +135,16 @@ function do_test() {
   var expected = [ "initial", "unset", "auto", "inherit", "revert", "revert-layer" ];
   ok(testValues(values, expected), "property margin's values.");
 
+  var prop = "inset";
+  var values = InspectorUtils.getCSSValuesForProperty(prop);
+  var expected = [ "initial", "unset", "auto", "inherit", "revert", "revert-layer" ];
+
+  if(SpecialPowers.getBoolPref("layout.css.anchor-positioning.enabled")) {
+    expected.push("anchor");
+  }
+
+  ok(testValues(values, expected), "property inset's values.");
+
   // Test property with "normal"
   var prop = "font-style";
   var values = InspectorUtils.getCSSValuesForProperty(prop);

servo/components/style/values/generics/position.rs

@@ -8,6 +8,7 @@
 use std::fmt::Write;
 
 use style_traits::CssWriter;
+use style_traits::SpecifiedValueInfo;
 use style_traits::ToCss;
 
 use crate::values::animated::ToAnimatedZero;
@@ -262,7 +263,6 @@ impl<N> ToAnimatedZero for AspectRatio<N> {
     Debug,
     MallocSizeOf,
     PartialEq,
-    SpecifiedValueInfo,
     ToCss,
     ToShmem,
     ToAnimatedValue,
@@ -286,6 +286,19 @@ pub enum GenericInset<P, LP> {
     ),
 }
 
+impl<P, LP> SpecifiedValueInfo for GenericInset<P, LP>
+where
+    LP: SpecifiedValueInfo,
+{
+    fn collect_completion_keywords(f: style_traits::KeywordsCollectFn) {
+        LP::collect_completion_keywords(f);
+        f(&["auto"]);
+        if static_prefs::pref!("layout.css.anchor-positioning.enabled") {
+            f(&["anchor"]);
+        }
+    }
+}
+
 impl<P, LP> GenericInset<P, LP> {
     /// `auto` value.
     #[inline]