LibJS: Add fast path TimeZone conversion to PlainDate#toZonedDateTime

This is a normative chane in the Temporal spec.

See: fcab1af

Userland/Libraries/LibJS/Runtime/Temporal/PlainDatePrototype.cpp

@@ -525,23 +525,33 @@ JS_DEFINE_NATIVE_FUNCTION(PlainDatePrototype::to_zoned_date_time)
 
     // 3. If Type(item) is Object, then
     if (item.is_object()) {
-        // a. Let timeZoneLike be ? Get(item, "timeZone").
+        // a. If item has an [[InitializedTemporalTimeZone]] internal slot, then
-        auto time_zone_like = TRY(item.as_object().get(vm.names.timeZone));
+        if (is<TimeZone>(item.as_object())) {
-
+            // i. Let timeZone be item.
-        // b. If timeZoneLike is undefined, then
+            time_zone = &item.as_object();
-        if (time_zone_like.is_undefined()) {
-            // i. Let timeZone be ? ToTemporalTimeZone(item).
-            time_zone = TRY(to_temporal_time_zone(vm, item));
 
             // ii. Let temporalTime be undefined.
         }
-        // c. Else,
+        // b. Else,
         else {
-            // i. Let timeZone be ? ToTemporalTimeZone(timeZoneLike).
+            // i. Let timeZoneLike be ? Get(item, "timeZone").
-            time_zone = TRY(to_temporal_time_zone(vm, time_zone_like));
+            auto time_zone_like = TRY(item.as_object().get(vm.names.timeZone));
 
-            // ii. Let temporalTime be ? Get(item, "plainTime").
+            // ii. If timeZoneLike is undefined, then
-            temporal_time_value = TRY(item.as_object().get(vm.names.plainTime));
+            if (time_zone_like.is_undefined()) {
+                // 1. Let timeZone be ? ToTemporalTimeZone(item).
+                time_zone = TRY(to_temporal_time_zone(vm, item));
+
+                // 2. Let temporalTime be undefined.
+            }
+            // iii. Else,
+            else {
+                // 1. Let timeZone be ? ToTemporalTimeZone(timeZoneLike).
+                time_zone = TRY(to_temporal_time_zone(vm, time_zone_like));
+
+                // 2. Let temporalTime be ? Get(item, "plainTime").
+                temporal_time_value = TRY(item.as_object().get(vm.names.plainTime));
+            }
         }
     }
     // 4. Else,

Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainDate/PlainDate.prototype.toZonedDateTime.js

@@ -74,6 +74,26 @@ describe("correct behavior", () => {
         expect(zonedDateTime.calendar).toBe(plainDate.calendar);
         expect(zonedDateTime.timeZone.id).toBe("UTC");
     });
+
+    test("time zone fast path returns if it is passed a Temporal.TimeZone instance", () => {
+        const plainDate = new Temporal.PlainDate(2021, 7, 6);
+
+        // This is obseravble via there being no property lookups (avoiding a "timeZone" property lookup in this case)
+        let madeObservableHasPropertyLookup = false;
+        class TimeZone extends Temporal.TimeZone {
+            constructor() {
+                super("UTC");
+            }
+
+            get timeZone() {
+                madeObservableHasPropertyLookup = true;
+                return this;
+            }
+        }
+        const timeZone = new TimeZone();
+        plainDate.toZonedDateTime(timeZone);
+        expect(madeObservableHasPropertyLookup).toBeFalse();
+    });
 });
 
 describe("errors", () => {