Reestablish feature gate for RangeFrom in slices

src/test/ui/half-open-range-patterns/slice_pattern_syntax_problem2.rs

@@ -0,0 +1,10 @@
+// run-pass
+
+fn main() {
+    let xs = [13, 1, 5, 2, 3, 1, 21, 8];
+    if let [3..=14, ..] = xs {
+        /* this variant must pass for now, unfortunately.
+         * This test is included here to help inform a future plan for these.
+         */
+    };
+}