Reestablish feature gate for RangeFrom in slices

src/test/ui/half-open-range-patterns/slice_pattern_syntax_problem1.rs

@@ -0,0 +1,9 @@
+// Instead of allowing the previous case, maintain the feature gate for slice patterns for now.
+fn main() {
+    let xs = [13, 1, 5, 2, 3, 1, 21, 8];
+    let [a @ 3.., b @ ..3, c @ 4..6, ..] = xs;
+    //~^ `X..` patterns in slices are experimental
+    //~| half-open range patterns are unstable
+    //~| exclusive range pattern syntax is experimental
+    //~| exclusive range pattern syntax is experimental
+}