add test for nested pattern

tests/ui/pattern/slice-patterns-nested.rs

@@ -0,0 +1,15 @@
+// check-pass
+#![allow(unused_variables)]
+
+struct Zeroes;
+struct Foo<T>(T);
+
+impl Into<[usize; 3]> for Zeroes {
+    fn into(self) -> [usize; 3] {
+        [0; 3]
+    }
+}
+
+fn main() {
+    let Foo([a, b, c]) = Foo(Zeroes.into());
+}