flatten nested slice patterns in HAIR construction

nested slice patterns have the same functionality as non-nested
ones, so flatten them in HAIR construction.

Fixes #26158.

src/test/compile-fail/issue-26158.rs

@@ -0,0 +1,16 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(slice_patterns)]
+
+fn main() {
+    let x: &[u32] = &[];
+    let &[[ref _a, ref _b..]..] = x; //~ ERROR refutable pattern
+}

src/test/run-pass/vec-matching.rs

@@ -144,6 +144,20 @@ fn e() {
     assert_eq!(c, 1);
 }
 
+fn f() {
+    let x = &[1, 2, 3, 4, 5];
+    let [a, [b, [c, ..].., d].., e] = *x;
+    assert_eq!((a, b, c, d, e), (1, 2, 3, 4, 5));
+
+    let x: &[isize] = x;
+    let (a, b, c, d, e) = match *x {
+        [a, [b, [c, ..].., d].., e] => (a, b, c, d, e),
+        _ => unimplemented!()
+    };
+
+    assert_eq!((a, b, c, d, e), (1, 2, 3, 4, 5));
+}
+
 pub fn main() {
     a();
     b();
@@ -151,4 +165,5 @@ pub fn main() {
     c();
     d();
     e();
+    f();
 }