auto merge of #16590 : nham/rust/str_searcher_underflow, r=alexcrichton

This incidentally fixes #16589, because it will cause `MatchIndices` to use `NaiveSearcher` instead of `TwoWaySearcher`, but I'm not sure #16589 should be closed until the underlying problem in `TwoWaySearcher` is found.

src/libcore/str.rs

@@ -562,7 +562,7 @@ enum Searcher {
 impl Searcher {
     fn new(haystack: &[u8], needle: &[u8]) -> Searcher {
         // FIXME: Tune this.
-        if needle.len() > haystack.len() - 20 {
+        if needle.len() + 20 > haystack.len() {
             Naive(NaiveSearcher::new())
         } else {
             let searcher = TwoWaySearcher::new(needle);

src/libcoretest/lib.rs

@@ -29,4 +29,5 @@
 mod raw;
 mod result;
 mod slice;
+mod str;
 mod tuple;

src/libcoretest/str.rs

@@ -0,0 +1,14 @@
+// Copyright 2014 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.
+
+#[test]
+fn strslice_issue_16589() {
+    assert!("bananas".contains("nana"));
+}