core: Make TwoWaySearcher reset its prefix memory when shifting by by…

…teset

Closes #16878.

src/libcore/str.rs

@@ -479,6 +479,9 @@ impl TwoWaySearcher {
                     ((haystack[self.position + needle.len() - 1] & 0x3f)
                      as uint)) & 1 == 0 {
                 self.position += needle.len();
+                if !long_period {
+                    self.memory = 0;
+                }
                 continue 'search;
             }
 

src/libcoretest/str.rs

@@ -26,6 +26,12 @@ fn strslice_issue_16589() {
     check_contains_all_substrings("012345678901234567890123456789bcdabcdabcd");
 }
 
+#[test]
+fn strslice_issue_16878() {
+    assert!(!"1234567ah012345678901ah".contains("hah"));
+    assert!(!"00abc01234567890123456789abc".contains("bcabc"));
+}
+
 
 #[test]
 fn test_strslice_contains() {