Improve the tests for vector self appending.

src/test/run-pass/string-self-append.rs

@@ -0,0 +1,19 @@
+// xfail-stage0
+
+use std;
+import std::str;
+
+fn main()
+{
+    // Make sure we properly handle repeated self-appends.
+    let str a = "A";
+    auto i = 20;
+    auto expected_len = 1u;
+    while (i > 0) {
+        log_err str::byte_len(a);
+        assert (str::byte_len(a) == expected_len);
+        a += a;
+        i -= 1;
+        expected_len *= 2u;
+    }
+}

src/test/run-pass/vec-self-append.rs

@@ -1,12 +1,19 @@
 // xfail-stage0
 
+use std;
+import std::vec;
+
 fn main()
 {
     // Make sure we properly handle repeated self-appends.
     let vec[int] a = [0];
     auto i = 20;
+    auto expected_len = 1u;
     while (i > 0) {
+        log_err vec::len(a);
+        assert (vec::len(a) == expected_len);
         a += a;
         i -= 1;
+        expected_len *= 2u;
     }
 }