test: Add two tests for issue #6272.

src/test/run-fail/borrowck-wg-autoderef-and-autoborrowvec-combined-fail-issue-6272.rs

@@ -0,0 +1,21 @@
+// error-pattern:borrowed
+
+// Issue #6272. Tests that freezing correctly accounts for all the
+// implicit derefs that can occur and freezes the innermost box. See
+// the companion test
+//
+//     run-pass/borrowck-wg-autoderef-and-autoborrowvec-combined-issue-6272.rs
+//
+// for a detailed explanation of what is going on here.
+
+fn main() {
+    let a = @mut [3i];
+    let b = @mut [a];
+    let c = @mut b;
+
+    // this should freeze `a` only
+    let x: &mut [int] = c[0];
+
+    // hence this should fail
+    a[0] = a[0];
+}