Fix rust_vec constructor assertion failure caused by slow path of upcall_vec_grow. Add testcase.

src/rt/rust_upcall.cpp

@@ -381,7 +381,7 @@ upcall_vec_grow(rust_task *task, rust_vec *v, size_t n_bytes, uintptr_t is_gc)
     LOG_UPCALL_ENTRY(task);
     rust_dom *dom = task->dom;
     task->log(rust_log::UPCALL|rust_log::MEM,
-             "upcall vec_grow(%" PRIxPTR ", %" PRIdPTR
+             "upcall vec_grow(0x%" PRIxPTR ", %" PRIdPTR
              "), alloc=%" PRIdPTR ", fill=%" PRIdPTR,
              v, n_bytes, v->alloc, v->fill);
     size_t alloc = next_power_of_two(sizeof(rust_vec) + v->fill + n_bytes);
@@ -411,7 +411,8 @@ upcall_vec_grow(rust_task *task, rust_vec *v, size_t n_bytes, uintptr_t is_gc)
             return NULL;
         }
         v->deref();
-        v = new (mem) rust_vec(dom, alloc, v->fill, &v->data[0]);
+        v = new (mem) rust_vec(dom, alloc, v->fill,
+                               v->fill ? &v->data[0] : NULL);
     }
     I(dom, sizeof(rust_vec) + v->fill <= v->alloc);
     return v;

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

@@ -1,10 +1,25 @@
 // -*- rust -*-
 
-fn main() {
+fn fast_growth() {
   let vec[int] v = vec(1,2,3,4,5);
   v += vec(6,7,8,9,0);
+
   log v.(9);
   check(v.(0) == 1);
   check(v.(7) == 8);
   check(v.(9) == 0);
 }
+
+fn slow_growth() {
+  let vec[int] v = vec();
+  let vec[int] u = v;
+  v += vec(17);
+
+  log v.(0);
+  check (v.(0) == 17);
+}
+
+fn main() {
+  fast_growth();
+  slow_growth();
+}