diff --git a/src/rt/rust_task.h b/src/rt/rust_task.h
index 5513cd9e282..05c34cec587 100644
--- a/src/rt/rust_task.h
+++ b/src/rt/rust_task.h
@@ -5,6 +5,8 @@
 #ifndef RUST_TASK_H
 #define RUST_TASK_H
 
+#include <map>
+
 #include "util/array_list.h"
 
 #include "context.h"
@@ -113,6 +115,8 @@ rust_task : public kernel_owned<rust_task>, rust_cond
 
     rust_obstack dynastack;
 
+    std::map<void *,type_desc *> local_allocs;
+
     // Only a pointer to 'name' is kept, so it must live as long as this task.
     rust_task(rust_scheduler *sched,
               rust_task_list *state,
diff --git a/src/rt/rust_upcall.cpp b/src/rt/rust_upcall.cpp
index 79291bdd3a3..5fabb5cd1ab 100644
--- a/src/rt/rust_upcall.cpp
+++ b/src/rt/rust_upcall.cpp
@@ -67,6 +67,8 @@ upcall_malloc(rust_task *task, size_t nbytes, type_desc *td) {
 
     void *p = task->malloc(nbytes, "tdesc", td);
 
+    task->local_allocs[p] = td;
+
     LOG(task, mem,
         "upcall malloc(%" PRIdPTR ", 0x%" PRIxPTR ") = 0x%" PRIxPTR,
         nbytes, td, (uintptr_t)p);
@@ -84,6 +86,8 @@ upcall_free(rust_task *task, void* ptr, uintptr_t is_gc) {
     DLOG(sched, mem,
              "upcall free(0x%" PRIxPTR ", is_gc=%" PRIdPTR ")",
              (uintptr_t)ptr, is_gc);
+
+    task->local_allocs.erase(ptr);
     task->free(ptr, (bool) is_gc);
 }