diff --git a/src/rt/rust_cc.cpp b/src/rt/rust_cc.cpp
index 8129f60fc7c..87b9f62e106 100644
--- a/src/rt/rust_cc.cpp
+++ b/src/rt/rust_cc.cpp
@@ -1,45 +1,191 @@
 // Rust cycle collector. Temporary, but will probably stick around for some
 // time until LLVM's GC infrastructure is more mature.
 
-#include <cstdio>
-#include <cstdlib>
-#include <map>
-#include <vector>
 #include "rust_gc.h"
 #include "rust_internal.h"
 #include "rust_shape.h"
 #include "rust_task.h"
+#include <cstdio>
+#include <cstdlib>
+#include <map>
+#include <vector>
+#include <stdint.h>
 
 #undef DPRINT
 #define DPRINT(fmt,...)     fprintf(stderr, fmt, ##__VA_ARGS__)
 
 namespace cc {
 
+// Internal reference count computation
+
+typedef std::map<void *,uintptr_t> irc_map;
+
+class irc : public shape::data<irc,shape::ptr> {
+    friend class shape::data<irc,shape::ptr>;
+
+    irc_map ircs;
+
+    irc(const irc &other, const shape::ptr &in_dp)
+    : shape::data<irc,shape::ptr>(other.task, other.align, other.sp,
+                                  other.params, other.tables, in_dp),
+      ircs(other.ircs) {}
+
+    irc(const irc &other,
+        const uint8_t *in_sp,
+        const shape::type_param *in_params,
+        const rust_shape_tables *in_tables = NULL)
+    : shape::data<irc,shape::ptr>(other.task,
+                                  other.align,
+                                  in_sp,
+                                  in_params,
+                                  in_tables ? in_tables : other.tables,
+                                  other.dp),
+      ircs(other.ircs) {}
+
+    irc(const irc &other,
+        const uint8_t *in_sp,
+        const shape::type_param *in_params,
+        const rust_shape_tables *in_tables,
+        shape::ptr in_dp)
+    : shape::data<irc,shape::ptr>(other.task,
+                                  other.align,
+                                  in_sp,
+                                  in_params,
+                                  in_tables,
+                                  in_dp),
+      ircs(other.ircs) {}
+
+    irc(rust_task *in_task,
+        bool in_align,
+        const uint8_t *in_sp,
+        const shape::type_param *in_params,
+        const rust_shape_tables *in_tables,
+        uint8_t *in_data,
+        irc_map &in_ircs)
+    : shape::data<irc,shape::ptr>(in_task, in_align, in_sp, in_params,
+                                  in_tables, in_data),
+      ircs(in_ircs) {}
+
+    void walk_vec(bool is_pod, uint16_t sp_size) {
+        if (is_pod || shape::get_dp<void *>(dp) == NULL)
+            return;     // There can't be any outbound pointers from this.
+
+        std::pair<uint8_t *,uint8_t *> data_range(get_vec_data_range(dp));
+        if (data_range.second - data_range.first > 10000)
+            abort();    // FIXME: Temporary sanity check.
+
+        irc sub(*this, data_range.first);
+        shape::ptr data_end = sub.end_dp = data_range.second;
+        while (sub.dp < data_end) {
+            sub.walk_reset();
+            align = true;
+        }
+    }
+
+    void walk_tag(shape::tag_info &tinfo, uint32_t tag_variant) {
+        shape::data<irc,shape::ptr>::walk_variant(tinfo, tag_variant);
+    }
+
+    void walk_box() {
+        shape::data<irc,shape::ptr>::walk_box_contents();
+    }
+
+    void walk_fn() {
+        shape::data<irc,shape::ptr>::walk_fn_contents(dp);
+    }
+
+    void walk_obj() {
+        shape::data<irc,shape::ptr>::walk_obj_contents(dp);
+    }
+
+    void walk_res(const shape::rust_fn *dtor, unsigned n_params,
+                  const shape::type_param *params, const uint8_t *end_sp,
+                  bool live) {
+        while (this->sp != end_sp) {
+            this->walk();
+            align = true;
+        }
+    }
+
+    void walk_subcontext(irc &sub) { sub.walk(); }
+
+    void walk_box_contents(irc &sub, shape::ptr &ref_count_dp) {
+        if (!ref_count_dp)
+            return;
+
+        // Bump the internal reference count of the box.
+        if (ircs.find((void *)dp) == ircs.end())
+            ircs[(void *)dp] = 1;
+        else
+            ++ircs[(void *)dp];
+
+        // Do not traverse the contents of this box; it's in the allocation
+        // somewhere, so we're guaranteed to come back to it (if we haven't
+        // traversed it already).
+    }
+
+    void walk_struct(const uint8_t *end_sp) {
+        while (this->sp != end_sp) {
+            this->walk();
+            align = true;
+        }
+    }
+
+    void walk_variant(shape::tag_info &tinfo, uint32_t variant_id,
+                      const std::pair<const uint8_t *,const uint8_t *>
+                      variant_ptr_and_end);
+
+    template<typename T>
+    inline void walk_number() { /* no-op */ }
+
+public:
+    static void compute_ircs(rust_task *task, irc_map &ircs);
+};
+
 void
-do_cc(rust_task *task) {
-    std::map<void *,type_desc *>::iterator begin(task->local_allocs.begin());
-    std::map<void *,type_desc *>::iterator end(task->local_allocs.end());
+irc::walk_variant(shape::tag_info &tinfo, uint32_t variant_id,
+                  const std::pair<const uint8_t *,const uint8_t *>
+                  variant_ptr_and_end) {
+    irc sub(*this, variant_ptr_and_end.first, tinfo.params);
+
+    assert(variant_id < 256);   // FIXME: Temporary sanity check.
+
+    const uint8_t *variant_end = variant_ptr_and_end.second;
+    while (sub.sp < variant_end) {
+        sub.walk();
+        align = true;
+    }
+}
+
+void
+irc::compute_ircs(rust_task *task, irc_map &ircs) {
+    std::map<void *,type_desc *>::iterator begin(task->local_allocs.begin()),
+                                           end(task->local_allocs.end());
     while (begin != end) {
-        void *p = begin->first;
+        uint8_t *p = reinterpret_cast<uint8_t *>(begin->first);
         type_desc *tydesc = begin->second;
 
-        DPRINT("marking allocation: %p, tydesc=%p\n", p, tydesc);
+        DPRINT("determining internal ref counts: %p, tydesc=%p\n", p, tydesc);
 
         // Prevents warnings for now
-        (void)p;
-        (void)tydesc;
-#if 0
         shape::arena arena;
         shape::type_param *params =
             shape::type_param::from_tydesc(tydesc, arena);
-        mark mark(task, true, tydesc->shape, params, tydesc->shape_tables, p);
-        mark.walk();
-#endif
+        irc irc(task, true, tydesc->shape, params, tydesc->shape_tables, p,
+                ircs);
+        irc.walk();
 
         ++begin;
     }
 }
 
+
+void
+do_cc(rust_task *task) {
+    irc_map ircs;
+    irc::compute_ircs(task, ircs);
+}
+
 void
 maybe_cc(rust_task *task) {
     // FIXME: We ought to lock this.