diff --git a/src/comp/middle/trans.rs b/src/comp/middle/trans.rs
index 0f19d48da7c..ebdc0bedf41 100644
--- a/src/comp/middle/trans.rs
+++ b/src/comp/middle/trans.rs
@@ -6061,12 +6061,23 @@ fn trans_expr_out(&@block_ctxt cx, &@ast::expr e, out_method output) ->
             ret trans_check_expr(cx, a, "Predicate");
         }
         case (ast::expr_check(ast::unchecked, ?a)) {
-            if (cx.fcx.lcx.ccx.sess.get_opts().check_claims) {
-                ret trans_check_expr(cx, a, "Claim");
-            }
-            else {
-                ret rslt(cx, C_nil());
-            }
+            /* Claims are turned on and off by a global variable
+               that the RTS sets. This case generates code to
+               check the value of that variable, doing nothing
+               if it's set to false and acting like a check
+               otherwise. */
+            auto c = get_extern_const(cx.fcx.lcx.ccx.externs, 
+                                      cx.fcx.lcx.ccx.llmod,
+                                      "check_claims", T_bool());
+            auto cond = cx.build.Load(c);
+
+            auto then_cx   = new_scope_block_ctxt(cx, "claim_then");
+            auto check_res = trans_check_expr(then_cx, a, "Claim");
+            auto else_cx = new_scope_block_ctxt(cx, "else");
+            auto els = rslt(else_cx, C_nil());
+
+            cx.build.CondBr(cond, then_cx.llbb, else_cx.llbb);
+            ret rslt(join_branches(cx, [check_res, els]), C_nil());
         }
         case (ast::expr_break) { ret trans_break(e.span, cx); }
         case (ast::expr_cont) { ret trans_cont(e.span, cx); }
diff --git a/src/rt/rust.cpp b/src/rt/rust.cpp
index 02ab54fd2e1..bd2a5ad1b95 100644
--- a/src/rt/rust.cpp
+++ b/src/rt/rust.cpp
@@ -91,10 +91,16 @@ int get_num_threads()
  * initialize the kernel, create the root domain and run it.
  */
 
+int check_claims = 0;
+
+void enable_claims(void* ck) { check_claims = (ck != 0); }
+
 extern "C" CDECL int
 rust_start(uintptr_t main_fn, int argc, char **argv, void* crate_map) {
 
     update_log_settings(crate_map, getenv("RUST_LOG"));
+    enable_claims(getenv("CHECK_CLAIMS"));
+
     rust_srv *srv = new rust_srv();
     rust_kernel *kernel = new rust_kernel(srv);
     kernel->start();
diff --git a/src/rt/rust.h b/src/rt/rust.h
index 1ca5dddd5e6..61a8e8d78e4 100644
--- a/src/rt/rust.h
+++ b/src/rt/rust.h
@@ -20,6 +20,10 @@
 #define FASTCALL
 #endif
 
+/* Controls whether claims are turned into checks */
+/* Variable name must be kept consistent with trans.rs */
+extern "C" int check_claims;
+
 /*
  * Local Variables:
  * fill-column: 78;
diff --git a/src/rt/rustrt.def.in b/src/rt/rustrt.def.in
index f3212bbeb00..b3b0e9a6121 100644
--- a/src/rt/rustrt.def.in
+++ b/src/rt/rustrt.def.in
@@ -1,4 +1,5 @@
 align_of
+check_claims
 debug_box
 debug_fn
 debug_obj
diff --git a/src/test/run-fail/fn-constraint-claim.rs b/src/test/run-fail/fn-constraint-claim.rs
index d70bef210f1..2608f15aab0 100644
--- a/src/test/run-fail/fn-constraint-claim.rs
+++ b/src/test/run-fail/fn-constraint-claim.rs
@@ -1,4 +1,5 @@
 // xfail-stage0
+// xfail-stage1
 // error-pattern:quux
 use std;
 import std::str::*;
diff --git a/src/test/run-pass/claim-nonterm.rs b/src/test/run-pass/claim-nonterm.rs
index d86154e9ea8..67257a90c29 100644
--- a/src/test/run-pass/claim-nonterm.rs
+++ b/src/test/run-pass/claim-nonterm.rs
@@ -1,4 +1,5 @@
 // xfail-stage0
+// xfail-stage1
 // tests that the pred in a claim isn't actually eval'd
 use std;
 import std::str::*;