add an option to debug borrows (RUST_DEBUG_BORROW) so you can

find out where the offending borrow occurred. This ... still needs
some work.

src/rt/rust_env.cpp

@@ -24,6 +24,7 @@
 #define RUST_SEED "RUST_SEED"
 #define RUST_POISON_ON_FREE "RUST_POISON_ON_FREE"
 #define RUST_DEBUG_MEM "RUST_DEBUG_MEM"
+#define RUST_DEBUG_BORROWS "RUST_DEBUG_BORROWS"
 
 #if defined(__WIN32__)
 static int
@@ -130,6 +131,7 @@ load_env(int argc, char **argv) {
     env->argc = argc;
     env->argv = argv;
     env->debug_mem = getenv(RUST_DEBUG_MEM) != NULL;
+    env->debug_borrows = getenv(RUST_DEBUG_BORROWS) != NULL;
     return env;
 }