Fix future reference count

Author: phischu

libraries/common/io/promise.effekt

@@ -5,6 +5,7 @@ import io
 // Futures
 // -------
 
+/// Must be filled exactly once and waited exactly once
 extern type Future[T]
 
 namespace future {

libraries/llvm/io.c

@@ -342,6 +342,7 @@ void c_future_fill(struct Pos future, struct Pos value) {
         case EMPTY: {
             f->state = FILLED;
             f->payload.value = value;
+            erasePositive(future);
             break;
         }
         case FILLED: {
@@ -354,7 +355,8 @@ void c_future_fill(struct Pos future, struct Pos value) {
         }
         case WAITED: {
             Stack stack = f->payload.stack;
-            free(f);
+            f->state = EMPTY;
+            erasePositive(future);
             resume_Pos(stack, value);
             break;
         }
@@ -367,11 +369,13 @@ void c_future_wait(struct Pos future, Stack stack) {
         case EMPTY: {
             f->state = WAITED;
             f->payload.stack = stack;
+            erasePositive(future);
             break;
         }
         case FILLED: {
             struct Pos value = f->payload.value;
-            free(f);
+            f->state = EMPTY;
+            erasePositive(future);
             resume_Pos(stack, value);
             break;
         }