Count Enumerable#count with an unsigned member of MEMO

Only imemo_count_up and imemo_count_value treat the count as unsigned, so keep cnt as rb_len_t for the other users of MEMO and give them their own member.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

enum.c

@@ -222,7 +222,7 @@ imemo_count_up(struct MEMO *memo)
         RUBY_ASSERT(RB_TYPE_P(memo->u3.value, T_BIGNUM));
         MEMO_V3_SET(memo, rb_int_succ(memo->u3.value));
     }
-    else if (++memo->u3.cnt == 0) {
+    else if (++memo->u3.ucnt == 0) {
         /* overflow */
         MEMO_V3_SET(memo, rb_int_succ(ULEN2NUM(RB_ULEN_MAX)));
     }
@@ -236,7 +236,7 @@ imemo_count_value(struct MEMO *memo)
         return memo->u3.value;
     }
     else {
-        return ULEN2NUM(memo->u3.cnt);
+        return ULEN2NUM(memo->u3.ucnt);
     }
 }
 

internal/imemo.h

@@ -108,7 +108,7 @@ struct rb_imemo_cdhash {
 };
 
 /* Set on imemo_memo when u3 holds a VALUE that GC must mark.
- * When unset, u3 is a non-VALUE (cnt/state). */
+ * When unset, u3 is a non-VALUE (cnt/ucnt/state). */
 #define MEMO_U3_IS_VALUE IMEMO_FL_USER0
 
 /*! MEMO
@@ -121,6 +121,7 @@ struct MEMO {
     const VALUE v2;
     union {
         rb_len_t cnt;
+        rb_ulen_t ucnt;
         long state;
         const VALUE value;
     } u3;