Remove 1 allocation in Enumerable#each_with_index (#11868)

* Remove 1 allocation in Enumerable#each_with_index

Previously, each call to Enumerable#each_with_index allocates 2
objects, one for the counting index, the other an imemo_ifunc passed
to `self.each` as a block.

Use `struct vm_ifunc::data` to hold the counting index directly to
remove 1 allocation.

* [DOC] Brief summary for usages of `struct vm_ifunc`

1 files changed, 11 insertions, 0 deletions

diff --git a/vm_eval.c b/vm_eval.c
index ff8e2e6780..469b89c03a 100644
--- a/vm_eval.c
+++ b/vm_eval.c
@@ -2702,6 +2702,17 @@ rb_current_realfilepath(void)
     return Qnil;
 }
 
+// Assert that an internal function is running and return
+// the imemo object that represents it.
+struct vm_ifunc *
+rb_current_ifunc(void)
+{
+    // Search VM_FRAME_MAGIC_IFUNC to see ifunc imemos put on the iseq field.
+    VALUE ifunc = (VALUE)GET_EC()->cfp->iseq;
+    RUBY_ASSERT_ALWAYS(imemo_type_p(ifunc, imemo_ifunc));
+    return (struct vm_ifunc *)ifunc;
+}
+
 void
 Init_vm_eval(void)
 {