Kernel#lambda: return forwarded block as non-lambda proc

Before this commit, Kernel#lambda can't tell the difference between a
directly passed literal block and one passed with an ampersand.

A block passed with an ampersand is semantically speaking already a
non-lambda proc. When Kernel#lambda receives a non-lambda proc, it
should simply return it.

Implementation wise, when the VM calls a method with a literal block, it
places the code for the block on the calling control frame and passes a
pointer (block handler) to the callee. Before this commit, the VM
forwards block arguments by simply forwarding the block handler, which
leaves the slot for block code unused when a control frame forwards its
block argument. I use the vacant space to indicate that a frame has
forwarded its block argument and inspect that in Kernel#lambda to detect
forwarded blocks.

This is a very ad-hoc solution and relies *heavily* on the way block
passing works in the VM. However, it's the most self-contained solution
I have.

[Bug #15620]

1 files changed, 7 insertions, 1 deletions

diff --git a/vm_core.h b/vm_core.h
index cd7bf74b5d..12c3ac3775 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -763,7 +763,7 @@ typedef struct rb_control_frame_struct {
     const rb_iseq_t *iseq;	/* cfp[2] */
     VALUE self;			/* cfp[3] / block[0] */
     const VALUE *ep;		/* cfp[4] / block[1] */
-    const void *block_code;     /* cfp[5] / block[2] */ /* iseq or ifunc */
+    const void *block_code;     /* cfp[5] / block[2] */ /* iseq or ifunc or forwarded block handler */
     VALUE *__bp__;              /* cfp[6] */ /* outside vm_push_frame, use vm_base_ptr instead. */
 
 #if VM_DEBUG_BP_CHECK
@@ -1494,6 +1494,12 @@ vm_block_handler_verify(MAYBE_UNUSED(VALUE block_handler))
 	      (vm_block_handler_type(block_handler), 1));
 }
 
+static inline int
+vm_cfp_forwarded_bh_p(const rb_control_frame_t *cfp, VALUE block_handler)
+{
+    return ((VALUE) cfp->block_code) == block_handler;
+}
+
 static inline enum rb_block_type
 vm_block_type(const struct rb_block *block)
 {