ZJIT: Use CFP_PC in warn_unused_block for outlined PC (ruby#16649)

warn_unused_block uses the caller's PC to build a dedup key for the
unused block warning table. When called from ZJIT-compiled code via
rb_vm_send, cfp->pc may be stale (poisoned or uninitialized) because
ZJIT stores the real PC in cfp->jit_return instead.

Use CFP_PC() which reads from jit_return when available, falling back
to cfp->pc for interpreter frames. This prevents the dedup key from
being based on a garbage PC value that could randomly collide with
prior entries, suppressing warnings that should be emitted.

Author: k0kubun

vm_insnhelper.c

@@ -3219,7 +3219,7 @@ vm_callee_setup_arg(rb_execution_context_t *ec, struct rb_calling_info *calling,
     if (UNLIKELY(!ISEQ_BODY(iseq)->param.flags.use_block &&
                  calling->block_handler != VM_BLOCK_HANDLER_NONE &&
                  !(vm_ci_flag(calling->cd->ci) & (VM_CALL_OPT_SEND | VM_CALL_SUPER)))) {
-        warn_unused_block(vm_cc_cme(cc), iseq, (void *)ec->cfp->pc);
+        warn_unused_block(vm_cc_cme(cc), iseq, (void *)CFP_PC(ec->cfp));
     }
 
     if (LIKELY(!(vm_ci_flag(ci) & VM_CALL_KW_SPLAT))) {

zjit/src/codegen_tests.rs

@@ -5544,3 +5544,30 @@ fn test_send_block_to_method_not_using_block() {
         test
     "), @"42");
 }
+
+#[test]
+fn test_send_block_unused_warning_emitted_from_jit() {
+    // When ZJIT compiles a send with a block as a dynamic dispatch fallback
+    // (gen_send -> rb_vm_send), warn_unused_block uses cfp->pc for the dedup
+    // key. We save cfp->pc before calling rb_vm_send so the key is stable
+    // and won't spuriously collide with prior entries in the dedup table.
+    assert_snapshot!(inspect(r#"
+        $warnings = []
+        module Warning
+          def warn(message, category: nil)
+            $warnings << message
+          end
+        end
+
+        def m_unused_block_warn_test = 42
+
+        def test
+          $VERBOSE = true
+          m_unused_block_warn_test {}
+          $warnings.any? { |w| w.include?("may be ignored") }
+        end
+
+        test
+        test
+    "#), @"true");
+}