YJIT: Handle special case of splat and rest lining up (#7422)

If you have a method that takes rest arguments and a splat call that
happens to line up perfectly with that rest, you can just dupe the
array rather than move anything around. We still have to dupe, because
people could have a custom to_a method or something like that which
means it is hard to guarantee we have exclusive access to that array.

Example:

```ruby
def foo(a, b, *rest)
end

foo(1, 2, *[3, 4])
```

1 files changed, 22 insertions, 0 deletions

diff --git a/bootstraptest/test_yjit.rb b/bootstraptest/test_yjit.rb
index 7c2a50c4e0..d7dd0cd460 100644
--- a/bootstraptest/test_yjit.rb
+++ b/bootstraptest/test_yjit.rb
@@ -3632,3 +3632,25 @@ assert_normal_exit %q{
 
   entry
 }
+
+# Test that splat and rest combined
+# properly dupe the array
+assert_equal "[]", %q{
+  def foo(*rest)
+    rest << 1
+  end
+
+  def test(splat)
+    foo(*splat)
+  end
+
+  EMPTY = []
+  custom = Object.new
+  def custom.to_a
+    EMPTY
+  end
+
+  test(custom)
+  test(custom)
+  EMPTY
+}