Signal is open source. They absolutely do always comply with US warrants. They have never provided any information to US law enforcement, because they can't access it. They literally have no way of accessing the information contained inside the texts. The most they could provide is metadata, but they currently aren't collecting that. I also think if they started, it would not work well for their user base. You can see all their requests for information, and the responses they gave, here: https://signal.org/bigbrother/