See the test in

oli-obk/cargo_metadata@ea84dc3

There, the for message in stream_deserilizer loop loops infinitelly, because serde repeteadly tries to parse the same invalid portion of input.

I think the correct behavior here is to "poison" the Iterator after the first error and return None. I can also imagine some error-recovey strategy (like skipping to the next utf8 codepoint), but poisoning seems more reliable to me.