Here's a negative signal I'm seeing often:

When a developer that usually consumes the language starts critiquing the language.

I could go on as to why it's a bad signal, psychologically, but let's just say that empirically it usually doesn't come from a good place, it's more like a developer raising the stakes of their application failing and blaming the language.

Sure one out of a thousand devs might be the next Linus Torvalds and develop the next Rust. But the odds aren't great.