tracing subscriber has a lot of default features

[matklad]

I see that tracing-subscriber has a lot default features. This feels a bit wrong to me: I understand that this is the crate which should be used by libraries implementing subscribers. Ie, it's an intermediate library. In this context, authors of subscribers might forget to include default-features = false, and induce deps on final consumers. See rust-lang/chalk#686 (comment) for an example of this.

I wonder if maybe this is wrong crate factoring? Maybe subscriber should be just a cfged non-default module of the tracing crate?

Super unclear about what's the best solution here, but it's definitely a problem that rust-analyzer pulls in chrono, and I need to investigate which of my dependencies done this :)

[hawkw]

I think it's definitely worth reconsidering this as part of the Tracing 0.2 work — for the next breaking change, I think we may want to factor out all of the subscriber implementations into a separate crate from the core trait definitions.