core: add conversion from Option<Level> to LevelFilter

[davidbarsky]

This PR resolves a previously unreported regression where Option<Level> was no longer a valid LevelFilter. Below is a minimal reproduction of the issue:

tracing_subscriber::fmt().with_max_level(Some(tracing::Level::WARN)).init();

I've added an From<Option<Level>> for LevelFilter implementation which might require an MSRV version bump to 1.41 to support the rebalanced coherence changes.

[hawkw]

which might require an MSRV version bump to 1.41 to support the rebalanced coherence changes.

I don't think that's the case --- it appears to compile on 1.40.0. The From impl is for a type that's defined in the current crate, so there's no potential coherence issue. :)

[hawkw]

The implementation looks fine. It seems like we accidentally removed the Into<Option<Level>> impl, can we fix that before merging this?

Also, it might be nice to add examples of the Option<Level> conversions in the docs for LevelFilter? Not a blocker for the fix though.

[davidbarsky]

I don't think that's the case --- it appears to compile on 1.40.0. The From impl is for a type that's defined in the current crate, so there's no potential coherence issue. :)

Nice! I was worried about the Option in the bounds, but you're right. If I understand the rebalancing coherence RFC correctly, we'd only run into the issue the RFC seeks to address if there are generics in impl clause, which there are not.

[hawkw]

I don't think that's the case --- it appears to compile on 1.40.0. The From impl is for a type that's defined in the current crate, so there's no potential coherence issue. :)

Nice! I was worried about the Option in the bounds, but you're right. If I understand the rebalancing coherence RFC correctly, we'd only run into the issue the RFC seeks to address if there are generics in impl clause, which there are not.

Yup!

[hawkw]

lgtm