Configurable log level for #[instrument(err)]

[nitnelave]

Feature Request

I'd like to be able to specify the log level that Err returns are printed at.

Crates

tracing::instrument

Motivation

Currently it's hardcoded as error, but I have some functions that I'd like to instrument where returning an error is not especially unusual (think of for instance "is the user logged in yet?": any error eventually gets folded into "no, the user is not logged in"). In such cases, I'd like to be able to lower the level to WARN or INFO or even DEBUG.

Proposal

How about an additional attribute err_level?

#[instrument(level="debug", ret, err, err_level="info")]
fn my_fun() -> Result<int> { Ok(42) }

Alternatives

I could avoid using the instrument macro and annotate every single caller with an event, but that's not really the spirit of the crate :)

[hawkw]

This is similar to the discussion #2328, which is about configuring the log level for the event generated by the ret argument to the attribute. As I mentioned in a comment on that discussion (#2328 (comment)), I think it definitely makes sense to make the level of ret events configurable, so I see no reason we shouldn't do it for err as well. I proposed a slightly different syntax, which would look like this:

#[instrument(level="debug", ret, err(level = "info"))]
fn my_fun() -> Result<int> { Ok(42) }

which i think might be a bit nicer, as it's slightly more concise, and introduces the possiblity to add additional configuration options for the ret and err events as "arguments" to those parameters. And, we wouldn't have to handle the case of "what happens if an #[instrument] attribute has err_level = "info" but no err?".

I think adding this should probably be fairly straightforward, at least for a contributor who's familiar with proc-macro implementation. If you're interested in opening a PR to add something like this, I'm happy to answer any questions you might have!

[nitnelave]

Alright, that sounds good! In the spirit of the discussions you linked to, I'd probably add the same feature for ret.

I'll try to have a swing at it, though it's my first foray into procedural macros. But hey, there's a start to everything!