#[instrument] does not warn on unrecognized options

[jonhoo]

Bug Report

Version

tracing 0.1.14

Description

The following code currently compiles without warnings or errors:

use tracing::instrument;

#[instrument(debug)]
fn foo() {}

A particularly unobservant user (like myself) may try this without reading the documentation carefully, and will likely be very confused why foo shows up even at info level. It'd be good to warn when unsupported attribute options are used, and ideally also suggest what to use instead (level = "debug") for things like debug/trace/etc. We probably don't want it to be a hard error for forward-compatibility.

Separately, wouldn't it be kind of nice to support this short-hand syntax? :p

[hawkw]

#672 will make instrument error on unrecognized syntax. I'm not sure if we can easily emit warnings rather than errrors from proc macros, but I can take a look at it.

Separately, wouldn't it be kind of nice to support this short-hand syntax? :p

Sure, we could probably add something like that!

[hawkw]

Well, it turns out that there isn't currently a good way to emit custom warnings from proc macros on stable (there is a nightly-only API, though!).

However, I figured out a bit of a hack to turn the failure on unrecognized input from #627 into a warning:

it's not the greatest, since it's not actually a deprecation, but at least it emits an actual warning, and works on stable. Unlike #672, this is not a breaking change.