tracing-subscriber: can't filter events by field

[stevenengler]

Bug Report

Version

tracing-test v0.1.0
├── tracing v0.1.34
│   ├── tracing-attributes v0.1.21 (proc-macro)
│   └── tracing-core v0.1.26
└── tracing-subscriber v0.3.11
    ├── tracing v0.1.34 (*)
    ├── tracing-core v0.1.26 (*)
    └── tracing-log v0.1.3
        └── tracing-core v0.1.26 (*)

Platform

Linux hostname 5.13.0-40-generic #45~20.04.1-Ubuntu SMP Mon Apr 4 09:38:31 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

Description

This seems like such a simple issue that I'm guessing I must be doing something wrong, but when I follow the documentation it's not working as I'd expect.

The EnvFilter documentation says that the field should match on events:

field matches on fields within spans. Field names can also be supplied without a value and will match on any Span or Event that has a field with that name. For example: [span{field=\"value\"}]=debug, [{field}]=trace.

When I run the following code:

#[tracing::instrument]
fn test() {
    tracing::event!(tracing::Level::INFO, abc = 5, "In test fn");
}

fn main() {
    tracing_subscriber::fmt::init();
    test();
}

with:

$ RUST_LOG='tracing_test[test{abc}]=trace' cargo run
$ RUST_LOG='tracing_test[test{abc=5}]=trace' cargo run

The event is not printed (there is no output) in either case. The test{abc} filter doesn't work when the EnvFilter documentation seems to imply that it should.

If instead I run it without the field filter:

$ RUST_LOG='tracing_test[test]=trace' cargo run

The INFO test: tracing_test: In test fn abc=5 line is printed.

[hawkw]

The directive [test{abc}] matches a span whose name is test with a field named abc. To match an event with a field named abc, you would need to write just [{abc}].

The match expressions [test{abc=5}] or [{abc=5}] will not match an event, because field value filters are currently only supported for spans.

Sorry if the documentation is unclear, if you have any suggestions for how we can make this clearer, that would be great.