tracing_journald does not forward to journald if data is passed as named fields

[s-m-e]

Bug Report

Version

Full tree for the below little example:

tracingtest v0.1.0 (/home/foo/Desktop/RUST/tracingtest)
├── tracing v0.1.41
│   ├── pin-project-lite v0.2.16
│   ├── tracing-attributes v0.1.30 (proc-macro)
│   │   ├── proc-macro2 v1.0.95
│   │   │   └── unicode-ident v1.0.18
│   │   ├── quote v1.0.40
│   │   │   └── proc-macro2 v1.0.95 (*)
│   │   └── syn v2.0.104
│   │       ├── proc-macro2 v1.0.95 (*)
│   │       ├── quote v1.0.40 (*)
│   │       └── unicode-ident v1.0.18
│   └── tracing-core v0.1.34
│       └── once_cell v1.21.3
├── tracing-journald v0.3.1
│   ├── libc v0.2.174
│   ├── tracing-core v0.1.34 (*)
│   └── tracing-subscriber v0.3.19
│       ├── nu-ansi-term v0.46.0
│       │   └── overload v0.1.1
│       ├── serde v1.0.219
│       ├── serde_json v1.0.140
│       │   ├── itoa v1.0.15
│       │   ├── memchr v2.7.5
│       │   ├── ryu v1.0.20
│       │   └── serde v1.0.219
│       ├── sharded-slab v0.1.7
│       │   └── lazy_static v1.5.0
│       ├── smallvec v1.15.1
│       ├── thread_local v1.1.9
│       │   └── cfg-if v1.0.1
│       ├── tracing-core v0.1.34 (*)
│       ├── tracing-log v0.2.0
│       │   ├── log v0.4.27
│       │   ├── once_cell v1.21.3
│       │   └── tracing-core v0.1.34 (*)
│       └── tracing-serde v0.2.0
│           ├── serde v1.0.219
│           └── tracing-core v0.1.34 (*)
└── tracing-subscriber v0.3.19 (*)

Platform

Linux xxx 6.8.0-60-generic _63~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Tue Apr 22 19:00:15 UTC 2 x86_64 x86_64 x86_64 GNU/Linux (Ubuntu 22 LTS)

Crates

Probably tracing_journald?

Description

Consider the following minimal example:

use tracing::{Level, debug, warn};
use tracing_journald::layer as journald_layer;
use tracing_subscriber::filter::LevelFilter;
use tracing_subscriber::fmt::layer;
use tracing_subscriber::layer::SubscriberExt;
use tracing_subscriber::util::SubscriberInitExt;
use tracing_subscriber::{Layer, registry};

fn tracing_init() {
    let mut layers = Vec::new();

    {
        let filter = LevelFilter::from_level(Level::DEBUG); // implicit "max level"?
        let layer = layer()
            .json()
            .with_current_span(false)
            .with_filter(filter)
            .boxed();
        layers.push(layer);
    }

    match journald_layer() {
        Ok(layer) => {
            let filter = LevelFilter::from_level(Level::WARN); // implicit "max level"?
            let layer = layer
                .with_syslog_identifier("FOO_BAR".to_string())
                .with_filter(filter)
                .boxed();
            layers.push(layer);
        }
        Err(_) => {
            panic!("failed to attached to journald");
        } // journald is typically only available on Linux systems
    }

    registry().with(layers).init();
}

fn main() {
    tracing_init();
    debug!(msg = "Goes to STDOUT (as json) as expected");
    warn!(msg = "Goes to STDOUT (as json), but not to JOURNALD, although it should?");
    warn!("Goes to both STDOUT (as json) *and* JOURNALD as expected");
}

On stdout, it results in:

{"timestamp":"2025-06-26T19:28:50.708470Z","level":"DEBUG","fields":{"msg":"Goes to STDOUT (as json) as expected"},"target":"tracingtest"}
{"timestamp":"2025-06-26T19:28:50.708506Z","level":"WARN","fields":{"msg":"Goes to STDOUT (as json), but not to JOURNALD, although it should?"},"target":"tracingtest"}
{"timestamp":"2025-06-26T19:28:50.708529Z","level":"WARN","fields":{"message":"Goes to both STDOUT (as json) *and* JOURNALD as expected"},"target":"tracingtest"}

In journald, it results in:

Jun 26 21:28:50 e3 FOO_BAR[1432418]: Goes to both STDOUT (as json) *and* JOURNALD as expected

The first call of warn! which has a field named msg instead of passing the data anonymously does not make it to journald (silently, no error or warning of any kind).

[s-m-e]

Logging with e.g. error!(traceback = "abc") actually results in an entry at journald's end, with a field along the lines of F_TRACEBACK="abc". Am I assuming correctly that the names of fields must match journald's nomenclature to be forwarded?