tracing can't use type as field

[BratSinot]

Bug Report

Version

tracing = "0.1.35"
tracing-subscriber = { version = "0.3.1", default-features = false, features = ["smallvec", "fmt", "tracing-log", "env-filter", "json", "time"] }

Description

Can't use type as field name. This code give output r#type:

use tracing::info;
use tracing_subscriber::fmt;

fn main() {
    fmt().json().flatten_event(true).init();

    info!(r#type = "foo", "bar");
}

{"timestamp":"2022-07-01T09:31:30.528058Z","level":"INFO","message":"bar","r#type":"foo","target":"playground2"}

If we change r#type on type code doesn't build:

    info!(type = "foo", "bar");

error: no rules expected the token `type`
 --> src/main.rs:7:5
  |
7 |     info!(type = "foo", "bar");
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ no rules expected this token in macro call
  |
  = note: this error originates in the macro `$crate::fieldset` (in Nightly builds, run with -Z macro-backtrace for more info)

[hawkw]

Field names are matched as Rust identifiers, as discussed in the documentation here. Keywords, such as type, match, or fn, are not valid identifiers.

The "raw identifier" (r#) syntax is supported by Rust, but requires special handling by tracing subscribers to strip the r# if it is present, which it appears the JSON formatter does not currently support. This could be fixed.

As an alternative, though, consider using a quoted field name. This is matched as a string literal, rather than an identifier. For example,

info!("type" = "foo", "bar");