SpanTrace with fmt layer includes ANSI codes in fields

[ColonelThirtyTwo]

Bug Report

Version

tracing v0.1.44
tracing-attributes v0.1.31
tracing-core v0.1.36
tracing-error v0.2.1
tracing-subscriber v0.3.22

Platform

Linux [hostname redacted] 6.18.7-arch1-1 #1 SMP PREEMPT_DYNAMIC Sat, 24 Jan 2026 00:47:39 +0000 x86_64 GNU/Linux

Description

If the tracing registry contains a fmt layer that has ANSI colors turned on, the fields string returned by SpanTrace::with_spans will as well. This also includes SpanTrace's Display implementation.

Example code:

use tracing_subscriber::{layer::SubscriberExt as _, util::SubscriberInitExt as _};

fn main() {
    tracing_subscriber::registry()
        .with(tracing_subscriber::fmt::layer())
        .with(tracing_error::ErrorLayer::default())
        .init();

    let _guard = tracing::error_span!("test span", foo="bar").entered();
    let st = tracing_error::SpanTrace::capture();
    st.with_spans(|_md, fields| {
        println!("{:?}", fields);
        true
    });
    println!("{:?}", st.to_string());
}

Output:

"\u{1b}[3mfoo\u{1b}[0m\u{1b}[2m=\u{1b}[0m\"bar\""
"   0: test_tracing_error::test span\n           with \u{1b}[3mfoo\u{1b}[0m\u{1b}[2m=\u{1b}[0m\"bar\"\n             at src/main.rs:9"

Calling .with_ansi(false) on the fmt layer or removing it entirely results in plaintext output:

"foo=\"bar\""
"   0: test_tracing_error::test span\n           with foo=\"bar\"\n             at src/main.rs:9"

The documentation for with_spans doesn't mention that it emits ANSI escape codes. It makes reading fields in spantraces more difficult in places that don't support ANSI codes.

In particular, #3378 means that doing tracing::error("{:?}", color_eyre_error) will display ANSI escapes for the span fields, even if you turn color_eyre colors off.