Dear rust-tracing-core maintainers,

Currently, rust-tracing-core assumes that Rust niche optimization[1]
remains the
same, even though Rust makes no such guarantee. As of Rust 1.97, this
hardcoded
assumption is no longer true, meaning that the
metadata::tests::level_filter_reprs
test will break[2]:

167s ---- metadata::tests::level_filter_reprs stdout ----
167s
167s thread 'metadata::tests::level_filter_reprs' (5315) panicked at
src/metadata.rs:1119:13:
167s assertion `left == right` failed: repr changed for LevelFilter::OFF
167s left: 5
167s right: 18446744073709551615
167s stack backtrace:
167s 0: __rustc::rust_begin_unwind
167s at /usr/src/rustc-1.97.1/library/std/src/panicking.rs:689:5
167s 1: core::panicking::panic_fmt
167s at /usr/src/rustc-1.97.1/library/core/src/panicking.rs:80:14
167s 2: core::panicking::assert_failed_inner
167s at /usr/src/rustc-1.97.1/library/core/src/panicking.rs:434:23
167s 3: core::panicking::assert_failed::<usize, usize>
167s at /usr/src/rustc-1.97.1/library/core/src/panicking.rs:394:5
167s 4: tracing_core::metadata::tests::level_filter_reprs
167s at ./src/metadata.rs:1119:13
167s 5: tracing_core::metadata::tests::level_filter_reprs::{closure#0}
167s at ./src/metadata.rs:1102:28
167s 6: <tracing_core::metadata::tests::level_filter_reprs::{closure#0} as
core::ops::function::FnOnce<()>>::call_once
167s at /usr/src/rustc-1.97.1/library/core/src/ops/function.rs:250:5
167s 7: <fn() -> core::result::Result<(), alloc::string::String> as
core::ops::function::FnOnce<()>>::call_once
167s at /usr/src/rustc-1.97.1/library/core/src/ops/function.rs:250:5
167s note: Some details are omitted, run with `RUST_BACKTRACE=full` for a
verbose backtrace.

Naturally, this bug will not impact Debian until Rust is updated to 1.97.
However, it is affecting Ubuntu and being tracked as LP: #2167368.

I have attached a patch which cherry-picks the upstream fix[3]. The fix
makes it so the crate works with all Rust versions past the MSRV -- I
successfully ran all tests with both 1.93.1 and 1.97.1, and I verified that
the package builds against Debian unstable.

Please let me know if you need anything else.

Best wishes,

Max

[1]: https://www.0xatticus.com/posts/understanding_rust_niche/
[2]:
https://autopkgtest.ubuntu.com/results/autopkgtest-stonking/stonking/amd64/r/rust-tracing-core/20260912_123714_a8bc3@/log.gz

[3]: https://github.com/tokio-rs/tracing/pull/3537

We believe that the bug you reported is fixed in the latest version of
rust-tracing-core, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1147945@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Max Gilmour <max.gilmour@canonical.com> (supplier of updated rust-tracing-core package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Tue, 15 Sep 2026 09:12:44 -0700
Source: rust-tracing-core
Built-For-Profiles: derivative.ubuntu noudeb
Architecture: source
Version: 0.1.36-2
Distribution: unstable
Urgency: medium
Maintainer: Debian Rust Maintainers <pkg-rust-maintainers@alioth-lists.debian.net>
Changed-By: Max Gilmour <max.gilmour@canonical.com>
Closes: 1147945
Launchpad-Bugs-Fixed: 2167368
Changes:
 rust-tracing-core (0.1.36-2) unstable; urgency=medium
 .
   * Team upload
   * Package tracing-core 0.1.36 from crates.io using debcargo 2.8.4
   * Cherry-pick proposed patch to fix build with rustc 1.97 (LP: #2167368)
     (Closes: #1147945)
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