`--timings` chart no longer shows all dependencies

[kpreid]

Problem

cargo build --timings has stopped having a full set of dependency lines in the graph.

Before:

After:

(Both screenshots taken while hovering on num-traits.)

Steps

1. Pick any large project, say https://github.com/gfx-rs/wgpu.git
2. Run cargo +nightly-2025-09-11 clean -p libm && cargo +nightly-2025-09-11 build -p wgpu --lib --timings twice (to produce a reproducible and reasonably small graph)
3. Do the same for nightly-2025-09-12.
4. Observe that the latter timings graph is missing some lines the former has.

Possible Solution(s)

Based on the presence of a legend in the latter only, this is probably caused by #15923. cc @Kobzol

Notes

@rustbot label +regression-from-stable-to-nightly

Version

This is the first nightly version that demonstrates the problem:

cargo 1.91.0-nightly (24bb93c38 2025-09-10)
release: 1.91.0-nightly
commit-hash: 24bb93c388fb8c211a37986539f24a819dc669d3
commit-date: 2025-09-10
host: aarch64-apple-darwin
libgit2: 1.9.1 (sys:0.20.2 vendored)
libcurl: 8.7.1 (sys:0.4.83+curl-8.15.0 system ssl:(SecureTransport) LibreSSL/3.3.6)
ssl: OpenSSL 3.5.2 5 Aug 2025
os: Mac OS 15.6.1 [64-bit]

[weihanglo]

Actually #15923 is in beta already, if that is the root cause.

[kpreid]

I can confirm that the problem manifests in rustc 1.91.0-beta.4 (aa7859c0d 2025-09-27).

[weihanglo]

Proposed a fix: #16055

[weihanglo]

Beta backport PR: #16057
(we anyway need another backport #16032 when FCP ends)