`-Zthreads=8` starts panicking a lot more frequently since today's nightly

[flippette]

Pretty much what the title says, I've narrowed it down to -Zthreads=8 and today's nightly with the following:

$ rustup toolchain list
nightly-2024-09-05-x86_64-unknown-linux-gnu
nightly-2024-09-06-x86_64-unknown-linux-gnu
nightly-x86_64-unknown-linux-gnu (default)
$ rustc +nightly-2024-09-05 -V
rustc 1.83.0-nightly (4ac7bcbaa 2024-09-04)
$ rustc +nightly-2024-09-06 -V
rustc 1.83.0-nightly (9c01301c5 2024-09-05)
$ rustc -V
rustc 1.83.0-nightly (26b5599e4 2024-09-06)
$ for i in $(seq 1 64); do cargo +nightly-2024-09-05 build -q; cargo clean -q; done
$ echo $?
0
$ for i in $(seq 1 64); do cargo +nightly-2024-09-06 build -q; cargo clean -q; done
$ echo $?
0
$ for i in $(seq 1 64); do env RUSTFLAGS="-Zthreads=1" cargo build -q; cargo clean; done
$ echo $?
0
$ for i in $(seq 1 64); do cargo build -q; cargo clean -q; done
<almost immediately ICEs>

I've attached some rustc backtraces.
rustc-ice-2024-09-07T19_02_57-44339.txt
rustc-ice-2024-09-07T19_40_47-164056.txt
rustc-ice-2024-09-07T19_40_45-162317.txt
rustc-ice-2024-09-07T19_40_44-161289.txt

[matthiaskrgr]

probably duplicate of #129112 but made more prevalent by next solver

[jieyouxu]

Is it possible to provide a link to or a snippet of a reproducer? I know it probably happens to various crates, but it still would be nice to have something that reproduces.

[nuudlman]

See #130089 for an attached repro.

[flippette]

The repro case is basically the same as #130089, but with other crates mentioned in the ICE logs from the issue comment.