ICE when running `cargo fmt -- --config-path ../foo/bar.toml` with unstable `ignore` feature

[davidgauch]

How to reproduce:

docker run -it rustlang/rust:nightly-slim
root@26e4ec332226:/# mkdir foo
echo "ignore=[\"baz\"]" > foo/bar.toml
cargo new app
rustup component add rustfmt
cd app
cargo fmt -- --config-path ../foo/bar.toml

rustc-ice-2026-03-25T23_58_59-228.txt

[aznszn]

MCVE

❯ mkdir a
❯ echo "ignore=[\"baz\"]" > a/bar.toml
❯ mkdir b
❯ touch b/x.rs
❯ rustfmt +nightly --config-path a/bar.toml b/*

ICE

❯ rustfmt +nightly --config-path a/bar.toml b/*

thread 'main' (2802464) panicked at /rust/deps/ignore-0.4.25/src/gitignore.rs:232:9:
path is expected to be under the root
stack backtrace:
   0:        0x10cc46454 - <<std[744602acdec27e7b]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[c30870ac2befe340]::fmt::Display>::fmt
   1:        0x109dc6d48 - core[c30870ac2befe340]::fmt::write
   2:        0x10cc5dc44 - <std[744602acdec27e7b]::sys::stdio::unix::Stderr as std[744602acdec27e7b]::io::Write>::write_fmt
   3:        0x10cc1cb58 - std[744602acdec27e7b]::panicking::default_hook::{closure#0}
   4:        0x10cc389b0 - std[744602acdec27e7b]::panicking::default_hook
   5:        0x10aab7964 - std[744602acdec27e7b]::panicking::update_hook::<alloc[7e6774b5f2aa1473]::boxed::Box<rustc_driver_impl[769d867097b16494]::install_ice_hook::{closure#1}>>::{closure#0}
   6:        0x10cc38d14 - std[744602acdec27e7b]::panicking::panic_with_hook
   7:        0x10cc1cc2c - std[744602acdec27e7b]::panicking::panic_handler::{closure#0}
   8:        0x10cc135a4 - std[744602acdec27e7b]::sys::backtrace::__rust_end_short_backtrace::<std[744602acdec27e7b]::panicking::panic_handler::{closure#0}, !>
   9:        0x10cc1e1d0 - __rustc[a6860c59d39e6e7e]::rust_begin_unwind
  10:        0x10cd04f58 - core[c30870ac2befe340]::panicking::panic_fmt
  11:        0x1046ce0ec - <rustfmt_nightly[846573da15513c59]::ignore_path::IgnorePathSet>::is_match
  12:        0x104673ba8 - <core[c30870ac2befe340]::iter::adapters::filter::Filter<alloc[7e6774b5f2aa1473]::collections::btree::map::IntoIter<rustfmt_nightly[846573da15513c59]::config::file_lines::FileName, rustfmt_nightly[846573da15513c59]::modules::Module>, rustfmt_nightly[846573da15513c59]::formatting::format_project<rustfmt_nightly[846573da15513c59]::Session<std[744602acdec27e7b]::io::stdio::Stdout>>::{closure#1}> as core[c30870ac2befe340]::iter::traits::iterator::Iterator>::next
  13:        0x10465f798 - <scoped_tls[a71c053c2ca60a89]::ScopedKey<rustc_span[640806b751288239]::SessionGlobals>>::with::<<rustfmt_nightly[846573da15513c59]::Session<std[744602acdec27e7b]::io::stdio::Stdout>>::format_input_inner::{closure#0}, core[c30870ac2befe340]::result::Result<rustfmt_nightly[846573da15513c59]::FormatReport, rustfmt_nightly[846573da15513c59]::ErrorKind>>
  14:        0x10465c280 - rustfmt[9b590d4af149c254]::format_and_emit_report::<std[744602acdec27e7b]::io::stdio::Stdout>
  15:        0x10466d178 - rustfmt[9b590d4af149c254]::execute
  16:        0x104669900 - rustfmt[9b590d4af149c254]::main
  17:        0x10466822c - std[744602acdec27e7b]::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
  18:        0x104668778 - std[744602acdec27e7b]::rt::lang_start::<()>::{closure#0}
  19:        0x10cc37018 - std[744602acdec27e7b]::rt::lang_start_internal
  20:        0x1046759f8 - _main

error: the compiler unexpectedly panicked. This is a bug

note: we would appreciate a bug report: https://github.com/rust-lang/rustfmt/issues/new?labels=bug

note: please make sure that you have updated to the latest nightly

note: please attach the file at `/Users/azan.ali/rustc-ice-2026-03-26T09_15_23-76631.txt` to your bug report

note: rustc 1.96.0-nightly (80d0e4be6 2026-03-25) running on aarch64-apple-darwin

query stack during panic:
end of query stack

so the issue is originating in the ignore crate

@ytmimi

[aznszn]

@rustbot claim