Add regression test for issue 6300

[matthewhughes934]

It looks like this issue has been fixed, so add a test to ensure we
never regress.

You may ask when this was fixed, the best I could do was bisect to
a merge commit: d97c4fb, via:

# use --first-parent so we don't try and build commits that were
# originally added in rust-lang/rust, since it's a pain to find the
# correct toolchain to build those against
$ git bisect start --term-new=fixed --term-old=broken --first-parent main 46cb7d322055625c07823dea8fdfad4c07f98754
$ git bisect run ./bisect.sh

Where bisect.sh is:

#!/usr/bin/env bash

set -o errexit

if ! cargo build --quiet --bin rustfmt
then
  echo "failed to build" >&2
  # exit: magic code that tells git-bisect to skip this commit (see
  # git-bisect(1))
  exit 125
fi

export LD_LIBRARY_PATH="$(rustc --print sysroot)/lib${LD_LIBRARY_PATH:+:${LD_LIBRARY_PATH}}"
# reverse the exit code: the basis of 'git bisect' is that the 'new' commit is
# bad, the 'old' commit is good (e.g. there's a bug in HEAD but not HEAD~4,
# which commit introduced that?). But we want the _opposite_
! ./target/debug/rustfmt --check ./samples/bugs.rs >/dev/null

Closes: #6300

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

[ytmimi]

Thank you!

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