error_on_unformatted doesn't actually make cargo fmt exit with non-zero

[Veetaha]

I tried this command in my repo.

$ cargo fmt --all -- --check --config unstable_features=true --config error_on_unformatted=true
error[internal]: not formatted because a comment would be lost
  --> /home/path/redacted/mod.rs:55
   |
92 |                 match rp.kind {
   |
   = note: set `error_on_unformatted = false` to suppress the warning against comments or string literals

warning: rustfmt has failed to format. See previous 1 errors.

I expected the command to exit with a non-zero status code to make sure it fails on CI, but it didn't. I suppose it's because the error[internal] is not actually an error, but a warning or smth like a non-failing error? I think this is a bug.

Versions

This behavior reproduces both on stable 1.74.0 and on nightly:

$cargo +nightly fmt --version
rustfmt 1.7.0-nightly (190f4c9 2024-01-09)
$ cargo +nightly --version    
cargo 1.77.0-nightly (2ce45605d 2024-01-04)

References

Tracking issue reference: #3392

[AaronKutch]

I ran into this same issue, a temporary workaround is to check if the stderr is non-empty