There is a regression in the interaction between Dune, ppxlib and OCaml 5.4.0, which results in some compiler errors being reported with missing location information. This is fairly disruptive in practice (as it breaks "jump-to-error" workflows.)

I believe that the issue is the one discussed in ocaml/ocaml#12991, but this compiler-side issue is fairly difficult to find so I thought that opening another issue where users might look for them could be helpful.

Reproduction steps

In an empty temporary directory:

echo > a.mli
echo "let () = NonExistingModule.run ()" > a.ml
echo "(lang dune 3.0)" > dune-project
echo "(library (name test) (preprocess (pps ppx_here)))" > dune
dune build

Observed output:

File "_none_", line 1:              
Error: Unbound module NonExistingModule

Expected output:

File "a.ml", line 1:              
Error: Unbound module NonExistingModule

(This works properly with OCaml 5.3 and older compiler versions.)