dune subst  --debug
Error: stat(test): No such file or directory
Raised by primitive operation at Stdune__Path.stat_exn in file
  "otherlibs/stdune/src/path.ml", line 1322, characters 17-40
Called from Main__Subst.is_a_source_file in file "bin/subst.ml", line 19,
  characters 5-25
Called from Main__Subst.subst.(fun) in file "bin/subst.ml", line 360,
  characters 7-28
Called from Stdlib__List.iter in file "list.ml", line 110, characters 12-15               Called from Fiber__Core.O.(>>|).(fun) in file "vendor/fiber/src/core.ml",                   line 252, characters 36-41
Called from Fiber__Scheduler.exec in file "vendor/fiber/src/scheduler.ml",