melange: crash when using ppx in melange library

[eWert-Online]

Expected Behavior

I would like to use a ppx like ppx_deriving or ppx_yojson_conv in a melange library
or be notified, that using ppx's is currently not possible.

Actual Behavior

dune crashes with the "I must not crash. Uncertainty is the mind-killer ..." message

Reproduction

A repository with an opam setup to reproduce the issue can be found here:
https://github.com/ewert-online/melange-dune-test

1. Add a dune library with the following contents:

   (library
    (name shared)
    (modes melange)
    (preprocess
     (pps ppx_deriving.make)))
   

2. Use this library in a melange.emit stanza

   (melange.emit
    (target es6)
    (alias melange)
    (libraries shared)
    (module_system es6))
   

3. Run dune build @melange

Specifications

• Version of dune: 3.6.0-450-gc8f392f
• Version of ocaml: 4.14.1
• Operating system (distribution and version): macOS Ventura 13.0.1

Additional information

output of dune build @melange --verbose:

Click to expand...

Shared cache: disabled
Workspace root: /Users/ewert/Documents/Projekte/melange-dune-test
Auto-detected concurrency: 8
Running[0]: /Users/ewert/Documents/Projekte/melange-dune-test/_opam/bin/ocamlc.opt -config > /var/folders/2p/nbzvd_q158gc1f9t74cpg79w0000gn/T/dune_ece0a0_output
Dune context:                   
{ name = "default"
; kind = "default"
; profile = Dev
; merlin = true
; for_host = None
; fdo_target_exe = None
; build_dir = In_build_dir "default"
; toplevel_path =
   Some
     External
       "/Users/ewert/Documents/Projekte/melange-dune-test/_opam/lib/toplevel"
; ocaml_bin =
   External "/Users/ewert/Documents/Projekte/melange-dune-test/_opam/bin"
; ocaml =
   Ok
     External
       "/Users/ewert/Documents/Projekte/melange-dune-test/_opam/bin/ocaml"
; ocamlc =
   External
     "/Users/ewert/Documents/Projekte/melange-dune-test/_opam/bin/ocamlc.opt"
; ocamlopt =
   Ok
     External
       "/Users/ewert/Documents/Projekte/melange-dune-test/_opam/bin/ocamlopt.opt"
; ocamldep =
   Ok
     External
       "/Users/ewert/Documents/Projekte/melange-dune-test/_opam/bin/ocamldep.opt"
; ocamlmklib =
   Ok
     External
       "/Users/ewert/Documents/Projekte/melange-dune-test/_opam/bin/ocamlmklib.opt"
; env =
   map
     { "DUNE_OCAML_HARDCODED" :
         "/Users/ewert/Documents/Projekte/melange-dune-test/_opam/lib"
     ; "DUNE_OCAML_STDLIB" :
         "/Users/ewert/Documents/Projekte/melange-dune-test/_opam/lib/ocaml"
     ; "DUNE_SOURCEROOT" :
         "/Users/ewert/Documents/Projekte/melange-dune-test"
     ; "INSIDE_DUNE" :
         "/Users/ewert/Documents/Projekte/melange-dune-test/_build/default"
     ; "MANPATH" :
         "/Users/ewert/Documents/Projekte/melange-dune-test/_build/install/default/bin"
     ; "OCAMLFIND_IGNORE_DUPS_IN" :
         "/Users/ewert/Documents/Projekte/melange-dune-test/_build/install/default/lib"
     ; "OCAMLPATH" :
         "/Users/ewert/Documents/Projekte/melange-dune-test/_build/install/default/lib"
     ; "OCAMLTOP_INCLUDE_PATH" :
         "/Users/ewert/Documents/Projekte/melange-dune-test/_build/install/default/lib/toplevel"
     ; "OCAML_COLOR" : "always"
     ; "OPAMCOLOR" : "always"
     }
; findlib_paths =
   [ External "/Users/ewert/Documents/Projekte/melange-dune-test/_opam/lib"
   ]
; arch_sixtyfour = true
; natdynlink_supported = true
; supports_shared_libraries = true
; ocaml_config =
   { version = "4.14.1"
   ; standard_library_default =
       "/Users/ewert/Documents/Projekte/melange-dune-test/_opam/lib/ocaml"
   ; standard_library =
       "/Users/ewert/Documents/Projekte/melange-dune-test/_opam/lib/ocaml"
   ; standard_runtime = "the_standard_runtime_variable_was_deleted"
   ; ccomp_type = "cc"
   ; c_compiler = "cc"
   ; ocamlc_cflags =
       [ "-O2"; "-fno-strict-aliasing"; "-fwrapv"; "-pthread" ]
   ; ocamlc_cppflags = [ "-D_FILE_OFFSET_BITS=64" ]
   ; ocamlopt_cflags =
       [ "-O2"; "-fno-strict-aliasing"; "-fwrapv"; "-pthread" ]
   ; ocamlopt_cppflags = [ "-D_FILE_OFFSET_BITS=64" ]
   ; bytecomp_c_compiler =
       [ "cc"
       ; "-O2"
       ; "-fno-strict-aliasing"
       ; "-fwrapv"
       ; "-pthread"
       ; "-D_FILE_OFFSET_BITS=64"
       ]
   ; bytecomp_c_libraries = [ "-lm"; "-lpthread" ]
   ; native_c_compiler =
       [ "cc"
       ; "-O2"
       ; "-fno-strict-aliasing"
       ; "-fwrapv"
       ; "-pthread"
       ; "-D_FILE_OFFSET_BITS=64"
       ]
   ; native_c_libraries = [ "-lm" ]
   ; native_pack_linker = [ "ld"; "-r"; "-o" ]
   ; cc_profile = []
   ; architecture = "amd64"
   ; model = "default"
   ; int_size = 63
   ; word_size = 64
   ; system = "macosx"
   ; asm = [ "cc"; "-c"; "-Wno-trigraphs" ]
   ; asm_cfi_supported = true
   ; with_frame_pointers = false
   ; ext_exe = ""
   ; ext_obj = ".o"
   ; ext_asm = ".s"
   ; ext_lib = ".a"
   ; ext_dll = ".so"
   ; os_type = "Unix"
   ; default_executable_name = "a.out"
   ; systhread_supported = true
   ; host = "x86_64-apple-darwin22.1.0"
   ; target = "x86_64-apple-darwin22.1.0"
   ; profiling = false
   ; flambda = false
   ; spacetime = false
   ; safe_string = true
   ; exec_magic_number = "Caml1999X031"
   ; cmi_magic_number = "Caml1999I031"
   ; cmo_magic_number = "Caml1999O031"
   ; cma_magic_number = "Caml1999A031"
   ; cmx_magic_number = "Caml1999Y031"
   ; cmxa_magic_number = "Caml1999Z031"
   ; ast_impl_magic_number = "Caml1999M031"
   ; ast_intf_magic_number = "Caml1999N031"
   ; cmxs_magic_number = "Caml1999D031"
   ; cmt_magic_number = "Caml1999T031"
   ; natdynlink_supported = true
   ; supports_shared_libraries = true
   ; windows_unicode = false
   }
}
Actual targets:                 
- recursive alias @melange
Internal error, please report upstream including the contents of _build/log.
Description:
("Option.value_exn", {})
Raised at Stdune__code_error.raise in file
"otherlibs/stdune/src/code_error.ml", line 11, characters 30-62
Called from Fiber__core.O.(>>|).(fun) in file "otherlibs/fiber/src/core.ml",
line 250, characters 36-41
Called from Fiber__scheduler.exec in file "otherlibs/fiber/src/scheduler.ml",
line 73, characters 8-11
-> required by ("<unnamed>", ())
-> required by
 ("build-alias", { dir = In_build_dir "default/src"; name = "melange" })
-> required by ("toplevel", ())

I must not crash.  Uncertainty is the mind-killer. Exceptions are the
little-death that brings total obliteration.  I will fully express my cases. 
Execution will pass over me and through me.  And when it has gone past, I
will unwind the stack along its path.  Where the cases are handled there will
be nothing.  Only I will remain.

[rgrinberg]

Thanks for reporting. The issue is caused by the fact that ppx_deriving is using a template for the META file instead of using dune's metadata. All we can in this case is improve the error message.

If you want to actually use ppx_deriving, you need to do two things:

1. remove the META file template
2. add (modes melange) to its runtime.