dune subst fails with opam pin -k path when no version can be determined

[Alidra]

##Description
With the latest version of Dune, running:

opam pin -k path .

fails when the project contains a watermark such as:

let version = "%%VERSION%%"

The failure occurs during the dune subst step automatically invoked by opam:

[ERROR] The compilation of dune-watermark-demo.~dev failed at "dune subst".

=== ERROR while compiling dune-watermark-demo.~dev ===========================#
context 2.1.5 | linux/x86_64 | ocaml.5.3.0 | pinned(file:///home/abdelghani/Repos/playwith/dune-watermark-demo)
path ~/.opam/default/.opam-switch/build/dune-watermark-demo.~dev
command ~/.opam/opam-init/hooks/sandbox.sh build dune subst
exit-code 1
output
File "bin/main.ml", line 1, characters 15-26:
1 | let version = "%%VERSION%%"
^^^^^^^^^^^
Error: repository does not contain any version information

Expected Behavior

opam pin -k path . should not fail merely because Dune cannot determine a version for the repository.

In particular, the presence of %%VERSION%% should not make a local path pin unusable when the repository is initialized, regardless of whether a Git tag exists.

Actual Behavior

dune subst fails as soon as it encounters %%VERSION%% and cannot derive version information from the repository:

Error: repository does not contain any version information

This prevents the pinned package from being built.

Reproduction

The issue can be reproduced with the dune-watermark-demo example:

dune-watermark-demo/
├── dune-project
├── bin/
│ ├── dune
│ └── main.ml
└── .git/

dune-project:

(lang dune 3.20)
(name dune_watermark_demo)

bin/dune:

(executable
(name main))

bin/main.ml:

let version = "%%VERSION%%"

let () =
Printf.printf "Version : %s\n" version

The problem occurs in both of the following situations:

The Git repository is initialized, but no tag has been created.
The Git repository is initialized and a tag has been created.

In both cases, running:

opam pin -k path .

ultimately invokes:

dune subst

which fails with:

Error: repository does not contain any version information

Specifications

• Version of dune (output of dune --version): 3.23.0
• Version of ocaml (output of ocamlc --version): 5.3.0
• Operating system (distribution and version): Ubuntu 22.04.5 LTS. 64 bits

Additional information

This affects projects using Dune watermarks such as %%VERSION%% and relying on:

opam pin -k path ...

for local development.

The issue is especially surprising because the repository may already be a valid Git repository and may even contain a tag, yet dune subst still reports that no version information is available.

[Alizter]

I believe the issue is that -k path stops copying the .git information that dune subst uses to actually substitute the version. If you do opam pin . instead, it should work. It's not yet clear if we can do anything better if we don't have VCS info to fetch the version in this case.

[Alidra]

Thank you @Alizter for your answer.
Please note however that -k path used to work with previous versions of Dune (3.7 for instance)

[Alizter]

I checked and this is version guarded behind (dune lang 3.17) so picking something lower should prevent issues. I don't think anybody bothered to preserve or document the wanted behaviour here however.

Relying on an error being ignored also seems strange, I don't know of any workarounds at the moment other than avoiding dune subst all together. We have dune-build-info which can provide similar functionality in a different manner.