opam-nix does not error out when pinning dependencies failed, making debugging difficult

[kentookura]

I would like there to be better error handling when pinning dependencies. I had added a *.opam.template file and the opam file was updated after running dune build. The OCaml environment built successfully, but the pinned dependency was not at the specified version. It turns out that there was an error when pinning the dependency which I only discovered when trying to build the repository with vanilla opam. The error was caused by the fact that the repo that I pinned contained multiple packages which my project depended on, so there was a bounds issue until I added a pin-depends entry for each package in the repo.

I hope I managed to explain what happened clearly enough.

Describe the solution you'd like

Vanilla opam actually fails in this scenario, so at the very least I would like there to be a message that pinning the dependencies failed

[balsoft]

Can you provide an example repo?

[kentookura]

https://github.com/kentookura/opam-nix-pins

[kentookura]

Note that pinning ppx_irmin is necessary even though the project has no explicit dependency on that library.