best way to pull in a file from a sibling directory?

[greglearns]

My flake.nix file is right next to the rust build.rs file, and build.rs needs to load a file ../spec/service.proto. All of these files are I've read the documentation https://crane.dev/source-filtering.html but I'm relatively new to nix, I'm not sure how to make this work, and I've tried a few times to make it work and I can't figure it out. I have this directory structure:
|root
|.git <--- all of the files below have been git added and so the flake should be able to find them.
|-- rust
|-- flake.nix
|-- build.rs <-- this needs to read the ../spec/service.proto file.
|-- src/
|-- spec/service.proto

Any suggestions?

P.S. Thank you for crane!

[ipetkov]

Hi @greglearns ! Could you share your flake.nix so I can see how the build is configured?

Two possible situations come to mind:

1. Is spec/service.proto staged with git at all? (e.g. have you run git add -N spec/service.proto?) Flakes can only see files which are staged (or committed) with git and aren't listed in .gitignore
2. You mentioned you've looked at source filtering. One quick thing to check is setting src = ./.; which will not do any additional source filtering (besides ignoring untracked files). If the build succeeds with this then its a matter of tweaking the source filter code, otherwise it might be something else!

[greglearns]

https://gist.github.com/greglearns/a8f63f33040248411f5ef281dde51e33

1. spec/service.proto is staged in git. The git repo is at the root level, with ./rust/flake.nix and ./spec/service.proto being subdirectories under the root directory where the .git dir lives.

2. I think the problem is that the flake is in the ./rust/ directory, so src = ./. is only looking in the ./rust directory, but the proto file is in the ./spec directory.

3. If I copy ./spec/service.proto into the ./rust directory as ./spec/copied-service.proto, and then change the ./rust/build.rs file to point to the copied version at ./rust/copied-service.proto file, then run nix build then it all works. But. I can't do that since the spec/service.proto is shared by other projects and can't be moved.

[ipetkov]

Ah I see! I had misunderstood the original description and didn't realize that flake.nix was inside of the rust directory.

As far as I understand, flakes do not allow accessing files in parent directories (only sub-directories relative to their location) so there's two approaches which come to mind for resolving this:

1. The simplest is to move flake.nix to the root of the repo. Newer versions of nix will automatically look for a flake.nix in a parent directory so you can theoretically still run things like nix build etc. inside of ./rust
2. It may be possible to add a symlink in ./rust/spec -> ../spec and get that to work, assuming that when the flake is evaluated it performs a deep copy of the symlink's contents in the store instead of copying the link itself. I haven't actually tested this so if it doesn't work, option 1 above would be my go to!

Hope this helps!