Flake inputs: git submodules/recursive checkout

[SomeoneSerge]

Is your feature request related to a problem? Please describe.

Can't just add git url to inputs to fetch sources with submodules.

Describe the solution you'd like

A recursive: bool field in flake inputs available when url refers to a git repo

Describe alternatives you've considered

Not using flake.lock or flake inputs, but instead using fetchFromGitHub

Additional context
Add any other context or screenshots about the feature request here.

[ruler501]

It would also be good if it copied over submodules when building devShell for a local flake. A project I'm working on needs to reference files in a git submodule to make its devShell, but the flake for it doesn't work currently since nix develop does not see files in submodules.

[ruler501]

It took me a while to parse through how nix develop fetches the files. I would propose to change src/nix/command.hh:138 like this

-    std::string _installable{"."};
+    std::string _installable{".?submodules=1"};

That would allow you to not have it be unconditional for all locally referenced repos.
It would also be good if we could add it to the github scheme, but looking at the code that seems very non-trivial and people can always fall back to the git reference instead.

[con-f-use]

By the way, to add insult to injury, nix doesn't even tell that it's because you're using something from a submodule. It just says something like:

error: getting status of '/nix/store/qjj8lgnp70m9842n2qr4bljb3hrd04dh-source/mysubmodule': No such file or directory

For context, that is if "mysubmodule" is a submodule of my current flake project managed in git and I'm using a file from that project somewhere. A file that physically exists on the drive, because the project repository was checked out recursively.
That error message could be much better and suggest a resolution.