Is your feature request related to a problem?

• • Nix currently resolves git+file:relative/path according to the process working directory instead of the flake.nix base directory. This is unreliable. Evaluation should be invariant under the process working directory, or it would violate the ability to be used as a flake input, among other things.
  • fetchTree does not have a base directory argument (and no implicit argument either, because that'd also be bad)
• Therefore fetchTree does not have the information to resolve relative paths.

Solution for Flakes users

Users will be able to use a relative path reference since Nix 2.26 and they can enable submodules declaratively, planned for release in 2.27

• Improve support for relative path inputs #10089
• Add inputs.self.submodules flake attribute #12421

{
  # For Nix 2.27:
  inputs.self.submodules = true;
  # instead of: inputs.foo.url = "git+file://./my-submodule";
  inputs.foo.url = ./my-submodule;
}

For Nix 2.26, instead of inputs.self.submodules, use:

nix build .?submodules=true#my-package

Proposed solution (for fetchTree)

• Disallow git relative path inputs in fetchTree
• Either
  • Resolve git relative path inputs in call-flake.nix (with a primop that does it? - unclear if that's future proof enough for reproducibility)
  • add a baseDirectory argument to fetchTree

Alternative solution

Forbid altogether. This blocks some users:

• Flake input git+file:./${submodule} no longer works #9708

Additional context

• fetchers/git: make relative path absolute for local repo #12107
• Warn against the use of relative 'git+file:' flake inputs #12277

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