Method to copy lock entry from one flake to another

[peterwaller-arm]

Describe the bug

I expected nix flake update --inputs-from ~/another-flake nixpkgs to copy the input from another-flake to the flake in $PWD.

It fails with:

warning: not writing lock file of flake 'git+file:///path-to-current-flake' because it has an unlocked input ('github:NixOS/nixpkgs/nixos-unstable?narHash=sha256-<hash>')

~/another-flake has a lockfile entry for nixpkgs which includes a rev and a narHash.

It seems to me that rather than failing, a useful natural behaviour would be to make use of the lock file entry in ~/another-flake. I'm not totally certain though what the intended behaviour of inputs-from is, and whether what I'm asking for is really a new feature request with a different name than --inputs-from.

Steps To Reproduce

1. Have two flakes
2. From the directory of one, the nixpkgs input from one to the other: nix flake update --inputs-from ~/another-flake nixpkgs
3. Observe warning and that the lockfile is left unmodified.

Expected behavior

I expected the flake lock entry for the requested input (nixpkgs) to be essentially copied from one to the other.

Metadata

nix (Nix) 2.31.2

Additional context

If I supply --option allow-dirty-locks true, then the command succeeds, but it records the ref rather than the rev into the locked entry.

There is --reference-lock-file, but this does not seem to do what I'm after either.

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

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