Sharing devenv.lock in monorepos

Based on a comment I found in #2209 this does not seem possible (referenced below)

The more general question: How can I inherit the exact versions from one devenv in a monorepo in a second devenv.

Is there a way of doing this that i am missing (apart from ref-linking the urls in the devenv.yaml). And what are the technical blockers to make this possible?

My exact use case: The monorepo structure (simplified):

• parent
  • devenv.nix (let's call it P)
  • frontend
    • devenv.nix (let's call it F)
  • backend
    • devenv.nix (let's call it B)

F sets up everything to run frontend, B backend. P contains all the wiring to allow devenv up to run the whole repo locally. This is extremely valuable and reproducible. However, now we still have a script in the parent to update all devenvs in lockstep, to make sure the nixpkgs version of F is the same as P, otherwise F often does not compile in P. Since different teams might manage F and B, keeping P in sync is potentially brittle and the only non-manual workaround would be a github action to relock all devenvs collectively. Which is not really what we want.

The docs say:

!!! warn "Locked inputs and follows"

"Follows" are fetched and re-locked using the URI of the followed inputs. The external project's lock file will not constrain the revision.

For example, it's common to reference nixpkgs-unstable without specifying a revision. The input is locked to a specific revision when fetched and this information is saved to either a devenv.lock or flake.lock file.

Following this input from another project will re-fetch the latest revison, which may be different from the lcoked revision used in the original project.

This is the comment referencing the commit where this doc was added.

It's a feature of Nix Flakes. Instead of providing a url, you can use follows to reference any other input by name, including their nested inputs.

I've added this information to the docs: bee405dc72a23f93002fdda03046177995d9bb01

Originally posted by @sandydoo in #2209