Exporting an API to allow custom external helpers inside _tool files

[fionera]

Currently it is not possible to wrap the cue software and register custom built-in functions and that is totally correct as it would make debugging way more complicated and would potentially allow non-hermetic use. But since _tool.cue files are allowed to break these premises I would like to see a way to register custom built-in functions for use inside a _tool.cue file.

My Usecase

I have a fairly big Kubernetes config that gets generated and validated via CUE. We deploy from a local machine and use gopass as store for our secrets. I already build a small wrapper that lists and fetches the secrets into a CUE _tool environment which is fairly simple and allows to just have non-concrete values inside the config that then get filled automagically. Since some changes inside the config it takes nearly forever to evaluate (way over 2 minutes) and requires tens of gigabytes of memory.

For now I "just" patch a custom built-in into CUE and break the guarantees by directly calling gopass from the custom built-in. This reduced the time from minutes to seconds and is fine for me for now until I either find a better solution or the evaluator has matured more to the point of not taking this much time or us switching to a different way of deploying things.

I already got recommendations of just moving the combination step out to external tools but I exactly use CUE because I can do everything in one tool.

Is that a reasonable thing or did I interpret the use wrong?