Adding error context in Nix

I always have a tough time debugging Nix stack traces, especially when they occur deep in the module system.

Luckily there’s a nice way to add error annotations to the stack trace to help debugging:

$ nix-instantiate \
    -E '(builtins.addErrorContext "Blame the assert!" (assert false; 10))' \
    --show-trace
error: Blame the assert!
assertion failed at (string):1:48

$ nix-instantiate \
    -E '(builtins.addErrorContext "Blame the assert!" (throw "not this time"))' \
    -show-trace
error: Blame the assert!
not this time

We’re not using this a lot in in nixpkgs. Maybe we should use it more often!

$ git grep addErrorContext | wc -l
6