Enter debugger on builtins.trace with an option

[9999years]

Motivation

With --debugger --debugger-on-trace, builtins.trace calls will start the interactive debugger, exposing stack trace information.

Context

Closes #1610.

I recently complained that my Nix project was emitting warnings with no clear provenance:

$ nix develop
trace: warning: `pythonForBuild` (from `python*`) has been renamed to `pythonOnBuildForHost`
$ rg pythonForBuild | wc -l
0

With this patch, it's easy to print a backtrace:

nix develop --debugger --debugger-on-trace
trace: warning: `pythonForBuild` (from `python*`) has been renamed to `pythonOnBuildForHost`
Welcome to Nix 2.21.0pre20240131_dirty. Type :? for help.

nix-repl> :bt

0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
/nix/store/hg65h51xnp74ikahns9hyf3py5mlbbqq-source/overrides/default.nix:132:27

   131|
   132|       bootstrappingBase = pkgs.${self.python.pythonAttr}.pythonForBuild.pkgs;
      |                           ^
   133|     in

Then, we can look at the source path to see that it's a checkout of poetry2nix:

$ head /nix/store/hg65h51xnp74ikahns9hyf3py5mlbbqq-source/README.md
[![](https://github.com/nix-community/poetry2nix/workflows/CI/badge.svg)](https://github.com/nix-community/poetry2nix/actions?query=branch%3Amaster+workflow%3ACI)
[![Chat on Matrix](https://matrix.to/img/matrix-badge.svg)](https://matrix.to/#/#poetry2nix:blad.is)

# poetry2nix

Nice!

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[9999years]

Added a changelog entry.

Note: If this PR is merged, #9916 should be merged as well or the behavior of :quit becomes confusing.

[fricklerhandwerk]

Please add links to documentation of debugger and builtins.trace. People who start reading from the command reference otherwise won't know what either means or where to find that information.

[pennae]

splitting it like this could also be a source of confusion since --debugger is not implied. perhaps it'd be better to implicitly enable the debugger when this setting is set and change the name to something that signifies such, eg --debugger-on-trace?

[9999years]

Done.