nix/doc/manual/rl-next/debugger-on-trace.md

synopsis	prs
Enter the `--debugger` when `builtins.trace` is called if `debugger-on-trace` is set	9914

If the debugger-on-trace option is set and --debugger is given, builtins.trace calls will behave similarly to builtins.break and will enter the debug REPL. This is useful for determining where warnings are being emitted from.