Forbid nested debuggers

[9999years]

Motivation

The nested debugger behavior is extremely confusing as a user and challenging to maintain invariants for (see the disclaimers about its behavior we get to remove now). The debugger should more-or-less behave like the REPL.

Before:

$ nix eval --file test.nix --debugger
info: breakpoint reached

      at «none»:0: (source not available)


Starting REPL to allow you to inspect the current state of the evaluator.

Welcome to Nix 2.18.1. Type :? for help.

nix-repl> builtins.throw "what"
error: what


Starting REPL to allow you to inspect the current state of the evaluator.

Welcome to Nix 2.18.1. Type :? for help.

nix-repl>

After:

$ nix eval --file test.nix --debugger
info: breakpoint reached


Starting REPL to allow you to inspect the current state of the evaluator.

Welcome to Nix 2.20.0pre20231222_dirty. Type :? for help.

nix-repl> builtins.throw "what"
error:
       … while calling the 'throw' builtin
         at «string»:1:1:
            1| builtins.throw "what"
             | ^

       error: what

nix-repl>

Priorities and Process

Add :+1: to pull requests you find important.

The Nix maintainer team uses a GitHub project board to schedule and track reviews.