Debugger cannot get locals in some instances

[lf-]

cc @DieracDelta this was the bug I was talking about

Describe the bug

The debugger sometimes doesn't have locals. This can be quite frustrating and it is a major reason I don't reach for the debugger as much as I'd like to.

Steps To Reproduce

bug.nix:

let r = [];
x = builtins.throw r;
in x

$ nix eval --debugger -f bug.nix
error: cannot coerce a list to a string

       at /home/jade/dev/nixpkgs/bug.nix:2:5:

            1| let r = [];
            2| x = builtins.throw r;
             |     ^
            3| in x


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

Welcome to Nix 2.13.3. Type :? for help.

nix-repl> r
error: undefined variable 'r'

       at «string»:1:1:

            1| r
             | ^

nix-repl> :env
Env level 0
builtins true false null scopedImport import isNull break abort throw derivationStrict placeholder baseNameOf dirOf removeAttrs map toString fetchMercurial fetchTree
 fetchTarball fetchGit fromTOML derivation 

Expected behavior

I should be able to get the value of r

nix-env --version output

nix-env (Nix) 2.13.3

Additional context

Add any other context about the problem here.

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[kjeremy]

I can reproduce this with:
a.nix

let a = 1; b = 2; in builtins.break b == 2

nix eval --debugger -f a.nix

[lf-]

Yeah I've hit many cases isomorphic to yours @kjeremy. I think it might be that the variables are defined at the top level or something? The exact conditions that trigger this aren't clear but this bug really sucks.

[lf-]

possible workaround: (_: builtins.break _) some-expr