Enter debugger more reliably in let expressions and function calls

[9999years]

Motivation

Currently, if you attempt to evaluate this file with the debugger:

let
  a = builtins.trace "before inner break" (
    builtins.break "hello"
  );
  b = builtins.trace "before outer break" (
    builtins.break a
  );
in
  b

Nix will correctly enter the debugger at builtins.break a, but if you ask it to :continue, it'll skip over the builtins.break "hello" expression entirely:

$ nix eval --file xxx-test.nix --debugger
trace: before outer break
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> :c
trace: before inner break
"hello"

This is because the set of debug traces is empty by the time Nix gets to the second call:

nix/src/libexpr/primops.cc

Line 757 in 0127d54

if (state.debugRepl && !state.debugTraces.empty()) {

With this patch, Nix will correctly enter the debugger both times:

$ nix eval --file xxx-test.nix --debugger
trace: before outer break
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> :c
trace: before inner break
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> :c
"hello"

Context

Fixes #6649.

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[thufschmitt]

Can't we just replace that by

Suggested change

	"while evaluating a '%1%' expression",
	"let"
	"while evaluating a 'let' expression"

and not add the formatting logic in makeDebugTraceStacker?

[thufschmitt]

Ah nvm, I didn't notice that makeDebugTraceStacker was already using hintfmt, just with a hardcoded number of 2 elements 😬 .

It makes sense to make it a bit more generic then

[9999years]

This also colors "let" magenta like the other things in error messages.

[thufschmitt]

Oh, right :) This thing is still too magical for me