`builtins.trace` should print lambda location

[roberth]

Is your feature request related to a problem? Please describe.

There's no reason why trace shouldn't be able to do what the repl printer can:

nix-repl> builtins.trace lib.showOption {}
trace: <LAMBDA>
{ }

nix-repl> lib.showOption
«lambda @ /home/user/h/nixpkgs/lib/options.nix:322:16»

Describe the solution you'd like

At least print the location like the repl does.

More ideas:

• print closure info when debugger is enabled
• when the builtins.trace argument is a thunk that points to a string interpolation, copy the thunk and evaluate the string with a flag that allows the lambda to be rendered as a source location (I don't think we have a nice ast node for this though. Add a new method to Expr for this kind of evaluation?)

Describe alternatives you've considered

Use the debugger.

Additional context

[9999years]

Fixed with #9606.

$ nix repl nixpkgs
Welcome to Nix 2.21.0pre20240131_dirty. Type :? for help.

Loading installable 'flake:nixpkgs#'...
Added 5 variables.
nix-repl> builtins.trace lib.showOption {}
trace: «lambda showOption @ /nix/store/ildml01gk7v91fba35whklllfi343lvq-source/lib/options.nix:411:16»
{ }

[roberth]

Thank you @9999years!

More ideas:

• print closure info when debugger is enabled

Better to be implemented as a repl command instead. This is would be quite a lot of info, and it's easy to get a lambda result by mistake. That'd be annoying.

• when the builtins.trace argument is a thunk that points to a string interpolation, copy the thunk and evaluate the string with a flag that allows the lambda to be rendered as a source location (I don't think we have a nice ast node for this though. Add a new method to Expr for this kind of evaluation?)

Not sure. Idea also covered by

• Structured markup for throw and trace messages that make Nix more robust #7795