Only print the first line of errors in values

[9999years]

Before:

nix-repl> legacyPackages.aarch64-darwin.pythonPackages.APScheduler
«error: Package ‘python-2.7.18.7’ in /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/pkgs/development/interpreters/python/cpython/2.7/default.nix:335 is marked as insecure, refusing to evaluate.

Known issues:
 - Python 2.7 has reached its end of life after 2020-01-01. See https://www.python.org/doc/sunset-python-2/.

You can install it anyway by allowing this package, using the
following methods:

a) To temporarily allow all insecure packages, you can use an environment
   variable for a single invocation of the nix tools:

     $ export NIXPKGS_ALLOW_INSECURE=1

   Note: When using `nix shell`, `nix build`, `nix develop`, etc with a flake,
         then pass `--impure` in order to allow use of environment variables.

b) for `nixos-rebuild` you can add ‘python-2.7.18.7’ to
   `nixpkgs.config.permittedInsecurePackages` in the configuration.nix,
   like so:

     {
       nixpkgs.config.permittedInsecurePackages = [
         "python-2.7.18.7"
       ];
     }

c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
   ‘python-2.7.18.7’ to `permittedInsecurePackages` in
   ~/.config/nixpkgs/config.nix, like so:

     {
       permittedInsecurePackages = [
         "python-2.7.18.7"
       ];
     }

»

After:

nix-repl> legacyPackages.aarch64-darwin.pythonPackages.APScheduler
«error: Package ‘python-2.7.18.7’ in /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/pkgs/development/interpreters/python/cpython/2.7/default.nix:335 is marked as insecure, refusing to evaluate.»

(Probably nix repl should print the whole error for top-level errors, but that's a separate issue. Update: Fixed in #10209.)

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[thufschmitt]

I like this a lot (these multi-line errors embedded in the printed expressions are just unreadable), but we should probably hint somehow that there's more stuff in the error message and give people a way to see it in full

[9999years]

give people a way to see it in full

Yes, they can print the expression directly (see #10209). I'm not sure it's necessary to indicate that there's more content -- these previews are mostly just intended to show that a certain sub-expression (list item or attribute value) had an error. We don't show the stack trace either (see #9928).

[9999years]

I also think it would be nice to have an option to configure this behavior, but that can be added later.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-03-11-nix-team-meeting-132/42960/1