lib/generators: withRecursion: don't break attr-sets with special attrs

[Ma27]

Description of changes

Note: This is an alternative implementation for #168327. Please also read the discussion about the pros and cons of this vs the other PRs in #168327 before deciding which patch is preferable.

cc @Profpatsch @K900 @infinisil @lheckemann

---

Closes #168327

The issue reported there can be demonstrated with the following
expression:

→ nix-instantiate --eval -E "with import ./. {}; pkgs.lib.options.showDefs [ { file = \"foo\"; value = pkgs.rust.packages.stable.buildRustPackages; } ]"
error: attempt to call something which is not a function but a string

       at /home/ma27/Projects/nixpkgs/lib/trivial.nix:442:35:

          441|   isFunction = f: builtins.isFunction f ||
          442|     (f ? __functor && isFunction (f.__functor f));
             |                                   ^
          443|

Basically, if a __functor is in an attribute-set at depth-limit,
__functor will be set to "<unevaluated>". This however breaks
lib.isFunction which checks for a __functor by invoking __functor
with f itself.

The same issue - "magic" attributes being shadowed by withRecursion -
also applies to others such as
__pretty/__functionArgs/__toString.

Since these attributes have a low-risk of causing a stack overflow
(because these are flat attr-sets or even functions), ignoring them in
withRecursion seems like a valid solution.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[Profpatsch]

For the record, I’m not happy with the direction the generators are taking.

Stuff like limiting recursion depth just does not belong into these functions, it should be a separate mechanism. I don’t have enough time to prevent this stuff from happening, but I’m kinda sad about it.

[Profpatsch]

Like, these functions do not do anything complicated, yet they are getting absurdly complex.

[Profpatsch]

I might have misunderstood, withRecursion is outside of toYAML, a new and it looks like undocumented function which I don’t know what it’s for.