simple flake spams "warning: Use stdenv.tests instead. passthru is a mkDerivation detail." on eval

[ckp95]

Steps To Reproduce

I'm trying to follow a Flakes tutorial here:

https://serokell.io/blog/practical-nix-flakes#basic-flake-structure

I got to the point of a trivial Flake that just re-exports the hello package:

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs";
  };

  outputs = { self, nixpkgs }: {
    packages.x86_64-linux.hello = nixpkgs.legacyPackages.x86_64-linux.hello;
  };
}

I ran nix eval .#hello to see what would happen, but it just spammed the following line in my terminal forever until I pressed CTRL-C:

trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.

Expected behavior

I don't know the expected behaviour because I'm still learning Flakes, but whatever it is, it shouldn't spam an unhelpful error forever.

Version info

$ nix --version
nix (Nix) 2.18.1

I run NixOS 23.11

[thufschmitt]

nix eval is a fairly low-level command designed to directly integrate with the language, and will blindly recurse into anything. For some reason, the derivations exposed by Nixpkgs are a cyclic data structure, so trying to directly evaluate a derivation is bound to loop forever.

I think there are three paths from here (in order of preference as far as I'm concerned):

1. Fix Nixpkgs stdenv to not yield infinitely recursive data structures (why does it even need that?)
2. Do like the old nix-instantiate --eval command which didn't deeply evaluate anything by default (and had a --strict mode for that)
3. Keep the current behaviour.

nix eval is in the queue of commands to be stabilized (#7701), so it will soon be a good time to re-visit it

[arianvp]

Worse. this even segfaults on my machine. Very confusing:

arian@Arians-MacBook-Pro mercury-intranet % nix eval nixpkgs#gcc
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
trace: warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
zsh: segmentation fault  nix eval nixpkgs#gcc

[thufschmitt]

@arianvp this was changed just last week: #10200

On Nix master:

 $ nix eval nixpkgs#gcc  
«derivation /nix/store/svc566dmzacxdvdy6d1w4ahhcm9qc8zf-gcc-wrapper-12.3.0.drv»