Assertion 'maybeOutputPath' failed in void main_nix_build

[chlohal]

Describe the bug

I have a bug that seems to be triggered by impure derivations. The build crashes with this message:

terminating due to unexpected unrecoverable internal error: Assertion 'maybeOutputPath' failed in void main_nix_build(int, char**) at ../nix-build/nix-build.cc:730
Nix crashed. This is a bug. Please report this at https://github.com/NixOS/nix/issues with the following information included:

std::terminate() called without exception
Stack trace:
 0# 0x00005B20B869C141 in nix-build
 1# 0x00007C4EEE4BF1AA in /nix/store/0p8b2lqk47fvxm9hc6c8mnln5l8x51q1-gcc-14.3.0-lib/lib/libstdc++.so.6
 2# std::unexpected() in /nix/store/0p8b2lqk47fvxm9hc6c8mnln5l8x51q1-gcc-14.3.0-lib/lib/libstdc++.so.6
 3# 0x00007C4EEF2DB0DF in /nix/store/bcy8w5fsd9fjkdzl2h1mwr0c3b03k3wl-nix-util-2.34.6/lib/libnixutil.so.2.34.6
 4# __wrap___assert_fail in nix-build
 5# 0x00005B20B8775F48 in nix-build
 6# 0x00005B20B8714115 in nix-build
 7# nix::handleExceptions(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::fun<void ()>) in /nix/store/bcy8w5fsd9fjkdzl2h1mwr0c3b03k3wl-nix-util-2.34.6/lib/libnixutil.so.2.34.6
 8# 0x00005B20B8671143 in nix-build
 9# 0x00007C4EEE02A4D8 in /nix/store/vr7ds8vwbl2fz7pr221d5y0f8n9a5wda-glibc-2.40-218/lib/libc.so.6
10# __libc_start_main in /nix/store/vr7ds8vwbl2fz7pr221d5y0f8n9a5wda-glibc-2.40-218/lib/libc.so.6
11# 0x00005B20B8675C85 in nix-build

Steps To Reproduce

Make a file test.nix with the following:

(import <nixpkgs> { }).stdenv.mkDerivation {
  name = "reproducer";
  __impure = true;

  buildCommand = ''
    mkdir $out
  '';
}

Run nix-build test.nix. It fails with the given error.

Expected behavior

The derivation should produce an empty output folder

Metadata

nix-env (Nix) 2.34.6

Additional context

The derivation works as expected when __impure = true; is removed

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates
• reviewed the contributing guide

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