nix copy builds drv files

[dasJ]

Describe the bug

nix copy is supposed to copy full closures and not build anything.
However, it does build stuff when copying drv files.

Steps To Reproduce

1. Create broken.nix with

derivation {
  name = "test";
  system = "x86_64-linux";
  builder = "this will error";
}

1. nix copy $(nix-instantiate broken.nix) --to ssh://a-string-that-does-not-matter

Expected behavior

The file should just be copied.

nix-env --version output

nix-env (Nix) 2.4pre20200521_00b562c

Additional context

cc @ajs124

[nixos-discourse]

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

https://discourse.nixos.org/t/to-flake-or-not-to-flake/10047/7

[dasJ]

Doubt anyone will ever change their decision on this "design choice".

[edolstra]

FYI, nix copy has a flag --derivation to copy the derivation rather than its outputs.