Recently, the minimal Nix version was bumped to 2.0. There are several drawbacks to this:
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It has only been 6 months since Nix 2.0 was initially released. This is a very aggressive timeline.
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Users of less than 1 year will find their installations unable to support Nixpkgs.
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Those users would solve this by trying to upgrade Nix. But this is now impossible using Nix 1 and the current HEAD of nixpkgs.
$ nix-env -f https://github.com/NixOS/nixpkgs/archive/master.tar.gz -iA nix error: evaluation aborted with the following error message: ‘ This version of Nixpkgs requires Nix >= 2.0, please upgrade: -
By extension, you can not upgrade directly from NixOS 17.09 to 18.09.
The pros stated in the PR are “Placeholders are just too convenient,” but I understand there are likely others and would like to hear them. Please provide your thoughts and discuss whether this requirement should be kept or reverted before 18.09 is considered stable.