mill: removing MILL_VERSION variable

[scalavision]

Motivation for this change

MILL_VERSION block the use of previous installed versions of mill. When removing this variable you can do:

MILL_VERSION=0.7.4 mill myproject.run

This has been tested manually by first installing 0.7.4, then 0.8.0, and running the command above.

Things done

• [x ] Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • [x ] NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• [x ] Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• [x ] Ensured that relevant documentation is up to date
• [x ] Fits CONTRIBUTING.md.

[veprbl]

Do you think you should use --set-default instead?

nixpkgs/pkgs/build-support/setup-hooks/make-wrapper.sh

Lines 18 to 19 in 6e9b71f

	# --set-default VAR VAL : like --set, but only adds VAR if not already set in
	# the environment

[scalavision]

thanks for the feedback, I didn't know about that setting :-)

There is a variable setting the default version, here in the mill launcher script:

https://github.com/lihaoyi/mill/blob/master/mill#L6

Isn't it cleaner to let the tool handle this, and let the user override it if necessary? I have no strong opinions about this though. Is there any advantage to let nix handle this? Will it be more reproducible / reliable?

The reason I removed it, was because a user wanted to be able to handle this himself. I agree with that, and adding this was probably a mistake on my part.

[veprbl]

I assumed that it was set in the wrapper for a reason. If you believe that we don’t need it anymore, then let’s remove it.

[m-a-t]

The .mill-wrapped script will call the installed mill version (given by ${version}) by default, there is no replacement necessary for the deleted line.
Removing this line was requested by me, as it allows to use a different mill version as explained in http://www.lihaoyi.com/mill/#overriding-mill-versions

@scalavision You can even override mill version to be a version you never downloaded before, mill will automatically fetch it.

I tested it locally and I would be happy if this could be merged.

[veprbl]

LGTM