libexpr: add sha256 noop argument to fetch

[zimbatm]

Nix 1.12 is going to support that argument fully. This addition is only
there to allow forward-compatibility.

[edolstra]

This seems dangerous to me. It creates a false impression that the hash is verified.

[zimbatm]

I'm trying to find a solution because 1.11 is going to be around for a long time. The most ideal would to backport, but that was also rejected in #1299.

The main use-case I am trying to solve is to pin nixpkgs. Something like:

let
  repo = builtins.fromJSON (builtins.readFile ./src.json);
in
  builtins.fetchTarball {
    url = "https://github.com/${repo.owner}/${repo.repo}/tarball/${repo.rev}";
    #sha256 = repo.sha256;
  }

(also any solution that imports <nixpkgs> doesn't work when NIX_PATH is being set to that file).

But because the sha256 is missing, github.com gets hit on each invocation. There is also a workaround with the cache TTL that has it's own set of problems.

Even if it's introduced in 1.12, not everybody is going to upgrade overnight. Also the release date for 1.12 is unknown. I want a backport that can be pushed to the 17.03 and 17.09 channels so we can start use this argument, even if it's just a noop.

[zimbatm]

Would it be acceptable if that argument would emit some sort of warning instead?

[zimbatm]

Closing this as @taktoa's solution is a viable workaround: https://gist.github.com/taktoa/4ec8b0cb6d07857400b07a830ad99558