Add nix-run and myEnvRun

[rickynils]

This adds nix-run, which is a thin wrapper around nix-build.
nix-run calls nix-build, and then executes the resulting build.
If no executable artifact is built, nix-runs outputs an error
message.

myEnvRun calls myEnvFun and builds a script that directly runs
the load-env-* script.

Together, nix-run and myEnvRun allows you to set up an environment
that can be loaded in this way:

envs.nix:
{
gcc = myEnvRun {
name = "gcc";
buildInputs = [ gcc ];
};
}

$ nix-run -A gcc envs.nix

You end up directly in your environment without having to do
nix-env -i. You will always have a fresh environment and you
don't have to pollute you profile with a lot of env packages.

[edolstra]

Hm, I'm still not entirely convinced about the use case for this package. Is this really all that much better than saying nix-build -A gcc; ./result/bin/gcc? Apart from that:

The nix-run package lacks meta attributes. In particular, a longDescription would be useful given that there is no other documentation.

Filenames should not be in camel case, so please use my-env-run.nix instead of myEnvRun.nix. Also, since it's not a package itself, it should be moved to build-support.

[rickynils]

@edolstra I'll add metadata and documentation once we think this is good to have.

I'm open for other suggestions. What I want is what you get today with myEnvFun, but without having to do nix-env -i, because you end up with an outdated environment.

[the-kenny]

I like the idea of adding nix-run. It's a good way to run programs you just need once (like openssl to generate an ssl certificate) without cluttering your profile (via nix-env) or the filesystem (nix-build, result symlink).

[bjornfor]

If it doesn't do much more than nix-build -A foo; ./result/bin/foo (which I use a lot) I'm afraid I don't see the point in it. I never run nix-env -i for testing temporary stuff, I use nix-build and execute.

[ocharles]

I don't see the draw back with this contribution. It resolves itches two people have, and doesn't require any real changes to anything we already have. So it's a +1 from me.

[rickynils]

I've moved myEnvRun.nix to build-support/my-env-run/default.nix and added meta information.

[shlevy]

@edolstra Let me know if you think I should revert, but I think this is general enough to be useful to some in the community.

[edolstra]

Well, we now have nix-shell -p, e.g.

$ nix-shell -p hello --command hello
Hello, world!

Doesn't that remove the need for this function?

[shlevy]

Ah true, that pretty much covers this. @rickynils do you still think there's a use for nix-run or myEnvRun now that nix-shell has -p?

[rickynils]

@shlevy @edolstra I guess nix-shell could suffice, and it is more general.

[shlevy]

Reverted in a330e24