[Nix-dev] warning if trying to install a package but it fails an assert.

[Erick Tryzelaar]

I had a problem today trying to build git, but nix-env said that there
was no matching derivation:

> nix-env -f . -i git
error: selector `git' matches no derivations

It took some time to track down what was going on with the help of
#nixos. Turns out when I tried to enable git-svn support in  my
.nixpkgs/config.nix:

{
  git = {
    svnSupport = true;
    guiSupport = true;
    useEmacs = false;
  };
}

I left out setting the svn perl bindings:

  subversion = {
    perlBindings = true;
  };

This was tripping this assert in .../git/default.nix:

assert svnSupport -> (subversion != null && perlLibs != [] &&
subversion.perlBindings);

But there was no warning or message explaining what was going on. It
would help me a lot next time I run into this if nix gave me a more
detailed  explanation of what happened. Any chance this could be
added?