defaultApp should have type 'derivation' - add better error message?

[patryk4815]

Describe the bug
Hi. Can we add better error message? I just wasted a couple of hours finding out why that doesn't work on nix 2.8.

error: attribute 'defaultApp.aarch64-linux' should have type 'derivation'

Steps To Reproduce

[thufschmitt]

I think there’s a plain bug here (I guess a consequence of #6389): defaultApp should be an app, not a plain derivation.

That being said, what would you expect as an error message here? Agree that the current one isn’t optimal, but I’d take any input as to how it could be improved ;)

[tomberek]

We played around with language like “Apps can only be in and must be in the apps output attribute and have type app”. It’s an if-and-only-if situation.

The other issue is the usage of defaultApp which is being deprecated and probably led to more confusion. Currently only “app.” should have apps.

[thufschmitt]

The other issue is the usage of defaultApp which is being deprecated and probably led to more confusion. Currently only “app.” should have apps.

I think that even if it’s deprecated, it would be nice to have `defaultApp` be checked properly (I mean it’s _only_ deprecated, not removed)

[wlcx]

For anyone searching for this error due to using nix 2.8 with a flake.nix with defaultApp, e.g. something like the below:

rec {
 # snip
 packages.my-project = naersk-lib.buildPackage {
    pname = "my-project";
    root = ./.;
  };
  defaultPackage = packages.my-project;

  # `nix run`
  apps.my-project = utils.lib.mkApp {
    drv = packages.my-project;
  };
  defaultApp = apps.my-project;
  # snip
}

(taken directly from the readme of nix-community/naersk)

You should replace defaultApp with just apps.default. Same with defaultPackage, though that seems to be more lenient with what it accepts. So, instead of the above, something like:

# snip
rec {
  packages.default = naersk-lib.buildPackage {
    pname = "my-project";
    root = ./.;
  };

  apps.default = utils.lib.mkApp { drv = packages.default;}; 
}
# snip