Great! It seems to work wonderfully, when importing overlays that does not need inputs.

But how can you handle with overlays that require inputs like this?

hmm, at that point its a file expected to be an overlay. I don't know of a way to distinguish moduleArgs -> overlay from overlay. After one application, both are functions, and after two applications the overlay needs to use the params, so I need the real args.

I guess it can be deffered to overlay evaluation time like i did in bundlers. I could potentially make that work but even then, the preferred way to write that would be:

final: prev: (import prev.inputs.rust-overlay) final prev

that should already work.
I'll see if i can make the proposed y
syntax work though

Yet another idea. Supporting wildcards in the alias like this:

nixDirAliases = {
  nixosModules = [ "nixos/modules/*"]
};

So you could support the layout:

The right side can be a path, yes; problem is the left side which doesn't support nesting.
I was considering having it recurse, but even then not sure if that would work since the key is not a concrete option.

for that you could write (assuming the dirs are dirs you want to load as attrs):

devShells = {
  ansible = flakelight.importDir "ansible/nix";
  autotest = flakelight.importDir "autotest/nix";
  dbusmock = flakelight.importDir "dbus-mock/nix";
};

or as I'd more likely write:

devShells = lib.genAttrs [ "ansible" "autotest" "dbus-mock" ] (n: flakelight.importDir "${n}/nix";

The right side can be a path, yes; problem is the left side which doesn't support nesting.
I was considering having it recurse, but even then not sure if that would work since the key is not a concrete option.

for that you could write (assuming the dirs are dirs you want to load as attrs):

devShells = {
  ansible = flakelight.importDir "ansible/nix";
  autotest = flakelight.importDir "autotest/nix";
  dbusmock = flakelight.importDir "dbus-mock/nix";
};

or as I'd more likely write:

devShells = lib.genAttrs [ "ansible" "autotest" "dbus-mock" ] (n: flakelight.importDir "${n}/nix");

The right side can be a path, yes; problem is the left side which doesn't support nesting.
I was considering having it recurse, but even then not sure if that would work since the key is not a concrete option.

for that you could write (assuming the dirs are dirs you want to load as attrs):

devShells = {
  ansible = flakelight.importDir "ansible/nix";
  autotest = flakelight.importDir "autotest/nix";
  dbusmock = flakelight.importDir "dbus-mock/nix";
};

or as I'd more likely write:

devShells = lib.genAttrs
  [ "ansible" "autotest" "dbus-mock" ]
  (n: flakelight.importDir "${n}/nix");

Yeah, for this usecase, I'd recommend putting the namespace in the file name rather than dir since with dirs its different in the filesystem and the nix code. nix flake show or nix eval .#nixosModules --apply __attrNames would show all of them without namespacing. IMO making it .#nixosModules.desktops-gnome.nix would be more descriptive.

though if you want that layout, i'd set nixDirAliases.nixosModules = [ "nixos/modules" ] and then have nixos/modules/default.nix be:

{flakelight, lib, ...}:
lib.pipe ./. [
  builtins.readDir
  (lib.filterAttrs (_: v: v == "directory")
  lib.attrNames
  (map (d: flakelight.importDir (./. + d)))
  (lib.foldr (a: b: a // b) {})
] 

I'm not sure how the glob would match in general, and what to do about matching files etc. I think this case would be good to specify how it should be loaded.

I guess the idea is just to use directories to better organize modules.

Your suggestion with using default.nix is the exact thing I want to minimize and ideally avoid.
It is hard to read for users without any Nix experience (I really try to keep the Nix files simple, so they are primarily declarative), and again it is boilerplate, that ideally it could be fully avoided with e.g. the wildcard feature.

I guess the idea is just to use directories to better organize modules.

Your suggestion with using default.nix is the thing I want to minimize and ideally avoid.
It is hard to read for users without any Nix experience (I really try to keep the Nix files simple, so they are primarily declarative), and again it is boilerplate, that ideally it could be fully avoided with e.g. the wildcard feature.