lang/nix: add ivy version of helm-nixos-options

And make it nix-mode's lookup-documentation handler, so pressing `K` on
an option will prefill the ivy search.

modules/lang/nix/autoload.el

@@ -0,0 +1,41 @@
+;;; lang/nix/autoload.el -*- lexical-binding: t; -*-
+
+(defun +nix--options-action (candidate)
+  (switch-to-buffer-other-window
+   (nixos-options-doc-buffer
+    (nixos-options-get-documentation-for-option candidate))))
+
+;;;###autoload
+(defun +nix/lookup-option (&optional initial-input)
+  "Look up documentation on a nix option."
+  (interactive
+   (list
+    ;; REVIEW Must be a better way to do this
+    (when (and (looking-at-p "[a-zA-Z0-9-_\\.]")
+               (not (doom-point-in-string-or-comment-p)))
+      (buffer-substring-no-properties
+       (save-excursion
+         (skip-chars-backward "^ ")
+         (point))
+       (save-excursion
+         (skip-chars-forward "^ ")
+         (point))))))
+  (cond ((featurep! :completion helm)
+         (require 'helm-nixos-options)
+         ;; REVIEW We reimplment `helm-nixos-options' so we can supply
+         ;; `initial-input'. Maybe use `helm-attrset' instead?
+         (helm :sources `(,(helm-source-nixos-options-search))
+               :buffer "*helm-nixos-options*"
+               :input initial-input))
+        ((featurep! :completion ivy)
+         (require 'nixos-options)
+         (ivy-read "NixOS options: "
+                   nixos-options
+                   :require-match t
+                   :initial-input initial-input
+                   :action #'+nix--options-action
+                   :caller '+nix/options))
+        ;; TODO Add general `completing-read' support
+        ((user-error "No search engine is enabled. Enable helm or ivy!")))
+  ;; Tell lookup module to let us handle things from here
+  'deferred)