Autocompletion options for Nix (nixd) (#4593)

* Nix autocompletion options

* Support for formatting with nixd

* Revert nixd lsp name

clients/lsp-nix.el

@@ -54,9 +54,57 @@
   :type 'string
   :package-version '(lsp-mode . "8.0.0"))
 
+(lsp-defcustom lsp-nix-nixd-formatting-command nil
+  "External formatter command with arguments.
+
+  Example: [ \"nixpkgs-fmt\" ]"
+  :type 'lsp-string-vector
+  :group 'lsp-nix-nixd
+  :lsp-path "nixd.formatting.command"
+  :package-version '(lsp-mode . "9.0.1"))
+
+(lsp-defcustom lsp-nix-nixd-nixpkgs-expr nil
+  "This expression will be interpreted as \"nixpkgs\" toplevel
+  Nixd provides package, lib completion/information from it.
+
+  Resource Usage: Entries are lazily evaluated, entire nixpkgs takes 200~300MB
+  for just \"names\". Package documentation, versions, are evaluated by-need.
+
+  Example: \"import <nixpkgs> { }\""
+  :type 'string
+  :group 'lsp-nix-nixd
+  :lsp-path "nixd.nixpkgs.expr"
+  :package-version '(lsp-mode . "9.0.1"))
+
+(lsp-defcustom lsp-nix-nixd-nixos-options-expr nil
+  "Option set for NixOS option completion. If this is omitted, the default
+  search path (<nixpkgs>) will be used.
+
+  Example:
+  \"(builtins.getFlake \"/home/lyc/flakes\").nixosConfigurations.adrastea.options\""
+  :type 'string
+  :group 'lsp-nix-nil
+  :lsp-path "nixd.options.nixos.expr"
+  :package-version '(lsp-mode . "9.0.1"))
+
+(lsp-defcustom lsp-nix-nixd-home-manager-options-expr nil
+  "Option set for home-manager option completion.
+
+  Example:
+  \"(builtins.getFlake \"/home/lyc/flakes\").nixosConfigurations.adrastea.options\""
+  :type 'string
+  :group 'lsp-nix-nil
+  :lsp-path "nixd.options.home-manager.expr"
+  :package-version '(lsp-mode . "9.0.1"))
+
 (lsp-register-client
  (make-lsp-client :new-connection (lsp-stdio-connection (lambda () lsp-nix-nixd-server-path))
                   :major-modes '(nix-mode nix-ts-mode)
+                  :initialized-fn (lambda (workspace)
+                                    (with-lsp-workspace workspace
+                                      (lsp--set-configuration
+                                       (lsp-configuration-section "nixd"))))
+                  :synchronize-sections '("nixd")
                   :server-id 'nixd-lsp
                   :priority -1))