lsp-go: Add possibility to wrap gopls binary (mostly for nix)

[ibizaman]

Fixes #1933 for lsp-go.

This allows for example to pick the go version that's provided in a repository's flake.nix or by home-manager or NixOS. This follows the example of lsp-haskell.

In my init.el I have:

(use-package nix-sandbox)

(defun my/nix--lsp-go-wrapper (args)
  (if-let ((sandbox (nix-current-sandbox)))
      (apply 'nix-shell-command sandbox args)
    args))

(setq lsp-go-server-path "gopls"
      lsp-go-server-wrapper-function 'my/nix--lsp-go-wrapper)

One of my repos provides the go binary with such a flake.nix (abridged version):

{
  outputs = { ... }: {
    devShells.default = pkgs.mkShell {
      packages = [ pkgs.go ];
    };
  };
}

And a shell.nix (needed for the nix-sandbox emacs package):

(import (
  let
    lock = builtins.fromJSON (builtins.readFile ./flake.lock);
  in fetchTarball {
    url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
    sha256 = lock.nodes.flake-compat.locked.narHash;
  }
) {
  src =  ./.;
}).shellNix

And also, the gopls binary is provided by home-manager:

$ type gopls
gopls is $HOME/.nix-profile/bin/gopls
$ type go
go not found
$ nix develop

bash-5.2$ type gopls
gopls is $HOME/.nix-profile/bin/gopls
bash-5.2$ type go
go is /nix/store/5p6bk8gmc8m8sm0766nb5qmxwp3f64gj-go/bin/go

Now, if I open a file in that repo, I get the go binary from the flake.nix:

(nix-executable-find (nix-current-sandbox) "go")
"/nix/store/5p6bk8gmc8m8sm0766nb5qmxwp3f64gj-go/bin/go"

(nix-executable-find (nix-current-sandbox) "gopls")
"/Users/pierre/.nix-profile/bin/gopls"

[yyoncho]

(lambda () (lsp-go--server-command)) this could be: lsp-go--server-command

[yyoncho]

Thank you for contributing to lsp-mode! I will do some restructuring to allow the use of this OOTB without additional config.

[ibizaman]

That was a pleasure. Thanks for accepting the PR :)

[toniz4]

Shouldn't lsp-go-server-path be lsp-go-gopls-server-path? Without manually setting lsp-go-server-path lsp-mode doesn't even try to start gopls