There’s several options, depending on what you want. The most commonly used option is the language server.

Oh nice, didn't know about it. (I have migrated from vim to neovim and half of it doesn't work for me yet)

I wonder why this is preferred over codegen (during build), possibly using some kind of annotations?

We've not had any proposals for a design like that. We are open to proposals though! I wrote a blog post detailing the process here: https://lpil.uk/blog/how-to-add-metaprogramming-to-gleam/

The language server code action :)