Jump to declaration for nixos module `config` attributes

[Atemu]

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nixd is able to jump to NixOS option declarations from the point they're set but not the points where their value is read using the config argument.

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It'd be great if nixd could also jump to option declarations from config.option.name.here

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It's common for config attributes to be put into another variable via let:

{ config, ... }:

let
  cfg = config.foo.bar;
in
{
  bar.foo = cfg.baz;
}

nixd should be able to resolve this redirection and jump to the declaration of options.foo.bar.baz when attempting to jump to the declaration of cfg.baz.

[inclyc]

Generally I don't like to add more heuristics. I'd expect a mathmatical deducing model to address all these stuff.

Much similar problem also reported in #600, where we cannot deal with nixos options declared inside config.

I think finally a type system should be added in Nix language, we can do this in lsp first (like python's type annotation).

@luochen1990 do you have some ideas?

[luochen1990]

A type system in lsp is cool, but there is indeed two different mechanism we need to have if we choose to implement all these in lsp instead of a new language -- the type system for nixos modules, and the type system for normal nix codes. They are very different but intuitively can be fused into a single one, but I don't know how to fuse them yet

[Atemu]

I don't think proper typing is going to happen in Nix any time soon. If work were to start now, I'd expect it to be usable no sooner than a decade later.

I also don't understand how a type system would be relevant here as this should be able to use a similar code-path to the current jump to option declaration; regardless of its type.