nil

This is inherently a Hard Problem™, since completions may require evaluating arbitrary derivations (e.g. building a custom Linux kernel). For "what symbols are available", the nil LSP implementation[1] works for anything in scope that doesn't require evaluation. It also includes completions for the stdlib and NixOS options (in certain contexts). Another LSP implementation is nixd[2], which is trying to tackle the problem of evaluations for completion. [1] https://github.com/oxalica/nil/ [2] https://github.com/nix-community/nixd

I most frequently used https://github.com/oxalica/nil and there is a C++ one I haven't tried that reuses code directly from the NixCpp implementation.

This looks nice! As someone interested in developing a client for it, I'm interested in couple of things: what are the features supported by it currently, the tweak-able configuration that can be passed to it and the various code action available. I like the way nil language server has documented it (https://github.com/oxalica/nil/tree/main/docs). Is there something equivalent available for this ?