Use :Inspect for checking highlights

:Inspect will check highlights at the cursor postion. This can help you figure out what text is affected by.

Inspecting lua state

You can run lua from the command line:

:lua vim.notify('hi!')

You can inspect any global/requireable variables from the command line

:=require('lazy')

This is shorthand for:

:lua vim.print(require('lazy'))

vim.print(...) is a wrapper around print(vim.inspect(...)) which also returns the arguments that are passed in. If you get an error and properly follow the stacktrace, you can wrap anything you want to inspect with it:

local foo = bar:gsub("asdf", "hjkl")
--          ^^^^^^^^ let's say you get an error here:
-- foo is unaffected no matter what you wrap with vim.print:
local foo = vim.print(bar):gsub("asdf", "hjkl")
-- 

Get familiar with stdpath()s

Neovim has documented “standard paths” that are used by Neovim and plugins to store things in consistent locations, defaulting to sensible locations on Unix/Windows. Largely speaking, lazy.nvim installs plugins under the data directory, so for lazy.nvim users, if you get a stacktrace where you see an error for a plugin, but can’t see the full directory name, you likely want to start under the result of:

:echo stdpath('data')