feat(#2415): add :NvimTreeHiTest (#2664)

* feat(#2415): add :NvimTreeHiTest

* feat(#2415): split out appearance diagnostics

lua/nvim-tree/appearance/diagnostics.lua

@@ -0,0 +1,80 @@
+local appearance = require "nvim-tree.appearance"
+
+local M = {}
+
+---@class HighlightDisplay for :NvimTreeHiTest
+---@field group string nvim-tree highlight group name
+---@field links string link chain to a concretely defined group
+---@field def string :hi concrete definition after following any links
+local HighlightDisplay = {}
+
+---@param group string nvim-tree highlight group
+---@return HighlightDisplay
+function HighlightDisplay:new(group)
+  local o = {}
+  setmetatable(o, self)
+  self.__index = self
+
+  o.group = group
+  local concrete = o.group
+
+  -- maybe follow links
+  local links = {}
+  local link = vim.api.nvim_get_hl(0, { name = o.group }).link
+  while link do
+    table.insert(links, link)
+    concrete = link
+    link = vim.api.nvim_get_hl(0, { name = link }).link
+  end
+  o.links = table.concat(links, " ")
+
+  -- concrete definition
+  local ok, res = pcall(vim.api.nvim_cmd, { cmd = "highlight", args = { concrete } }, { output = true })
+  if ok and type(res) == "string" then
+    o.def = res:gsub(".*xxx *", "")
+  else
+    o.def = ""
+  end
+
+  return o
+end
+
+function HighlightDisplay:render(bufnr, fmt, l)
+  local text = string.format(fmt, self.group, self.links, self.def)
+
+  vim.api.nvim_buf_set_lines(bufnr, l, -1, true, { text })
+  vim.api.nvim_buf_add_highlight(bufnr, -1, self.group, l, 0, #self.group)
+end
+
+---Run a test similar to :so $VIMRUNTIME/syntax/hitest.vim
+---Display all nvim-tree highlight groups, their link chain and actual definition
+function M.hi_test()
+  local displays = {}
+  local max_group_len = 0
+  local max_links_len = 0
+
+  -- build all highlight groups, name only
+  for _, highlight_group in ipairs(appearance.HIGHLIGHT_GROUPS) do
+    local display = HighlightDisplay:new(highlight_group.group)
+    table.insert(displays, display)
+    max_group_len = math.max(max_group_len, #display.group)
+    max_links_len = math.max(max_links_len, #display.links)
+  end
+
+  -- create a buffer
+  local bufnr = vim.api.nvim_create_buf(false, true)
+
+  -- render and highlight
+  local l = 0
+  local fmt = string.format("%%-%d.%ds %%-%d.%ds %%s", max_group_len, max_group_len, max_links_len, max_links_len)
+  for _, display in ipairs(displays) do
+    display:render(bufnr, fmt, l)
+    l = l + 1
+  end
+
+  -- finalise and focus the buffer
+  vim.api.nvim_buf_set_option(bufnr, "modifiable", false)
+  vim.cmd.buffer(bufnr)
+end
+
+return M