feat(statusline): vim.diagnostic.status() #33723

Problem:
Not easy to get a status string for diagnostics.

Solution:
- Add vim.diagnostic.status().
- Add it to the default 'statusline'.

runtime/lua/vim/diagnostic.lua

@@ -2840,4 +2840,42 @@ function M.fromqflist(list)
   return diagnostics
 end
 
+--- Returns formatted string with diagnostics for the current buffer.
+--- The severities with 0 diagnostics are left out.
+--- Example `E:2 W:3 I:4 H:5`
+---
+--- To customise appearance, set diagnostic signs text with
+--- ```lua
+--- vim.diagnostic.config({
+---   signs = { text = { [vim.diagnostic.severity.ERROR] = 'e', ... } }
+--- })
+--- ```
+---@param bufnr? integer Buffer number to get diagnostics from.
+---                      Defaults to 0 for the current buffer
+---
+---@return string
+function M.status(bufnr)
+  vim.validate('bufnr', bufnr, 'number', true)
+  bufnr = bufnr or 0
+  local counts = M.count(bufnr)
+  local user_signs = vim.tbl_get(M.config() --[[@as vim.diagnostic.Opts]], 'signs', 'text') or {}
+  local signs = vim.tbl_extend('keep', user_signs, { 'E', 'W', 'I', 'H' })
+  local result_str = vim
+    .iter(pairs(counts))
+    :map(function(severity, count)
+      return ('%s:%s'):format(signs[severity], count)
+    end)
+    :join(' ')
+
+  return result_str
+end
+
+vim.api.nvim_create_autocmd('DiagnosticChanged', {
+  group = vim.api.nvim_create_augroup('nvim.diagnostic.status', {}),
+  callback = function(ev)
+    vim.api.nvim__redraw({ buf = ev.buf, statusline = true })
+  end,
+  desc = 'diagnostics component for the statusline',
+})
+
 return M