nixpkgs/neovim: show diagnostic code in float

.config/nixpkgs/neovim/lsp.lua

@@ -1,6 +1,16 @@
--- only show virtual text for WARN and higher
 vim.diagnostic.config({
-  virtual_text = { severity = { min = vim.diagnostic.severity.WARN } }
+  -- only show virtual text for WARN and higher
+  virtual_text = { severity = { min = vim.diagnostic.severity.WARN } },
+  -- show diagnostic code in float if available
+  float = {
+    prefix = function(diagnostic, i)
+      if diagnostic.code == nil then
+        return string.format("[%s] ", vim.diagnostic.severity[diagnostic.severity])
+      else
+        return string.format("[%s] ", diagnostic.code)
+      end
+    end,
+  },
 })
 
 local nvim_lsp = require('lspconfig')