fix(lsp): unclear error on malformed server responses #40422

Problem:
Malformed server responses with `null` (vim.NIL) values, do not show
clear error messages.


Solution:
Assert two known, specific cases, to avoid user confusion.

runtime/lua/vim/diagnostic/_float.lua

@@ -211,6 +211,7 @@ function M.open(opts, ...)
 
     --- @type lsp.DiagnosticRelatedInformation[]
     local related_info = vim.tbl_get(diagnostic, 'user_data', 'lsp', 'relatedInformation') or {}
+    assert(related_info ~= vim.NIL, 'server response has invalid (null) relatedInformation')
     -- Below the diagnostic, show its LSP related information (if any) in the form of file name and
     -- range, plus description.
     for _, info in ipairs(related_info) do

runtime/lua/vim/lsp/diagnostic.lua

@@ -83,6 +83,7 @@ end
 --- @return table?
 local function tags_lsp_to_vim(diagnostic, client_id)
   local tags ---@type table?
+  assert(diagnostic.tags ~= vim.NIL, 'server response has invalid (null) tags')
   for _, tag in ipairs(diagnostic.tags or {}) do
     if tag == protocol.DiagnosticTag.Unnecessary then
       tags = tags or {}

test/functional/lua/diagnostic_spec.lua

@@ -3657,6 +3657,22 @@ describe('vim.diagnostic', function()
       )
     end)
 
+    it('errors when LSP relatedInformation is null', function()
+      matches(
+        'server response has invalid %(null%) relatedInformation',
+        pcall_err(
+          exec_lua,
+          [[
+          local diagnostic = _G.make_warning('Some warning', 1, 1, 1, 3)
+          diagnostic.user_data = { lsp = { relatedInformation = vim.NIL } }
+          vim.api.nvim_win_set_buf(0, _G.diagnostic_bufnr)
+          vim.diagnostic.set(_G.diagnostic_ns, _G.diagnostic_bufnr, { diagnostic })
+          vim.diagnostic.open_float({ header = false, scope = 'buffer' })
+        ]]
+        )
+      )
+    end)
+
     it('works with the old signature', function()
       eq(
         { '1. Syntax error' },

test/functional/plugin/lsp/diagnostic_spec.lua

@@ -7,6 +7,8 @@ local clear = n.clear
 local exec_lua = n.exec_lua
 local eq = t.eq
 local neq = t.neq
+local matches = t.matches
+local pcall_err = t.pcall_err
 
 local create_server_definition = t_lsp.create_server_definition
 
@@ -300,6 +302,20 @@ describe('vim.lsp.diagnostic', function()
         end)
       )
     end)
+
+    it('errors when diagnostic tags is null', function()
+      matches(
+        'server response has invalid %(null%) tags',
+        pcall_err(exec_lua, function()
+          vim.lsp.diagnostic.on_publish_diagnostics(nil, {
+            uri = fake_uri,
+            diagnostics = {
+              vim.tbl_extend('force', _G.make_error('Diagnostic', 0, 0, 0, 0), { tags = vim.NIL }),
+            },
+          }, { client_id = client_id })
+        end)
+      )
+    end)
   end)
 
   describe('vim.lsp.diagnostic.on_diagnostic', function()