Fix completion crash on plugin suggestions without default_args

apps/language_server/lib/language_server/providers/completion.ex

@@ -592,11 +592,14 @@ defmodule ElixirLS.LanguageServer.Providers.Completion do
     capture_before? = context.capture_before?
 
     for s <- suggestions do
+      # suggestions from third party plugins may not set optional keys like default_args
+      default_args = Map.get(s, :default_args) || 0
+
       default_arg_variants =
-        if s.type in [:function, :macro] and s.default_args > 0 do
+        if s.type in [:function, :macro] and default_args > 0 do
           max_arity_name = function_name_with_parens?(s.name, s.arity, locals_without_parens)
 
-          for i <- s.default_args..1//-1,
+          for i <- default_args..1//-1,
               capture_before? or !signature_help_supported or
                 max_arity_name !=
                   function_name_with_parens?(s.name, s.arity - i, locals_without_parens) do

apps/language_server/test/providers/completion_test.exs

@@ -45,6 +45,24 @@ defmodule ElixirLS.LanguageServer.Providers.CompletionTest do
     assert first_item.preselect == true
   end
 
+  test "plugin function suggestion without default_args does not crash completion" do
+    text = """
+    defmodule MyModule do
+      def fun do
+        legacy_plugin_fun
+      end
+    end
+    """
+
+    {line, char} = {3, 22}
+    parser_context = ParserContextBuilder.from_string(text, {line, char})
+
+    {:ok, %GenLSP.Structures.CompletionList{items: items}} =
+      Completion.completion(parser_context, line, char, @supports)
+
+    assert Enum.any?(items, &(&1.label == "legacy_plugin_fun"))
+  end
+
   test "keyword param option text edit uses 0-based LSP positions" do
     text = """
     defmodule MyModule do

apps/language_server/test/support/plugins/legacy_format_plugin.ex

@@ -0,0 +1,30 @@
+defmodule ElixirLS.LanguageServer.Test.LegacyFormatPlugin do
+  @moduledoc false
+  # Emits a function suggestion in the shape of older third party plugins
+  # (no default_args key). Only reacts to the hint `legacy_plugin_fun`.
+  @behaviour ElixirLS.LanguageServer.Plugin
+
+  @impl true
+  def reduce("legacy_plugin_fun", _env, _buffer_metadata, _cursor_context, acc) do
+    suggestion = %{
+      type: :function,
+      name: "legacy_plugin_fun",
+      arity: 1,
+      def_arity: 1,
+      args: "value",
+      args_list: ["value"],
+      origin: "Legacy.Plugin.Builtins",
+      spec: "@spec legacy_plugin_fun(value :: atom()) :: atom()",
+      metadata: %{},
+      snippet: nil,
+      summary: "A legacy plugin function",
+      needed_import: nil,
+      needed_require: nil,
+      visibility: :public
+    }
+
+    {:cont, %{acc | result: acc.result ++ [suggestion]}}
+  end
+
+  def reduce(_hint, _env, _buffer_metadata, _cursor_context, acc), do: {:cont, acc}
+end