Filter modules during collection in IEx autocomplete (#15143)

Instead of collecting all modules as strings, sorting the full list, and
then linearly scanning for matching prefixes, filter modules during
collection and only sort the (much smaller) set of matches.

Benchmarked with 261 loaded modules, compared against the original code
(before the previous two optimizations) and the current code:

  match_modules("Elixir.Enum"):
    before PR:  333 µs, 13.6 KB heap
    current:     91 µs,  1.6 KB heap
    proposed:    30 µs,  0.5 KB heap  (11x faster, 26x less heap)

  match_modules("er"):
    before PR:  345 µs, 13.8 KB heap
    current:     97 µs,  2.9 KB heap
    proposed:    39 µs,  0.4 KB heap  (9x faster, 33x less heap)

Author: dkuku

lib/iex/lib/iex/autocomplete.ex

@@ -609,31 +609,33 @@ defmodule IEx.Autocomplete do
   end
 
   defp match_modules(hint, elixir_root?) do
-    get_modules(elixir_root?)
-    |> :lists.usort()
-    |> Enum.drop_while(&(not String.starts_with?(&1, hint)))
-    |> Enum.take_while(&String.starts_with?(&1, hint))
-  end
+    modules =
+      for mod <- :erlang.loaded(),
+          str = Atom.to_string(mod),
+          String.starts_with?(str, hint),
+          do: str
 
-  defp get_modules(true) do
-    ["Elixir.Elixir"] ++ get_modules(false)
-  end
+    modules =
+      if elixir_root? and String.starts_with?("Elixir.Elixir", hint),
+        do: ["Elixir.Elixir" | modules],
+        else: modules
 
-  defp get_modules(false) do
-    modules = Enum.map(:erlang.loaded(), &Atom.to_string/1)
+    modules =
+      case :code.get_mode() do
+        :interactive -> modules ++ match_modules_from_applications(hint)
+        _otherwise -> modules
+      end
 
-    case :code.get_mode() do
-      :interactive -> modules ++ get_modules_from_applications()
-      _otherwise -> modules
-    end
+    :lists.usort(modules)
   end
 
-  defp get_modules_from_applications do
+  defp match_modules_from_applications(hint) do
     for [app] <- loaded_applications(),
         {:ok, modules} = :application.get_key(app, :modules),
-        module <- modules do
-      Atom.to_string(module)
-    end
+        module <- modules,
+        str = Atom.to_string(module),
+        String.starts_with?(str, hint),
+        do: str
   end
 
   defp loaded_applications do