Provide completions for protocol functions

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

@@ -285,6 +285,34 @@ defmodule ElixirLS.LanguageServer.Providers.Completion do
     end
   end
 
+  defp from_completion_item(
+         %{
+           type: :protocol_function,
+           args: args,
+           spec: _spec,
+           name: name,
+           summary: summary,
+           arity: arity,
+           origin: origin
+         },
+         context
+       ) do
+    def_str = if(context[:def_before] == nil, do: "def ")
+
+    full_snippet = "#{def_str}#{snippet(name, args, arity)} do\n\t$0\nend"
+    label = "#{def_str}#{function_label(name, args, arity)}"
+
+    %__MODULE__{
+      label: label,
+      kind: :interface,
+      detail: "#{origin} protocol function",
+      documentation: summary,
+      insert_text: full_snippet,
+      priority: 2,
+      filter_text: name
+    }
+  end
+
   defp from_completion_item(%{type: :field, name: name, origin: origin}, _context) do
     %__MODULE__{
       label: to_string(name),

apps/language_server/mix.exs

@@ -10,6 +10,7 @@ defmodule ElixirLS.LanguageServer.Mixfile do
       config_path: "config/config.exs",
       deps_path: "../../deps",
       lockfile: "../../mix.lock",
+      elixirc_paths: elixirc_paths(Mix.env()),
       build_embedded: false,
       start_permanent: true,
       build_per_environment: false,
@@ -31,4 +32,7 @@ defmodule ElixirLS.LanguageServer.Mixfile do
       {:dialyxir, "~> 1.0.0-rc.6", runtime: false}
     ]
   end
+
+  defp elixirc_paths(:test), do: ["lib", "test/support"]
+  defp elixirc_paths(_), do: ["lib"]
 end

apps/language_server/test/providers/completion_test.exs

@@ -55,4 +55,26 @@ defmodule ElixirLS.LanguageServer.Providers.CompletionTest do
       assert(Enum.any?(items, fn %{"label" => label} -> label == lfn end))
     end
   end
+
+  test "provides completions for protocol functions" do
+    text = """
+    defimpl ElixirLS.LanguageServer.Fixtures.ExampleProtocol, for: MyModule do
+
+    #^
+    end
+    """
+
+    {line, char} = {1, 1}
+    TestUtils.assert_has_cursor_char(text, line, char)
+    {:ok, %{"items" => items}} = Completion.completion(text, line, char, true)
+
+    completions =
+      items
+      |> Enum.filter(&(&1["detail"] =~ "protocol function"))
+      |> Enum.map(& &1["label"])
+
+    assert completions == [
+             "def my_fun(example,arg)"
+           ]
+  end
 end

apps/language_server/test/support/fixtures/example_protocol.ex

@@ -0,0 +1,11 @@
+defprotocol ElixirLS.LanguageServer.Fixtures.ExampleProtocol do
+  @moduledoc """
+  ExampleProtocol protocol used in tests.
+  """
+
+  @doc """
+  Does what `my_fun` does for `t`
+  """
+  @spec my_fun(t, integer) :: binary
+  def my_fun(example, arg)
+end