Link Module.function/arity to hexdocs in exception messages (#1263)

Idea by @Gazler

This is similar to ExDoc autolinking, but only works for the simple
Elixir `Module.function/arity` format

lib/plug/debugger.ex

@@ -233,7 +233,7 @@ defmodule Plug.Debugger do
       assigns =
         Keyword.merge(assigns,
           conn: conn,
-          message: message,
+          message: maybe_autolink(message),
           markdown: markdown,
           style: style,
           banner: banner,
@@ -549,4 +549,40 @@ defmodule Plug.Debugger do
   defp maybe_merge_dark_styles(style, default_dark_style) do
     Map.put(style, :dark, default_dark_style)
   end
+
+  defp maybe_autolink(message) do
+    splitted =
+      Regex.split(~r/`[A-Z][A-Za-z0-9_.]+\.[a-z][A-Za-z0-9_!?]*\/\d+`/, message,
+        include_captures: true,
+        trim: true
+      )
+
+    Enum.map(splitted, &maybe_format_function_reference/1)
+    |> IO.iodata_to_binary()
+  end
+
+  defp maybe_format_function_reference("`" <> reference = text) do
+    reference = String.trim_trailing(reference, "`")
+
+    with {:ok, m, f, a} <- get_mfa(reference),
+         url when is_binary(url) <- get_doc(m, f, a, Application.get_application(m)) do
+      ~s[<a href="#{url}" target="_blank">`#{h(reference)}`</a>]
+    else
+      _ -> h(text)
+    end
+  end
+
+  defp maybe_format_function_reference(text), do: h(text)
+
+  def get_mfa(capture) do
+    [function_path, arity] = String.split(capture, "/")
+    {arity, ""} = Integer.parse(arity)
+    parts = String.split(function_path, ".")
+    {function_str, parts} = List.pop_at(parts, -1)
+    module = Module.safe_concat(parts)
+    function = String.to_existing_atom(function_str)
+    {:ok, module, function, arity}
+  rescue
+    _ -> :error
+  end
 end

lib/plug/templates/debugger.html.eex

@@ -884,7 +884,7 @@
                 <small>at <%= h method(@conn) %></small>
                 <small class="path"><%= h @conn.request_path %></small>
             </h5>
-            <code><pre class="exception-details-text"><%= h @message %></pre></code>
+            <code><pre class="exception-details-text"><%= @message %></pre></code>
         </header>
 
         <%= for %{label: label, encoded_handler: encoded_handler} <- @actions do %>

mix.exs

@@ -26,7 +26,8 @@ defmodule Plug.MixProject do
         groups_for_extras: groups_for_extras(),
         source_ref: "v#{@version}",
         source_url: "https://github.com/elixir-plug/plug"
-      ]
+      ],
+      test_ignore_filters: [&String.starts_with?(&1, "test/fixtures/")]
     ]
   end
 

test/plug/adapters/test/conn_test.exs

@@ -161,13 +161,13 @@ defmodule Plug.Adapters.Test.ConnTest do
   end
 
   test "use existing conn.remote_ip if exists" do
-    conn_with_remote_ip = %Plug.Conn{conn(:get, "/") | remote_ip: {151, 236, 219, 228}}
+    conn_with_remote_ip = %{conn(:get, "/") | remote_ip: {151, 236, 219, 228}}
     child_conn = Plug.Adapters.Test.Conn.conn(conn_with_remote_ip, :get, "/", foo: "bar")
     assert child_conn.remote_ip == {151, 236, 219, 228}
   end
 
   test "use existing conn.port if exists" do
-    conn_with_port = %Plug.Conn{conn(:get, "/") | port: 4200}
+    conn_with_port = %{conn(:get, "/") | port: 4200}
     child_conn = Plug.Adapters.Test.Conn.conn(conn_with_port, :get, "/", foo: "bar")
     assert child_conn.port == 4200
   end

test/plug/debugger_test.exs

@@ -599,4 +599,30 @@ defmodule Plug.DebuggerTest do
 
     assert conn.resp_body =~ "<span class=\"code\">  end</span>"
   end
+
+  test "links to hexdocs" do
+    conn =
+      conn(:get, "/foo/bar")
+      |> put_req_header("accept", "text/html")
+      |> render([], fn -> raise "please use `Plug.Conn.send_resp/3` instead" end)
+
+    assert conn.resp_body =~
+             ~r(<a href="https://hexdocs.pm/plug/.*/Plug.Conn.html#send_resp/3" target="_blank">`Plug.Conn.send_resp/3`</a>)
+  end
+
+  test "does not create new atoms" do
+    conn =
+      conn(:get, "/foo/bar")
+      |> put_req_header("accept", "text/html")
+      |> render([], fn ->
+        raise "please use `NotExisting.not_existing_atom_for_test_does_not_create_new_atom/1` instead"
+      end)
+
+    assert conn.resp_body =~
+             ~r(`NotExisting.not_existing_atom_for_test_does_not_create_new_atom/1`)
+
+    assert_raise ArgumentError, fn ->
+      String.to_existing_atom("not_existing_atom_for_test_does_not_create_new_atom")
+    end
+  end
 end