Support returning {:ok, payload} from adapter.inform/3

lib/plug/conn.ex

@@ -1320,10 +1320,19 @@ defmodule Plug.Conn do
   `get_http_protocol/1` to retrieve the protocol and version.
   """
   @spec inform(t, status, Keyword.t()) :: t
-  def inform(%Conn{} = conn, status, headers \\ []) do
+  def inform(%Conn{adapter: {adapter, _}} = conn, status, headers \\ []) do
     status_code = Plug.Conn.Status.code(status)
-    adapter_inform(conn, status_code, headers)
-    conn
+
+    case adapter_inform(conn, status_code, headers) do
+      :ok ->
+        conn
+
+      {:ok, payload} ->
+        put_in(conn.adapter, {adapter, payload})
+
+      {:error, :not_supported} ->
+        conn
+    end
   end
 
   @doc """
@@ -1339,7 +1348,10 @@ defmodule Plug.Conn do
       :ok ->
         conn
 
-      _ ->
+      {:ok, payload} ->
+        put_in(conn.adapter, {adapter, payload})
+
+      {:error, :not_supported} ->
         raise "inform is not supported by #{inspect(adapter)}." <>
                 "You should either delete the call to `inform!/3` or switch to an " <>
                 "adapter that does support informational such as Plug.Cowboy"
@@ -1356,8 +1368,9 @@ defmodule Plug.Conn do
     raise AlreadySentError
   end
 
-  defp adapter_inform(%Conn{adapter: {adapter, payload}}, status, headers),
-    do: adapter.inform(payload, status, headers)
+  defp adapter_inform(%Conn{adapter: {adapter, payload}}, status, headers) do
+    adapter.inform(payload, status, headers)
+  end
 
   @doc """
   Request a protocol upgrade from the underlying adapter.

lib/plug/conn/adapter.ex

@@ -131,7 +131,7 @@ defmodule Plug.Conn.Adapter do
   should be returned.
   """
   @callback inform(payload, status :: Conn.status(), headers :: Keyword.t()) ::
-              :ok | {:error, term}
+              :ok | {:ok, payload()} | {:error, term()}
 
   @doc """
   Attempt to upgrade the connection with the client.

test/plug/conn_test.exs

@@ -438,6 +438,11 @@ defmodule Plug.ConnTest do
     )
   end
 
+  test "inform!/3 performs an informational request" do
+    conn = conn(:get, "/foo") |> inform!(103, [{"link", "</style.css>; rel=preload; as=style"}])
+    assert {103, [{"link", "</style.css>; rel=preload; as=style"}]} in sent_informs(conn)
+  end
+
   test "upgrade_adapter/3 requests an upgrade" do
     conn = conn(:get, "/foo") |> upgrade_adapter(:supported, opt: :supported)
     assert {:supported, [opt: :supported]} in sent_upgrades(conn)