Add __RELATIVEFILE__ to PLUG_EDITOR replacements

lib/plug/debugger.ex

@@ -91,6 +91,11 @@ defmodule Plug.Debugger do
   Or, using Visual Studio Code:
 
       vscode://file/__FILE__:__LINE__
+
+  You can also use `__RELATIVEFILE__` if your project path is different from
+  the running application. This is useful when working with Docker containers.
+
+      vscode://file//path/to/your/project/__RELATIVEFILE__:__LINE__
   """
 
   @already_sent {:plug_conn, :sent}
@@ -338,7 +343,7 @@ defmodule Plug.Debugger do
        doc: doc,
        clauses: clauses,
        args: args,
-       link: editor && get_editor(source, line, editor)
+       link: editor && get_editor(source, file, line, editor)
      }, index + 1}
   end
 
@@ -458,9 +463,10 @@ defmodule Plug.Debugger do
     end
   end
 
-  defp get_editor(file, line, editor) do
+  defp get_editor(source, file, line, editor) do
     editor
-    |> :binary.replace("__FILE__", URI.encode(Path.expand(file)))
+    |> :binary.replace("__FILE__", URI.encode(Path.expand(source)))
+    |> :binary.replace("__RELATIVEFILE__", URI.encode(file))
     |> :binary.replace("__LINE__", to_string(line))
     |> h
   end

test/plug/debugger_test.exs

@@ -332,7 +332,7 @@ defmodule Plug.DebuggerTest do
     assert conn.resp_body =~ "<script>oops</script>"
   end
 
-  test "uses PLUG_EDITOR" do
+  test "uses PLUG_EDITOR with __FILE__" do
     System.put_env("PLUG_EDITOR", "hello://open?file=__FILE__&line=__LINE__")
 
     conn = stack([{Plug.Conn, :unknown, 1, file: "lib/plug/conn.ex", line: 1}])
@@ -344,6 +344,16 @@ defmodule Plug.DebuggerTest do
     assert conn.resp_body =~ "hello://open?file=#{file}&line=10000"
   end
 
+  test "uses PLUG_EDITOR with __RELATIVEFILE__" do
+    System.put_env("PLUG_EDITOR", "hello://open?file=__RELATIVEFILE__&line=__LINE__")
+
+    conn = stack([{Plug.Conn, :unknown, 1, file: "lib/plug/conn.ex", line: 1}])
+    assert conn.resp_body =~ "hello://open?file=lib/plug/conn.ex&line=1"
+
+    conn = stack([{GenServer, :call, 2, file: "lib/gen_server.ex", line: 10_000}])
+    assert conn.resp_body =~ "hello://open?file=lib/gen_server.ex&line=10000"
+  end
+
   test "styles can be overridden" do
     conn = put_req_header(conn(:get, "/boom"), "accept", "text/html")