Fix interpolation helper for Elixir 1.12

lib/credo/code/interpolation_helper.ex

@@ -130,7 +130,7 @@ defmodule Credo.Code.InterpolationHelper do
 
     # TODO: this seems to be wrong. the closing """ determines the
     #       indentation, not the first line of the heredoc.
-    padding_in_first_line = determine_padding_at_start_of_line(first_line_in_heredoc)
+    padding_in_first_line = determine_heredoc_padding_at_start_of_line(first_line_in_heredoc)
 
     list
     |> find_interpolations(source)
@@ -215,7 +215,18 @@ defmodule Credo.Code.InterpolationHelper do
     nil
   end
 
-  defp determine_padding_at_start_of_line(line, regex \\ ~r/^\s+/) do
+  if Version.match?(System.version(), ">= 1.12.0-rc") do
+    # Elixir >= 1.12.0
+    #
+    defp determine_heredoc_padding_at_start_of_line(_line), do: 0
+  else
+    # Elixir < 1.12.0
+    #
+    defp determine_heredoc_padding_at_start_of_line(line),
+      do: determine_padding_at_start_of_line(line, ~r/^\s+/)
+  end
+
+  defp determine_padding_at_start_of_line(line, regex) do
     regex
     |> Regex.run(line)
     |> List.wrap()