build code both in OTP20 and OTP21 without deprecation warnings

otp_compat_transform.erl

-module(otp_compat_transform).
-export([parse_transform/2]).
-export([add_otp21_nowarn/1]). % Export to avoid unused function warning on OTP20

%% OTP_RELEASE is defined in OTP21, not before
-ifdef(OTP_RELEASE).
parse_transform(AST, _) ->
  add_otp21_nowarn(AST).
-else.
parse_transform(AST, _) ->
  AST.
-endif.


add_otp21_nowarn(Exprs) ->
  {Before, [ModAttr|After]} = lists:splitwith(fun not_module_attr/1, Exprs),
  SuddenlyDeprecated = [{erlang, get_stacktrace, 0}, {ssl, ssl_accept, 2}],
  NoWarnAttr = {attribute, 0, compile, {nowarn_deprecated_function, SuddenlyDeprecated}},
  Before ++ [ModAttr, NoWarnAttr] ++ After.

not_module_attr({attribute, _, module, _}) ->
  true;
not_module_attr(_) ->
  false.