Add a poor man's implementation of `{enable_feature,Feature}`

There is ongoing work, which will result in an EEP and a pull request,
for a way to conditionally enable new or experimental features in
the compiler and runtime system.

This commit implements this feature in a crude way, to allow us to
selectively make `maybe` and `else` keywords instead of atoms. That
will allow us to compile source code that use `maybe` and `else`
as atoms. (They are quite a lot of code in OTP that uses `maybe` as
an atom.)

Note that as implemented in this commit, for the
`{enable_feature,maybe_expr}` to have any effect, it must be given as an
option to the compiler, **not** in the source code for the module.

1 files changed, 13 insertions, 0 deletions

diff --git a/lib/compiler/src/compile.erl b/lib/compiler/src/compile.erl
index ec0414cdda..847ea484ba 100644
--- a/lib/compiler/src/compile.erl
+++ b/lib/compiler/src/compile.erl
@@ -1018,12 +1018,25 @@ do_parse_module(DefEncoding, #compile{ifile=File,options=Opts,dir=Dir}=St) ->
                         false ->
                             1
                     end,
+
+    %% FIXME: Rewrite this when the enable feature EEP has been implemented.
+    ResWordFun = case proplists:get_value(enable_feature, Opts, []) of
+                     maybe_expr ->
+                         fun('maybe') -> true;
+                            ('else') -> true;
+                            (Other) -> erl_scan:reserved_word(Other)
+                         end;
+                     _ ->
+                         fun erl_scan:reserved_word/1
+                 end,
+
     R = epp:parse_file(File,
                        [{includes,[".",Dir|inc_paths(Opts)]},
                         {source_name, SourceName},
                         {macros,pre_defs(Opts)},
                         {default_encoding,DefEncoding},
                         {location,StartLocation},
+                        {reserved_word_fun,ResWordFun},
                         extra]),
     case R of
 	{ok,Forms0,Extra} ->