Handle singleton records in erlang_to_fate

I realized this case needed special handling in hz_sophia, but didn't
get around to covering it properly in the older hz_aaci analogues.

While I was at it, I went and improved the error paths for record elements.

src/hz_aaci.erl

@@ -796,6 +796,10 @@ coerce_map_to_record(O, N, MemberTypes, Map) ->
     case zip_record_fields(MemberTypes, Map) of
         {ok, Zipped} ->
             case coerce_zipped_bindings(Zipped, to_fate, field) of
+                {ok, [SingleElem]} ->
+                    % Singleton records aren't implemented as FATE tuples at
+                    % all.
+                    {ok, SingleElem};
                 {ok, Converted} ->
                     {ok, {tuple, list_to_tuple(Converted)}};
                 Errors ->
@@ -821,10 +825,18 @@ coerce_record_to_map(O, N, MemberTypes, Tuple) ->
             single_error({record_too_few_terms, O, N, Tuple});
         {error, too_many_terms} ->
             single_error({record_too_many_terms, O, N, Tuple});
-        Errors ->
+        {error, Errors} ->
-            Errors
+            correct_record_error_paths(Names, Errors)
     end.
 
+correct_record_error_paths(Names, Errors) ->
+    CorrectOne = fun({Error, [{record_element, N} | Path]}) ->
+                     FieldName = lists:nth(N + 1, Names),
+                     {Error, [{record_element, FieldName} | Path]}
+                 end,
+    Corrected = lists:map(CorrectOne, Errors),
+    {error, Corrected}.
+
 zip_record_fields(Fields, Map) ->
     case lists:mapfoldl(fun zip_record_field/2, {Map, []}, Fields) of
         {_, {_, Missing = [_|_]}} ->
@@ -915,6 +927,11 @@ fate_to_erlang({O, N, {variant, Variants}}, {variant, _, Tag, Tuple}) ->
     Terms = tuple_to_list(Tuple),
     {Name, TermTypes} = lists:nth(Tag + 1, Variants),
     coerce_variant2(O, N, Variants, Name, Tag, TermTypes, Terms, from_fate);
+fate_to_erlang({O, N, {record, [SingleMemberType]}}, Data) ->
+    % Singleton records aren't implemented as FATE tuples at all.
+    % Pretend they are, so we can get the full error indexing of the
+    % non-singletone case.
+    coerce_record_to_map(O, N, [SingleMemberType], {Data});
 fate_to_erlang({O, N, {record, MemberTypes}}, {tuple, Tuple}) ->
     coerce_record_to_map(O, N, MemberTypes, Tuple);
 fate_to_erlang({O, N, {unknown_type, _}}, Data) ->
@@ -1120,6 +1137,25 @@ record_substitution_test() ->
     {ok, {[], Output}} = get_function_signature(AACI, "f"),
     check_roundtrip(Output, #{"a" => 123, "b" => 456}, {tuple, {123, 456}}).
 
+singleton_record_substitution_test() ->
+    Contract = "
+        contract C =
+            record single('t) = { it: 't }
+            entrypoint f(): single(int) = { it = 1 }
+            entrypoint g(): single(single(int)) = { it = { it = 2 } }
+            entrypoint h(): single(int * int) = { it = (3, 4) }
+    ",
+    {ok, AACI} = aaci_from_string(Contract),
+    {ok, {[], FOutput}} = get_function_signature(AACI, "f"),
+    check_roundtrip(FOutput, #{"it" => 123}, 123),
+    {ok, {[], GOutput}} = get_function_signature(AACI, "g"),
+    check_roundtrip(GOutput, #{"it" => #{"it" => 123}}, 123),
+    {ok, {[], HOutput}} = get_function_signature(AACI, "h"),
+    check_roundtrip(HOutput, #{"it" => {123, 456}}, {tuple, {123, 456}}),
+    % Also check that records have accurate paths, since the implementation for
+    % record error paths is a bit fiddly.
+    {error, [{{tuple_too_many_terms, _, _, _}, [{record_element, "it"}]}]} = fate_to_erlang(HOutput, {tuple, {1, 2, 3}}).
+
 tuple_substitution_test() ->
     Contract = "
         contract C =