Fix deduplication of -enable-experimental-feature

swiftpkg/internal/bzl_selects.bzl

@@ -201,13 +201,42 @@ No Starlark expression members were generated for {}\
 
     return scg.new_expr(*expr_members)
 
+def _unwrap_list(values, kind_handlers = {}):
+    """Converts the provied values into unstructured types using the information in the \
+    kind handlers.
+
+    Args:
+        values: A `list` of values that are processed and added to the output.
+        kind_handlers: A `dict` of king handler `struct` values
+            (`bzl_selects.new_kind_handler`).
+
+    Returns:
+        A list where `struct` values are unwrapped
+    """
+    result = []
+    for v in values:
+        v_type = type(v)
+        if v_type != "struct":
+            if v_type == "list":
+                result.extend(v)
+            else:
+                result.append(v)
+            continue
+
+        # We are assuming that the select will always result in a list.
+        # Hence, we wrap the transformed value in a list.
+        kind_handler = kind_handlers.get(v.kind, _noop_kind_handler)
+        result.extend(lists.flatten(kind_handler.transform(v.value)))
+    return result
+
 bzl_selects = struct(
     default_condition = _bazel_select_default_condition,
     new = _new,
     new_from_build_setting = _new_from_build_setting,
     new_from_target_dependency_condition = _new_from_target_dependency_condition,
     new_kind_handler = _new_kind_handler,
     to_starlark = _to_starlark,
+    unwrap_list = _unwrap_list,
 )
 
 # NEED TO CONVERT:

swiftpkg/internal/swiftpkg_build_files.bzl

@@ -106,7 +106,7 @@ def _swift_target_build_file(pkg_ctx, target):
     if len(defines) > 0:
         attrs["defines"] = bzl_selects.to_starlark(defines)
     if len(copts) > 0:
-        attrs["copts"] = bzl_selects.to_starlark(copts)
+        attrs["copts"] = bzl_selects.unwrap_list(copts)
 
     res_build_file = _handle_target_resources(
         pkg_ctx,