Check struct.{to_json,to_proto} are built-in functions

`to_json`/`to_proto` function have been removed from `structs`.

Check that they are built-in functions before eliminating the keys.

Tests are gated behind `bazel_features.rules.no_struct_field_denylist` to prevent breakage on old Bazel when running the tests.

Author: mattyclarkson

MODULE.bazel

@@ -21,6 +21,7 @@ bazel_dep(name = "rules_pkg", version = "1.0.1", dev_dependency = True)
 bazel_dep(name = "rules_testing", version = "0.6.0", dev_dependency = True)
 bazel_dep(name = "rules_cc", version = "0.0.17", dev_dependency = True)
 bazel_dep(name = "rules_shell", version = "0.3.0", dev_dependency = True)
+bazel_dep(name = "bazel_features", version = "1.32.0", dev_dependency = True)
 
 # Needed for bazelci and for building distribution tarballs.
 # If using an unreleased version of bazel_skylib via git_override, apply

lib/structs.bzl

@@ -14,6 +14,20 @@
 
 """Skylib module containing functions that operate on structs."""
 
+_built_in_function = type(str)
+
+def _is_built_in_function(value):
+    """Returns True if v is an instance of a built-in function.
+
+    Args:
+      v: The value whose type should be checked.
+
+    Returns:
+      True if v is an instance of a built-in function, False otherwise.
+    """
+
+    return type(value) == _built_in_function
+
 def _to_dict(s):
     """Converts a `struct` to a `dict`.
 
@@ -31,7 +45,7 @@ def _to_dict(s):
     return {
         key: getattr(s, key)
         for key in dir(s)
-        if key != "to_json" and key != "to_proto"
+        if not ((key == "to_json" or key == "to_proto") and _is_built_in_function(getattr(s, key)))
     }
 
 def _merge(first, *rest):

tests/structs_tests.bzl

@@ -14,9 +14,13 @@
 
 """Unit tests for structs.bzl."""
 
+load("@bazel_features//:features.bzl", "bazel_features")
 load("//lib:structs.bzl", "structs")
 load("//lib:unittest.bzl", "asserts", "unittest")
 
+def _placeholder():
+    pass
+
 def _to_dict_test(ctx):
     """Unit tests for structs.to_dict."""
     env = unittest.begin(ctx)
@@ -42,6 +46,24 @@ def _to_dict_test(ctx):
         structs.to_dict(struct(a = 1, b = struct(bb = 1))),
     )
 
+    # Older Bazel denied creating `struct` with `to_json`/`to_proto`
+    if not bazel_features.rules.no_struct_field_denylist:
+        return unittest.end(env)
+
+    # Test `to_json`/`to_proto` values are propagated
+    asserts.equals(
+        env,
+        {"to_json": 1, "to_proto": 2},
+        structs.to_dict(struct(to_json = 1, to_proto = 2)),
+    )
+
+    # Test `to_json`/`to_proto` functions are propagated
+    asserts.equals(
+        env,
+        {"to_json": _placeholder, "to_proto": _placeholder},
+        structs.to_dict(struct(to_json = _placeholder, to_proto = _placeholder)),
+    )
+
     return unittest.end(env)
 
 to_dict_test = unittest.make(_to_dict_test)