Cope with field_transformer being a generator

This worked on 25.1.0 but was inadvertently broken by #1401

Author: ashb

src/attr/_make.py

@@ -450,6 +450,9 @@ def _transform_attrs(
     if field_transformer is not None:
         attrs = field_transformer(cls, attrs)
 
+        if inspect.isgenerator(attrs):
+            attrs = tuple(attrs)
+
     # Check attr order after executing the field_transformer.
     # Mandatory vs non-mandatory attr order only matters when they are part of
     # the __init__ signature and when they aren't kw_only (which are moved to

tests/test_hooks.py

@@ -217,6 +217,16 @@ class C:
         assert "CAttributes" == fields_type.__name__
         assert issubclass(fields_type, tuple)
 
+    def test_hook_generator(self):
+        def hook(cls, attribs):
+            yield from attribs
+
+        @attr.s(auto_attribs=True, field_transformer=hook)
+        class Base:
+            x: int
+
+        assert ["x"] == [a.name for a in attr.fields(Base)]
+
 
 class TestAsDictHook:
     def test_asdict(self):