Cope with field_transformer being a generator (#1417)

* Cope with `field_transformer` being a generator

This worked on 25.1.0 but was inadvertently broken by #1401

* Update src/attr/_make.py

Co-authored-by: Ash Berlin-Taylor <ash_github@firemirror.com>

---------

Co-authored-by: Hynek Schlawack <hs@ox.cx>

Author: ashb

src/attr/_make.py

@@ -448,7 +448,7 @@ def _transform_attrs(
     attrs = base_attrs + own_attrs
 
     if field_transformer is not None:
-        attrs = field_transformer(cls, attrs)
+        attrs = tuple(field_transformer(cls, attrs))
 
     # Check attr order after executing the field_transformer.
     # Mandatory vs non-mandatory attr order only matters when they are part of

tests/test_hooks.py

@@ -217,6 +217,22 @@ class C:
         assert "CAttributes" == fields_type.__name__
         assert issubclass(fields_type, tuple)
 
+    def test_hook_generator(self):
+        """
+        Ensure that `attrs.fields` are correctly recorded when field_transformer is a generator
+
+        Regression test for #1417
+        """
+
+        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):