Add optimization for default on_setattr w/ no work to do

Otherwise we'd end up with an explicit setattr every time.

src/attr/_make.py

@@ -59,6 +59,8 @@
 # Unique object for unequivocal getattr() defaults.
 _sentinel = object()
 
+_ng_default_on_setattr = setters.pipe(setters.convert, setters.validate)
+
 
 class _Nothing(object):
     """
@@ -722,13 +724,16 @@ def __init__(
             self._cls_dict["__delattr__"] = _frozen_delattrs
 
             self._wrote_own_setattr = True
-        elif on_setattr == setters.validate:
+        elif on_setattr == _ng_default_on_setattr:
             for a in attrs:
                 if a.validator is not None:
                     break
+                if a.converter is not None:
+                    break
             else:
-                # If class-level on_setattr is set to validating, but there's
-                # no field to validate, pretend like there's no on_setattr.
+                # If class-level on_setattr is set to convert + validate, but
+                # there's no field to convert or validate, pretend like there's
+                # no on_setattr.
                 self._on_setattr = None
 
         if getstate_setstate:

src/attr/_next_gen.py

@@ -8,7 +8,13 @@
 from attr.exceptions import UnannotatedAttributeError
 
 from . import setters
-from ._make import NOTHING, _frozen_setattrs, attrib, attrs
+from ._make import (
+    NOTHING,
+    _frozen_setattrs,
+    _ng_default_on_setattr,
+    attrib,
+    attrs,
+)
 
 
 def define(
@@ -92,7 +98,7 @@ def wrap(cls):
 
         # By default, mutable classes convert & validate on setattr.
         if frozen is False and on_setattr is None:
-            on_setattr = [setters.convert, setters.validate]
+            on_setattr = _ng_default_on_setattr
 
         # However, if we subclass a frozen class, we inherit the immutability
         # and disable on_setattr.