Fix warnings with attrs 19.2 and fix object assertions

attrs 19.2 deprecated cmp in favor of the dataclass-ish eq/order duo.

This causes deprecation warnings that in turn break some of the cool new deep
object comparisons. Since we at attrs expected this to be a problem, it shipped
with helpers to write backward and forward compatible code.

This PR uses that and avoids changed to minimal versions.

src/_pytest/assertion/util.py

@@ -8,6 +8,7 @@ from typing import Optional
 import _pytest._code
 from _pytest import outcomes
 from _pytest._io.saferepr import saferepr
+from _pytest.compat import attrs_has_eq
 
 # The _reprcompare attribute on the util module is used by the new assertion
 # interpretation code and assertion rewriter to detect this plugin was
@@ -112,6 +113,18 @@ def isattrs(obj):
     return getattr(obj, "__attrs_attrs__", None) is not None
 
 
+if attrs_has_eq:
+
+    def attrsfieldhaseq(a):
+        return a.eq
+
+
+else:
+
+    def attrsfieldhaseq(a):
+        return a.cmp
+
+
 def isiterable(obj):
     try:
         iter(obj)
@@ -375,7 +388,7 @@ def _compare_eq_cls(left, right, verbose, type_fns):
         fields_to_check = [field for field, info in all_fields.items() if info.compare]
     elif isattrs(left):
         all_fields = left.__attrs_attrs__
-        fields_to_check = [field.name for field in all_fields if field.cmp]
+        fields_to_check = [field.name for field in all_fields if attrsfieldhaseq(field)]
 
     same = []
     diff = []

src/_pytest/compat.py

@@ -354,3 +354,10 @@ if sys.version_info < (3, 5, 2):  # pragma: no cover
 
     def overload(f):  # noqa: F811
         return f
+
+
+attrs_has_eq = getattr(attr, "__version_info__", (0, 0)) >= (19, 2)
+if attrs_has_eq:
+    attrs_no_eq = {"eq": False}
+else:
+    attrs_no_eq = {"cmp": False}

src/_pytest/mark/structures.py

@@ -8,6 +8,7 @@ from typing import Set
 import attr
 
 from ..compat import ascii_escaped
+from ..compat import attrs_no_eq
 from ..compat import getfslineno
 from ..compat import NOTSET
 from _pytest.outcomes import fail
@@ -367,7 +368,7 @@ class NodeKeywords(MutableMapping):
         return "<NodeKeywords for node {}>".format(self.node)
 
 
-@attr.s(cmp=False, hash=False)
+@attr.s(hash=False, **attrs_no_eq)  # type: ignore
 class NodeMarkers:
     """
     internal structure for storing marks belonging to a node

testing/test_assertion.py

@@ -9,6 +9,7 @@ import pytest
 from _pytest import outcomes
 from _pytest.assertion import truncate
 from _pytest.assertion import util
+from _pytest.compat import attrs_no_eq
 
 
 def mock_config():
@@ -687,7 +688,7 @@ class TestAssert_reprcompare_attrsclass:
         @attr.s
         class SimpleDataObject:
             field_a = attr.ib()
-            field_b = attr.ib(cmp=False)
+            field_b = attr.ib(**attrs_no_eq)
 
         left = SimpleDataObject(1, "b")
         right = SimpleDataObject(1, "b")