always show full diff in ci

follow-up to #1314, for similar reasons

closes #9023

changelog/9023.feature.rst

@@ -0,0 +1,4 @@
+
+Full diffs are now always shown for equality assertions of iterables when
+`CI` or ``BUILD_NUMBER`` is found in the environment, even when ``-v`` isn't
+used.

src/_pytest/assertion/util.py

@@ -287,7 +287,7 @@ def _surrounding_parens_on_own_lines(lines: List[str]) -> None:
 def _compare_eq_iterable(
     left: Iterable[Any], right: Iterable[Any], verbose: int = 0
 ) -> List[str]:
-    if not verbose:
+    if not verbose and not running_on_ci():
         return ["Use -v to get the full diff"]
     # dynamic import to speedup pytest
     import difflib

testing/test_assertion.py

@@ -13,6 +13,7 @@ import pytest
 from _pytest import outcomes
 from _pytest.assertion import truncate
 from _pytest.assertion import util
+from _pytest.monkeypatch import MonkeyPatch
 from _pytest.pytester import Pytester
 
 
@@ -448,6 +449,25 @@ class TestAssert_reprcompare:
         assert verbose_expl is not None
         assert "\n".join(verbose_expl).endswith(textwrap.dedent(expected).strip())
 
+    def test_iterable_full_diff_ci(
+        self, monkeypatch: MonkeyPatch, pytester: Pytester
+    ) -> None:
+        pytester.makepyfile(
+            r"""
+            def test_full_diff():
+                left = [0, 1]
+                right = [0, 2]
+                assert left == right
+        """
+        )
+        monkeypatch.setenv("CI", "true")
+        result = pytester.runpytest()
+        result.stdout.fnmatch_lines(["E         Full diff:"])
+
+        monkeypatch.delenv("CI", raising=False)
+        result = pytester.runpytest()
+        result.stdout.fnmatch_lines(["E         Use -v to get the full diff"])
+
     def test_list_different_lengths(self) -> None:
         expl = callequal([0, 1], [0, 1, 2])
         assert expl is not None