assertion/rewrite: fix test crash on assert failure with terminalreporter disabled

The `config.get_terminal_writer()` in `assertrepr_compare` (=> the
function injected by assertion rewriting for every `assert`) requires
the `terminalreporter` plugin, so it crashed when the plugin is
disabled.

Fix #14377.

Author: bluetech

src/_pytest/assertion/__init__.py

@@ -205,7 +205,11 @@ def pytest_sessionfinish(session: Session) -> None:
 def pytest_assertrepr_compare(
     config: Config, op: str, left: Any, right: Any
 ) -> list[str] | None:
-    highlighter = config.get_terminal_writer()._highlight
+    if config.pluginmanager.has_plugin("terminalreporter"):
+        highlighter = config.get_terminal_writer()._highlight
+    else:
+        # Keep it plaintext when not using terminalrepoterer (#14377).
+        highlighter = util.dummy_highlighter
     return util.assertrepr_compare(
         op=op,
         left=left,

testing/test_assertion.py

@@ -24,7 +24,13 @@ class TerminalWriter:
         def _highlight(self, source, lexer="python"):
             return source
 
+    class PluginManager:
+        def has_plugin(self, name: str) -> bool:
+            return True
+
     class Config:
+        pluginmanager = PluginManager()
+
         def get_terminal_writer(self):
             return TerminalWriter()
 

testing/test_assertrewrite.py

@@ -2404,3 +2404,22 @@ def test_saferepr_unbounded(self):
             _saferepr(self.Help)
             == f"<class '{Path(__file__).stem}.{self.__class__.__name__}.Help'>"
         )
+
+
+def test_assertion_failure_when_terminalreporter_is_disabled(
+    pytester: Pytester,
+) -> None:
+    """Assertion rewriting doesn't crash when the terminalreporter plugin is
+    disabled (#14378)."""
+    pytester.makepyfile(
+        """
+        import pytest
+
+        def test():
+            with pytest.raises(AssertionError) as excinfo:
+                assert 0 == 1
+            assert excinfo.value.args[0] == 'assert 0 == 1'
+        """
+    )
+    reprec = pytester.inline_run("-p", "no:terminalreporter")
+    reprec.assertoutcome(passed=1)