fixup! Color the full diff that pytest shows as a diff

src/_pytest/assertion/util.py

@@ -7,8 +7,10 @@ from typing import Any
 from typing import Callable
 from typing import Iterable
 from typing import List
+from typing import Literal
 from typing import Mapping
 from typing import Optional
+from typing import Protocol
 from typing import Sequence
 from unicodedata import normalize
 
@@ -17,7 +19,6 @@ from _pytest import outcomes
 from _pytest._io.saferepr import _pformat_dispatch
 from _pytest._io.saferepr import saferepr
 from _pytest._io.saferepr import saferepr_unlimited
-from _pytest._io.terminalwriter import TerminalWriter
 from _pytest.config import Config
 
 # The _reprcompare attribute on the util module is used by the new assertion
@@ -34,6 +35,11 @@ _assertion_pass: Optional[Callable[[int, str, str], None]] = None
 _config: Optional[Config] = None
 
 
+class _HighlightFunc(Protocol):
+    def __call__(self, source: str, lexer: Literal["diff", "python"] = "python") -> str:
+        """Apply highlighting to the given source."""
+
+
 def format_explanation(explanation: str) -> str:
     r"""Format an explanation.
 
@@ -228,7 +234,7 @@ def assertrepr_compare(
 
 
 def _compare_eq_any(
-    left: Any, right: Any, writer: TerminalWriter, verbose: int = 0
+    left: Any, right: Any, highlighter: _HighlightFunc, verbose: int = 0
 ) -> List[str]:
     explanation = []
     if istext(left) and istext(right):
@@ -249,7 +255,7 @@ def _compare_eq_any(
             # field values, not the type or field names. But this branch
             # intentionally only handles the same-type case, which was often
             # used in older code bases before dataclasses/attrs were available.
-            explanation = _compare_eq_cls(left, right, writer, verbose)
+            explanation = _compare_eq_cls(left, right, highlighter, verbose)
         elif issequence(left) and issequence(right):
             explanation = _compare_eq_sequence(left, right, verbose)
         elif isset(left) and isset(right):
@@ -258,7 +264,7 @@ def _compare_eq_any(
             explanation = _compare_eq_dict(left, right, verbose)
 
         if isiterable(left) and isiterable(right):
-            expl = _compare_eq_iterable(left, right, writer, verbose)
+            expl = _compare_eq_iterable(left, right, highlighter, verbose)
             explanation.extend(expl)
 
     return explanation
@@ -327,7 +333,7 @@ def _surrounding_parens_on_own_lines(lines: List[str]) -> None:
 def _compare_eq_iterable(
     left: Iterable[Any],
     right: Iterable[Any],
-    writer: TerminalWriter,
+    highligher: _HighlightFunc,
     verbose: int = 0,
 ) -> List[str]:
     if verbose <= 0 and not running_on_ci():
@@ -353,7 +359,7 @@ def _compare_eq_iterable(
     # "right" is the expected base against which we compare "left",
     # see https://github.com/pytest-dev/pytest/issues/3333
     explanation.extend(
-        writer._highlight(
+        highligher(
             "\n".join(
                 line.rstrip()
                 for line in difflib.ndiff(right_formatting, left_formatting)
@@ -510,7 +516,7 @@ def _compare_eq_dict(
 
 
 def _compare_eq_cls(
-    left: Any, right: Any, writer: TerminalWriter, verbose: int
+    left: Any, right: Any, highlighter: _HighlightFunc, verbose: int
 ) -> List[str]:
     if not has_default_eq(left):
         return []
@@ -557,7 +563,9 @@ def _compare_eq_cls(
             ]
             explanation += [
                 indent + line
-                for line in _compare_eq_any(field_left, field_right, writer, verbose)
+                for line in _compare_eq_any(
+                    field_left, field_right, highlighter, verbose
+                )
             ]
     return explanation