assertion: add a Protocol for rewrite hook

Mostly to fix the TODO, doesn't have much semantic meaning.

Also fixes a related typo in import inside `TYPE_CHECKING` block.

Author: bluetech

src/_pytest/assertion/__init__.py

@@ -6,6 +6,7 @@
 from collections.abc import Generator
 import sys
 from typing import Any
+from typing import Protocol
 from typing import TYPE_CHECKING
 
 from _pytest.assertion import rewrite
@@ -82,15 +83,18 @@ def register_assert_rewrite(*names: str) -> None:
         if not isinstance(name, str):
             msg = "expected module names as *args, got {0} instead"  # type: ignore[unreachable]
             raise TypeError(msg.format(repr(names)))
+    rewrite_hook: RewriteHook
     for hook in sys.meta_path:
         if isinstance(hook, rewrite.AssertionRewritingHook):
-            importhook = hook
+            rewrite_hook = hook
             break
     else:
-        # TODO(typing): Add a protocol for mark_rewrite() and use it
-        # for importhook and for PytestPluginManager.rewrite_hook.
-        importhook = DummyRewriteHook()  # type: ignore
-    importhook.mark_rewrite(*names)
+        rewrite_hook = DummyRewriteHook()
+    rewrite_hook.mark_rewrite(*names)
+
+
+class RewriteHook(Protocol):
+    def mark_rewrite(self, *names: str) -> None: ...
 
 
 class DummyRewriteHook:

src/_pytest/config/__init__.py

@@ -70,7 +70,7 @@
 
 
 if TYPE_CHECKING:
-    from _pytest.assertions.rewrite import AssertionRewritingHook
+    from _pytest.assertion.rewrite import AssertionRewritingHook
     from _pytest.cacheprovider import Cache
     from _pytest.terminal import TerminalReporter
 
@@ -397,7 +397,8 @@ class PytestPluginManager(PluginManager):
     """
 
     def __init__(self) -> None:
-        import _pytest.assertion
+        from _pytest.assertion import DummyRewriteHook
+        from _pytest.assertion import RewriteHook
 
         super().__init__("pytest")
 
@@ -443,7 +444,7 @@ def __init__(self) -> None:
             self.enable_tracing()
 
         # Config._consider_importhook will set a real object if required.
-        self.rewrite_hook = _pytest.assertion.DummyRewriteHook()
+        self.rewrite_hook: RewriteHook = DummyRewriteHook()
         # Used to know when we are importing conftests after the pytest_configure stage.
         self._configured = False