diff -r 8d9767bf4adb -r 54d9f496f07a mercurial/interfaces/modules.py
--- a/mercurial/interfaces/modules.py	Fri Oct 04 23:23:24 2024 -0400
+++ b/mercurial/interfaces/modules.py	Sat Oct 05 15:00:37 2024 -0400
@@ -50,3 +50,26 @@
 
     xdiffblocks: Optional[BDiffBlocksFnc]
     """This method is currently only available in the ``cext`` module."""
+
+
+class CharEncoding(Protocol):
+    """A Protocol class for the various charencoding module implementations."""
+
+    def isasciistr(self, s: bytes) -> bool:
+        """Can the byte string be decoded with the ``ascii`` codec?"""
+
+    def asciilower(self, s: bytes) -> bytes:
+        """convert a string to lowercase if ASCII
+
+        Raises UnicodeDecodeError if non-ASCII characters are found."""
+
+    def asciiupper(self, s: bytes) -> bytes:
+        """convert a string to uppercase if ASCII
+
+        Raises UnicodeDecodeError if non-ASCII characters are found."""
+
+    def jsonescapeu8fast(self, u8chars: bytes, paranoid: bool) -> bytes:
+        """Convert a UTF-8 byte string to JSON-escaped form (fast path)
+
+        Raises ValueError if non-ASCII characters have to be escaped.
+        """