Don't show docs links for plugin error codes (#16383)

Fixes #16375

Author: ilevkivskyi

mypy/errorcodes.py

@@ -274,3 +274,6 @@ def __hash__(self) -> int:
     "General",
     sub_code_of=MISC,
 )
+
+# This copy will not include any error codes defined later in the plugins.
+mypy_error_codes = error_codes.copy()

mypy/errors.py

@@ -8,7 +8,7 @@
 from typing_extensions import Literal, TypeAlias as _TypeAlias
 
 from mypy import errorcodes as codes
-from mypy.errorcodes import IMPORT, IMPORT_NOT_FOUND, IMPORT_UNTYPED, ErrorCode
+from mypy.errorcodes import IMPORT, IMPORT_NOT_FOUND, IMPORT_UNTYPED, ErrorCode, mypy_error_codes
 from mypy.message_registry import ErrorMessage
 from mypy.options import Options
 from mypy.scope import Scope
@@ -560,6 +560,7 @@ def add_error_info(self, info: ErrorInfo) -> None:
             and not self.options.hide_error_codes
             and info.code is not None
             and info.code not in HIDE_LINK_CODES
+            and info.code.code in mypy_error_codes
         ):
             message = f"See {BASE_RTD_URL}-{info.code.code} for more info"
             if message in self.only_once_messages: