diff --git a/trio/_core/_local.py b/trio/_core/_local.py
index 27252bc78d..f1cbb3e61f 100644
--- a/trio/_core/_local.py
+++ b/trio/_core/_local.py
@@ -17,7 +17,7 @@ class _NoValue:
 
 
 @final
-@attr.s(eq=False, hash=False, slots=False)
+@attr.s(eq=False, hash=False, slots=True)
 class RunVarToken(Generic[T], metaclass=NoPublicConstructor):
     _var: RunVar[T] = attr.ib()
     previous_value: T | type[_NoValue] = attr.ib(default=_NoValue)
diff --git a/trio/_core/_tests/test_guest_mode.py b/trio/_core/_tests/test_guest_mode.py
index bd0ef9ce69..be9d4cb9c2 100644
--- a/trio/_core/_tests/test_guest_mode.py
+++ b/trio/_core/_tests/test_guest_mode.py
@@ -26,15 +26,18 @@
 import pytest
 from outcome import Outcome
 from pytest import MonkeyPatch, WarningsRecorder
-from typing_extensions import TypeAlias
 
 import trio
 import trio.testing
 from trio._channel import MemorySendChannel
+from trio.abc import Instrument
 
 from ..._util import signal_raise
 from .tutil import buggy_pypy_asyncgens, gc_collect_harder, restore_unraisablehook
 
+if TYPE_CHECKING:
+    from typing_extensions import TypeAlias
+
 T = TypeVar("T")
 InHost: TypeAlias = Callable[[object], None]
 
@@ -346,7 +349,7 @@ async def get_woken_by_host_deadline(watb_cscope: trio.CancelScope) -> None:
                 # 'sit_in_wait_all_tasks_blocked', we want the test to
                 # actually end. So in after_io_wait we schedule a second host
                 # call to tear things down.
-                class InstrumentHelper(trio._abc.Instrument):
+                class InstrumentHelper(Instrument):
                     def __init__(self) -> None:
                         self.primed = False
 
diff --git a/trio/_ssl.py b/trio/_ssl.py
index 77d3b80140..57f41c11d2 100644
--- a/trio/_ssl.py
+++ b/trio/_ssl.py
@@ -239,7 +239,7 @@ def done(self) -> bool:
 
 _State = _Enum("_State", ["OK", "BROKEN", "CLOSED"])
 
-# TODO: variance
+# invariant
 T_Stream = TypeVar("T_Stream", bound=Stream)
 
 
diff --git a/trio/_tests/test_file_io.py b/trio/_tests/test_file_io.py
index d438a9fb10..5d617a0661 100644
--- a/trio/_tests/test_file_io.py
+++ b/trio/_tests/test_file_io.py
@@ -244,13 +244,15 @@ async def test_aclose_cancelled(path: pathlib.Path) -> None:
     assert f.closed
 
 
-async def test_detach_rewraps_asynciobase() -> None:
-    raw = io.BytesIO()
-    buffered = io.BufferedReader(raw)  # type: ignore[arg-type] # ????????????
+async def test_detach_rewraps_asynciobase(tmp_path: pathlib.Path) -> None:
+    tmp_file = tmp_path / "filename"
+    tmp_file.touch()
+    with open(tmp_file, mode="rb", buffering=0) as raw:
+        buffered = io.BufferedReader(raw)
 
-    async_file = trio.wrap_file(buffered)
+        async_file = trio.wrap_file(buffered)
 
-    detached = await async_file.detach()
+        detached = await async_file.detach()
 
-    assert isinstance(detached, AsyncIOWrapper)
-    assert detached.wrapped is raw
+        assert isinstance(detached, AsyncIOWrapper)
+        assert detached.wrapped is raw
diff --git a/trio/_tests/test_highlevel_generic.py b/trio/_tests/test_highlevel_generic.py
index 4b2008c08c..3e9fc212a8 100644
--- a/trio/_tests/test_highlevel_generic.py
+++ b/trio/_tests/test_highlevel_generic.py
@@ -27,8 +27,9 @@ async def aclose(self) -> None:
 class RecordReceiveStream(ReceiveStream):
     record: list[str | tuple[str, int | None]] = attr.ib(factory=list)
 
-    async def receive_some(self, max_bytes: int | None = None) -> None:  # type: ignore[override]
+    async def receive_some(self, max_bytes: int | None = None) -> bytes:
         self.record.append(("receive_some", max_bytes))
+        return b""
 
     async def aclose(self) -> None:
         self.record.append("aclose")
diff --git a/trio/_tests/test_ssl.py b/trio/_tests/test_ssl.py
index 58e069f239..4a4ba95ecd 100644
--- a/trio/_tests/test_ssl.py
+++ b/trio/_tests/test_ssl.py
@@ -8,10 +8,9 @@
 from contextlib import asynccontextmanager, contextmanager
 from functools import partial
 from ssl import SSLContext
-from typing import Any, AsyncIterator, Iterator, NoReturn
+from typing import TYPE_CHECKING, Any, AsyncIterator, Iterator, NoReturn
 
 import pytest
-from typing_extensions import TypeAlias
 
 from trio import StapledStream
 from trio._core import MockClock
@@ -46,6 +45,9 @@
     memory_stream_pair,
 )
 
+if TYPE_CHECKING:
+    from typing_extensions import TypeAlias
+
 # We have two different kinds of echo server fixtures we use for testing. The
 # first is a real server written using the stdlib ssl module and blocking
 # sockets. It runs in a thread and we talk to it over a real socketpair(), to
diff --git a/trio/_tests/test_subprocess.py b/trio/_tests/test_subprocess.py
index e4713b3950..8c32f1c49b 100644
--- a/trio/_tests/test_subprocess.py
+++ b/trio/_tests/test_subprocess.py
@@ -20,7 +20,6 @@
 
 import pytest
 from pytest import MonkeyPatch, WarningsRecorder
-from typing_extensions import TypeAlias
 
 from .. import (
     ClosedResourceError,
@@ -38,6 +37,9 @@
 from ..lowlevel import open_process
 from ..testing import MockClock, assert_no_checkpoints, wait_all_tasks_blocked
 
+if TYPE_CHECKING:
+    from typing_extensions import TypeAlias
+
 if sys.platform == "win32":
     SignalType: TypeAlias = None
 else:
diff --git a/trio/_tests/test_sync.py b/trio/_tests/test_sync.py
index 448ead15da..3f9ca88650 100644
--- a/trio/_tests/test_sync.py
+++ b/trio/_tests/test_sync.py
@@ -1,16 +1,18 @@
 from __future__ import annotations
 
 import weakref
-from typing import Callable, Union
+from typing import TYPE_CHECKING, Callable, Union
 
 import pytest
-from typing_extensions import TypeAlias
 
 from .. import _core
 from .._sync import *
 from .._timeouts import sleep_forever
 from ..testing import assert_checkpoints, wait_all_tasks_blocked
 
+if TYPE_CHECKING:
+    from typing_extensions import TypeAlias
+
 
 async def test_Event() -> None:
     e = Event()
diff --git a/trio/_tests/test_threads.py b/trio/_tests/test_threads.py
index 4fa1e4bedf..77650bf569 100644
--- a/trio/_tests/test_threads.py
+++ b/trio/_tests/test_threads.py
@@ -9,7 +9,6 @@
 import weakref
 from functools import partial
 from typing import (
-    TYPE_CHECKING,
     AsyncGenerator,
     Awaitable,
     Callable,
@@ -462,12 +461,10 @@ async def test_run_in_worker_thread_limiter(
         # Mutating them in-place is OK though (as long as you use proper
         # locking etc.).
         class state:
-            if TYPE_CHECKING:
-                ran: int
-                high_water: int
-                running: int
-                parked: int
-            pass
+            ran: int
+            high_water: int
+            running: int
+            parked: int
 
         state.ran = 0
         state.high_water = 0
diff --git a/trio/testing/_fake_net.py b/trio/testing/_fake_net.py
index 2b1d2c8b34..fbe2670f46 100644
--- a/trio/testing/_fake_net.py
+++ b/trio/testing/_fake_net.py
@@ -24,7 +24,6 @@
 )
 
 import attr
-from typing_extensions import Buffer, Self
 
 import trio
 from trio._util import NoPublicConstructor, final
@@ -33,7 +32,7 @@
     from socket import AddressFamily, SocketKind
     from types import TracebackType
 
-    from typing_extensions import TypeAlias
+    from typing_extensions import Buffer, Self, TypeAlias
 
 IPAddress: TypeAlias = Union[ipaddress.IPv4Address, ipaddress.IPv6Address]