diff --git a/docs/source/conf.py b/docs/source/conf.py
index a5abeb0dca..39eda12ad5 100755
--- a/docs/source/conf.py
+++ b/docs/source/conf.py
@@ -67,6 +67,7 @@
     ("py:class", "types.FrameType"),
     ("py:class", "P.args"),
     ("py:class", "P.kwargs"),
+    ("py:class", "RetT"),
     # TODO: figure out if you can link this to SSL
     ("py:class", "Context"),
     # TODO: temporary type
diff --git a/pyproject.toml b/pyproject.toml
index 0900f3a7d1..a23f7f5db9 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -53,6 +53,7 @@ module = [
     "trio._core._entry_queue",
     "trio._core._local",
     "trio._core._unbounded_queue",
+    "trio._core._thread_cache",
     "trio._deprecate",
     "trio._dtls",
     "trio._file_io",
diff --git a/trio/_core/_thread_cache.py b/trio/_core/_thread_cache.py
index cc272fc92c..d8f5a94c5b 100644
--- a/trio/_core/_thread_cache.py
+++ b/trio/_core/_thread_cache.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import ctypes
 import ctypes.util
 import sys
@@ -5,10 +7,12 @@
 from functools import partial
 from itertools import count
 from threading import Lock, Thread
-from typing import Callable, Optional, Tuple
+from typing import Any, Callable, Generic, TypeVar
 
 import outcome
 
+RetT = TypeVar("RetT")
+
 
 def _to_os_thread_name(name: str) -> bytes:
     # ctypes handles the trailing \00
@@ -17,18 +21,20 @@ def _to_os_thread_name(name: str) -> bytes:
 
 # used to construct the method used to set os thread name, or None, depending on platform.
 # called once on import
-def get_os_thread_name_func() -> Optional[Callable[[Optional[int], str], None]]:
-    def namefunc(setname: Callable[[int, bytes], int], ident: Optional[int], name: str):
+def get_os_thread_name_func() -> Callable[[int | None, str], None] | None:
+    def namefunc(
+        setname: Callable[[int, bytes], int], ident: int | None, name: str
+    ) -> None:
         # Thread.ident is None "if it has not been started". Unclear if that can happen
         # with current usage.
         if ident is not None:  # pragma: no cover
             setname(ident, _to_os_thread_name(name))
 
-    # namefunc on mac also takes an ident, even if pthread_setname_np doesn't/can't use it
+    # namefunc on Mac also takes an ident, even if pthread_setname_np doesn't/can't use it
     # so the caller don't need to care about platform.
     def darwin_namefunc(
-        setname: Callable[[bytes], int], ident: Optional[int], name: str
-    ):
+        setname: Callable[[bytes], int], ident: int | None, name: str
+    ) -> None:
         # I don't know if Mac can rename threads that hasn't been started, but default
         # to no to be on the safe side.
         if ident is not None:  # pragma: no cover
@@ -110,9 +116,13 @@ def darwin_namefunc(
 name_counter = count()
 
 
-class WorkerThread:
-    def __init__(self, thread_cache):
-        self._job: Optional[Tuple[Callable, Callable, str]] = None
+class WorkerThread(Generic[RetT]):
+    def __init__(self, thread_cache: ThreadCache) -> None:
+        self._job: tuple[  # type: ignore[no-any-unimported]
+            Callable[[], RetT],
+            Callable[[outcome.Outcome[RetT]], object],
+            str | None,
+        ] | None = None
         self._thread_cache = thread_cache
         # This Lock is used in an unconventional way.
         #
@@ -130,7 +140,7 @@ def __init__(self, thread_cache):
             set_os_thread_name(self._thread.ident, self._default_name)
         self._thread.start()
 
-    def _handle_job(self):
+    def _handle_job(self) -> None:
         # Handle job in a separate method to ensure user-created
         # objects are cleaned up in a consistent manner.
         assert self._job is not None
@@ -161,7 +171,7 @@ def _handle_job(self):
             print("Exception while delivering result of thread", file=sys.stderr)
             traceback.print_exception(type(e), e, e.__traceback__)
 
-    def _work(self):
+    def _work(self) -> None:
         while True:
             if self._worker_lock.acquire(timeout=IDLE_TIMEOUT):
                 # We got a job
@@ -185,10 +195,16 @@ def _work(self):
 
 
 class ThreadCache:
-    def __init__(self):
-        self._idle_workers = {}
-
-    def start_thread_soon(self, fn, deliver, name: Optional[str] = None):
+    def __init__(self) -> None:
+        self._idle_workers: dict[WorkerThread[Any], None] = {}
+
+    def start_thread_soon(  # type: ignore[no-any-unimported]
+        self,
+        fn: Callable[[], RetT],
+        deliver: Callable[[outcome.Outcome[RetT]], object],
+        name: str | None = None,
+    ) -> None:
+        worker: WorkerThread[RetT]
         try:
             worker, _ = self._idle_workers.popitem()
         except KeyError:
@@ -200,7 +216,11 @@ def start_thread_soon(self, fn, deliver, name: Optional[str] = None):
 THREAD_CACHE = ThreadCache()
 
 
-def start_thread_soon(fn, deliver, name: Optional[str] = None):
+def start_thread_soon(  # type: ignore[no-any-unimported]
+    fn: Callable[[], RetT],
+    deliver: Callable[[outcome.Outcome[RetT]], object],
+    name: str | None = None,
+) -> None:
     """Runs ``deliver(outcome.capture(fn))`` in a worker thread.
 
     Generally ``fn`` does some blocking work, and ``deliver`` delivers the
diff --git a/trio/_tests/verify_types.json b/trio/_tests/verify_types.json
index 55ba3b32d7..5a0c59e33e 100644
--- a/trio/_tests/verify_types.json
+++ b/trio/_tests/verify_types.json
@@ -7,11 +7,11 @@
     "warningCount": 0
   },
   "typeCompleteness": {
-    "completenessScore": 0.9186602870813397,
+    "completenessScore": 0.9202551834130781,
     "exportedSymbolCounts": {
       "withAmbiguousType": 0,
-      "withKnownType": 576,
-      "withUnknownType": 51
+      "withKnownType": 577,
+      "withUnknownType": 50
     },
     "ignoreUnknownTypesFromImports": true,
     "missingClassDocStringCount": 1,
@@ -101,7 +101,6 @@
       "trio.lowlevel.reschedule",
       "trio.lowlevel.spawn_system_task",
       "trio.lowlevel.start_guest_run",
-      "trio.lowlevel.start_thread_soon",
       "trio.lowlevel.temporarily_detach_coroutine_object",
       "trio.lowlevel.wait_readable",
       "trio.lowlevel.wait_writable",