add deprecation warning for specifying strict_exception_groups=False

src/trio/_core/_run.py

@@ -33,6 +33,7 @@
 
 from .. import _core
 from .._abc import Clock, Instrument
+from .._deprecate import warn_deprecated
 from .._util import NoPublicConstructor, coroutine_or_error, final
 from ._asyncgens import AsyncGenerators
 from ._concat_tb import concat_tb
@@ -997,6 +998,15 @@ def open_nursery(
           and ultimately removed in a future version of Trio.
 
     """
+    # only warn if explicitly set to falsy, not if we get it from the global context.
+    if strict_exception_groups is not None and not strict_exception_groups:
+        warn_deprecated(
+            "open_nursery(strict_exception_groups=False)",
+            version="0.24.1",
+            issue=2929,
+            instead="the default value of True and rewrite exception handlers to handle ExceptionGroups",
+        )
+
     if strict_exception_groups is None:
         strict_exception_groups = GLOBAL_RUN_CONTEXT.runner.strict_exception_groups
 
@@ -2244,6 +2254,13 @@ def run(
           propagates it.
 
     """
+    if strict_exception_groups is not None and not strict_exception_groups:
+        warn_deprecated(
+            "trio.run(..., strict_exception_groups=False)",
+            version="0.24.1",
+            issue=2929,
+            instead="the default value of True and rewrite exception handlers to handle ExceptionGroups",
+        )
 
     __tracebackhide__ = True
 
@@ -2350,6 +2367,14 @@ def my_done_callback(run_outcome):
     For the meaning of other arguments, see `trio.run`.
 
     """
+    if strict_exception_groups is not None and not strict_exception_groups:
+        warn_deprecated(
+            "trio.start_guest_run(..., strict_exception_groups=False)",
+            version="0.24.1",
+            issue=2929,
+            instead="the default value of True and rewrite exception handlers to handle ExceptionGroups",
+        )
+
     runner = setup_runner(
         clock,
         instruments,

src/trio/_core/_tests/test_run.py

@@ -2551,6 +2551,9 @@ def _create_kwargs(strictness: bool | None) -> dict[str, bool]:
     return {"strict_exception_groups": strictness}
 
 
+@pytest.mark.filterwarnings(
+    "ignore:.*strict_exception_groups=False:trio.TrioDeprecationWarning"
+)
 @pytest.mark.parametrize("run_strict", [True, False, None])
 @pytest.mark.parametrize("open_nursery_strict", [True, False, None])
 @pytest.mark.parametrize("multiple_exceptions", [True, False])
@@ -2591,6 +2594,9 @@ def run_main() -> None:
             run_main()
 
 
+@pytest.mark.filterwarnings(
+    "ignore:.*strict_exception_groups=False:trio.TrioDeprecationWarning"
+)
 @pytest.mark.parametrize("strict", [True, False, None])
 async def test_nursery_collapse(strict: bool | None) -> None:
     """
@@ -2630,6 +2636,9 @@ async def test_cancel_scope_no_cancellederror() -> None:
     assert not scope.cancelled_caught
 
 
+@pytest.mark.filterwarnings(
+    "ignore:.*strict_exception_groups=False:trio.TrioDeprecationWarning"
+)
 @pytest.mark.parametrize("run_strict", [False, True])
 @pytest.mark.parametrize("start_raiser_strict", [False, True, None])
 @pytest.mark.parametrize("raise_after_started", [False, True])

src/trio/_tests/test_deprecate_strict_exception_groups_false.py

@@ -0,0 +1,61 @@
+from typing import Awaitable, Callable
+
+import pytest
+
+import trio
+
+
+async def test_deprecation_warning_open_nursery() -> None:
+    with pytest.warns(
+        trio.TrioDeprecationWarning, match="strict_exception_groups=False"
+    ) as record:
+        async with trio.open_nursery(strict_exception_groups=False):
+            ...
+    assert len(record) == 1
+    async with trio.open_nursery(strict_exception_groups=True):
+        ...
+    async with trio.open_nursery():
+        ...
+
+
+def test_deprecation_warning_run() -> None:
+    async def foo() -> None: ...
+
+    async def foo_nursery() -> None:
+        # this should not raise a warning, even if it's implied loose
+        async with trio.open_nursery():
+            ...
+
+    async def foo_loose_nursery() -> None:
+        # this should raise a warning, even if specifying the parameter is redundant
+        async with trio.open_nursery(strict_exception_groups=False):
+            ...
+
+    def helper(fun: Callable[..., Awaitable[None]], num: int) -> None:
+        with pytest.warns(
+            trio.TrioDeprecationWarning, match="strict_exception_groups=False"
+        ) as record:
+            trio.run(fun, strict_exception_groups=False)
+        assert len(record) == num
+
+    helper(foo, 1)
+    helper(foo_nursery, 1)
+    helper(foo_loose_nursery, 2)
+
+
+def test_deprecation_warning_start_guest_run() -> None:
+    # "The simplest possible "host" loop."
+    from .._core._tests.test_guest_mode import trivial_guest_run
+
+    async def trio_return(in_host: object) -> str:
+        await trio.lowlevel.checkpoint()
+        return "ok"
+
+    with pytest.warns(
+        trio.TrioDeprecationWarning, match="strict_exception_groups=False"
+    ) as record:
+        trivial_guest_run(
+            trio_return,
+            strict_exception_groups=False,
+        )
+    assert len(record) == 1

src/trio/_tests/test_subprocess.py

@@ -582,9 +582,6 @@ async def do_stuff() -> None:
     with RaisesGroup(RaisesGroup(Matcher(ValueError, "^foo$"))):
         _core.run(do_stuff, strict_exception_groups=True)
 
-    with pytest.raises(ValueError, match="^foo$"):
-        _core.run(do_stuff, strict_exception_groups=False)
-
 
 async def test_warn_on_failed_cancel_terminate(monkeypatch: pytest.MonkeyPatch) -> None:
     original_terminate = Process.terminate