sim.core: add a warning about async generators on Python <= 3.12

Author: agrif

amaranth/sim/core.py

@@ -191,6 +191,13 @@ async def testbench(ctx):
         which they were added. If two testbenches share state, or must manipulate the design in
         a coordinated way, they may rely on this execution order for correctness.
 
+        .. warning::
+
+            On Python 3.12 and earlier, async generators (:py:`async` functions that also
+            :py:`yield`) are not cleaned up reliably when the simulator exits. To make sure context
+            managers or :py:`finally` blocks inside async generators run properly, use Python 3.13
+            or later.
+
         Raises
         ------
         :exc:`RuntimeError`
@@ -248,6 +255,13 @@ async def process(ctx):
             :py:`await ctx.tick().sample(...)`. Such state is visible in a waveform viewer,
             simplifying debugging.
 
+        .. warning::
+
+            On Python 3.12 and earlier, async generators (:py:`async` functions that also
+            :py:`yield`) are not cleaned up reliably when the simulator exits. To make sure context
+            managers or :py:`finally` blocks inside async generators run properly, use Python 3.13
+            or later.
+
         Raises
         ------
         :exc:`RuntimeError`