Replace uses of types.coroutine with a class

[TeamSpen210]

After seeing a discuss.python.org thread about finally deprecating types.coroutine, I thought I'd try pre-emptively removing the two internal places in Trio which uses it. Both are to yield one value, to terminate the await stack. Also removed a typing.cast(), though we could have actually fixed that without the class change.

Instead of a class, we could also just copy over the code that sets the relevant bitflag in the code object, but this is much more robust and future proof. I imagine that flag will also be removed at some point.

@oremanj since you just posted in that thread.

[oremanj]

This is a valid workaround. I think it is less clear and probably slower, though. I would prefer not to make the change unless there is an actual improvement from it or upstream removes the capability we’re currently relying on. I still think the latter would be a mistake, but it’s outside of our control.

[TeamSpen210]

I did try a few micro-benchmarks (running a few million checkpoint()s), the difference was tiny. Adding a few dummy calls to int() was enough to cancel it out for example.

[A5rocks]

Could you get some concrete numbers? I think pyperf timeit works for microbenchmarks. (and then pyperf compare_to for a nice table)

[A5rocks]

You can use yield inside an async function! It just doesn't do what we want.

[TeamSpen210]

Quite true.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.93728%. Comparing base (bb63c53) to head (97c25fe).
⚠️ Report is 71 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##                main       #3266   +/-   ##
=============================================
  Coverage   99.93728%   99.93728%           
=============================================
  Files            126         126           
  Lines          19132       19133    +1     
  Branches        1296        1296           
=============================================
+ Hits           19120       19121    +1     
  Misses            10          10           
  Partials           2           2           

Files with missing lines	Coverage Δ
src/trio/_core/_tests/test_run.py	100.00000% <100.00000%> (ø)
src/trio/_core/_traps.py	100.00000% <100.00000%> (ø)

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[A5rocks]

Let's close this until there's actually some consensus; we'll have time before any change lands and then arrives in a version we support.