Allow mixed sync/async context managers in `async with` statements

[Zac-HD (Zac Hatfield-Dodds)]

@graingert

you could make an as_acm that doesn’t yield to the event loop

@Jelle

It would be more straightforward to implement if you could use async with even if all the context managers are sync; this seems harmless.

The problem here is that if we don’t yield to the event loop, or if we allow all-sync context managers, this breaks the syntactic visibility of checkpoints.

Trio goes to considerable lengths to ensure that every time you see the await, async for, or async with keyword, it’s executing a checkpoint where the event loop has a chance to cancel the task or schedule other work^[1]. This is vital to avoid deadlocks in a cooperative-concurrency system; I wrote a whole linter and Anthropic’s codebase puts considerable effort into maintaining this property.

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1. or raises an exception; the __aexit__ of a nursery also has slightly more complicated rules ↩︎