SE-0472: Starting tasks synchronously from caller context

[ktoso (Konrad 'ktoso' Malawski 🐟🏴‍☠️)]

@filip-sakel

Perhaps I'm missing something, but doesn't this violate the principle of forward progress? I saw the relevant paragraph in the proposal:

The proposed Task.startSynchronously API forms an async context on the calling thread/task/executor, and therefore allows us to call into async code, at the risk of overhanging [emphasis mine] on the calling executor. So while this should be used sparingly, it allows entering an asynchronous context synchronously.

Not really any more than any nonisolated async function under the mode Swift is adopting now due to [Focused re-review] SE-0461: Run nonisolated async functions on the caller's actor by default

It's a tradeoff between overhangs and excessive switching, which for async functions we realized was the wrong tradeoff. Here we're not changing any default behavior of Task, but we are offering a way to opt into the slightly more overhanging version of it with similar implications and tradeoffs.