[vasama]

Introduction

This PR introduces a CRTP alternative to stop callbacks. This allows reducing the sizes of many operation states for two reasons:

1. __optional wrappers can be eliminated, because the stoppable base tracks its own state.
2. Pointers to the operation states stored in their stop callbacks can be eliminated, because a reference to the operation state can be derived from a reference to the stop callback base.

See also P3301: inplace_stoppable_base.

Changes in this PR

• A concrete implementation for inplace_stop_token: inplace_stoppable_base
• A generic implementation based on stop callbacks.
• stoppable_base_for_t alias template, analogous to stop_callback_for_t.

Measurements

This table shows sizes of operation state objects, using three different stop token types, before and after the changes. Measured on x86-64 Ubuntu 26.04 using Clang 22.

Operation state	inplace_stop_token	std::stop_token	never_stop_token
when_all	120 → 104	136	80 → 72
when_any	120 → 104	136	72
split	88 → 72	104	40
ensure_started	88 → 72	104	40
merge_each	80 → 64	96	40 → 32
async_scope future operation	80 → 64	96	40 → 32
async_scope future state (heap)	224 → 208 (always inplace)	–	–
timed_thread_scheduler schedule_after	192 → 176	208	144
io_uring schedule_after	184 → 152	200 → 184	144 → 120

Note

Similar changes were made to a few Windows or CUDA specific components, but changes in the relevant operation state sizes were not measured.

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[ericniebler]

this is great! thanks!

[ericniebler]

/ok to test 6688192