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59b6cce418
Aiming for the design where `heavier_once_cell::OnceCell` is initialized by a future factory lead to awkwardness with how `LayerInner::get_or_maybe_download` looks right now with the `loop`. The loop helps with two situations: - an eviction has been scheduled but has not yet happened, and a read access should cancel the eviction - a previous `LayerInner::get_or_maybe_download` that canceled a pending eviction was canceled leaving the `heavier_once_cell::OnceCell` uninitialized but needing repair by the next `LayerInner::get_or_maybe_download` By instead supporting detached initialization in `heavier_once_cell::OnceCell` via an `OnceCell::get_or_detached_init`, we can fix what the monolithic #7030 does: - spawned off download task initializes the `heavier_once_cell::OnceCell` regardless of the download starter being canceled - a canceled `LayerInner::get_or_maybe_download` no longer stops eviction but can win it if not canceled Split off from #7030. Cc: #5331