Sample

Distinguish non-blocking race initialization from blocking sync initialization in once_cell, proving that race::OnceBox executes all concurrent closures and drops loser allocations, permits re-entrancy without deadlocking, and does not poison on panics

sha256:bf34c4b96e5d2e5c6749e27e05216e4f465933461e70adea2f1b2170b3c1b021

PUBLISHED L3_CONTRACT_PASS MIT-0

Case

Goal: Distinguish non-blocking race initialization from blocking sync initialization in once_cell, proving that race::OnceBox executes all concurrent closures and drops loser allocations, permits re-entrancy without deadlocking, and does not poison on panics HOW
Packages: once_cell 1.21.4
Environment: rust
Created: 2026-08-16T08:11:04Z

assert race::OnceBox executes all 16 contending closures concurrently while immediately dropping all 15 losing heap allocations and storing exactly one winner
assert sync::OnceCell blocks contending threads and executes its initialization closure exactly once across all 16 threads
assert race::OnceBox allows re-entrant initialization where inner init wins and outer value is discarded, whereas unsync::OnceCell panics with 'reentrant init' and sync::OnceCell deadlocks
assert panics during get_or_init do not poison sync::OnceCell or race::OnceBox, leaving them uninitialized and available for subsequent successful initialization
assert sync::OnceCell::set returns Err(T), sync::OnceCell::try_insert returns Err((&T, T)), race::OnceBox::set returns Err(Box<T>), and race::OnceNonZeroUsize/OnceBool return Err(()) on collision
assert race::OnceBox::clone on an initialized cell deep-clones into an independent heap allocation, while an uninitialized clone remains empty
assert race::OnceRef stores borrowed references without heap allocation and rejects subsequent writes with Err(())

rust 1 · CONTAINER_RUN · compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS · cargo@1 · 2026-08-16 · ed25519:d91480838ac982c9