Sample

Decide what still needs once_cell now that std has OnceLock, LazyLock, cell::OnceCell and cell::LazyCell, by measuring which once_cell APIs have a stable std equivalent on this rustc and which are still gated

sha256:36b55cf55782c3258dcb2509e721eb8edf4affdd15e52754fe01d12bb5fa27c9

Case

Goal

Decide what still needs once_cell now that std has OnceLock, LazyLock, cell::OnceCell and cell::LazyCell, by measuring which once_cell APIs have a stable std equivalent on this rustc and which are still gated MIGRATION

Packages

once_cell 1.21.4

Environment

rust 1

Created

2026-08-14T13:27:13Z

Contract

• assert a second OnceLock::set neither panics nor overwrites, and hands the rejected value straight back inside Err while the first value stays put
• assert get_or_init on an already filled cell never calls its closure, and that take() needs &mut so a shared cell stays write-once
• assert once_cell::sync::OnceCell::set has the identical hands-it-back contract
• assert get_or_init runs its closure exactly once when 16 threads arrive together, counted with an AtomicUsize, and that every thread leaves with the winner's value
• assert set() called by the same 16 threads instead runs 16 times, returns exactly one Ok, and hands every loser its own value back rather than the winner's
• assert a LazyLock static has not run its initialiser before the first deref, and runs it exactly once across 8 threads doing 1000 lookups each
• assert LazyLock::force hands back the same allocation as deref, and that once_cell::sync::Lazy builds the same table with the same laziness
• assert std LazyLock implements DerefMut and force_mut on this rustc, so neither is a once_cell-only feature any more
• assert a const LazyLock compiles and re-runs its initialiser once per use site: three uses, three builds, no warning
• assert once_cell::sync::Lazy in a const does exactly the same, so the trap belongs to const and not to the crate
• assert the same three uses of a static LazyLock build once and hand out a single address
• assert std::cell::OnceCell and std::cell::LazyCell cover once_cell::unsync, with the same set/get contract and the same first-use laziness
• assert the measured sizes: the unsync cell saves nothing over OnceLock for a u32 and eight bytes for a String
• compile a probe file with rustc to prove OnceLock::get_or_try_init is E0658 behind the once_cell_try feature, tracking issue 109737
• compile a probe file with rustc to prove OnceLock::try_insert is E0658 behind the once_cell_try_insert feature
• compile probe files with rustc to prove OnceLock::wait and LazyLock::force_mut do compile on this rustc
• compile a probe file with rustc to prove std::cell::OnceCell is not Sync, with rustc naming std::sync::OnceLock as the replacement
• assert once_cell's get_or_try_init leaves the cell empty after a failed init so the next caller retries, and stops calling the closure once one succeeds
• assert the hand-rolled get-then-set stand-in runs its initialiser once per thread, 16 out of 16, while still handing every caller the same value

Files

• Cargo.lock
• Cargo.toml
• csx.json
• src/main.rs

Origin Seeder

anonymous

Verification receipts

• rust 1 · CONTAINER_RUN · compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS · cargo@1 · 2026-08-14 · ed25519:a2ec939a4c60e243
• · CONTAINER_RUN · compile:SKIPPED · contract:FAIL · load:SKIPPED · resolve:PASS · cargo@1 · 2026-08-14 · ed25519:7da3f610facce8bf