std: cache mach_timebase_info on Apple#156915
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`mach_timebase_info` returns boot-time constants that never change for the lifetime of the process. The current code path on every Apple target invokes it on every call to `Instant::into_mach_absolute_time_ceil`, which is on the hot path of `std::thread::sleep_until`. A cache for exactly this value existed in the original implementation (rust-lang/rust PR 77727, 2020). It was removed in the `clock_gettime` migration (PR 116238, 2023) when `mach_timebase_info` stopped being used by `Instant::now`. The trap was re-introduced uncached when `sleep_until` was implemented for Apple (PR 151004, 2026). Restore the cache using `OnceLock<(u32, u32)>` in a private helper, scoped to the existing `#[cfg(target_vendor = "apple")]` gate. No API change, no behavior change.
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Fixes #156914
mach_timebase_inforeturns boot-time constants that never change for the lifetime of the process. The current code path on every Apple target invokes it on every call toInstant::into_mach_absolute_time_ceil, which is on the hot path ofstd::thread::sleep_until.A cache existed for exactly this reason in #77727 (2020). It was removed in #116238 (2023) when
clock_gettimereplacedmach_timebase_infoforInstant::now. The trap was re-introduced uncached in #151004 (2026) whensleep_untilwas implemented for Apple.This PR restores the cache using
OnceLock<(u32, u32)>in a private helper, scoped to the existing#[cfg(target_vendor = "apple")]gate.No API change, no behavior change. Per-call cost on
x86_64-apple-darwin/ macOS 15.7.7 /rustc -O: ~3.4 ns → ~1.1 ns (10M-iter microbench in #156914). Mirrors the pattern from #77727.