Struct Guard 

pub struct Guard { /* private fields */ }

RAII timing guard. Records elapsed time on drop.

16 bytes: fits in two registers on both x86_64 (rax+rdx) and aarch64 (x0+x1), eliminating all memory stores from the measurement window.

Uses a raw hardware counter (rdtsc / cntvct_el0) instead of Instant::now() to minimize clock-read cost. The tsc-to-nanosecond conversion happens in drop_cold, outside the measurement window.

Implementations

impl Guard

pub fn check(&self)

Check for thread migration after an .await point.

If the Guard has migrated to a different thread, pushes a phantom StackEntry onto the new thread’s stack so that children can update it via the normal parent fast path. Also registers the Guard as migrated so the original thread can clean up the orphaned entry.

No-op if still on the same thread. Idempotent: safe to call multiple times (skips if phantom already exists on current stack).

Trait Implementations

impl Drop for Guard

fn drop(&mut self)

Inlined so the counter read (rdtsc/cntvct_el0) happens at the drop site as a single inline instruction. compiler_fence prevents the compiler from hoisting Guard field loads before the counter read.