Add sync option to -Z threads to force synchronization on one thread

[Zoxc]

This adds a sync option to -Z threads to force synchronization while still using one thread. This is useful to measure overhead of synchronization without the noise of additional threads.

[rustbot]

r? @folkertdev

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[folkertdev]

A small nit, after that r=me

View changes since this review

[folkertdev]

I think this will now be clearer as

Suggested change

	pub(crate) fn parse_threads(slot: &mut Option<usize>, v: Option<&str>) -> bool {
	let n = match v {
	Some("sync") => {
	*slot = Some(1);
	return true;
	}
	None => false,
	Some(s) => match s.parse().ok() {
	Some(0) => std::thread::available_parallelism().map_or(1, NonZero::<usize>::get),
	Some(i) => i,
	None => return false,
	},
	None => return false,
	};
	// We want to cap the number of threads here to avoid large numbers like 999999 and compiler panics.
	// This solution was suggested here https://github.com/rust-lang/rust/issues/117638#issuecomment-1800925067
	*slot = slot.clone().min(MAX_THREADS_CAP);
	ret
	let n = n.min(MAX_THREADS_CAP);
	*slot = (n > 1).then_some(n);
	true
	}
	pub(crate) fn parse_threads(slot: &mut Option<usize>, v: Option<&str>) -> bool {
	let Some(s) = v else { return false };
	// Configures to use synchronization despite only using one thread.
	if s == "sync" {
	*slot = Some(1);
	return true;
	}
	let n = match s.parse().ok() {
	Some(0) => std::thread::available_parallelism().map_or(1, NonZero::<usize>::get),
	Some(i) => i,
	None => return false,
	};
	// We want to cap the number of threads here to avoid large numbers like 999999 and compiler panics.
	// This solution was suggested here https://github.com/rust-lang/rust/issues/117638#issuecomment-1800925067
	let n = n.min(MAX_THREADS_CAP);
	*slot = (n > 1).then_some(n);
	true
	}

maybe with another comment on the (n > 1).then_some(n) line.

[rustbot]

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