[rami3l]

Cherry-picked from #5194.

GitHub seems to be silently downgrading the experience on macOS runners by lowering available RAM. This is not always the case, but when it triggers, rustup emits this warning and the snapshots get messed up:

warn: using single-threaded unpacking due to low memory (ram budget:  32.0 MiB < 512.0 MiB threshold), set RUSTUP_IO_THREADS to override

This is only possible when the branch in L500 is triggered without also triggering the error!() call (minimum_ram is set to exactly 32MiB):

rustup/src/diskio/mod.rs

Lines 490 to 505 in 71668d2

	pub(super) fn unpack_ram(io_chunk_size: usize, budget: Option<usize>) -> usize {
	const RAM_ALLOWANCE_FOR_RUSTUP_AND_BUFFERS: usize = 200 * 1024 * 1024;
	let minimum_ram = io_chunk_size * 2;
	let default_max_unpack_ram = match effective_limits::memory_limit() {
	Ok(effective)
	if effective as usize > minimum_ram + RAM_ALLOWANCE_FOR_RUSTUP_AND_BUFFERS =>
	{
	effective as usize - RAM_ALLOWANCE_FOR_RUSTUP_AND_BUFFERS
	}
	Ok(_) => minimum_ram,
	Err(e) => {
	error!("{:#}", anyhow!(e).context("can't determine memory limit"));
	minimum_ram
	}
	};

... which means the available RAM is less than 200MiB. Please note that this only happens to macOS runners; Windows and Linux runners don't have this problem.

I think this warn!() message is still valuable, but don't think it can be that interesting to the user in such circumstances, so I downgraded it from warn!() to debug!().

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.