Build is always executed if mtimes are missing

[matklad]

Today a user handled me a rar archive (from a windows machine) with a Cargo package. When I extracted it on my linux machine, I've noticed that cargo build always rebuilds the project, instead of doing zero builds. Looks like the problem is that extracted files miss mtimes, so we always rebulid in here:

cargo/src/cargo/ops/cargo_rustc/fingerprint.rs

Lines 250 to 252 in f46d485

	let should_rebuild = match (*on_disk_mtime, *previously_built_mtime) {
	(None, None) => false,
	(Some(_), None) | (None, Some(_)) => true,

Seems like a niche use case, and I am not sure how to fix it properly (touching files seems inappropriate?), but lets log the issue anyways!

[alexcrichton]

Hm I'm not actually sure how an mtime can be missing... Did the archive include the target directory with fingerprints?

[matklad]

Hm, that's super weird. I've tried to reproduce it now, and I can't =/ I would say it was a hallucination, if not for the fact that I happen to have one of terminal tabs with experiments laying around, which clearly shows rebuild happening: https://gist.github.com/matklad/0fe6554ecf473b3af9e511e197921614

Did the archive include the target directory with fingerprints?

Yeah, but I think I've tried removing the target directory, and rebuilds still happened, but given that I can't reproduce even the original issue, I am not sure in anything now...

[matklad]

For the context, the upstream issue is intellij-rust/intellij-rust#1644, which also looks rather mysterious...

[eminence]

Triage: This issue is quite old, and seemingly hard to reproduce. There's a lot of discussion (some even recent!) in #6529 about moving away from mtimes and to try things like checking hashes.

Given this issue here doesn't seem particularly actionable, it probably makes the most sense to just close it.

[ehuss]

Yea, without a clear reproduction this probably isn't actionable.
Also, the fingerprint code has changed a lot since this was written, and it may be possible that it no longer behaves the same.