Provide a way to run build.rs and then cancel the build because it decides nothing has changed

[golddranks]

There is the rerun-if-changed and rerun-if-env-changed which are very useful to avoid needless reruns. However, I'd like to rerun when there has been a new commit, because I'm using build.rs to embed the commit hash and title into the binary. build.rs can't find this out using just filesystem and environment variable monitoring. Currently I pass cargo:rerun-if-changed=.git, but it's prone to false positives, because any git operation will trigger rerun.

It would be useful if build.rs could be built and run, but it could then decide by itself whether anything has changed or not. Then I could check the latest git commit inside build.rs and act according to that. There is surely other cases too where "env var changed" and "file modified" don't cut it as the only rerun triggers; one might want to check if the contents of a file changed, or do some further processing, like parse a file and check if a specific field changed.

[golddranks]

Btw. a quick question: would a feature like this need an RFC, or would it be sufficient to just implement it? I'm a bit out of the loop how the decision process works W.R.T Cargo features.

[fenhl]

So the idea would be to use something like cargo:rerun-if-script-changed=git rev-parse --verify HEAD, right? That would be very useful.

[dtolnay]

I think this is a near duplicate of #3404.

[ehuss]

Yea, seems like the same, closing as a duplicate of #3404.