rustc_tools_util: does not emit any "rerun-if-changed"

[RalfJung]

In Miri, a while ago we replaced vergen by rustc_tools_util. However, unlike vergen, rustc_tools_util does not emit any "rerun-if-changed" and therefore the git hash it shows will not be properly updated on rebuilds.

[matthiaskrgr]

hmmm, never noticed this myself 😅
I checked how vergen does this and but the proposed cargo:rerun-if-changed=.git/HEAD is equally broken, as this the file is only modified, when you SWITCH a branch.

However, when you do an empty commit on top of your current branch, you stay on the same branch/symbolic ref ref: refs/heads/master so git does not see any reason to touch the file (and rightfully so).
Thus not a regression? :P

[matthiaskrgr]

Right now i'm not sure if this can be solved while

1. making sure an empty commit retriggers a build
2. making sure when crate sources are NOT from a git repo root (crates io or rustc subrepo, basically no .git/), we do not trigger a rebuild every single time because the .git is not found

:/

[samueltardieu]

Wouldn't this cover all cases?

• Check if .git/HEAD is present, and if it is rerun-if-changed for .git/HEAD
• And in this case, if git/HEAD contains a ref: REF_PATH, rerun-if-changed for $REF_PATH

[RalfJung]

Yeah that seems to be what vergen does as well:

• If using one of the git enabled libraries, will emit cargo:rerun-if-changed=.git/HEAD. This is done to ensure any git instructions are regenerated when commits are made.
• If using one of the git enabled libraries, will emit cargo:rerun-if-changed=.git/<path_to_ref>. This is done to ensure any git instructions are regenerated when commits are made.