Cargo mtime issue preventing effective caching on git dependencies

[0o-de-lally]

Hi all I work on a few repos with hundreds of external dependencies at the root level. Many of those are specified as git paths. @Swatinem pointed out this issue to me related to mtimes.

TL;DR: The cargo caching of GitHub dependencies has a known issue with being cached by timestamp, and not hash. rust-lang/cargo#6529

It's not as simple as saving ~/.cargo/git/checkouts since the timestamp gets updated. See this message for what seems to be happening: the cargo git dependencies appear to be more recent than the checked out code.
rust-lang/cargo#6529 (comment)

The solutions are more convoluted. A number of people have workarounds:
rust-lang/cargo#6529 (comment)

Elsewhere people have tried to reset mtime in the working tree (not deps) with this https://github.com/chetan/git-restore-mtime-action. Though this would set the mtime to the commit date, which if my understanding is correct, this could make matters worse. Though it did produce a slightly shorter compile time, shaving one minute off: without vs with. But a properly cached git dependencies should bring compile time to zero.
paradigmxyz/reth@a22c274