Ability to have per-workspace registry/git cache

[devsnek]

Describe the problem you are trying to solve

An open source project I'm working on uses a number of tools for building the final artifacts. One of the many tools used is cargo. Cargo is the only tool among them which forces cached data to be stored in a single global location. Many of the contributors are not programmers, and we'd like to avoid cluttering their computers with files they don't even know about.

Describe the solution you'd like

Ideally, cargo could have an option to put all the relevant cached objects in target/.cargo or something like that. It is worth mentioning that this should not affect how configuration resolution works (this is why $CARGO_HOME is undesirable).

Notes

I'm happy to code this, just need the cargo gods to come to a decision on whether the feature is wanted in the first place.

[Eh2406]

Would it make sense to use cargo vender for your use case?

[devsnek]

@Eh2406 i don't think so. it would be undesirable to hold all those files in version control.

[joshtriplett]

Would it suffice to have a separate environment variable to specify the cargo configuration file location? That way you could set CARGO_HOME without affecting configuration file resolution.

Another option: you could detect if the user already has a cargo directory, and only set CARGO_HOME if they don't.

[devsnek]

Maybe a separate env variable to set the git/crates cache location?

[joshtriplett]

@devsnek Well, hopefully one day those will go in XDG_CACHE_HOME, which you could set arbitrarily, but that's going to take a while.

[jonhoo]

As a quick note, this seems very related to #6452.