Use platform-defined directories for cargo state

[spacekookie]

Continuation of PR #8063.

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This PR changes the cargo config system to use the XDG spec directories, instead of using $HOME/.cargo.

Currently it only uses XDG_DATA_HOME and XDG_CACHE_HOME. There was some discussion in #1734 about also using the new XDG_BIN_HOME directory, but this is not yet implemented.

The priority of paths cargo will check is as follows:

1. Use $CARGO_HOME, if it is set
2. Use $CARGO_CACHE_DIR, $CARGO_CONFIG_DIR, etc, if they are set
3. If no environment variables are set, and $HOME/.cargo is present,
   use that
4. Finally, use the platform-default directory paths

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

To ensure I understand the current state of things, this -- #8063 (comment) -- is still the status here I think? That is, I think the RFC feedback still needs synthesis/digestion and perhaps a follow-up RFC is in order?

[spacekookie]

To ensure I understand the current state of things, this -- #8063 (comment) -- is still the status here I think? That is, I think the RFC feedback still needs synthesis/digestion and perhaps a follow-up RFC is in order?

Sure thing! I will have a look at the existing RFC and some of the feedback and post the work in progress I come up with here.

Somewhat unrelated, but is it possible to turn an PR back into a draft PR?

[spacekookie]

Okay I read the OG RFC and most of the comments (which largely don't add any relevant rationale), and started a PR on my own RFC repo here: spacekookie/rfcs#1 to have discussions on (and keep this thread free for the technical implementation)

[bors]

☔ The latest upstream changes (presumably #9175) made this pull request unmergeable. Please resolve the merge conflicts.

[DianaNites]

If no environment variables are set, and $HOME/.cargo is present,
use that

How could an existing $HOME/.cargo be migrated?

There are also some third-party tools that store stuff in .cargo, such as cargo-audit, at $HOME/.cargo/advisory-db

Maybe another environment variable could be set for 3rd party subcommands, giving a path where its okay to put stuff? That way they dont need to re-implement cargo logic

[oxalica]

How could an existing $HOME/.cargo be migrated?

We can write the migration guide in cargo reference and produce a warning with the URL . Most users should only handle credential, config and bin. Other cache can be simply removed.

There are also some third-party tools that store stuff in .cargo, such as cargo-audit, at $HOME/.cargo/advisory-db

Third-party tools are individual programs. They should have their own data or cache directory instead of cargo's. We can request them to switch directories. But they would not be affected by cargo changes.

[tailhook]

Is using directories = "1" intentional? (while there is already 3.0.2)

[ehuss]

I'm gonna close this due to inactivity, and per the followup discussion on the RFC linked above. Thanks!