Add safe-directories.

[ehuss]

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This is an implementation of RFC 3279.

[ehuss]

Questions/Notes:

• I did not add any sort of "unstable" flag to enable this feature. Which approach do you think would be good?

  • Merge this on a separate branch, and create a testing release from that?
  • Merge on master, and revert it if a release needs to be made before we commit to this feature?
  • Add some unstable gate, such as a CLI flag or environment variable like RUSTUP_UNSTABLE_SAFE_DIRECTORIES=true to mirror the Cargo one. This might be the most desirable, but requires changing a bunch of things to support it.

• Should I bump DEFAULT_METADATA_VERSION? I think the only consequence of not doing that is that an older version of rustup may delete the safe_directories settings. If it is bumped, then older rustups will just fail, which seems worse.

• I can't tell why some code uses macros like info! and others use a Notification.

• I can't tell why some code is in rustup_mode.rs and some is in common.rs.

• The ownership checking code is copied from Cargo until a new cargo_util is published.

[kinnison]

How did the cargo part of this go Eric? I'm reluctant to start validating this until we know Cargo makes sense.

[ehuss]

I need to follow up on some things, and I have been procrastinating. 🐌