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r? @weihanglo

this is related to #14951 , also #13586 (comment)

Can we make sure this has a test? I'm not sure if the intent here is to ensure it stays consistent if the tree is moved, perhaps just a freshness test would suffice?

@Dirbaio are you still interested in this. We might need a test as mentioned above.

I'll mark this as draft temporarily. When ready, feel free to tell @rustbot ready.

I'm not sure if we want this change. It fixes hash instability when you move the workspace and a path dep outside the workspace together, but it creates a new instability when you move the workspace only.

The former happens if you refer to another crate by relative path, typically inside the same repo, but aren't using a workspace. The latter happens if you refer to some crate by absolute path, for example if they're separate repos or the source is installed "system-wide" somehow.

So it depends on which scenario we think is more likely. I've personally ran into the former but not the latter, but that's only one data point.

If you guys agree it's better to always use relative paths I'll rebase this PR and add a test.