Pared: A new crate for projected reference-counted pointers

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This mimics the aliasing constructor of C++'s shared_ptr, but with a (hopefully) safe interface that doesn't let you store a reference to anything other than a reference that can be obtained from the original data.

It could be useful in situations where you only want to expose a part of an Arc or a Rc to some thread or a part of your code. Unlike general-purpose crates like yoke, this should be very simple to use.

I would greatly appreciate any code review, soundness bug reveals, and/or PRs.

Special thanks to @CAD97, @alice and @steffahn for answering on my previous posts as great information sources.

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Can't test right now, but from a glance it looks like it might suffer from the same bug as `OwningRef::map` is unsound · Issue #71 · Kimundi/owning-ref-rs · GitHub

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You're right, it does

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I fixed it for 0.2.0 by requiring that T: 'static. It doesn't break any usage examples, and while it's more restrictive this way, it shouldn't be in the way of any intended usage for this.

Thank you for pointing this out, I didn't realize the missing lifetime bound on the return value of the closure. I tested it with references, but it didn't occur to me to check with references to references.

I'm moderately annoyed that I couldn't figure out how to bind the return type of FnOnce to the lifetime it's using in the reference, but this shouldn't be too bad either.

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You got the 'static bound backwards in the from_rc and from_arc methods (the input type if U but you bounded T)

Also, you choose to bound both T and U in Parc::project, while in Prc::project you only bounded T.

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:man_facepalming:

I accidentally deleted the tests that were supposed to catch those kinds of errors, and kept a duplicate there instead.

0.2.1 it is, at least I can say that API change is a bugfix

Thank you for these, I really appreciate it.

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