Remove impl PinCoerceUnsized for Pin

[Darksonn]

The safety requirements for PinCoerceUnsized are essentially that the type does not have a malicious Deref or DerefMut impl. However, the Pin type is fundamental, so the end-user can provide their own implementation of DerefMut for Pin<&SomeLocalType>, so it's possible for Pin to have a malicious DerefMut impl.

Thus, remove the unsound implementation of PinCoerceUnsized for the Pin type.

This PR is a breaking change, but it fixes #85099.

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[Darksonn]

Note that all other fundamental types that implement PinCoerceUnsized prevent this unsoundness by having an unconditional impl of either !DerefMut or DerefMut, both of which prevent the end user from providing an unsound impl of DerefMut.

Note that this also means that making Arc or Rc fundamental would be unsound unless an impl of !DerefMut is added.

[fee1-dead]

r? libs

@bors2 try

[rust-bors]

⌛ Trying commit f360584 with merge e658ebe…

To cancel the try build, run the command @bors try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: e658ebe (e658ebe4ffa1b515cd2ef06dd428d0d063e2ae6b, parent: 0f353363965ebf05e0757f7679c800b39c51a07e)

[fee1-dead]

@craterbot check

[craterbot]

👌 Experiment pr-144896 created and queued.
🤖 Automatically detected try build e658ebe
🔍 You can check out the queue and this experiment's details.

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[craterbot]

🚧 Experiment pr-144896 is now running

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[craterbot]

🎉 Experiment pr-144896 is completed!
📊 2 regressed and 7 fixed (676659 total)
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[Darksonn]

Seems like there are no real regressions. The openssl one it lists seems to be unrelated.

[Darksonn]

@rustbot label +I-lang-nominated

[traviscross]

We talked about this in the lang call. Why does this remove the impl rather than taking one of the approaches that had seemed more favored in the discussion in #85099?

[Darksonn]

You're right. Looking at this again I think the right solution is preventing the end-user from implementing DerefMut. That would make the impl of PinCoerceUnsized follow the safety requirements.