Rollup merge of #125048 - dingxiangfei2009:stable-deref, r=amanieu

PinCoerceUnsized trait into core

cc ``@Darksonn`` ``@wedsonaf`` ``@ojeda``

This is a PR to introduce a `PinCoerceUnsized` trait in order to make trait impls generated by the proc-macro `#[derive(SmartPointer)]`, proposed by [RFC](https://github.com/rust-lang/rfcs/blob/e17e19ac7ad1c8ccad55d4babfaee1aa107d1da5/text/3621-derive-smart-pointer.md#pincoerceunsized-1), sound. There you may find explanation, justification and discussion about the alternatives.

Note that we do not seek stabilization of this `PinCoerceUnsized` trait in the near future. The stabilisation of this trait does not block the eventual stabilization process of the `#[derive(SmartPointer)]` macro. Ideally, use of `DerefPure` is more preferrable except this will actually constitute a breaking change. `PinCoerceUnsized` emerges as a solution to the said soundness hole while avoiding the breaking change. More details on the `DerefPure` option have been described in this [section](https://github.com/rust-lang/rfcs/blob/e17e19ac7ad1c8ccad55d4babfaee1aa107d1da5/text/3621-derive-smart-pointer.md#derefpure) of the RFC linked above.

Earlier discussion can be found in this [Zulip stream](https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/Pin.20and.20soundness.20of.20unsizing.20coercions) and [rust-for-linux thread](https://rust-lang.zulipchat.com/#narrow/stream/425075-rust-for-linux/topic/.23.5Bderive.28SmartPointer.29.5D.20and.20pin.20unsoundness.20rfc.233621).

try-job: dist-various-2

Author: matthiaskrgr