Commit209703a

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Rollup merge of#125072 - Darksonn:pin-dyn-dispatch-sound, r=jhpratt

Add test for dynamic dispatch + Pin::new soundnessWhile working on [the `#[derive(SmartPointer)]` RFC][1], I realized that the soundness of <code>impl [DispatchFromDyn](https://doc.rust-lang.org/stable/std/ops/trait.DispatchFromDyn.html) for [Pin](https://doc.rust-lang.org/stable/std/pin/struct.Pin.html)</code> relies on the restriction that you can't implement [`Unpin`](https://doc.rust-lang.org/stable/std/marker/trait.Unpin.html) for trait objects.As far as I can tell, the relevant error exists to solve some unrelated issues with coherence. To avoid cases where `Pin` is made unsound due to changes in the coherence-related errors, add a test that verifies that unsound use of `Pin` and `DispatchFromDyn` does not become allowed in the future.[1]:rust-lang/rfcs#3621

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	`1`	`+use std::marker::PhantomPinned;`
	`2`	`+use std::pin::Pin;`
	`3`	`+`
	`4`	`+traitMyUnpinTrait{`
	`5`	`+fninto_pinned_type(self:Pin<&mutSelf>) ->Pin<&mutPhantomPinned>;`
	`6`	`+}`
	`7`	`+implMyUnpinTraitforPhantomPinned{`
	`8`	`+fninto_pinned_type(self:Pin<&mutSelf>) ->Pin<&mutPhantomPinned>{`
	`9`	`+self`
	`10`	`+}`
	`11`	`+}`
	`12`	`+implUnpinfordynMyUnpinTrait{}//~ ERROR E0321`
	`13`	`+`
	`14`	`+// It would be unsound for this function to compile.`
	`15`	`+fnpin_it(not_yet_pinned:&mutPhantomPinned) ->Pin<&mutPhantomPinned>{`
	`16`	`+Pin::new(not_yet_pinnedas&mutdynMyUnpinTrait).into_pinned_type()`
	`17`	`+}`
	`18`	`+`
	`19`	`+fnmain(){}`

Lines changed: 9 additions & 0 deletions

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	`1`	+error[E0321]: cross-crate traits with a default impl, like `Unpin`, can only be implemented for a struct/enum type, not `(dyn MyUnpinTrait + 'static)`
	`2`	`+ --> $DIR/pin-dyn-dispatch-sound.rs:12:1`
	`3`	`+ \|`
	`4`	`+LL \| impl Unpin for dyn MyUnpinTrait {}`
	`5`	`+ \| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type`
	`6`	`+`
	`7`	`+error: aborting due to 1 previous error`
	`8`	`+`
	`9`	+For more information about this error, try `rustc --explain E0321`.

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