Remove T: Send + Sync bound from Arc::downcast

[maxdexh]

Discussion on zulip

Implements the change described in rust-lang/libs-team#859.
T-libs can pick if they want to skip the ACP or not.

@rustbot label needs-fcp T-libs

[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

 impl<A: Allocator> Arc<dyn Any + Send + Sync, A> {
 // stable diff
-    fn downcast<T: Any + Send + Sync>(self) -> Result<Arc<T, A>, Arc<dyn Any + Send + Sync, A>>;
+    fn downcast<T: Any>(self) -> Result<Arc<T, A>, Arc<dyn Any + Send + Sync, A>>;
 // unstable diff
-    fn downcast_unchecked<T: Any + Send + Sync>(self) -> Arc<T, A>;
+    fn downcast_unchecked<T: Any>(self) -> Arc<T, A>;
 }

@rfcbot fcp merge libs

I think this bound is probably further relaxable to T: 'static + ?Sized, but if we do that it can be follow-up across all of the downcast APIs on Box/Arc/Rc.

[rust-rfcbot]

@Mark-Simulacrum has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @cuviper
• @dtolnay
• @joshtriplett
• @the8472
• @thomcc

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/libs-ping: FCP proposed for libs, please feel free to register concerns.
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[maxdexh]

I think this bound is probably further relaxable to T: 'static + ?Sized, but if we do that it can be follow-up across all of the downcast APIs on Box/Arc/Rc.

Do you mean by returning Err for all unsized types? Ah I see, you'd also relax the bound on is. Yeah that should work