Add Box::downcast() for dyn Any + Send + Sync

[sdroege]

Looks like a plain omission, but unfortunately I just needed that in my code :)

[rust-highfive]

r? @cramertj

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[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.channel         := nightly
configure: rust.debug-assertions := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
skip untracked path cpu-usage.csv during rustfmt invocations
skip untracked path src/doc/book/ during rustfmt invocations
skip untracked path src/doc/rust-by-example/ during rustfmt invocations
skip untracked path src/llvm-project/ during rustfmt invocations
Diff in /checkout/library/alloc/src/boxed.rs at line 1387:
     pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
         if self.is::<T>() {
             unsafe {
-                let (raw, alloc): (*mut (dyn Any + Send + Sync), _) = Box::into_raw_with_allocator(self);
+                let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
+                    Box::into_raw_with_allocator(self);
                 Ok(Box::from_raw_in(raw as *mut T, alloc))
         } else {
         } else {
Running `"/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/rustfmt" "--config-path" "/checkout" "--edition" "2018" "--unstable-features" "--skip-children" "--check" "/checkout/library/alloc/src/boxed.rs"` failed.
If you're running `tidy`, try again with `--bless`. Or, if you just want to format code, run `./x.py fmt` instead.
Build completed unsuccessfully in 0:00:21

[KodrAus]

This does just seem like an omission to me.

@rfcbot fcp merge

This adds Box<dyn Any + Send + Sync>::downcast, which already exists for Box<dyn Any> and Box<dyn Any + Send>.

[KodrAus]

This could be added as unstable first, but that doesn’t seem necessary to me.

[sdroege]

@rfcbot fcp merge

@KodrAus I believe this did not work, maybe because the T-libs label was added later? Or at least it does not show up on rfcbot.rs

[GuillaumeGomez]

ping @rust-lang/libs , the fcg-merge instruction failed I think?

[m-ou-se]

@KodrAus in #80945 (comment):

This does just seem like an omission to me.

@rfcbot fcp merge

This adds Box<dyn Any + Send + Sync>::downcast, which already exists for Box<dyn Any> and Box<dyn Any + Send>.

@rfcbot fcp merge

[rfcbot]

Team member @m-ou-se has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @KodrAus
• @dtolnay
• @m-ou-se
• @sfackler
• @withoutboats

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!

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[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

The RFC will be merged soon.

[m-ou-se]

@bors r+

[bors]

📌 Commit 12014d2 has been approved by m-ou-se