Add Fn* blanket impls for Arc

[bsodmike]

Hi all,

Apologies if I've opened this issue in the wrong place; I want to work on extending Box<dyn Fn()> to Arc.

The Boxed implementation has been discussed here: #48055 and added in this PR #59500. Can I use this existing RFC or should we open a new one?

The feature gate could be called arc_closure_impls. This discrepancy was discussed by @jonhoo in his latest "Crust of the Rust" episode covering closures etc.

[ibraheemdev]

You can just open a pull request adding the implementations, it's not possible to feature gate trait implementations, and you likely don't need an RFC for a smaller change like this. This probably also applies to other pointers as well.

[bsodmike]

Got it, thanks!

[bsodmike]

Having looked at this much closer, I've learned the following:

• Due to Fn's trait-hierarchy and trait bounds defined on FnMut (specifically), we are unable to add the Fn impl to Arc<T>.
• One idea is to consider Arc<Mutex<T>> but this is not possible as Arc sits in alloc and Mutex is in std.

Based on discussions in Discord:

• Backward compatibility would break uses of alloc::sync::Arc in #![no_std] crates

My last progress: master...bsodmike:arc_fn_impls

Thanks, in no particular order to Discord users helping out:

• T-Dark
• Giuschi
• @jonhoo for the original idea.

[bsodmike]

Thanks to Giuschi, we realized there was another approach. Will open a PR shortly.

[bsodmike]

Feel free to close this, thanks!