Add Clone impls for extern "C" and unsafe fns

[lilyball]

We only implemented Clone on extern "Rust" fns (for up to 8
parameters). This didn't cover extern "C" or unsafe (or
unsafe extern "C") fns, but there's no reason why they shouldn't be
cloneable as well.

The new impls are marked unstable because the existing impl for extern "Rust" fns is.

Fixes #24161.

[rust-highfive]

r? @pcwalton

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[alexcrichton]

@bors: r+ df95719

Thanks!

[richo]

I noodled on this a bit more generically. There are plenty of other supported ABI's in rust, which should all get a Clone impl, right?

I'm happy to do the copypasting for the other abi's, I'm just wondering if this is going to end up accidentally combinatorial.

[Kimundi]

A way to be generic over all ABIs would be nice:

impl<const ABI: &str, ...T, U> extern ABI fn(...T) -> U { ... }

[richo]

I just did it with More Macros™

[lilyball]

Supporting this for other ABIs would be nice as well. I didn't do it because I don't know offhand what other ABIs are available and I didn't really want to produce a huge explosion of Clone impls.

I agree that it would be nice to be able to be generic over ABIs, but I don't know how that would really work, as the ABI isn't a value but is in fact a syntactic element (even though it uses quotes).

[richo]

tbc, I'm +1 on this change. I'll PR my thing in the next few days, and then we can bikeshed on that PR?

[Kimundi]

@kballard Well it would probably involve adding values to the typesystem and redefining the syntax of a function type to take a string value at that position - nothing easily done without a bunch of RFCs post 1.0 though :).

[alexcrichton]

I would rather not add bindings to all ABIs that we support for now. With specialization we will be able to day impl<T: Copy> Clone for T which will solve many of these problems. For now this is otherwise just implementing Clone for the most common function parameters found in structures that want to use #[derive(Clone)]

[bors]

⌛ Testing commit df95719 with merge 0e5e669...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt