Expose begin_unwind somewhere besides libstd

[kmcallister]

It used to be in librustrt but that's gone now. This is currently blocking my ability to enable derive(Rand) and derive(Rustc{En,De}codable) for no-std crates.

[Gankra]

CC @aturon was this intended?

[aturon]

We haven't had a great stabilization story for this usage, and this was a byproduct of moving rustrt into std.

It should be easy enough to expose something on an experimental basis, but will take more thought to develop a long-term, stable design.

@kmcallister If you wanted to make a PR just making this stuff public under std::rt, I'd be open to that.

cc @alexcrichton

[alexcrichton]

Due to recent changes to stability lints we now have two stable functions:

• http://doc.rust-lang.org/std/rt/unwind/fn.begin_unwind.html
• http://doc.rust-lang.org/std/rt/unwind/fn.begin_unwind_fmt.html

I believe, however, that the motivation for this issue is to mirror the same structure in libstd as it is in libcore to allow #[macro_reexport] in the standard library. Both these deriving modes, however, rely on libraries that depend on the standard library, so I'm not sure how much more relevant the motivation is any more.

[steveklabnik]

begin_unwind is gone entirely, and i believe that panic handlers cover this use-case today. Is this still a valid bug?

[alexcrichton]

Nah I don't think this is valid any more