The #[panic_handler] attribute can be applied to non-functions

[phil-opp]

It's possible to apply the panic_handler attribute to an extern crate statement without errror:

#[panic_handler]
extern crate some_example_crate;

It only causes a linker error:

rust-lld: error: undefined symbol: rust_begin_unwind

(When the panic_handler attribute is not defined there is a much better "#[panic_handler] function required, but not found" error. There is also a nice error when the function signature is wrong. But it seems like the above case is not handled.)

[Centril]

cc @japaric

[japaric]

I'm very busy this week and probably won't have time to look at this until next Wednesday but if someone wants to take a stab at this I would look for the logic used by #[global_allocator]: that attribute gives you "error: allocators must be statics" when applied to non-static items.

[pnkfelix]

Visited for triage. Does not seem super critical to resolve immediately. Putting on the Release milestone (rather than RC2) to reflect this, and also downgrading priority to P-medium.