rustc should complain when language items are not correctly implemented.

[glandium]

While filing issue #48339, I made a mistake in how I was defining the panic_fmt language item. It was missing both a pub and #[no_mangle]. While rustc does complain when the language item is not present when building with no-std, it doesn't complain when it is present, but badly setup, and that leads to the error happening at the linker level. But by the time the linker errors out, the function has a different name, and the reason why it happens is not clear.

/checkout/src/libcore/panicking.rs:71: undefined reference to `rust_begin_unwind'

is the error as it appears, despite the language item having a different name (panic_fmt), and the function as defined in the source might have yet another name.

[ollie27]

Dupe of #9307?

[glandium]

Surely related, but that bug doesn't talk about pub and no_mangle, only about the function signatures (arguments and return value, which I've also noticed the compiler doesn't care about)

[pietroalbini]

Duplicate of #9307, closing this.

Even if that issue doesn't mention pub and no_mangle explicitly, I'm confident that will be checked as part of the function signature (if the check will be implemented in the future).