How to define panic_fmt is not documented well

[habnabit]

This includes on https://doc.rust-lang.org/book/no-stdlib.html and https://doc.rust-lang.org/core/. The former has an incorrect signature on panic_fmt and the latter doesn't mention that you need a #[lang] annotation. The relationship between panic_fmt and rust_begin_unwind (the former is a lang item, and the latter is a symbol that the former produces) has been confusing people on #rust recently, and should probably be documented as well.

For reference, the correct way to define it is:

#[lang="panic_fmt"]
#[unwind]
extern fn panic_fmt(args: ::core::fmt::Arguments, file: &'static str, line: u32) -> ! {
    // ...
}

Every part of that signature (except the function name) is necessary; any omission can cause unclear or misleading errors, or lead to the function being called with the wrong ABI.

[oli-obk]

related: #9307

[steveklabnik]

Sigh, I just ran into this today. Will send a PR tomorrow.

[steveklabnik]

apparently tomorrow never came. I am willing to mentor anyone who wants to add this

[pepyakin]

It seems https://doc.rust-lang.org/book/no-stdlib.html is no longer exists
So what is preferred way of declaring panic_fmt lang item?

[steveklabnik]

https://doc.rust-lang.org/unstable-book/language-features/lang-items.html#writing-an-executable-without-stdlib

[pepyakin]

Thanks!