Change:
This is the most annoying and most useless error message ever if you don't know how to get a backtrace. The docs should state how to get one. Without a backtrace it's only helpful if you already know where the error isn't always the case. This: Returns:
A better option may be not the full traceback by default, but just make panic point to user's code, not code from std, as Perl's does, by unwinding stack trace up to the point where it goes out of "trusted" std code.
I know all the problems with unwinding, but it's already there when you print traceback anyway, why not make another step to users?
Nice! Now it's at least documented.

Discussion:
`panic!` is a macro, similar to `println!()` that we’ve already seen. Unlike `println!()`, `panic!()` causes the current thread of execution to crash with <del> the given message. </del> <ins> the given message. Because this function will cause a crash, it will never return, and so it has the type ‘`!`’, which is read ‘diverges’. </ins> <del> Because this function will cause a crash, it will never return, and so it has the type ‘`!`’, which is read ‘diverges’. A diverging function can be used as any type: </del> <ins> If you add a main function that calls `diverges()` and run it, you’ll get some output that looks like this: ```text thread ‘<main>’ panicked at ‘This function never returns!’, hello.rs:2 ``` If you want more information, you can get a backtrace by setting the `RUST_BACKTRACE` environment variable: ```text $ RUST_BACKTRACE=1 ./diverges thread '<main>' panicked at 'This function never returns!', hello.rs:2 stack backtrace: 1:     0x7f402773a829 - sys::backtrace::write::h0942de78b6c02817K8r 2:     0x7f402773d7fc - panicking::on_panic::h3f23f9d0b5f4c91bu9w 3:     0x7f402773960e - rt::unwind::begin_unwind_inner::h2844b8c5e81e79558Bw 4:     0x7f4027738893 - rt::unwind::begin_unwind::h4375279447423903650 5:     0x7f4027738809 - diverges::h2266b4c4b850236beaa 6:     0x7f40277389e5 - main::h19bb1149c2f00ecfBaa 7:     0x7f402773f514 - rt::unwind::try::try_fn::h13186883479104382231 8:     0x7f402773d1d8 - __rust_try 9:     0x7f402773f201 - rt::lang_start::ha172a3ce74bb453aK5w 10:     0x7f4027738a19 - main 11:     0x7f402694ab44 - __libc_start_main 12:     0x7f40277386c8 - <unknown> 13:                0x0 - <unknown> ``` `RUST_BACKTRACE` also works with Cargo’s `run` command: ```text $ RUST_BACKTRACE=1 cargo run Running `target/debug/diverges` thread '<main>' panicked at 'This function never returns!', hello.rs:2 stack backtrace: 1:     0x7f402773a829 - sys::backtrace::write::h0942de78b6c02817K8r 2:     0x7f402773d7fc - panicking::on_panic::h3f23f9d0b5f4c91bu9w 3:     0x7f402773960e - rt::unwind::begin_unwind_inner::h2844b8c5e81e79558Bw 4:     0x7f4027738893 - rt::unwind::begin_unwind::h4375279447423903650 5:     0x7f4027738809 - diverges::h2266b4c4b850236beaa 6:     0x7f40277389e5 - main::h19bb1149c2f00ecfBaa 7:     0x7f402773f514 - rt::unwind::try::try_fn::h13186883479104382231 8:     0x7f402773d1d8 - __rust_try 9:     0x7f402773f201 - rt::lang_start::ha172a3ce74bb453aK5w 10:     0x7f4027738a19 - main 11:     0x7f402694ab44 - __libc_start_main 12:     0x7f40277386c8 - <unknown> 13:                0x0 - <unknown> ``` A diverging function can be used as any type: </ins> ```should_panic # fn diverges() -> ! {