Stabilize PanicInfo::message() and PanicMessage

[StackOverflowExcept1on]

Resolves #66745

This stabilizes the PanicInfo::message() and PanicMessage.

Demonstration of custom panic handler:

#![no_std]
#![no_main]

extern crate libc;

#[no_mangle]
extern "C" fn main() -> libc::c_int {
    panic!("I just panic every time");
}

#[panic_handler]
fn my_panic(panic_info: &core::panic::PanicInfo) -> ! {
    use arrayvec::ArrayString;
    use core::fmt::Write;

    let message = panic_info.message();
    let location = panic_info.location().unwrap();

    let mut debug_msg = ArrayString::<1024>::new();
    let _ = write!(&mut debug_msg, "panicked with '{message}' at '{location}'");

    if debug_msg.try_push_str("\0").is_ok() {
        unsafe {
            libc::puts(debug_msg.as_ptr() as *const _);
        }
    }

    unsafe { libc::exit(libc::EXIT_FAILURE) }
}

$ cargo +stage1 run --release
panicked with 'I just panic every time' at 'src/main.rs:8:5'

• FCP: Tracking issue for PanicInfo::message #66745 (comment)

r? libs-api

[rustbot]

This PR modifies tests/run-make/. If this PR is trying to port a Makefile
run-make test to use rmake.rs, please update the
run-make port tracking issue
so we can track our progress. You can either modify the tracking issue
directly, or you can comment on the tracking issue and link this PR.

cc @jieyouxu

[SkiFire13]

    let mut debug_msg = ArrayString::<1024>::new();
    let _ = write!(&mut debug_msg, "panicked with '{message}' at '{location}'");
    let _ = debug_msg.try_push_str("\0");

    unsafe {
        libc::puts(debug_msg.as_ptr() as *const _);

Nitpick since it's just a secondary part of the example, but wouldn't this be unsound if the write! writes at least 1024 characters? Then the \0 won't be written to debug_msg and puts will read out of bounds.

[StackOverflowExcept1on]

@SkiFire13 if number of characters exceeds 1024, write_str will fail and be ignored due to the presence of let _ = write!(...), and then null terminator will be written to debug_msg. Thus, the program will output panicked with '. But this is just an example of how this might be organized in an embedded system.

Update: added correct handling of the null terminator in the case when write!() filled the entire buffer.

[tgross35]

It would probably be good to add an example in the API docs since the other methods have one. Can't use arrayvec::ArrayString, but you can just use std types with a note that it works even when String/Vec aren't available.

[StackOverflowExcept1on]

@tgross35 we already have minimal example in the documentation: https://doc.rust-lang.org/nightly/core/panic/struct.PanicInfo.html#method.message. Although it may not show how to format panic message without allocator and standard library.

[tgross35]

Was looking at the stable docs and missed that, thanks

[m-ou-se]

@bors r+ rollup

[bors]

📌 Commit bb00657 has been approved by m-ou-se

It is now in the queue for this repository.