Throw core::panic!("message") as &str instead of String.

[m-ou-se]

This makes core::panic!("message") consistent with std::panic!("message"), which throws a &str and not a String.

This also makes any other panics from core::panicking::panic result in a &str rather than a String, which includes compiler-generated panics such as the panics generated for mem::zeroed().

---

Demonstration:

use std::panic;
use std::any::Any;

fn main() {
    panic::set_hook(Box::new(|panic_info| check(panic_info.payload())));

    check(&*panic::catch_unwind(|| core::panic!("core")).unwrap_err());
    check(&*panic::catch_unwind(|| std::panic!("std")).unwrap_err());
}

fn check(msg: &(dyn Any + Send)) {
    if let Some(s) = msg.downcast_ref::<String>() {
        println!("Got a String: {:?}", s);
    } else if let Some(s) = msg.downcast_ref::<&str>() {
        println!("Got a &str: {:?}", s);
    }
}

Before:

Got a String: "core"
Got a String: "core"
Got a &str: "std"
Got a &str: "std"

After:

Got a &str: "core"
Got a &str: "core"
Got a &str: "std"
Got a &str: "std"

[rust-highfive]

r? @joshtriplett

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[joshtriplett]

Seems reasonable to me, but I think a libs person needs to review this one.

[m-ou-se]

a libs person needs to review this one.

@rust-lang/libs

[Amanieu]

@bors r+

[bors]

📌 Commit 9890217 has been approved by Amanieu