Diagnostic for duplicate `#[panic_handler]` shouldn't talk about lang items

[jonas-schievink]

Given the following code:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=eda11ad1f58bb9579e1a515f26b30846

#![no_std]
#![no_main]

use core::panic::PanicInfo;

#[panic_handler]
fn panik(_: &PanicInfo) -> ! {
    loop {}
}

#[panic_handler]
fn panik2(_: &PanicInfo) -> ! {
    loop {}
}

The current output is:

error[E0152]: found duplicate lang item `panic_impl`
  --> src/lib.rs:12:1
   |
12 | fn panik2(_: &PanicInfo) -> ! {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: the lang item is first defined here
  --> src/lib.rs:7:1
   |
7  | fn panik(_: &PanicInfo) -> ! {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0152`.

"lang items" are an internal, perma-unstable implementation detail and should not be mentioned in this error message.

The error should probably also suggest a resolution, namely removing one of the panic handler functions.

This error is hit fairly often by people new to bare-metal Rust.

[jakubadamw]

This error is hit fairly often by people new to bare-metal Rust.

Also those who attempt to define a panic handler in a non-no_std program.