no_std entrypoint diagnostic suggests invalid `#[no_mangle]` syntax in Edition 2024

[mousecrusher2]

Code

#![no_std]

use core::panic::PanicInfo;

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

fn main() {}

Current output

error: using `fn main` requires the standard library
  |
  = help: use `#![no_main]` to bypass the Rust generated entrypoint and declare a platform specific entrypoint yourself, usually with `#[no_mangle]`

error: aborting due to 1 previous error

Desired output

error: using `fn main` requires the standard library
  |
  = help: use `#![no_main]` to bypass the Rust generated entrypoint and declare a platform specific entrypoint yourself, usually with `#[unsafe(no_mangle)]`

error: aborting due to 1 previous error

Rationale and extra context

In Edition 2024, the no_mangle attribute must be written using the unsafe-attribute syntax:
#[unsafe(no_mangle)]

The current diagnostic therefore suggests syntax that is rejected under the edition being compiled.

I compiled with:

rustc --edition=2024 -C panic=abort repro.rs

#[unsafe(no_mangle)] is also supported in earlier editions, so the diagnostic should be able to use this spelling unconditionally rather than branching based on the crate edition.

Other cases

Rust Version

$ rustc --version --verbose
rustc 1.97.1 (8bab26f4f 2026-07-14)
binary: rustc
commit-hash: 8bab26f4f68e0e26f0bb7960be334d5b520ea452
commit-date: 2026-07-14
host: x86_64-pc-windows-msvc
release: 1.97.1
LLVM version: 22.1.6

Anything else?

No response

[mejrs]

We're moving away from this phrasing (including the hash/brackets) for that reason, so this should be changed to "with the no_mangle attribute"

cc @nnethercote who is working on improving the consistency in these messages

[nnethercote]

We're moving away from this phrasing (including the hash/brackets)

It's complicated... when the error message directly suggests text, I think it's reasonable to include the #[/] delimiters. Excluding the delimeters works better when the error message points at actual code.