`panic_handler` can be an `unsafe fn`

This issue is a variant of #134225. I am filing this issue on behalf of @zesterer, who discovered this.

This code currently compiles:

#![no_std]

#[panic_handler]
unsafe fn handle(_: &core::panic::PanicInfo) -> ! {
    loop {}
}

Presumably, this causes this unsafe function to be called if a panic happens. This is unsound, as this calls an unsafe function without considering its safety requirements.

Cursory github search for code that relies on this compiling: https://github.com/search?q=language%3Arust+%2F%23%5C%5Bpanic_handler%5C%5D%5Cnunsafe%2F&type=code

Meta

Reproducible on the playground with version 1.100.0-nightly (2026-09-17 330d317121e16b5db8e5)

Reproduce the example in the playground with the reported nightly version, then read issue #134225 and trace the compiler handling for the #[panic_handler] attribute. The fix is complete when an unsafe panic handler is no longer accepted without accounting for its safety requirements, with regression coverage for this example.