core::panic!() cannot be used everywhere std::panic!() can.

[m-ou-se]

This compiles:

fn main() {
    let x = &5;
    x = &std::panic!()
}

This does not:

fn main() {
    let x = &5;
    x = &core::panic!()
}

error[E0308]: mismatched types
 --> src/main.rs:3:9
  |
3 |     x = &core::panic!()
  |         ^^^^^^^^^^^^^^^ expected integer, found `!`
  |
  = note: expected reference `&{integer}`
             found reference `&!`

The cause seems to be that std::panic!() expands to { some_function(..) } while core::panic!() expands to some_function(..), without { .. }. Adding { .. } in the core::panic!() macro_rules 'fixes' it.

[m-ou-se]

#80848 is an attempt at fixing this, by adding { .. } in the macro_rules! panic in core.

But as mentioned in #80848 (comment), the braces were not added in std::panic!() for this purpose, and seem to be left there by accident.

Is this a bug? Should this behave the same with and without those braces?

[eddyb]

The never type never ceases to surprise, does it? cc @nikomatsakis

I'd be worried about changing typeck behavior here, personally, I'm not even sure it would survive crater, and could have subtler implications still. Adding the braces to core::panic! should be fine.