#[repr(u64)], #[repr(C, u64)], #[repr(u64, C)] have different behavior

[lambda-fairy]

Code

use std::mem;

#[repr(u64)]
enum BookHorses { TwilightSparkle }

#[repr(C, u64)]
enum PurpleHorses { TwilightSparkle }

#[repr(u64, C)]
enum PrincessHorses { TwilightSparkle }

fn main() {
    println!("{}", mem::size_of::<BookHorses>());
    println!("{}", mem::size_of::<PurpleHorses>());
    println!("{}", mem::size_of::<PrincessHorses>());
}

Output

8
4
8

i.e. when the compiler sees multiple repr arguments, it takes the first one and ignores the rest.

Expected behavior

Either all enums should have the same size, or the latter enums should not compile.